Organization Of Shoot Apical Meristem Research Articles

All together now: Cellular and molecular aspects of leaf development in lycophytes, ferns, and seed plants

Recent advances in plant developmental genetics together with rapid accumulation of transcriptomic data on plants from divergent lineages provide an exciting opportunity to explore the evolution of plant morphology. To understand leaf origin in sporophytes of land plants, we have combined the available molecular and structural data on development of leaves with different morphologies in different plant lineages: clubmosses, spikemosses, leptosporangiate ferns, ophioglossioid ferns, marattioid ferns, whisk ferns, horsetails, and conifers. Specifically, we address the peculiarities of proximo-distal, ad/abaxial, and lateral development; presence/absence of mesophyll differentiation into palisade and spongy parenchyma; and type of leaf vascular bundles (collateral and bicollateral). Furthermore, taxon-specific and morphology-specific features of leaf development are considered in the context of the organization of shoot apical meristems (SAMs)—monoplex, simplex, or duplex—that produce leaf primordia. The data available imply that cellular patterns of leaf initiation correlate strongly with the structure of the SAMs but not with further leaf development or morphology. The later stages of leaf development are neither correlated with SAM structure nor with taxonomy. Occurrence and, if available, patterns of expression of homologs of the angiosperm genes responsible for the development of adaxial (ARPandC3HDZ) and abaxial (YABBYandKANADI) leaf domains, or establishment of the leaf marginal meristem (WOX) are discussed. We show that there is no correlation in the set of homologs of TFs that regulate abaxial and adaxial leaf domain development between leaves containing only spongy and no palisade mesophyll (of spikemosses, clubmosses, whisk ferns, horsetails, and most conifers), and leaves differentiated into palisade and spongy mesophyll (of leptosporangiate ferns,Ginkgo,Gnetum, and angiosperms). Expression of three out of four regulators of leaf development in primordia of both leaves and sporangia—C3HDZin spikemosses and whisk ferns,YABBYin clubmosses andKANADIin spikemosses and horsetails—indicates that a sporangium developmental program could have been co-opted as a “precursor program” for the origin of microphylls and euphylls. Additionally, expression of leaf development regulators in SAMs of spikemosses (ARP,C3HDZ, andKANADI), clubmosses (YABBY), leptosporangiate ferns (C3HDZ), and horsetails (C3HDZandKANADI) indicates that at least some mechanisms of SAM regulation were co-opted as well in the pre-program of leaf precursors.

Topological properties accurately predict cell division events and organization of shoot apical meristem in Arabidopsis thaliana.

ABSTRACTCell division and the resulting changes to the cell organization affect the shape and functionality of all tissues. Thus, understanding the determinants of the tissue-wide changes imposed by cell division is a key question in developmental biology. Here, we use a network representation of live cell imaging data from shoot apical meristems (SAMs) in Arabidopsis thaliana to predict cell division events and their consequences at the tissue level. We show that a support vector machine classifier based on the SAM network properties is predictive of cell division events, with test accuracy of 76%, which matches that based on cell size alone. Furthermore, we demonstrate that the combination of topological and biological properties, including cell size, perimeter, distance and shared cell wall between cells, can further boost the prediction accuracy of resulting changes in topology triggered by cell division. Using our classifiers, we demonstrate the importance of microtubule-mediated cell-to-cell growth coordination in influencing tissue-level topology. Together, the results from our network-based analysis demonstrate a feedback mechanism between tissue topology and cell division in A. thaliana SAMs.

Transcriptome Dynamics of Epidermal Reprogramming during Direct Shoot Regeneration in Torenia fournieri.

abstractShoot regeneration involves reprogramming of somatic cells and de novo organization of shoot apical meristems (SAMs). In the best-studied model system of shoot regeneration using Arabidopsis, regeneration is mediated by the auxin-responsive pluripotent callus formation from pericycle or pericycle-like tissues according to the lateral root development pathway. In contrast, shoot regeneration can be induced directly from fully differentiated epidermal cells of stem explants of Torenia fournieri (Torenia), without intervening the callus mass formation in culture with cytokinin; yet, its molecular mechanisms remain unaddressed. Here, we characterized this direct shoot regeneration by cytological observation and transcriptome analyses. The results showed that the gene expression profile rapidly changes upon culture to acquire a mixed signature of multiple organs/tissues, possibly associated with epidermal reprogramming. Comparison of transcriptomes between three different callus-inducing cultures (callus induction by auxin, callus induction by wounding and protoplast culture) of Arabidopsis and the Torenia stem culture identified genes upregulated in all the four culture systems as candidates of common factors of cell reprogramming. These initial changes proceeded independently of cytokinin, followed by cytokinin-dependent, transcriptional activations of nucleolar development and cell cycle. Later, SAM regulatory genes became highly expressed, leading to SAM organization in the foci of proliferating cells in the epidermal layer. Our findings revealed three distinct phases with different transcriptomic and regulatory features during direct shoot regeneration from the epidermis in Torenia, which provides a basis for further investigation of shoot regeneration in this unique culture system.

Identification and Evolution of the WUSCHEL-Related Homeobox Protein Family in Bambusoideae.

Bamboos (Bambusoideae) are fast-growing species due to their rapid growth rate and ability to reproduce annually via cloned buds produced on the rhizome. WUSCHEL-related homeobox (WOX) genes have been reported to regulate shoot apical meristem organization, lateral organ formation, cambium and vascular proliferation, and so on, but have rarely been studied in bamboos. In this study, the WOXs of both herbaceous bamboo species (12 OlaWOXs and nine RguWOXs) and woody bamboo species (18 GanWOXs, 27 PheWOXs, and 26 BamWOXs) were identified and categorized into three clades based on their phylogenetic relationship—ancient, intermediate, or WUS clade. Polyploidy is the major driver of the expansion of the bamboo WOX family. Eight conserved domains, besides the homeodomain, were identified by comparatively analyzing the WOXs of dicot and monocot species. Intensive purifying selection pressure in the coding region of specific domains explained the functional similarity of WOXs between different species. For Bambusoideae WOXs, polyploidy is the major driver of the expansion of the WOX family. Stronger purifying selection was found in orthologous WOXs of Bambusoideae, especially for WOX4s and WOX5s, which are conserved not only at the translational levels, but also at the genome level. Several conserved cis-acting elements were discovered at similar position in the promoters of the orthologous WOXs. For example, AP2/ERF protein-binding elements and B3 protein-binding elements were found in the promoters of the bamboo WOX4, while MYB protein-binding elements and Dof protein-binding elements were found in the promoters of bamboo WOX5, and MADS protein-binding sites was found in the promoters of bamboo WUS, WOX3, and WOX9. These conserved positions may play an important role in regulating the expression of bamboo WOXs. Our work provides insight into the origin and evolution of bamboo WOXs, and will facilitate functional investigations of the clonal propagation of bamboos.

Identification and co-evolution pattern of stem cell regulator miR394s and their targets among diverse plant species

BackgroundMicro RNAs (miRNAs), a class of small non-coding RNAs, have been implicated in various aspects of plant development. miR394 is required for shoot apical meristem organization, stem cell maintenance and abiotic stress responses in Arabidopsis, where it functions by negatively regulating the transcript level of target LEAF CURLING RESPONSIVENESS (LCR), which is an F-box protein-coding gene. The evolutionary conservation of stem cell regulatory miR394-LCR module among plants remains elusive.ResultsOur study has identified 79 miR394 and 43 target sequences across 40 plant species using various homology based search tools and databases, and analysed their co-evolution pattern. We customised an annotation workflow which computationally validates 20 novel miR394s from 14 plant species. Independent phylogenetic trees were reconstructed with precursor MIR394s, mature miR394s, and their target sequences along with complementary miR394 binding sites. The phylogeny revealed that mature sequences of miR394s as well as their targets belonging to the F-box protein encoding gene families, were highly conserved. Though, miR394–3p were complementary to miR394s/miR394–5p, they clustered separately.ConclusionThe existence and separate clustering of miR394–3p and miR394s/miR394–5p indicate their independent regulation. The phylogeny also suggests that miR394s had evolved at the beginning of gymnosperm-angiosperm divergence. Despite strong conservation, some level of sequence variation in miR394s and the complementary binding sites of their targets suggests possible functional diversification of miR394-LCR mediated stem cell regulation in plants.

Trans-splicing of plastid rps12 transcripts, mediated by AtPPR4, is essential for embryo patterning in Arabidopsis thaliana.

AtPPR4-mediated trans-splicing of plastid rps12 transcripts is essential for key embryo morphogenetic events such as development of cotyledons, determination of provascular tissue, and organization of the shoot apical meristem (SAM), but not for the formation of the protodermal layer. Members of the pentatricopeptide repeat (PPR) containing protein family have emerged as key regulators of the organelle post-transcriptional processing and to be essential for proper plant embryo development. In this study, we report the functional characterization of the AtPPR4 (At5g04810) gene encoding a plastid nucleoid PPR protein. In-situ hybridization analysis reveals the presence of AtPPR4 transcripts already at the transition stage of embryo development. As a consequence, embryos lacking the AtPPR4 protein arrest their development at the transition/early-heart stages and show defects in the determination of the provascular tissue and organization of SAM. This complex phenotype is due to the specific role of AtPPR4 in the trans-splicing of the plastid rps12 transcripts, as shown by northern and slot-blot hybridizations, and the consequent defect in 70S ribosome accumulation and plastid protein synthesis, in agreement with the role proposed for the maize orthologue, ZmPPR4.

Proteomic Identification of Putative MicroRNA394 Target Genes in Arabidopsis thaliana Identifies Major Latex Protein Family Members Critical for Normal Development

Expression of the F-Box protein Leaf Curling Responsiveness (LCR) is regulated by microRNA, miR394, and alterations to this interplay in Arabidopsis thaliana produce defects in leaf polarity and shoot apical meristem organization. Although the miR394-LCR node has been documented in Arabidopsis, the identification of proteins targeted by LCR F-box itself has proven problematic. Here, a proteomic analysis of shoot apices from plants with altered LCR levels identified a member of the Latex Protein (MLP) family gene as a potential LCR F-box target. Bioinformatic and molecular analyses also suggested that other MLP family members are likely to be targets for this post-translational regulation. Direct interaction between LCR F-Box and MLP423 was validated. Additional MLP members had reduction in protein accumulation, in varying degrees, mediated by LCR F-Box. Transgenic Arabidopsis lines, in which MLP28 expression was reduced through an artificial miRNA technology, displayed severe developmental defects, including changes in leaf patterning and morphology, shoot apex defects, and eventual premature death. These phenotypic characteristics resemble those of Arabidopsis plants modified to over-express LCR Taken together, the results demonstrate that MLPs are driven to degradation by LCR, and indicate that MLP gene family is target of miR394-LCR regulatory node, representing potential targets for directly post-translational regulation mediated by LCR F-Box. In addition, MLP28 family member is associated with the LCR regulation that is critical for normal Arabidopsis development.

HANABA TARANUregulates the shoot apical meristem and leaf development in cucumber (Cucumis sativusL.)

The shoot apical meristem (SAM) is essential for continuous organogenesis in higher plants, while the leaf is the primary source organ and the leaf shape directly affects the efficiency of photosynthesis. HANABA TARANU (HAN) encodes a GATA3-type transcription factor that functions in floral organ development, SAM organization, and embryo development in Arabidopsis, but is involved in suppressing bract outgrowth and promoting branching in grass species. Here the function of the HAN homologue CsHAN1 was characterized in cucumber, an important vegetable with great agricultural and economic value. CsHAN1 is predominantly expressed at the junction of the SAM and the stem, and can partially rescue the han-2 floral organ phenotype in Arabidopsis. Overexpression and RNAi of CsHAN1 transgenic cucumber resulted in retarded growth early after embryogenesis and produced highly lobed leaves. Further, it was found that CsHAN1 may regulate SAM development through regulating the WUSCHEL (WUS) and SHOOT MERISTEMLESS (STM) pathways, and mediate leaf development through a complicated gene regulatory network in cucumber.

Ribonuclease J is required for chloroplast and embryo development in Arabidopsis.

Chloroplasts perform many essential metabolic functions and their proper development is critically important in embryogenesis. However, little is known about how chloroplasts function in embryogenesis and more relevant components need to be characterized. In this study, we show that Arabidopsis Ribonuclease J (RNase J) is required for chloroplast and embryo development. Mutation of AtRNJ led to albino ovules containing aborted embryos; the morphological development of rnj embryos was disturbed after the globular stage. Observation of ultrastructures indicated that these aborted embryos may result from impaired chloroplast development. Furthermore, by analyzing the molecular markers of cell fate decisions (STM, FIL, ML1, SCR, and WOX5) in rnj embryos, we found that this impairment of chloroplast development may lead to aberrant embryo patterning along the apical-basal axis, indicating that AtRNJ is important in initiating and maintaining the organization of shoot apical meristems (SAMs), cotyledons, and hypocotyls. Moreover, the transport and response of auxin in rnj embryos was found to be disrupted, suggesting that AtRNJ may be involved in auxin-mediated pathways during embryogenesis. Therefore, we speculate that RNJ plays a vital role in embryo morphogenesis and apical-basal pattern formation by regulating chloroplast development.

Arabidopsis A BOUT DE SOUFFLE is a putative mitochondrial transporter involved in photorespiratory metabolism and is required for meristem growth at ambient CO2 levels

Photorespiratory metabolism is essential in all oxygenic photosynthetic organisms. In plants, it is a highly compartmentalized pathway that involves chloroplasts, peroxisomes, mitochondria and the cytoplasm. The metabolic pathway itself is well characterized, and the enzymes required for its function have been identified. However, very little information is available on the transport proteins that catalyze the high metabolic flux between the involved compartments. Here we show that the A BOUT DE SOUFFLE (BOU) gene, which encodes a mitochondrial carrier, is involved in photorespiration in Arabidopsis. BOU was found to be co-expressed with photorespiratory genes in leaf tissues. The knockout mutant bou-2 showed the hallmarks of a photorespiratory growth phenotype, an elevated CO(2) compensation point, and excessive accumulation of glycine. Furthermore, degradation of the P-protein, a subunit of glycine decarboxylase, was demonstrated for bou-2, and is reflected in strongly reduced glycine decarboxylase activity. The photorespiration defect in bou-2 has dramatic consequences early in the seedling stage, which are highlighted by transcriptome studies. In bou-2 seedlings, as in shm1, another photorespiratory mutant, the shoot apical meristem organization is severely compromised. Cell divisions are arrested, leading to growth arrest at ambient CO(2) . Although the specific substrate for the BOU transporter protein remains elusive, we show that it is essential for the function of the photorespiratory metabolism. We hypothesize that BOU function is linked with glycine decarboxylase activity, and is required for normal apical meristems functioning in seedlings.

CaHAM is autoregulated and regulates CaSTM expression and is required for shoot apical meristem organization in pepper

The angiosperm shoot apical meristem (SAM) is characterized by tightly organized cell layers and zones. The SAM's organization allows it to maintain its indeterminate nature while producing determinate lateral organs. Alterations in SAM gene expression partly account for the immense diversity in plant architecture. The GRAS protein family gene HAIRY MERISTEM (HAM) is an important regulator of SAM organization in Petunia and Arabidopsis. Here we describe CaHAM loss-of-function pepper mutants characterized by an arrested SAM following the formation of several leaves on the primary stem, complete inhibition of axillary meristem development, an expanded tunica domain and trichome formation on the SAM epidermis. CaHAM is expressed in the periphery of the SAM and in the vasculature of young leaves throughout plant development, reaching its highest level in the reproductive growth stage. Analysis of the effect of CaHAM loss-of-function on its own expression showed that CaHAM is negatively autoregulated. Furthermore, CaHAM negatively regulates the expression level and pattern of pepper SHOOT MERISTEMLESS (CaSTM), which is required to maintain the SAM in an undifferentiated state. We conclude that CaHAM is regulated to achieve adjusted functional levels and has a conserved role in controlling SAM maintenance, organization and axillary meristem formation.

Novel Insights from Live‐imaging in Shoot Meristem Development

Microscopic imaging of fluorescent reporters for key meristem regulators in live tissues is emerging as a powerful technique, enabling researchers to observe dynamic spatial and temporal distribution of hormonal and developmental regulators in living cells. Aided by time-lapse microphotography, new types of imaging acquisition and analysis software, and computational modeling, we are gaining significant insights into shoot apical meristem (SAM) behavior and function. This review is focused on summarizing recent advances in the understanding of SAM organization, development, and behavior derived from live-imaging techniques. This includes the revelation of mechanical forces in microtubule-controlled anisotropic growth, the role of the CLV-WUS network in the specification of peripheral zone and central zone cells, the multiple feedback loops involving cytokinin in controlling WUS expression, auxin dynamics in determining the position of new primordia, and, finally, sequence of regulatory events leading to de novo assembly of shoots from callus in culture. Future studies toward formulating "digital SAM" that incorporates multi-dimensional data ranging from images of SAM morphogenesis to a genome-scale expression map of SAM will greatly enhance our ability to understand, predict, and manipulate SAM, containing the stem cells that give rise to all above ground parts of a plant.

BARD1 may be renamed ROW1 because it functions mainly as a REPRESSOR OF WUSCHEL1

Human BRCA1 (BRreast CAncer susceptible gene1) is known to involve in cell-cycle control, transcriptional regulation, DNA recombination, DNA repair and many other processes. hBARD1 (BRCA1-Associated Ring Domain 1) forms heterodimer via its N-terminal conserved RING domain with BRCA1. In Arabidopsis, two genes, At4g21070 and At1g04020, that share N-terminal RING domain and C-terminal BRCT (for BRCA1 C-Terminal) domains with no substantial similarities for other motifs, have been identified. AtBRCA1 was induced by γ-ray while AtBARD1 was required for DNA repair. Recently, we find that AtBARD1 may function to confine WUS transcription in the shoot apical meristem organization center, together with the ATPase-dependent chromatin remodeling factor, SYD. In bard1-3 Arabidopsis knock-out mutant, WUS was released to the outer layers and expressed at extremely high level comparing to wild-type. Our data suggest that BARD1 mainly function as a REPRESSOR OF WUSCHEL1 (ROW1). Extensive motif analyses carried out here showed that ROW1 possesses substantial sequence identity with a reported transcription repressor, MLL and also a potential PHD domain which recognizes histone tail codes, in its uncharacterized middle region. We suggest that ROW1 represses transcription in a chromatin-related mechanism.

Mutation ofArabidopsis BARD1Causes Meristem Defects by Failing to ConfineWUSCHELExpression to the Organizing Center

Stem cell fate in the Arabidopsis thaliana shoot apical meristem (SAM) is controlled by WUSCHEL (WUS) and CLAVATA. Here, we examine BARD1 (for BRCA1-associated RING domain 1), which had previously been implicated in DNA repair functions; we find that it also regulates WUS expression. We observed severe SAM defects in the knockout mutant bard1-3. WUS transcripts accumulated >238-fold in bard1-3 compared with the wild type and were located mainly in the outermost cell layers instead of the usual organizing center. A specific WUS promoter region was recognized by nuclear protein extracts obtained from wild-type plants, and this protein-DNA complex was recognized by antibodies against BARD1. The double mutant (wus-1 bard1-3) showed prematurely terminated SAM structures identical to those of wus-1, indicating that BARD1 functions through regulation of WUS. BARD1 overexpression resulted in reduced WUS transcript levels, giving a wus-1-like phenotype. Either full-length BARD1 or a clone that encoded the C-terminal domain (BARD1:C-ter;bard1-3) was sufficient to complement the bard1-3 phenotype, indicating that BARD1 functions through its C-terminal domain. Our data suggest that BARD1 regulates SAM organization and maintenance by limiting WUS expression to the organizing center.

Signaling network for stem cell maintenance and functioning inarabidopsis shoot apical meristem

The shoot apical meristem (SAM) domain, which contains a self-renewing population of undifferentiated pluripotent cells, produces all of a plant’s post-embryonic aerial organs. Intra- and intercellular signaling networks are essential to the maintenance of SAM size and structure, and for coordinating the formation and patterning of new organs. A key regulatory system for meristem functioning consists of a non-cell-autonomous WUSCHEL (WUS)-CLAVATA (CLV) regulatory loop and class I KNOX-related signaling in the shoot apex. Meristem activity is also dependent on short- and long-distance signaling from the organizing center, organ boundaries/primordia, and distant organs. Here, we provide an overview of SAM organization and present our current knowledge about the signaling network for stem cell maintenance and functioning in the vegetative SAM in Arabidopsis. Transcription factors belonging to the WOX, KNOX, HD-ZIP, MYB, SAND, AP2, and NAC domain protein families, as well as microRNA, play central roles in this network, along with hormonal cross-talk. We also discuss regulation at different levels, such as for protein interactions, and transcriptional and epigenetic controls. Our intent is to show how various signals are integrated to maintain a stem cell niche in the SAM.

A stochastic approach to phyllotactic patterns analysis

A statistical method is presented to characterize the degree of order in phyllotactic systems. We developed equations allowing the theoretical estimation of the number of leaves regularly distributed (spiral or verticillate) in a partially random phyllotactic system. The equations are simple and accurate enough to make quantitative predictions concerning the organization of different phyllotactic patterns (verticillate, distichous, spiral and random). This method can bring out patterns that are not visible a priori on a planar representation of the shoot apex. As a case study, the method was applied to the quantitative analysis of the sho mutants recently produced by Itoh et al. [2000. SHOOT ORGANIZATION genes regulate shoot apical meristem organization and the pattern of leaf primordium initiation in Rice. Plant Cell 12, 2161–2174]. By using our method, it was possible to predict the number of leaves distributed in distichous or random patterns on these phyllotactic mutants.

Alterations of the glutathione redox state improve apical meristem structure and somatic embryo quality in white spruce (Picea glauca)

In white spruce, an improvement of somatic embryo number and quality can be achieved through experimental manipulations of the endogenous levels of reduced (GSH) and oxidized (GSSG) glutathione. An optimal protocol for embryo production included an initial application of GSH in the maturation medium, followed by replacement with GSSG during the remaining maturation period. Under these conditions, the overall embryo population more than doubled, and the percentage of fully developed embryos increased from 22% to almost 70%. These embryos showed improved post-embryonic growth and conversion frequency. Structural studies revealed remarkable differences between embryo types, especially in storage product deposition pattern and organization of the shoot apical meristem (SAM). Compared with their control counterparts, glutathione-treated embryos accumulated a larger amount of starch during the early stages of development, and more protein and lipid bodies during the second half of development. Differences were also noted in the organization of SAMs. Shoot meristems of control embryos were poorly organized and were characterized by the presence of intercellular spaces, which caused separation of the subapical cells. Glutathione-treated embryos had well-organized meristems composed of tightly packed cells which lack large vacuoles. The improved organization of the shoot apical meristems in treated embryos was ascribed to a lower production of ethylene. Differences in meristem structure between control and treated embryos were also related to the localization pattern of HBK1, a shoot apical meristem 'molecular marker' gene with preferential expression to the meristematic cells of the shoot pole. Expression of this gene, which was localized to the apical cells in control embryos, was extended to the subapical cells of treated embryos. Overall, it appears that meristem integrity and embryo quality are under the direct control of the glutathione redox state.

Phenotypic alterations in Arabidopsis thaliana plants caused by Rhodococcus fascians infection.

Arabidopsis thaliana (L.) Heynh. plants were challenged with Rhodococcus fascians at several developmental stages and using different inoculation procedures. A variety of morphological alterations was scored on the infected plants; some of them resembled phenotypes of A. thaliana mutants in their shoot apical meristem (SAM) organization. Infection with R. fascians did not affect SAM organization in wild type nor in SAM mutants. Anatomical studies on the new organs formed after infection with R. fascians demonstrated extensive bacterial colonization. Colonization and concomitant production of specific signals are the likely cause of malformations.

Developmental events and shoot apical meristem gene expression patterns during shoot development in Arabidopsis thaliana.

Critical developmental and gene expression profiles were charted during the formation of shoots from root explants in Arabidopsis tissue culture. Shoot organogenesis is a two-step process involving pre-incubation on an auxin-rich callus induction medium (CIM) during which time root explants acquire competence to form shoots during subsequent incubation on a cytokinin-rich shoot induction medium (SIM). At a histological level, the organization of shoot apical meristems (SAMs) appears to occur during incubation on SIM about the time of shoot commitment, i.e. the transition from hormone-dependent to hormone-independent shoot development. Genes involved in SAM formation, such as SHOOTMERISTEMLESS (STM) and CLAVATA1 (CLV1), were upregulated at about the time of shoot commitment, while WUSCHEL (WUS) was upregulated somewhat earlier. Genes required for STM expression, such as CUP-SHAPED COTYLEDON 1 and 2 (CUC1 and 2) were upregulated prior to shoot commitment. Gene expression patterns were determined for two GFP enhancer trap lines with tissue-specific expression in the SAM, including one line reporting on CUC1 expression. CUC1 was generally expressed in callus tissue during early incubation on SIM, but later CUC1 was expressed more locally in presumptive sites of shoot formation. In contrast, the expression pattern of the enhancer trap lines during zygotic embryogenesis was more localized to the presumptive SAM even in early stages of embryogenesis.

FASCIATA Genes for Chromatin Assembly Factor-1 in Arabidopsis Maintain the Cellular Organization of Apical Meristems

Postembryonic development of plants depends on the activity of apical meristems established during embryogenesis. The shoot apical meristem (SAM) and the root apical meristem (RAM) have similar but distinct cellular organization. Arabidopsis FASCIATA1 ( FAS1) and FAS2 genes maintain the cellular and functional organization of both SAM and RAM, and FAS gene products are subunits of the Arabidopsis counterpart of chromatin assembly factor-1 (CAF-1). fas mutants are defective in maintenance of the expression states of WUSCHEL ( WUS) in SAM and SCARECROW ( SCR) in RAM. We suggest that CAF-1 plays a critical role in the organization of SAM and RAM during postembryonic development by facilitating stable maintenance of gene expression states.