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Stomata exert considerable effects on global carbon and water cycles by mediating gas exchange and water vapour 1 , 2 . Stomatal closure prevents water loss in response to dehydration and limits pathogen entry 3 , 4 . However, prolonged stomatal closure reduces photosynthesis and transpiration and creates aqueous apoplasts that promote colonization by pathogens. How plants dynamically regulate stomatal reopening in a changing climate is unclear. Here we show that the secreted peptides SMALL PHYTOCYTOKINES REGULATING DEFENSE AND WATER LOSS (SCREWs) and the cognate receptor kinase PLANT SCREW UNRESPONSIVE RECEPTOR (NUT) counter-regulate phytohormone abscisic acid (ABA)- and microbe-associated molecular pattern (MAMP)-induced stomatal closure. SCREWs sensed by NUT function as immunomodulatory phytocytokines and recruit SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) co-receptors to relay immune signalling. SCREWs trigger the NUT-dependent phosphorylation of ABA INSENSITIVE 1 (ABI1) and ABI2, which leads to an increase in the activity of ABI phosphatases towards OPEN STOMATA 1 (OST1)—a key kinase that mediates ABA- and MAMP-induced stomatal closure 5 , 6 —and a reduction in the activity of S-type anion channels. After induction by dehydration and pathogen infection, SCREW–NUT signalling promotes apoplastic water loss and disrupts microorganism-rich aqueous habitats to limit pathogen colonization. The SCREW–NUT system is widely distributed across land plants, which suggests that it has an important role in preventing uncontrolled stomatal closure caused by abiotic and biotic stresses to optimize plant fitness. A plant endogenous peptide-receptor signaling pathway termed SCREW–NUT is described; it counteracts microbe-associated molecular pattern (MAMP)- and abscisic acid-induced stomatal closure to regulate the reopening of stomata after biotic and abiotic stresses.

Perception of biotic and abiotic stresses often leads to stomatal closure in plants 1 , 2 . Rapid influx of calcium ions (Ca 2+ ) across the plasma membrane has an important role in this response, but the identity of the Ca 2+ channels involved has remained elusive 3 , 4 . Here we report that the Arabidopsis thaliana Ca 2+ -permeable channel OSCA1.3 controls stomatal closure during immune signalling. OSCA1.3 is rapidly phosphorylated upon perception of pathogen-associated molecular patterns (PAMPs). Biochemical and quantitative phosphoproteomics analyses reveal that the immune receptor-associated cytosolic kinase BIK1 interacts with and phosphorylates the N-terminal cytosolic loop of OSCA1.3 within minutes of treatment with the peptidic PAMP flg22, which is derived from bacterial flagellin. Genetic and electrophysiological data reveal that OSCA1.3 is permeable to Ca 2+ , and that BIK1-mediated phosphorylation on its N terminus increases this channel activity. Notably, OSCA1.3 and its phosphorylation by BIK1 are critical for stomatal closure during immune signalling, and OSCA1.3 does not regulate stomatal closure upon perception of abscisic acid—a plant hormone associated with abiotic stresses. This study thus identifies a plant Ca 2+ channel and its activation mechanisms underlying stomatal closure during immune signalling, and suggests specificity in Ca 2+ influx mechanisms in response to different stresses. A study in Arabidopsis thaliana shows that the immune receptor-associated cytosolic kinase BIK1 phosphorylates OSCA1.3 and identifies OSCA1.3 as the pathogen-responsive Ca 2+ -permeable channel that regulates stomatal closure.

Mammalian peptide hormones propagate extracellular stimuli from sensing tissues to appropriate targets to achieve optimal growth maintenance 1 . In land plants, root-to-shoot signalling is important to prevent water loss by transpiration and to adapt to water-deficient conditions 2 , 3 . The phytohormone abscisic acid has a role in the regulation of stomatal movement to prevent water loss 4 . However, no mobile signalling molecules have yet been identified that can trigger abscisic acid accumulation in leaves. Here we show that the CLAVATA3/EMBRYO-SURROUNDING REGION-RELATED 25 (CLE25) peptide transmits water-deficiency signals through vascular tissues in Arabidopsis , and affects abscisic acid biosynthesis and stomatal control of transpiration in association with BARELY ANY MERISTEM (BAM) receptors in leaves. The CLE25 gene is expressed in vascular tissues and enhanced in roots in response to dehydration stress. The root-derived CLE25 peptide moves from the roots to the leaves, where it induces stomatal closure by modulating abscisic acid accumulation and thereby enhances resistance to dehydration stress. BAM receptors are required for the CLE25 peptide-induced dehydration stress response in leaves, and the CLE25–BAM module therefore probably functions as one of the signalling molecules for long-distance signalling in the dehydration response. In an Arabidopsis model, the CLE25 peptide acts as a root-to-shoot signalling molecule that modulates abscisic acid expression to close stomata and enhance resistance to dehydration.

Stomata regulate gas exchange between plants and atmosphere by integrating opening and closing signals. Stomata open in response to low CO 2 concentrations to maximize photosynthesis in the light; however, the mechanisms that coordinate photosynthesis and stomatal conductance have yet to be identified. Here we identify and characterize CBC1/2 (CONVERGENCE OF BLUE LIGHT (BL) AND CO 2 1/2), two kinases that link BL, a major component of photosynthetically active radiation (PAR), and the signals from low concentrations of CO 2 in guard cells. CBC1/CBC2 redundantly stimulate stomatal opening by inhibition of S-type anion channels in response to both BL and low concentrations of CO 2 . CBC1/CBC2 function in the signaling pathways of phototropins and HT1 (HIGH LEAF TEMPERATURE 1). CBC1/CBC2 interact with and are phosphorylated by HT1. We propose that CBCs regulate stomatal aperture by integrating signals from BL and CO 2 and act as the convergence site for signals from BL and low CO 2. Stomata open in response to low CO 2 conditions in the light to maximise photosynthesis. Here, Hiyama et al. identify two kinases that promote stomatal opening by inhibiting S-type anion channels downstream of phototropin and HT1 thereby acting as a convergence point for blue light and CO 2 signaling.

Developmental outcomes are shaped by the interplay between intrinsic and external factors. The production of stomata—essential pores for gas exchange in plants—is extremely plastic and offers an excellent system to study this interplay at the cell lineage level. For plants, light is a key external cue, and it promotes stomatal development and the accumulation of the master stomatal regulator SPEECHLESS (SPCH). However, how light signals are relayed to influence SPCH remains unknown. Here, we show that the light-regulated transcription factor ELONGATED HYPOCOTYL 5 (HY5), a critical regulator for photomorphogenic growth, is present in inner mesophyll cells and directly binds and activates STOMAGEN . STOMAGEN, the mesophyll-derived secreted peptide, in turn stabilizes SPCH in the epidermis, leading to enhanced stomatal production. Our work identifies a molecular link between light signaling and stomatal development that spans two tissue layers and highlights how an environmental signaling factor may coordinate growth across tissue types. Light promotes stomatal development in plants. Here Wang et al . show that light stimulates stomatal development via the HY5 transcription factor which induces expression of STOMAGEN, a mesophyll-derived secreted peptide, that in turn leads to stabilization of a master regulator of stomatal development in the epidermis.

Present-day forests use water more efficiently, probably owing to the effect of increased atmospheric carbon dioxide on leaf stomata, which partially close to maintain a near-constant level of carbon dioxide inside the leaves despite increasing atmospheric levels. Large increase in forest water-use efficiency Theory suggests that rising atmospheric CO 2 concentrations should increase the efficiency with which plants use water, but the actual magnitude of this effect in natural forest ecosystems remains unknown. An analysis of long-term measurements of carbon and water fluxes from forest research sites across the Northern Hemisphere has identified an unexpectedly large increase in water-use efficiency during the past two decades, coinciding with an increase of atmospheric CO 2 from 350 to 400 parts per million. This trend is often accompanied by concurrent increases in rates of photosynthetic uptake and carbon sequestration. The authors suggest partial closure of stomata — to maintain constant CO 2 concentrations in the plant leaves — as the most likely explanation for the observed trend in water-use efficiency. The results are inconsistent with current theory and terrestrial biosphere models. Terrestrial plants remove CO 2 from the atmosphere through photosynthesis, a process that is accompanied by the loss of water vapour from leaves 1 . The ratio of water loss to carbon gain, or water-use efficiency, is a key characteristic of ecosystem function that is central to the global cycles of water, energy and carbon 2 . Here we analyse direct, long-term measurements of whole-ecosystem carbon and water exchange 3 . We find a substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades. We systematically assess various competing hypotheses to explain this trend, and find that the observed increase is most consistent with a strong CO 2 fertilization effect. The results suggest a partial closure of stomata 1 —small pores on the leaf surface that regulate gas exchange—to maintain a near-constant concentration of CO 2 inside the leaf even under continually increasing atmospheric CO 2 levels. The observed increase in forest water-use efficiency is larger than that predicted by existing theory and 13 terrestrial biosphere models. The increase is associated with trends of increasing ecosystem-level photosynthesis and net carbon uptake, and decreasing evapotranspiration. Our findings suggest a shift in the carbon- and water-based economics of terrestrial vegetation, which may require a reassessment of the role of stomatal control in regulating interactions between forests and climate change, and a re-evaluation of coupled vegetation–climate models.

The global carbon and water cycles are governed by the coupling of CO2 and water vapour exchanges through the leaves of terrestrial plants, controlled by plant adaptations to balance carbon gains and hydraulic risks. We introduce a trait-based optimality theory that unifies the treatment of stomatal responses and biochemical acclimation of plants to environments changing on multiple timescales. Tested with experimental data from 18 species, our model successfully predicts the simultaneous decline in carbon assimilation rate, stomatal conductance and photosynthetic capacity during progressive soil drought. It also correctly predicts the dependencies of gas exchange on atmospheric vapour pressure deficit, temperature and CO2. Model predictions are also consistent with widely observed empirical patterns, such as the distribution of hydraulic strategies. Our unified theory opens new avenues for reliably modelling the interactive effects of drying soil and rising atmospheric CO2 on global photosynthesis and transpiration.Using trait-based optimality theory that unifies stomatal responses and acclimation of plants to changing environments, this study builds a model of the coupling of CO2 and water vapour exchanges through the leaves. This successfully predicts the simultaneous decline in carbon assimilation, stomatal conductance and photosynthetic capacity during progressive droughts.

Stomatal pores allow gas exchange between plant and atmosphere. Stomatal development is regulated by multiple intrinsic developmental and environmental signals. Here, we show that spatially patterned hydrogen peroxide (H 2 O 2 ) plays an essential role in stomatal development. H 2 O 2 is remarkably enriched in meristemoids, which is established by spatial expression patterns of H 2 O 2 -scavenging enzyme CAT2 and APX1 . SPEECHLESS (SPCH), a master regulator of stomatal development, directly binds to the promoters of CAT2 and APX1 to repress their expression in meristemoid cells. Mutations in CAT2 or APX1 result in an increased stomatal index. Ectopic expression of CAT2 driven by SPCH promoter significantly inhibits the stomatal development. Furthermore, H 2 O 2 activates the energy sensor SnRK1 by inducing the nuclear localization of the catalytic α-subunit KIN10, which stabilizes SPCH to promote stomatal development. Overall, these results demonstrate that the spatial pattern of H 2 O 2 in epidermal leaves is critical for the optimal stomatal development in Arabidopsis. Stomatal development is regulated by multiple intrinsic developmental and environmental signals. Here, the authors show that spatially patterned hydrogen peroxide activates the energy sensor SnRK1 to stabilize the SPCH transcription factor and optimize stomatal development in Arabidopsis.

Stomatal cell lineage is an archetypal example of asymmetric cell division (ACD), which is necessary for plant survival 1 – 4 . In Arabidopsis thaliana , the GLYCOGEN SYNTHASE KINASE3 (GSK3)/SHAGGY-like kinase BRASSINOSTEROID INSENSITIVE 2 (BIN2) phosphorylates both the mitogen-activated protein kinase (MAPK) signalling module 5 , 6 and its downstream target, the transcription factor SPEECHLESS (SPCH) 7 , to promote and restrict ACDs, respectively, in the same stomatal lineage cell. However, the mechanisms that balance these mutually exclusive activities remain unclear. Here we identify the plant-specific protein POLAR as a stomatal lineage scaffold for a subset of GSK3-like kinases that confines them to the cytosol and subsequently transiently polarizes them within the cell, together with BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL), before ACD. As a result, MAPK signalling is attenuated, enabling SPCH to drive ACD in the nucleus. Moreover, POLAR turnover requires phosphorylation on specific residues, mediated by GSK3. Our study reveals a mechanism by which the scaffolding protein POLAR ensures GSK3 substrate specificity, and could serve as a paradigm for understanding regulation of GSK3 in plants. POLAR, identified in a survey of the protein interactome of BRASSINOSTEROID INSENSITIVE 2 in Arabidopsis thaliana , has a key role in coordinating cell polarity and enabling asymmetric cell division.

The formation of stomata and leaf mesophyll airspace must be coordinated to establish an efficient and robust network that facilitates gas exchange for photosynthesis, however the mechanism by which this coordinated development occurs remains unclear. Here, we combine microCT and gas exchange analyses with measures of stomatal size and patterning in a range of wild, domesticated and transgenic lines of wheat and Arabidopsis to show that mesophyll airspace formation is linked to stomatal function in both monocots and eudicots. Our results support the hypothesis that gas flux via stomatal pores influences the degree and spatial patterning of mesophyll airspace formation, and indicate that this relationship has been selected for during the evolution of modern wheat. We propose that the coordination of stomata and mesophyll airspace pattern underpins water use efficiency in crops, providing a target for future improvement. Gas exchange for photosynthesis occurs via stomata on the leaf surface and the airspace in the underlying mesophyll tissue. Here, the authors show that stomatal function modulates mesophyll airspace formation and that their coordinated development influences water use efficiency in crops