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This article is a Commentary on Meigas et al. (2024), 243: 513–518. [ABSTRACT FROM AUTHOR]

Summary: Stomatal movement involves ion transport across the plasma membrane (PM) and vacuolar membrane (VM) of guard cells. However, the coupling mechanisms of ion transporters in both membranes and their interplay with Ca2+ and pH changes are largely unclear.Here, we investigated transporter networks in tobacco guard cells and mesophyll cells using multiparametric live‐cell ion imaging and computational simulations.K+ and anion fluxes at both, PM and VM, affected H+ and Ca2+, as changes in extracellular KCl or KNO3 concentrations were accompanied by cytosolic and vacuolar pH shifts and changes in [Ca2+]cyt and the membrane potential. At both membranes, the K+ transporter networks mediated an antiport of K+ and H+. By contrast, net transport of anions was accompanied by parallel H+ transport, with differences in transport capacity for chloride and nitrate. Guard and mesophyll cells exhibited similarities in K+/H+ transport but cell type‐specific differences in [H+]cyt and pH‐dependent [Ca2+]cyt signals. Computational cell biology models explained mechanistically the properties of transporter networks and the coupling of transport across the PM and VM.Our integrated approach indicates fundamental principles of coupled ion transport at membrane sandwiches to control H+/K+ homeostasis and points to transceptor‐like Ca2+/H+‐based ion signaling in plant cells. [ABSTRACT FROM AUTHOR]

This article is an interview with Rob Roelfsema, a plant scientist who specializes in studying ion transport systems in plants, particularly in guard cells that regulate stomatal movements. Roelfsema discusses his interest in plant science, his decision to pursue a career in research, and what motivates him in his work. He also mentions his role models and favorite papers from the journal New Phytologist. Roelfsema's research focuses on understanding how plants adapt to environmental conditions through ion transport systems. He is a University Professor at the Department of Molecular Plant Biology and Biophysics at the University of Würzburg in Germany. [Extracted from the article]

Summary: Guard cells control the opening of stomatal pores in the leaf surface, with the use of a network of protein kinases and phosphatases. Loss of function of the CBL‐interacting protein kinase 23 (CIPK23) was previously shown to decrease the stomatal conductance, but the molecular mechanisms underlying this response still need to be clarified.CIPK23 was specifically expressed in Arabidopsis guard cells, using an estrogen‐inducible system. Stomatal movements were linked to changes in ion channel activity, determined with double‐barreled intracellular electrodes in guard cells and with the two‐electrode voltage clamp technique in Xenopus oocytes.Expression of the phosphomimetic variant CIPK23T190D enhanced stomatal opening, while the natural CIPK23 and a kinase‐inactive CIPK23K60N variant did not affect stomatal movements. Overexpression of CIPK23T190D repressed the activity of S‐type anion channels, while their steady‐state activity was unchanged by CIPK23 and CIPK23K60N.We suggest that CIPK23 enhances the stomatal conductance at favorable growth conditions, via the regulation of several ion transport proteins in guard cells. The inhibition of SLAC1‐type anion channels is an important facet of this response. [ABSTRACT FROM AUTHOR]

Summary: A trait coordination network is constructed through intercorrelations of functional traits, which reflect trait‐based adaptive strategies. However, little is known about how these networks change across spatial scales, and what drivers and mechanisms mediate this change.This study bridges that gap by integrating functional traits related to plant carbon gain and water economy into the coordination network of Siberian elm (Ulmus pumila), a eurybiont that survives along a 3800 km environmental gradient from humid forest to arid desert.Our results demonstrated that both stomatal density and stomatal size reached a physiological threshold at which adjustments in these traits were not sufficient to cope with the increased environmental stress. Network analysis further revealed that the mechanism for overcoming this threshold, the stomatal opening ratio, gratio, was represented by the highest values for centrality across different spatial scales, and therefore mediated the changes in the trait coordination network along environmental gradients. The mediating roles manifested as creating the highest maximum theoretical stomatal conductance (gsmax) but lowest possible gratio for pathogen defense in humid regions, while maintaining the gratio 'sweet spot' (c. 20% in this region) for highest water use efficiency in semihumid regions, and having the lowest gsmax and highest gratio for gas exchange and leaf cooling in arid regions.These results suggested that the stomatal traits related to control of stomatal movement play fundamental roles in balancing gas exchange, leaf cooling, embolism resistance and pathogen defense. These insights will allow more accurate model parameterization for different regions, and therefore better predictions of species' responses to global change. [ABSTRACT FROM AUTHOR]

Summary: Whereas the role of calcium ions (Ca2+) in plant signaling is well studied, the physiological significance of pH‐changes remains largely undefined.Here we developed CapHensor, an optimized dual‐reporter for simultaneous Ca2+ and pH ratio‐imaging and studied signaling events in pollen tubes (PTs), guard cells (GCs), and mesophyll cells (MCs). Monitoring spatio‐temporal relationships between membrane voltage, Ca2+‐ and pH‐dynamics revealed interconnections previously not described.In tobacco PTs, we demonstrated Ca2+‐dynamics lag behind pH‐dynamics during oscillatory growth, and pH correlates more with growth than Ca2+. In GCs, we demonstrated abscisic acid (ABA) to initiate stomatal closure via rapid cytosolic alkalization followed by Ca2+ elevation. Preventing the alkalization blocked GC ABA‐responses and even opened stomata in the presence of ABA, disclosing an important pH‐dependent GC signaling node. In MCs, a flg22‐induced membrane depolarization preceded Ca2+‐increases and cytosolic acidification by c. 2 min, suggesting a Ca2+/pH‐independent early pathogen signaling step. Imaging Ca2+ and pH resolved similar cytosol and nuclear signals and demonstrated flg22, but not ABA and hydrogen peroxide to initiate rapid membrane voltage‐, Ca2+‐ and pH‐responses.We propose close interrelation in Ca2+‐ and pH‐signaling that is cell type‐ and stimulus‐specific and the pH having crucial roles in regulating PT growth and stomata movement. [ABSTRACT FROM AUTHOR]

Summary: Guard cell membrane ion transport and metabolism are deeply interconnected, and their coordinated regulation is integral to stomatal opening. Whereas ion transport is exceptionally well understood, how guard cell metabolism influences stomatal movements is less well known. Organic metabolites, such as malate and sugars, fulfill several functions in guard cells during stomatal opening as allosteric activators, counter‐ions, energy source and osmolytes. However, their origin and exact fate remain debated. Recent work revealed that the guard cell carbon pool regulating stomatal function and plant growth is mostly of mesophyll origin, highlighting a tight correlation between mesophyll and guard cell metabolism. This review discusses latest research into guard cell carbon metabolism and its impact on stomatal function and whole plant physiology. [ABSTRACT FROM AUTHOR]

Summary: Low concentrations of CO2 cause stomatal opening, whereas [CO2] elevation leads to stomatal closure. Classical studies have suggested a role for Ca2+ and protein phosphorylation in CO2‐induced stomatal closing. Calcium‐dependent protein kinases (CPKs) and calcineurin‐B‐like proteins (CBLs) can sense and translate cytosolic elevation of the second messenger Ca2+ into specific phosphorylation events. However, Ca2+‐binding proteins that function in the stomatal CO2 response remain unknown.Time‐resolved stomatal conductance measurements using intact plants, and guard cell patch‐clamp experiments were performed.We isolated cpk quintuple mutants and analyzed stomatal movements in response to CO2, light and abscisic acid (ABA). Interestingly, we found that cpk3/5/6/11/23 quintuple mutant plants, but not other analyzed cpk quadruple/quintuple mutants, were defective in high CO2‐induced stomatal closure and, unexpectedly, also in low CO2‐induced stomatal opening. Furthermore, K+‐uptake‐channel activities were reduced in cpk3/5/6/11/23 quintuple mutants, in correlation with the stomatal opening phenotype. However, light‐mediated stomatal opening remained unaffected, and ABA responses showed slowing in some experiments. By contrast, CO2‐regulated stomatal movement kinetics were not clearly affected in plasma membrane‐targeted cbl1/4/5/8/9 quintuple mutant plants.Our findings describe combinatorial cpk mutants that function in CO2 control of stomatal movements and support the results of classical studies showing a role for Ca2+ in this response. [ABSTRACT FROM AUTHOR]

Summary: The coordinated control of ion transport across the two major membranes of differentiated plant cells, the plasma and the vacuolar membranes, is fundamental in cell physiology. The stomata responses to the fluctuating environmental conditions are an illustrative example. Indeed, they rely on the coordination of ion fluxes between the different cell compartments. The cytosolic environment, which is an interface between intracellular compartments, and the activity of the ion transporters localised in the different membranes influence one each other. Here we analyse the molecular mechanisms connecting and modulating the transport processes at both the plasma and the vacuolar membranes of guard cells. [ABSTRACT FROM AUTHOR]

Summary: When plants emerged from their aquatic origins to colonise land, they needed to avoid desiccation while still enabling gas and water exchange with the environment. The solution was the development of a waxy cuticle interrupted by epidermal pores, known as stomata. Despite the importance of stomata in plant physiology and their contribution to global water and carbon cycles, our knowledge of the genetic basis of stomatal development is limited mostly to the model dicot, Arabidopsis thaliana. This limitation is particularly troublesome when evaluating grasses, whose members represent our most agriculturally significant crops. Grass stomatal development follows a trajectory strikingly different from Arabidopsis and their uniquely shaped four‐celled stomatal complexes are especially responsive to environmental inputs. Thus, understanding the development and regulation of these efficient complexes is of particular interest for the purposes of crop engineering. This review focuses on genetic regulation of grass stomatal development and prospects for the future, highlighting discoveries enabled by parallel comparative investigations in cereal crops and related genetic model species such as Brachypodium distachyon. See also the Commentary on this article by Catalán & Vogel, 227: 1587–1590. [ABSTRACT FROM AUTHOR]

Stomatal movement involves ion transport across the plasma membrane (PM) and vacuolar membrane (VM) of guard cells. However, the coupling mechanisms of ion transporters in both membranes and their interplay with Ca 2+ and pH changes are largely unclear. Here, we investigated transporter networks in tobacco guard cells and mesophyll cells using multiparametric live-cell ion imaging and computational simulations. K + and anion fluxes at both, PM and VM, affected H + and Ca 2+ , as changes in extracellular KCl or KNO 3 concentrations were accompanied by cytosolic and vacuolar pH shifts and changes in [Ca 2+ ] cyt and the membrane potential. At both membranes, the K + transporter networks mediated an antiport of K + and H + . By contrast, net transport of anions was accompanied by parallel H + transport, with differences in transport capacity for chloride and nitrate. Guard and mesophyll cells exhibited similarities in K + /H + transport but cell type-specific differences in [H + ] cyt and pH-dependent [Ca 2+ ] cyt signals. Computational cell biology models explained mechanistically the properties of transporter networks and the coupling of transport across the PM and VM. Our integrated approach indicates fundamental principles of coupled ion transport at membrane sandwiches to control H + /K + homeostasis and points to transceptor-like Ca 2+ /H + -based ion signaling in plant cells., (© 2023 The Authors New Phytologist © 2023 New Phytologist Foundation.)

This article is a Commentary on Cai et al. (2021), 230: 1201–1213. [ABSTRACT FROM AUTHOR]

Contents1018I.1018II.1019III.1022IV.1025V.1026VI.10291030References1030 Summary: Stomata are leaf epidermal structures consisting of two guard cells surrounding a pore. Changes in the aperture of this pore regulate plant water‐use efficiency, defined as gain of C by photosynthesis per leaf water transpired. Stomatal aperture is actively regulated by reversible changes in guard cell osmolyte content. Despite the fact that guard cells can photosynthesize on their own, the accumulation of mesophyll‐derived metabolites can seemingly act as signals which contribute to the regulation of stomatal movement. It has been shown that malate can act as a signalling molecule and a counter‐ion of potassium, a well‐established osmolyte that accumulates in the vacuole of guard cells during stomatal opening. By contrast, their efflux from guard cells is an important mechanism during stomatal closure. It has been hypothesized that the breakdown of starch, sucrose and lipids is an important mechanism during stomatal opening, which may be related to ATP production through glycolysis and mitochondrial metabolism, and/or accumulation of osmolytes such as sugars and malate. However, experimental evidence supporting this theory is lacking. Here we highlight the particularities of guard cell metabolism and discuss this in the context of the guard cells themselves and their interaction with the mesophyll cells. [ABSTRACT FROM AUTHOR]

Date palm Phoenix dactylifera is a desert crop well adapted to survive and produce fruits under extreme drought and heat. How are palms under such harsh environmental conditions able to limit transpirational water loss?, Here, we analysed the cuticular waxes, stomata structure and function, and molecular biology of guard cells from P. dactylifera., To understand the stomatal response to the water stress phytohormone of the desert plant, we cloned the major elements necessary for guard cell fast abscisic acid ( ABA) signalling and reconstituted this ABA signalosome in Xenopus oocytes. The Phoenix SLAC1-type anion channel is regulated by ABA kinase Pd OST1. Energy-dispersive X-ray analysis ( EDXA) demonstrated that date palm guard cells release chloride during stomatal closure. However, in Cl− medium, Pd OST1 did not activate the desert plant anion channel Pd SLAC1 per se. Only when nitrate was present at the extracellular face of the anion channel did the OST1-gated Pd SLAC1 open, thus enabling chloride release. In the presence of nitrate, ABA enhanced and accelerated stomatal closure., Our findings indicate that, in date palm, the guard cell osmotic motor driving stomatal closure uses nitrate as the signal to open the major anion channel SLAC1. This initiates guard cell depolarization and the release of anions together with potassium. [ABSTRACT FROM AUTHOR]

Christoph-Martin holds a doctoral degree and a Habilitation in Plant Nutrition and was a postdoctoral fellow at Hohenheim University (Germany, 2016) and at the University of Kiel (Germany, 2011-2014). Keywords: chlorine; drought; guard cells; magnesium; plant nutrition; salt stress EN chlorine drought guard cells magnesium plant nutrition salt stress 1998 1999 2 02/07/22 20220301 NES 220301 What inspired your interest in plant science? Chlorine, drought, guard cells, magnesium, salt stress, plant nutrition. [Extracted from the article]

Summary: Both photosynthesis (A) and stomatal conductance (gs) respond to changing irradiance, yet stomatal responses are an order of magnitude slower than photosynthesis, resulting in noncoordination between A and gs in dynamic light environments.Infrared gas exchange analysis was used to examine the temporal responses and coordination of A and gs to a step increase and decrease in light in a range of different species, and the impact on intrinsic water use efficiency was evaluated.The temporal responses revealed a large range of strategies to save water or maximize photosynthesis in the different species used in this study but also displayed an uncoupling of A and gs in most of the species. The shape of the guard cells influenced the rapidity of response and the overall gs values achieved, with different impacts on A and Wi. The rapidity of gs in dumbbell‐shaped guard cells could be attributed to size, whilst in elliptical‐shaped guard cells features other than anatomy were more important for kinetics.Our findings suggest significant variation in the rapidity of stomatal responses amongst species, providing a novel target for improving photosynthesis and water use. [ABSTRACT FROM AUTHOR]

The control of stomatal aperture involves reversible changes in the concentration of osmolytes in guard cells. Sucrose has long been proposed to have an osmolytic role in guard cells. However, direct evidence for such a role is lacking. Furthermore, recent evidence suggests that sucrose may perform additional roles in guard cells. Here, we provide an update covering the multiple roles of sucrose in guard cell regulation, highlighting the knowledge accumulated regarding spatiotemporal differences in the synthesis, accumulation, and degradation of sucrose as well as reviewing the role of sucrose as a metabolic connector between mesophyll and guard cells. Analysis of transcriptomic data from previous studies reveals that several genes encoding sucrose and hexose transporters and genes involved in gluconeogenesis, sucrose and trehalose metabolism are highly expressed in guard cells compared with mesophyll cells. Interestingly, this analysis also showed that guard cells have considerably higher expression of C4-marker genes than mesophyll cells. We discuss the possible roles of these genes in guard cell function and the role of sucrose in stomatal opening and closure. Finally, we provide a perspective for future experiments which are required to fill gaps in our understanding of both guard cell metabolism and stomatal regulation. [ABSTRACT FROM AUTHOR]

During infection plants recognize microbe-associated molecular patterns ( MAMPs), and this leads to stomatal closure. This study analyzes the molecular mechanisms underlying this MAMP response and its interrelation with ABA signaling., Stomata in intact Arabidopsis thaliana plants were stimulated with the bacterial MAMP flg22, or the stress hormone ABA, by using the noninvasive nanoinfusion technique. Intracellular double-barreled microelectrodes were applied to measure the activity of plasma membrane ion channels., Flg22 induced rapid stomatal closure and stimulated the SLAC1 and SLAH3 anion channels in guard cells. Loss of both channels resulted in cells that lacked flg22-induced anion channel activity and stomata that did not close in response to flg22 or ABA. Rapid flg22-dependent stomatal closure was impaired in plants that were flagellin receptor ( FLS2)-deficient, as well as in the ost1-2 ( Open Stomata 1) mutant, which lacks a key ABA-signaling protein kinase. By contrast, stomata of the ABA protein phosphatase mutant abi1-1 ( ABscisic acid Insensitive 1) remained flg22-responsive., These data suggest that the initial steps in flg22 and ABA signaling are different, but that the pathways merge at the level of OST1 and lead to activation of SLAC1 and SLAH3 anion channels. [ABSTRACT FROM AUTHOR]

Changes in cytosolic free calcium ([Ca2+]cyt) are an early and essential element of signalling networks activated by the perception of pathogen-associated molecular patterns ( PAMPs), such as flg22. The flg22-induced calcium signal has been described on whole-plant, but not on single-cell scale so far. Also, the Ca2+ sources and channels contributing to its generation are still obscure., Ratiometric fluorescence imaging employing the calcium reporter Yellow Cameleon 3.6 was performed to analyse the flg22-induced calcium signature in single guard cells of Arabidopsis thaliana. Calcium stores and channel types involved in its generation were determined by a pharmacological approach., In contrast to the calcium signal determined on whole-plant level, the signature on single-cell level is not characterized by one sustained response, but by oscillations in [Ca2+]cyt. These oscillations were abolished by EGTA and lanthanum, as well as by U73122, neomycin and TMB-8, but only partially or not at all affected by inhibitors of glutamate receptor-like channels and cyclic nucleotide-gated channels., Our analyses suggest that the response observed on whole-plant level is the summary of oscillations occurring in single cells. Parallel to external calcium, influx via channels located at internal stores contributes to the signal. [ABSTRACT FROM AUTHOR]

The stomatal behavior of ferns provides an excellent system for disentangling responses to different environmental signals, which balance carbon gain against water loss., Here, we measured responses of stomatal conductance ( gs) to irradiance, CO2, and vapor pressure deficit ( VPD) for 13 phylogenetically diverse species native to open and shaded habitats, grown under high- and low-irradiance treatments. We tested two main hypotheses: that plants adapted and grown in high-irradiance environments would have greater responsiveness to all stimuli given higher flux rates; and that species' responsiveness to different factors would be correlated because of the relative simplicity of fern stomatal control., We found that species with higher light-saturated gs had larger responses, and that plants grown under high irradiance were more responsive to all stimuli. Open habitat species showed greater responsiveness to irradiance and CO2, but lower responsiveness to VPD; a case of plasticity and adaptation tending in different directions. Responses of gs to irradiance and VPD were positively correlated across species, but CO2 responses were independent and highly variable., The novel finding of correlations among stomatal responses to different stimuli suggests coordination of hydraulic and photosynthetic signaling networks modulating fern stomatal responses, which show distinct optimization at growth and evolutionary time-scales. [ABSTRACT FROM AUTHOR]

Stomata control gaseous fluxes between the internal leaf air spaces and the external atmosphere. Guard cells determine stomatal aperture and must operate to ensure an appropriate balance between CO2 uptake for photosynthesis (A) and water loss, and ultimately plant water use efficiency (WUE). A strong correlation between A and stomatal conductance (gs) is well documented and often observed, but the underlying mechanisms, possible signals and metabolites that promote this relationship are currently unknown. In this review we evaluate the current literature on mesophyll-driven signals that may coordinate stomatal behaviour with mesophyll carbon assimilation. We explore a possible role of various metabolites including sucrose and malate (from several potential sources; including guard cell photosynthesis) and new evidence that improvements in WUE have been made by manipulating sucrose metabolism within the guard cells. Finally we discuss the new tools and techniques available for potentially manipulating cell-specific metabolism, including guard and mesophyll cells, in order to elucidate mesophyll-derived signals that coordinate mesophyll CO2 demands with stomatal behaviour, in order to provide a mechanistic understanding of these processes as this may identify potential targets for manipulations in order to improve plant WUE and crop yield. [ABSTRACT FROM AUTHOR]

Two-component signalling ( TCS) systems play important roles in cytokinin and ethylene signalling in Arabidopsis thaliana. Although the involvement of histidine kinases ( AHKs) in drought stress responses has been described, their role and that of histidine phosphotransferases ( AHPs) in guard cell signalling remain to be fully elucidated., Here, we investigated the roles of TCS genes, the histidine phosphotransferase AHP2 and the histidine kinases AHK2 and AHK3, previously reported to play roles in cytokinin and abscisic acid ( ABA) signalling., We show that AHP2 is present in the nucleus and the cytoplasm, and is involved in light-induced opening. We also present evidence that there is some redistribution of AHP2 from the nucleus to the cytoplasm on addition of ABA. In addition, we provide data to support a role for the cytokinin receptors AHK2 and AHK3 in light-induced stomatal opening and, by inference, in controlling the stomatal sensitivity to ABA., Our results provide new insights into the operation of TCS in plants, cross-talk in stomatal signalling and, in particular, the process of light-induced stomatal opening. [ABSTRACT FROM AUTHOR]

1121I.1121II.1122III.1125IV.1129V.11331134References1134 Summary: Guard cells regulate stomatal pore size through integration of both endogenous and environmental signals; they are widely recognized as providing a key switching mechanism that maximizes both the efficient use of water and rates of CO2 exchange for photosynthesis; this is essential for the adaptation of plants to water stress. Reactive oxygen species (ROS) are widely considered to be an important player in guard cell signalling. In this review, we focus on recent progress concerning the role of ROS as signal molecules in controlling stomatal movement, the interaction between ROS and intrinsic and environmental response pathways, the specificity of ROS signalling, and how ROS signals are sensed and relayed. However, the picture of ROS‐mediated signalling is still fragmented and the issues of ROS sensing and the specificity of ROS signalling remain unclear. Here, we review some recent advances in our understanding of ROS signalling in guard cells, with an emphasis on the main players known to interact with abscisic acid signalling. [ABSTRACT FROM AUTHOR]

Information about several papers discussed at the 29th New Phytologist Symposium: Stomata in Manchester, England in July 2012 on the global impact of stomatic pores and guard cells is presented. Topics include stomatal development and evolution, climate models and improved crop production. Its logo is also provided which shows the role of such cells in regulating worldwide issues such as climate change and breeding of sustainable crop.

The Arabidopsis guard cell anion channel SLAC1 is essential for stomatal closure in response to various endogenous and environmental stimuli. Interestingly, here we reveal an unexpected impairment of slac1 alleles on stomatal opening., We report that mutations in SLAC1 unexpectedly slow stomatal opening induced by light, low CO2 and elevated air humidity in intact plants and that this is caused by the severely reduced activity of inward K+ (K+in) channels in slac1 guard cells., Expression of channels and transporters involved in stomatal opening showed small but significant reductions in transcript levels in slac1 guard cells; however, this was deemed insufficient to explain the severely impaired K+in channel activity in slac1. We further examined resting cytosolic Ca2+ concentration ([Ca2+]cyt) and K+in channel sensitivity to [Ca2+]cyt in slac1. These experiments showed higher resting [Ca2+]cyt in slac1 guard cells and that reducing [Ca2+]cyt to < 10 nM rapidly restored the activity of K+in channels in slac1 closer to wild-type levels., These findings demonstrate an unanticipated compensatory feedback control in plant stomatal regulation, which counteracts the impaired stomatal closing response of slac1, by down-regulating stomatal opening mechanisms and implicates enhanced [Ca2+]cyt sensitivity priming as a mechanistic basis for the down-regulated K+in channel activity. [ABSTRACT FROM AUTHOR]

The possible roles of K+ channels in plant adaptation to high Na+ conditions have not been extensively analyzed. Here, we characterize an inward Shaker K+ channel, MIRK (melon inward rectifying K+ channel), cloned in a salt-tolerant melon (Cucumis melo) cultivar, and show that this channel displays an unusual sensitivity to Na+. MIRK expression localization was analyzed by reverse-transcription PCR (RTPCR). MIRK functional analyses were performed in yeast (growth tests) and Xenopus oocytes (voltage-clamp). MIRK-type activity was revealed in guard cells using the patch-clamp technique. MIRK is an inwardly rectifying Shaker channel belonging to the 'KAT' subgroup and expressed in melon leaves (especially in guard cells and vasculature), stems, flowers and fruits. Besides having similar features to its close homologs, MIRK displays a unique property: inhibition of K+ transport by external Na+. In Xenopus oocytes, external Na+ affected both inward and outward MIRK currents in a voltage-independent manner, suggesting a blocking site in the channel external mouth. The degree of MIRK inhibition by Na+, which is dependent on the Na+ / K+ concentration ratio, is predicted to have an impact on the control of K+ transport in planta upon salt stress. Expressed in guard cells, MIRK might control Na+ arrival to the shoots via regulation of stomatal aperture by Na+. [ABSTRACT FROM AUTHOR]

Hydrogen sulphide (H2S) has been proposed as the third gasotransmitter. In animal cells, H2S has been implicated in several physiological processes. H2S is endogenously synthesized in both animals and plants by enzymes with L-Cys desulphydrase activity in the conversion of L-Cys to H2S, pyruvate and ammonia. The participation of H2S in both stomatal movement regulation and abscisic acid (ABA)-dependent induction of stomatal closure was studied in epidermal strips of three plant species (Vicia faba, Arabidopsis thaliana and Impatiens walleriana). The effect of H2S on stomatal movement was contrasted with leaf relative water content (RWC) measurements of whole plants subjected to water stress. In this work we report that exogenous H2S induces stomatal closure and this effect is impaired by the ATP-binding cassette (ABC) transporter inhibitor glibenclamide; scavenging H2S or inhibition of the enzyme responsible for endogenous H2S synthesis partially blocks ABA-dependent stomatal closure; and H2S treatment increases RWC and protects plants against drought stress. Our results indicate that H2S induces stomatal closure and participates in ABAdependent signalling, possibly through the regulation of ABC transporters in guard cells. [ABSTRACT FROM AUTHOR]

•The pharmacology has been further investigated of the two transport systems mediating potassium (rubidium) (K+(Rb+)) release from the guard cell vacuole, responsible, respectively, for the resting efflux and abscisic acid (ABA)-induced transient stimulation of efflux, and for the transient stimulation induced by hypotonic treatment. •Here, the effects of fusicoccin and of butyrate-induced cytoplasmic acidification on 86Rb efflux were measured in isolated guard cells of Commelina communis. •Fusicoccin (10 μM) inhibited the resting efflux at the tonoplast and the ABA-induced transient, but had no effect on the hypotonic transient. All three processes were inhibited by cytoplasmic acidification. Fusicoccin did not inhibit efflux at the plasmalemma. •As the hypotonic response is inhibited by cytoplasmic acidification but not by fusicoccin, the effect of fusicoccin on the resting efflux and ABA response must be direct, and not the result of fusicoccin-induced cytoplasmic acidification. The collected tonoplast efflux properties resemble those of TPC1 (two-pore channel) rather than TPK1 (two-pore K channel). The flux and TPC1 are both activated by Ca2+, but inhibited by phenylarsine oxide and by cytoplasmic acidification. The flux is inhibited by fusicoccin. TPC1 is inhibited by 14-3-3 proteins and has the C-terminal sequence STSDT, a type III binding site for 14-3-3 proteins, of the kind involved in fusicoccin binding. [ABSTRACT FROM AUTHOR]

Contents Guard cells can integrate and process multiple complex signals from the environment and respond by opening and closing stomata in order to adapt to the environmental signal. Over the past several years, considerable research progress has been made in our understanding of the role of reactive oxygen species (ROS) as essential signal molecules that mediate abscisic acid (ABA)-induced stomatal closure. In this review, we discuss hydrogen peroxide (H2O2) generation and signalling, H2O2-induced gene expression, crosstalk and the specificity between ABA and H2O2 signalling, and the cellular mechanism for ROS sensing in guard cells. This review focuses especially on the points of connection between ABA and H2O2 signalling in guard cells. The fundamental progress in understanding the role of ABA and ROS in guard cells will continue to provide a rational basis for biotechnological improvements in the development of drought-tolerant crop plants with improved water-use efficiency. [ABSTRACT FROM AUTHOR]

• Pharmacological agents were used to investigate the possible involvement of actin in signalling chains associated with abscisic acid (ABA)-induced ion release from the guard cell vacuole, a process which is absolutely essential for stomatal closure. • Effects on the ABA-induced transient stimulation of tonoplast efflux were measured, using 86Rb in isolated guard cells of Commelina communis, together with effects on stomatal apertures. • In the response to 10 µm ABA (triggered by Ca2+ influx rather than internal Ca2+ release), jasplakinolide (stabilizing actin filaments) and latrunculin B (depolymerizing actin filaments) had opposite effects. Both closure and the vacuolar efflux transient were inhibited by jasplakinolide but enhanced by latrunculin B. At 10 µm ABA prevention of mitogen-activated protein (MAP) kinase activation by PD98059 partially inhibited closure and reduced the efflux transient. By contrast, latrunculin B inhibited the efflux transient at 0.1 µm ABA (involving internal Ca2+ release rather than Ca2+ influx). • The results suggest that 10 µm ABA activates Ca2+-dependent vacuolar ion efflux via a Ca2+-permeable influx channel which is maintained closed by interaction with F-actin. A MAP kinase is also involved, in a chain similar to that postulated for Ca2+-dependent gene expression in cold acclimation. [ABSTRACT FROM AUTHOR]

• In grapevine, the penetration and sporulation of Plasmopara viticola occur via stomata, suggesting functional relationships between guard cells and the pathogen. This assumption was supported by our first observation that grapevine ( Vitis vinifera cv. Marselan) cuttings infected by P. viticola wilted more rapidly than healthy ones when submitted to water starvation. • Here, complementary approaches measuring stomatal conductance and infrared thermographic and microscopic observations were used to investigate stomatal opening/closure in response to infection. • In infected leaves, stomata remained open in darkness and during water stress, leading to increased transpiration. This deregulation was restricted to the colonized area, was not systemic and occurred before the appearance of symptoms. Cytological observations indicated that stomatal lock-open was not related to mechanical forces resulting from the presence of the pathogen in the substomatal cavity. In contrast to healthy leaves, stomatal closure in excised infected leaves could not be induced by a water deficit or abscisic acid (ABA) treatment. However, ABA induced stomatal closure in epidermal peels from infected leaves, indicating that guard cells remained functional. • These data indicate that the oomycete deregulates guard cell functioning, causing significant water losses. This effect could be attributed to a nonsystemic compound, produced by the oomycete or by the infected plant, which inhibits stomatal closure or induces stomatal opening; or a reduction of the back-pressure exerted by surrounding epidermal cells. Both hypotheses are under investigation. [ABSTRACT FROM AUTHOR]

Summary • Under red light in ambient CO2 guard cells of faba bean (Vicia faba ) fix CO2 and accumulate sucrose, causing stomata to open. We examined whether at [CO2 ] low enough to limit guard cell photosynthesis stomata would open when illuminated with red (R) or far-red (FR) light. • After illumination with R or FR in buffered KCl solutions, net stomatal opening was c . 3 µm (R and FR) in air containing 210–225 µl l-1 CO2 and was 5 µm (R) or 6.5 µm (FR) in air containing 40–50 µl l-1 CO2 . Opening was fully inhibited by 3-(3,4-dichlorophenyl)-1,1 dimethyl urea, the calmodulin antagonist W-7, the ser/thr kinase inhibitor ML-9, and sodium orthovanadate, but not by dithiothreitol, which inhibits formation of zeaxanthin, the blue light photoreceptor of guard cells. • Stomatal opening was accompanied by K+ uptake and starch loss. Similar results were obtained when leaves were exposed to conditions designed to lower intercellular leaf [CO2 ]. • These data suggest that the guard cell chloroplasts transduce reduced [CO2 ], activating stomatal opening through an ion uptake mechanism that depends on chloroplastic photosynthetic electron transport and that shares downstream components of the blue light signal transduction cascade. [ABSTRACT FROM AUTHOR]

The post-cytokinetic guard cells of Asplenium nidus display a prominent perinuclear microtubule system and a few microtubules under the periclinal walls. Afterwards, microtubules appear on the whole surface of the ventral wall, whereas those below the periclinal walls proliferate and tend to become parallel with the ventral wall. The perinuclear microtubules gradually diminish but persist in later stages of guard cell differentiation. In post- cytokinetic guard cells, actin is found in the perinuclear cytoplasm and in the cortical cytoplasm lining all the walls. In differentiating guard cells, the following cortical microtubules and actin filament 'systems' appear in succession: (a) radial microtubule and actin filament arrays beneath the periclinal walls converging on the stomatal pore region, (b) anticlinal microtubule bundles, which are co-localized with actin filaments, along the ventral wall outlining the region of the stomatal pore, (c) periclinal microtubules and actin filaments on the polar ventral wall ends. These cytoskeletal systems, except for the radial actin filaments, persist in advanced stages of guard cell differentiation. Instead of the radial actin filaments, a prominent actin filament reticulum is organized under the margins of the developing wall thickenings of the stomatal pore. In addition, an extensive endoplasmic actin filament reticulum develops around the plastids. It seems likely that the successive microtubule systems in guard cells are formed by putative microtubule organizing centres operating in a definite spatial arid temporal succession. Guard cell morphogenesis is the outcome of a definite process, in which the cortical microtubule cytoskeleton plays the primary role, implicated in the deposition of cellulose microfibrils and probably of the local wall thickenings. Callose or a callose-like glucan is deposited on the whole surface of the nascent ventral wall and in the wall regions where thickenings are deposited. Finally, the guard cells of Asplenium assume a kidney shape and display polar hypostomatic swellings. Particular structural features established in guard cell mother cells affect guard cell morphogenesis. [ABSTRACT FROM AUTHOR]

Calcium ions are known to play an important part in signal transduction in stomatal guard cells. In Commelina communis L., stomatal opening in isolated epidermis is strongly inhibited if the calcium concentration in the incubation medium is 0.1 mol m-3 or greater. It can be assumed that in the intact leaf, the apoplastic concentration of free calcium in the vicinity of the guard cells must be kept below this level if interference with stomatal functioning is to be avoided. When C. communis was grown with 15 mol m-3 calcium in the rhizosphere, the concentration of fret calcium in the xylem sap in the shoot was found to he 3.76 mol m-3. A mechanism is clearly needed for reducing this concentration as the sap traverses the apoplast between the xylem and the stomatal guard cells, Evidence is presented here that the deposition of calcium oxalate in cells of the leaf achieves the necessary regulation. The protective role of the six specialized subsidiary cells in this species appears to he specially important. It is suggested that the regulation of apoplastic free calcium may take an important contribution to the effective stomatal control of gas exchange. [ABSTRACT FROM AUTHOR]