Re-examining meristems through the lens of evo-devo

The concept of the meristem was introduced in 1858 to characterize multicellular, formative, and proliferative tissues that give rise to the entire plant body, based on observations of vascular plants. Although its original definition did not encompass bryophytes, this concept has been used and continuously refined over the past 165 years to describe the diverse apices of all land plants. Here, we re-examine this matter in light of recent evo-devo research and show that, despite displaying high anatomical diversity, land plant meristems are unified by shared genetic control. We also propose a modular view of meristem function and highlight multiple evolutionary mechanisms that are likely to have contributed to the assembly and diversification of the varied meristems during the course of plant evolution.

The Role of CLE Peptides in the Suppression of Mycorrhizal Colonization of Tomato

Symbioses with beneficial microbes are widespread in plants, but these relationships must balance the energy invested by the plants with the nutrients acquired. Symbiosis with arbuscular mycorrhizal (AM) fungi occurs throughout land plants, but our understanding of the genes and signals that regulate colonization levels is limited, especially in non-legumes. Here, we demonstrate that in tomato, two CLV3/EMBRYO-SURROUNDING REGION (CLE) peptides, SlCLE10 and SlCLE11, act to suppress AM colonization of roots. Mutant studies and overexpression via hairy transformation indicate that SlCLE11 acts locally in the root to limit AM colonization. Indeed, SlCLE11 expression is strongly induced in AM-colonized roots, but SlCLE11 is not required for phosphate suppression of AM colonization. SlCLE11 requires the FIN gene that encodes an enzyme required for CLE peptide arabinosylation to suppress mycorrhizal colonization. However, SlCLE11 suppression of AM does not require two CLE receptors with roles in regulating AM colonization, SlFAB (CLAVATA1 ortholog) or SlCLV2. Indeed, multiple parallel pathways appear to suppress mycorrhizal colonization in tomato, as double mutant studies indicate that SlCLV2 and FIN have an additive influence on mycorrhizal colonization. SlCLE10 appears to play a more minor or redundant role, as cle10 mutants did not influence intraradical AM colonization. However, the fact that cle10 mutants had an elevated number of hyphopodia and that ectopic overexpression of SlCLE10 did suppress mycorrhizal colonization suggests that SlCLE10 may also play a role in suppressing AM colonization. Our findings show that CLE peptides regulate AM colonization in tomato and at least SlCLE11 likely requires arabinosylation for activity.

What a tangled web it weaves: auxin coordination of stem cell maintenance and flower production

Robust agricultural yields require consistent flower production throughout fluctuating environmental conditions. Floral primordia are produced in the inflorescence meristem, which contains a pool of continuously dividing stem cells. Daughter cells of these divisions either retain stem cell identity or are pushed to the SAM periphery, where they become competent to develop into floral primordia after receiving the appropriate signal. Thus, flower production is inherently linked to regulation of the stem cell pool. The plant hormone auxin promotes flower development throughout its early phases and has been shown to interact with the molecular pathways regulating stem cell maintenance. Here, we will summarize how auxin signaling contributes to stem cell maintenance and promotes flower development through the early phases of initiation, outgrowth, and floral fate establishment. Recent advances in this area suggest that auxin may serve as a signal that integrates stem cell maintenance and new flower production.

Complex peptide hormone signaling in plant stem cells

Peptide hormones influence diverse aspects of plant development through highly coordinated cell-cell signaling pathways. Many peptide hormone families play key roles in stem cell maintenance across land plants. In this review, we focus on recent work in two conserved peptide hormone families, CLAVATA3/EMBRYO-SURROUNDING REGION (CLEs) and ROOT MERISTEM GROWTH FACTOR (RGFs), and their roles in regulating plant stem cells. We discuss recent work establishing downstream crosstalk between peptide hormones and other conserved signaling mechanisms in meristem maintenance as well as highlight advances in peptide hormone gene identification that provide important context for CLE/RGF family evolution across diverse plant lineages. CLE and RGF gene families have greatly expanded in angiosperms, contributing to the complex genetic regulation of stem cell homeostasis observed in model systems over the last 30 years. Peptide hormone duplications have resulted in genetic compensation mechanisms that ensure robust development through the function of paralogous genes. Broad conservation of genetic compensation across angiosperms highlights the importance of these mechanisms in developmental signaling and understanding their regulation could inform broader understanding of morphological diversity and evolutionary innovation.

Peptide Effectors in Phytonematode Parasitism and Beyond

Peptide signaling is an emerging paradigm in molecular plant-microbe interactions with vast implications for our understanding of plant-nematode interactions and beyond. Plant-like peptide hormones, first discovered in cyst nematodes, are now recognized as an important class of peptide effectors mediating several different types of pathogenic and symbiotic interactions. Here, we summarize what has been learned about nematode-secreted CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) peptide effectors since the last comprehensive review on this topic a decade ago. We also highlight new discoveries of a diverse array of peptide effectors that go beyond the CLE peptide effector family in not only phytonematodes but in organisms beyond the phylum Nematoda.

Cytokinin-CLAVATA cross-talk is an ancient mechanism regulating shoot meristem homeostasis in land plants

SignificancePlants grow from their tips. The gametophore (shoot-like organ) tip of the moss Physcomitrium patens is a single cell that performs the same functions as those of multicellular flowering plants, producing the cells that make leaves and regenerating new stem cells to maintain the shoot tip. Several pathways, including CLAVATA and cytokinin hormonal signaling, regulate stem cell abundance in flowering plants and in mosses, although the mechanisms whereby these pathways regulate stem cell abundance and their conservation between these plant lineages is poorly understood. Using moss, we investigated how PpCLAVATA and cytokinin signaling interact. Overall, we found evidence that PpCLAVATA and cytokinin signaling interact similarly in moss and flowering plants, despite their distinct anatomies, life cycles, and evolutionary distance.

CLAVATA modulates auxin homeostasis and transport to regulate stem cell identity and plant shape in a moss

The CLAVATA pathway is a key regulator of stem cell function in the multicellular shoot tips of Arabidopsis, where it acts via the WUSCHEL transcription factor to modulate hormone homeostasis. Broad-scale evolutionary comparisons have shown that CLAVATA is a conserved regulator of land plant stem cell function, but CLAVATA acts independently of WUSCHEL-like (WOX) proteins in bryophytes. The relationship between CLAVATA, hormone homeostasis and the evolution of land plant stem cell functions is unknown. Here we show that in the moss, Physcomitrella (Physcomitrium patens), CLAVATA affects stem cell activity by modulating hormone homeostasis. CLAVATA pathway genes are expressed in the tip cells of filamentous tissues, regulating cell identity, filament branching, plant spread and auxin synthesis. The receptor-like kinase PpRPK2 plays the major role, and Pprpk2 mutants have abnormal responses to cytokinin, auxin and auxin transport inhibition, and show reduced expression of PIN auxin transporters. We propose a model whereby PpRPK2 modulates auxin gradients in filaments to determine stem cell identity and overall plant form. Our data indicate that CLAVATA-mediated auxin homeostasis is a fundamental property of plant stem cell function, probably exhibited by the last shared common ancestor of land plants.

PpGRAS12 acts as a positive regulator of meristem formation in Physcomitrium patens

This study focused on the key regulatory function of Physcomitrium patens GRAS12 gene underlying an increasing plant complexity, an important step in plant terrestrialization and the evolutionary history of life. The miR171-GRAS module has been identified as a key player in meristem maintenance in angiosperms. PpGRAS12 is a member of the GRAS family and a validated target for miR171 in Physcomitrium (Physcomitrella) patens. Here we show a regulatory function of miR171 at the gametophytic vegetative growth stage and targeted deletion of the PpGRAS12 gene adversely affects sporophyte production since fewer sporophytes were produced in ΔPpGRAS12 knockout lines compared to wild type moss. Furthermore, highly specific and distinct growth arrests were observed in inducible PpGRAS12 overexpression lines at the protonema stage. Prominent phenotypic aberrations including the formation of multiple apical meristems at the gametophytic vegetative stage in response to elevated PpGRAS12 transcript levels were discovered via scanning electron microscopy. The production of multiple buds in the PpGRAS12 overexpression lines similar to ΔPpCLV1a/1b disruption mutants is accompanied by an upregulation of PpCLE and downregulation of PpCLV1, PpAPB, PpNOG1, PpDEK1, PpRPK2 suggesting that PpGRAS12 acts upstream of these genes and negatively regulates the proposed pathway to specify simplex meristem formation. As CLV signaling pathway components are not present in the chlorophytic or charophytic algae and arose with the earliest land plants, we identified a key regulatory function of PpGRAS12 underlying an increasing plant complexity, an important step in plant terrestrialization and the evolutionary history of life.

Control of Arabidopsis shoot stem cell homeostasis by two antagonistic CLE peptide signalling pathways

Stem cell homeostasis in plant shoot meristems requires tight coordination between stem cell proliferation and cell differentiation. In Arabidopsis, stem cells express the secreted dodecapeptide CLAVATA3 (CLV3), which signals through the leucine-rich repeat (LRR)-receptor kinase CLAVATA1 (CLV1) and related CLV1-family members to downregulate expression of the homeodomain transcription factor WUSCHEL (WUS). WUS protein moves from cells below the stem cell domain to the meristem tip and promotes stem cell identity, together with CLV3 expression, generating a negative feedback loop. How stem cell activity in the meristem centre is coordinated with organ initiation and cell differentiation at the periphery is unknown. We show here that the CLE40 gene, encoding a secreted peptide closely related to CLV3, is expressed in the SAM in differentiating cells in a pattern complementary to that of CLV3. CLE40 promotes WUS expression via BAM1, a CLV1-family receptor, and CLE40 expression is in turn repressed in a WUS-dependent manner. Together, CLE40-BAM1-WUS establish a second negative feedback loop. We propose that stem cell homeostasis is achieved through two intertwined pathways that adjust WUS activity and incorporate information on the size of the stem cell domain, via CLV3-CLV1, and on cell differentiation via CLE40-BAM1.

WOX going on: CLE peptides in plant development

The development of plant tissues requires cell-cell communication facilitated by chemical and peptide hormones, including small signaling peptides in the CLAVATA3/EMBRYO-SURROUNDING REGION (CLE) family. The paradigmatic CLE signaling peptide CLAVATA3 regulates the size of the shoot apical meristem and the expression of the stem cell-promoting WUSCHEL transcription factor through an unknown mechanism. This review discusses recent advances in CLE signaling, showing that CLE pathways are conserved in bryophytes, that CLE peptides in Arabidopsis thaliana regulate stem cell identity and cell division in root tissues, and connections to auxin biosynthesis in regulating flower and leaf development. These advances shed light on potential WUSCHEL family-independent aspects of CLE signaling and the overlap between CLE and auxin signaling.

In da club: the cytoplasmic kinase MAZZA joins CLAVATA signaling and dances with CLV1-like receptors

This article comments on:

Blümke P, Schlegel J, Gonzalez-Ferrer C, Becher S, Pinto K, Monaghan J, Simon R. 2021. Receptor-like cytoplasmickinase MAZZA mediates developmental processes with CLAVATA1 family receptors in Arabidopsis. Journal of Experimental Botany 72, 4853–4870.

Receptor-like cytoplasmic kinase MAZZA mediates developmental processes with CLAVATA1 family receptors in Arabidopsis

The receptor-like kinases (RLKs) CLAVATA1 (CLV1) and BARELY ANY MERISTEMs (BAM1-BAM3) form the CLV1 family (CLV1f), which perceives peptides of the CLV3/EMBRYO SURROUNDING REGION (ESR)-related (CLE) family within various signaling pathways of Arabidopsis thaliana. CLE peptide signaling, which is required for meristem size control, vascular development, and pathogen responses, involves the formation of receptor complexes at the plasma membrane. These complexes comprise RLKs and co-receptors in varying compositions depending on the signaling context, and regulate expression of target genes, such as WUSCHEL (WUS). How the CLE signal is transmitted intracellularly after perception at the plasma membrane is not known in detail. Here, we found that the membrane-associated receptor-like cytoplasmic kinase (RLCK) MAZZA (MAZ) and additional members of the Pti1-like protein family interact in vivo with CLV1f receptors. MAZ, which is widely expressed throughout the plant, localizes to the plasma membrane via post-translational palmitoylation, potentially enabling stimulus-triggered protein re-localization. We identified a role for a CLV1-MAZ signaling module during stomatal and root development, and redundancy could potentially mask other phenotypes of maz mutants. We propose that MAZ, and related RLCKs, mediate CLV1f signaling in a variety of developmental contexts, paving the way towards understanding the intracellular processes after CLE peptide perception.

The role of CLAVATA signalling in the negative regulation of mycorrhizal colonization and nitrogen response of tomato

Plants form mutualistic nutrient-acquiring symbioses with microbes, including arbuscular mycorrhizal fungi. The formation of these symbioses is costly, and plants employ a negative feedback loop termed autoregulation of mycorrhizae (AOM) to limit formation of arbuscular mycorrhizae (AM). We provide evidence for the role of one leucine-rich repeat receptor-like kinase (FAB), a hydroxyproline O-arabinosyltransferase enzyme (FIN), and additional evidence for one receptor-like protein (SlCLV2) in the negative regulation of AM formation in tomato. Reciprocal grafting experiments suggest that the FAB gene acts locally in the root, while the SlCLV2 gene may act in both the root and the shoot. External nutrients including phosphate and nitrate can also strongly suppress AM formation. We found that FAB and FIN are required for nitrate suppression of AM but are not required for the powerful suppression of AM colonization by phosphate. This parallels some of the roles of legume homologues in the autoregulation of the more recently evolved symbioses with nitrogen-fixing bacteria leading to nodulation. This deep homology in the symbiotic role of these genes suggests that in addition to the early signalling events that lead to the establishment of AM and nodulation, the autoregulation pathway might also be considered part of the common symbiotic toolkit that enabled plants to form beneficial symbioses.

WUSCHEL in the shoot apical meristem: old player, new tricks

The maintenance of the stem cell niche in the shoot apical meristem, the structure that generates all of the aerial organs of the plant, relies on a canonical feedback loop between WUSCHEL (WUS) and CLAVATA3 (CLV3). WUS is a homeodomain transcription factor expressed in the organizing centre that moves to the central zone to promote stem cell fate. CLV3 is a peptide whose expression is induced by WUS in the central zone and that can move back to the organizing centre to inhibit WUS expression. Within the past 20 years since the initial formulation of the CLV-WUS feedback loop, the mechanisms of stem cell maintenance have been intensively studied and the function of WUS has been redefined. In this review, we highlight the most recent advances in our comprehension of the molecular mechanisms of WUS function, of its interaction with other transcription factors and hormonal signals, and of its connection to environmental signals. Through this, we will show how WUS can integrate both internal and external cues to adapt meristem function to the plant environment.

An Evolutionarily Conserved Coreceptor Gene Is Essential for CLAVATA Signaling in Marchantia polymorpha

Growth and development of land plants are controlled by CLAVATA3/EMBRYO SURROUNDING REGION-related (CLE) family of peptide hormones. In contrast to the genetic diversity of CLE family in flowering plants, the liverwort Marchantia polymorpha possesses a minimal set of CLE, MpCLE1(TDIF homolog), and MpCLE2 (CLV3 homolog). MpCLE1 and MpCLE2 peptides exert distinct function at the apical meristem of M. polymorpha gametophyte via specific receptors, MpTDIF RECEPTOR (MpTDR) and MpCLAVATA1 (MpCLV1), respectively, both belonging to the subclass XI of leucine-rich repeat receptor-like kinases (LRR-RLKs). Biochemical and genetic studies in Arabidopsis have shown that TDR/PXY family and CLV1/BAM family recognize the CLE peptide ligand in a heterodimeric complex with a member of subclass-II coreceptors. Here we show that three LRR-RLK genes of M. polymorpha are classified into subclass II, representing three distinct subgroups evolutionarily conserved in land plants. To address the involvement of subclass-II coreceptors in M. polymorpha CLE signaling, we performed molecular genetic analysis on one of them, MpCLAVATA3 INSENSITIVE RECEPTOR KINASE (MpCIK). Two knockout alleles for MpCIK formed narrow apical meristems marked by _prom MpYUC2:GUS marker, which were not expanded by MpCLE2 peptide treatment, phenocopying Mpclv1. Loss of sensitivity to MpCLE2 peptide was also observed in gemma cup formation in both Mpclv1 and Mpcik. Biochemical analysis using a Nicotiana benthamiana transient expression system revealed weak association between MpCIK and MpCLV1, as well as MpCIK and MpTDR. While MpCIK may also participate in MpCLE1 signaling, our data show that the conserved CLV3-CLV1-CIK module functions in M. polymorpha, controlling meristem activity for development and organ formation for asexual reproduction.

CLAVATA Signaling Ensures Reproductive Development in Plants across Thermal Environments

The ability to thrive in diverse environments requires that species maintain development and reproduction despite dynamic conditions. Many developmental processes are stabilized through robust signaling pathways that cooperatively ensure proper development.¹ During reproduction, plants like Arabidopsis thaliana continuously generate flowers on growing indeterminate inflorescences.² Flower primordia initiation and outgrowth depends on the hormone auxin and is robust across diverse environments.^3-6 Here, we show that reproductive development under different thermal conditions requires the integration of multiple pathways regulating auxin-dependent flower production. In colder/ambient temperatures, the receptor complex CLAVATA2/CORYNE (CLV2/CRN) is necessary for continuous flower outgrowth during inflorescence development. CLV2/CRN signaling is independent of CLAVATA1 (CLV1)-related receptor signaling but involves the CLAVATA3 INSENSITIVE RECEPTOR KINASE (CIK) family co-receptors, with higher order cik mutant combinations phenocopying clv2/crn flower outgrowth defects. Developing crn inflorescences display reduced auxin signaling, and restoration of auxin biosynthesis is sufficient to restore flower outgrowth in colder and ambient temperatures. In contrast, at higher temperatures, both clv2/crn signaling and heat-induced auxin biosynthesis via YUCCA family genes are synergistically required to maintain flower development. Our work reveals a novel mechanism integrating peptide hormone and auxin signaling in the regulation of flower development across diverse thermal environments.

The Roles of Plant Hormones and Their Interactions with Regulatory Genes in Determining Meristem Activity

Plants, unlike animals, have developed a unique system in which they continue to form organs throughout their entire life cycle, even after embryonic development. This is possible because plants possess a small group of pluripotent stem cells in their meristems. The shoot apical meristem (SAM) plays a key role in forming all of the aerial structures of plants, including floral meristems (FMs). The FMs subsequently give rise to the floral organs containing reproductive structures. Studies in the past few decades have revealed the importance of transcription factors and secreted peptides in meristem activity using the model plant Arabidopsis thaliana. Recent advances in genomic, transcriptomic, imaging, and modeling technologies have allowed us to explore the interplay between transcription factors, secreted peptides, and plant hormones. Two different classes of plant hormones, cytokinins and auxins, and their interaction are particularly important for controlling SAM and FM development. This review focuses on the current issues surrounding the crosstalk between the hormonal and genetic regulatory network during meristem self-renewal and organogenesis.

CLAVATA Was a Genetic Novelty for the Morphological Innovation of 3D Growth in Land Plants

How genes shape diverse plant and animal body forms is a key question in biology. Unlike animal cells, plant cells are confined by rigid cell walls, and cell division plane orientation and growth rather than cell movement determine overall body form. The emergence of plants on land coincided with a new capacity to rotate stem cell divisions through multiple planes, and this enabled three-dimensional (3D) forms to arise from ancestral forms constrained to 2D growth. The genes involved in this evolutionary innovation are largely unknown. The evolution of 3D growth is recapitulated during the development of modern mosses when leafy shoots arise from a filamentous (2D) precursor tissue. Here, we show that a conserved, CLAVATA peptide and receptor-like kinase pathway originated with land plants and orients stem cell division planes during the transition from 2D to 3D growth in a moss, Physcomitrella. We find that this newly identified role for CLAVATA in regulating cell division plane orientation is shared between Physcomitrella and Arabidopsis. We report that roles for CLAVATA in regulating cell proliferation and cell fate are also shared and that CLAVATA-like peptides act via conserved receptor components in Physcomitrella. Our results suggest that CLAVATA was a genetic novelty enabling the morphological innovation of 3D growth in land plants.

The Art of Self-Control - Autoregulation of Plant-Microbe Symbioses

Plants interact with diverse microbes including those that result in nutrient-acquiring symbioses. In order to balance the energy cost with the benefit gained, plants employ a systemic negative feedback loop to control the formation of these symbioses. This is particularly well-understood in nodulation, the symbiosis between legumes and nitrogen-fixing rhizobia, and is known as autoregulation of nodulation (AON). However, much less is understood about the autoregulation of the ancient arbuscular mycorrhizal symbioses that form between Glomeromycota fungi and the majority of land plants. Elegant physiological studies in legumes have indicated there is at least some overlap in the genes and signals that regulate these two symbioses but there are major gaps in our understanding. In this paper we examine the hypothesis that the autoregulation of mycorrhizae (AOM) pathway shares some elements with AON but that there are also some important differences. By reviewing the current knowledge of the AON pathway, we have identified important directions for future AOM studies. We also provide the first genetic evidence that CLV2 (an important element of the AON pathway) influences mycorrhizal development in a non-legume, tomato and review the interaction of the autoregulation pathway with plant hormones and nutrient status. Finally, we discuss whether autoregulation may play a role in the relationships plants form with other microbes.

Perception of root-active CLE peptides requires CORYNE function in the phloem vasculature

Arabidopsis root development is orchestrated by signaling pathways that consist of different CLAVATA3/EMBRYO SURROUNDING REGION (CLE) peptide ligands and their cognate CLAVATA (CLV) and BARELY ANY MERISTEM (BAM) receptors. How and where different CLE peptides trigger specific morphological or physiological changes in the root is poorly understood. Here, we report that the receptor‐like protein CLAVATA 2 (CLV2) and the pseudokinase CORYNE (CRN) are necessary to fully sense root‐active CLE peptides. We uncover BAM3 as the CLE45 receptor in the root and biochemically map its peptide binding surface. In contrast to other plant peptide receptors, we found no evidence that SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) proteins act as co‐receptor kinases in CLE45 perception. CRN stabilizes BAM3 expression and thus is required for BAM3‐mediated CLE45 signaling. Moreover, protophloem‐specific CRN expression complements resistance of the crn mutant to root‐active CLE peptides, suggesting that protophloem is their principal site of action. Our work defines a genetic framework for dissecting CLE peptide signaling and CLV/BAM receptor activation in the root.

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.

Shared and distinct functions of the pseudokinase CORYNE (CRN) in shoot and root stem cell maintenance of Arabidopsis

Stem cell maintenance in plants depends on the activity of small secreted signaling peptides of the CLAVATA3/EMBRYO SURROUNDING REGION (CLE) family, which, in the shoot, act through at least three kinds of receptor complexes, CLAVATA1 (CLV1) homomers, CLAVATA2 (CLV2) / CORYNE (CRN) heteromers, and CLV1/CLV2/CRN multimers. In the root, the CLV2/CRN receptor complexes function in the proximal meristem to transmit signals from the CLE peptide CLE40. While CLV1 consists of an extracellular receptor domain and an intracellular kinase domain, CLV2, a leucine-rich repeat (LRR) receptor-like protein, and CRN, a protein kinase, have to interact to form a receptor-kinase complex. The kinase domain of CRN has been reported to be catalytically inactive, and it is not yet known how the CLV2/CRN complex can relay the perceived signal into the cells, and whether the kinase domain is necessary for signal transduction at all. In this study we show that the kinase domain of CRN is actively involved in CLV3 signal transduction in the shoot apical meristem of Arabidopsis, but it is dispensable for CRN protein function in root meristem maintenance. Hence, we provide an example of a catalytically inactive pseudokinase that is involved in two homologous pathways, but functions in distinctively different ways in each of them.

Arabidopsis CLAVATA1 and CLAVATA2 receptors contribute to Ralstonia solanacearum pathogenicity through a miR169-dependent pathway

Bacterial wilt caused by Ralstonia solanacearum is one of the most destructive bacterial plant diseases. Although many molecular determinants involved in R. solanacearum adaptation to hosts and pathogenesis have been described, host components required for disease establishment remain poorly characterized. Phenotypical analysis of Arabidopsis mutants for leucine-rich repeat (LRR)-receptor-like proteins revealed that mutations in the CLAVATA1 (CLV1) and CLAVATA2 (CLV2) genes confer enhanced disease resistance to bacterial wilt. We further investigated the underlying mechanisms using genetic, transcriptomic and molecular approaches. The enhanced resistance of both clv1 and clv2 mutants to the bacteria did not require the well characterized CLV signalling modules involved in shoot meristem homeostasis, and was conditioned by neither salicylic acid nor ethylene defence-related hormones. Gene expression microarray analysis performed on clv1 and clv2 revealed deregulation of genes encoding nuclear transcription factor Y subunit alpha (NF-YA) transcription factors whose post-transcriptional regulation is known to involve microRNAs from the miR169 family. Both clv mutants showed a defect in miR169 accumulation. Conversely, overexpression of miR169 abrogated the resistance phenotype of clv mutants. We propose that CLV1 and CLV2, two receptors involved in CLV3 perception during plant development, contribute to bacterial wilt through a signalling pathway involving the miR169/NF-YA module.

Expanding the Regulatory Network for Meristem Size in Plants

The remarkable plasticity of post-embryonic plant development is due to groups of stem-cell-containing structures called meristems. In the shoot, meristems continuously produce organs such as leaves, flowers, and stems. Nearly two decades ago the WUSCHEL/CLAVATA (WUS/CLV) negative feedback loop was established as being essential for regulating the size of shoot meristems by maintaining a delicate balance between stem cell proliferation and cell recruitment for the differentiation of lateral primordia. Recent research in various model species (Arabidopsis, tomato, maize, and rice) has led to discoveries of additional components that further refine and improve the current model of meristem regulation, adding new complexity to a vital network for plant growth and productivity.

Beyond the meristems: similarities in the CLAVATA3 and INFLORESCENCE DEFICIENT IN ABSCISSION peptide mediated signalling pathways

Plants form new organs throughout their lives; this requires a balance between cell proliferation and differentiation, and between the generation and loss of organs. To do this, plants must maintain a population of stem cells within the meristems, and at the same time, closely control the identity and position of cells at the meristem boundaries as they differentiate to new leaf or flower primordia. Once developed, organs may need to be shed, either as a controlled developmental decision-such as floral abscission after pollination, or as a response to disease, environmental stress, and predators. Cell wall degradation at specialized abscission zone (AZ) cells needs to occur for this to take place, but since there is little cell rearrangement in plants, cell separation events are also important for plant architecture. In this Opinion paper we discuss the role of two peptide ligand signalling systems that control stem cell homeostasis and cell separation, respectively. We draw parallels between the signalling pathways and explore on the commonalities of the downstream components activated and controlled by the signalling peptides. We provide evidence for AZ cells having a meristem identity and discuss the role of identical KNOTTED-LIKE HOMEOBOX (KNOX) transcription factors in meristem maintenance and abscission. Lastly we explore the evolutionary relationship between the pathways.

Enhanced resistance to soybean cyst nematode Heterodera glycines in transgenic soybean by silencing putative CLE receptors

CLE peptides are small extracellular proteins important in regulating plant meristematic activity through the CLE-receptor kinase-WOX signalling module. Stem cell pools in the SAM (shoot apical meristem), RAM (root apical meristem) and vascular cambium are controlled by CLE signalling pathways. Interestingly, plant-parasitic cyst nematodes secrete CLE-like effector proteins, which act as ligand mimics of plant CLE peptides and are required for successful parasitism. Recently, we demonstrated that Arabidopsis CLE receptors CLAVATA1 (CLV1), the CLAVATA2 (CLV2)/CORYNE (CRN) heterodimer receptor complex and RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2), which transmit the CLV3 signal in the SAM, are required for perception of beet cyst nematode Heterodera schachtii CLEs. Reduction in nematode infection was observed in clv1, clv2, crn, rpk2 and combined double and triple mutants. In an effort to develop nematode resistance in an agriculturally important crop, orthologues of Arabidopsis receptors including CLV1, CLV2, CRN and RPK2 were identified from soybean, a host for the soybean cyst nematode Heterodera glycines. For each of the receptors, there are at least two paralogues in the soybean genome. Localization studies showed that most receptors are expressed in the root, but vary in their level of expression and spatial expression patterns. Expression in nematode-induced feeding cells was also confirmed. In vitro direct binding of the soybean receptors with the HgCLE peptide was analysed. Knock-down of the receptors in soybean hairy roots showed enhanced resistance to SCN. Our findings suggest that targeted disruption of nematode CLE signalling may be a potential means to engineer nematode resistance in crop plants.

Reevaluation of the CLV3-receptor interaction in the shoot apical meristem: dissection of the CLV3 signaling pathway from a direct ligand-binding point of view

The CLAVATA signaling pathway is a key component of the network that controls stem cell renewal and differentiation in Arabidopsis thaliana. CLAVATA3 (CLV3) is a post-translationally arabinosylated secreted peptide signal that regulates WUSHEL (WUS) transcription to affect the balance of stem cell differentiation and proliferation in the shoot apical meristem (SAM). Known membrane-localized receptors involved in the perception of CLV3 signaling include CLV1, the CLV2/CORYNE (CRN) complex and RPK2. The CLV3 peptide can directly bind to CLV1; however, it is unclear whether the CLV3 peptide directly binds to CLV2 or RPK2. In this study, we re-evaluated the direct interaction between CLV3 and its receptors by photoaffinity labeling with photoactivatable arabinosylated CLV3. We showed that CLV2 and RPK2 exhibited no direct binding to the CLV3 peptide. Further analysis showed that the receptor kinase BAM1 directly binds the CLV3 peptide. A loss-of-function clv1 bam1 double mutant exhibited a large number of stem cells that accumulated in the SAM and was insensitive to exogenous treatment with the arabinosylated CLV3 peptide. WUS gene transcripts were up-regulated, and the region of WUS expression was enlarged at the SAM in the clv1 bam1 double mutant. These results indicate that CLV1 and BAM1 are direct receptors that are sufficient to affect the regulatory network controlling stem cell number in the SAM. In contrast, the CLV2/CRN complex and RPK2 are not involved in direct ligand interactions but may act as co-receptors.

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

The shoot apical meristem (SAM) acts as a reservoir for stem cells. The central zone (CZ) harbors stem cells. The stem cell progenitors differentiate in the adjacent peripheral zone and in the rib meristem located just beneath the CZ. The SAM is further divided into distinct clonal layers: the L1 epidermal, L2 sub-epidermal and L3 layers. Collectively, SAMs are complex structures that consist of cells of different clonal origins that are organized into functional domains. By employing fluorescence-activated cell sorting, we have generated gene expression profiles of ten cell populations that belong to different clonal layers as well as domains along the central and peripheral axis. Our work reveals that cells in distinct clonal layers exhibit greater diversity in gene expression and greater transcriptional complexity than clonally related cell types in the central and peripheral axis. Assessment of molecular functions and biological processes reveals that epidermal cells express genes involved in pathogen defense: the L2 layer cells express genes involved in DNA repair pathways and telomere maintenance, and the L3 layers express transcripts involved in ion balance and salt tolerance besides photosynthesis. Strikingly, the stem cell-enriched transcriptome comprises very few hormone-responsive transcripts. In addition to providing insights into the expression profiles of hundreds of transcripts, the data presented here will act as a resource for reverse genetic analysis and will be useful in deciphering molecular pathways involved in cell type specification and their functions.

POLTERGEIST and PLL1 are required for stem cell function with potential roles in cell asymmetry and auxin signaling

Stem cell establishment and maintenance are essential for the largely post-embryonic developmental patterning that occurs in higher plants. Plant embryos establish two stem cell populations at the shoot and root meristems which then function to continuously generate shoot and root tissues, respectively. Research has uncovered entirely separate sets of regulators for the shoot and root stem cells, raising questions about the origin of the later-evolving root meristem and the relationship between the two meristems. We have recently demonstrated that the related Arabidopsis phosphatases POL and PLL1 are essential for both shoot and root stem cell maintenance and that they act in each population by promoting expression of related WOX transcription factors. Furthermore, pol pll1 mutant embryos exhibit defects in key asymmetric divisions in the early embryo. We hypothesize that the primary functions of POL and PLL1 are to establish and/or maintain stem cell polarity. Here we show additional data linking POL/PLL1 function to the proper polarization and localization of auxin signaling during embryogenesis.

A large family of genes that share homology with CLE domain in Arabidopsis and rice

Thirty-one CLAVATA3/ENDOSPERM SURROUNDING REGION (ESR)-related (CLE) proteins are encoded in the Arabidopsis genome, and they are supposed to function as dodecapeptides with two hydroxyproline residues. Twenty-six synthetic CLE peptides, corresponding to the predicted products of the 31 CLE genes, were examined in Arabidopsis and rice. Nineteen CLE peptides induced root meristem consumption, resulting in the short root phenotype in Arabidopsis and rice, whereas no CLE peptides affected the shoot apical meristem in rice. Database searches revealed 47 putative CLE genes in the rice genome. Three of the rice CLE genes, OsCLE502, OsCLE504 and OsCLE506, encode CLE proteins with multiple CLE domains, which are not found in the Arabidopsis genome, and polyproline region was found between these CLE domains. These results indicate conserved and/or diverse CLE functions in each plant species.

The Protein Phosphatases POL and PLL1 are Signaling Intermediates for Multiple Pathways in Arabidopsis

The development of plants from seedlings to mature adult organisms requires the precise maintenance of the stem cell populations at meristems. In Arabidopsis, the CLAVATA (CLV) pathway is vital for homeostatic regulation of stem cell populations at shoot and floral meristems. Two components of this pathway are the protein phosphatase type 2 C proteins, POLTERGEIST (POL) and PLL1, which positively regulate stem cell specification. Recent studies by our laboratory demonstrated that POL and PLL1 are vital not only for meristem regulation but also for other essential developmental processes including those regulating the formation of the central vasculature and the basal half of the embryo. These results suggest that POL and PLL1 are components of multiple pathways, not just the CLV pathway. Furthermore, considering the processes that are disrupted in the pol pll1 double mutants it is possible that POL and PLL1 are required for certain asymmetric cell divisions during development.

The Shoot Apical Meristem Size Regulated by FON4 in Rice

CLAVATA pathway is one of best-characterized signaling pathway involves in the regulation of meristem development in Arabidopsis. Increasing evidence indicated that this pathway also exist in the monocots as well as in the dicots. We have recently identified FON4 in rice as an ortholog of CLV3 in Arabidopsis. FON4 is putative ligand of FON1, which play a role in restricting the meristem size in rice. FON4 and CLV3 are the members of CLE gene family, which encode small functional secreted peptide with a conserved 14-amino acid motif (CLE motif) near or at the C termini.