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Plant Roots The Hidden Half Amram Eshel, Tom Beeckman

Cellular Patterning of the Root Meristem: Genes and Signals

Publication details https://www.routledgehandbooks.com/doi/10.1201/b14550-6 Kimberly L. Gallagher Published online on: 17 Apr 2013

How to cite :- Kimberly L. Gallagher. 17 Apr 2013, Cellular Patterning of the Root Meristem: Genes and Signals from: Plant Roots, The Hidden Half CRC Press Accessed on: 27 Sep 2021 https://www.routledgehandbooks.com/doi/10.1201/b14550-6

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The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The publisher shall not be liable for an loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 sidering multiple different taxa. In this edition, I will focus on focus will I edition, this In taxa. different multiple sidering an provided con- function and and form RAM of RAM analysis comparative excellent the of properties mechanical and cal physi- the discussed chapter this book, this of editions previous In form. they how and function, they how structured, howare to they given been has attention much plant, adult the of sues signals. abiotic and tobiotic appropriately respond to optimize water,and and light to like resources of acquisition structure, the overall its adjust to plasticity the plant the allows plan body adult the of development Postembryonic respectively). the RAM, and (SAM apices: meristems apical root root and shoot and coor- shoot the of in cells action stem of groups the dinated through postembryonically patterned is body plant adult the of much because is This seedling. minated ger- newly the in lacking completely or underdeveloped be may structures These organs. and appendages lateral multiple with structure elaborated highly a often is plan body plant adult The to adult. the when one a newly compares emerged seedling plant minia- true not in is same The and toes. 10 adult 10fingers, legs, 2 the arms, 2 ture: essentially is it pattern, overall to respect With parent. the to relates child infant an how see to easy is It I. Pennsylvania of University Gallagher L. Kimberly © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis

As the SAM and RAM collectively give rise to all of the tis- the of all to rise give collectively RAM and SAM the As Introduction XV. References XIV. XIII. XII. VI. III. II. XI. V. IV. I. VIII. X. VII. IX.

Conclu Import Stem Cell N Cell Stem Stem Cell i Cell Stem Reform Structu Sys Root SAM Definit Anatomi Introduction Import Contro Signifi Change ...... Cellular Patterning of the Root ...... cance of an Open versus a Closed Meristem aClosed versus Open of an cance ion of the Termion of the “Meristem” ance of Cell Signaling in Root Growth Root in Signaling of Cell ance l of Meristem Size l of Meristem ance of the QC in Maintaining Root Growth Root Maintaining in QC of the ance sions...... re of the RAM re of the ation and Maintenance of aQC Maintenance and ation s in the Patterning of the Meristem of the Patterning the s in tem cal Coordinates cal Meristem: Genes and Signals n the Root Meristem Root n the iche Concept in the Root the in iche Concept ...... is apical to the cells labeled “b” and “c.” Conversely “basal” is is “basal” Conversely “c.” and “b” labeled cells the to apical 3.1B is Figure in “a” labeled cell epidermal the example, For tip. is toward the described, being or feature structure to relative the that, location a describes “apical” term the shoot, and both root the in Consequently 3.1A ). and (Figure shoot the respectively as “apex,” to root referred are root the of tip the the of tip and shoot the convention Therefore, By 2005). here. defined al. be et will terms (Baluska these division cell and growth of directions relative defining to respect with particularly lit- erature the in terminology anatomical of use inconsistent is There II. RAM. tothe for comparison abasis as SAM of the description brief a and terms anatomical of definitions provide I RAM, the to Prior discussing root. of the patterning the roles in important play clearly very they here, coverage although modest only given are topics these signaling, hormone and root the of patterning sepa- vascular on entirely are concentrate that there book this As in chapters rate systems. possible, other When to system. drawn are this comparisons in done been and has formation maintenance RAM of analysis genetic and molecular the of to paid is attention Particular angiosperms. of RAMs the of maintenance and regulation the ...... a natomical Coordinates ...... development much as Arabidopsis 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 -21 -21 -15 -10 -18 -16 -11 -14 -4 -8 -8 -3 -3 -2 -1 -1 -1 Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 terms periclinal and anticlinal are very helpful in describing describing in helpful very are anticlinal and periclinal the terms While organ. the in layers cell of number the elongation increase cell and of plane the within occur largely divisions cell the In axis. proximal–distal the in cells of number the increases division cell growth. anticlinal of root type The of axis primary the and epidermis the to dicular The elongation. root of axis “1”labeled divisions 3.1Bin Figure perpen- and C have occurred orientations: the to two parallel in or occur perpendicular can divisions cell anticlinal the In root, surface. organ the to angle) right a (at that wall perpendicular is cell new a create divisions cell Anticlinal the epidermis. usually organ, the of surface the to relative division cell orientation of the describe to used are 3.1D) (Figure “periclinal” root-ward. is mal proxi- shoot, the in whereas shoot-ward root, proximal, the is In and “root-ward” terms 2010).et al. (Baskin “shoot-ward” in favor and distal of proximal the adopted have some Recently “b.” to proximal is “c” cell 3.1B, Figure In “distal.” is away whereas junction, root–shoot the toward indicates “Proximal” junction). root–hypocotyl the (usually shoot the and root the of junction the to relative feature a of position the describe to used also are collet zone in in zone collet the as 3.1A; to by Dolan referred Figure in (dottedshoot line the joins root the where point the toward and apex the from away to adds right the on one the whereas body, plant cap. the root the to contributes left the on division (C).The in indicated are divisions cell (D).“T” in images the to refer divisions these with associated numbers The white. in cells the in lines black bold as shown are divisions (C), (B) and oriented both In the an through slice transverse (B)A labeled. are terms Directional line. dashed the 3.1 FIGURE 3 © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis -2 With respect to cell division, the terms “anticlinal” and and “anticlinal” terms the division, cell to respect With root meristem. The lowercase labels “a,” “b,” and “c” on the column of cells in (B) are for orientation and are referenced in the text. thetext. in are referenced and in (B)are for orientation cells “b,” of and“a,” column thelabels “c” on The lowercase meristem. root Arabidopsis

Anatom Arabidopsis root-ward proximal root-ward (A) ical coordinates. (A) Cartoon image of a hypothetical seedling. The junction between the root and shoot is indicated by indicated is shoot and root the between junction The seedling. hypothetical a of image Cartoon (A) coordinates. ical Apical Basal distal ). The terms “proximal” and “distal” “distal” and “proximal” terms The ). Root RAM SAM Shoot root, periclinal periclinal root, Arabidopsis shoot-ward proximal Basal shoot-ward Apical distal (C)

(B) T 1 2 eeal casfe a tps f eitm. n eiig a more defining thethat oftheEsau suggested definition termmodern meristem, In meristems. of types as classified generally to add are (whichboth today although respectively), cambia bark, and cork the vasculature and vascular the to have applied not been would meristem term the conception, Therefore, original cambia. its cork in and vascular the of those existing like an organ, to tissues new added that those from an organ produced entire that cells of populations distinguish to was term (von RAMs 1858).Nägeli ofand Part forthe motivation new developing this SAM the example, for organs, entire to rise give to able are that plant the in cells von of populations Wilhelm to refer to Carl Nägeli by 1858 in coined was meristem term The III. cell. of the axis tothe relative orientation of and the orientation maydivision, cell be described the explaining in useful are anticlinal and periclinal than other descriptors case, In this oblique are to root the surface. divisions most and of cell formative the 3.3E F) is and Figure a tetrahedral in cell apical the example, For surface. organ the to or perpendicular parallel not oriented are clearly not are cells divisions when when or divisions cuboidal to referring when confusing be can they meristem, root the in divisions cell of orientation the Lam. (discussed in more detail later; see see later; detail more in (discussed Lam. filiculoides Azolla

root. (C) Medial longitudinal cross section through through section cross longitudinal (C)Medial root. Arabidopsis 3 Definition of the of Definition 1 2 3

b a c T (D) T 3 2 1 Anticlinal Periclinal Anticlinal erm “Meerm Root Structure ristem” Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 central zone of the meristem extends over a portion of the cells cells the of portion a over extends meristem the of zone central The angiosperms. in gene expression and division cell ference in dif- describe to used be can but1943) (Foster gymnosperms in differences histological upon 1992).based (Medford originally were zones (RZ) These zone rib the and (PZ), zone peripheral (CZ), zone the central organization—the of ferentcellular zones created. is of cells mass acentral so irregular is corpus the in divisions of orientation the contrast, In layers. cell L1 L2 and single maintain they so, doing in and anticlinally, divide to tend tunica the in body). Cells the (literally corpus the comprises L3 The meristem. the of covering outer tunic)—the the (literally tunica the comprise SAM the of layers L2 and L1 In corpus. divisions, the and cell tunica the of regions: two into orientation the upon based divided, is stem organization model, the meri- the and ofbehavior In the SAM. the tunica–corpus describe to today used widely are that 1943) Foster (from model zonation meristem the and 1924) Schmidt by (originated model tunica-corpus the models: of two lishment estab- the to lead has development SAM of regularity The way. predictable a in proceeds generally tissues and organs lateral of 1996). Sussex and Szymkowiak by (reviewed lineage than rather position determined by are plant the in decisions fate cell most though as appears it examined, when Indeed position. its by determined is fate cell Instead, organs. lateral of formation the in invariant nor predetermined neither is identity chimeras cellular that L1–L2 shown have and ablations cell with experiments Instead, layers. these in cells the between differences developmental ent inher- to due not is this However, 2003). Battey and Tooke by (reviewed tissue vascular the layer, L3 the and epidermis the to rise gives generally layer L1 The may cells. of layers and multiple contain organized less is layer L3 the whereas cells, of sheets single are layers L2 and L1 the general In layer. cell the outermost being L1 the with L3, and L2, L1, the labeled generally are that layers distinct three to two into organized is angiosperms SAM ofThe most meristems. or floral axillary, thorns, be leaves, may formed organs the development, of stage the and plant the developing between organ (Esau primordia 1965). on Depending in lies that structure dome-shaped to flat a generally is and axis tipthetheshoot of at is located The SAM SAM. of the action the of All in the the tissues aboveground plant are produced through IV. chapter. this in later discussed be will which animals, in to stem cells similar are cells meristem way, this In 1965). (Esau organ the of life the throughout state undifferentiated relatively competent and a mitotically in tained main- is meristem the that in cells of clones small or cells new produce to organ an within occasionally divide that cells from distinct are cells meristem that out points Esau organs. or sues, tis- cells, new produce to plant the of life the throughout divide that cells active mitotically of group a of composed is meristem Root Meristem of the Patterning Cellular © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis Overlaid upon the tunica-corpus organization are three dif- three are organization tunica-corpus the upon Overlaid formation the SAM, the in invariant not is fate cell Although

S A M the the Arabidopsis RAM makes no lateral organs; instead, when cells in the RAM RAM the in The cells ). when 3.2 instead, Figure organs; in lateral no green makes RAM com- in colored that (false cells cap of root layers the prise protective by apex its at covered is (RAM). meristem apical root the in divisions on relies roots these of all of the growth formed once origin, of independent However 2009). et al. Rebouillat by (reviewed roots lateral the or root seminal the of the formation on or no effect little but have roots, of crown tion forma- the inhibit that mutations are there of For root. example, type each of formation inhibit that mutations specific are there origins, developmental different have roots crown and lateral, seminal, As systems). root rice in observed is branching order fifth- to (up laterals additional generate themselves of can some slender), which and large (both roots lateral to rise give ally gener- roots crown the and seminal the Both roots. adventitious considered are therefore and tissue root to opposed as plant the growth. of of stem the stage from emerge and rice in seedling prevalent are roots Crown the through only short persisting is root lived, rice seminal the The emerges. germination root Upon (embryonic) rice. seminal is this of example prime A 2006). et al. (Aloni roots system lateral branched root of multiply composed fibrous a form that roots multiple of adventitious and emergence the lateral with correlated often is growth of Cessation growth). (determinate ceases growth then of and period time limited a for grows it lived; short often is root pri- mary the monocots, In 6). Chapter (see root parent the of layer pericycle the within from emerge and taproot the than smaller generally are roots Lateral 2008). et al. (Shishkova rare probably are roots to indeterminate Truly said 1960). is (Sinnot plant indeterminate the be of life the throughout grow) maintains to least ability at (orthe grow to continues that root primary A plant. the of life the throughout grow to continue may that root tap- prominent a forming lived, long often is root primary the dicots, 1961). In (Clowes root) (primary radicle the is seed ing germinat- the from emerge to structure first the plants, most In V. 2003). TookeVernoux Battey 2002; and and Traas by (reviewed meristem the of flanks the at are produced appendages lateral and increased is plant the of height the way, this In 1997). Hake and (Kerstetter tissues stem the to ily primar- cells contribute and SAM the of height the increase RZ the in Divisions zones. peripheral and central the between diate zones and at peripheral and central divides a rate that is interme- as RZ The the subtends organ differentiate primordia. eventually cells these As and CZ the from CZ. further pushed are the cells daughter their in divide, cells sister their than rapidly more divide zone peripheral the in Cells place. takes initiation organ of toward periphery the CZ the cells into daughter where PZ, the their displace so doing clear in and now divide do CZ is the of cells it the However,that species. many of RAM the in found once to center quiescent cells the were akin to thought be quiescent, CZ largely the of cells The layers. corpus and tunica the in Unlike the SAM, the RAM is a subapical structure that that structure subapical a is RAM the SAM, the Unlike r oot System 3- 3 Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 Rost 1996; Groot et al. 2003; Rost 2010). Rost 2003; et al. Groot 1996; Rost and body and root Baum 1993; et al. the (Dolan cap/columella in root the in distally proximally both situated cells mature more with cap root the mature below position least subapical a the occupy cells which in cells of continuum a of comprised is then root The cells. cap root of number of constant relatively a maintenance the in resulting off, sloughed of are cells cap root apical the most the RAM, are the in cells divisions columella through new added As columella. as differentiate imme- they diately Instead, differentiation. before divisions of series In cap. root the enter division cell during apically Cells displaced are that endodermis. apex root the the in in form strips Casparian the and differentiation zone the in 3.2) (Figure epidermis hairs the root from emerge example, For adopt to fates. begin developmental cells respective differentia- which their in cell region the the is is which zone zone, tion elongation the to Basal expansion occurs. cell rapid where zone, elongation root the enter- into and ing meristem root the the exiting of zone eventually before peripheral SAM) the in cells of behavior the to (similar times several to one into divide generally basally they root, the of body displaced the are cells As 1961). (Clowes to root the body apically of the to either contribute to basally or displaced cap root the are to contribute cells daughter their divide, red. in colored false are cells root initial the and green, in colored false been has cap root The labeled. are root the of zones different The Wu). Shuang by provided (kindly root an through section medial single blue–stained Toluidine 3.2 FIGURE 3 © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis -4

Se c (See Primordia Root hair Developmental zones in the root. root. the in zones Developmental insert.) olor Arabidopsis Elongation zone Meristem , these cells do not undergo a undergo not do cells these , Arabidopsis that have closed meristems such as as such meristems have closed that the be state. may ancestral open that suggest Seago and Heimsch meristems, open have to tend angiosperms basal many As apex. root the organization in of degree the and cap root the from is body root the distinct how upon based subclassifications different into fifteen organization open and closed the out breaking sification, clas- Guttenberg’s extended Seago and Heimsch Likewise lies. fami- many in organization meristem closed and open both of examples were However,there groups. other than organization closed exclusively an have to likely more were angiosperms that found Seago and Heimsch comparisons, these From eudicots.” and monocots, angiosperms, of basal 132 orders and 45 families Seago “over and examining Guttenberg workof the upon Heimsch (2008)expanded by work Recent body. root the and to cap the basally to apically both added that daughters produce to divide would tip root the in cells progenitor single did instead, not; meristem open an with Roots layers. cell cap root and sue tis- ground distinct clonally had meristem closed a with Roots “open.”or “closed” either as roots classified Guttenberg related, (1960). Based upon whether the root cap and cortex were clonally by Guttenberg descriptions early in important also was cap root 1917; (Schuepp grew root the Clowes 1961). as off sloughed cells regularly these as were not, did divisions Kappe-type whereas root, the of diameter the increased divisions Körper-type that noted also Schuepp development. with changed or root to dif- root from fered boundary the others in whereas consistent, and clear was cap the and Korper the between boundary the plants, some in that found Schuepp base. the toward pointed T the cap, the in T pointed toward in the crosshatch the root which apex, whereas a produced body root the (root in Kappe cap). the Divisions and body) (root Körper the into root the divided Schuepp junction, of this orientation upon the Based cells. related of clonally series in a to longitudinal from of reorientation transverse division cell the by formed apparently were 3.1C These T-junctions ).(Figure a of formation the T-junction in resulted files cell these of gence when files weretwo withcell thefile, onecontinuous cellconver- anddifferent roots various that noted through sections gitudinal howdescribe the root grows, Schuepp (1917) fixed lon- examined of isthe This model Körper–Kappe divisions. cell (Clowesorientation 1961). prevailing To the of basis the on domains two into tunica– the to root the it separates likened because organization of SAM model corpus been has growth root of descriptions early the of One behavior. and development its describing inter- in est much root been has primary there the roots, of lateral cells the formed the once of and all to rise gives RAM the As VI. tal to the tip of the stele (stellar pole). This observation led to led progenitor of dedicated population observation a subapical that proposal the This pole). (stellar stele the of tip the to tal dis- is Scheupp’softhat T) crosshatch or head (i.e.,the vergence con- of point a to files cell individual back trace to possible is it In longitudinal sections through the RAM of many species species many of RAM the through sections longitudinal In the and body root the into apex root the of separation The

Structure of the Structure RA Arabidopsis M , rice, and maize, maize, and , rice, Root Structure Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 columella initial; S, stele initial; CEI, cortical endodermal initial; LRCI, lateral root cap initial. cap root lateral LRCI, initial; endodermal cortical CEI, initial; stele S, initial; columella CI, center; quiescent color. QC, same the with labeled are types cell common All by lowercase. indicated are types Cell letters. capital with labeled et al., B. Gunning, upon (Based cap. root the to rise give cell an apical with the below (RCI) labeled and initials colors cap Root (M).different in shown are merophytes three and green, in shown is (AC) cell apical The (F). in close-up and (E) in an shown 2010). of tip The et al. Courdert upon based (Images (E/LRC) initial an share cap root lateral and epidermis the and distinct, are in contrast, In CCEI. the from derived (p) are epidermis (x) the and exodermis the tissue and ground (s), the schlerenchyma rice (e), (c)In endodermis layers. cortex the multiple and including tissue ground the in cells specialized different multiple has root rice the the of in (D). structures in The isshown and enlarged (C) (B).in meristem rice Thecells initial the 3.3 FIGURE variously been have cells initial The 1965). (Esau initials as to referred identities now generally are 1961). Clowes 3.3; cells cell (Figure These predictable and organ the within positions able predict- with files cell of number constant a to rise give to way in a predictable divide cells these of In absence the perturbation, thelayerscell ofthethat root. growing maintains been identified has apex the in region localized a roots many in out, borne tally experimen- been not has identity cell and lineage cell between relationship strict a While fate. cell determines lineage cell that calyptro- a B), and 3.3A Arabidopsis like (Figure dicots in common is dermatogen example following the in illustrated As proposed. was a calyptro-dermatogen related, ally clon- were epidermis the Clowes and cap 1874; root the where 1961). cases In (Janczewski calyptrogens the histogen, fourth a added Janczewski Later animals. in layers germ primary three respectively 1870).(Hanstein to In werethe conception, the histogens similar tissues, stele and cortex, epidermis, the produced that cells new of source continuous a provided histogens These plerome. the and periblem, the dermatogen, the exist: histogens distinct three that proposed and histogens progenitor cells these of in the SAM (Clowescells 1961; Barlow 1976). labeled Hanstein population CZ to the similar roots, of these tip at the exists cells Cellular Patterning of the Root Meristem of the Patterning Cellular © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis Implicit in Hanstein’s theory of histogenesis was the concept concept the was histogenesis of theory Hanstein’s in Implicit (B)

(See c (See CED

Cellular organization of three root meristems. meristems. root three of organization Cellular insert.) olor E/LRC CEI (A) QC S Arabidopsis CI QC e e c

p . (C) (D) Rice in in cells initial that the of behavior The divisions root. the of length the transit-amplifying increase in symmetrically divide layers cell or additional generate to divisions) asymmetri- T Körper divide (in also cally itself may meristem the of center the from displaced is that daughter The cap. root the and the stele between position its retains cell initial the while cap, root or the that add division one cell files to cell the plant each asymmetric body with cell linear the of points end the at cells the as initials meristem root the define et al. (1993). Scheres et al. (1969),Dolan and Seago (1965), Esau from ideas essential combines it (1996) as et al. Scheres of that is here used be will that definition The behavior. and root the in position both upon based defined ( daughters their and cells initial the of divisions asymmetric the through generated are cells cap root and epidermis, cortex, the filiculoides Azolla s opsd f h cluel ad h ltrl ot a (LRC) cap root lateral the and collumella the of composed is which cap, root the is (S). apex stele root the Atthe comprise cle pericy- the with along which tissues, of vascular cylinder central a surround layers cell four These (e),pericycle. and endodermis (p), layers of epidermis (c), concentric cortex of single composed Figures C QC QC Upon germination the primary root of Uponthe primary germination Arabidopsis thaliana Arabidopsis CI S is shown in (A) and cropped and enlarged to show to enlarged and cropped (A)and in shown is Arabidopsis 3.3 and and 3.3 CCEI (E)

LRCI 3.4 wtr en poie od xmls f how of examples good provide fern) (water e

). c M and rice roots are similar. However, similar. are roots rice and Arabidopsis

M c s s Azolla M c (L.) Heynh., Heynh., (L.) c x x p Planta (F) , 143, 121, 1978.). Initial cells are are cells 143,121,, 1978.). Initial

Oryza sativa Oryza RCI RCI M is thaliana Arabidopsis Arabidopsis AC L. (rice), and (rice),and L. root is is root Azolla , these tissue tissue these , 3- 5 Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 and symmetrically in the meristem until they are displaced displaced are they until meristem the in symmetrically and transversely divide to continue will cortex and endodermis the in cells formed, Once layers. cell cortical and endodermal arate (a QC and periclinally divides Körper T division) to produce sep- the from proximally displaced is which daughter), endodermal (cortical CED the cell, daughter a produce to anticlinally once source of each CEI the Generally divides cortex and endodermis. ultimate the are CEIs B). The and 3.4A and B and 3.3A (Figures (CEIs)the ofQC on initials are visible flanks the endodermal cal zone. meristematic the in cells transit- of number the the increase transiently in cells amplifying (anticlinal) divisions divisions, cell formative these Following cap. root lateral the endoder- and and epidermis the and cortex mis the to rise give tier third and second the in initials common contrast, In respectively. files, cell stele and columella the to rise give initials of tiers lower and upper the in cells of initial divisions dedicated 1993). and anticlinal Periclinal et al. Dolan by defined (QC; center as quiescent the encircle and into tiers that three are located the between stele and the root cap arranged are cells initial The B). and 3.3A (Figure daughters ate immedi- their and cells initial the are root of the pattern the ing maintain- for responsible primarily cells The 2002). et al. Baum 2000; Scheres and Rost Benfey 1996; and Baum 1996; 1994, et al. Scheres 1993; et al. (Dolan apex root the in occur that divisions cell stereotypical of series a through postembryonically tained gener- is root main- then 1994) the and et al. (Scheres embryogenesis of during ated pattern cellular The 1993). al. et (Dolan initial; initial. columella CI, endodermal initial; cap cortical root CEI, lateral epidermis E/LRC, division. periclinal inhibits SMB whereas division, cell periclinal promotes initials the in expression (blue).FEZ (E) SMB and (magenta) FEZ of (D)Expression orange). (lighter tissue ground and stele the throughout present and (C) activity. SCR and SHR (CED)both daughter requires endodermal cortical the in line) shown division (dotted (B)movement. SHR asymmetric The indicate endodermis the in dots yellow the endodermis; the moves into and stele 3.4 FIGURE 3 © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis -6 In longitudinal cross sections through the root tip, the corti- the tip, root the through sections cross longitudinal In

(See co (See Proteins involved in in involved Proteins lor insert.) (A) (B) CEI SCR SHR E CED C root patterning. (A) SHR (yellow) and SCR (green) as SHR is transcribed in the the in (A) transcribed is SHR as (green) (yellow)SCR SHR and patterning. root Arabidopsis (C) (E) E/LRC SCZ et al. 1993; Baum and Rost 1996). Following the division of the the of division the Following 1996). Rost and Baum 1993; et al. 3.3B (Figures QC the to distal diately imme- are that (CIs) cells initial of divisions transverse the by produced are cells columella The cap. root lateral the mella and (Miyashima et al. 2009). et al. (Miyashima ( in pat-mutations as suggested been also has terning radial in RNAs small for role A 2002). et al. (Mylona mis epider- the through tearing from ofinside root, the dramatically ten Hove et al. 2010). In the absence of absence the 2010). In et al. Hove ten 2010; et al. Pernas 2010; (Doerner epidermis as differentiating from layer tissue ground Cortex the outer in cells prevents specification cortex. the specifying for required is factors, scription ( XIV ofspecification the in endodermis more(discussed detail for independently required also is protein SHR The 2000). et al. Helariutta 1996; et al. Laurenzio Di B; and 3.4A (Figure CEDs ( SCARECROW GRAS family transcription factors, factors, transcription family GRAS 1993).et al. initials for( and cortex endodermis initials separate produce to periclinally divide may itself CEI the types, However,file. cell in the daughters ineco- more other undivided or two are there that so periclinally divides cell CED the before delay a be can there ecotypes, some In zone. elongation the into SCZ AGO1 In In Genetic analysis of has shown that root two related analysis Genetic patterning ; Helariutta et al. 2000; Nakajima et al. 2001). et al. Nakajima 2000; et al. ; Helariutta ) (Figure 3.4C), a protein with similarity to ) heat 3.4C),(Figure shock tran- a similarity protein with Arabidopsis or ) CIs expression is highest in the QC and initials (dark orange) initials QC and the in is highest SCZ expression cause extra ground tissue layers in the root root the in layers tissue ground extra cause HYL1 (D) SCR h ro cp s opsd f oh h colu- the both of composed is cap root the ), promote the asymmetric divisions of the the of divisions asymmetric the promote ), SMB FEZ E/LRC Figures 3.3B Figures ( SHORT-ROOT and 3.4D and E) (Dolan (Dolan and E) 3.4D and SCZ , root hairs emerge emerge hairs root , and 3.4B; Dolan Root Structure ARGONAUTE 1 SCHIZORIZA SHR Section Section ) and and ) Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 role in cell identity (Willemsen et al. 2008). et al. (Willemsen identity cell role in et al. 2010).(Bennett FEZ to In appears cap contrast, not play root a direct the of maturation the controls also BEARSKIN2, and LRC. and BEARSKIN1 columella proteins, NAC-domain other two the with along SMB in layers cell extra and divisions cell of sion eaae rm l ohr el aes f h ro (lws 1994). (Clowes root the of layers cell other all from separate layers and related, the cell root are clonally cap dermal is entirely epi- and cortex the rice, In (Clowes2000). monocots and dicots between differs cells root cap and epidermal, root ontogeny cortical, the of and the types), different the between varies layers cortex of number (the ( cylinder 3.3C vascular the surround that cells of rings into of bition E/LRC and CI the inhi- tobe of appears SMB function The primary in cells. daughter specifically expressed is SMB cell division. periclinal promotes and initials E/LRC and columella the both in expressed is protein FEZ The initials. E/LRC and mella ( factors, NAC transcription two domain–containing that showed (2008) al. et Willemsen pathways. tory regula- share they related, COLLRC the notthe are and clonally although Interestingly soil. the root be through lost grows the as placed into the elongation ofzone, whereas cells the root cap will dis- be eventually will epidermis the in cells division, continued With files. these in of cells number the to increase epidermis the in the LRC occur and divisions Once transit-amplifying formed, types. cell two these of formation the to leads that divisions sequence of the in flexibility some therefore is There 2006). (Campilho et al. cells LRC new of formation the in epi- resulted as dermis develop to predicted have would one cells in divisions cell periclinal imaging time-lapse During invariant. not is sions divi- these of timing However,the 1996). Rost and Baum 1993; et al. LRC, (Dolan epidermis the produce the divisions produce anticlinal whereas cells E/LRCI the of divisions cell periclinal E/LRCI to ForCIs. QC the apical are encircle the and most the part, cells The 3.4).and 3.3 (Figures epidermis the and cap root lateral the both produce E/LRCIs) initials; cap root lateral mal epider- (the cap root lateral the to rise give that cells the mella, species 1981). (Clowes these of caps root the in cells columella new of the source near only the not are generally initials columella the division that of suggests which QC, rate lowest the the of with region cap columella the root in divisions cell of gradation a is For cap. root the both in in example, divisions proliferative other are in however, there 2008); species, al. et in Willemsen 1993; columella al. et the (Dolan of production the in divisions transit-amplifying no are There cells. these in granules of starch accumulation the by evidenced is This columella. as entiate differ- cells daughter displaced apically the whereas cells, initial as remain QC the with contact in are that cells daughter the CI, Cellular Patterning of the Root Meristem of the Patterning Cellular © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis SMB Similar to the the to Similar n otat o h ddctd ntas ht rdc te colu- the produce that initials dedicated the to contrast In and and ) ( ) or loss of loss or FEZ and and 3.4D Figure in cells outside of the initials, as ectopic expres- ectopic as initials, the of outside cells in FEZ D ). In rice however, there are multiple layers of cortex cortex of layers multiple are however, there rice In ). root, rice roots are also organized organized also are roots rice root, Arabidopsis Helianthus annuus Helianthus Arabidopsis and rice as it does most between activity results in ectopic periclinal periclinal ectopic in results activity SMB E ), regulate divisions in both the colu- the both in divisions regulate ), L. and and L. and and FEZ Zea mays Zea SOMBRERO Arabidopsis L., there there L., Figure Figure of all of the cells of the root body, but is instead more akin to akin more instead is but body, root the of cells the of all of of cell apical the that contended some have tip, root the at cell apical other single a have in that systems root experiments upon Based 1981b). Polito and Gifford root the 1978; et al. to (Gunning cortex rise outer the and give epidermis, the will hairs, cell outer The cortex. inner the and endodermis, the pericycle, the rise including give tissue will vascular cell the to inner The cell. outer an and inner an duces (darkin green cell daughter of apical the division first The file. the in of number cells the sions to increase divi- transit-amplifying of series a in anticlinally divide mately ulti- then and files cell of number the increase to divisions) (T times several of periclinally packets cells) divide that (essentially produce to what referred are merophytes as divisions cell mative for-These cell. ofmother the faces proximal three to the parallel oriented are walls cell new the which in cell apical the of sions cell. initial single a of divisions regular the by related—produced clonally (Clowes root cap cells 1961).are these roots in cells all Therefore, of population steady a maintain to face apical of its along root the growth the throughout continuously divides cell apical Instead, cap. the root the for initials of group separate no is there genus the of ferns However for that the most of its so growth, initials cap root the produce to face distal its along divides cell apical the root, the of development the thought in early is that It cap. root the except body, plant the of cells all to rise gives that cell” “apical pyramidal-shaped single a and initial cells cap root the are cap root the to Proximal cap. root a by Polito and 1981a,b). At the its apex, ( cell initial single a from derived are the of stele) and cortex, dermis, colu- and LRC cells. mella the for cells initial separate be roots, to rice appear mature there the In 2009). al. et Rebouillat by (reviewed cylinder vascular the in arise types cell various how describe to However, oriented. are some done work been divisions has these how or population cell stele the sustain to required are cells tial ini- howmany 2010). unclear Itis et al. (Coudert soils saturated water- in survive to root the help layers cell These and endodermis. laterals), some of exception the with roots all in enchyma aer- into differentiate generally will which (cortex, mesodermis (x), schlerenchyma (s), exodermis root: the of layers cell cortical to times several produce of all periclinally divides the specialized position endodermal the occupying cell the contrast, In lineage. cell epidermal the produce to of divisions series cell a in transit-amplifying anticlinally divides file cell epidermal the in cell The resulting cells. precursor endodermal the and epidermal the generate to periclinally divides that cell daughter a produce to anticlinally divides CEEI the cortex, and epidermis To the form cells. QC the as tier same the occupies CEEI) initial, epidermis initial (commonendodermis cell epidermal Thethe QC. cortical surround that tiers three in arranged are rice in cells The initial The cells of the of cells The to contrast In and rice, all of the cell types (epi- types cell the of all rice, and Arabidopsis root body are produced by divi- produced are body root filiculoides A. root has a closed meristem. root a meristem. has closed A. filiculoides have an open meristem; meristem; open an have Marsilea (water fern) root root fern) (water filiculoides A. root is covered filiculoides A. and and 3.3E Figure is not the source source the not is Azolla Figure 3.3E and Figure F ; Gifford Gifford ; F ) pro- 3- 7 Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 limiting H limiting defines the transition zone and the switch from cell division to cell elongation. cell to division cell from switch the and zone transition the defines elongation zone, a gradient of H of gradient a zone, elongation the (C)In constant. is relatively kept themeristem of thesize and off, fall levels gibberellin and root growth, in Later signals, increases. MZ, meristem of the size cytokinin the the inhibit In of gibberellins levels suppressed. high is meristem, of the growth the in signaling Early auxin and cytokinin. inhibits and differentiation division cell promotes toward auxin predisposed are cells predominates cytokinin where (TZ) zone transition 3.5 FIGURE cells free-living individual cells, border produce be to may it ability that the suggest (2006) al. et Hamamoto other? the sus ver- configuration one of (Clowes advantage capacity the is growth then what 1961). So in differences any with correlate to appear not do differences these 1981).However,1961, (Clowes quiescence of gradation a often with is there roots and in meristem, is closed a it as defined well as not often is center escent qui- the open an meristem, with roots in cap. and body Likewise the root between thereandis notseparation aclear defined, well less is meristems open of organization the discrete whereas cells; of in tiers arranged often are meristems closed in cells initial The cap. root the and body root the to cells contribute to divide they how on potentially and arranged are meristem the in cells meristem as either open or closed coveys on information how the their of organization the upon based roots of classification The VII. 1981). aQC. Grev. lack therefore Hook and (Kurth roots Azolla and 1982) Kurth and (Gifford Michx scirpoides in cells apical the for shown been also has This root. adult the of cells the of all provides and active mitotically is cell apical The incorrect. be to (1980) this howevershown et al. have Nitayangkura and 1958b)(1956a, Clowes by Experiments ini- tials. actual the were merophytes first the and quiescent largely of cell apical large the that was thought The and rice of cells QC the 3 © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis -8

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2 levels in the transition zone. An O An zone. transition the in levels sus aClosed Meristem Regu lation of meristem size. (A) The relative actions of cytokinin and auxin control the size of the meristem (MZ) in (B). In the the In (B). in (MZ) meristem the of size the control auxin and cytokinin of actions relative The (A) size. meristem of lation (A) Cytokinin Auxin (reviewed by Gifford 1983).Gifford by (reviewed Arabidopsis 2 O 2 (with levels increasing basally) promotes differentiation. A mobile protein, UPB1, facilitates this gradient by gradient this UPB1,facilitates protein, mobile A differentiation. promotes basally) increasing levels (with SHY2 ARR12 ARR1 2 − gradient in the meristem promotes cell division. The overlap between these two ROS gradients gradients two ROS these between overlap The division. cell promotes meristem the in gradient was was filiculoides A. PINS Gibberellin vestita Marsilea RGA Equisetum Equisetum (B) VIII. tomaize. compared as Helianthus in cells border potential of pool larger a to contribute may This days. 3 take would process this maize in whereas days, 1.5 every mately RAMs, respective their in cells active mitotically of distribution the in ining Clowes that (1981) observations of additional the exam- made in one Interestingly attack. abiotic to and biotic of resistance terms in meristems closed with those to advantage an have may fore there- meristems open with 1998).Roots et al. (Hawes grows it as damage root from intact the soil—protecting the in guards as act to is cells border of functions the of one that has suggested Itbeen mucilage. binding aluminum even and antibiotics duce of geneto pattern pro- expression their change border root, cells the from release upon that pea in shown has (1993,1995) et al. do the than environment roots a with Bingham closed meristem. and shed considerably produce (7.5–200 times) more live border meristems into cells open with Roots enzymes. wall-degrading cell- of activity the through cap root the from derived are that h rt o cl dvso i te eitm ut qa te ae at ratewhich cells the “enter” equal musteach meristemof the the other two in zones. division Often this cell is of not ratethe the zonesconstant,remaintoindividual ofthese ofeach sizeFor the stem and elongation zones (Figure 3.5B; Baluska et al. 1990, 2010). meri- the betweentransition of region a ferentiation with zones, The root apex isdivided into the meristematic, elongation,and dif- and maize roots was that due to differences differences to due that was roots maize and Helianthus

is expected to replace its root cap approxi- cap root its replace to expected is Helianthus MZ TZ EZ Control of Meristem Size (C) O O H H High High 2 2 2 2 O O – – 2 2 UPB Root Structure Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 Cellular Patterning of the Root Meristem of the Patterning Cellular tm s yai ad ht ioi atvt i te i my delay may differentiation. tip the in activity mitotic that and dynamic is stem meri- the of size the elongation,that cellindicating and division beexaggerated byinhibitortreatingroots an the with of cell both change in the position of the differentiationzone. This effectcould 0.19 mm change in the height of the meristem, they every foundfor a 1 instead,mm meristem; the of height the in decrease the to equivalentmovementdirectly differentiation not thezone was of differentiationzone moved apically towardthe roottip. The apical thethe meristem, divisionin decreased cell rate the of as is, That differentiationslowly in roots. dividing tip the closerzone was to correlatedthedistance to with thedifferentiation zone that sothe inversely was meristem the of height Interestingly,the mm. 2.0 to returned heightmeristem the decreased, index mitotic the as developmentgationzone)inLater 1.9increased2.7 mm. fromto elon-the androot capmeristem the(thebetween theof distance developmentrootofwhenthe divisions were frequent,height the (1988)Baummeristem, open the andRost in an earlyfound that meristematicdifferentiationand root, which the zonespea has of the examining in example,For root. developmentthe ofthe out relativetheandcase, ofsizes each of regions the change through- 1998a,b; Shishkova et al. 2008). et al. Shishkova 1998a,b; et al. (Zhu growth of cessation complete a precedes often This size. in shrinks meristem the and decrease, divisions cell root, the is meristem of largely stable. However, later size of in the growth the the and differentiation, cell matches division cell ment, develop- root in stage this At 2007). Ioioet al. (Dello maximum and fall, to begin levels gibberellin germination, after 5 day by However, differentiation. cell over predominates division cell and root, the in high are levels ellin 2007; gibber- growth, of days al. few first the et During 2009). et al. Achard Ioio (Dello inhibitors kinase cyclin-dependent of decreased in results turn in which (RGAs), acid gibberellic of repressors the of tion degrada- the promoting by pathway this into feeds Gibberellin inhibits auxin whereas factors), transcription Cytokinin of transport. expression promotes auxin polar consequently and proteins of PIN expression the inhibits which genes, of auxin-induced ily through ( SHORT signal HYPOCOTYL2 cytokinin and auxin Both division. and cell inhibit differentiation cell promote cytokinin of levels high zone transition the in cell meristem, promote the of outside contrast, cytokinin In of division. levels low and meristem, auxin the of In 2012). levels al. high et Durbak 2009; al. et Galinha by ( gibberellin and cytokinin, auxin, ticular in size meristem of regulators primary The meristem. the of size the increase zone expansion the into cells of progression sloworthe meristem the in divisions promote either that factors Therefore, tip. root the through section epidermal cross longitudinal or medial a of side one on cortex cells unexpanded of number the counting by ecotypes in the size of the root meristem. Mouchel et al. (2004) et al. Mouchel meristem. root the of size the in ecotypes © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis In In In In the size of the meristem is generally measured measured generally is meristem the of size the Arabidopsis there is a natural variation between different different between variation natural a is there Arabidopsis are the phytohormones, in par- in phytohormones, the are Arabidopsis expression and decreased expression expression decreased and expression SHY2 (through the the (through SHY2 SHY2 ), a member of the Aux/IAA fam- ), a member of Aux/IAA the expression reaches its its reaches expression SHY2 Figure 3.5A Figure and and ARR1 expression. expression. SHY2 ; reviewed reviewed ; ARR12 restoration of the BRI1 receptor is sufficient to rescue rescue to sufficient is receptor BRI1 the of restoration epidermal-specific as epidermis the by regulated be to appears the of rescue for ficient suf- was vasculature root the throughout expression that cluded con- et al. Mouchel (2006) for BRX, requirements tissue-specific analyzing In root. the in primary growth meristem the regulate that tissues determine to Similar 2001). conducted al. been et have Pien experiments 1999; 1997, al. et (Fleming growth organ ground above brassinosteroid-dependent regulating sue tis- primary the is epidermis the that the suggesting tissue), ground in not (but epidermis the in perception brassinosteroid of autonomous restoration cell the by is rescued deficient) mutants and of the cell-to-cell transport or perception of tissue-specific tissue-specific of perception In or factors. growth transport cell-to-cell the of result the be may organ an of development the in tissue par- ticular a of primacy the Alternatively growth. constrain may ers al. lay- et L2 and L1 the (Fleming that 2001),suggesting plants et al. Pien 1997, 1999; tobacco and tomato in formation leaf induce to sufficient is “loosening” cell-wall promoting by cells between tension reduces which EXPANSINS, of SAM the of ers For or in application outer the expression lay- plant. localized the example, the within cells all encase that walls cell of network rigid the by imposed are on that tissues constraints different or between growth signals mechanical differential from result may findings Sach’s others. than organ or plant the of opment devel- the over control more exert tissues some that suggested 2009). et al. Scacchi 2006; et al. Briggs 2004; et al. have both positive and negative on effects which brassinosteroids, and auxins between loop feedback a of that 2009), indicating et al. the (Scacchi nucleus in BRX of degradation auxin-induced by and brassinolide of application the by moderately) (albeit regulated negatively is 2009). et al. In (Scacchi contrast, forrequired its function is which nucleus, the into translocation its promotes of expression the Auxin increases also expression. gene auxin-induced in step” “rate-limiting through brassinosteroids of production that suggest results These inhibitors. brassinosteroid with roots BRX the in lost entirely almost was auxin) exogenous of (uponapplication sion expres- gene However,auxin-responsive not. was auxin length; to brassinolide of ( DWARF the of tion upregula- through brassinosteroids of biosynthesis the in tion plants with the the with plants to compared when respectively, 70%, and 50% by approximately meristem the and zone elongation the of size the reduces allele named They locus this examined. individuals the between lengths root in differences the of 80% approximately for accounts single that locus a identifying variation, natural this exploit to able were al wr b Scs rvee b Ptr ad oo 1996) Tomos and Peters by (reviewed Sachs by work Early brassinosteroid-insensitive 1 brassinosteroid-insensitive activity is required for auxin signaling. Indeed, the the Indeed, signaling. auxin for required is activity BRX CPD ( RADIX BREVIS roots, an effect that was mimicked by treating treating by mimicked was that effect an roots, brx CONSTITUTIVE PHOTOMORPHOGENESIS AND AND PHOTOMORPHOGENESIS CONSTITUTIVE ) gene. Consistent with this finding, application application finding, this with Consistent gene. ) allele. The The allele. BRX Arabidopsis roots was able to partially rescue root root rescue partially to able was roots brx brx roots. In contrast, signal perception perception signal contrast, In roots. BRX the dwarf phenotype of the the of phenotype dwarf the ( (Mouchel et al. 2006) and and 2006) et al. (Mouchel BRX gene was shown to func- to shown was gene BRX bri1 ). Homozygosity for the the for Homozygosity ). ) (brassinosteroid receptor receptor ) (brassinosteroid appears to be the the be to appears BRX activity (Mouchel BRX activity is at the center atis the BRX expression expression BRX bri1 roots roots 3- cpd brx 9 Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 nymi corniculata ( species different five in that showed (2003) et al. 2004)—Chapman et al. (Groot open diate interme- and open meristem—basic open of types different two between distinguishes that scheme labeling a Using meristem. ofpotential the root ofdifferentiation the the and eventual signal growth the in change a indicate may conformation, open an to closed a from particularly meristem, the of organization the in one closed a into (Clowes 1981). meristem open an of conversion the changes what to led unclear was It lineage. cell common a cap had and cortex cells the that so cells columella to rise giving layers cell cortex the in divisions periclinal with correlated often was arrangement open an into meristem closed a of conversion The open. becoming meristems closed frequently more and closed to changing meristems open apparently both of examples with growth, root during organization meristem in changes frequent and Heimsch (1976) Armstrong found of roots, species different twelve examining In signals. the developmental both and to environment response in changes it instead, static; not is RAM of the organization the however that is highlights argument this What 2008). Seago and (Heimsch configuration meristem open an with out start indeed that roots of examples multiple citing contention this dispute convincingly Seago and Heimsch tion. meri- germina- after generated is closed arrangement open the a that and stem have form embryonic mature their in roots all that argued have Many meristem. the of patterning the also but development Not with of change only does size the meristem the IX. meristem. entire the therefore and root of the layers other in growth of rate the that limit may endodermis the of indicating growth reduced 2008) al. of et size (Ubeda-Tomas the meristem in root decrease the a in results endodermis the in GAI of allele insensitive) (degradation dominant a of Expression mis. endoder- the instead but epidermis, the nor stele the neither be to appears gibberellins for action of site critical the contrast, In BRI1. via epidermis the to signal may BRX1 via vasculature the in production brassinosteroid 2011).Therefore, et al. (Hacham 3 These results indicate a connection between an open meristem meristem open an between connection a indicate results These species. root the upon transformation depending days 41 This and 14 lost. between took between was body root distinction the and clear cap root the the and cytoplasmic, densely less became cells irregular, became shapes cell decreased, generally of the organization the meristem in which the size of the root cap root determinacy. In there species, all was in ashift wide-ranging preceded that growth of deceleration gradual afollowed by then uponburst was ofthat germination growth was initial an species five all in open observed they What growth. intermediate root of cessation to prior an to changed organization the meristem, © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis -10 Experiments by Chapman et al. (2003) suggest that changes changes that suggest (2003) et al. Chapman by Experiments Urb., and and Urb.,

of th thePatterning in Changes L., L., e Meristem inhs caryophyllus Dianthus Salvia farinacea Salvia Clarkia unguiculata Clarkia Epling) with initially a closed closed a initially with Epling) L., L., lmnaha hiero- Blumenbachia Lindl., Lindl., Oxalis Oxalis organized, the cells were highly vacuolated, there was no visible visible no was there vacuolated, highly were cells the organized, of dis- meristems were medium, low-phosphate the on grown roots, 12, Arabidopsis day By 2005). al. et (Sanchez-Calderon process this accelerate further can medium low-phosphorus on Growing 1998a,b). et al. (Zhu growth root of to closed open at a about with cessation 28 coincides which days, of RAM the sand, in grown When meristem. the of organization the in changes obvious no of root 2007). et al. ­comp genetic strong a indicating tissue, callus from generated are that in roots of 2003).iset al. recapitulated process This (Rodríguez-Rodríguez apex the at form differ- hairs root tip and root entiates, the meristem, the of loss the of consequence a As lost. eventually is it until decreases also cap root the of size the process, this During vacuolated. and enlarged become cells initial the decreases; meristem QC. the of size the obvious grows, root the As any lacks meristem the and stele, the of base at present the are cells initial The stele. central a and tissue layers, ground 4–5 layer, cell epidermal an of composed is root the of body the germination, Upon body. root and cap root the duce of root primary to pro- apex in the divide cells The open an has meristem; gummosus growth. root determinate stitutively con- of example interesting particularly a provides (Engelm.), activity. meristematic of loss the accelerate can conditions growth pea, of those like roots determinate factor.” constitutively However,for even environmental an by usually induced is “determinacy Nonconstitutive environment. the of independently occurs that development” root of part natural a “is potential meristematic loss of the roots, determinate For constitutively nonconstitutive. and constitutive growth: root determinate of types two defined (2008) et al. Shishkova context, this In apex. root the of tiation differen- and root the eventual may growth determinate indicate growth of cessation contrast, In reactivation. signals pro- gramming or conditions until arrested simply but meristematic still are apex the in cells the which 1967)in et al. Clowes by (asused “dormancy” root with correlates growth of cessation the cases, In some conditions. or due to growth unfavorable programming due either to genetic cease growth and meristem the in divisions cell which in period a by followed be then may phase terminate inde- This maintained. is growth root overall and zones, tiation replenish the that are cells lost to the elongation and the differen- able to are meristem the within divisions cell phase, this During of developmentis rapid “indeterminate.” and where root growth et al. (2008) point out that for most phase roots, there is an initial Shishkova control. environmental and endogenous both under of the organization and meristem forthe potential root is growth root growth. to determinate a switch mayopen signal meristem acquired an Therefore meristem. closed a with out roots started in that only but potential growth limited a and organization When grown on rich medium in tissue culture, the primary primary the culture, tissue in medium rich on grown When cactus, desert the of root The the that indicates species different multiple of Examination onent to determinate growth in this species (Shishkova (Shishkova species this in growth determinate to onent is maintained for more than 1 month with with month 1 than more for maintained is Arabidopsis seedlings switches from from switches seedlings Arabidopsis gummosus Stenocereus roots roots Arabidopsis Root Structure gummosus S. S. S. Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 rice, which have short-lived determinate primary roots, the the like roots, primary plants In determinate short-lived soils. have favorable which more rice, access potentially and explore to roots lateral the allow to root primary the from away resources divert may growth root determinate phosphate, low ter access to water. In the case of of case the In water. to access ter bet- for surface soil the to close so produced are early roots lateral that terminates which root, primary the of establishment of case the in gummosus S. (as determinacy constitutive et al. Therefore, Aloni 2006). 1999; Hasenstein and (Zhang tip root the off ting cut- to akin determi- dominance, that apical of release a suggest causes al. results growth nate These et 2011). al. et Sanchez-Calderon Lucas 2003; 2005; al. et (Rodríguez-Rodríguez scr low-phosphate-grown the cactus, and/or pea. cactus desert in growth determinate to switch the regulating in role a play SCR and/or SHR of levels the in changes perhaps 2004), (Bolle cies As ceased. growth root that time the at tissue ground the comprised layer cell tical four to one, in from layers cell cortex of number the in decrease (2003)a noted et al. Rodríguez-Rodríguez layers. tissue ground of number the in a decrease perhaps indicating height decreased, and meristem activity meristematic the as decreased also root pea the of eter diam- (1988), the Baum and al. Rost of et experiments the Helariutta In 1996; 2000). al. et Laurenzio (Di epidermis the and pericycle the between in two of instead layer tissue only ground oneof presence the is germination, of time the at apparent is in defect obvious most The other medium. growth plant standard on grown when mutants scr-4 in 3–5days takes tip. process This the root to proceeds differentiation and lost, is QC the irregular, becomes cells initial the of division the grow, they However,as The 2000). et al. Helariutta 1996; et al. Laurenzio (Di disorganized become either of alleles Null growth. determinate precocious have that roots for look to growth. root maintain to meristem the in required are cells of number critical a that a et al. 2005), perhaps (Sanchez-Calderon indicating was observed expressed still 10cells approximately when 8, day at medium high-phosphate to shifted were roots if However, recovered. be could growth root meristem, the in cells or 7, 6 days at 11 13 CycB1;1:uidA and between had still roots when medium high-phosphate to back shifted were 14,By day of no expression germination. after 4 day by division cell of rate the in decrease low-phosphate the the using in plants activity mitotic of Examination 1988). to what zone in tiation pea was similar observed (Rost and Baum differen- the of migration apical an and activity meristematic of loss a indicating tip, the to close formed had hairs root and QC, Cellular Patterning of the Root Meristem of the Patterning Cellular © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis One consequence of determinate root growth in the desert desert the in growth root determinate of consequence One is control root growth that factors the One way to understand mutants is the emergence of lateral and adventitious roots roots adventitious and lateral of emergence the is mutants and and shr ) may be a timing mechanism that allows for the the for allows that mechanism timing a be may ) so that a single endodermal and cor- so a that endodermal single S. gummosus or SHR marker revealed a significant significant a revealed marker CycB1;1:uidA mutants initially have a closed meristem. meristem. closed a have initially mutants scr and and SHR result in roots whose meristems meristems whose roots in result SCR was observed. If plants CycB1;1:uidA plants If observed. was are found in many plant spe- plant many in found are SCR shr-2 and shr-2 5–7 and in days mutants Arabidopsis Arabidopsis CycB1;1:uidA mutants, which which mutants, scr-4 , and the the and , roots grown on on grown roots , no recovery recovery no , -expressing -expressing shr and and minate root growth are are growth root minate 2011). et al. (Lucas root primary smaller a of presence the in growth plant in the patterning of the root, particularly the vasculature vasculature the particularly root, the Torrey1976). and (Feldman changes of with patterning correlate QC the the of in size the in changes cases, may some QC In 1984). the (Clowes root of the of size life the The throughout large. change in than QCs small in mitotic higher activity is there trend, a as of Likewise those meristems. than closed initials) cap root the to (normalized frequently more divide meristems open with roots of cells QC the general In quiescence. of degree highest the showing QC the of center the at cells with QC, same the in cells between even and same plant, the on roots different between between roots, of varies species different quiescence of degree relative The 1993). al. et sions in the root. the sions in divi- cell for maintaining important may be QC the that gesting sug- maintenance, QC affect that signaling auxin in defects and arrest cycle cell causes biosynthesis of glutathione inhibition cal as a agent redoxacts that buffer. orEither genetic pharmacologi- reducing a is Glutathione biosynthesis. glutathione in enzyme γ root the of When cells. number the to component genetic strong a root, indicating primary the as divisions of number same the after nated termi- formed it that laterals the and roots, lateral form to able Only emergence. hair root and tissue vascular of tion forma- apical the by evidenced as tip root the of differentiation and cells vacuolated highly showed also mutants Both mutants. these in meristems the of functionality limited indicating type) in the in cells of number the in was there no increase Atafterfile. germination, 4 dayscell an average roots ofcontain 17 per cells and 17epidermal cortical germination 1995).Upon et al. (Cheng Arabidopsis and 2010). For et al. Coudert 2009; et al. (Rebouillat soils wet in better plant the supports mechanically that root branched tiply mul- a create may roots adventitious and lateral of production 1984), and in in and 1984), repens Trifolium in cells QC of number the contrast, In 1000. nearly to 200 from of those like QCs large with roots some 1956).(Clowes In activity low mitotic ing indicat- precursors, acid nucleic or nucleotides incorporation slow) radiolabeled (or of weak their of cells basis other the from on root the distinguished in be can QC The layers. corti- cell and cal endodermal at the stele between the of convergence pole of point apical the the at lies that activity of mitotic region a low is 1958a), (1956a,b, Clowes by defined as QC, The X. -glutamylcysteine synthetase (Vernoux et al. 2000), the first first the 2000), al. et (Vernoux synthetase -glutamylcysteine Extreme examples of mutations that cause precocious deter- precocious cause that mutations of examples Extreme (approximately onefold compared to 10-fold in the wild wild the 10-foldin to compared onefold(approximately rml2

scr Mai of QC the in Importance mutants, the emergence of laterals may help support support help may laterals of emergence the mutants, ntaining ntaining Arabidopsis gene was cloned, it was shown to encode encode to shown was it cloned, was gene RML1 L. is usually small, 2 cells on average (Clowes (Clowes average on cells 2 small, usually is L. Zea mays Zea mutants and a very limited increase increase limited very a and mutants rml1 ( meristemless root R 4–7 cells comprise the QC (Dolan (Dolan QC the comprise cells 4–7 , the number of QC cells can range range can cells QC of number the , oot Gro wth and and rml1 Arabidopsis was was rml1 rml2 3- of ) shr 11 Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 om rm sal ru o (paety dsraie cls at cells disorganized (apparently) of group small a from form QC and meristem the plants, some in Instead, cell. hypophyseal to a toformroot. make a fail hypophysisthat fail MP) of alleles loss-of-function or BDL of alleles dominant (e.g., QC. as cell apical the and columella as cell basal the of proper forspecification the required is signaling nin signaling cytoki- and auxin of cytokinin partitioning 2008).This Sheen and (Muller inhibit which AAR15, and ARR7 of sion promotes cell expres- basal the in Auxin cells. apical the natesin predomi- cytokinin whereas cell, basal the to segregate to seem (Schlereth cell et al. 2010). levels auxin high division, this During basal a and lens-shapedcell apical toproduce an divides then sis hypophy- The hypophysis. the as cell this specify the to of suspensor cell apical the into embryo the of region basal the from moves TMO7 expression, Upon proper. embryo the of region basal the in protein (TMO7) 7 MONOPTEROUSOF TARGET the of expression the allowing repression from factor scription (BDL). Degradation of BDL frees the MONOPTEROS (MP) tran- ofofbase the degradation the embryo in BODENLOSthe results Friml et al. 2004; Michniewicz et al. 2007). The auxinmaximum at 2001;et al. Benjamins of hypophysisthe (Christensen 2000; et al. specification with interfere signaling auxin disrupt that Factors embryo. the of domain basal the in maximum auxin an mentof establish- the with correlates hypophysis the of recruitment The form from the hypophysis, which is recruited from the suspensor. proper the with exception of the QC and the CIs. TheQC andembryo CIs the of cells all to rise give will which cell, apical an and produce which cell, formssuspensor a basal the extra-embryonic the tilization, fer- 1994).After et al. Scheres 3.6; (Figure development embryo 2005). Feldman and In (Jiang QC a development, their in point promotes expression of ARR7 and ARR15 inhibiting cytokinin (CK) signaling. Cytokinin is required in the upper lens-shaped cell to specify the the Development et al., B. specify to Scheres, from cell modified lens-shaped upper (Images the QC. in required is Cytokinin signaling. (CK) cytokinin inhibiting ARR15 and ARR7 of expression promotes auxin High lower daughter. to the segregated is auxin high divides, hypophysis the when embryo, of the growth in Later (HYP). hypophysis the as cell this specifies auxin high with along This suspensor. the of cell top the into TMO7,moves of which expression allows This BDL. by MP of sion 3.6 FIGURE 3 © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis -12 o al pce seiy h Q truh erimn o a of recruitment through QC the specify species all Not some at least at have, roots all that suggested been has It Arabidopsis

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mutants

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both both the in of QC absent markers the either are or misexpressed Likewise medium. normal-phosphate to root the returning when growth of root rescue to loss inability the with The correlated 8 by day markers QC growth. root of degree some maintained and cells QC recognizable had initially roots the low-phosphatemedium, grew (2005) al. et Sanchez-Calderon et al. 1980),When determinate. and is root constitutively growth des and 2003) et al. in (Rodríguez-Rodríguez gummosus S. example, For 2008). et al. (Shishkova grows root the as QC their lose progressively they or entirely QC a lack either growth root determinate nonconstitutively or constitutively show that Roots 1968). Evert and Alfieri 1958a,b; (Clowes embryonically post- forms QC the mitotically and emergence, radicle are of time the at meristem active root the in cells all species, other In 2005). Feldman and Jiang 1968; Guttenberg (von pole root the h Q. ieie iia rsls ee on we ros were roots when found were results similar Likewise QC. the Thebasis ofbackwithin traced wasthis recovery to divisions cell conditions. growing normal to back transferred were plants the (referred to by Clowes could when resume as dormancy), growth quiescence imposed of state a after that found treatment cold or wounding, drought, through perturbed was growth root in which (1967) Clowes potential a by Experiments is cells. QC replacement The of replaced. source be must cells these grow, continue to to root the for order In process. aging normal the due or to wounding to due either lost are cells initial root, the of growth the forDuring replacement. as a cell reservoir QC serves the that suggests first The growth. indeterminate supporting in growth. for indeterminate requisite pre- a is QC a of presence the that suggest results These growth. root determinate constitutively show that mutants double and 2001) et al. other in and Nakajima 2000; et al. HYP Two mechanisms have been proposed for the role of the QC QC the of role the for proposed been have Twomechanisms

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auxin High edig on seedlings Arabidopsis single single Arabidopsis Root Structure A. filiculoi- A. Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 Cellular Patterning of the Root Meristem of the Patterning Cellular show constitutively determinate root growth. These roots lack lack roots These growth. root determinate constitutively show In genetically. population. cell initial the a maintains signal QC-derived that concluded was it studies, ablation these From signal weakens. the to respond to cells surrounding the of ability the or intensity in wanes signal QC the either ages root the as perhaps in older roots, is that observed is something As this endodermis. and cortex to separately rise gave that initial) endodermal tical cor- common a to (asopposed initials dedicated essentially had now roots These fate. cell CED the adopting from cells CEI the prevents cells QC contacting the of presence the that suggesting to to in butthe a CEDs, divide similar manner did so periclinally continued QC ablated an cell with contact in CEI cells normally. behaved QC cells intact the with contact in those whereas mella, the ablated QC asto failed and colu- divide differentiated instead Willemsen were that and with in contact (1997) initials columella found the that Berg den van replacement, before frame time the in ablated (Feldman specifically was roots cells QC four the of 1976).one maize only However, if in excised surgically were cells QC the and Berg den (van 1997;Willemsen Xu wereet al. results maximum 2006). found Similar when auxin new that the region a with in correlated stele, the in cells by replaced rapidly was QC the in ablated were lost. also is meristem the that predict would one lost, was QC the If competence. and fate cell both thefromaffect QC should Therefore,distance tion. differentia- and expansion cell of process the begin and QC the of influence the from removed significantly are they once sion divisions) divi- cease finally and cell signal QC-derived of the less even receive transit-amplifying undergo which (those stem meri- larger the in Cells of differentiation. some degree undergo cells initial the When from QC the and hence are displaced cells daughter their divide, initials. as signal specified this therefore of are concentration and highest the receive QC the with contact direct in are that cells the that such manner dependent concentration- a in behavior cell affects that signal short-range an in population thethat QC cell is a produces assumption The state. undifferentiated initial the maintains that center nizing orga- an as acts QC the that suggest results These differentiate. terminally and divisions cease cells initial surrounding the QC, all in that vation obser- upon the largely based is growth indeterminate maintain meristem. afunctional reform and cells injured any replace to divide then can QC the released, is stressor the When loss. from protected sense a in are cells these activity, mitotic low has QC the Since damage. most the suffer would daughters their and cells initial the that means This ing. divid- actively are that cells damage The preferentially radiation ing 1962). ioniz- or (Wilcox drought, treatment, cold like overwintering stressors that is thought during cedar incense of (Clowes radiation to 1963,ionizing exposed 1965) or in the roots © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis The concept of the QC as an “organizer” has also been tested tested been also has “organizer” an as QC the of concept The QC the comprise that cells four the hypothesis, this test To or promote may QC the which by mechanism second The root, replacement was delayed. In the the In delayed. was replacement root, Arabidopsis mutants both the primary and lateral roots roots lateral and primary the both mutants scr mutants, which lack a functional functional a lack which mutants, Arabidopsis root. Following ablation, the the ablation, Following root. Arabidopsis of one to does two reducecells notthis signal. significantly loss and QC, ofthe may division inhibits that providemobileasignal cells initial the example, For lost. is cells initial of ulation pop- significant a if divides only QC the that be may It ments. ablationexperi- bythe caused damage limited duemaythe tobe This systems. other in suggested been has as cells, initial lost the replace to QC the by division of indication no was there ments, experi- these in 1997). that noteworthy Willemsen It is and Berg (van status cell den Berg the initial 1995;et al. maintains den van werethat cells positionthat ablated, indicating relative to QC the E/LRCI replace functionally and divide to ablewere cells cortex to produce endodermis functional and cortex cell layers. Likewise periclinally divided turn in CED the CED; aproduce to clinally anti- divided They CEI. a ofbehavior the adopted cells daughter merly occupied by that ablated CEI. In position,this the pericycle for-position the invade and periclinally divide could layer cycle peri- adjacent the from cells ablated, were cells CEI If ablations. ofquality the cell. initial This inherenthypothesis was also testedsome using cell upon based not and QC the to relative position upon based meristem the in status cell progenitor their achieve 2010). et al. Jiang 2007; et al. (Rymen scription tran- of level the at regulated be may activity RBR maize in that and identity QC with correlate also may maize in activity RBR that indicating cells, stem proximal the or cap root the to pared lowroot, oflevels maize the In differentiation. inhibit therefore and mechanism) tified uniden- yet a (through activity RBR reduce to is QC the in SCR of the phenotype maintenance cell stem the rescue partially can RBR Although against RNAi meristem, root the mitosis. in expressed uniformly is promoteRBR which factors, transcription E2F inhibits and differentiation cellular promotes both RBR 2005). et al. (Wildwater activity (RBR) retinoblastoma-related of tion downregula- the by bypassed partially be can maintenance root plants. scr in growth determinate the loss in ofprecocious results that defect the QC is the primary that suggest results These inhibited. was QC and cells initial the of differentiation however, root; wild-type completely a to tive root of rate rela- the were ofdecreased meristem the size overall the and QC, growth rescued a with roots the In growth. root maintain to required is QC indeterminate wild-type a that of suggesting growth, reestablishment enabled tissue, ground the of Restoration descendants. their and CEI the or wild-type QC the either in replaced (2003)activity SCR al. et Sabatini growth, root taining main- for responsible is types cell these of which To and test 2004). CEIs, et al. the Gallagher 1997; Benfey QC, and (Malamy the cells daughter their endodermis, the in expressed is tein pro- SCR the roots, wild-type In 1996). et al. Laurenzio (Di tip root the to proceeds divide eventually differentiation initials and abnormally surrounding the and cells QC and former lost, the are both markers QC growth, root determinate to the Upon switch QC. a maintain to fail and layer cell endodermal an A prediction of the QC as “organizer” model is cells that initial for requirement the that suggest results recent More mutant. These results indicate that one theindicate of roles of results These mutant. scr-4 activity in the QC, but not in the CEI and and CEI the in not but QC, the in activity SCR are found in the QC com-as found the are in ZmRBR2;1 in in SCR 3- 13 Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 tional equivalent of equivalent tional can rescue rescue can state. undifferentiated and a active in mitotically cells stem surrounding the maintain OC the from signals that in organizing QC the to the similar is OC the of as (OC). function center The to referred SAM the of center the in region meristems. floral and shoot the sion of the hypophysis. In the mature root, root, mature the In hypophysis. divi- the of sion asymmetric the after immediately QC the form will that cell lens-shaped upper the in embryogenesis during initiated is rescue can ber of this family, family, this of ber transcription is factor in the WOX5 2006). et al. (Xu pathways 2007) et al. and 2004), et al. the the 2003),et al. Sabatini 2000; et al. 1996; Helariutta et al. Laurenzio (Di previously) (discussed genes SCR and SHR the are QC the of aQC. reformation with maximum to auxin tothe respond able are cells all not that indicate results These 1988). Jones and 1976;Rost (Feldman stump root the to angles right at form roots lateral Instead, specified. not is meristem new a removed, are also cells root the transit-amplifying cap, the and the QC, the initials, to addition in If limits. its has meristem the respecify to auxin of ability the Interestingly 1988). Jones and in (Rost seen pea are results Similar QC. new a of establishment the ing follow- active only is However, meristem reforms. meristem this new a and cells), pericycle the in (predominately stump the in root 1976). (Feldman are initiated divisions maize cell In experiments, these the from excised is initials) and (QC tip the which in experiments in demonstrated been has meristem self-organization root the of Indeed, 2007). al. et (Grieneisen the cells recruits initial turn in QC the and in QC system the specifies self-organizing auxin a which for potential the up sets then sig- nal auxin this of primacy The meristem. root the maintain to al. et required is (Xu signal auxin continuous QC a that indicates original This 2006). the to proximal generally maximum, the of loss the new auxin of position the either QC theat of or respecification QC in results signal auxin this of disruption pharmacologi- or cal Genetic 2009). in al. et auxin (Petersson of tip production root the localized and 2007) al. 2005; et al. Grieneisen et (Blilou shoot the from efflux—PIN carriers) influx—AUX auxin and (via auxin of movement cell-to-cell tip the root the by at maintained is QC the of position the corre- with which lates maximum, auxin An QC. the of formation onic embry- during is it as and auxin, is formation root mature QC the in for maintenance signals key the of One QC. the of part become cells which determines also lineage than rather position that suggests 2006) 1997; Xu et al. Willemsen and Berg (van den that The thefinding QC isallreplaced when arefourcells ablated XI. 3 © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis -14 Downstream of auxin in the formation and maintenance of maintenance and formation the in auxin of Downstream ( 5 HOMEOBOX RELATED WUSCHEL r Mai eformation and mutants (Sarkar et al. 2007). 2007). et al. (Sarkar mutants wus mutants and expression of expression and mutants wox5 WUSCHEL ( ntenance of aQC ( PLETHORA WUS WUS . Correct . expression of Correct , is required for maintenance of both both of maintenance for required is , WUS ) gene family. The mem- founding PLT is expressed in a defined defined a in expressed is WUS ) (Aida et al. 2004; Galinha Galinha 2004; et al. (Aida ) is expressed expressed is WOX5 is the func- the WOX5 is expression expression WOX5 in the OC OC the in WOX5 ) (Haecker (Haecker WOX5) WUS in the root region of the QC with reduced quiescence at the periphery. at the quiescence reduced with QC of the region mayThis indicate a inregionthiscentral of quiescence increased approxi- to corresponds 2003a) mately the center of the region bydefined et al.Umeda al. (1999a,b). et (Kamiya overlap QHB which in region The QC. the as interpreted been has region This expression. of devoid largely is that tip the at cells) four than more (containing region a with meristem the in sion showed (1999a,b) roots broad expres- rice by Umeda et al. the in (CDKs) divisions cell of markers of examination Likewise QC. large a suggests grasses in structure QC of 1984)1971, (1956a, Clowes by Analysis rice. in region QC entire the define not may and and of the maintenance QC but are largely independent of the similar of the four genes. of Loss similar both 2007). (Aida et al. meristem Galinha root 2004; the et al. in cells initial surrounding QC the the upon and converge that domains overlapping partially in and of region overlapping the However, 2004). et al. Haecker 2003a,b; et al. (Kamiya root the of tip the at cells four in intact be to likely as rice, is regulation This 2004). al. et (Haecker that indicating in lost or reduced is expression WOX5 of that to similar is ment of role the that suggests studies sion in the the in PLT2 or presence ofin the expression exogenously applied auxin of Analysis root. a lacking entirely mutants, function) and plt3, between mutants Quadruple growth. root embryonic and plt2, between mutants Triple 2004). et al. (Aida initials and QC the of activity the and maintenance the both in functions PLT that indicating tip, root the at hairs root of initial formation and the cells, of differentiation markers, QC of loss the in results rice the of WOX5/WOX7 expression homolog, 2007; the of Werr Analysis 2007). and al. et (Nardmann Nardmann poplar and sorghum, podium, 2007). et al. Nardmann 2007; in homolog WOX5of related closely WOX7most As the is initials. proximal ofthe maintenance in function other generedundantly may that suggesting determinate, precociously not is growth however, root cells; stem proximal of the of effects structure the on mild wox5 are There 2007). al. et (Sarkar state ferentiated undif- an in cells initial columella of the fate the maintaining in WOX5for role autonomous non-cell a suggesting cells columella, as stem distal the of differentiation and QC abnormal an in the the that shows in the QC (Haecker et al. 2004). Loss of Loss 2004). et al. (Haecker QC the in The The There are homologs of homologs are There Arabidopsis also affect auxin activity through upregulation of the of upregulation through activity auxin affect PLTsalso are AP2 class transcription factors that are expressed expressed are that factors transcription class PLT4AP2 are pathways (Aida et al. 2004). 2004). al. et (Aida pathways SHR/SCR mp ( PLETHORA and and QHB , ( plt4 are similar to the the to similar are plt3 (NPH4) double mutants mutants double(NPH4) 4 hypocotyl non-phototropic and and PLT1 , this is a likely candidate (Nardmann and Werr and (Nardmann candidate likely a is this , babyboom is downstream of both SHR and SCR SCR and SHR both of downstream is WOX5 expression largely overlap, particularly in in overlap,particularly largely expression SCR PLT act downstream of auxin. In turn turn In auxin. of downstream act PLT2 ) genes are also downstream of auxin in of auxin ) downstream genes are also in rice, as well as ectopic expres- ectopic as well as rice, in QHB ) resemble resemble ) in in WOX5 in rice, maize, brachy- maize, rice, WOX5/WOX7 in mutants with no post- no with mutants rml1/2 and and PLT1 Arabidopsis (nulls) or (nulls) mp in rice root develop- root rice in QHB PLT1 and and and shr-1 wox are the most most the are PLT2 PLT1 fu 5 PLT2 expression Root Structure . In In . and and SCR mutants, mutants, scr-4 , nction results results nction (gain of (gain BDL L2 PLT3 PLT2, Arabidopsis Arabidopsis PLT1 and plt1, plt2, plt2, plt1, and and SCR WOX5 QHB plt1, , Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 anism by which anism a mech- suggesting kinases cyclin-dependent and cyclins B-type to rescue to rescue o peuae xrsin of expression upregulate to ( both of expression the by ofbut expression expression, of gradient the creates what precisely not2009).known It is and and differentiation. cell and endoreduplication inhibits expression whose ligase E3 by in behavior cell of effect the on Based differentiation. for required expression levels of levels expression ACTIVATION2B PLT of levels moderate cells, transit-amplifying the In fate. cell stem of concentrations high initials, surrounding the and QC the In initials. the and QC the in of levels expression est quiescence (Jiang et al. 2010). et al. (Jiang quiescence levels of high PLT,that root, do not promote maize atthe in least indicating cells, these in mitosis of activation with concomitant of levels the root, maize the of decapping However,upon quiescence. promote els lev-PLT highest the that speculate to tempting is it maize, and both in initials proximal the to relative QC the in is approximately initials 4:1. Based upon the high levels of of ratio the maize In 2009). Scheres and (Kornet defects meristem root producing 2010) suggest similar role for the 2010) similar suggest Feldman and (Jiang maize 2011) and Xue and (Li rice in genes of Examination 2007). et al. (Galinha meristem the of tion Cellular Patterning of the Root Meristem of the Patterning Cellular (reviewed by Jiang and Feldman 2005). The difference between between difference The 2005). Feldman and Jiang by (reviewed niche the itwith associate others whereas cells, stem as cells QC the classify some QC, the of activities different the upon Based XII. concentration. PLT2 upon depends PLTs the of function the Interestingly cells. transit-amplifying and initial the in function these species. of expression ectopic that and auxin of downstream specification accumulation. the that auxin suggest results these of Collectively absence the in cells columella of tion forma- the and marker QC-25 the of expression both show that of expression ectopic in delay is a there as significant of auxin downstream directly tions neither However, levels. auxin between and feedback PLT suggesting proteins PIN4 and PIN3, PIN1, © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis A1 ... GCN5 a.k.a. HAG1 The function of the PLTs is not restricted to the QC; they also also they QC; the the PLTs to of function is not The restricted can bypass the need for an auxin maximum in forma- in maximum auxin an for need the bypass can PLT expression after auxin application. One of the effects of effects the of One application. auxin after PLTexpression PLT promote mitotic activity, and even lower levels of levels lower even and activity, mitotic promote PLT2 , PLT3

is RBR; however, downregulation of however, RBR; is downregulation Stem Cell in the the Stem in Cell plt1 . Loss of HPY2 expression leads to a reduction in both both in reduction a to leads expression HPY2 of Loss . , and mutants (Galinha et al. 2007). et al. (Galinha mutants Arabidopsis HPY2 PLT PLT4 ( zmPLT ADA2b zmPLT s function in the root meristem (Ishida et al. et al. (Ishida root the meristem in s function PLT are all expressed in the RAM with the high- is the induction of ectopic meristems PLT1 ofmeristems ectopic induction the is is expressed downstream of both both of downstream expressed is ) and and ) ( expression actually increase in the QC QC the in increase actually expression 1 , expression in the QC relative to the the to relative QC the in expression , ) as loss-of-function alleles reduce the the reduce alleles loss-of-function as ) and and , an obvious candidate for regulation for regulation , candidate obvious an ACETYLTRANSFERASE HISTONE IH PLOIDY2 HIGH PLT1 ALTERATION/DEFICIENCY IN IN ALTERATION/DEFICIENCY 2 ) and abolish the PLT gradient PLTgradient the abolish and ) PLT and and R s in maintaining the QC in in QC the maintaining s in oot Meri PLT2 PLT QC genes mediate nor PLT1 PLT RBR is regulated in part part in regulated is ( HPY2 has been shown been has is not sufficient not is sufficient PLT2 PLT stem Arabidopsis func- PLT2 , SUMO a ), levels on levels promote PLT zmPLT PLT1 PLT1 PLT PLT are are , competent cells. It is thought that the first cells to be lost in the in lost be to cells first the that thought is It cells. competent of mitotically to deplete are all required doses higher still population and cells, second the kill radiation of doses Higher crypt. to the repopulate become activated which exist, still populations cell stem potential two actively least at but the killed, are cells radiation, stem cycling of doses low to moderate With 1986). Potten and (Ijiri crypt the within cells stem of populations ent differ- functionally indeed are there that shown have irradiated 2010). Clevers and Li 2006; et al. Crosnier 1997; Loeffler and Potten by (reviewed stem potential CBCs to the of are or damage due to loss activated be may that cells cells LRC the that thought is It quiescent. relatively are cells these staining, BrDU the dilute would sions (BrDU).divi- As cell of of a bromodeoxyuridine retention pulse their to due named so (LRCs) cells label-retaining the are cells CBC Above the in culture. a reform mini-gut can that cells stem multipotent are cells CBC cells. Paneth among the crypt of at the base exist cells (CBC) crypt, columnar intestinal crypt-based an cycling actively within example, For cells. stem actual of population a near or within exist cells in these often and these of animals, examples multiple are There quiescent. they largely are examined, time the at but cells, stem as behave to ability the have cells stem potential contrast, In population. cell stem the maintaining actively and cell—they stem proliferative, a undifferentiated, are of of time characteristic the key at the that fulfill ones are examination cells stem Actual poten- cells. and stem cells tial stem actual cells: stem of types different two between distinguish to useful is it state), future uncertain some in act will they how cells (i.e., potential stem their upon as based defined that are out point In they cell, cells. stem stem label the regard assigning (1997) Loeffler and Potten how at look “stemness.” their in probably differences are there and of cells populations identical not are they that clear However, is it cells. stem as classified be could meristem root the in cells initial the and cells QC the both definition, this 2010). By Clevers and Li 2008; Spradling and 1997; Morrison Loeffler and (Potten organ or tissue the within cells other to respect with state undif- ferentiated relatively a and cells daughter its to compared division a lowto ratewounding, regenerate orafter organ an oftissue cell arethe to ability stem are cells ascribed that often characteristics Other cells. daughter its of one least at to identity cell stem on” “pass to able is cell stem the division each with self- that so to maintain and divide to ability the are They cells. stem of nitions defi- in appear universally that features two are However, it.”there see I when it know I but define, to to regard hard “It’s in pornography: Stewart Justice Court Supreme U.S. of that to lar simi- conclusion a to lead can definition acceptable generally a formulating 1997), Loeffler and Potten 1989; andWatt Hall see examplesways different in (for cells stem define people different “since that conclusion the to came (1997) et al. Morrison cells, (see cells niche and stem of definition their in lies groups two these Experiments in which the cells of the intestinal crypt are are crypt intestinal the of cells the which in Experiments to useful is it cells, initial versus QC the of context the In for more details). In a review article on stem stem on article review a In details). more for XIII Section 3- 15 Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 and and of expression subepidermal root with the meristem, in cells petal), or cotyledon, (root, tissue callus the of source the of independent resembled, they instead, cells; tiated undifferen- not were they examined, were tissues callus the of (Atta profiles expression the growth When 2009). et al. plant Rebouillat 2009; in et al. cells these for role special a perhaps suggesting poles, xylem the with associated also are rice in roots to crown rise give that cells the Interestingly, pericycle. lack that in cells the root the orpoles with xylem in associated cells organs opment, see Chapter 6.) that Thesesuggest plantmay cells results devel- roots lateral in role pericycle the about details more (For damage. or stress to response in also but formation root of lateral process the during only not meristems novo de generate to able are that cells stem potential of population large a represent 2010).thereforemay et al. (Sugimoto pericycle The of callus tion forma- the in role key a plays pericycle the that suggests again which state, competent” in a “mitotically cells pericycle the tains main- that protein nuclear-localized a is ALF4 roots. lateral of ( FORMATION the in mutation a harboring ined in exam- were callus of generation the in participating cells actual the when However, redifferentiation. by followed (callus) state naïve or embryonic an to back root) or leaf mature (from tion dedifferentia- thought asof aof is cellular often process gression of concentrations pro- This new plant. phytohormones) entirely an appropriate into producing the of application the through (again coaxed then hor- is tissue of callus The callus. produce to cocktail mones appropriate the with treated are tissues plant mature wounding), after to opposed (as culture tissue in callus of formation the During injury. wounded following of regenerate to capacity organs the (2) and culture in tissue callus from seedlings entire regenerate to ability the (1) observations: two al. et (Sugimoto animal 2011). the The notion of of the totipotent plantcellis largely based upon regions defined within cells found stem of pockets small to limited is potential regenerative where cells, animal This isto in contrast plant ororgans. specific entire the able either to regenerate appropriate conditions under cells— stem potential essentially are cells plant all that idea the 2005). Feldman and by Jiang (reviewed type general same the of variations are cells these that indicating as initials structural QC, the in cells the and initials functional as the classified are surround QC that cells initial dividing actively the Barlow, by proposed nomenclature in Indeed, cells. stem potential as cells QC the and cells stem actual as cells initial classify and clature nomen- animal the adopt to tempting is it behavior, this upon lost, and the QC to activates repopulate the root Based meristem. are cells by initial Clowes (1963, the irradiation, 1965). performed after is, That experiments irradiation root the to similar very are surface) their on least (at experiments irradiation crypt the root, the of framework the In 2008). et al. Barker 1997; Loeffler and Potten 1986; Potten and (Ijiri LRCs the then and cells, CBC followed bythe amplification, transit are undergoing those crypt 3 © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis -16 One of the problems in defining stem cells in plants has been been has plants in cells stem defining in problems the of One PLT1 Arabidopsis . Interestingly callus formation was blocked in plants plants in blocked was formation callus Interestingly . ALF4 , they tended to be either pole xylem pericycle ) gene, which is required for the initiation initiation the for required is which gene, ) BRAT AEA ROOT LATERAL ABERRANT SHR, SCR, WOX5, load than the initial cells. initial the than load genetic less with cells stem of population protected a represent may cells QC Therefore, 2009). Sablowski and (Fulcher spared are cells QC the process, this During pathway. death cell a ing activat- by respond they that and DSB to sensitive particularly are (DSBs),daughters their and cells breaks initial the RAM the in showedthat double-strand chiefly 2009), al. et Yoshiyama 2007; Hays and (Curtis damage DNA at looking papers Recent mutation. and damage cellular to sensitive less are they active, mitotically not are cells these Because cells. stem potential cent, quies- of population a having to seen is benefit similar A cells. stem the of load genetic the reduce may this reduced; is divide to need cells stem actual the that times of number the cells, sit tran- the to function proliferative a assigning By potential. tive proliferative even in the absence of havethey are stem cells, some regenera- tissue cells transit As root. rice the in epidermis the cell different adopt to fates as is seen in the development of the layers ground tissue and asymmetrically divide also may selves them- that cells daughter in resulting asymmetric be may sions divi- these of Some tissue. the in cells of number the amplify to times several to one divide cells these pathway, this Along tion. differentia- to lead will that pathway ona be to thought are cells These self-maintain. to ability the lack but cells stem of aspects some have which cells, transit are extremes two In these divide. between to the ability the is lost has other that the cell at differentiated mature and proliferation for capacity sustained has a that cell stem naïve the is spectrum the of end one At ties. identi- cellular different of continuum a exist may there an organ, within that suggest (1997) Loeffler and Potten cells. boring neigh- or daughter their of behavior the to relative also but ior potential. erative the planthave within regen- cells and only that specific potential developmental their in cells animal than different that not all be and spermatogenesis in Drosophila, the progeny of the germ germ the of progeny the Drosophila, in spermatogenesis oogenesis both and during Likewise 2008). et al. (Barker cell stem intestinal of properties into the adopt can back they crypt, the of transplanted base the are they if crypt; intestinal the in cells transit-amplifying the for case the be to appears This fate. cell stem a into induced be may and niche the enter also may tiation differen- terminal undergone not have which However, cell). cells, transit a susceptible as to refer would (1997) Loeffler and Potten (what cells colony-forming first-generation become identity and cell stem their lose they niche, the from displaced are cells As cell. stem the of potential proliferative the maintaining in role a played cells niche the maturation, preventing By cells. stem the of maturation the preventing as cells associated these saw He behavior.” [their] “determine that cells other with tion associa- in found are cells stem organism, the within that posed (HSCs).pro- He cells stem hematopoietic to (1978)respect with Theconcept of astem cell “niche”was firstsuggested by Schofield XIII. Stem cells behav- notfuture are Stem defined their upon cells based only

in th in Stem Niche Cell Concept e R oot Root Structure Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 Cellular Patterning of the Root Meristem of the Patterning Cellular niche. A true niche should function independently of resident of independently function should niche true A niche. a of presence the indicate itself by not does signals intercellular for requirement “a that stated who (2000) Spradling and defined Xie by as concept niche cell stem and the Jiang from by deviates Feldman definition the that noted be should it However,meristem. root the in overlapping largely are populations two as (founderstem both cells that the QC cells) and suggesting niche cells the classify (2005) Feldman and Jiang cell. niche a consid- ered is type cell neither where systems several in cells between stem signaling of examples are there addition, In pop- ulations. two these between differences reinforce fact in may cells stem and quiescent active between signaling mutual that (2010)suggest Clevers and Li niche. the of part as QC the of sification the does presence notof function the necessitate clas- a signaling However, 2007). Ivanov 2005; Feldman and Jiang by (reviewed population cell stem a to opposed as to niche a as some QC the consider led has function signaling This initials fate. the cell of stem a division adopt asymmetric through QC the with tact come that into con- capacity, as cells inductive and maintenance cell stem both have to appears QC the cells, cap the Like 2000). cells in QC the types. to likened been cell have ovariole the different in cells the cap The between drawn been have analogies in its to geometry the process of root development in 1997). Willemsen and Berg den van 1995; et al. Berg den (van generated were cells stem new that so initial contacting the in division of orientation the affected cell initial the root a of ablation the to where et al. Berg Den similar van of is results This cells. daughter the of fate the hence and division the of cell orientation ence GSC affects of a neighboring as for the the pres- GSC is important stem maintenance, also cell between signaling However, population. GSC the maintain ing signal- direct through and niche the comprise cells cap the thought that is It oogenesis. during GSCs ablated replace and sion in the cap cells (GSC) with ofstem cells contact in line germ ability the apex. root the in maintain populations cell initial signals and QC the both two These QC. the at maxima with gradient the and auxin an both is there root, the In weaker. are sis reside at the edges of mito- the niche where inhibiting the signals would cells stem popula- However, actual the maintained. be cell would tion stem quiescent the highest, are signals inhibitory where niche center ofAt the the population. cell stem active and niche maypromote attime a (potential) both same quiescent the the that suggest Trumpp (2006) and Wilson 2008). Interestingly Spradling and (Morrison cells stem quiescent or divi- active of through sion or niche the into back cells transit of migration cell via maystem occur animals in native niche of the Repopulation population. the of loss upon cells new recruit to popula- and cell tion stem the maintain to both is niche the of role the Therefore, 2008). Spradling and (Xie Morrison 2000; production Spradling their and after generations three least at for cells replacement as serve to ability the (GSCs)retain cells stem line © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis As the process of oogenesis in in of oogenesis process the As by illustrated classically is niche cell stem a of Repopulation and the GSCs to the initials (Xie and Spradling Spradling and (Xie initials the to GSCs the and Arabidopsis Drosophila Drosophila germarium to change their pattern of divi- pattern to cell their change germarium is strikingly similar similar strikingly is Drosophila Arabidopsis PLT , tial stem cell population (as discussed earlier) as opposed to a to opposed as earlier) discussed (as population cell stem tial poten- a as QC the considers one 2010).if However, Birnbaum ble in the absence of a stem niche (Senacells Sena et al. and 2009; The root that regeneration is as QC. evidence possi- this authors interpret a maintain to fail that genotypes in reduced) slightly Arabidopsis 1976). mation (Feldman of meristem the in was the not case This refor- to prior QC the of establishment the in resulted root the of regeneration maize, In tip. the reformed and divided stump the within layers all from regenerate. cells process, regeneration the could During tip root entire the removed, were meristem proximal However, and root cap, QC, meristem. the entire if the tion of root if the the primary was ablation unsuccessful removed regenera- 1988), Jones and Rost 1976; (Feldman pea and maize with experiments in seen was As root meristem. the in positions wild-type of tips surgi- the who ablated (2009) cally et al. Sena by results recent explain to help limited. severely is organ intact an where movementplants relative within of cells in true particularly be may This 2008). Spradling and Morrison 2006; move(Scadden to niche the of ability the and niche the of positioning the in flexibility more to lead an may niche in epithelial types cell specialized heterologous for requirement a of lack the that animals in suggested been Ithas growth. maintain and population cell stem entire the of behavior to the help orchestrate then cells stem active and quiescent the between tions interac- Homogeneous cells. stem active become and auxin of levels lower experience initials surrounding the whereas (QC), levels ofand high express auxin of levels highest the with Cells cells. stem surrounding and QC the of activity the induces that niche it 2007), ofepithelial niche as an root the stem cell to al. think may be useful et (Grieneisen meristem root the of maintenance and formation for signal primary the is maximum auxin an As 2007). and Spradling (Nystul type cell continuous specific any with contacts no maintain they which a in in environment exist dynamic (FSCs) cells follicle ovarian example, For control stemness. that constituents” non-cellular other and matrix lular “composed ofmay be extracel- niches some that suggests (2006) the niche. defines domain Scadden a Instead, localized stemness. control that types” cell of specialized “devoid are niches thelial” “epi- contrast, In in population. cell stem exist active they an of absence and the cells, stem the from separate are cells cells. These stem resident the of fate the control ovariole) the of end anterior the in cells cap distinct the (e.g., niche the niches, in cells specific “stromal” and In 2008). Spradling and Morrison 2006; (Scadden niches epithelial and stromal between made is population. cell stem as a classified be should QC the systems, animal in are they that way same the in meristem root significant. be the to are applied cell niche and may cell stem terms the if Therefore, growth root to contribution the systems, in population cell initial contribute the QC stem tothe may cells.” While not significantly Considering the root meristem as an epithelial niche may also root the as an epithelial meristem Considering distinction a animals In root? the in then niche the is What . In fact, . root In tip fact, regeneration possible was (although PLT and therefore stem become quiescent cells Arabidopsis roots at different different at roots Arabidopsis , it does divide, , and in other it does divide, WOX5 , SCR 3- 17 , Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 13 amino acids long. In the CZ of the SAM, the the SAM, the of CZ the In long. acids 13amino approximately are form functional and processed their in that peptides secreted CLEs, the are pathways these in molecules ing of the OC in the SAM (for recent review, see Katsir et al. 2011). Katsir see review, (forrecent SAM the in OC the of WUS of expression the repress and receptors CLV1 and CLV2/CRN, CLV3 The layers. L3 the of region upper and L2, L1, the in expressed root growth. RGF1 is expressed in the QC and columella initials and moves out from these cells forming a gradient (indicated by the arrows). arrows). the by expression. (indicated gradient a forming cells these from out of moves patterns and expression The initials columella and QC the in expressed is RGF1 growth. root maintain that peptides identified recently (B) RGFsare cells. QC the to WOX5expression restricting ACR4 receptor the to binds it where initials 3.7 FIGURE the CLAVATA is their this of and example cells understood best stem the the Perhaps ofdaughters. function normal the maintain to required is signaling cell-to-cell root, the and shoot the both In XIV. Wagersby Jones and 2008). (reviewed niches new of generation novo de the and new cells stem generating cells stem of examples clear are there animals In cells. niche the not population cell stem the of part are cells QC the concept, niche epithelial the of case proper.” the In cells stem replace know,cannot we as far as which, niches,’ animal cell ‘stem to comparison in difference remarkable a is QC the of as serve a This can feature source of derivatives cells new initials. “…QC that stated (2010) who et al. Tullio by suggested was animals as to compared as plants in niche the of definitions new distinct for need the eliminates also niche epithelial an as stem cell by mediated responses stem in defects to due determinate is growth but reforms, the meristem In regenerated. functional are niche a and plants, wild-type In divisions. limited undergo to able is cell transit-amplifying the maximum, auxin new a of establishment the to prior frame time the In reforms. tip root the and cells, responsive in identity cell stem induces it achieved, is maximum auxin the Once tip. the at increased idly rap- levels auxin ablation following concept, niche epithelial an of context the In QC. a of lack the by demonstrated not is niche a of absence the then niche, “stromal” componenta of signaling 3 © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis -18 and therefore both the number of stem cells and the size size the and cells stem of number the both therefore and peptide signals to cells in the L3 that express the RPK2, RPK2, the express that L3 the in cells to signals peptide

in in Signaling of Cell Importance (

CLV R Pept oot Gro )– ide signals in the root meristem. (A) The CLE40 peptide is expressed in the columella and moves basally into the columella columella the into basally moves and columella the in expressed is peptide CLE40 The (A) meristem. root the in signals ide WUS (A) PLT and signaling pathway. The primary signal- primary The pathway. signaling and and RGF2 CLE40 WOX5 wth SCR are also shown. Protein localization is not known for these two RGFs. Darker colors indicate higher higher indicate colors Darker RGFs. two these for known not is localization Protein shown. also are RGF3 or scr . Considering the root the . meri- Considering mutants, the niche niche the mutants, plt Stem cellmaintenance ACR4 CLV3 peptide is is peptide WOX5 the process of colonization. Expression of the CLE encoding encoding CLE the Hg-SYV46 of Expression colonization. of process the during peptides CLE release and express that species non-plant are there specific, plant largely are peptides CLE While well. nematodes as parasitic cyst-forming by co-opted been pathway have may this Remarkably pathway. CLV–WUS the of use the produce cells shoot that suggest and roots results may adapted have differently differentiated root, the produce in cells stem while shoot, the in addition In domains. on turn to been shown to move [Yadavcells between et al. 2011]) is required and overlap partially domains the SAM, the in root that is the shoot and the in theCLV–WUS between difference pathways important An tials. beyond expression the butQC, has no ini- ontheeffect proximal of expansion an and initials columella the of expansion of sion as ectopic expres- RAM, the play a also tides role in maintaining In the mainly expressed in the innermost layer of central columella columella central of layer innermost the in expressed mainly are RGF3 and RGF2 while cells, stem columella and QC the to that revealed 2010). hybridization In situ et al. that Zhou 2010; et al. Matsuzaki peptides 3.7B; (Figure (TPST) sul- fotransferase tyrosylprotein homologous by modification redundant posttranslational undergo functionally three (RGFs): factors growth meristem root the is development root 2005). (Wang way et al. CLV–WUS the in path- functional are pathogens plant these by quiescent center cells (QC). Loss of Loss (QC). cells center quiescent ARABIDOPSIS the through 4 (ACR4)CRINKLY acts receptor to restrict peptide CLE40 The 3.7A). (Figure columella the in cells differentiated the in 2009). expressed is al. et (Stahl root the the as in identified signal CLV been endogenous has CLE40 Recently 2005). 2004, al. et in Mutations root growth. of termination to leads genes) these by encoded peptides cue the the cue Another recently discovered class of signaling peptides in in peptides signaling of class discovered recently Another Arabidopsis CLV3 , clv3 gene from gene from CLV3 (B) G Stemcellmaintenance RGP mutant, suggesting that the CLE peptides secreted secreted peptides CLE the that suggesting mutant, CLE19 . In the root, root, the In . root, it has longthat CLE pep- been suspected RGF1 , or , Heterodera glycines Heterodera clv2 CLE40 TA celldivision mloae hs hntp (Fiers phenotype this ameliorate WOX5 RGF3 RGF2 WUS (or application of the purified purified the of application (or and and cle40 (which itself has recently recently has itself (which WOX5 Ichinohe is able to is res- Ichinohe or CLV40 ACR4 expression to expression the is confined RGF1confined is Root Structure CLV3 are in distinct distinct in are CLV40 leads to an an to leads and and . These These . WOX5 CLE40 CLV3, WUS Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 Cellular Patterning of the Root Meristem of the Patterning Cellular tion and mitotic activity in the root meristem. root the in activity mitotic and tion sure ofroot both elonga- PDand et al. decreases 2000) (Sivaguru clo- the induces aluminum to plants of exposure conditions. example, For environmental to responsive are and time mental develop- over change PD of capacities transport and the structure Likewise control. developmental under are domains these of maintenance and formation The domains. symplastic creates cells between Theand of lated network degree cells. connectivity related and both unre- is possible between continuity symplastic Thus, walls. cell existing already into inserted are and division cell after form PD Secondary PD. primary by poten- connected least tially) (at are therefore cells related clonally All tubules. derived reticulum endoplasmic around material membrane and PD wall of new cell deposition by the Primary cytokinesis during formed are cells. neighboring of cytoplasm the connect that channels (PD) Plasmodesmata intercellular are membrane-lined 1978;(Gunning maintenance Zhu et al. 1998a; Baum et al. 2002). may be a forrequirement signaling meristem ing symplastic that suggest- connectivity, in a plasmodesmatal related with decrease and Arabidopsis in (PD).both In plasmodesmata through signaling is pathway lent preva- other The development. root in functions that pathways of PLT2 expression ectopic or peptides sulfonated purified of the application external either by rescued partially be can phenotype This meristem. reduced Loss the either pathways. of PLT and PIN7) and (PIN3 auxin the of lation forof the maintenance root regu- stem niche cell through in required part are TPST1 associated the and RGFs the that revealed et al. 2010). region (Matsuzaki analysis the meristematic Genetic into diffuse RGFs that showed However,immunostaining cells. of the SAM. For example, in in example, For SAM. the of organization the in mirrored often is plants vascular lower and ferns of meristem root the in cell apical single a of presence The in growth may be one to of a contributing the to factors switch determinate connectivity symplastic in decrease a that suggest results These 4. at week lowest PDD the detected with 4, 2 and weeks between observed was that growth root in decline the preceded PDD in decrease significant a Likewise germination. post 2 week and 1 week between seen was that growth root of rate the in increase rapid a preceded PDD in increase This week. 1 to up mination ger- of time the from significantly increased PD of density the tissues, all In PD. of density highest the had tissue ground the within andthestele within cells related thatclonally They found as the root (PDD) in aged. the RAM ofcells all the walls between Arabidopsis in as that indicating merophytes, of were the made observations same The divisions. earlier as PD of density to same failed the cells maintain apical the that noted he division, 35th the After of cell apical the that (1978)showed Gunning well. as filiculoides © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis In In signaling the of one only is peptides mobile of secretion The Symplastic signaling may also affect meristem organization. organization. meristem affect also may signaling Symplastic divided 55 times (on average) before growth ceased. ceased. growth before average) (on times 55 divided Azolla Zhu et al. (1998a,b) examined PD densities densities PD examined (1998a,b) al. et Zhu Arabidopsis roots. This is likely the case for roots of islikely This roots. Arabidopsis , decreased PDD precedes determinate root growth. root determinate PDD precedes , decreased or the the or RGFs Azolla , a switch to determinate growth , is a cor-growth to switch determinate results in a stunted root with a with root stunted a in results TPST . both the SAM and the the and SAM the both Equisetum A. A. in the stele but also in the endodermis and the QC, indicating indicating QC, the and endodermis the in also but stele the in only not present is protein however,the stele; the in exclusively 2010).The et al. Carlsbecker 2000; et al. (Helariutta cells protoxylem replace cells disrupted, metaxylem is and QC their layer, cell endodermal an lack mutants The patterning. cellular in defects and growth determinate of roots the earlier, mentioned 2011). et al. As Koizumi 2009; Benfey and Gallagher 2007; et al. et al.(Nakajima 2001; et al. Gallagher Cui 2004; et al. 2007; Welch in homeostasis. developmentroot and in molecules fore represent a signaling number oflarge potential there- These proteins cells. between moving proteins 2006) et al. (Lee domain MYB and box, bZIP,MADS NAC, factors Dof, including transcription of types different many with widespread also was movement for capacity The types. tissue different tiple approxi- for movement mately mul- 16% between Movement proteins. cell-to-cell of these occurred for evidence found They factors. transcription tissue-specific 61 of for domain localization and protein transcription of domain the al. et examined Lee (2006) cells, between move factors root-of transcription fraction expressed what determine to effort microRNA, an In mRNAs, proteins. and to metabolites small from PD, through by primary PD. sisters its of all to connected highly is that cell apical single a to meristem the directly limits to cells non-sister therefore between communicate and PD that secondary make suggest to authors inability The the cell. apical single a with meristems “fern-like” had PD primary make to able only were that those while cells, initial multiple with SAMs angiosperm-like had PD secondary and primary both made PD. that lycopods secondary Interestingly, make to ability their in divided were lycopods The SAM. their in cell apical single a had and PD primary only made (2007) Hiratsuka and the Imaichi by of examined pteridophytes all contrast, In SAMs. multi-initial have nonrelated they and and cells) related clonally both (between PD secondary and cells) related clonally (between PD primary both make can Angiosperms network. tion of the SAM and the plasmodesmatal and organiza- the between found was pteridophytes, correlation strong a lycopods, gymnosperms, angiosperms, in structure SAM examining In development. during communicate to for cells need the upon based is that explanation compelling a vide pro- (2007) Hiratsuka and Imaichi example, For meristem. the in in cells initial of limitation number and size the inherent restricts that groups these an reflect also Scheres may and it (Bennett However, 2010). populations cell stem these of both for origin common a suggest may RAM the and SAM the between similarities structural The 2007). Hiratsuka and (Imaichi SAMs the and RAMs the in initials multiple have plants where sperms toangio- extends to of produce cells the all shoot. This similarity its along faces divides that cell apical a tetrahedral-shaped single of SAM the Likewise merophytes. to rise give that cells apical active mitotically prominent contain RAM Perhaps one of the best-characterized symplastic signals signals symplastic best-characterized the of one Perhaps pass to able are that molecules different of range a are There root growth is the SHR transcription factor factor transcription SHR the is growth root Arabidopsis mutants show precocious precocious show mutants shr-2 transcript is expressed expressed is transcript SHR contains contains filiculoides A. shr-2 3- 19 Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 tive feedback loops that control SHR movement (Figure 3.8; 3.8; (Figure movement SHR control that loops feedback tive nega- and positive SHR-dependent of sets interesting revealed root. tothe provide support may structural some that tissue lignified and which suberized layer,a both endodermal is itself an of absence the in tissue vascular thickened mature, of formation the speeds that loop feedback a with root the provides also system a such that speculate could ment165/6. is one miRNA way of to a active create One gradient miRNA165/6 acts in a manner. dosage-dependent Graded move- stele. However, et al. (2010) from Carlsbecker results that suggest 165/6 miRNA the in activating to SHR directly opposed is as nal sig- reciprocal this of value adaptive the what unclear Itis ways. edu.cn/PMRD/ database, (PMRD plants in widely found are 165/6 2011).isoforms miRNA Vaten et al. 2010; et al. (Carlsbecker tissue vascular the of patterning the in functions the a stele activates microRNA which is itself a mobile that signal from SHR of movement which in pathway signaling reciprocal metaxylem defects in in defects metaxylem the phenocopy site binding miRNA the in mutations with PHB mRNA, target its inhibits it where tissue stele the into moves turn in which miRNA165/6, of expression initiates SHR endodermis, the In xylem. of the ing is This 2010). al. et is dependent foran signal required endodermal because pattern- Carlsbecker 2009; Benfey and Gallagher pat- vascular the the in seen nor defects terning radial the neither rescue SHR of forms cycle cell 2011). et al. (Gardiner the of regulation SHR-dependent and SHR of mentmove- show also tissue leaf in experiments as root, the to cific 2001; al. pathwayalso is et al. not 2007). The spe- Laajanen SHR et signaling (Sassa configuration meristem open an with roots in and monocots in functions pathway signaling SHR/SCR the that suggest results These endodermis. the includes which cap, root lateral the and tissue stele the between arc an in expressed expression of a D-type cyclin, ofexpression cyclin, a D-type the activate SCR et al. and SHR both 2001). cells, CED and CEI the (Nakajima In QC the in expression own its maintains turn SHR factor,in which layer, transcription SCR of the expression cell upregulates neighboring the In 3.8). and 3.4A (Figures ers lay- cell neighboring these into stele the from moves SHR that 3 the homologs ( of SCR the In 2008). al. et (Sole t in ally related (Kamiya et al. 2003; Bolle 2004). In In Don, D. 2004). Bolle 2003; et al. (Kamiya related ally clon- are epidermis the and cortex the where rice of patterning radial the in intact is pathway this that suggest data interaction of patterns expression 2010). et al. (Sozzani formation cortex middle during development, root endodermis. in and Later cortex separate a of formation the in results © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis -20 Research into the mechanisms by which SHR moves has has moves SHR which by mechanisms the into Research Movement of SHR is required for its function. Nonmobile Nonmobile function. its for required is SHR of Movement homologs are found in multiple species of plants. The The plants. of species multiple in found are homologs SHR he CED triggers the asymmetric periclinal cell division that that division cell periclinal asymmetric the triggers CED he is expressed in a pattern similar to similar pattern a in expressed is PrSHR ) perhaps suggesting conserved patterning path- patterning conserved suggesting perhaps ) L. and and L. sylvestris Pinus and and PsySCR shr-2 and and OsSHR1 mutants (Gallagher et al. 2004; 2004; et al. (Gallagher shr-2mutants . SHR therefore is at the center of a of center the at is therefore SHR . expression is upregulated upregulated is expression CYCD6;1 PHABULOSA cyc D6;1. E PsSCR in rice along with with along rice in OsSCR http://bioinformatics.cau. , respectively) are both both are respectively) , xpression xpression of . Dominant alleles of alleles Dominant . L., L., sativum Pisum radiata Pinus Arabidopsis cyc D6;1 D6;1 scription factor named named factor scription tran- 14) (subfamily bHLH novel a is patterning root in role a formed. are that layers tissue of ground numbers the movement therefore may be a by mechanism which roots control of SHR Regulation root. layers the ofin number the cell increase homozygosity (e.g., for movement SHR increase that 2011). Interestingly conditions al. et (Koizumi tissue ground the in sions or homozygotes siel-4 (e.g., activity SHR or SHR of movement reduce that Mutations cells. endodermal of nuclei the in SHR concentrating for nism mecha- a provide may then This it. inhibits it endodermis, the in whereas movement, own its promotes SHR stele, the in that 2011). et al. (Koizumi indicate endodermis into the results These SHR of movement promotes and cells stele of cytoplasm the in SHR with interacts that protein endosome-associated an is SIEL SCR. and SHR both of downstream is LETHAL) EMBRYONIC SIEL (for protein a called SHR-interacting SHR INTERACTING of expression stele the Interestingly, cells. endodermal of nuclei SHR stele trapping orin the into through presumably the cortex, JACKDAW movement into the back of inhibit either SHR (JKD) and SCR factors, MAGPIE (MGP), transcription and one factor SHR-independent transcription SHR-dependent two endo- the In dermis 2004). al. et (Gallagher endodermis stele the the into nor into back neither moves and localized nuclear sively exclu- is SHR endodermis, the In and endodermis. the cytoplasm into moves the and nucleus the both in SHR present the is protein cells, stele 2011). In et al. Gallagher Koizumi 2007; 2009; Benfey et al. Welchand 2007; et al. Cui 2004; et al. Sena species (ROS). In the root, there are two major ROS, hydrogen ROS, major two are there root, the In (ROS). species oxygen reactive of levels the of regulation the through size stem meri- and growth root regulates 2010).UPB1 et al. Wells2010; unknown mechanism, indicated by the dashed arrow) promote the the promote arrow) movement. SHR promotes which protein, dashed SIEL of the expression the by indicated mechanism, an unknown (though SCR and SHR stele, the In endodermis. the in centrations con- SHR increasing potentially SHR of localization nuclear and promote endodermis the of out SHR of movement inhibit JKD and MGP, MGP. and SCR (endo) on both SCR, it endodermis where turns the into 3.8 FIGURE One of the latest mobile transcription factors shown to play to shown factors transcription mobile latest the of One or SCR RNAi) (Cui et al. 2007; Welch et al. 2007) also also 2007) Welchet al. 2007; et al. (Cui RNAi) SCR or jkd-4

SHR sig SHR SHR Stele SHR SIEL SHR naling. The SHR protein moves out of the stele stele the of out moves protein SHR The naling. shr-2 divi- ectopic in heterozygotes) result UPBEAT1 SCR SHR SHR noCortex Endo SHR MGP ( UPB1 JKD ) (Tsukagoshi et al. al. et (Tsukagoshi ) Root Structure Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 Examination of Examination comitant decrease in the elongation zone (Tsukagoshi et al.con- 2010).a and meristem root the of expansion an in resulting root the in gradients ROS UPB1affect mutationsin Loss-of-function cells. movesbetween UPB that suggesting elongationzones, and transition the in cells of nuclei the in primarily detected is tein well as plant stem cells. stem plant as well as animal regulating in ROS for role a 2010) Jonsson indicating and Eliasson by ROS(reviewed of levels the altering by reversed ferentiation.O As aias ht a raiy ec t fr H form to react readily can that radicals hydroxyl of formation the through loosening cell-wall induces auxin zone, elongation the In ROS. of generation the through root the is also in to signal is thought One auxin of ways that the becomecells differentiated (Tsukagoshi2010). et al. whereas lower levels of O lowerofwhereaslevels ). The overlap of O of overlap The 3.5C). (Figure zone a model in which high levels of O expression of several peroxidases. Tsukagoshi et al. (2010) propose (LRC)ofcap root lateral UPB1 2007). et al. Dunand 2007; Penel and (Dunand zone transition O in the meristem, whereas H whereas meristem, the in in the relative levels of the two ROS. Once the ratio of H of ratio the Once ROS. two the of levels relative the in changes subtlecontrolledby be can behavior cell gradients, ping erties that are associated with stem cells in animals including including animals in cells stem with associated are that erties prop- key have cells Initial cellular predictable. and stable and are identities constant organiza- relatively the maintained is that root the so of tion ways stereotypical in divide meristem awith closed in roots cells initial The initials. termed cells active mitotically of populations are there meristems, all in However, plants. of species different between differs meristem root the of organization The meristems. of action the through bryonically In plants, much of the of patterning the organism postem- occurs XV. peroxides (H cells (HSC) are kept in a relatively quiescent state in the bone of O concentration the the by limiting marrow in state quiescent relatively a in kept are (HSC) cells have been shown. For also stem hematopoietic example, animals in of growth opmentregulation however, the for roles ROS in as is probablyThis the not roleonly permeability. for inROSdevel- PD in changes be may UPB1 ROS on and auxin of effects the of one that possible is it PD. Nonetheless, movevia also can they if known plasma is not It via carriers. efflux cells and influx between localized membrane move Auxins 2011). al. et Alfonso Benitez- 2009; Jackson and (Benitez-Alfonso PD block and tion forma- callose to ofinduce generation ROSROS the as known is of regulation the through function would auxin, UPB1and like elongation (Mori cell et al. 2009). It orthat mobile signals, is interesting division cell either promote could auxin circumstances neat ih O with interact Cellular Patterning of the Root Meristem of the Patterning Cellular © 2013 by Taylor & Francis Group, LLC Group, Taylor by © 2013 &Francis 2 − UPB1 is not alone in regulating the levels of ROS in the root. root. the in ROS of levels the regulating in alone not UPB1is reacheslevel, its cell maximal proliferation stops entirely, and mRNA is present in the vascular tissue as well as in the the in as well as tissue vascular the in present is mRNA

Conclusions 2 O 2 ) and oxygens radical (O 2 UPB1 o om O form to 2 − and H and targets revealed that UPB1 repressed the the repressed UPB1 that revealed targets Arabidopsis 2 − 2 and high levels of H oflevels high and O 2 2 O 2 − form inverse but partially overlap- form inverse but partially Teeoe dpnig pn the upon depending Therefore, . 2 is highest in the root elongationroot the in highest is 2 − promote proliferation,cellular roots, while the UPB1pro- the while roots, 2 − and H and 2 − ). O 2 2 -. Quiescence can be can -. Quiescence O 2 2 2 − O . Auxin can also also can Auxin . levels are highest 2 2 O constitutes the the constitutes 2 promotedif- 2 O 2 to Benitez-Alfonso Y, Jackson D. 2009. Redox homeostasis regulates Baluska F,Baluska Kubica HauskrechtS, Postmitotic1990. M. isodiamet- and apical is What 2005. et al. TI PW,F,Barlow BaluskaBaskin 2009. al. et E Boucheron-Dubuisson L, Laurens R, Atta of reorganizationOntogenetic 1976. C. 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