The BASL polarity protein controls a MAPK signaling feedback loop in asymmetric cell division

Chemical genetics reveals cross-activation of plant developmental signaling by the immune peptide-receptor pathway

Cells sense and integrate multiple signals to coordinate development and defence. A receptor-kinase signaling pathway for plant stomatal development shares components with the immunity pathway. The mechanism ensuring their signal specificities remains unclear. Using chemical genetics, here we report the identification of a small molecule, kC9, that triggers excessive stomatal differentiation by inhibiting the canonical ERECTA receptor-kinase pathway. kC9 binds to and inhibits the downstream MAP kinase MPK6, perturbing its substrate interaction. Strikingly, activation of immune signaling by a bacterial flagellin peptide nullified kC9's effects on stomatal development. This cross-activation of stomatal development by immune signaling depends on the immune receptor FLS2 and occurs even in the absence of kC9 if the ERECTA-family receptor population becomes suboptimal. Furthermore, proliferating stomatal-lineage cells are vulnerable to the immune signal penetration. Our findings suggest that the signal specificity between development and immunity can be ensured by MAP Kinase homeostasis reflecting the availability of upstream receptors, thereby providing a novel view on signal specificity.

Spatially resolved proteomics of the Arabidopsis stomatal lineage identifies polarity complexes for cell divisions and stomatal pores

Cell polarity is used to guide asymmetric divisions and create morphologically diverse cells. We find that two oppositely oriented cortical polarity domains present during the asymmetric divisions in the Arabidopsis stomatal lineage are reconfigured into polar domains marking ventral (pore-forming) and outward-facing domains of maturing stomatal guard cells. Proteins that define these opposing polarity domains were used as baits in miniTurboID-based proximity labeling. Among differentially enriched proteins, we find kinases, putative microtubule-interacting proteins, and polar SOSEKIs with their effector ANGUSTIFOLIA. Using AI-facilitated protein structure prediction models, we identify potential protein-protein interaction interfaces among them. Functional and localization analyses of the polarity protein OPL2 and its putative interaction partners suggest a positive interaction with mitotic microtubules and a role in cytokinesis. This combination of proteomics and structural modeling with live-cell imaging provides insights into how polarity is rewired in different cell types and cell-cycle stages.

Performance and morphology of several soybean varieties and responses to pests and diseases in South Sulawesi

Soybeans are a commodity that is widely grown by farmers in rainfed rice fields in South Sulawesi. One of the determining factors in increasing soybean productivity in South Sulawesi is the type of variety. The aim of this research was to determine the characteristics, morphology and response to pests and diseases in several soybean varieties planted in rainfed rice fields in South Sulawesi. This research was carried out in Allepolea Village, Maros Regency in 2022 using a Randomized Block Design with 13 treatments and 3 replications. Varieties tested as treatments include: 1) Derap-1, 2) Devon-2, 3) Deja-1, 4) Anjasmoro, 5) Dena-2, 6) Dena-1, 7) Gepak Kuning, 8) Grobogan, 9) Devon-1, 10) Dega-1, 11) Deja-2, 12) Demas-1, and 13) Detap-1. The results showed that of the 13 varieties tested, the highest height was found in Devon-2 (33.67 cm) and Detap-1 (31.67 cm) in the vegetative phase and in the generative phase in Detap-1 (75.53 cm) and Gepak Yellow (74.67 cm). The largest number of branches is in Dena-1 (3.13 branches). The highest nitrogen content was found in Devon-1 (12.64 m2 per g). The largest leaf area was Detap-1 (4.15 cm2) and Gepak Kuning (4.15 cm2). The highest number of stomata was in Dena-1 (42.80 μm) and Deja-1 (44.00 μm). The highest stomata width was found in Gepak Kuning (2.76 μm). The lowest level of leaf damage due to attacks by Valanga sp (Acrididae) occurred in Grobogan (6.89 %) and Dega-1 (7.35 %). The lowest level of pod damage due to Nezara viridula attack was in Devon-2 (3.56 %) and Dena-2 (3.64 %). The lowest level of leaf damage due to Phaedonia inclusa attack occurred in Dega-1 (4.37 %), Dena-2 (4, 12 %), and Grobogan (4.69 %). Seed damage due to Cercospora sp attack was lowest on Dena-2 (0.81 %). The highest seed yield was in Dena-2 (3.78 t ha-1) and the lowest in Anjasmoro (1.93 t ha-1) and Deja-2 (2.02 t ha-1).

Membrane-associated NRPM proteins are novel suppressors of stomatal production in Arabidopsis

In Arabidopsis, stomatal development and patterning require tightly regulated cell division and cell-fate differentiation that are controlled by key transcription factors and signaling molecules. To identify new regulators of stomatal development, we assay the transcriptomes of plants bearing enriched stomatal lineage cells that undergo active division. A member of the novel regulators at the plasma membrane (NRPM) family annotated as hydroxyproline-rich glycoproteins was identified to highly express in stomatal lineage cells. Overexpressing each of the four NRPM genes suppressed stomata formation, while the loss-of-function nrpm triple mutants generated severely overproduced stomata and abnormal patterning, mirroring those of the erecta receptor family and MAPKKK yoda null mutants. Manipulation of the subcellular localization of NRPM1 surprisingly revealed its regulatory roles as a peripheral membrane protein instead of a predicted cell wall protein. Further functional characterization suggests that NRPMs function downstream of the EPF1/2 peptide ligands and upstream of the YODA MAPK pathway. Genetic and cell biological analyses reveal that NRPM may promote the localization and function of the ERECTA receptor proteins at the cell surface. Therefore, we identify NRPM as a new class of signaling molecules at the plasma membrane to regulate many aspects of plant growth and development.

ABA guides stomatal proliferation and patterning through the EPF-SPCH signaling pathway in Arabidopsis thaliana

Adaptation to dehydration stress requires plants to coordinate environmental and endogenous signals to inhibit stomatal proliferation and modulate their patterning. The stress hormone abscisic acid (ABA) induces stomatal closure and restricts stomatal lineage to promote stress tolerance. Here, we report that mutants with reduced ABA levels, xer-1, xer-2 and aba2-2, developed stomatal clusters. Similarly, the ABA signaling mutant snrk2.2/2.3/2.6, which lacks core ABA signaling kinases, also displayed stomatal clusters. Exposure to ABA or inhibition of ABA catabolism rescued the increased stomatal density and spacing defects observed in xer and aba2-2, suggesting that basal ABA is required for correct stomatal density and spacing. xer-1 and aba2-2 displayed reduced expression of EPF1 and EPF2, and enhanced expression of SPCH and MUTE. Furthermore, ABA suppressed elevated SPCH and MUTE expression in epf2-1 and epf1-1, and partially rescued epf2-1 stomatal index and epf1-1 clustering defects. Genetic analysis demonstrated that XER acts upstream of the EPF2-SPCH pathway to suppress stomatal proliferation, and in parallel with EPF1 to ensure correct stomatal spacing. These results show that basal ABA and functional ABA signaling are required to fine-tune stomatal density and patterning.

Rotating Stomata Measurement Based on Anchor-Free Object Detection and Stomata Conductance Calculation

Stomata play an essential role in regulating water and carbon dioxide levels in plant leaves, which is important for photosynthesis. Previous deep learning-based plant stomata detection methods are based on horizontal detection. The detection anchor boxes of deep learning model are horizontal, while the angle of stomata is randomized, so it is not possible to calculate stomata traits directly from the detection anchor boxes. Additional processing of image (e.g., rotating image) is required before detecting stomata and calculating stomata traits. This paper proposes a novel approach, named DeepRSD (deep learning-based rotating stomata detection), for detecting rotating stomata and calculating stomata basic traits at the same time. Simultaneously, the stomata conductance loss function is introduced in the DeepRSD model training, which improves the efficiency of stomata detection and conductance calculation. The experimental results demonstrate that the DeepRSD model reaches 94.3% recognition accuracy for stomata of maize leaf. The proposed method can help researchers conduct large-scale studies on stomata morphology, structure, and stomata conductance models.

Experimental validation of the mechanism of stomatal development diversification

Stomata are the structures responsible for gas exchange in plants. The established framework for stomatal development is based on the model plant Arabidopsis, but diverse patterns of stomatal development have been observed in other plant lineages and species. The molecular mechanisms behind these diversified patterns are still poorly understood. We recently proposed a model for the molecular mechanisms of the diversification of stomatal development based on the genus Callitriche (Plantaginaceae), according to which a temporal shift in the expression of key stomatal transcription factors SPEECHLESS and MUTE leads to changes in the behavior of meristemoids (stomatal precursor cells). In the present study, we genetically manipulated Arabidopsis to test this model. By altering the timing of MUTE expression, we successfully generated Arabidopsis plants with early differentiation or prolonged divisions of meristemoids, as predicted by the model. The epidermal morphology of the generated lines resembled that of species with prolonged or no meristemoid divisions. Thus, the evolutionary process can be reproduced by varying the SPEECHLESS to MUTE transition. We also observed unexpected phenotypes, which indicated the participation of additional factors in the evolution of the patterns observed in nature. This study provides novel experimental insights into the diversification of meristemoid behaviors.

The wheat VQ motif-containing protein TaVQ4-D positively regulates drought tolerance in transgenic plants

VQ motif-containing proteins play important roles in plant abiotic and biotic stresses. In this study, we cloned the VQ protein gene TaVQ4-D that is induced by drought stress. Arabidopsis and wheat plants overexpressing TaVQ4-D showed increased tolerance to drought stress. In contrast, wheat lines in which TaVQ4-D expression had been silenced showed decreased drought tolerance. Under drought stress conditions, the contents of superoxide dismutase and proline increased and the content of malondialdehyde decreased in transgenic wheat plants overexpressing TaVQ4-D compared with the wild type. At the same time, the expression of reactive oxygen species-scavenging-related genes and stress-related genes was up-regulated. However, plants of TaVQ4-D-silenced wheat lines showed decreased activities of antioxidant enzymes and reduced expression of some stress-related and antioxidant-related genes. In addition, the TaVQ4-D protein physically interacts with two mitogen-activated protein kinases (MPK3 and MPK6) and plays a role in plant drought stress as the phosphorylated substrates of MPK3 and MPK6. In summary, the results of our study suggest that TaVQ4-D can positively regulate drought stress tolerance in wheat.

Arabidopsis stomatal lineage cells establish bipolarity and segregate differential signaling capacity to regulate stem cell potential

Cell polarity combined with asymmetric cell divisions (ACDs) generates cellular diversity. In the Arabidopsis stomatal lineage, a single cortical polarity domain marked by BASL orients ACDs and is segregated to the larger daughter to enforce cell fate. We discovered a second, oppositely positioned polarity domain defined by OCTOPUS-LIKE (OPL) proteins, which forms prior to ACD and is segregated to the smaller (meristemoid) daughter. Genetic and misexpression analyses show that OPLs promote meristemoid-amplifying divisions and delay stomatal fate progression. Polarity mediates OPL segregation into meristemoids but is not required for OPL function. OPL localization and activity are largely independent of other stomatal polarity genes and of the brassinosteroid signaling components associated with OPLs in other contexts. While OPLs are unique to seed plants, ectopic expression in the liverwort Marchantia suppressed epidermal fate progression, suggesting that OPLs engage ancient and broadly conserved pathways to regulate cell division and cell fate.

Polarizing to the challenge: New insights into polarity-mediated division orientation in plant development

Land plants depend on oriented cell divisions that specify cell identities and tissue architecture. As such, the initiation and subsequent growth of plant organs require pathways that integrate diverse systemic signals to inform division orientation. Cell polarity is one solution to this challenge, allowing cells to generate internal asymmetry both spontaneously and in response to extrinsic cues. Here, we provide an update on our understanding of how plasma membrane-associated polarity domains control division orientation in plant cells. These cortical polar domains are flexible protein platforms whose positions, dynamics, and recruited effectors can be modulated by varied signals to control cellular behavior. Several recent reviews have explored the formation and maintenance of polar domains during plant development [1-4], so we focus here on substantial advances in our understanding of polarity-mediated division orientation from the last five years to provide a current snapshot of the field and highlight areas for future exploration.

Signaling in plant development and immunity through the lens of the stomata

The proper development and function of stomata - turgor-driven valves for efficient gas-exchange and water control - impact plant survival and productivity. It has become apparent that various receptor kinases regulate stomatal development and immunity. Although stomatal development and immunity occur over different cellular time scales, their signaling components and regulatory modules are strikingly similar, and often shared. In this review, we survey the current knowledge of stomatal development and immunity signaling components, and provide a synthesis and perspectives on the key concepts to further understand the conservation and specificity of these two signaling pathways.

Cortical polarity ensures its own asymmetric inheritance in the stomatal lineage to pattern the leaf surface

Asymmetric cell divisions specify differential cell fates across kingdoms. In metazoans, preferential inheritance of fate determinants into one daughter cell frequently depends on polarity-cytoskeleton interactions. Despite the prevalence of asymmetric divisions throughout plant development, evidence for analogous mechanisms that segregate fate determinants remains elusive. Here, we describe a mechanism in the Arabidopsis leaf epidermis that ensures unequal inheritance of a fate-enforcing polarity domain. By defining a cortical region depleted of stable microtubules, the polarity domain limits possible division orientations. Accordingly, uncoupling the polarity domain from microtubule organization during mitosis leads to aberrant division planes and accompanying cell identity defects. Our data highlight how a common biological module, coupling polarity to fate segregation through the cytoskeleton, can be reconfigured to accommodate unique features of plant development.

Revisiting the role of MAPK signalling pathway in plants and its manipulation for crop improvement

The mitogen-activated protein kinase (MAPK) pathway is an important signalling event associated with every aspect of plant growth, development, yield, abiotic and biotic stress adaptation. Being a central metabolic pathway, it is a vital target for manipulation for crop improvement. In this review, we have summarised recent advancements in understanding involvement of MAPK signalling in modulating abiotic and biotic stress tolerance, architecture and yield of plants. MAPK signalling cross talks with reactive oxygen species (ROS) and abscisic acid (ABA) signalling events in bringing about abiotic stress adaptation in plants. The intricate involvement of MAPK pathway with plant's pathogen defence ability has also been identified. Further, recent research findings point towards participation of MAPK signalling in shaping plant architecture and yield. These make MAPK pathway an important target for crop improvement and we discuss here various strategies to tweak MAPK signalling components for designing future crops with improved physiology and phenotypes.

Extensive embryonic patterning without cellular differentiation primes the plant epidermis for efficient post-embryonic stomatal activities

Plant leaves feature epidermal stomata that are organized in stereotyped patterns. How does the pattern originate? We provide transcriptomic, imaging, and genetic evidence that Arabidopsis embryos engage known stomatal fate and patterning factors to create regularly spaced stomatal precursor cells. Analysis of embryos from 36 plant species indicates that this trait is widespread among angiosperms. Embryonic stomatal patterning in Arabidopsis is established in three stages: first, broad SPEECHLESS (SPCH) expression; second, coalescence of SPCH and its targets into discrete domains; and third, one round of asymmetric division to create stomatal precursors. Lineage progression is then halted until after germination. We show that the embryonic stomatal pattern enables fast stomatal differentiation and photosynthetic activity upon germination, but it also guides the formation of additional stomata as the leaf expands. In addition, key stomatal regulators are prevented from driving the fate transitions they can induce after germination, identifying stage-specific layers of regulation that control lineage progression during embryogenesis.

A HD-ZIP transcription factor specifies fates of multicellular trichomes via dosage-dependent mechanisms in tomato

Hair-like structures are shared by most living organisms. The hairs on plant surfaces, commonly referred to as trichomes, form diverse types to sense and protect against various stresses. However, it is unclear how trichomes differentiate into highly variable forms. Here, we show that a homeodomain leucine zipper (HD-ZIP) transcription factor named Woolly controls the fates of distinct trichomes in tomato via a dosage-dependent mechanism. The autocatalytic reinforcement of Woolly is counteracted by an autoregulatory negative feedback loop, creating a circuit with a high or low Woolly level. This biases the transcriptional activation of separate antagonistic cascades that lead to different trichome types. Our results identify the developmental switch of trichome formation and provide mechanistic insights into the progressive fate specification in plants, as well as a path to enhancing plant stress resistance and the production of beneficial chemicals.

Asymmetric cell division in plant development

Asymmetric cell division (ACD) is a fundamental process that generates new cell types during development in eukaryotic species. In plant development, post-embryonic organogenesis driven by ACD is universal and more important than in animals, in which organ pattern is preset during embryogenesis. Thus, plant development provides a powerful system to study molecular mechanisms underlying ACD. During the past decade, tremendous progress has been made in our understanding of the key components and mechanisms involved in this important process in plants. Here, we present an overview of how ACD is determined and regulated in multiple biological processes in plant development and compare their conservation and specificity among different model cell systems. We also summarize the molecular roles and mechanisms of the phytohormones in the regulation of plant ACD. Finally, we conclude with the overarching paradigms and principles that govern plant ACD and consider how new technologies can be exploited to fill the knowledge gaps and make new advances in the field.

Emerging roles of protein phosphorylation in regulation of stomatal development

Stomata, tiny epidermal spores, control gas exchange between plants and their external environment, thereby playing essential roles in plant development and physiology. Stomatal development requires rapid regulation of components in signaling pathways to respond flexibly to numerous intrinsic and extrinsic signals. In support of this, reversible phosphorylation, which is particularly suitable for rapid signal transduction, has been implicated in this process. This review highlights the current understanding of the essential roles of reversible phosphorylation in the regulation of stomatal development, most of which comes from the dicot Arabidopsis thaliana. Protein phosphorylation tightly controls the activity of SPEECHLESS (SPCH)-SCREAM (SCRM), the stomatal lineage switch, and the activity of several mitogen-activated protein kinases and receptor kinases upstream of SPCH-SCRM, thereby regulating stomatal cell differentiation and patterning. In addition, protein phosphorylation is involved in the establishment of cell polarity during stomatal asymmetric cell division. Finally, cyclin-dependent kinase-mediated protein phosphorylation plays essential roles in cell cycle control during stomatal development.

Conserved signalling components coordinate epidermal patterning and cuticle deposition in barley

Faced with terrestrial threats, land plants seal their aerial surfaces with a lipid-rich cuticle. To breathe, plants interrupt their cuticles with adjustable epidermal pores, called stomata, that regulate gas exchange, and develop other specialised epidermal cells such as defensive hairs. Mechanisms coordinating epidermal features remain poorly understood. Addressing this, we studied two loci whose allelic variation causes both cuticular wax-deficiency and misarranged stomata in barley, identifying the underlying genes, Cer-g/ HvYDA1, encoding a YODA-like (YDA) MAPKKK, and Cer-s/ HvBRX-Solo, encoding a single BREVIS-RADIX (BRX) domain protein. Both genes control cuticular integrity, the spacing and identity of epidermal cells, and barley's distinctive epicuticular wax blooms, as well as stomatal patterning in elevated CO₂ conditions. Genetic analyses revealed epistatic and modifying relationships between HvYDA1 and HvBRX-Solo, intimating that their products participate in interacting pathway(s) linking epidermal patterning with cuticular properties in barley. This may represent a mechanism for coordinating multiple adaptive features of the land plant epidermis in a cultivated cereal.

The functional specificity of ERECTA-family receptors in Arabidopsis stomatal development is ensured by molecular chaperones in the endoplasmic reticulum

Stomata are epidermal pores that control gas exchange between plants and the atmosphere. In Arabidopsis, the ERECTA family (ERECTAf) receptors, including ERECTA, ERECTA-LIKE 1 (ERL1) and ERL2, redundantly play pivotal roles in enforcing the 'one-cell-spacing' rule. Accumulating evidence has demonstrated that the functional specificities of receptors are likely associated with their differential subcellular dynamics. The endoplasmic reticulum (ER)-resident chaperone complex SDF2-ERdj3B-BiP functions in many aspects of plant development. We employed pharmacological treatments combined with cell biological and biochemical approaches to demonstrate that the abundance of ERECTA was reduced in the erdj3b-1 mutant, but the localization and dynamics of ERECTA were not noticeably affected. By contrast, the erdj3b mutation caused the retention of ERL1/ERL2 in the ER. Furthermore, we found that the function of SDF2-ERdj3B-BiP is implicated with the distinct roles of ERECTAf receptors. Our findings establish that the ERECTAf receptor-mediated signaling in stomatal development is ensured by the activities of the ER quality control system, which preferentially maintains the protein abundance of ERECTA and proper subcellular dynamics of ERL1/ERL2, prior to the receptors reaching their destination - the plasma membrane - to execute their functions.

Protein polarization: Spatiotemporal precisions in cell division and differentiation

Specification of cell polarity is vital to normal cell growth, morphogenesis, and function. As other eukaryotes, plants generate cellular polarity that is coordinated with tissue polarity and organ axes. In development, new cell types are generated by stem-cell division and differentiation, a process often involving proteins that are polarized to cortical domains at the plasma membrane. In the past decade, pioneering work using the model plant Arabidopsis identified multiple proteins that are polarized in dividing cells to instruct divisional behaviors and/or specify cell fates. In this review, we use these polarized cell-division regulators as example to summarize key mechanisms underlying protein polarization in plant cells. Recent progress underscores that self-organizing amplification processes are commonly involved in establishing cell polarity, and cellular polarity is influenced by both tissue-level and local mechanochemical cues. In addition, protein polarization during asymmetric cell division shows a distinct feature of temporal control in the stomatal lineage. We further discuss possible coordination between protein polarization and the progression of cell cycle in this developmental context.

OsMPK4 promotes phosphorylation and degradation of IPA1 in response to salt stress to confer salt tolerance in rice

Salt stress adversely affects plant growth, development, and crop yield. Rice (Oryza sativa L.) is one of the most salt-sensitive cereal crops, especially at the early seedling stage. Mitogen-activated protein kinase (MAPK/MPK) cascades have been shown to play critical roles in salt response in Arabidopsis. However, the roles of the MPK cascade signaling in rice salt response and substrates of OsMPK remain largely unknown. Here, we report that the salt-induced OsMPK4-Ideal Plant Architecture 1 (IPA1) signaling pathway regulates the salt tolerance in rice. Under salt stress, OsMPK4 could interact with IPA1 and phosphorylate IPA1 at Thr180, leading to degradation of IPA1. Genetic evidence shows that IPA1 is a negative regulator of salt tolerance in rice, whereas OsMPK4 promotes salt response in an IPA1-dependent manner. Taken together, our results uncover an OsMPK4-IPA1 signal cascade that modulates the salt stress response in rice and sheds new light on the breeding of salt-tolerant rice varieties.

Division site determination during asymmetric cell division in plants

During development, both animals and plants exploit asymmetric cell division (ACD) to increase tissue complexity, a process that usually generates cells dissimilar in size, morphology, and fate. Plants lack the key regulators that control ACD in animals. Instead, plants have evolved two unique cytoskeletal structures to tackle this problem: the preprophase band (PPB) and phragmoplast. The assembly of the PPB and phragmoplast and their contributions to division plane orientation have been extensively studied. However, how the division plane is positioned off the cell center during asymmetric division is poorly understood. Over the past 20 years, emerging evidence points to a critical role for polarly localized membrane proteins in this process. Although many of these proteins are species- or cell type specific, and the molecular mechanism underlying division asymmetry is not fully understood, common features such as morphological changes in cells, cytoskeletal dynamics, and nuclear positioning have been observed. In this review, we provide updates on polarity establishment and nuclear positioning during ACD in plants. Together with previous findings about symmetrically dividing cells and the emerging roles of developmental cues, we aim to offer evolutionary insight into a common framework for asymmetric division-site determination and highlight directions for future work.

Dichotomy of the BSL phosphatase signaling spatially regulates MAPK components in stomatal fate determination

MAPK signaling modules play crucial roles in regulating numerous biological processes in all eukaryotic cells. How MAPK signaling specificity and strength are tightly controlled remains a major challenging question. In Arabidopsis stomatal development, the MAPKK Kinase YODA (YDA) functions at the cell periphery to inhibit stomatal production by activating MAPK 3 and 6 (MPK3/6) that directly phosphorylate stomatal fate-determining transcription factors for degradation in the nucleus. Recently, we demonstrated that BSL1, one of the four BSL protein phosphatases, localizes to the cell cortex to activate YDA, elevating MPK3/6 activity to suppress stomatal formation. Here, we showed that at the plasma membrane, all four members of BSL proteins contribute to the YDA activation. However, in the nucleus, specific BSL members (BSL2, BSL3, and BSU1) directly deactivate MPK6 to counteract the linear MAPK pathway, thereby promoting stomatal formation. Thus, the pivotal MAPK signaling in stomatal fate determination is spatially modulated by a signaling dichotomy of the BSL protein phosphatases in Arabidopsis, providing a prominent example of how MAPK activities are integrated and specified by signaling compartmentalization at the subcellular level.

The wild grass Brachypodium distachyon as a developmental model system

The arrival of cheap and high-throughput sequencing paired with efficient gene editing technologies allows us to use non-traditional model systems and mechanistically approach biological phenomena beyond what was conceivable just a decade ago. Venturing into different model systems enables us to explore for example clade-specific environmental responses to changing climates or the genetics and development of clade-specific organs, tissues and cell types. We-both early career researchers working with the wild grass model Brachypodium distachyon-want to use this review to (1) highlight why we think B. distachyon is a fantastic grass developmental model system, (2) summarize the tools and resources that have enabled discoveries made in B. distachyon, and (3) discuss a handful of developmental biology vignettes made possible by using B. distachyon as a model system. Finally, we want to conclude by (4) relating our personal stories with this emerging model system and (5) share what we think is important to consider before starting work with an emerging model system.

Deceleration of the cell cycle underpins a switch from proliferative to terminal divisions in plant stomatal lineage

Differentiation of specialized cell types requires precise cell-cycle control. Plant stomata are generated through asymmetric divisions of a stem-cell-like precursor followed by a single symmetric division that creates paired guard cells surrounding a pore. The stomatal-lineage-specific transcription factor MUTE terminates the asymmetric divisions and commits to differentiation. However, the role of cell-cycle machineries in this transition remains unknown. We discover that the symmetric division is slower than the asymmetric division in Arabidopsis. We identify a plant-specific cyclin-dependent kinase inhibitor, SIAMESE-RELATED4 (SMR4), as a MUTE-induced molecular brake that decelerates the cell cycle. SMR4 physically and functionally associates with CYCD3;1 and extends the G1 phase of asymmetric divisions. By contrast, SMR4 fails to interact with CYCD5;1, a MUTE-induced G1 cyclin, and permits the symmetric division. Our work unravels a molecular framework of the proliferation-to-differentiation switch within the stomatal lineage and suggests that a timely proliferative cell cycle is critical for stomatal-lineage identity.

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During stomatal differentiation, asymmetric divisions are faster than terminal divisions

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Upon commitment to differentiation, MUTE induces the cell-cycle inhibitor SMR4

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SMR4 decelerates the asymmetric cell division cycle via selective binding to cyclin D

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Regulating duration of the G1 phase is critical for epidermal cell fate specification

MAP kinase cascades in plant development and immune signaling

Mitogen-activated protein kinase (MAPK) cascades are important signaling modules regulating diverse biological processes. During the past 20 years, much progress has been made on the functions of MAPK cascades in plants. This review summarizes the roles of MAPKs, known MAPK substrates, and our current understanding of MAPK cascades in plant development and innate immunity. In addition, recent findings on the molecular links connecting surface receptors to MAPK cascades and the mechanisms underlying MAPK signaling specificity are also discussed.

Misregulation of MYB16 expression causes stomatal cluster formation by disrupting polarity during asymmetric cell divisions

Stomatal pores and the leaf cuticle regulate evaporation from the plant body and balance the tradeoff between photosynthesis and water loss. MYB16, encoding a transcription factor involved in cutin biosynthesis, is expressed in stomatal lineage ground cells, suggesting a link between cutin biosynthesis and stomatal development. Here, we show that the downregulation of MYB16 in meristemoids is directly mediated by the stomatal master transcription factor SPEECHLESS (SPCH) in Arabidopsis thaliana. The suppression of MYB16 before an asymmetric division is crucial for stomatal patterning, as its overexpression or ectopic expression in meristemoids increased stomatal density and resulted in the formation of stomatal clusters, as well as affecting the outer cell wall structure. Expressing a cutinase gene in plants ectopically expressing MYB16 reduced stomatal clustering, suggesting that cutin affects stomatal signaling or the polarity setup in asymmetrically dividing cells. The clustered stomatal phenotype was rescued by overexpressing EPIDERMAL PATTERNING FACTOR2, suggesting that stomatal signaling was still functional in these plants. Growing seedlings ectopically expressing MYB16 on high-percentage agar plates to modulate tensile strength rescued the polarity and stomatal cluster defects of these seedlings. Therefore, the inhibition of MYB16 expression by SPCH in the early stomatal lineage is required to correctly place the polarity protein needed for stomatal patterning during leaf morphogenesis.

Cell biology of the leaf epidermis: Fate specification, morphogenesis, and coordination

As the outermost layer of plants, the epidermis serves as a critical interface between plants and the environment. During leaf development, the differentiation of specialized epidermal cell types, including stomatal guard cells, pavement cells, and trichomes, occurs simultaneously, each providing unique and pivotal functions for plant growth and survival. Decades of molecular-genetic and physiological studies have unraveled key players and hormone signaling specifying epidermal differentiation. However, most studies focus on only one cell type at a time, and how these distinct cell types coordinate as a unit is far from well-comprehended. Here we provide a review on the current knowledge of regulatory mechanisms underpinning the fate specification, differentiation, morphogenesis, and positioning of these specialized cell types. Emphasis is given to their shared developmental origins, fate flexibility, as well as cell cycle and hormonal controls. Furthermore, we discuss computational modeling approaches to integrate how mechanical properties of individual epidermal cell types and entire tissue/organ properties mutually influence each other. We hope to illuminate the underlying mechanisms coordinating the cell differentiation that ultimately generate a functional leaf epidermis.

Pole position: How plant cells polarize along the axes

Having a sense of direction is a fundamental cellular trait that can determine cell shape, division orientation, or function, and ultimately the formation of a functional, multicellular body. Cells acquire and integrate directional information by establishing discrete subcellular domains along an axis with distinct molecular profiles, a process known as cell polarization. Insight into the principles and mechanisms underlying cell polarity has been propelled by decades of extensive research mostly in yeast and animal models. Our understanding of cell polarity establishment in plants, which lack most of the regulatory molecules identified in other eukaryotes, is more limited, but significant progress has been made in recent years. In this review, we explore how plant cells coordinately establish stable polarity axes aligned with the organ axes, highlighting similarities in the molecular logic used to polarize both plant and animal cells. We propose a classification system for plant cell polarity events and nomenclature guidelines. Finally, we provide a deep phylogenetic analysis of polar proteins and discuss the evolution of polarity machineries in plants.

Connected function of PRAF/RLD and GNOM in membrane trafficking controls intrinsic cell polarity in plants

Cell polarity is a fundamental feature underlying cell morphogenesis and organismal development. In the Arabidopsis stomatal lineage, the polarity protein BASL controls stomatal asymmetric cell division. However, the cellular machinery by which this intrinsic polarity site is established remains unknown. Here, we identify the PRAF/RLD proteins as BASL physical partners and mutating four PRAF members leads to defects in BASL polarization. Members of PRAF proteins are polarized in stomatal lineage cells in a BASL-dependent manner. Developmental defects of the praf mutants phenocopy those of the gnom mutants. GNOM is an activator of the conserved Arf GTPases and plays important roles in membrane trafficking. We further find PRAF physically interacts with GNOM in vitro and in vivo. Thus, we propose that the positive feedback of BASL and PRAF at the plasma membrane and the connected function of PRAF and GNOM in endosomal trafficking establish intrinsic cell polarity in the Arabidopsis stomatal lineage.

Using quantitative methods to understand leaf epidermal development

As the interface between plants and the environment, the leaf epidermis provides the first layer of protection against drought, ultraviolet light, and pathogen attack. This cell layer comprises highly coordinated and specialised cells such as stomata, pavement cells and trichomes. While much has been learned from the genetic dissection of stomatal, trichome and pavement cell formation, emerging methods in quantitative measurements that monitor cellular or tissue dynamics will allow us to further investigate cell state transitions and fate determination in leaf epidermal development. In this review, we introduce the formation of epidermal cell types in Arabidopsis and provide examples of quantitative tools to describe phenotypes in leaf research. We further focus on cellular factors involved in triggering cell fates and their quantitative measurements in mechanistic studies and biological patterning. A comprehensive understanding of how a functional leaf epidermis develops will advance the breeding of crops with improved stress tolerance.

Opposite polarity programs regulate asymmetric subsidiary cell divisions in grasses

Grass stomata recruit lateral subsidiary cells (SCs), which are key to the unique stomatal morphology and the efficient plant-atmosphere gas exchange in grasses. Subsidiary mother cells (SMCs) strongly polarise before an asymmetric division forms a SC. Yet apart from a proximal polarity module that includes PANGLOSS1 (PAN1) and guides nuclear migration, little is known regarding the developmental processes that form SCs. Here, we used comparative transcriptomics of developing wild-type and SC-less bdmute leaves in the genetic model grass Brachypodium distachyon to identify novel factors involved in SC formation. This approach revealed BdPOLAR, which forms a novel, distal polarity domain in SMCs that is opposite to the proximal PAN1 domain. Both polarity domains are required for the formative SC division yet exhibit various roles in guiding pre-mitotic nuclear migration and SMC division plane orientation, respectively. Nonetheless, the domains are linked as the proximal domain controls polarisation of the distal domain. In summary, we identified two opposing polarity domains that coordinate the SC division, a process crucial for grass stomatal physiology.

Evolution of polarity protein BASL and the capacity for stomatal lineage asymmetric divisions

Asymmetric and oriented stem cell divisions enable the continued production of patterned tissues. The molecules that guide these divisions include several "polarity proteins" that are localized to discrete plasma membrane domains, are differentially inherited during asymmetric divisions, and whose scaffolding activities can guide division plane orientation and subsequent cell fates. In the stomatal lineages on the surfaces of plant leaves, asymmetric and oriented divisions create distinct cell types in physiologically optimized patterns. The polarity protein BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) is a major regulator of stomatal lineage division and cell fate asymmetries in Arabidopsis, but its role in the stomatal lineages of other plants is unclear. Here, using phylogenetic and functional assays, we demonstrate that BASL is a eudicot-specific polarity protein. Dicot BASL orthologs can polarize in heterologous systems and rescue the Arabidopsis BASL mutant. The more widely distributed BASL-like proteins, although they share BASL's conserved C-terminal domain, are neither polarized nor do they function in asymmetric divisions of the stomatal lineage. Comparison of BASL protein localization and loss of function BASL phenotypes in Arabidopsis and tomato revealed previously unappreciated differences in how asymmetric cell divisions are employed for pattern formation in different species. This multi-species analysis therefore provides insight into the evolution of a unique polarity regulator and into the developmental choices available to cells as they build and pattern tissues.

Establishing asymmetry: stomatal division and differentiation in plants

In the leaf epidermis, stomatal pores allow gas exchange between plants and the environment. The production of stomatal guard cells requires the lineage cells to divide asymmetrically. In this Insight review, we describe an emerging picture of how intrinsic molecules drive stomatal asymmetric cell division in multidimensions, from transcriptional activities in the nucleus to the dynamic assembly of the polarity complex at the cell cortex. Given the significant roles of stomatal activity in plant responses to environmental changes, we incorporate recent advances in external cues feeding into the regulation of core molecular machinery required for stomatal development. The work we discuss here is mainly based on the dicot plant Arabidopsis thaliana with summaries of recent progress in the monocots.

The Arabidopsis stomatal polarity protein BASL mediates distinct processes before and after cell division to coordinate cell size and fate asymmetries

In many land plants, asymmetric cell divisions (ACDs) create and pattern differentiated cell types on the leaf surface. In the Arabidopsis stomatal lineage, BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) regulates division plane placement and cell fate enforcement. Polarized subcellular localization of BASL is initiated before ACD and persists for many hours after the division in one of the two daughters. Untangling the respective contributions of polarized BASL before and after division is essential to gain a better understanding of its roles in regulating stomatal lineage ACDs. Here, we combine quantitative imaging and lineage tracking with genetic tools that provide temporally restricted BASL expression. We find that pre-division BASL is required for division orientation, whereas BASL polarity post-division ensures proper cell fate commitment. These genetic manipulations allowed us to uncouple daughter-cell size asymmetry from polarity crescent inheritance, revealing independent effects of these two asymmetries on subsequent cell behavior. Finally, we show that there is coordination between the division frequencies of sister cells produced by ACDs, and this coupling requires BASL as an effector of peptide signaling.

Stomatal development in the context of epidermal tissues

BACKGROUND

Stomata are adjustable pores on the surface of plant shoots for efficient gas exchange and water control. The presence of stomata is essential for plant growth and survival, and the evolution of stomata is considered as a key developmental innovation of the land plants, allowing colonization on land from aquatic environments some 450 million years ago. In the past two decades, molecular genetic studies using the model plant Arabidopsis thaliana identified key genes and signalling modules that regulate stomatal development: master regulatory transcription factors that orchestrate cell state transitions and peptide-receptor signal transduction pathways, which, together, enforce proper patterning of stomata within the epidermis. Studies in diverse plant species, ranging from bryophytes to angiosperm grasses, have begun to unravel the conservation and uniqueness of the core modules in stomatal development.

SCOPE

Here, I review the mechanisms of stomatal development in the context of epidermal tissue patterning. First, I introduce the core regulatory mechanisms of stomatal patterning and differentiation in the model species A. thaliana. Subsequently, experimental evidence is presented supporting the idea that different cell types within the leaf epidermis, namely stomata, hydathodes pores, pavement cells and trichomes, either share developmental origins or mutually influence each other's gene regulatory circuits during development. Emphasis is placed on extrinsic and intrinsic signals regulating the balance between stomata and pavement cells, specifically by controlling the fate of stomatal-lineage ground cells (SLGCs) to remain within the stomatal cell lineage or differentiate into pavement cells. Finally, I discuss the influence of intertissue layer communication between the epidermis and underlying mesophyll/vascular tissues on stomatal differentiation. Understanding the dynamic behaviours of stomatal precursor cells and their differentiation in the broader context of tissue and organ development may help design plants tailored for optimal growth and productivity in specific agricultural applications and a changing environment.

HEAT SHOCK PROTEIN 90 proteins and YODA regulate main body axis formation during early embryogenesis

The YODA (YDA) kinase pathway is intimately associated with the control of Arabidopsis (Arabidopsis thaliana) embryo development, but little is known regarding its regulators. Using genetic analysis, HEAT SHOCK PROTEIN 90 (HSP90) proteins emerge as potent regulators of YDA in the process of embryo development and patterning. This study is focused on the characterization and quantification of early embryonal traits of single and double hsp90 and yda mutants. HSP90s genetic interactions with YDA affected the downstream signaling pathway to control the development of both basal and apical cell lineage of embryo. Our results demonstrate that the spatiotemporal expression of WUSCHEL-RELATED HOMEOBOX 8 (WOX8) and WOX2 is changed when function of HSP90s or YDA is impaired, suggesting their essential role in the cell fate determination and possible link to auxin signaling during early embryo development. Hence, HSP90s together with YDA signaling cascade affect transcriptional networks shaping the early embryo development.

Spatiotemporal control of miR398 biogenesis, via chromatin remodeling and kinase signaling, ensures proper ovule development

The coordinated development of sporophytic and gametophytic tissues is essential for proper ovule patterning and fertility. However, the mechanisms regulating their integrated development remain poorly understood. Here, we report that the Swi2/Snf2-Related1 (SWR1) chromatin-remodeling complex acts with the ERECTA receptor kinase-signaling pathway to control female gametophyte and integument growth in Arabidopsis thaliana by inhibiting transcription of the microRNA gene MIR398c in early-stage megagametogenesis. Moreover, pri-miR398c is transcribed in the female gametophyte but is then translocated to and processed in the ovule sporophytic tissues. Together, SWR1 and ERECTA also activate ARGONAUTE10 (AGO10) expression in the chalaza; AGO10 sequesters miR398, thereby ensuring the expression of three AGAMOUS-LIKE (AGL) genes (AGL51, AGL52, and AGL78) in the female gametophyte. In the context of sexual organ morphogenesis, these findings suggest that the spatiotemporal control of miRNA biogenesis, resulting from coordination between chromatin remodeling and cell signaling, is essential for proper ovule development in Arabidopsis.

Chromatin phosphoproteomics unravels a function for AT-hook motif nuclear localized protein AHL13 in PAMP-triggered immunity

In many eukaryotic systems during immune responses, mitogen-activated protein kinases (MAPKs) link cytoplasmic signaling to chromatin events by targeting transcription factors, chromatin remodeling complexes, and the RNA polymerase machinery. So far, knowledge on these events is scarce in plants and no attempts have been made to focus on phosphorylation events of chromatin-associated proteins. Here we carried out chromatin phosphoproteomics upon elicitor-induced activation of Arabidopsis The events in WT were compared with those in mpk3, mpk4, and mpk6 mutant plants to decipher specific MAPK targets. Our study highlights distinct signaling networks involving MPK3, MPK4, and MPK6 in chromatin organization and modification, as well as in RNA transcription and processing. Among the chromatin targets, we characterized the AT-hook motif containing nuclear localized (AHL) DNA-binding protein AHL13 as a substrate of immune MAPKs. AHL13 knockout mutant plants are compromised in pathogen-associated molecular pattern (PAMP)-induced reactive oxygen species production, expression of defense genes, and PAMP-triggered immunity. Transcriptome analysis revealed that AHL13 regulates key factors of jasmonic acid biosynthesis and signaling and affects immunity toward Pseudomonas syringae and Botrytis cinerea pathogens. Mutational analysis of the phosphorylation sites of AHL13 demonstrated that phosphorylation regulates AHL13 protein stability and thereby its immune functions.

A spatiotemporal molecular switch governs plant asymmetric cell division

Asymmetric cell division (ACD) requires protein polarization in the mother cell to produce daughter cells with distinct identities (cell-fate asymmetry). Here, we define a previously undocumented mechanism for establishing cell-fate asymmetry in Arabidopsis stomatal stem cells. In particular, we show that polarization of the protein phosphatase BSL1 promotes stomatal ACD by establishing kinase-based signalling asymmetry in the two daughter cells. BSL1 polarization in the stomatal ACD mother cell is triggered at the onset of mitosis. Polarized BSL1 is inherited by the differentiating daughter cell, where it suppresses cell division and promotes cell-fate determination. Plants lacking BSL proteins exhibit stomatal overproliferation, which demonstrates that the BSL family plays an essential role in stomatal development. Our findings establish that BSL1 polarization provides a spatiotemporal molecular switch that enables cell-fate asymmetry in stomatal ACD daughter cells. We propose that BSL1 polarization is triggered by an ACD checkpoint in the mother cell that monitors the establishment of division-plane asymmetry.

Transcriptional profiling reveals signatures of latent developmental potential in Arabidopsis stomatal lineage ground cells

In many developmental contexts, cell lineages have variable or flexible potency to self-renew. What drives a cell to exit from a proliferative state and begin differentiation, or to retain the capacity to divide days or years later is not clear. Here we exploit the mixed potential of the stomatal lineage ground cell (SLGC) in the Arabidopsis leaf epidermis as a model to explore how cells might balance potential to differentiate with a reentry into proliferation. By generating transcriptomes of fluorescence-activated cell sorting-isolated populations that combinatorically define SLGCs and integrating these data with other stomatal lineage datasets, we find that SLGCs appear poised between proliferation and endoreduplication. Furthermore, we found the RNA polymerase II-related mediator complex interactor DEK and the transcription factor MYB16 accumulate differentially in the stomatal lineage and influence the extent of cell proliferation during leaf development. These findings suggest that SLGC latent potential is maintained by poising of the cell cycle machinery, as well as general and site-specific gene-expression regulators.

Exocyst subunit Exo70B2 is linked to immune signaling and autophagy

During the immune response, activation of the secretory pathway is key to mounting an effective response, while gauging its output is important to maintain cellular homeostasis. The Exo70 subunit of the exocyst functions as a spatiotemporal regulator by mediating numerous interactions with proteins and lipids. However, a molecular understanding of the exocyst regulation remains challenging. We show that, in Arabidopsis thaliana, Exo70B2 behaves as a bona fide exocyst subunit. Conversely, treatment with the salicylic acid (SA) defence hormone analog benzothiadiazole (BTH), or the immunogenic peptide flg22, induced Exo70B2 transport into the vacuole. We reveal that Exo70B2 interacts with AUTOPHAGY-RELATED PROTEIN 8 (ATG8) via two ATG8-interacting motives (AIMs) and its transport into the vacuole is dependent on autophagy. In line with its role in immunity, we discovered that Exo70B2 interacted with and was phosphorylated by the kinase MPK3. Mimicking phosphorylation had a dual impact on Exo70B2: first, by inhibiting localization at sites of active secretion, and second, it increased the interaction with ATG8. Phosphonull variants displayed higher effector-triggered immunity (ETI) and were hypersensitive to BTH, which induce secretion and autophagy. Our results suggest a molecular mechanism by which phosphorylation diverts Exo70B2 from the secretory into the autophagy pathway for its degradation, to dampen secretory activity.

Shouting out loud: signaling modules in the regulation of stomatal development

Stomata are small pores on the surface of land plants that facilitate gas exchange for photosynthesis while minimizing water loss. The function of stomata is pivotal for plant growth and survival. Intensive research on the model plant Arabidopsis (Arabidopsis thaliana) has discovered key peptide signaling pathways, transcription factors, and polarity components that together drive proper stomatal development and patterning. In this review, we focus on recent findings that have revealed co-option of peptide-receptor kinase signaling modules-utilized for diverse developmental processes and immune response. We further discuss an emerging connection between extrinsic signaling and intrinsic polarity modules. These findings have further enlightened our understanding of this fascinating developmental process.

Quantitative and dynamic cell polarity tracking in plant cells

Quantitative information on the spatiotemporal distribution of polarised proteins is central for understanding cell-fate determination, yet collecting sufficient data for statistical analysis is difficult to accomplish with manual measurements. Here we present Polarity Measurement (Pome), a semi-automated pipeline for the quantification of cell polarity and demonstrate its application to a variety of developmental contexts. Pome analysis reveals that, during asymmetric cell divisions in the Arabidopsis thaliana stomatal lineage, polarity proteins BASL and BRXL2 are more asynchronous and less mutually dependent than previously thought. A similar analysis of the linearly arrayed stomatal lineage of Brachypodium distachyon revealed that the MAPKKK BdYDA1 is segregated and polarised following asymmetrical divisions. Our results demonstrate that Pome is a versatile tool, which by itself or combined with tissue-level studies and advanced microscopy techniques can help to uncover new mechanisms of cell polarity.

RNA polymerase II associated proteins regulate stomatal development through direct interaction with stomatal transcription factors in Arabidopsis thaliana

RNA polymerase II (Pol II) associated proteins (RPAPs) have been ascribed diverse functions at the cellular level; however, their roles in developmental processes in yeasts, animals and plants are very poorly understood. Through screening for interactors of NRPB3, which encodes the third largest subunit of Pol II, we identified RIMA, the orthologue of mammalian RPAP2. A combination of genetic and biochemical assays revealed the role of RIMA and other RPAPs in stomatal development in Arabidopsis thaliana. We show that RIMA is involved in nuclear import of NRPB3 and other Pol II subunits, and is essential for restraining division and for establishing cell identity in the stomatal cell lineage. Moreover, plant RPAPs IYO/RPAP1 and QQT1/RPAP4, which interact with RIMA, are also crucial for stomatal development. Importantly, RIMA and QQT1 bind physically to stomatal transcription factors SPEECHLESS, MUTE, FAMA and SCREAMs. The RIMA-QQT1-IYO complex could work together with key stomatal transcription factors and Pol II to drive cell fate transitions in the stomatal cell lineage. Direct interactions with stomatal transcription factors provide a novel mechanism by which RPAP proteins may control differentiation of cell types and tissues in eukaryotes.

Phytohormones and their crosstalk in regulating stomatal development and patterning

Phytohormones play important roles in regulating various aspects of plant growth and development as well as in biotic and abiotic stress responses. Stomata are openings on the surface of land plants that control gas exchange with the environment. Accumulating evidence shows that various phytohormones, including abscisic acid, jasmonic acid, brassinosteroids, auxin, cytokinin, ethylene, and gibberellic acid, play many roles in the regulation of stomatal development and patterning, and that the cotyledons/leaves and hypocotyls/stems of Arabidopsis exhibit differential responsiveness to phytohormones. In this review, we first discuss the shared regulatory mechanisms controlling stomatal development and patterning in Arabidopsis cotyledons and hypocotyls and those that are distinct. We then summarize current knowledge of how distinct hormonal signaling circuits are integrated into the core stomatal development pathways and how different phytohormones crosstalk to tailor stomatal density and spacing patterns. Knowledge obtained from Arabidopsis may pave the way for future research to elucidate the effects of phytohormones in regulating stomatal development and patterning in cereal grasses for the purpose of increasing crop adaptive responses.

Tuning self-renewal in the Arabidopsis stomatal lineage by hormone and nutrient regulation of asymmetric cell division

Asymmetric and self-renewing divisions build and pattern tissues. In the Arabidopsis stomatal lineage, asymmetric cell divisions, guided by polarly localized cortical proteins, generate most cells on the leaf surface. Systemic and environmental signals modify tissue development, but the mechanisms by which plants incorporate such cues to regulate asymmetric divisions are elusive. In a screen for modulators of cell polarity, we identified CONSTITUTIVE TRIPLE RESPONSE1, a negative regulator of ethylene signaling. We subsequently revealed antagonistic impacts of ethylene and glucose signaling on the self-renewing capacity of stomatal lineage stem cells. Quantitative analysis of cell polarity and fate dynamics showed that developmental information may be encoded in both the spatial and temporal asymmetries of polarity proteins. These results provide a framework for a mechanistic understanding of how nutritional status and environmental factors tune stem-cell behavior in the stomatal lineage, ultimately enabling flexibility in leaf size and cell-type composition.

SPEECHLESS and MUTE Mediate Feedback Regulation of Signal Transduction during Stomatal Development

Stomatal density, spacing, and patterning greatly influence the efficiency of gas exchange, photosynthesis, and water economy. They are regulated by a complex of extracellular and intracellular factors through the signaling pathways. After binding the extracellular epidermal patterning factor 1 (EPF1) and 2 (EPF2) as ligands, the receptor-ligand complexes activate by phosphorylation through the MAP-kinase cascades, regulating basic helix-loop-helix (bHLH) transcription factors SPEECHLESS (SPCH), MUTE, and FAMA. In this review, we summarize the molecular mechanisms and signal transduction pathways running within the transition of the protodermal cell into a pair of guard cells with a space (aperture) between them, called a stoma, comprising asymmetric and symmetric cell divisions and draw several functional models. The feedback mechanisms involving the bHLH factors SPCH and MUTE are not fully recognized yet. We show the feedback mechanisms driven by SPCH and MUTE in the regulation of EPF2 and the ERECTA family. Intersections of the molecular mechanisms for fate determination of stomatal lineage cells with the role of core cell cycle-related genes and stabilization of SPCH and MUTE are also reported.

Ceramides mediate positional signals in Arabidopsis thaliana protoderm differentiation

The differentiation of distinct cell types in appropriate patterns is a fundamental process in the development of multicellular organisms. In Arabidopsis thaliana, protoderm/epidermis differentiates as a single cell layer at the outermost position. However, little is known about the molecular nature of the positional signals that achieve correct epidermal cell differentiation. Here, we propose that very-long-chain fatty acid-containing ceramides (VLCFA-Cers) mediate positional signals by stimulating the function of ARABIDOPSIS THALIANA MERISTEM LAYER1 (ATML1), a master regulator of protoderm/epidermis differentiation, during lateral root development. We show that VLCFA-Cers, which are synthesized predominantly in the outermost cells, bind to the lipid-binding domain of ATML1. Importantly, this cell type-specific protein-lipid association alters the activity of ATML1 protein and consequently restricts its expression to the protoderm/epidermis through a transcriptional feedback loop. Furthermore, establishment of a compartment, enriched with VLCFA-containing sphingolipids, at the outer lateral membrane facing the external environment may function as a determinant of protodermal cell fate. Taken together, our results indicate that VLCFA-Cers play a pivotal role in directing protoderm/epidermis differentiation by mediating positional signals to ATML1.This article has an associated 'The people behind the papers' interview.

Stomatal Lineage Control by Developmental Program and Environmental Cues

Stomata are micropores that allow plants to breathe and play a critical role in photosynthesis and nutrient uptake by regulating gas exchange and transpiration. Stomatal development, therefore, is optimized for survival and growth of the plant despite variable environmental conditions. Signaling cascades and transcriptional networks that determine the birth, proliferation, and differentiation of a stomate have been identified. These networks ensure proper stomatal patterning, density, and polarity. Environmental cues also influence stomatal development. In this review, we highlight recent findings regarding the developmental program governing cell fate and dynamics of stomatal lineage cells at the cell state- or single-cell level. We also overview the control of stomatal development by environmental cues as well as developmental plasticity associated with stomatal function and physiology. Recent advances in our understanding of stomatal development will provide a route to improving photosynthesis and water-stress resilience of crop plants in the climate change we currently face.