Top PDF Morphogenesis of the Arabidopsis Shoot Apical Meristem

Morphogenesis of the Arabidopsis Shoot Apical Meristem

Morphogenesis of the Arabidopsis Shoot Apical Meristem

Previous work in this field has shown that rules must exist to control the positioning of new cell walls in the meristem in order to produce the patterns (size, shape, and connectedness) that are observed[43, 56]. Here we determined that the rules are not simple, and in fact the rules seem to be different depending on the location in the meristem. This might be a result of flower founder cells changing their “preferred” division polarity prior to the rapid growth characteristic of flower primordia. This is similar to the observation in vegetative meristems of pea plants where leaf founder cells often divide periclinally (parallel to the surface of the meristem) about one half plastochron before primordial growth where non-founder cells would usually divide anticlinally (perpendicular to the surface)[58, 59]. This is different than the phenomenon observed in Arabidopsis inflorescence meristems where periclinal divisions are not observed, but similar in that the divisions which don’t abide by the modern interpretation of Errera’s rule or our potential minimization rule are found preferentially near the edge, possibly in regions of flower founder cells. One explanation for the deviation from the rules near the perimeter of the meristem is that it helps make the elongated cells commonly observed in the boundary region of new primordia.
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Arabidopsis HD Zip II transcription factors control apical embryo development and meristem function

Arabidopsis HD Zip II transcription factors control apical embryo development and meristem function

The Arabidopsis genome encodes ten Homeodomain-Leucine zipper (HD-Zip) II proteins. ARABIDOPSIS THALIANA HOMEOBOX 2 ( ATHB2 ), HOMEOBOX ARABIDOPSIS THALIANA 1 ( HAT1 ), HAT2 , HAT3 and ATHB4 are regulated by changes in the red/far red light ratio that induce shade avoidance in most of the angiosperms. Here, we show that progressive loss of HAT3, ATHB4 and ATHB2 activity causes developmental defects from embryogenesis onwards in white light. Cotyledon development and number are altered in hat3 athb4 embryos, and these defects correlate with changes in auxin distribution and response. athb2 gain-of-function mutation and ATHB2 expression driven by its promoter in hat3 athb4 result in significant attenuation of phenotypes, thus demonstrating that ATHB2 is functionally redundant to HAT3 and ATHB4. In analogy to loss-of-function mutations in HD-Zip III genes, loss of HAT3 and ATHB4 results in organ polarity defects, whereas triple hat3 athb4 athb2 mutants develop one or two radialized cotyledons and lack an active shoot apical meristem (SAM). Consistent with overlapping expression pattern of HD-Zip II and HD-Zip III gene family members, bilateral symmetry and SAM defects are enhanced when hat3 athb4 is combined with mutations in PHABULOSA ( PHB ), PHAVOLUTA ( PHV ) or REVOLUTA ( REV ). Finally, we show that ATHB2 is part of a complex regulatory circuit directly involving both HD-Zip II and HD-Zip III proteins. Taken together, our study provides evidence that a genetic system consisting of HD-Zip II and HD- Zip III genes cooperates in establishing bilateral symmetry and patterning along the adaxial-abaxial axis in the embryo as well as in controlling SAM activity.
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Real time lineage analysis reveals oriented cell divisions associated
with morphogenesis at the shoot apex of Arabidopsis thaliana

Real time lineage analysis reveals oriented cell divisions associated with morphogenesis at the shoot apex of Arabidopsis thaliana

measured; in this case the region is marked as P-4 and it is located between P-1 and P-2 (Fig. 6A). The cell division analysis in these cells revealed a contrasting pattern in comparison with the cells that form the next set of primordia. The cell division patterns of P-4 cells that occurred over a period of 66 hours are projected onto the final time point in Fig. 9A,B. Cells in the intervening regions divided in random orientation and the resultant lineages appeared as square blocks rather than as columns of cells. For simplicity in comparison, the lineages in the rest of the meristem are not represented, although similar differential patterns of cell division did occur there. However, not all of the lineages that follow oriented cell divisions ultimately became part of primordia (for example, the lineage marked in blue in Fig. 7L, which was retained in the SAM proximal to the primordium, and only a part of the lineage marked in yellow became part of the primordium). This observation suggests that the axis of growth does not entirely constrain the fate of these lineages. Seven plastochrons from two different plants were examined and all of them revealed similar cellular behavior. Similar analysis was carried out on cells in the L2 layer, and the resultant lineages were projected onto the final time point. Since it was not possible to represent all the cells located at different depths in the L2 layer in one section, the cells were represented in individual optical sections at different depths (Fig. 8B-D). This analysis revealed that cells preferentially divided parallel to the axis of primordium growth. Even though individual cell divisions in the corpus could be followed, it was not possible to serially reconstruct complete lineages due to a lack of resolution in the distal-most regions, owing to the curvature of the SAM. However, cells located in the corpus region of primordia in P3 stages could be mapped; this corresponds to the time the flower primordium begins to acquire height. During these stages, periclinal divisions could be observed
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MAX1and MAX2 control shoot lateral branching in Arabidopsis

MAX1and MAX2 control shoot lateral branching in Arabidopsis

First order branching is enhanced at both vegetative and reproductive stages in max1 and max2 mutants The timing and extent of axillary shoot growth depends on node position along the shoot axis, often resulting in a characteristic apical-basal pattern. Arabidopsis wild type shows two distinct patterns of lateral shoot development, which depend on the developmental stage (Hempel and Feldman, 1994; Grbic´ and Bleecker, 1996; Stirnberg et al., 1999). During the vegetative phase, axillary shoot meristems initiate in the axils of leaf primordia at some distance from the primary shoot apical meristem, and axillary shoot development progresses in parallel with development of the subtending leaf. This results in an acropetal progression of vegetative axillary shoot development. The second pattern, characteristic for the reproductive phase, is a basipetal progression of outgrowth of lateral inflorescences, which originate from axillary shoot meristems that arise even in the axils of the youngest leaf primordia in close proximity to the primary shoot apical meristem. In order to study the effect of max1-1 and max2-1 on these patterns of lateral shoot development, we determined the phyllotactic sequence of wild-type and mutant shoots, dissected the leaves with their associated axillary shoots from the shoot axis and recorded axillary shoot growth at consecutive node positions. Arabidopsis axillary shoots are connected to their subtending leaves as they originate from cells at the leaf base (Stirnberg et al., 1999; Long and Barton, 2000).
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CLAVATA WUSCHEL signaling in the shoot meristem

CLAVATA WUSCHEL signaling in the shoot meristem

The generation of a signal: CLV3 and related peptides CLV3 is a founding member of the CLAVATA3/EMBRYO SURROUNDING REGION (ESR) CLE peptide family, the members of which can be identified by sequence similarity to CLV3 and the maize ESR gene products, which are expressed in the developing endosperm surrounding the embryo (Clark et al., 1995; Opsahl-Ferstad et al., 1997). In Arabidopsis, there are 24 expressed CLE family members, which share a conserved 14 amino acid sequence motif termed the CLE-box and have been implicated in stem cell maintenance in the SAM, the root apical meristem (RAM) and the vascular cambium (Casamitjana-Martínez et al., 2003; Cock and McCormick, 2001; Fletcher et al., 1999; Ito et al., 2006; Stahl et al., 2009). CLV3 is expressed as a pre-pro peptide only in stem cells of the SAM, and the processed peptide is secreted (Fletcher et al., 1999; Rojo et al., 2002). In the underlying cells of the OC, CLV3 peptide is perceived by at least four different receptor-like proteins to repress WUS activity (Brand et al., 2000; Fiers et al., 2005; Hobe et al., 2003; Müller et al., 2008; Schoof et al., 2000). Accordingly, repression of WUS by CLV3 results in fewer stem cells being maintained, and, ultimately, in a reduction in CLV3 production (Brand et al., 2000; Schoof et al., 2000). This feedback loop enables the stem cell compartment and the OC domain to maintain their size, by adjusting relative to each other, and it was found that this system can robustly buffer SAM size when CLV3 levels are varied up to tenfold (Müller et al., 2006). For example, a surge in CLV3 signal activity would result in rapid downregulation of WUS, followed by a loss of responsiveness of the system to ongoing CLV3 signaling during a refractory period (Müller et al., 2006). What causes this loss of responsiveness is not known, but it could be due to depletion of receptors from the plasma membrane or the temporary modification of downstream signaling, such as hyperphosphorylation (Nimchuk et al., 2011b).
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TOPLESS mediates brassinosteroid control of shoot boundaries and root meristem development in Arabidopsis thaliana

TOPLESS mediates brassinosteroid control of shoot boundaries and root meristem development in Arabidopsis thaliana

The transcription factor BRI1-EMS-SUPRESSOR 1 (BES1) is a master regulator of brassinosteroid (BR)-regulated gene expression. BES1 together with BRASSINAZOLE-RESISTANT 1 (BZR1) drive activated or repressed expression of several genes, and have a prominent role in negative regulation of BR synthesis. Here, we report that BES1 interaction with TOPLESS (TPL), via its ERF-associated amphiphilic repression (EAR) motif, is essential for BES1-mediated control of organ boundary formation in the shoot apical meristem and the regulation of quiescent center (QC) cell division in roots. We show that TPL binds via BES1 to the promoters of the CUC3 and BRAVO targets and suppresses their expression. Ectopic expression of TPL leads to similar organ boundary defects and alterations in QC cell division rate to the bes1-d mutation, while bes1-d defects are suppressed by the dominant interfering protein encoded by tpl-1, with these effects respectively correlating with changes in CUC3 and BRAVO expression. Together, our data unveil a pivotal role of the co- repressor TPL in the shoot and root meristems, which relies on its interaction with BES1 and regulation of BES1 target gene expression.
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Coordination of meristem and boundary functions by transcription factors in the SHOOT MERISTEMLESS regulatory network

Coordination of meristem and boundary functions by transcription factors in the SHOOT MERISTEMLESS regulatory network

The Arabidopsis homeodomain transcription factor SHOOT MERISTEMLESS (STM) is crucial for shoot apical meristem (SAM) function, yet the components and structure of the STM gene regulatory network (GRN) are largely unknown. Here, we show that transcriptional regulators are overrepresented among STM-regulated genes and, using these as GRN components in Bayesian network analysis, we infer STM GRN associations and reveal regulatory relationships between STM and factors involved in multiple aspects of SAM function. These include hormone regulation, TCP-mediated control of cell differentiation, AIL/PLT-mediated regulation of pluripotency and phyllotaxis, and specification of meristem-organ boundary zones via CUC1. We demonstrate a direct positive transcriptional feedback loop between STM and CUC1, despite their distinct expression patterns in the meristem and organ boundary, respectively. Our further finding that STM activates expression of the CUC1-targeting microRNA miR164c combined with mathematical modelling provides a potential solution for this apparent contradiction, demonstrating that these proposed regulatory interactions coupled with STM mobility could be sufficient to provide a mechanism for CUC1 localisation at the meristem-organ boundary. Our findings highlight the central role for the STM GRN in coordinating SAM functions.
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A high resolution gene expression map of the Arabidopsis shoot meristem stem cell niche

A high resolution gene expression map of the Arabidopsis shoot meristem stem cell niche

A spatiotemporal orchestration of gene expression and cell behaviors is crucial for cell fate specification (Baurle and Laux, 2003; Reddy, 2008). The stem cells in shoot apical meristems (SAMs) of higher plants differentiate into various above-ground organs (Steeves and Sussex, 1989). Traditionally, SAMs have been divided into distinct domains on the basis of their function and cytological criteria (Lyndon, 1998; Steeves and Sussex, 1989; Xie et al., 2009). The central zone (CZ) harbors a set of pluripotent stem cells (Reddy and Meyerowitz, 2005). The stem cell progeny differentiate into leaves or flowers within the adjacent peripheral zone (PZ). Upon specification of leaf or flower primordia, specific cellular behaviors and differentiation events lead to the formation of boundary regions that separate developing organs from SAMs, whereas cells in the rib meristem (RM), situated just below the CZ, differentiate to give rise to the stem. Collectively, stem cell daughters progress through
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The Arabidopsis OBERON1 and OBERON2 genes encode plant
homeodomain finger proteins and are required for apical meristem
maintenance

The Arabidopsis OBERON1 and OBERON2 genes encode plant homeodomain finger proteins and are required for apical meristem maintenance

Maintenance of the stem cell population located at the apical meristems is essential for repetitive organ initiation during the development of higher plants. Here, we have characterized the roles of OBERON1 ( OBE1 ) and its paralog OBERON2 ( OBE2 ), which encode plant homeodomain finger proteins, in the maintenance and/or establishment of the meristems in Arabidopsis . Although the obe1 and obe2 single mutants were indistinguishable from wild-type plants, the obe1 obe2 double mutant displayed premature termination of the shoot meristem, suggesting that OBE1 and OBE2 function redundantly. Further analyses revealed that OBE1 and OBE2 allow the plant cells to acquire meristematic activity via the WUSCHEL - CLAVATA pathway, which is required for the maintenance of the stem cell population, and they function parallel to the SHOOT MERISTEMLESS gene, which is required for preventing cell differentiation in the shoot meristem. In addition, obe1 obe2 mutants failed to establish the root apical meristem, lacking both the initial cells and the quiescent center. In situ hybridization revealed that expression of PLETHORA and SCARECROW , which are required for stem cell specification and maintenance in the root meristem, was lost from obe1 obe2 mutant embryos. Taken together, these data suggest that the OBE1 and OBE2 genes are functionally redundant and crucial for the maintenance and/or establishment of both the shoot and root meristems.
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ERECTA family genes coordinate stem cell functions between the epidermal and internal layers of the shoot apical meristem

ERECTA family genes coordinate stem cell functions between the epidermal and internal layers of the shoot apical meristem

ER-family loss of function renders the SAM insensitive to CLV3 peptide treatment in a tissue layer-specific manner (Fig. 5). However, it is unlikely that ER-family proteins directly perceive the CLV3 signal, as direct binding of the CLV3 peptide to its corresponding receptor CLV1 has been unambiguously demonstrated (Ogawa et al., 2008). Several EPIDERMAL PATTERNING FACTOR-LIKE (EPFL) secreted peptides have been identified as ligands for ER-family proteins in stomatal patterning, inflorescence morphogenesis and leaf serration (Abrash et al., 2011; Lee et al., 2015, 2012; Tameshige et al., 2016; Uchida et al., 2012). Therefore, EPFL-family members may act as a yet- uncharacterized signal upstream of the ER family in stem cell maintenance. Accordingly, we found that EPFL1 and EPFL2 are expressed in the shoot apex (Fig. S10), although it remains unclear whether these genes act in regulating SAM functions. In this study, we show that the localized ER expression in the epidermis is sufficient to rescue the misexpression of stem cell marker CLV3 in the epidermis of ER-family mutants, whereas ER is expressed throughout the shoot apices (Uchida et al., 2013; Fig. S4A). Given that the shoot apex is a complex tissue composed of multiple domains, such as OC, the boundary region, the rib zone and the initiating primordia of lateral organs, it will be important in future research to delineate the individual functions of ER-family proteins in each domain and to elucidate whether the SAM-expressed EPFLs
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STIMPY mediates cytokinin signaling during shoot meristem establishment in Arabidopsis seedlings

STIMPY mediates cytokinin signaling during shoot meristem establishment in Arabidopsis seedlings

As sucrose can restore stip mutant seedlings to normal growth (Wu et al., 2005), but fails to completely rescue the ahk2-2 ahk3-3 cre1- 12 mutants (Higuchi et al., 2004; Nishimura et al., 2004), we hypothesized that STIP acts downstream of cytokinin perception during seedling development, and that part of the cytokinin triple- receptor mutant phenotype might be due to the lack of STIP activity. To test this model, we examined STIP expression in two- day-old wild-type and ahk2-2 ahk3-3 cre1-12 seedlings using in situ hybridization. In wild-type seedlings, STIP RNA was detected in the shoot apical meristem (SAM), the emerging leaf (Fig. 1F) and the upper root meristematic zone (Fig. 1D). By contrast, although the cytokinin receptor mutants are morphologically similar to the wild type at this stage, STIP expression was much reduced in their shoot apex (Fig. 1G) and there was very little detectable STIP expression in the roots (Fig. 1E). We also found similar reductions in STIP mRNA levels in the roots of the type-B arr1-3 arr10-5 arr12-1 triple-mutant seedlings (Fig. 1H), which are nearly completely insensitive to cytokinin stimulation in the roots (Argyros et al., 2008; Yokoyama et al., 2007). Although we cannot rule out the possibility that the observed reductions in STIP expression are caused by morphological changes due to the lack of cytokinin signaling, these findings are consistent with our hypothesis that STIP acts downstream of the cytokinin signaling pathways.
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Control of embryonic meristem initiation in Arabidopsis by PHD finger protein complexes

Control of embryonic meristem initiation in Arabidopsis by PHD finger protein complexes

embryos of obe1 obe2 tta2 triple mutants from an obe1/+ obe2 tta2 mother plant. Among the progeny of such plants, 20% of embryos exhibited embryonic lethality (Table 3). obe1 obe2 tta2 did not show novel phenotypes in the basal region where the formation of the embryonic root meristem is already disrupted in obe1 obe2 and tta1 tta2 embryos. However, development of the apical region where cotyledon primordia and shoot apical meristem are produced was disturbed (Fig. 4A-C,E-G). During transition from triangular to heart stage in wild-type siblings in the same silique, cotyledon primordia had correctly emerged (Fig. 4A,B); however, emergence of cotyledon primordia was not observed in obe1 obe2 tta2 embryos (Fig. 4E,F). In addition, the apical region of obe1 obe2 tta2 embryos was abnormally expanded compared with wild-type siblings (Fig. 4B,F). obe1 obe2 tta2 triple mutant embryos arrested at the triangular stage (Fig. 4C,G). We further investigated other triple mutant combinations and found that all of them showed same phenotypes (data not shown). Finally, we investigated the phenotypes of obe1 obe2 tta1 tta2 quadruple mutants. We found that ~5% of embryos from obe1/+ obe2 tta1/+ tta2 mother plants were swollen when wild-type siblings were at the bent-cotyledon stage (Fig. 4D,H; Table 3). These data indicate that although the OBE1/2 and TTA1/2 pairs are not redundant in root formation, all four proteins function redundantly in development of the apical pole, as well as in progression beyond the triangular stage of embryogenesis.
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ULTRAPETALA1 encodes a SAND domain putative transcriptional
regulator that controls shoot and floral meristem activity in
Arabidopsis

ULTRAPETALA1 encodes a SAND domain putative transcriptional regulator that controls shoot and floral meristem activity in Arabidopsis

The pattern of ULT2 expression in inflorescence meristems, floral meristems and developing flowers appears to coincide perfectly with that of ULT1, yet ult2-1 T-DNA mutant plants do not display any shoot or floral phenotypes. Currently, we cannot exclude the possibility that the presence of very low levels of ULT2 protein translated from rare correctly spliced transcripts is sufficient for proper reproductive meristem activity in the ult2-1 mutant. Nonetheless, the presence of wild- type levels of ULT2 cannot compensate for the loss of ULT1 activity in reproductive meristems, whereas increasing ULT2 expression under the control of a dual 35S promoter can complement the ult1-1 mutation. This observation positions ULT2 as a functional duplicate of ULT1, and suggests that shoot and floral meristem activity may be sensitive to the dose of the ULT proteins. The necessity to fine tune the regulation of genes involved in meristem maintenance could explain the retention of both ULT factors in Arabidopsis. This implies that ULT1 and ULT2 are likely to have multiple common targets, the regulation of which is dependent on ULT dose. That the two genes have other independent targets, as well, is clear from the specific expression of ULT1 in leaf primordia and ULT2 in the embryonic root apical meristem.
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Specification of Arabidopsis floral meristem identity by
repression of flowering time genes

Specification of Arabidopsis floral meristem identity by repression of flowering time genes

We then tested if the floral phenotypes of loss of function of AP1 were partially caused by the activity of SVP, AGL24 and SOC1. The ap1-1 strong mutants exhibited at least two types of defects in floral meristem specification and perianth floral organ specification. The disturbed specification of floral meristems in ap1-1 was manifested by the phenotypes showing that flowers arising at basal positions of the ap1-1 inflorescence generated secondary flowers or inflorescences in the axils of the leaf-like first whorl organs on the elongated internodes, whereas flowers arising at median or apical positions generated fewer or no secondary flowers in the axils of first whorl organs without internode elongation (Bowman et al., 1993). The generation of secondary flowers or inflorescences in floral structures arising from individual floral meristems at basal positions of the main inflorescence was significantly reduced in the double mutants of ap1-1 agl24-1, ap1-1 svp-41 and ap1-1 soc1-2 compared with that in ap1-1 single mutants (Table 2 and Fig. 2B,D,F,H), and the phenotype of supernumerary inflorescence of ap1-1 was suppressed accordingly (Fig. 2A,C,E,G). In ap1-1 agl24-1 and ap1- 1 svp-41, most of the flower meristems developed as single flowers occasionally with secondary flowers, but without internode elongation (Fig. 2D,F). In the flowers of ap1-1 soc1-2, the number of secondary floral structures was reduced compared with that in ap1-1, but these floral structures usually developed like inflorescences with internode elongation (Table 2 and Fig. 2H). A close examination of the mean number of floral structures produced in each pedicel or peduncle of ap1-1 agl24-1, ap1-1 svp-41, and ap1-1 soc1-2 showed that loss of these genes function caused almost similar effect on reducing the ectopic floral structures in ap1-1 (Table 2). Triple mutants created by genetic crossing of the above double mutants further decreased the mean number of floral structures in each pedicel or peduncle (Table 2). These observations indicate that all these three genes partly contribute to the shoot characteristics in ap1-1 floral meristems, and that AP1 activity may be required for the regulation of expression of these genes.
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The SHOOTLESS2 and SHOOTLESS1 Genes Are Involved in Both Initiation and Maintenance of the Shoot Apical Meristem Through Regulating the Number of Indeterminate Cells

The SHOOTLESS2 and SHOOTLESS1 Genes Are Involved in Both Initiation and Maintenance of the Shoot Apical Meristem Through Regulating the Number of Indeterminate Cells

T HE shoot system is constructed throughout the dopsis, maize, and rice. As for the maintenance of the plant life cycle by the continuous activity of small Arabidopsis SAM, SHOOT MERISTEMLESS (STM) func- groups of cells called shoot apical meristems (SAMs). tions to keep central meristem cells indeterminate The SAM originates during embryogenesis and is later (Endrizzi et al. 1996; Clark et al. 1997). In strong stm responsible for generating the aboveground organs of alleles, the SAM is rarely formed, but, in a weak mutant the plant. Thus, understanding the developmental allele, abnormal plants develop, suggesting that STM is events that determine the aboveground architecture re- required for proper organization of the SAM (Barton quires detailed examination of the SAM. The SAM can and Poethig 1993; Endrizzi et al. 1996). The clavata be thought of as having two fundamental functions: self- (clv) mutations increase the number of cells in CZ, and perpetuation and the formation of lateral organs (for CLV genes seem to regulate the proliferation of CZ review see Steeves and Sussex 1989). The former is cells (Clark et al. 1993, 1995; Kayes and Clark 1998). executed by a cluster of infrequently dividing cells that Molecular and biochemical analyses have shed light on are positioned in the center of the SAM. This region intracellular events in CLV signaling. The CLV1 gene of the SAM is called the central zone (CZ). The latter encodes a serine/threonine receptor kinase (Clark et function is performed in the peripheral zone (PZ) that al. 1997), and CLV3 seems to act as its ligand (Fletcher surrounds the CZ. Alternatively, the SAM is viewed in et al. 1999; Brand et al. 2000). Further, CLV genes inter- terms of clonally distinct cell layers (L1, L2, and L3). act with WUSCHEL (WUS), a gene that is required for This stratification reflects the orientation of planes of stem cell identity, to establish a negative feedback loop cell divisions in different layers. Recent studies revealed between the stem cells and the underlying organizing that these zones and layers form separate symplasmic center (Brand et al. 2000; Laux et al. 1996; Mayer et domains (Rinne and van der Schoot 1998; Gisel et al. 1998; Schoof et al. 2000). In addition, the POLTER- al. 1999). For the SAM to function properly, the estab- GEIST gene functions downstream of the CLV genes lishment and maintenance of these zones and layers are and redundantly with WUS (Pogany et al. 1998; Yu et
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TRICOT encodes an AMP1 related carboxypeptidase that regulates root nodule development and shoot apical meristem maintenance in Lotus japonicus

TRICOT encodes an AMP1 related carboxypeptidase that regulates root nodule development and shoot apical meristem maintenance in Lotus japonicus

ROOT FORMATION 1 (HAR1) gene causes a hypernodulation phenotype (Wopereis et al., 2000; Krusell et al., 2002; Nishimura et al., 2002). Grafting experiments showed that this abnormal phenotype results from malfunction of the gene in the shoot. HAR1 encodes a putative leucine-rich repeat (LRR) receptor-like kinase and phylogenetic analysis indicates that the HAR1 protein belongs to a clade including Arabidopsis CLV1 and rice FLORAL ORGAN NUMBER 1, which is required for the maintenance of floral meristem (Suzaki et al., 2004; Oka-Kira and Kawaguchi, 2006). Recently, the AON-related KLAVIER (KLV) gene was shown to function in the shoot and to encode another type of LRR receptor- like kinase that has similarity to Arabidopsis RPK2 (Miyazawa et al., 2010). In addition, mutation or knockdown of the CLV2-like gene in L. japonicus and in pea causes a hypernodulation phenotype (Krusell et al., 2011). Overall, the findings from the various studies described above indicate that AON in legumes is controlled by a series of genes that are orthologous to genes with essential roles for the regulation of the SAM in other plant species. Additionally, the findings suggest that there might be a common genetic mechanism in the regulation of nodulation and SAM formation.
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The ULTRAPETALA1 Gene Functions Early in Arabidopsis Development to Restrict Shoot Apical Meristem Activity and Acts Through WUSCHEL to Regulate Floral Meristem Determinacy

The ULTRAPETALA1 Gene Functions Early in Arabidopsis Development to Restrict Shoot Apical Meristem Activity and Acts Through WUSCHEL to Regulate Floral Meristem Determinacy

Figure 3.—Inflorescence meristem and flower pheno- types of ult1 stm and ult1 wus double mutants. (A) A wild- type Ler inflorescence meri- stem. (B) An ult1-1 mutant in- florescence meristem, which generates flowers containing additional organs of all types, predominantly sepals and pet- als. (C) ult1-1 stm-11 plants pro- duce inflorescence meristems that generate a limited number of abnormal flowers. (D) Flow- ers produced by an ult1-1 stm- 11 inflorescence meristem con- tain fewer petals, stamens, and carpels than wild-type flowers do and resemble flowers gener- ated by plants carrying weak stm alleles. Infrequently, flow- ers form carpeloid structures in the center of the flower (arrow). (E) Plants carrying the weak stm-2 allele produce inflorescence meristems that generate a limited number of abnormal flowers. (F) Flowers produced by an stm-2 inflor- escence meristem lack the full complement of internal organs and fail to generate carpels. (G) ult1-1 stm-2 plants produce inflorescence meristems that generate more flowers than stm-2 inflorescence meristems do. (H) Flowers produced by an ult1-1 stm-2 inflorescence meristem contain internal organs and can form unfused carpels or a normal, fused gynoecium (arrows). (I) A wus-1 inflorescence meristem, which generates a small number of abnormal flowers in a disorganized phyllotactic pattern. ( J) ult1-1 wus-1 inflorescence meristems form many more flowers than do wus-1 single-mutant meristems in a normal spiral phyllotaxy. (K) Flowers produced by wus-1 plants lack the full complement of organs and generally terminate in a solitary stamen (arrow). (L) Flowers produced by ult1-1 wus-1 plants can form more sepals and petals than either wild-type or wus-1 flowers, but fail to form carpels and generally terminate in a solitary stamen (arrow).
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Regulation of Arabidopsis shoot apical meristem and lateral
organ formation by microRNA miR166g and its AtHD ZIP target
genes

Regulation of Arabidopsis shoot apical meristem and lateral organ formation by microRNA miR166g and its AtHD ZIP target genes

appearance of such foci of WUS expression in jba-1D plants can explain the observed meristem phenotype, where independent domains of WUS expression lead to the formation of discrete, ectopic meristems. In serial longitudinal sections through entire 11-day old jba-1D seedlings, no WUS expression was detected outside of the shoot apical meristem. In our pWUS::GUS experiments, we noted that GUS expression was detected in jba-1D meristems after a shorter incubation time than wild-type Col meristems (compare Fig. 3B and C), suggesting that cells in jba-1D SAMs might express higher levels of WUS mRNA than those in wild-type SAMs. To test this hypothesis we performed real-time quantitative RT- PCR (qRT-PCR) on aerial parts of 28-day-old wild-type and jba-1D plants using primers directed against CLV3 and WUS. We found that, while the relative level of CLV3 transcription was unchanged between wild-type and jba-1D plants, the level of WUS transcription was ~12-fold higher in jba-1D plants compared to wild-type plants (Fig. 3G). We next determined whether WUS function was sufficient to produce the jba meristem phenotypes by generating double mutants between jba-1D plants and plants carrying the wus-1 null allele. wus-1 jba-1D double mutant meristems were indistinguishable from wus-1 mutant meristems (see Fig. S2 in supplementary material), indicating that WUS activity is absolutely required to obtain the jba SAM phenotypes. Taken together, our evidence indicates that the presence of elevated levels of WUS transcription in jba-1D shoot apical meristems is sufficient to account for the fasciation and de novo meristem formation phenotypes.
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AMP1 and MP antagonistically regulate embryo and
meristem development in Arabidopsis

AMP1 and MP antagonistically regulate embryo and meristem development in Arabidopsis

The controls regulating the balance between proliferating and differentiating cells are only partially understood. Where amenable to genetic dissection, as in the shoot apical meristem (SAM), these controls seem to comprise antagonistic activities acting in specific zones (reviewed by Bäurle and Laux, 2003; Williams and Fletcher, 2005). Antagonistic activities might also control the size of other meristems. A mechanism related to that in the SAM has been proposed for the root meristem (Casamitjana-Martinez et al., 2003), and the formation of procambium in the leaf seems to occur in competition with mesophyll differentiation (Scarpella et al., 2004). Mutations in the presumptive glutamate carboxypeptidase AMP1 are associated with diverse morphological abnormalities including supernumerary cotyledons, shortened plastochrons and a bushy appearance, and are further characterized by cytokinin overproduction and upregulation of CYCD3;1 (Chaudhury et al., 1993; Chin-Atkins et al., 1996; Nogué et al., 2000a; Nogué et al., 2000b; Riou-Khamlichi et al., 1999). However, amp1 mutants are phenotypically distinct from both cytokinin or CYCD3;1- overproducing plants and it is unclear what primary defect could account for the various aspects of the amp1 phenotype. Despite a
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3DCellAtlas Meristem: a tool for the global cellular annotation of shoot apical meristems

3DCellAtlas Meristem: a tool for the global cellular annotation of shoot apical meristems

Modern imaging approaches enable the acquisition of 3D and 4D datasets capturing plant organ development at cellular resolution. Computational analyses of these data enable the digitization and analysis of individual cells. In order to fully harness the information encoded within these datasets, annotation of the cell types within organs may be performed. This enables data points to be placed within the context of their position and identity, and for equivalent cell types to be compared between samples. The shoot apical meristem (SAM) in plants is the apical stem cell niche from which all above ground organs are derived. We developed 3DCellAtlas Meristem which enables the complete cellular annotation of all cells within the SAM with up to 96% accuracy across all cell types in Arabidopsis and 99% accuracy in tomato SAMs. Successive layers of cells are identified along with the central stem cells, boundary regions, and layers within developing primordia. Geometric analyses provide insight into the morphogenetic pro‑ cess that occurs during these developmental processes. Coupling these digital analyses with reporter expression will enable multidimensional analyses to be performed at single cell resolution. This provides a rapid and robust means to perform comprehensive cellular annotation of plant SAMs and digital single cell analyses, including cell geometry and gene expression. This fills a key gap in our ability to analyse and understand complex multicellular biology in the apical plant stem cell niche and paves the way for digital cellular atlases and analyses.
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