Cell Fate and Cell Morphogenesis in Higher Plants

Cell fate and cell morphogenesis in higher plants

John W Schiefelbein

University of Michigan, Ann Arbor, USA

The differentiation of plant cells depends on the regulation of cell fate and cell morphogenesis. Recent studies have led to the identification of mutants and the cloning of genes that influence these processes. In several instances, the genes encode products with homeodomains or Myb or Myc DNA-binding domains.

Current Opinion in Genetics and Development 1994, 4:647451

Introduction mutants led eo the early notion that iUVl influences the Gte of leaf cells. Recent studies have shed light on the In higher plants, the formation and differentiation of normaI role of KNY. In localization experiments, KNl cells occurs in an orderly manner, ofien near meristems, is undetectable in leaves, but it is abundant in the api- which are groups of dividing cells chat act like permanent cal meristem and the immature axes of vegetative and stem cell populations. Cell differentiation in plants dif- floral shoots [7]. Also, the dominant Knl-N2 mutation fers horn that in animals because, for the most part, of a has been found to cause ectopic expression, such that lack of the relative cell movements that are characteristic KZVZ transcripts and KNl protein are detected in lateral of animal development. Also, the rigid plant cell wall veins of developing leaves (in addition to the normal makes the control of cell shape a critical aspect of cell accumulation in the meristem), which correlates well differentiation in plants. with the alterations in leaf cell fite and knot forma- The fate of undifferentiated plant cells is influenced pri- tion associated with Krzl mutants [7]. When the KNY marily by cell position rather than cell lineage [1,2]. cDNA is expressed in tobacco under the control of the Thus, one of the current goals in studies of plant cell strong cauliflower mosaic virus (G&IV) 35s promoter, differentiation is to determine how cells acquire and the plants display a reduction in shoot stature with rum- respond to positional cues during development. An- pled or lobed leaves and (in some plants) a loss of apical other goal is to understand how, once a particular &te dominance and severe dwarfism with meristematic tis- is adopted, cell morphogenesis is regulated to generate a sue forming in inappropriate places [8’]. These results cell of the appropriate shape. The application of molec- indicate that the KNl product is associated with inde- ular genetic approaches have led recently to an under- terminate growth and cell divisions, although the lack standing of some of the genes that participate in cell fite of recessive (loss-of-fimction) alleles of KN? prevents a specification or cell morphogenesis in plants. This review complete understanding of the role of this gene. will principally focus on these recent molecular and ge- In addition to the Kn 1 mutations, other dominant maize netic experiments, including a summary of the rapidly mutations affect leaf cell fate, including mutations of growing number of homeobox genes that influence plant the Rough sheath1 (RSl) and Liguleless (Lg3 and Lg4) cell development and some of rhe model systems that are genes [9]. A detailed examination of the Rsl-0 mu- being used to study cell fate and morphogenesis in plants. tant has shown that it is similar to Knl mutants; each possessesdominant neomorphic mutations that act non cell autonomously to cause a transformation ofblade tissue to sheath, although the Rsl mutant differs in that it Maize homeobox genes associated with cell fate affects the entire lateral dimension of the ligular region of the leaf, causes affected tissue to display attributes of The homeodomain is an approximately 60 amino acid both sheath and auricle, and does not show a dose re- DNA-binding domain found in a superfamily of eukary- sponse [ 10.1. Although the normal role of RS 1 cannot be otic proteins, some of which regulate cell fate [3,4]. The determined directly by analyzing the neomorphic mu- first plant homeobox gene to be isolated, KNOTTED1 tant phenotype, the RSl gene has recently been found (ml), is associated with dominant neomorphic mu- to encode a KNl-like homeodomain protein (P Becraft, tations that alter leaf development in maize and cause M Freeling, unpublished data cited in [9,10*]). Coinci- excess cell proliferation (knots) along lateral veins of the dentally, the RSI gene was one of several maize home- leaf blade ([S]; reviewed in [6]). The phenotype of Kn 1 obox genes cloned by members of the Hake laboratory

Abbreviations CaMV-cauliflower mosaic virus; Cll-GLABRAI; KNl-KNOlTEDl; RSl-Rough sheathl; TTGTRANSPARENT TESTA-GLABRA.

0 Current Biology Ltd 0959-437X 647 648 Diffkentiation and gene regulation

[5,11”] by virtue of their sequence similarity to KM. has an amino-terminal region similar to metal-binding The expression of four of these maize hotneobox genes domains (denoted the PHD-finger), but no direct ev- (HVI, RSI, KNOX3, and KZYOX8) has been examined idence for a role in plant development has (yet) been in detail, and in wild-type plants, transcripts 6om each obtained [18]. gene have been shown to accumulate in shoot meris- terns and the developing stem, but not in determinate lateral organs such as leaves or floral organs, nor in root meristems [ll”]. Overall, the pattern of expres- Control of epidermal cell fate in Arabidopsis sion of KNOX8 is similar to that of KM, whereas the RSY and KNOX3 genes are expressed in only a subset The formation of hair-bearing and hairless cells in the of KM-expre&g cells. The expression of these four shoot and root epidermis of Ambidopsis provides a useful genes in the vegetative shoot apical meristem appears to model for the study of cell fate specification. In the shoot predict the site of leaf initiation, as well as portions of epidermis, some of the cells differentiate into trichomes the segmentation unit of the shoot (‘phytomer’) [ll*.]. (leaf hairs), and genetic studies have shown that two The distinct regions of homeobox gene expression are loci, GLM3RA 1 (GL1) and TRANSPARENT TESTA- suggestive of roles for the gene products in defining re- GJYBRA (7TG). are required for the specification of a gional identities within the meristem which influence trichome cell. The CL1 gene encodes a product with the fate of differentiating cells. similarity to the DNA-binding domain of the my6 family of transcriptional regulators [19], and it appears to act locally in trichome precursor cells, as evidenced by in situ RNA hybridization [20’] and genetic mosaic analyses [21*]. Interestingly, two pieces of evidence indicate Cloned homeobox genes associated with that GL1 is not only required for specifying cell fate, developmental abnormalities but also to maintain trichome cell differentiation. First, a weak allele of the g/Y gene @l-2) which is associated The role of plant homeobox genes in cell differentiation with the loss of 27 amino acids from the carboxyl termi- has also been studied in other plant species. A KM-like nus of the protein generates a small number of trichomes, homeobox gene isolated from rice (OSH1) produces al- with some aborted trichomes that cease expanding be- terations in leaf morphology reminiscent of those in Kn 7 fore reaching full size [22]. Second, CL7 transcripts per- mutants when introduced (in multiple copies) into rice sist in the differentiating trichome cells after expression [12-l. When the OSHl cDNA is expressed in Arabidopsis in the surrounding leaf primordia has decreased [20*]. (under CaMV35S control) or tobacco (under the con- These results imply that CL1 may be needed to maintain trol of various promoters), plant morphology is altered the expression of genes involved in cell morphogenesis. dramatically, with abnormalities in shoots, leaves, and flowers, and the formation of excess meristematic tis- The other gene required for trichome cell specification, sue [12*,13]. The OSH? gene appears normally to be TT%, may encode or activate a homolog of the maize R expressed in vegetative shoot apices [12’] which, when gene product [23]. Trichome (and anthocyanin) produc- taken together with the cDNA data, implies that it (like tion is restored to rg mutant plants when a cDNA from KIV7) may be a regulator of cell differentiation in/near the Lc gene of the maize R fhily is introduced under the shoot meristem. the control of the CaMV35S promoter [23]. The R gene family members encode proteins with a basic domain Using reduced-stringency DNA hybridization, a large containing a helix-loop-helix (HLH) motif, similar to number of homeobox genes have been identified that found in the myc class of DNA-binding proteins, from Arubidopsis, and one subf&nily has been shown and an acidic domain characteristic of transcriptional to be characterized by a leucine zipper motif adja- activators [24,25]. In maize, the myc-related R gene cent to the homeodomain (denoted HD-Zip proteins) product and the myb-related C7 product are required [14-163. One HD-Zip gene, HAT4, may play a gen- to activate transcription of genes encoding flavonoid eral role in plant development. Its transcripts accumu- biosynthetic enzymes (reviewed in [26]). Thus, an at- late in all tissues examined, with highest levels in stems tractive possibility is that the TTG gene may encode and leaves [17-l. Transgenic plants expressing an anti- a myc-related product that interacts with the myb-re- sense CaMV35!&h!AT4 gene construct display a gen- lated CL1 to activate trichome cell differentiation in eral delay in development, whereas plants expressing a the developing shoot epidermis. The 7TG gene may Cah4V35s-HAT4 sense construct are tall and develop also play a role in trichome spacing, as the R-express- rapidly [17’]. It is not clear whether cell fates are altered ing Arubidopsis plants produce excess trichomes over the in these plants or whether the only effect is on the rate of leaf surface [23]. Trichome spacing is also affected in a cell formation and differentiation. A member of another newly isolated mutant, TRZp7YCHON (Try)), which ex- Arabidopsis homeobox gene subfamily has been identified hibits nests or clusters of trichomes on the leaf, implying because its product (HAT3.1) binds to a portion of the that the affected gene may normally prevent cells adja- photoregulated u&-E promoter [18]. HAT3.1 lacks the cent to a trichome precursor from differentiating in the leucine zipper motif found in the HD-Zip proteins and same manner [21*]. Cell fate and cell morphogenesis in higher plants Schiefelbein 649

In the root epidermis, cells difherentiate to form either meristem is composed of three tiers of putative initials root hair cells or hairless cells. In Arubidopsis, the fate of that divide regularly to generate the longitudinal files immature epidermal cells is related to their position; cells of cells in the various tissues [31*]. Cell morphogen- located over an anti&al wall separating underlying cor- esis can be analyzed along individual cell files, with tical cells differentiate into root hair bearing cells, and the ‘younger’ cells present near the root apex and ‘older’ other epidermal cells differentiate into mature hairless (more differentiated) cells further from the apex. Sev- cells. Recently, the 77G gene has been found to par- eral Arabidopsis root morphogenesis mutants have been ticipate in the specification of root epidermal cell fate, isolated and characterized recently [32,33*]. Three mu- as nfg mutants produce root-hair cells in all epidermal tants display excessive cell expansion that is limited positions, and Arabidopsis expressing the maize R gene primarily to a single cell layer in each mutant (cobra generate hairless cells in all positions [27]. This indi- mutant-expanded epidermis, sabre mutant-expanded cates that 27X normally acts at an early stage in root cortex, and lion’s tail mutant-expanded stele), indicat- epidermis differentiation to generate or respond to a ing that the affected genes regulate cell expansion in a position-dependent signal which influences cell f%e. cell type specific manner [33*]. Mutants in which mor- This also demonstrates overlap in the genetic control phogenesis of Arubidopsis root hair cells is affected have of cell differentiation in the shoot and root epidermis, also been identified and subdivided into those in which although, interestingly, alterations in 77G action affect either root hair emergence or root hair tip growth is af- the root and shoot epidermis in opposite ways. Recessive fected [34]. One of these mutants has been found to dis- ttg mutations cause excess root hair production and a lack play abnormal pollen tube growth as well, which suggests of trichomes, whereas R expression results in excess tri- that the aEected gene (TZPY) is required for the morpho- chomes and a reduction in root hair formation [23,27]. genesis of both of these tip-growing cell types [35]. Root It may be that the ground state is different in these two hair morphogenesis mutants have also been identified re- tissues, with a hairless cell the ground state for the shoot cently in maize [36]. epidermis and a hair-bearing cell the ground state for the root epidermis.

Conclusions Genes controlling plant cell morphogenesis Molecular genetic approaches have led to new insights Once cell fate is specified, the differentiating plant cell into the control of plant cell differentiation. One of the initiates a program of cell morphogenesis. The com- themes that is beginning to emerge from these studies plexity of this process is reflected by the finding of is that plant cell differentiation appears to be governed, at least 18 genes that are required for the morpho- at least in part, by regulatory molecules with putative genesis of a trichome cell in Arabidopsis [21*,28]. The DNA-binding domains related to those present in some genetic analysis of trichome formation indicates that it vertebrate and Drosophila proteins (e.g. homeodomains, is the result of many independent events, rather than a Myb, and Myc domains). In these instances, it will be sequential stepwise process, with some of the events be- important to determine how the distribution of these ing dependent on the cell.reaching a certain size [21*]. regulatory molecules is controlled during plant develop- One of the trichome morphogenesis genes, GUBRAZ ment, and what the downstream targets of these regu- (CL?), has been cloned recently [29”]. Plants homozy- latory molecules might be. There are many possible gous for a g/2 mutation produce trichomes with a lat- types of such downstream targets. Because cell posi- erally expanded shape, indicating a defect in trichome tion is known to be important in speci@ng fate, one outgrowth [21’,29”]. The product of the CL.2 gene is class of targets may be genes encoding components of the a novel type of plant homeodomain protein which lacks cell wall (plant extracellular matrix) involved in cell-cell the leucine zipper motif of the Arabidopsis HD-Zip pro- communication. In vertebrates and Drosophila, cell adhe- teins and the PHD-finger motif of the HAT3.1 protein sion and substrate adhesion molecules have been impli- [29**]. The CL2 transcripts are detected in individual cated recently as targets of some of the homeodomain developing trichome cells of the emerging leaf primor- proteins [37], and the in&rence of the cell wall in algal dia [29”], consistent with the results of genetic mosaic cell fate has recently been documented during early de- analysis [21*], indicating that CL? may activate down- velopment in Fucus [38’]. It is also possible that a class of stream genes associated with the expansion of trichome downstream target genes may tie into pathways of plant cells. hormone action, as hormones can markedly affect plant cell morphogenesis. Finally, another class of targets may Many investigators interested in plant cell morphogenesis be genes encoding regulators of the plant cytoskeleton, as have been attracted recently to the study of the .4rubidop- microtubule and microfilament systems reorganize dur- sir root. Major reasons are its small size and remarkably ing cell differentiation and influence the shape of plant simple structure [30*,31*]. The Arabidopsis root apical cells. 650 Differentiation and gene regulation 4

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No evidence was obtained to sup Schiefelbein JW: The I7G G&e is Required to.Specify Ep< port the possibility of direct interactions between the KNI and the other dennal Cell Fate and Cell Patterning in the Arabidquis Root. three genes in controlling their expression. The KNI protein is suggested Dev Biol 1994, in press. to act directly as an intercellular signal to explain differences between protein and RNA accumulation patterns. 28. Marks MD, Esch JJ: Trichome Formation in Arabidquis as a Genetic Model for Studying Cell Expansion. Curr Top Plant 12. Matsuoka M, lchikawa H, Saito A, Tada Y, Fujimura T, Kano- Biochem Physiol 1992, 11 :131-l 42. . Murakami Y: Expression of a Rice Homeohox Gene Causes Altered Mqhlogy of Transgenic Plank. Plant Cell 1993, 29. Rerie WC, Feldmann KA, Marks MD: The Gl.AURA2 Gene En- 5:1039-l 048. . . codes a Home&main Protein Required for Normal Trichome The isolation and characterization of a rice homeobox gene (OSHIJ very Development in Arabidopsis.. Genes Dev 1994, 8:138&1399. similar to the maize KNI gene is described. Expression of this gene in This reports the exciting finding of a new type of plant homeobox gene Arabidopsis was shown to alter vegetative and reproductive develop and shows that it is expressed in developing trichomes. The GL2 product ment. is likely to regulate cell expansion during trichome development. Cell fate and cell morphogenesis in higher plants Schiefelbein 651

30. Aeschbacher RA, Schiefelbein JW, Benfey PN: The Genetic and 35. Schiefelbein J, Calway M, Masucci J, Ford 5: Pdkn T&e and . Mokcukr Basis of Root Development. Annu Rev P/anr Physiol Root-Hair lip Growth is Disrupted in a Mutant of ARMdopsf MO/ Biol 1994, 45:25-G. t/t&au. Planr Physiol 1993, 103:979-985. A comprehensive review of recent studies of root development with particular emphasis on genetic approaches with Arabidopsis. 36. Wen T-J, Schnable PS: Analyses of Mutank of Three Genes that 31. Dolan L, Janmaat K, Willemsen V, Linstead P, Pcethig 5, Roberts lnfhmce Root Hair Development in Zea map (Gnunbreae) . K, Scheres B: Cellular Organization of the Arab&p& t/m/Jam Suggest that Root Hairs are Dkpensahk. Am / Bof 1994, Root. Devefqmenr 1993, 119:71-84. 81~833-842. This study characterizes the organization of the developing Arabidopsis root and describes its strikingly simple structure. A model is proposed 37. Edelman GM, Jones FS: Oukide and Downstream of the Home- for the organization of the root apical meristem, including the putative obox. ) Biol Chem 1993, 268:20683-20686. initials for each tissue. 32. Baskin TI, Betzner AS, Hoggart R, Cork A, Williamson RE: Root 38. Berger F, Taylor A, BrownI& C: Cell Fate Determination by Morphology Mutants in Arabidopais tbalfana. Aust / Plant Phy- . the Cell Wall in Eariy FINN Development. Science 1994, siol 1992, 19427-438. 263:1421-l 423. 33. Benfey PN, Linstead PI, Roberts K, Schiefelbein JW, Hauser The cell wall is implicated in the control of cell fate in the two-celled . M, Aeschbacher RA: Root Development in Arabidopsk Four embryo of Fucus spiralis. The contact of one of the two cells of the early Mutants with Dramatically Altered Root Morphogenesis. De- embryo with the isolated cell wall of the other caused a switch in cell vehpmeflt 1993, 11957-70. fate. This report outlines the isolation of root morphogenesis mutants of Ara- bidopsis. The characterization of four of these mutants shows that each of the affected genes largely controls the morphogenesis of a single cell layer. JW Schiefelbein, Department of Biology, 830 North University 34. Schiefelbein JW, Somerville C: Genetic Control of Root Hair Development in Arabbfopsfs tba/i;ml. P/anr Cell 1990, Avenue, University of Michigan, Ann Arbor. Michigan 48109- 2~235-243. 1048, USA.