Shoot Meristem Formation and Maintenance Michael Lenhard and Thomas Laux*

Shoot meristem formation and maintenance Michael Lenhard and Thomas Laux*

The shoot apical meristem of higher plants is a self-maintaining gests the existence of at least two ‘communication com- stem cell system which gives rise to the entire aboveground partments’ in the shoot meristem. Although it is not clear part of a plant. In the past year, genetic and molecular studies whether these compartments coincide with the classical have provided increasing insight into the processes of shoot CZ and PZ, an important conclusion of this work is that meristem formation and maintenance, as well as into the selective symplasmic coupling of cells could provide a relation between the apical meristem and its products. mechanism to restrict the spread of potential morphogens to separate regions in the shoot meristem. Addresses Lehrstuhl für Entwicklungsgenetik, Universität Tübingen, Auf der Clonal analyses in a number of plant species, including Morgenstelle 1, D-72076 Tübingen, Federal Republic of Germany Arabidopsis thaliana, provided evidence that all vegetative * e-mail: [email protected] and generative structures of the shoot are derived from one Current Opinion in Plant Biology 1999, 2:44–50 common set of stem cells [7–9]. These findings contradict the ‘méristème d’attente’ concept put forward by Buvat http://biomednet.com/elecref/1369526600200044 and colleagues some decades ago [10], which holds that © Elsevier Science Ltd ISSN 1369-5266 the germ cells are produced by a pool of stem cells set Abbreviations aside very early in development, similar to the germ line in CZ central zone animals. Recent clonal analysis in maize, however, has KAPP kinase-associated protein phosphatase taken up this concept again. The maize shoot meristem PZ peripheral zone forms a limited number of vegetative leaves before a ter- minal inflorescence, the tassel, is produced on the main Introduction axis. It was shown that the upper part of the tassel is One of the fundamental features of postembryonic devel- derived from a set of cells in the shoot meristem that does opment of higher plants is the reiterative formation of new not contribute to postembryonic vegetative growth, even if organs by the shoot meristem [1]. The shoot meristem is the extent of vegetative growth is artificially increased formed during embryogenesis and subsequently gives rise [11••]. The author concludes that some apical cells are set to internodes, leaves, axillery shoot meristems and flowers aside in the shoot meristem early in development to exclu- (Figure 1). The bases for this activity are the abilities of sively form tassel. This does not imply, however, that these the shoot meristem to: firstly, maintain a set of pluripotent cells become committed to the formation of the upper tas- stem cells in a central zone (CZ); secondly, to initiate sel early in development, rather that they could simply be organs from the progeny of the stem cells in a peripheral located in a position where they are not recruited during zone (PZ); and thirdly, to balance these two processes the vegetative phase of the maize shoot meristem. (reviewed in [2–5]). In the following review, we will dis- cuss papers of the past year with regard to four aspects: the Formation of the shoot meristem during organization of the shoot meristem; its formation during embryogenesis embryogenesis; the maintenance of an active shoot meris- The origin and development of the shoot meristem in tem in postembryonic growth; and finally the inter-relation the embryo have been discussed controversially between the shoot meristem and its products. (reviewed in [12,13]). On the basis of comparative morphology, it has been argued that when the shoot Organization of the shoot meristem meristem is first differentiated, the partitioning of the As stated above, the shoot meristem contains two cell pop- embryo apex simultaneously defines two regions of the ulations with distinct behaviors, that is, those in the center shoot meristem with separate functions, the cotyle- which remain pluripotent and those in the periphery which donary primordia at the periphery and the ‘apical initials contribute to organ formation and eventually differentiate. per se’ in the center, required for meristem perpetuation Thus, cell behavior must be co-ordinated within one pop- [13]. Kaplan concluded that the cotyledons represent the ulation and distinguished from that of the other first products of the shoot meristem, a view which is con- population, implying regulated intercellular communica- sistent with the observation that the shoot meristem tion. To address the question of whether specific specific gene SHOOTMERISTEMLESS (STM) [14] is cytoplasmic coupling is involved in this process, Rinne and only expressed in the central cells, and not in the pre- van Schoot microinjected fluorescent dyes into single epi- cursor cells of the cotyledons [13]. Recent molecular dermal cells of the birch shoot meristem and followed the studies of various genes implicated in embryonic shoot fluorescence spread [6••]. Intercellular coupling was meristem development, however, have demonstrated observed within a central and a peripheral region, but not that shoot meristem formation involves a succession of between these two regions, except for a transient period, events and that at the stage of cotyledon initiation not all possibly during the initiation of each new leaf. This sug- aspects are in place. Shoot meristem formation and maintenance Lenhard and Laux 45

Figure 1

Development of the Arabidopsis shoot meristem. The shoot meristem (arrow) arises * between the outgrowing cotyledonary * primordia during embryogenesis. In the mature embryo, the shoot meristem (arrow) has initiated the first true leaf primordia (*). In seedlings, the shoot meristem forms a shallow dome and gives rise to leaves (*). The * inflorescence meristem initiates floral meristems at its flanks. The oldest floral meristem (at right) has already formed the first Late heart stage Mature embryo Seedling whorl of organ primordia, the sepals (*). Scanning electron microscopy images. Inflorescence meristem

Floral meristems

Expression analysis of the shoot meristem gene derived from genetic analysis [18] and with the proposed WUSCHEL (WUS) has indicated a considerably earlier role of the maize knotted1 (kn1) gene, a putative STM start of shoot meristem development than previously ortholog [19]. STM appears to be functional from early thought [15••]. WUS is expressed in the four inner apical stages on, since at least the expression of another meristem cells of the 16-cell embryo and through several asymmet- gene, UNUSUAL FLORAL ORGANS (UFO), requires STM ric cell divisions its expression segregates with a subset of activity as early as the late globular stage [17••]. daughter cells which become located in the center of the shoot meristem primordium (Figure 2). Mutant analysis In the heart stage embryo, when cotyledonary primordia indicated that WUS is only necessary for the development are apparent, expression of a further meristem gene, of the shoot meristem, but not for those cell lineages CLAVATA1 (CLV1), is initiated within the embryo apex derived from early WUS-expressing cells that contribute independently of STM activity (Figure 2 [17••]). Mutations to the cotyledons [16]. The function of WUS at very early in CLV1 result in a progressive enlargement of the meris- embryo stages, before a shoot meristem is evident, is tem during postembryonic development (see below). The unclear. One possibility is that WUS functions to preserve late onset of its expression in the embryo suggests, howev- a pluripotent state in the cells required later in the emerg- er, that CLV1 may not play a prominent role in very early ing shoot meristem. stages of shoot meristem development. By contrast, the PRIMORDIA TIMING (PT) gene affects meristem size at At the late globular stage, when the embryo consists of these stages. pt-mutations cause a progressive enlargement about 100 cells, expression of the STM gene is initiated of the shoot meristem region from the globular embryo (Figure 2, [17••]), and this step is independent of WUS stage onward, but this defect regresses during later plant activity at earlier embryo stages [15••]. STM is expressed in development [20•]. Consistent with the temporal differ- a central region of the embryo apex that may correspond to ence in the manifestation of the respective phenotypes, the apical cells per se as well as in cells separating the double mutant analysis suggests that PT and CLV1 func- cotyledon primordia [17••]. In its absence these cells dif- tion in two independent processes. ferentiate and fused organs are formed, suggesting that STM may keep these cells from participating in organ for- In zll mutants [21] (allelic to pinhead [22]), the cells in mation. This interpretation is consistent with models the shoot meristem primordium do not maintain STM 46 Growth and development

Figure 2

Shoot meristem formation during Arabidopsis embryogenesis. The first indication of shoot Shoot meristem Shoot meristem development is the onset of Cotyledon WUS expression at the 16-cell stage, long before a shoot meristem is evident. Protoderm Subsequently, expression of STM and CLV1 Apical Hypocotyl is initiated. Initiation of STM expression is Basal independent of WUS activity and onset of CLV1 expression is independent of STM. The Root ZLL gene is necessary to maintain shoot meristem development at later embryo stages. Bars represent stages at which Zygote One-cell8-cell 16-cell Globular Heart Seedling mRNA is detected (WUS, STM, CLV1) or phenotypic defects are observed (ZLL). WUS Shaded regions in embryos represent approximate expression domains. STM CLV1 ZLL Current Opinion in Plant Biology

expression and differentiate, indicating that ZLL is as Newman elegantly put it [26], raising the question of required to maintain the meristematic cell status in the how the identity of these cells is specified. In wus mutants, apex of the embryo [23••]. ZLL codes for a member of a stem cells appear to be mis-specified and to have differen- novel gene family, including ARGONAUTE1, a gene tiated [16]. In contrast to stm mutations, however, cells in involved in leaf development [24••], and sequences wus apices are not recruited into organs, suggesting that derived from genomic sequencing projects, e.g. in WUS positively regulates cell fate rather than preventing humans and C. elegans. The recently cloned rabbit trans- organ formation. WUS was cloned and shown to encode a lation initiation factor eIF2C turned out to be another putative homeodomain protein of a novel subtype [15••]. member of this family [25], suggesting that ZLL and WUS is expressed in a small group of cells in the meristem AGO1 could be implicated in translational control. As center underneath the presumed position of the stem cells. mutations in ZLL result in specific defects, the gene A conceiveable model derived from these data is that could be involved in tissue- and/or stage-specific transla- WUS-expressing cells act as an organizing center confer- tional control. ZLL is expressed in the vascular precursor ring stem cell fate to overlying neighbors (Figure 3). This cells underlying the shoot meristem primordium from model implies similarities in the organization of shoot and earliest embryo stages on and in the embryo apex at later root meristems, since in the root meristem the stem cells stages [23••]. In which cells its expression is needed for also appear to be maintained by signaling from a central meristem development remains to be determined. organizing cell group, the quiescent center [27••] Interestingly, the requirement for ZLL in primary shoot (Figure 4). Similarities between shoot and root meristem meristem development is only transient. Once the first regulation have also been concluded from the study of the true leaf primordia are present, the shoot meristem Defective embryo and meristems (Dem) gene of tomato which appears to be able to self-maintain independently of ZLL affects cell divisions in both meristems [28•]. Because activity [23••]. organ primordia are also affected in the dem mutant, however, the significance of Dem for meristem development Thus, from these studies it follows that shoot meristem still needs to be determined. formation is a prolonged, dynamic process which begins during early embryo pattern formation. Also, whereas some Once the progeny of the stem cells have left the center of aspects of the formation of cotyledons are strikingly simi- the shoot meristem, they are recruited into organogenesis lar to that of leaves, important differences should not be and eventually differentiate. In Arabidopsis, this process is overseen, such as that not all molecular mechanisms of the promoted by the CLV and MGOUN (MGO) genes. postembryonic shoot meristem are in place when the cotyledons are initiated. CLV1, encoding a putative receptor kinase, and CLV3, which interacts genetically with CLV1, are likely to be com- The shoot meristem in ponents of a common signaling pathway, with mutations in postembryonic development either gene causing a progressive increase in meristem size Clonal analysis indicated that stem cells of the shoot meris- [29,30]. Previously two mutually not exclusive models for tem are not permanent but are instructed by positional their role had been proposed: CLV signaling could pro- information as ‘temporary occupants of a permanent office’ mote the entry of cells into organogenesis and/or Shoot meristem formation and maintenance Lenhard and Laux 47

negatively regulate the proliferation of meristem cells Figure 3 [29,31]. The latter alternative has recently been refuted by Laufs et al. These authors find that in clv3 meristems the STM/kN1 size of the central region of low mitotic activity is increased and the cells in this region divide even less frequently than in wild type [32••]. Thus, it appears likely that the CLV CLV MGO pathway primarily enhances the rate of a differentiation sc (PHAN) step during organ formation (Figure 3). p

To gain first insight into how CLV signaling is processed CZ PZ within the cell, two laboratories examined the biochemi- p WUS cal properties of CLV1 and its interaction with the kinase-associated protein phosphatase KAPP [33••,34••]. PZ The CLV1 intracellular domain can autophosphorylate RZ and appears to oligomerize with and transphosphorylate other CLV1 molecules. KAPP was found to be able to dephosphorylate CLV1 in vitro and the results of trans- Current Opinion in Plant Biology genic studies point at a role of KAPP as a negative regulator of CLV1 signaling in planta. As KAPP interacts Genes involved in the regulation of shoot meristem activity. WUS with various receptor-like kinases similar to CLV1 [35], expression in the basal part of the central zone (CZ) affects the state of the overlying stem cells. The CLV signaling pathway, including however, it may be a more general modulator of different CLV1and CLV3, promotes a differentiation step reflected by the receptor kinase pathways. transition of cells from the CZ to the PZ. This step is counteracted by STM in Arabidopsis, and the maize kn1 possibly plays a similar role. Mutations in the CLV2 gene result in an increase of shoot The MGO genes promote formation of organ primordia (p) from cells of the PZ. Expression of the PHAN gene in leaf primordia is required meristem height and an extra whorl of organs in flowers, for maintenance of shoot meristem activity. The figure combines data similar to weak clv1 and clv3 alleles [36•]. Organ devel- from Arabidopsis (WUS, STM, CLV, MGO), Antirrhinum (PHAN) and opment is also affected in clv2, however; for example the maize (kn). See text for details. sc, stem cells; RZ, rib zone. pedicel length of flowers is increased compared to wild type. clv1 and clv3 are epistatic to clv2 with respect to floral organ number, but additive with respect to pedicel In contrast to Arabidopsis, in some species the shoot meris- length. This suggests that CLV2 acts in the same pathway tem only forms an intrinsically limited number of as CLV1/3 to regulate meristem activity, whereas it seems structures. How is the meristem program terminated in to affect further organ development independently. The such species? One example is the maize spikelet meristem initiation of lateral organs by the shoot meristem also which gives rise to two floral meristems only. Mutations in requires the MGOUN (MGO) genes. mgo1 and mgo2 result the indeterminate spikelet1 (ids1) gene abolish spikelet in a reduction of the number of leaves and floral organs, meristem determinacy such that it gives rise to additional larger meristems and fasciation [37•]. In contrast to clv3 floral meristems, indicating a role of ids1 in meristem ter- (see above), mgo2 shoot meristems accumulate cells in mination. Chuck et al. showed that ids1 codes for a gene the PZ [32••]. As clv3 mgo double mutants are additive, related to APETALA2, which is required in Arabidopsis the genes appear to be involved in different steps of flower development [41••]. Although the precise regulato- organ formation: whereas CLV3 affects the rate of the ry mechanism for spikelet meristem determinacy is transition of cells from CZ to PZ, the MGO genes may unknown, one attractive hypothesis is that ids acts as a neg- affect the partitioning of PZ cells into organ ative regulator of those factors necessary for maintaining primordia (Figure 3). indeterminacy, such as kn1.

The maize homeobox gene kn1 appears to counteract dif- The relationship between the shoot meristem ferentiation of meristem cells and organ formation [19]. and leaves Recent overexpression studies, however, are consistent Is the shoot meristem autonomous and independent from with differences in the action of kn1 as well as in the devel- its products, or is there a mutual interaction between shoot opmental plasticity of leaf cells in monocotyledonous and meristem and organs? The latter view is supported by clas- dicotyledonous plants. Whereas overexpression of kn1 in sical studies demonstrating that continued activity of the tobacco produced ectopic meristems on leaves [38], no shoot meristem depends on hormonal supply from young such effect has been observed in studies with transgenic leaf primordia [42]. Recent studies on mutations primarily maize [39•] and barley [40•]. In barley, the only effect of affecting leaf development point into the same direction. ectopic expression of maize kn1 was the formation of addi- The phantastica (phan) mutation of Antirrhinum partly dis- tional florets on the awn of primary spikelets, suggesting rupts dorsoventrality of lateral organs, with ventral (abaxial, that kn1 might have induced inflorescence meristem fate lower leaf side) tissue present on the upper side of leaves, in cells of the normally determinate awn [40•]. and blocks the outgrowth of leaf primordia [43••]. The gene 48 Growth and development

Figure 4

Model for the maintenance of stem cells in shoot and root meristems. Analysis of the WUS gene and cell ablation studies allow for a model in which maintaining the state of stem CZ cells (lightly shaded) in shoot and root p PZ meristems requires information (white arrows) PZ from neighboring cell groups (darkly shaded), lrc lrc p the WUS expressing cells and the quiescent WUS QC center (QC), respectively. Progeny of the RZ stem cells in the surrouding regions (white areas) undergo differentiation, presumably integrating information (shaded arrows) from crc more mature tissues (for review see [2]). CZ, central zone; crc, central root cap; lrc, lateral root cap; p, leaf primordia; PZ, peripheral Current Opinion in Plant Biology zone; RZ, rib zone.

codes for a putative MYB transcription factor and is References and recommended reading expressed uniformly in young primordia of leaves and floral Papers of particular interest, published within the annual period of review, have been highlighted as: organs. When the formation of leaves is disrupted in condi- • of special interest tional phan alleles, shoot development discontinues, •• of outstanding interest indicating that leaf development is required for shoot meris- 1. Steeves TA, Sussex IM: Patterns in Plant Development. Cambridge: tem activity (Figure 3). The dominant phabulosa mutation of Cambridge University Press; 1989. Arabidopsis affects leaf development in a manner opposite to 2. Laux T, Mayer KFX: Cell fate regulation in the shoot meristem. Sem that of phantastica, causing a transformation of ventral into Cell Dev Biol 1998, 9:195-200. dorsal (adaxial, upper leaf side) leaf tissue to varying degrees 3. Barlow PW: The concept of the stem cell in the context of plant [44••]. Interestingly, in such dorsalized leaves, ectopic growth and development. In Stem Cells and Tissue Homeostasis. Edited by Lord BI, Potten CS, Cole RJ. Cambridge: Cambridge meristems are formed opposite to the leaf axils, at the lower University Press; 1978:87-113. side of the petiole base, indicating a correlation between 4. Meyerowitz EM: Genetic control of cell division patterns in dorsal leaf fate and the development of axillary shoot meris- developing plants. Cell 1997, 88:299-308. tems and as the authors put it, “a cyclical model for shoot 5. Clark SE: Organ formation at the vegetative shoot meristem. development: the shoot meristem makes leaves which in Plant Cell 1997, 9:1067-1076. turn are responsible for generating new shoot meristems”. 6. Rinne PL, van der Schoot C: Symplasmic fields in the tunica of the •• shoot apical meristem coordinate morphogenetic events. Development 1998, 125:1477-1485. Conclusions This paper suggests the existence of symplasmic isolation between a cen- The literature reviewed provides new insights into the tral and peripheral zone in the epidermis of the birch shoot meristem, which is only broken briefly with the initiation of each new leaf. The far-reaching mechanisms underlying the formation and maintenance of potential of such ‘communication compartments’ for the regulated the shoot meristem. Several interesting new mutants have exchange of potential morphogens and for the segregation of cell fates in the apex is discussed. been identified and several genes have been isolated that will considerably add to our understanding of these process- 7. Tilney-Basset RAE: Plant Chimeras. Baltimore: Edward Arnold; 1986. es. Nevertheless, despite extensive screens by several 8. Irish VF, Sussex IM: A fate map of the Arabidopsis embryonic shoot apical meristem. Development 1992, 115:745-753. laboratories, the number of regulators specific for shoot 9. Furner IJ, Pumfrey JE: Cell fate in the shoot apical meristem of meristem development that have been identified appears Arabidopsis thaliana. Development 1992, 115:755-764. small. Finding further novel components may require spe- 10. Buvat R: Structure, évolution et functionnement du méristème cific screening strategies, as exemplified by the isolation of apical de quelques dicotylédones. Annu Sci Nat Bot 1952, suppressors of the clv1 mutation [45]. With several impor- 13:199-300. [Title translation: Structure, evolution and function of apical meristems of some dicotyledons.] tant genes cloned which are involved in the regulation of 11. Irish EE: Additional vegetative growth in maize reflects expansion meristem cell fate and organ formation, the important ques- •• of fates in preexisting tissue, not additional divisions by apical tions can now be addressed of what is their cellular function, initials. Dev Biol 1998, 197:198-204. This paper reports a clonal analysis of extra vegetative leaves formed from which genes and processes are their targets and how are determinate maize meristems after explantation and in vitro culture. As these their functions integrated in an active shoot meristem. leaves are found not to be related to the apical cells which form the generative structures of the tassel, the author suggests that in contrast to the situ- ation in many other plant species the reproductive organs are formed by Acknowledgements cells in the maize apex that were set aside early in development. We gratefully acknowledge support from grants by the Deutsche Forschungsgemeinschaft to Thomas Laux and by a stipend from the 12. Laux T, Jürgens G: Embryogenesis: A new start in life. Plant Cell 1997, 9:989-1000. Boehringer Ingelheim Fond to Michael Lenhard. We thank the member of the Laux laboratory for helpful comments on the manuscript. We apologize to 13. Kaplan DR, Cooke TJ: Fundamental concepts in the those colleagues working in the field whose work was not mentioned due to embryogenesis of dicotyledons: a morphological interpretation of space constraints. embryo mutants. Plant Cell 1997, 9:1903-1919. Shoot meristem formation and maintenance Lenhard and Laux 49

14. Long JA, Moan EI, Medford JI, Barton MK: A member of the 28. Keddie JS, Carroll BJ, Thomas CM, Reyes ME, Klimyuk V, Holtan H, KNOTTED class of homeodomain proteins encoded by the STM • Gruissem W, Jones JD: Transposon tagging of the Defective gene of Arabidopsis. Nature 1996, 379:66-69. embryo and meristems gene of tomato. Plant Cell 1998, 10:877-888. 15. Mayer KFX, Schoof H, Haecker A, Lenhard M, Jürgens G, Laux T: The phenotype of the tomato dem mutant is described which exhibits a •• Role of WUSCHEL in regulating stem cell fate in the Arabidopsis severly disturbed organization and development of both shoot and root shoot meristem. Cell 1998, in press. meristems. Dem, encoding a novel protein, is not specific for meristems, The WUSCHEL gene, required for the specification of stem cells in the but is expressed in all tissues with organized cell division. However, the Arabidopsis shoot meristem, codes for a novel homeodomain protein. Its mRNA is notably absent from callus, suggesting a role in the coordination expression pattern suggests that stem cells in the shoot meristem are spec- of cell division. ified by an underlying cell group which is established as early as in the 16- cell embryo and becomes localized to its prospective domain of function by 29. Clark SE, Williams RW, Meyerowitz EM: The CLAVATA1 gene asymmetric cell divisions. These findings suggest profound similarities encodes a putative receptor-kinase that controls shoot and floral between different stem cell systems in animals and plants. meristem size in Arabidopsis. Cell 1997, 89:575-585. 16. Laux T, Mayer KFX, Berger J, Jürgens G: The WUSCHEL gene is 30. Clark SE, Running MP, Meyerowitz EM: CLAVATA3 is a specific required for shoot and floral meristem integrity in Arabidopsis. regulator of shoot and floral meristem development affecting the Development 1996, 122:87-96. same processes as CLAVATA1. Development 1995, 121:2057-2067. 17. 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MERISTEMLESS gene is required for maintenance of An excellent description of the morphology and distribution of mitoses in the undifferentiated cells in Arabidopsis shoot and floral meristems Arabidopsis shoot apex based on confocal laser scanning microscopy is and acts at a different regulatory level than the meristem genes reported. The results obtained for the wild type are then used as a framework WUSCHEL and ZWILLE. Plant J 1996, 10:967-979. for analyzing the defects in clv3 and mgo2 apices in greater detail, allowing important novel conclusions about the functions of these genes. 19. Vollbrecht E, Veit B, Sinha N, Hake S: The developmental gene Knotted-1 is a member of a maize homeobox gene family. Nature 33. Stone JM, Trotochaud AE, Walker JC, Clark SE: Control of meristem •• 1991, 350:241-243. development by CLAVATA1 receptor kinase and kinase-associated protein phosphatase interactions. Plant Physiol 20. Mordhorst AP, Voerman KJ, Hartog MV, Meijer EA, van Went J, 1998, 117:1217-1225. • Koornneef M, de Vries SC: Somatic embryogenesis in Arabidopsis Biochemical analysis demonstrates protein kinase activity for CLV1 and thaliana is facilitated by mutations in genes repressing the physical interaction of phosphorylated CLV1 with KAPP. KAPP proba- meristematic cell divisions. Genetics 1998, 149:549-563. bly functions as a negative regulator of CLV signaling, since reducing A detailed study of the effects of the pt, clv1 and clv3 mutations on KAPP mRNA can rescue the clv1 phenotype in a dose-dependent manner. somatic embryogenesis is reported, as well as an analysis of the genetic These findings provide important insights into the intracellular processing interactions between these genes, indicating a temporal succession in of CLV1 signals. their functions. 34. Williams RW, Wilson JM, Meyerowitz EM: A possible role for kinase 21. 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Zou C, Zhang Z, Wu S, Osterman JC: Molecular cloning and into organ primordia. characterization of a rabbit eIF2C protein. Gene 1998, 211:187-194. 38. Sinha NR, Williams RE, Hake S: Overexpression of the maize homeobox gene, KNOTTED-1, causes a switch from determinate 26. Newman IV: Patterns in the meristems of vascular plants. III. to indeterminate cell fates. Genes Dev. 1993, 7:787-795. Pursuing the patterns where no cell is a permanent cell. J Linn Soc Bot 1965, 59:185-214. 39. Zhang S, Williams-Carrier R, Jackson D, Lemaux PG: Expression of • CDC2Zm and KNOTTED1 during in vitro axillary shoot meristem 27. van den Berg C, Willemsen V, Hendriks G, Weisbeek P, Scheres B: proliferation and adventitious shoot meristem formation in maize •• Short-range control of cell differentiation in the Arabidopsis root (Zea mays L.) and barley (Hordeum vulgare L.). Planta 1998, meristem. Nature 1997, 390:287-289. 204:542-549. This important paper describes the effects of cell ablation in the quiescent Examining the expression patterns of CDC2Zm and knotted1 during the for- center of the root meristem on the neighboring stem cells. The results indi- mation of adventitious shoot meristems in vitro, the authors find no difference cate that short range signaling from the quiescent center to its immediate to the expression patterns observed in planta. kn1 overexpressing maize neighbors prevents their differentiation. plants show no signs of ectopic meristem formation. 50 Growth and development

40. Williams-Carrier RE, Lie YS, Hake S, Lemaux PG: Ectopic 43. Waites R, Selvadurai HR, Oliver IR, Hudson A: The PHANTASTICA • expression of the maize kn1 gene phenocopies the Hooded •• gene encodes a MYB transcription factor involved in growth and mutant of barley. Development 1997, 124:3737-3745. dorsoventrality of lateral organs in Antirrhinum. Cell 1998, Constitutive expression of maize kn1 in barley results in ectopic flowers, a 93:779-789. defect similar to that of the Hooded mutation of barley. Notably, no ectopic The PHAN gene, required for leaf dorsoventrality, was cloned and shown to shoot meristem formation is observed, as it has been reported in overex- encode a putative MYB transcription factor. Its expression pattern and the pression studies in tobacco. mutant phenotype suggest a role in the specification of leaf identity. PHAN is necessary for sustained meristem activity in a non-cell-autonomous manner, reinforcing the observation that shoot meristem activity is affected by 41. Chuck G, Meeley RB, Hake S: The control of maize spikelet signaling from leaves. •• meristem fate by the APETALA2-like gene indeterminate spikelet1. Genes Dev 1998, 12:1145-1154. 44. McConnell JR, Barton MK: Leaf polarity and meristem formation in This paper describes the cloning and analysis of the indeterminate spikelet1 •• Arabidopsis. Development 1998, 125:2935-2942. gene, that is required for determinacy of the first two branches (spikelets) of The dominant phb-d mutation which disrupts dorsoventrality of lateral organs the maize inflorescence. The possible implications of ids1 for inflorescence leads to the ectopic formation of axillary meristems on the lower side of leaf architecture in other grass species are discussed. petioles. This interesting effect suggests that dorsal (adaxial) leaf fate promotes the formation of axillary shoot meristems. 42. Shabde M, Murashige T: Hormonal requirements of excised 45. Pogany JA, Simon EJ, B KR, De Guzman BM, Yu LP, Trotchaud AE, Dianthus caryophyllus L. shoot apical meristem in vitro. Am J Clark SE: Identifying novel regulators of shoot meristem Botany 1977, 64:443-448. development. J Plant Res 1998, 111:307-313.