ACR4, a putative receptor kinase gene of Arabidopsis thaliana, that is expressed in the outer cell layers of embryos and plants, is involved in proper embryogenesis

CLE peptides act via the receptor-like kinase CRINKLY 4 in Physcomitrium patens gametophore development

The CLAVATA pathway plays a key role in the regulation of multicellular shoot and root meristems in flowering plants. In Arabidopsis, CLAVATA 3-like signaling peptides (CLEs) act via receptor-like kinases CLAVATA 1 and CRINKLY 4 (CR4). In the moss Physcomitrium patens, PpCLAVATA and PpCR4 were previously studied independently and shown to play conserved roles in the regulation of cell proliferation and differentiation. The plant calpain DEFECTIVE KERNEL 1 (DEK1) has been identified as another key regulator of cell division and cell fate in vascular plants and bryophytes. The functional interaction between CLAVATA, CR4, and DEK1 remains unknown. Here, we show that P. patens crinkly4 and dek1 mutants respond differently to CLE peptide treatments suggesting their distinct roles in the CLAVATA pathway. Reduced CLAVATA-mediated suppression of leafy shoot growth in Δcr4 mutants indicates that PpCR4 is involved in CLV3p perception, most likely as a receptor. The CLV3p strongly suppressed leaf vein development in Δcr4 mutants, suggesting that other receptors are involved in these processes and indicating a potential role of PpCR4 in organ sensitization to CLEs.

Identification and characterization of ACR gene family in maize for salt stress tolerance

Background

Members of the ACR gene family are commonly involved in various physiological processes, including amino acid metabolism and stress responses. In recent decades, significant progress has been made in the study of ACR genes in plants. However, little is known about their characteristics and function in maize.

Methods

In this study, ACR genes were identified from the maize genome, and their molecular characteristics, gene structure, gene evolution, gene collinearity analysis, cis-acting elements were analyzed. qRT-PCR technology was used to verify the expression patterns of the ZmACR gene family in different tissues under salt stress. In addition, Ectopic expression technique of ZmACR5 in Arabidopsis thaliana was utilized to identify its role in response to salt stress.

Results

A total of 28 ZmACR genes were identified, and their molecular characteristics were extensively described. Two gene pairs arising from segmented replication events were detected in maize, and 18 collinear gene pairs were detected between maize and 3 other species. Through phylogenetic analysis, three subgroups were revealed, demonstrating distinct divergence between monocotyledonous and dicotyledonous plants. Analysis of ZmACR cis-acting elements revealed the optional involvement of ZmACR genes in light response, hormone response and stress resistance. Expression analysis of 8 ZmACR genes under salt treatment clearly revealed their role in the response to salt stress. Ectopic overexpression of ZmACR5 in Arabidopsis notably reduced salt tolerance compared to that of the wild type under salt treatment, suggesting that ZmACR5 has a negative role in the response to salt stress.

Conclusion

Taken together, these findings confirmed the involvement of ZmACR genes in regulating salt stress and contributed significantly to our understanding of the molecular function of ACR genes in maize, facilitating further research in this field.

Emerging Roles of Receptor-like Protein Kinases in Plant Response to Abiotic Stresses

The productivity of plants is hindered by unfavorable conditions. To perceive stress signals and to transduce these signals to intracellular responses, plants rely on membrane-bound receptor-like kinases (RLKs). These play a pivotal role in signaling events governing growth, reproduction, hormone perception, and defense responses against biotic stresses; however, their involvement in abiotic stress responses is poorly documented. Plant RLKs harbor an N-terminal extracellular domain, a transmembrane domain, and a C-terminal intracellular kinase domain. The ectodomains of these RLKs are quite diverse, aiding their responses to various stimuli. We summarize here the sub-classes of RLKs based on their domain structure and discuss the available information on their specific role in abiotic stress adaptation. Furthermore, the current state of knowledge on RLKs and their significance in abiotic stress responses is highlighted in this review, shedding light on their role in influencing plant-environment interactions and opening up possibilities for novel approaches to engineer stress-tolerant crop varieties.

OsCIP1, a secreted protein, binds to and stabilizes OsCR4 to promote aleurone layer development, seed germination and early seedling growth in rice

The receptor kinase CRINKLY 4 (CR4) and its orthologs are known for their essential roles in cell differentiation and their shuttling between plasma membrane and cytoplasmic vesicles, a unique feature tied to their extracellular domain. However, the extracellular regulators of CR4 have been little known. Here we identified an OsCR4 Interacting Protein 1 (OsCIP1) (also named as OsLTPL36 in rice) by a yeast two-hybrid screen using the extracellular domain of OsCR4 (OsCR4E) as bait. OsCIP1/OsLTPL36 harbors a signal peptide and is localized to the outer surface of the plasma membrane. It interacted with the TNFR subdomain of OsCR4, causing an increase in OsCR4 recycling to the plasma membrane. oscip1, in which OsCR4 protein was decreased, exhibited thinner aleurone layer, late germination and delayed growth; while OsCIP1-overexpressing plants, in which OsCR4 protein was increased, displayed enhanced growth at the early seedling stage. OsCIP1 was cleaved between W61 and Q62, and the resulting C-terminal half exhibited a greater affinity for OsCR4E than did its precursor. Abolishing this cleavage site compromises OsCIP1's ability to promote seedling growth. Our results provide valuable clues for the regulation of CR4 activity and its functions in aleurone layer cell differentiation by a secreted small protein in rice.

Combining Fine Mapping, Whole-Genome Re-Sequencing, and RNA-Seq Unravels Candidate Genes for a Soybean Mutant with Short Petioles and Weakened Pulvini

A short petiole is an important agronomic trait for the development of plant ideotypes with high yields. However, the genetic basis underlying this trait remains unclear. Here, we identified and characterized a novel soybean mutant with short petioles and weakened pulvini, designated as short petioles and weakened pulvini (spwp). Compared with the wild type (WT), the spwp mutant displayed shortened petioles, owing to the longitudinally decreased cell length, and exhibited a smaller pulvinus structure due to a reduction in motor cell proliferation and expansion. Genetic analysis showed that the phenotype of the spwp mutant was controlled by two recessive nuclear genes, named as spwp1 and spwp2. Using a map-based cloning strategy, the spwp1 locus was mapped in a 183 kb genomic region on chromosome 14 between markers S1413 and S1418, containing 15 annotated genes, whereas the spwp2 locus was mapped in a 195 kb genomic region on chromosome 11 between markers S1373 and S1385, containing 18 annotated genes. Based on the whole-genome re-sequencing and RNA-seq data, we identified two homologous genes, Glyma.11g230300 and Glyma.11g230600, as the most promising candidate genes for the spwp2 locus. In addition, the RNA-seq analysis revealed that the expression levels of genes involved in the cytokinin and auxin signaling transduction networks were altered in the spwp mutant compared with the WT. Our findings provide new gene resources for insights into the genetic mechanisms of petiole development and pulvinus establishment, as well as soybean ideotype breeding.

The hidden half comes into the spotlight: Peeking inside the black box of root developmental phases

Efficient use of natural resources (e.g., light, water, and nutrients) can be improved with a tailored developmental program that maximizes the lifetime and fitness of plants. In plant shoots, a developmental phase represents a time window in which the meristem triggers the development of unique morphological and physiological traits, leading to the emergence of leaves, flowers, and fruits. Whereas developmental phases in plant shoots have been shown to enhance food production in crops, this phenomenon has remained poorly investigated in roots. In light of recent advances, we suggest that root development occurs in three main phases: root apical meristem appearance, foraging, and senescence. We provide compelling evidence suggesting that these phases are regulated by at least four developmental pathways: autonomous, non-autonomous, hormonal, and periodic. Root developmental pathways differentially coordinate organ plasticity, promoting morphological alterations, tissue regeneration, and cell death regulation. Furthermore, we suggest how nutritional checkpoints may allow progression through the developmental phases, thus completing the root life cycle. These insights highlight novel and exciting advances in root biology that may help maximize the productivity of crops through more sustainable agriculture and the reduced use of chemical fertilizers.

Receptor-like protein kinases in plant reproduction: Current understanding and future perspectives

Reproduction is a crucial process in the life span of flowering plants, and directly affects human basic requirements in agriculture, such as grain yield and quality. Typical receptor-like protein kinases (RLKs) are a large family of membrane proteins sensing extracellular signals to regulate plant growth, development, and stress responses. In Arabidopsis thaliana and other plant species, RLK-mediated signaling pathways play essential roles in regulating the reproductive process by sensing different ligand signals. Molecular understanding of the reproductive process is vital from the perspective of controlling male and female fertility. Here, we summarize the roles of RLKs during plant reproduction at the genetic and molecular levels, including RLK-mediated floral organ development, ovule and anther development, and embryogenesis. In addition, the possible molecular regulatory patterns of those RLKs with unrevealed mechanisms during reproductive development are discussed. We also point out the thought-provoking questions raised by the research on these plant RLKs during reproduction for future investigation.

Conserved and differentiated functions of CIK receptor kinases in modulating stem cell signaling in Arabidopsis

The shoot apical meristem (SAM) and root apical meristem (RAM) act as pools of stem cells that give rise to aboveground and underground tissues and organs in higher plants, respectively. The CLAVATA3 (CLV3)-WUSCHEL (WUS) negative-feedback loop acts as a core pathway controlling SAM homeostasis, while CLV3/EMBRYO SURROUNDING REGION (ESR) 40 (CLE40) and WUSCHEL-RELATED HOMEOBOX5 (WOX5), homologs of CLV3 and WUS, direct columella stem cell fate. Moreover, CLV3 INSENSITIVE KINASES (CIKs) have been shown to be essential for maintaining SAM homeostasis, whereas whether they regulate the distal root meristem remains to be elucidated. Here, we report that CIKs are indispensable for transducing the CLE40 signal to maintain homeostasis of the distal root meristem. We found that the cik mutant roots displayed disrupted quiescent center and delayed columella stem cell (CSC) differentiation. Biochemical assays demonstrated that CIKs interact with ARABIDOPSIS CRINKLY4 (ACR4) in a ligand-independent manner and can be phosphorylated by ACR4 in vitro. In addition, the phosphorylation of CIKs can be rapidly induced by CLE40, which partially depends on ACR4. Although CIKs act as conserved and redundant regulators in the SAM and RAM, our results demonstrated that they exhibit differentiated functions in these meristems.

Receptor-like kinase OsCR4 controls leaf morphogenesis and embryogenesis by fixing the distribution of auxin in rice

Cell differentiation is a key event in organ development; it involves auxin gradient formation, cell signaling, and transcriptional regulation. Yet, how these processes are orchestrated during leaf morphogenesis is poorly understood. Here, we demonstrate an essential role for the receptor-like kinase OsCR4 in leaf development. oscr4 loss-of-function mutants displayed short shoots and roots, with tiny, crinkly, or even dead leaves. The delayed outgrowth of the first three leaves and seminal root in oscr4 was due to defects in plumule and radicle formation during embryogenesis. The deformed epidermal, mesophyll, and vascular tissues observed in oscr4 leaves arose at the postembryo stage; the corresponding expression pattern of proOsCR4:GUS in embryos and young leaves suggests that OsCR4 functions in these tissues. Signals from the auxin reporter DR5rev:VENUS were found to be altered in oscr4 embryos and disorganized in oscr4 leaves, in which indole-3-acetic acid accumulation was further revealed by immunofluorescence. OsWOX3A, which is auxin responsive and related to leaf development, was activated extensively and ectopically in oscr4 leaves, partially accounting for the observed lack of cell differentiation. Our data suggest that OsCR4 plays a fundamental role in leaf morphogenesis and embryogenesis by fixing the distribution of auxin.

Comprehensive study and multipurpose role of the CLV3/ESR-related (CLE) genes family in plant growth and development

The CLAVATA3/endosperm surrounding region-related (CLE) is one of the most important signaling peptides families in plants. These peptides signaling are common in the cell to cell communication and control various physiological and developmental processes, that is cell differentiation and proliferation, self-incompatibility, and the defense response. The CLE signaling systems are conserved across the plant kingdom but have a diverse mode of action in various developmental processes in different species. In this review, we concise various methods of peptides identification, structure, and molecular identity of the CLE family, the developmental role of CLE genes/peptides in plants, environmental stimuli, and CLE family and some other novel progress in CLE genes/peptides in various crops, and so forth. According to previous literature, about 1,628 CLE genes were identified in land plants, which deeply explained the tale of plant development. Nevertheless, some important queries need to be addressed to get clear insights into the CLE gene family in other organisms and their role in various physiological and developmental processes. Furthermore, we summarized the power of the CLE family around the environment as well as bifunctional activity and the crystal structure recognition mechanism of CLE peptides by their receptors and CLE clusters functions. We strongly believed that the discovery of the CLE family in other organisms would provide a significant breakthrough for future revolutionary and functional studies.

MINI SEED 2 (MIS2) Encodes a Receptor-like Kinase that Controls Grain Size and Shape in Rice

BACKGROUND

Grain size is a key agronomic trait that is directly associated with grain yield in rice. Although several genes related to grain size in rice have been identified, our understanding of the mechanism of grain development is still limited.

RESULTS

In this study, we reported the characterization of a novel seed size mutant mini seed 2 (mis2), in which the grain showed reduced length, width and thickness along with wrinkled surface. Microscopic analysis revealed that the spikelet epidermal cell size was reduced but the cell number was increased in the mis2 mutant, suggesting that MIS2 controls grain size by coordinately regulating epidermal cell size and cell number. Map-based cloning revealed that MIS2 encodes a receptor-like kinase CRINKLY4 (CR4) which showed the highest expression in developing panicles. The MIS2 protein is localized primarily on the plasma membrane along with the endosome. However, the Arg258Gln mutation located in extracellular domain in the mis2 mutant disturbed its subcellular localization. Additionally, three major haplotypes of MIS2 were identified in the japonica, indica and aus rice cultivars. The 18-bp InDel (insertion and deletion) in the 5'-UTR (untranslated region) caused different expression level of MIS2 in haplotypes.

CONCLUSIONS

We reported a key role of OsCR4 in controlling grain size and shape by coordinately regulating epidermal cell size and cell number. The Arg258 in the extracellular seven-repeat domain is essential for the correct subcellular behavior and function of the OsCR4 protein.

SERK Receptor-like Kinases Control Division Patterns of Vascular Precursors and Ground Tissue Stem Cells during Embryo Development in Arabidopsis

During embryo development, the vascular precursors and ground tissue stem cells divide to renew themselves and produce the vascular tissue, endodermal cells, and cortical cells. However, the molecular mechanisms regulating division of these stem cells have remained largely elusive. In this study, we show that loss of function of SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) genes results in aberrant embryo development. Fewer cortical, endodermal, and vascular cells are generated in the embryos of serk1 serk2 bak1 triple mutants. WUSCHEL-RELATED HOMEOBOX 5 (WOX5) is ectopically expressed in vascular cells of serk1 serk2 bak1 embryos. The first transverse division of vascular precursors in mid-globular embryos and second asymmetric division of ground tissue stem cells in early-heart embryos are abnormally altered to a longitudinal division. The embryo defects can be partially rescued by constitutively activated mitogen-activated protein kinase (MAPK) kinase kinase YODA (YDA) and MAPK kinase MKK5. Taken together, our results reveal that SERK-mediated signals regulate division patterns of vascular precursors and ground tissue stem cells, likely via the YDA-MKK4/5 cascade, during embryo development.

ATML1 activity is restricted to the outermost cells of the embryo through post-transcriptional repressions

Cell fate determination in plants relies on positional cues. To investigate the position-dependent gene regulation in plants, we focused on shoot epidermal cell specification, which occurs only in the outermost cells. ATML1, which encodes an HD-ZIP class IV transcription factor, is a positive regulator of shoot epidermal cell identity. Despite the presence of a weak ATML1 promoter activity in the inner cells, ATML1 protein was detected mostly in the outermost cells, which suggests that ATML1 accumulation is inhibited in the inner cells. ATML1 nuclear localization was reduced in the epidermis and there was a positive, albeit weak, correlation between the amount of ATML1 in the nuclei and the expression of a direct target of ATML1. Nuclear accumulation of ATML1 was more strongly inhibited in the inner cells than in the outermost cells. Domain deletion analyses revealed that the ZLZ-coding sequence was necessary and partially sufficient for the post-transcriptional repression of ATML1 Our results suggest that post-transcriptional repressions contribute to the restriction of master transcriptional regulator activity in specific cells to enable position-dependent cell differentiation.

Characterization and enzymatic properties of protein kinase ACR4 from Arabidopsis thaliana

Serine/threonine-protein kinase-like protein ARABIDOPSIS CRINKLY4 (ACR4), a transmembrane protein of Arabidopsis thaliana, plays important roles in cell division and differentiation. Although accumulating studies shed light on the function of ACR4, the structure and catalytic mechanism of ACR4 remain to be elucidated. Here, we report the purification and enzymatic properties of the intracellular kinase domain (residues 464-799) of ACR4 (ACR4^IKD). Through Ni-affinity chromatography and gel filter chromatography methods, we successfully obtain high-purity ACR4^IKD protein from Escherichia coli. Dynamic light scattering and gel-filtration methods reveal that ACR4^IKD distributes with high homogeneity and exists as a monomer in solution. In addition, the ACR4^IKD protein has typical kinase activity with myelin basic protein (MBP) as the substrate. Our study may lay the foundation for structure determination of ACR4^IKD and further functional research, for example, screening significant substrates of ACR4 in Arabidopsis thaliana.

Fluctuations of the transcription factor ATML1 generate the pattern of giant cells in the Arabidopsis sepal

Multicellular development produces patterns of specialized cell types. Yet, it is often unclear how individual cells within a field of identical cells initiate the patterning process. Using live imaging, quantitative image analyses and modeling, we show that during Arabidopsis thaliana sepal development, fluctuations in the concentration of the transcription factor ATML1 pattern a field of identical epidermal cells to differentiate into giant cells interspersed between smaller cells. We find that ATML1 is expressed in all epidermal cells. However, its level fluctuates in each of these cells. If ATML1 levels surpass a threshold during the G2 phase of the cell cycle, the cell will likely enter a state of endoreduplication and become giant. Otherwise, the cell divides. Our results demonstrate a fluctuation-driven patterning mechanism for how cell fate decisions can be initiated through a random yet tightly regulated process.

Regulatory function of homeodomain-leucine zipper (HD-ZIP) family proteins during embryogenesis

Homeodomain-leucine zipper proteins (HD-ZIPs) form a plant-specific family of transcription factors functioning as homo- or heterodimers. Certain members of all four classes of this family are involved in embryogenesis, the focus of this review. They support auxin biosynthesis, transport and response, which are in turn essential for the apical-basal patterning of the embryo, radicle formation and outgrowth of the cotyledons. They transcriptionally regulate meristem regulators to maintain the shoot apical meristem once it is initiated. Some members are specific to the protoderm, the outermost layer of the embryo, and play a role in shoot apical meristem function. Within classes, homeodomain-leucine zippers tend to act redundantly during embryo development, and there are many examples of regulation within and between classes of homeodomain-leucine zippers. This indicates a complex network of regulation that awaits future experiments to uncover.

Tissue and Organ Initiation in the Plant Embryo: A First Time for Everything

Land plants can grow to tremendous body sizes, yet even the most complex architectures are the result of iterations of the same developmental processes: organ initiation, growth, and pattern formation. A central question in plant biology is how these processes are regulated and coordinated to allow for the formation of ordered, 3D structures. All these elementary processes first occur in early embryogenesis, during which, from a fertilized egg cell, precursors for all major tissues and stem cells are initiated, followed by tissue growth and patterning. Here we discuss recent progress in our understanding of this phase of plant life. We consider the cellular basis for multicellular development in 3D and focus on the genetic regulatory mechanisms that direct specific steps during early embryogenesis.

CLE peptide signaling and nitrogen interactions in plant root development

The CLAVATA signaling pathway is essential for the regulation of meristem activities in plants. This signaling pathway consists of small signaling peptides of the CLE family interacting with CLAVATA1 and leucine-rich repeat receptor-like kinases (LRR-RLKs). The peptide-receptor relationships determine the specificities of CLE-dependent signals controlling stem cell fate and differentiation that are critical for the establishment and maintenance of shoot and root apical meristems. Plants root systems are highly organized into three-dimensional structures for successful anchoring and uptake of water and mineral nutrients from the soil environment. Recent studies have provided evidence that CLE peptides and CLAVATA signaling pathways play pivotal roles in the regulation of lateral root development and systemic autoregulation of nodulation (AON) integrated with nitrogen (N) signaling mechanisms. Integrations of CLE and N signaling pathways through shoot-root vascular connections suggest that N demand modulates morphological control mechanisms and optimize N uptake as well as symbiotic N fixation in roots.

The growing story of (ARABIDOPSIS) CRINKLY 4

Receptor kinases play important roles in plant growth and development, but only few of them have been functionally characterized in depth. Over the past decade CRINKLY 4 (CR4)-related research has peaked as a result of a newly discovered role of ARABIDOPSIS CR4 (ACR4) in the root. Here, we comprehensively review the available (A)CR4 literature and describe its role in embryo, seed, shoot, and root development, but we also flag an unexpected role in plant defence. In addition, we discuss ACR4 domains and protein structure, describe known ACR4-interacting proteins and substrates, and elaborate on the transcriptional regulation of ACR4 Finally, we address the missing knowledge in our understanding of ACR4 signalling.

CLE peptides and their signaling pathways in plant development

Cell-to-cell communication is crucial for the coherent functioning of multicellular organisms, and they have evolved intricate molecular mechanisms to achieve such communication. Small, secreted peptide hormones participate in cell-to-cell communication to regulate various physiological processes. One such family of plant peptide hormones is the CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION-related (CLE) family, whose members play crucial roles in the differentiation of shoot and root meristems. Recent biochemical and genetic studies have characterized various CLE signaling modules, which include CLE peptides, transmembrane receptors, and downstream intracellular signaling components. CLE signaling systems are conserved across the plant kingdom but have divergent modes of action in various developmental processes in different species. Moreover, several CLE peptides play roles in symbiosis, parasitism, and responses to abiotic cues. Here we review recent studies that have provided new insights into the mechanisms of CLE signaling.

The phenotype of the CRINKLY4 deletion mutant of Physcomitrella patens suggests a broad role in developmental regulation in early land plants

Deletion of the ancestral gene of the land plant multigene family of receptor like kinase CR4 in Physcomitrella patens demonstrates involvement in developmental control of gametophytic and sporophytic organs. The CRINKLY4 (CR4) family of receptor kinases in angiosperms consists of three clades, one including CR4, the CR4-related CCR1 and CCR2, a second including CCR3 and CCR4 family members, and a third and more distant clade. In addition to crinkly leaves in maize, which gave rise to the mutant gene name, CR4 is implicated in ovule, embryo, flower and root development in Arabidopsis thaliana. In root tips of the same species the module including a CLAVATA3/ESR-related protein, an Arabidopsis CR4, a CLAVATA1 and a WUSCHEL-related homeobox 5 (CLE40-ACR4-CLV1-WOX5) is implicated in meristem cell regulation. In embryos and shoots, CR4 acts together with A. thaliana MERISTEM LAYER 1 and PROTODERMAL FACTOR 2 to promote A. thaliana epidermis differentiation. Phylogenetic analysis has demonstrated that early land plants, e.g. mosses carry a single ancestral CR4 gene, together with genes encoding the other members of the CLE40-ACR4-CLV1-WOX5 signaling module. Here we show that CR4 serves as a broad regulator of morphogenesis both in gametophyte phyllids, archegonia and in sporophyte epidermis of the moss Physcomitrella patens. The phenotype of the CR4 deletion mutant in moss provides insight into the role of the ancestral CR4 gene as a regulator of development in early land plants.

PP2A-3 interacts with ACR4 and regulates formative cell division in the Arabidopsis root

In plants, the generation of new cell types and tissues depends on coordinated and oriented formative cell divisions. The plasma membrane-localized receptor kinase ARABIDOPSIS CRINKLY 4 (ACR4) is part of a mechanism controlling formative cell divisions in the Arabidopsis root. Despite its important role in plant development, very little is known about the molecular mechanism with which ACR4 is affiliated and its network of interactions. Here, we used various complementary proteomic approaches to identify ACR4-interacting protein candidates that are likely regulators of formative cell divisions and that could pave the way to unraveling the molecular basis behind ACR4-mediated signaling. We identified PROTEIN PHOSPHATASE 2A-3 (PP2A-3), a catalytic subunit of PP2A holoenzymes, as a previously unidentified regulator of formative cell divisions and as one of the first described substrates of ACR4. Our in vitro data argue for the existence of a tight posttranslational regulation in the associated biochemical network through reciprocal regulation between ACR4 and PP2A-3 at the phosphorylation level.

A phylogenetic approach to study the origin and evolution of the CRINKLY4 family

Cell-cell communication plays a crucial role in plant growth and development and relies to a large extent on peptide ligand-receptor kinase signaling mechanisms. The CRINKLY4 (CR4) family of receptor-like kinases is involved in a wide range of developmental processes in plants, including mediating columella stem cell identity and differentiation in the Arabidopsis thaliana root tip. Members of the CR4 family contain a signal peptide, an extracellular part, a single-pass transmembrane helix and an intracellular cytoplasmic protein kinase domain. The main distinguishing features of the family are the presence of seven "crinkly" repeats and a TUMOR NECROSIS FACTOR RECEPTOR (TNFR)-like domain in the extracellular part. Here, we investigated the evolutionary origin of the CR4 family and explored to what extent members of this family are conserved throughout the green lineage. We identified members of the CR4 family in various dicots and monocots, and also in the lycophyte Selaginella moellendorffii and the bryophyte Physcomitrella patens. In addition, we attempted to gain insight in the evolutionary origin of different CR4-specific domains, and we could detect "crinkly" repeat containing proteins already in single celled algae. Finally, we related the presence of likely functional CR4 orthologs to its best described signaling module comprising CLAVATA3/EMBRYO SURROUNDING REGION-RELATED 40 (CLE40), WUSCHEL RELATED HOMEOBOX 5 (WOX5), CLAVATA 1 (CLV1), and ARABIDOPSIS CR4 (ACR4), and established that this module likely is already present in bryophytes and lycophytes.

ATML1 and PDF2 Play a Redundant and Essential Role in Arabidopsis Embryo Development

The epidermis of shoot organs in plants develops from the outermost layer (L1) of the shoot apical meristem. In Arabidopsis, a pair of homeobox genes, ARABIDOPSIS THALIANA MERISTEM LAYER1 (ATML1) and PROTODERMAL FACTOR2 (PDF2), play a role in regulating the expression of L1-specific genes. atml1-1 pdf2-1 double mutants show striking defects in the differentiation of shoot epidermal cells. However, because atml1-1 and pdf2-1 have a T-DNA inserted downstream of the respective homeobox sequences, these alleles may not represent null mutations. Here we characterized additional mutant alleles that have a T-DNA insertion at different positions of each gene. Double mutants of a strong atml1-3 allele with each pdf2 allele were found to cause embryonic arrest at the globular stage. Although with low frequency, all double mutant combinations of a weak atml1-1 allele with each pdf2 allele germinated and showed phenotypes defective in shoot epidermal cell differentiation. We further confirmed that transgenic induction of PDF2 fused to the Drosophila Engrailed repressor domain temporarily interferes with epidermal cell differentiation in the wild-type background. These results indicate that ATML1 and PDF2 act redundantly as a positive regulator of shoot epidermal cell differentiation and at least one copy of these genes is essential for embryo development.

An annotated database of Arabidopsis mutants of acyl lipid metabolism

We have constructed and annotated a web-based database of over 280 Arabidopsis genes that have characterized mutants associated with Arabidopsis acyl lipid metabolism. Mutants have played a fundamental role in gene discovery and in understanding the function of genes involved in plant acyl lipid metabolism. The first mutant in Arabidopsis lipid metabolism (fad4) was described in 1985. Since that time, characterization of mutants in more than 280 genes associated with acyl lipid metabolism has been reported. This review provides a brief background and history on identification of mutants in acyl lipid metabolism, an analysis of the distribution of mutants in different areas of acyl lipid metabolism and presents an annotated database (ARALIPmutantDB) of these mutants. The database provides information on the phenotypes of mutants, pathways and enzymes/proteins associated with the mutants, and allows rapid access via hyperlinks to summaries of information about each mutant and to literature that provides information on the lipid composition of the mutants. In addition, the database of mutants is integrated within the ARALIP plant acyl lipid metabolism website ( http://aralip.plantbiology.msu.edu ) so that information on mutants is displayed on and can be accessed from metabolic pathway maps. Mutants for at least 30% of the genes in the database have multiple names, which have been compiled here to reduce ambiguities in searches for information. The database should also provide a tool for exploring the relationships between mutants in acyl lipid-related genes and their lipid phenotypes and point to opportunities for further research.

Evidence for intermolecular interactions between the intracellular domains of the arabidopsis receptor-like kinase ACR4, its homologs and the Wox5 transcription factor

Arabidopsis CRINKLY4 (ACR4) is a receptor-like kinase (RLK) involved in the global development of the plant. The Arabidopsis genome encodes four homologs of ACR4 that contain sequence similarity and analogous architectural elements to ACR4, termed Arabidopsis CRINKLY4 Related (AtCRRs) proteins. Additionally, a signaling module has been previously proposed including a postulated peptide ligand, CLE40, the ACR4 RLK, and the WOX5 transcription factor that engage in a possible feedback mechanism controlling stem cell differentiation. However, little biochemical evidence is available to ascertain the molecular aspects of receptor heterodimerization and the role of phosphorylation in these interactions. Therefore, we have undertaken an investigation of the in vitro interactions between the intracellular domains (ICD) of ACR4, the CRRs and WOX5. We demonstrate that interaction can occur between ACR4 and all four CRRs in the unphosphorylated state. However, phosphorylation dependency is observed for the interaction between ACR4 and CRR3. Furthermore, sequence analysis of the ACR4 gene family has revealed a conserved ‘KDSAF’ motif that may be involved in protein-protein interactions among the receptor family. We demonstrate that peptides harboring this conserved motif in CRR3 and CRK1are able to bind to the ACR4 kinase domain. Our investigations also indicate that the ACR4 ICD can interact with and phosphorylate the transcription factor WOX5.

Themes and variations in cell type patterning in the plant epidermis

It has recently become evident that plant development, like animal development, has molecular patterning modules that are reused again and again to create different cell type patterns. Here we focus on three of these plant modules: (1) the MYB-bHLH-WD40 protein complex, (2) the transmembrane calpain protease DEFECTIVE KERNEL1 (DEK1), and (3) homeodomain leucine zipper (HD-ZIP) class IV transcription factors acting in concert with SIAMESE-related cyclin-dependent kinase inhibitors. These three modules initiate the patterning of multiple cell types in the plant epidermis: the regular spacing of trichomes (leaf hairs), the stripes of root hairs, diverse pigmentation patterns in petals, the scattering of giant cells, and the files of bulliform cells. Varied combinations of players and additional regulatory inputs partially account for the diversity of patterns that are generated by reusing the same molecular mechanisms.

Arabidopsis DELLA and two HD-ZIP transcription factors regulate GA signaling in the epidermis through the L1 box cis-element

Gibberellins (GAs) are plant hormones that affect plant growth and regulate gene expression differentially across tissues. To study the molecular mechanisms underlying GA signaling in Arabidopsis thaliana, we focused on a GDSL lipase gene (LIP1) induced by GA and repressed by DELLA proteins. LIP1 contains an L1 box promoter sequence, conserved in the promoters of epidermis-specific genes, that is bound by ATML1, an HD-ZIP transcription factor required for epidermis specification. In this study, we demonstrate that LIP1 is specifically expressed in the epidermis and that its L1 box sequence mediates GA-induced transcription. We show that this sequence is overrepresented in the upstream regulatory regions of GA-induced and DELLA-repressed transcriptomes and that blocking GA signaling in the epidermis represses the expression of L1 box-containing genes and negatively affects seed germination. We show that DELLA proteins interact directly with ATML1 and its paralogue PDF2 and that silencing of both HD-ZIP transcription factors inhibits epidermal gene expression and delays germination. Our results indicate that, upon seed imbibition, increased GA levels reduce DELLA protein abundance and release ATML1/PDF2 to activate L1 box gene expression, thus enhancing germination potential.

Bergmann D, De Smet I, Sozzani R, Weijers D, Brady SM. Omics and modelling approaches for understanding regulation of asymmetric cell divisions in arabidopsis and other angiosperm plants

BACKGROUND

Asymmetric cell divisions are formative divisions that generate daughter cells of distinct identity. These divisions are coordinated by either extrinsic ('niche-controlled') or intrinsic regulatory mechanisms and are fundamentally important in plant development.

SCOPE

This review describes how asymmetric cell divisions are regulated during development and in different cell types in both the root and the shoot of plants. It further highlights ways in which omics and modelling approaches have been used to elucidate these regulatory mechanisms. For example, the regulation of embryonic asymmetric divisions is described, including the first divisions of the zygote, formative vascular divisions and divisions that give rise to the root stem cell niche. Asymmetric divisions of the root cortex endodermis initial, pericycle cells that give rise to the lateral root primordium, procambium, cambium and stomatal cells are also discussed. Finally, a perspective is provided regarding the role of other hormones or regulatory molecules in asymmetric divisions, the presence of segregated determinants and the usefulness of modelling approaches in understanding network dynamics within these very special cells.

CONCLUSIONS

Asymmetric cell divisions define plant development. High-throughput genomic and modelling approaches can elucidate their regulation, which in turn could enable the engineering of plant traits such as stomatal density, lateral root development and wood formation.

Transcriptional regulation of cuticle biosynthesis

Plant cuticle is the hydrophobic protection layer that covers aerial plant organs and plays a pivotal role during plant development and interactions of plants with the environment. The mechanical structure and chemical composition of cuticle lipids and other secondary metabolites vary considerably between plant species, and in response to environmental stimuli and stresses. As the cuticle plays an important role in responses of plants to major abiotic stresses such as drought and high salinity, close attention has been paid to molecular processes underlying the stress-induced biosynthesis of cuticle components. This review addresses the genetic networks responsible for cuticle formation and in particular highlights the role of transcription factors that regulate cuticle formation in response to abiotic stresses.

Specification of epidermal cell fate in plant shoots

Land plants have evolved a single layer of epidermal cells, which are characterized by mostly anticlinal cell division patterns, formation of a waterproof coat called cuticle, and unique cell types such as stomatal guard cells and trichomes. The shoot epidermis plays important roles not only to protect plants from dehydration and pathogens but also to ensure their proper organogenesis and growth control. Extensive molecular genetic studies in Arabidopsis and maize have identified a number of genes that are required for epidermal cell differentiation. However, the mechanism that specifies shoot epidermal cell fate during plant organogenesis remains largely unknown. Particularly, little is known regarding positional information that should restrict epidermal cell fate to the outermost cell layer of the developing organs. Recent studies suggested that certain members of the HD-ZIP class IV homeobox genes are possible master regulators of shoot epidermal cell fate. Here, we summarize the roles of the regulatory genes that are involved in epidermal cell fate specification and discuss the possible mechanisms that limit the expression and/or activity of the master transcriptional regulators to the outermost cell layer in plant shoots.

A CLE-WOX signalling module regulates root meristem maintenance and vascular tissue development in rice

CLAVATA3 (CLV3)/ENDOSPERM SURROUNDING REGION (ESR)-related (CLE) proteins belong to a small peptide family conserved in plants. Recent studies in Arabidopsis and rice have revealed a key role for CLEs in mediating cell-cell communication and stem cell maintenance during plant development, but how CLE signalling controls root development in the rice remains largely unknown. Here it is shown that exogenous application of a synthetic dodeca-amino acid peptide corresponding to the CLE motif of the rice FON2-LIKE CLE PROTEIN2 (FCP2p) protein or overexpression of FCP2 terminates root apical meristem (RAM) activity and impairs late metaxylem formation. FCP2p treatment suppresses the expression of the rice QUIESCENT-CENTER-SPECIFIC HOMEOBOX (QHB) gene, a putative orthologue of Arabidopsis WUSCHEL (WUS)-RELATED HOMEOBOX 5 (WOX5) gene, in both quiescent centre and late metaxylem cells; whereas inducible overexpression of QHB reduces the sensitivity of rice to FCP2p treatment. These results together suggest that in rice RAM maintenance and late metaxylem development are probably controlled by the mutual regulation between FCP2 and QHB. Moreover, a cross-species peptide treatment experiment in Arabidopsis implies that FCP2 has both evolutionarily conserved and species-specific roles in root development.

Control of rice embryo development, shoot apical meristem maintenance, and grain yield by a novel cytochrome p450

Angiosperm seeds usually consist of two major parts: the embryo and the endosperm. However, the molecular mechanism(s) underlying embryo and endosperm development remains largely unknown, particularly in rice, the model cereal. Here, we report the identification and functional characterization of the rice GIANT EMBRYO (GE) gene. Mutation of GE resulted in a large embryo in the seed, which was caused by excessive expansion of scutellum cells. Post-embryonic growth of ge seedling was severely inhibited due to defective shoot apical meristem (SAM) maintenance. Map-based cloning revealed that GE encodes a CYP78A subfamily P450 monooxygenase that is localized to the endoplasmic reticulum. GE is expressed predominantly in the scutellar epithelium, the interface region between embryo and endosperm. Overexpression of GE promoted cell proliferation and enhanced rice plant growth and grain yield, but reduced embryo size, suggesting that GE is critical for coordinating rice embryo and endosperm development. Moreover, transgenic Arabidopsis plants overexpressing AtCYP78A10, a GE homolog, also produced bigger seeds, implying a conserved role for the CYP78A subfamily of P450s in regulating seed development. Taken together, our results indicate that GE plays critical roles in regulating embryo development and SAM maintenance.

Moderation of Arabidopsis root stemness by CLAVATA1 and ARABIDOPSIS CRINKLY4 receptor kinase complexes

BACKGROUND

The root system of higher plants originates from the activity of a root meristem, which comprises a group of highly specialized and long-lasting stem cells. Their maintenance and number is controlled by the quiescent center (QC) cells and by feedback signaling from differentiated cells. Root meristems may have evolved from structurally distinct shoot meristems; however, no common player acting in stemness control has been found so far.

RESULTS

We show that CLAVATA1 (CLV1), a key receptor kinase in shoot stemness maintenance, performs a similar but distinct role in root meristems. We report that CLV1 is signaling, activated by the peptide ligand CLAVATA3/EMBRYO SURROUNDING REGION40 (CLE40), together with the receptor kinase ARABIDOPSIS CRINKLY4 (ACR4) to restrict root stemness. Both CLV1 and ACR4 overlap in their expression domains in the distal root meristem and localize to the plasma membrane (PM) and plasmodesmata (PDs), where ACR4 preferentially accumulates. Using multiparameter fluorescence image spectroscopy (MFIS), we show that CLV1 and ACR4 can form homo- and heteromeric complexes that differ in their composition depending on their subcellular localization.

CONCLUSIONS

We hypothesize that these homo- and heteromeric complexes may differentially regulate distal root meristem maintenance. We conclude that essential components of the ancestral shoot stemness regulatory system also act in the root and that the specific interaction of CLV1 with ACR4 serves to moderate and control stemness homeostasis in the root meristem. The structural differences between these two meristem types may have necessitated this recruitment of ACR4 for signaling by CLV1.

Arabidopsis cuticular waxes: advances in synthesis, export and regulation

Cuticular waxes and cutin form the cuticle, a hydrophobic layer covering the aerial surfaces of land plants and acting as a protective barrier against environmental stresses. Very-long-chain fatty acid derived compounds that compose the cuticular waxes are produced in the endoplasmic reticulum of epidermal cells before being exported to the environmental face of the epidermis. Twenty years of genetic studies on Arabidopsis thaliana have led to the molecular characterization of enzymes catalyzing major steps in fatty acid elongation and wax biosynthesis. Although transporters required for wax export from the plasma membrane have been identified, intracellular and extracellular traffic remains largely unknown. In accordance with its major function in producing an active waterproof barrier, wax metabolism is up-regulated at the transcriptional level in response to water deficiency. However its developmental regulation is still poorly described. Here, we discuss the present knowledge of wax functions, biosynthesis and transport as well as the regulation of these processes.

Rice Crinkly4 receptor-like kinase positively regulates culm elongation and amino acid K532 is not essential for its kinase activity

Receptor-like kinases (RLKs) play important roles in multiple aspects of plant growth and development. As a member of the TNFR-like RLK subfamily, rice Crinkly4 (OsCR4) functions mainly in epidermal cell differentiation in many organs. Here we show that in addition to its essential role in epidermal cell differentiation in the palea and lemma, OsCR4 positively regulates rice culm elongation, similar to maize CR4. Although OsCR4 is an active kinase, like CR4 in maize and ACR4 in Arabidopsis, the conserved amino acid K532 in OsCR4 is not essential for its kinase activity in vitro. Whether other conserved amino acids are required for its kinase activity and the relationship between its activity and function in plant development remain to be investigated.

Peptides and receptors controlling root development

The growth of a plant's root system depends on the continued activity of the root meristem, and the generation of new meristems when lateral roots are initiated. Plants have developed intricate signalling systems that employ secreted peptides and plasma membrane-localized receptor kinases for short- and long-range communication. Studies on growth of the vascular system, the generation of lateral roots, the control of cell differentiation in the root meristem and the interaction with invading pathogens or symbionts has unravelled a network of peptides and receptor systems with occasionally shared functions. A common theme is the employment of conserved modules, consisting of a short signalling peptide, a receptor-like kinase and a target transcription factor, that control the fate and proliferation of stem cells during root development. This review intends to give an overview of the recent advances in receptor and peptide ligand-mediated signalling involved in root development.

Endosperm: food for humankind and fodder for scientific discoveries

The endosperm is an essential constituent of seeds in flowering plants. It originates from a fertilization event parallel to the fertilization that gives rise to the embryo. The endosperm nurtures embryo development and, in some species including cereals, stores the seed reserves and represents a major source of food for humankind. Endosperm biology is characterized by specific features, including idiosyncratic cellular controls of cell division and epigenetic controls associated with parental genomic imprinting. This review attempts a comprehensive summary of our current knowledge of endosperm development and highlights recent advances in this field.

Plasmodesmata paradigm shift: regulation from without versus within

Plant cells are surrounded by cellulosic cell walls, creating a potential challenge to resource sharing and information exchange between individual cells. To overcome this, plants have evolved channels called plasmodesmata that provide cytoplasmic continuity between each cell and its immediate neighbors. We first review plasmodesmata basics-their architecture, their origin, the types of cargo they transport, and their molecular components. The bulk of this review discusses the regulation of plasmodesmata formation and function. Historically, plasmodesmata research has focused intensely on uncovering regulatory or structural proteins that reside within or immediately adjacent to plasmodesmata. Recent findings, however, underscore that plasmodesmata are exquisitely sensitive to signals far removed from the plasmodesmal channel itself. Signals originating from molecules and pathways that regulate cellular homeostasis-such as reactive oxygen species, organelle-organelle signaling, and organelle-nucleus signaling-lead to astonishing alterations in gene expression that affect plasmodesmata formation and function.

Differences in protodermal cell wall structure in zygotic and somatic embryos of Daucus carota (L.) cultured on solid and in liquid media

The ultrastructure, cuticle, and distribution of pectic epitopes in outer periclinal walls of protodermal cells of Daucus carota zygotic and somatic embryos from solid and suspension culture were investigated. Lipid substances were present as a continuous layer in zygotic and somatic embryos cultured on solid medium. Somatic embryos from suspension cultures were devoid of cuticle. The ultrastructure of the outer walls of protodermis of embryos was similar in zygotic and somatic embryos from solid culture. Fibrillar material was observed on the surface of somatic embryos. In zygotic embryos, in cotyledons and root pectic epitopes recognised by the antibody JIM5 were observed in all cell walls. In hypocotyls of these embryos, these pectic epitopes were not present in the outer periclinal and anticlinal walls of the protodermis. In somatic embryos from solid media, distribution of pectic epitopes recognised by JIM5 was similar to that described for their zygotic counterparts. In somatic embryos from suspension culture, pectic epitopes recognised by JIM5 were detected in all cell walls. In the cotyledons and hypocotyls, a punctate signal was observed on the outside of the protodermis. Pectic epitopes recognised by JIM7 were present in all cell walls independent of embryo organs. In zygotic embryos, this signal was punctate; in somatic embryos from both cultures, this signal was uniformly distributed. In embryos from suspension cultures, a punctate signal was detected outside the surface of cotyledon and hypocotyl. These data are discussed in light of current models for embryogenesis and the influence of culture conditions on cell wall structure.

A few standing for many: embryo receptor-like kinases

Development of plant embryos is a complex and highly organized process, and experimental evidence indicates that intercellular signaling plays a major role. The recent identification of Receptor-Like Kinases (RLKs) and related Receptor-Like Cytoplasmic Kinases (RLCKs) with specific roles in Arabidopsis thaliana embryo development suggest important functions of intercellular signaling during embryogenesis. Despite the characterization of only a few RLKs and RLCKs with embryonic roles, expression data indicate that many RLKs and RLCKs with either post-embryonic functions or unknown functions are transcribed in Arabidopsis embryos. The functional characterization of a few members of this large kinase family is likely to represent only the tip of the iceberg, and we predict that many RLKs and RLCKs play major roles throughout embryo development.

Asymmetric cell division in land plants and algae: the driving force for differentiation

Asymmetric cell division generates two cells with different fates and has an important role in plant development. It produces distinct cell types and new organs, and maintains stem cell niches. To handle the constraints of having immobile cells, plants possess numerous unique features to obtain asymmetry, such as specific regulators of intrinsic polarity. Although several components have not yet been identified, new findings, together with knowledge from different developmental systems, now allow us to take an important step towards a mechanistic overview of asymmetric cell division in plants and algae. Strikingly, several key regulators are used for different developmental processes, and common mechanisms can be recognized.

Identification of in vitro autophosphorylation sites and effects of phosphorylation on the Arabidopsis CRINKLY4 (ACR4) receptor-like kinase intracellular domain: insights into conformation, oligomerization, and activity

Arabidopsis CRINKLY4 (ACR4) is a receptor-like kinase (RLK) that consists of an extracellular domain and an intracellular domain (ICD) with serine/threonine kinase activity. While genetic and cell biology experiments have demonstrated that ACR4 is important in cell fate specification and overall development of the plant, little is known about the biochemical properties of the kinase domain and the mechanisms that underlie the overall function of the receptor. To complement in planta studies of the function of ACR4, we have expressed the ICD in Escherichia coli as a soluble C-terminal fusion to the N-utilization substance A (NusA) protein, purified the recombinant protein, and characterized the enzymatic and conformational properties. The protein autophosphorylates via an intramolecular mechanism, prefers Mn(2+) over Mg(2+) as the divalent cation, and displays typical Michaelis-Menten kinetics with respect to ATP with an apparent K(m) of 6.67 ± 2.07 μM and a V(max) of 1.83 ± 0.18 nmol min(-1) mg(-1). Autophosphorylation is accompanied by a conformational change as demonstrated by circular dichroism, fluorescence spectroscopy, and limited proteolysis with trypsin. Analysis by nanoliquid chromatography and mass spectrometry revealed 16 confirmed sites of phosphorylation at Ser and Thr residues. Sedimentation velocity and gel filtration experiments indicate that the ICD has a propensity to oligomerize and that this property is lost upon autophosphorylation.

Opportunities and successes in the search for plasmodesmal proteins

The proteinaceous composition of plasmodesmata (PDs) is a puzzle for which pieces have proven particularly difficult to find. This review describes the numerous approaches that have been undertaken in the search for PD-associated proteins and what each has contributed to our understanding of PD structure and function. These approaches include immunolocalisation of known proteins, proteomic characterisation of PD-enriched tissue fractions, high-throughput screens of random cDNAs and mutant screens. In addition to components of the cytoskeleton, novel proteins with predicted or unknown functions have been identified. Many of these have properties that relate to the symplastic and/or apoplastic faces of the plasma membrane. Mutant screens have identified proteins involved in previously unconnected cell pathways such as ROS signalling, implicating ROS in PD formation and regulation. Proteins associated with callose synthesis and degradation have also been identified and characterised, providing considerable weight to the hypothesis that callose deposition around the neck of the PD pore is one mechanism by which the PD aperture is regulated. The techniques described in this review have been developed such that it is to be expected that a considerable number of new PD proteins will be identified in coming years to fill in further detail of the structure and functional mechanisms of these dynamic pores.

Endoreplication controls cell fate maintenance

Cell-fate specification is typically thought to precede and determine cell-cycle regulation during differentiation. Here we show that endoreplication, also known as endoreduplication, a specialized cell-cycle variant often associated with cell differentiation but also frequently occurring in malignant cells, plays a role in maintaining cell fate. For our study we have used Arabidopsis trichomes as a model system and have manipulated endoreplication levels via mutants of cell-cycle regulators and overexpression of cell-cycle inhibitors under a trichome-specific promoter. Strikingly, a reduction of endoreplication resulted in reduced trichome numbers and caused trichomes to lose their identity. Live observations of young Arabidopsis leaves revealed that dedifferentiating trichomes re-entered mitosis and were re-integrated into the epidermal pavement-cell layer, acquiring the typical characteristics of the surrounding epidermal cells. Conversely, when we promoted endoreplication in glabrous patterning mutants, trichome fate could be restored, demonstrating that endoreplication is an important determinant of cell identity. Our data lead to a new model of cell-fate control and tissue integrity during development by revealing a cell-fate quality control system at the tissue level.

Differentiating cells often amplify their nuclear DNA content through a special cell-cycle variant, called endoreplication, in which cell division is skipped. Although this process is widespread from humans to plants, not much is currently known about the biological importance of endoreplication. Moreover, the control of cell-cycle activities has been thought to follow developmental decisions and the adoption of a specific cell fate. Here we have uncovered a previously unrecognized function of endoreplication in maintaining cell identity, presenting a striking example of how cell fate and cell-cycle progression are linked. Using leaf hairs on the reference plant Arabidopsis as a model, we show that compromising endoreplication leads to dedifferentiation of the newly forming leaf hair cell. Live observations of young Arabidopsis leaves revealed that dedifferentiating leaf hairs underwent repeated rounds of cell division and were re-integrated into the epidermal cell layer acquiring the typical characteristics of the surrounding epidermal cells. Conversely, promoting endoreplication in mutants that fail to develop hairs could at least partially restore their differentiation program. With this, our findings also pinpoint an important role of the social context of a cell, revealing a differentiation control system at the tissue level.