Osmo-sensitive and stretch-activated calcium-permeable channels in Vicia faba guard cells are regulated by actin dynamics

Mechanism of the Pulvinus-Driven Leaf Movement: An Overview

Leaf movement is a manifestation of plant response to the changing internal and external environment, aiming to optimize plant growth and development. Leaf movement is usually driven by a specialized motor organ, the pulvinus, and this movement is associated with different changes in volume and expansion on the two sides of the pulvinus. Blue light, auxin, GA, H+-ATPase, K+, Cl-, Ca2+, actin, and aquaporin collectively influence the changes in water flux in the tissue of the extensor and flexor of the pulvinus to establish a turgor pressure difference, thereby controlling leaf movement. However, how these factors regulate the multicellular motility of the pulvinus tissues in a species remains obscure. In addition, model plants such as Medicago truncatula, Mimosa pudica, and Samanea saman have been used to study pulvinus-driven leaf movement, showing a similarity in their pulvinus movement mechanisms. In this review, we summarize past research findings from the three model plants, and using Medicago truncatula as an example, suggest that genes regulating pulvinus movement are also involved in regulating plant growth and development. We also propose a model in which the variation of ion flux and water flux are critical steps to pulvinus movement and highlight questions for future research.

Mechanotransduction in the spotlight of mechano-sensitive channels

The study of mechanosensitive channels (MS) in living organisms has progressed considerably over the past two decades. The understanding of their roles in mechanosensation and mechanotransduction was consecrated by the awarding of the Nobel Prize in 2021 to A. Patapoutian for his discoveries on the role of MS channels in mechanoperception in humans. In this review, we first summarize the fundamental properties of MS channels and their mode of operation. Then in a second step, we provide an update on the knowledge on the families of MS channels identified in plants and the roles and functions that have been attributed to them.

Distinct Roles for KASH Proteins SINE1 and SINE2 in Guard Cell Actin Reorganization, Calcium Oscillations, and Vacuolar Remodeling

The linker of nucleoskeleton and cytoskeleton (LINC) complex is a protein complex spanning the inner and outer membranes of the nuclear envelope. Outer nuclear membrane KASH proteins interact in the nuclear envelope lumen with inner nuclear membrane SUN proteins. The paralogous Arabidopsis KASH proteins SINE1 and SINE2 function during stomatal dynamics induced by light-dark transitions and ABA. Previous studies have shown F-actin organization, cytoplasmic calcium (Ca²⁺) oscillations, and vacuolar morphology changes are involved in ABA-induced stomatal closure. Here, we show that SINE1 and SINE2 are both required for actin pattern changes during ABA-induced stomatal closure, but influence different, temporally distinguishable steps. External Ca²⁺ partially overrides the mutant defects. ABA-induced cytoplasmic Ca²⁺ oscillations are diminished in sine2-1 but not sine1-1, and this defect can be rescued by both exogenous Ca²⁺ and F-actin depolymerization. We show first evidence for nuclear Ca²⁺ oscillations during ABA-induced stomatal closure, which are disrupted in sine2-1. Vacuolar fragmentation is impaired in both mutants and is partially rescued by F-actin depolymerization. Together, these data indicate distinct roles for SINE1 and SINE2 upstream of this network of players involved in ABA-based stomatal closure, suggesting a role for the nuclear surface in guard cell ABA signaling.

Plant electrical signals: A multidisciplinary challenge

Plant electrical signals, an early event in the plant-stimulus interaction, rapidly transmit information generated by the stimulus to other organs, and even the whole plant, to promote the corresponding response and trigger a regulatory cascade. In recent years, many promising state-of-the-art technologies applicable to study plant electrophysiology have emerged. Research focused on expression of genes associated with electrical signals has also proliferated. We propose that it is appropriate for plant electrical signals to be considered in the form of a "plant electrophysiological phenotype". This review synthesizes research on plant electrical signals from a novel, interdisciplinary perspective, which is needed to improve the efficient aggregation and use of plant electrical signal data and to expedite interpretation of plant electrical signals.

Rapid movements in plants

Plant movements are generally slow, but some plant species have evolved the ability to move very rapidly at speeds comparable to those of animals. Whereas movement in animals relies on the contraction machinery of muscles, many plant movements use turgor pressure as the primary driving force together with secondarily generated elastic forces. The movement of stomata is the best-characterized model system for studying turgor-driven movement, and many gene products responsible for this movement, especially those related to ion transport, have been identified. Similar gene products were recently shown to function in the daily sleep movements of pulvini, the motor organs for macroscopic leaf movements. However, it is difficult to explain the mechanisms behind rapid multicellular movements as a simple extension of the mechanisms used for unicellular or slow movements. For example, water transport through plant tissues imposes a limit on the speed of plant movements, which becomes more severe as the size of the moving part increases. Rapidly moving traps in carnivorous plants overcome this limitation with the aid of the mechanical behaviors of their three-dimensional structures. In addition to a mechanism for rapid deformation, rapid multicellular movements also require a molecular system for rapid cell-cell communication, along with a mechanosensing system that initiates the response. Electrical activities similar to animal action potentials are found in many plant species, representing promising candidates for the rapid cell-cell signaling behind rapid movements, but the molecular entities of these electrical signals remain obscure. Here we review the current understanding of rapid plant movements with the aim of encouraging further biological studies into this fascinating, challenging topic.

AtSEC22 Regulates Cell Morphogenesis via Affecting Cytoskeleton Organization and Stabilities

The plant cytoskeleton forms a stereoscopic network that regulates cell morphogenesis. The cytoskeleton also provides tracks for trafficking of vesicles to the target membrane. Fusion of vesicles with the target membrane is promoted by SNARE proteins, etc. The vesicle-SNARE, Sec22, regulates membrane trafficking between the ER and Golgi in yeast and mammals. Arabidopsis AtSEC22 might also regulate early secretion and is essential for gametophyte development. However, the role of AtSEC22 in plant development is unclear. To clarify the role of AtSEC22 in the regulation of plant development, we isolated an AtSEC22 knock-down mutant, atsec22-4, and found that cell morphogenesis and development were seriously disturbed. atsec22-4 exhibited shorter primary roots (PRs), dwarf plants, and partial abortion. More interestingly, the atsec22-4 mutant had less trichomes with altered morphology, irregular stomata, and pavement cells, suggesting that cell morphogenesis was perturbed. Further analyses revealed that in atsec22-4, vesicle trafficking was blocked, resulting in the trapping of proteins in the ER and collapse of structures of the ER and Golgi apparatus. Furthermore, AtSEC22 defects resulted in impaired organization and stability of the cytoskeleton in atsec22-4. Our findings revealed essential roles of AtSEC22 in membrane trafficking and cytoskeleton dynamics during plant development.

The calcium transporter ANNEXIN1 mediates cold-induced calcium signaling and freezing tolerance in plants

The transient elevation of cytosolic free calcium concentration ([Ca²⁺ ]_cyt ) induced by cold stress is a well-established phenomenon; however, the underlying mechanism remains elusive. Here, we report that the Ca²⁺ -permeable transporter ANNEXIN1 (AtANN1) mediates cold-triggered Ca²⁺ influx and freezing tolerance in Arabidopsis thaliana. The loss of function of AtANN1 substantially impaired freezing tolerance, reducing the cold-induced [Ca²⁺ ]_cyt increase and upregulation of the cold-responsive CBF and COR genes. Further analysis showed that the OST1/SnRK2.6 kinase interacted with and phosphorylated AtANN1, which consequently enhanced its Ca²⁺ transport activity, thereby potentiating Ca²⁺ signaling. Consistent with these results and freezing sensitivity of ost1 mutants, the cold-induced [Ca²⁺ ]_cyt elevation in the ost1-3 mutant was reduced. Genetic analysis indicated that AtANN1 acts downstream of OST1 in responses to cold stress. Our data thus uncover a cascade linking OST1-AtANN1 to cold-induced Ca²⁺ signal generation, which activates the cold response and consequently enhances freezing tolerance in Arabidopsis.

Prolonged oxygen depletion in microwounded cells of Chara corallina detected with novel oxygen nanosensors

Primary physicochemical steps in microwounding of plants were investigated using electrochemical nano- and microprobes, with a focus on the role of oxygen in the wounding responses of individual plant cells. Electrochemical measurements of cell oxygen content were made with carbon-filled quartz micropipettes with platinum-coated tips (oxygen nanosensors). These novel platinum nanoelectrodes are useful for understanding cell oxygen metabolism and can be employed to study the redox biochemistry and biology of cells, tissues and organisms. We show here that microinjury of Chara corallina internodal cells with the tip of a glass micropipette is associated with a drastic decrease in oxygen concentration at the vicinity of the stimulation site. This decrease is reversible and lasts for up to 40 minutes. Membrane stretching, calcium influx, and cytoskeleton rearrangements were found to be essential for the localized oxygen depletion induced by cell wall microwounding. Inhibition of electron transport in chloroplasts or mitochondria did not affect the magnitude or timing of the observed response. In contrast, the inhibition of NADPH oxidase activity caused a significant reduction in the amplitude of the decrease in oxygen concentration. We suggest that the observed creation of localized anoxic conditions in response to cell wall puncture might be mediated by NADPH oxidase.

Model-driven discovery of calcium-related protein-phosphatase inhibition in plant guard cell signaling

The plant hormone abscisic acid (ABA) promotes stomatal closure via multifarious cellular signaling cascades. Our previous comprehensive reconstruction of the stomatal closure network resulted in an 81-node network with 153 edges. Discrete dynamic modeling utilizing this network reproduced over 75% of experimental observations but a few experimentally supported results were not recapitulated. Here we identify predictions that improve the agreement between model and experiment. We performed dynamics-preserving network reduction, resulting in a condensed 49 node and 113 edge stomatal closure network that preserved all dynamics-determining network motifs and reproduced the predictions of the original model. We then utilized the reduced network to explore cases in which experimental activation of internal nodes in the absence of ABA elicited stomatal closure in wet bench experiments, but not in our in silico model. Our simulations revealed that addition of a single edge, which allows indirect inhibition of any one of three PP2C protein phosphatases (ABI2, PP2CA, HAB1) by cytosolic Ca2+ elevation, resolves the majority of the discrepancies. Consistent with this hypothesis, we experimentally show that Ca2+ application to cellular lysates at physiological concentrations inhibits PP2C activity. The model augmented with this new edge provides new insights into the role of cytosolic Ca2+ oscillations in stomatal closure, revealing a mutual reinforcement between repeated increases in cytosolic Ca2+ concentration and a self-sustaining feedback circuit inside the signaling network. These results illustrate how iteration between model and experiment can improve predictions of highly complex cellular dynamics.

Guard Cell Microfilament Analyzer Facilitates the Analysis of the Organization and Dynamics of Actin Filaments in Arabidopsis Guard Cells

The actin cytoskeleton is involved in regulating stomatal movement, which forms distinct actin arrays within guard cells of stomata with different apertures. How those actin arrays are formed and maintained remains largely unexplored. Elucidation of the dynamic behavior of differently oriented actin filaments in guard cells will enhance our understanding in this regard. Here, we initially developed a program called ‘guard cell microfilament analyzer’ (GCMA) that enables the selection of individual actin filaments and analysis of their orientations semiautomatically in guard cells. We next traced the dynamics of individual actin filaments and performed careful quantification in open and closed stomata. We found that de novo nucleation of actin filaments occurs at both dorsal and ventral sides of guard cells from open and closed stomata. Interestingly, most of the nucleated actin filaments elongate radially and longitudinally in open and closed stomata, respectively. Strikingly, radial filaments tend to form bundles whereas longitudinal filaments tend to be removed by severing and depolymerization in open stomata. By contrast, longitudinal filaments tend to form bundles that are severed less frequently in closed stomata. These observations provide insights into the formation and maintenance of distinct actin arrays in guard cells in stomata of different apertures.

A Nck-associated protein 1-like protein affects drought sensitivity by its involvement in leaf epidermal development and stomatal closure in rice

Water deficit is a major environmental threat affecting crop yields worldwide. In this study, a drought stress-sensitive mutant drought sensitive 8 (ds8) was identified in rice (Oryza sativa L.). The DS8 gene was cloned using a map-based approach. Further analysis revealed that DS8 encoded a Nck-associated protein 1 (NAP1)-like protein, a component of the SCAR/WAVE complex, which played a vital role in actin filament nucleation activity. The mutant exhibited changes in leaf cuticle development. Functional analysis revealed that the mutation of DS8 increased stomatal density and impaired stomatal closure activity. The distorted actin filaments in the mutant led to a defect in abscisic acid (ABA)-mediated stomatal closure and increased ABA accumulation. All these resulted in excessive water loss in ds8 leaves. Notably, antisense transgenic lines also exhibited increased drought sensitivity, along with impaired stomatal closure and elevated ABA levels. These findings suggest that DS8 affects drought sensitivity by influencing actin filament activity.

Boolean Calcium Signalling Model Predicts Calcium Role in Acceleration and Stability of Abscisic Acid-Mediated Stomatal Closure

Inconsistent hypotheses have proposed Ca²⁺ as either being essential or irrelevant and redundant in ABA induced stomatal closure. This study integrates all available information from literature to define ABA signalling pathway and presents it in a systems view for clearer understanding of the role of Ca²⁺ in stomatal closure. Importantly, it incorporates into an Asynchronous Boolean model time delays sourced from an extensive literature search. The model predicted the timing of ABA events and mutant behaviour close to biology. It revealed biologically reported timing for Ca²⁺ activation and Ca²⁺ dynamics consistent with biology. It also predicts that Ca²⁺ elevation is not essential in stomatal closure but it can accelerate closure, consistent with previous findings, but our model further explains that acting as a mediator, Ca²⁺ accelerates stomatal closure by enhancing plasma membrane slowly activating anion channel SLAC1 and actin rearrangement. It shows statistical significance of Ca²⁺ induced acceleration of closure and that of Ca²⁺ induced acceleration of SLAC1 activation. Further, the model demonstrates that Ca²⁺ enhances resilience of closure to perturbation of important elements; especially, ROS pathway, as did previous ABA model, and even to the ABA signal disruption. It goes further to elucidate the mechanisms by which Ca²⁺ engenders stomatal closure in these perturbations.

Calcium imaging in Arabidopsis pollen cells using G-CaMP5

Calcium (Ca²⁺ ) signaling has been implicated in pollen germination and pollen tube growth. To date, however, we still know very little about how exactly Ca²⁺ signaling links to various physiological subcellular processes during pollen germination and pollen tube growth. Given that Ca²⁺ signaling is tightly related to the cytosolic concentration and dynamics of Ca²⁺ , it is vital to trace the dynamic changes in Ca²⁺ levels in order to decode Ca²⁺ signaling. Here, we demonstrate that G-CaMP5 serves well as an indicator for monitoring cytosolic Ca²⁺ dynamics in pollen cells. Using this probe, we show that cytosolic Ca²⁺ changes dramatically during pollen germination, and, as reported previously, Ca²⁺ forms a tip-focused gradient in the pollen tube and undergoes oscillation in the tip region during pollen tube growth. In particular, using G-CaMP5 allowed us to capture the dynamic changes in the cytosolic Ca²⁺ concentration ([Ca²⁺ ]_cyt ) in pollen tubes in response to various exogenous treatments. Our data suggest that G-CaMP5 is a suitable probe for monitoring the dynamics of [Ca²⁺ ]_cyt in pollen cells.

Actin filaments regulate the adhesion between the plasma membrane and the cell wall of tobacco guard cells

During the opening and closing of stomata, guard cells undergo rapid and reversible changes in their volume and shape, which affects the adhesion of the plasma membrane (PM) to the cell wall (CW). The dynamics of actin filaments in guard cells are involved in stomatal movement by regulating structural changes and intracellular signaling. However, it is unclear whether actin dynamics regulate the adhesion of the PM to the CW. In this study, we investigated the relationship between actin dynamics and PM-CW adhesion by the hyperosmotic-induced plasmolysis of tobacco guard cells. We found that actin filaments in guard cells were depolymerized during mannitol-induced plasmolysis. The inhibition of actin dynamics by treatment with latrunculin B or jasplakinolide and the disruption of the adhesion between the PM and the CW by treatment with RGDS peptide (Arg-Gly-Asp-Ser) enhanced guard cell plasmolysis. However, treatment with latrunculin B alleviated the RGDS peptide-induced plasmolysis and endocytosis. Our results reveal that the actin depolymerization is involved in the regulation of the PW-CW adhesion during hyperosmotic-induced plasmolysis in tobacco guard cells.

A mechanosensitive Ca2+ channel activity is dependent on the developmental regulator DEK1

Responses of cells to mechanical stress are thought to be critical in coordinating growth and development. Consistent with this idea, mechanically activated channels play important roles in animal development. For example, the PIEZO1 channel controls cell division and epithelial-layer integrity and is necessary for vascular development in mammals. In plants, the actual contribution of mechanoperception to development remains questionable because very few putative mechanosensors have been identified and the phenotypes of the corresponding mutants are rather mild. Here, we show that the Arabidopsis Defective Kernel 1 (DEK1) protein, which is essential for development beyond early embryogenesis, is associated with a mechanically activated Ca2+ current in planta, suggesting that perception of mechanical stress plays a critical role in plant development.

A new discrete dynamic model of ABA-induced stomatal closure predicts key feedback loops

Stomata, microscopic pores in leaf surfaces through which water loss and carbon dioxide uptake occur, are closed in response to drought by the phytohormone abscisic acid (ABA). This process is vital for drought tolerance and has been the topic of extensive experimental investigation in the last decades. Although a core signaling chain has been elucidated consisting of ABA binding to receptors, which alleviates negative regulation by protein phosphatases 2C (PP2Cs) of the protein kinase OPEN STOMATA 1 (OST1) and ultimately results in activation of anion channels, osmotic water loss, and stomatal closure, over 70 additional components have been identified, yet their relationships with each other and the core components are poorly elucidated. We integrated and processed hundreds of disparate observations regarding ABA signal transduction responses underlying stomatal closure into a network of 84 nodes and 156 edges and, as a result, established those relationships, including identification of a 36-node, strongly connected (feedback-rich) component as well as its in- and out-components. The network's domination by a feedback-rich component may reflect a general feature of rapid signaling events. We developed a discrete dynamic model of this network and elucidated the effects of ABA plus knockout or constitutive activity of 79 nodes on both the outcome of the system (closure) and the status of all internal nodes. The model, with more than 1024 system states, is far from fully determined by the available data, yet model results agree with existing experiments in 82 cases and disagree in only 17 cases, a validation rate of 75%. Our results reveal nodes that could be engineered to impact stomatal closure in a controlled fashion and also provide over 140 novel predictions for which experimental data are currently lacking. Noting the paucity of wet-bench data regarding combinatorial effects of ABA and internal node activation, we experimentally confirmed several predictions of the model with regard to reactive oxygen species, cytosolic Ca2+ (Ca2+c), and heterotrimeric G-protein signaling. We analyzed dynamics-determining positive and negative feedback loops, thereby elucidating the attractor (dynamic behavior) repertoire of the system and the groups of nodes that determine each attractor. Based on this analysis, we predict the likely presence of a previously unrecognized feedback mechanism dependent on Ca2+c. This mechanism would provide model agreement with 10 additional experimental observations, for a validation rate of 85%. Our research underscores the importance of feedback regulation in generating robust and adaptable biological responses. The high validation rate of our model illustrates the advantages of discrete dynamic modeling for complex, nonlinear systems common in biology.

Actin filament reorganisation controlled by the SCAR/WAVE complex mediates stomatal response to darkness

Stomata respond to darkness by closing to prevent excessive water loss during the night. Although the reorganisation of actin filaments during stomatal closure is documented, the underlying mechanisms responsible for dark-induced cytoskeletal arrangement remain largely unknown. We used genetic, physiological and cell biological approaches to show that reorganisation of the actin cytoskeleton is required for dark-induced stomatal closure. The opal5 mutant does not close in response to darkness but exhibits wild-type (WT) behaviour when exposed to abscisic acid (ABA) or CaCl2 . The mutation was mapped to At5g18410, encoding the PIR/SRA1/KLK subunit of the ArabidopsisSCAR/WAVE complex. Stomata of an independent allele of the PIR gene (Atpir-1) showed reduced sensitivity to darkness and F1 progenies of the cross between opal5 and Atpir-1 displayed distorted leaf trichomes, suggesting that the two mutants are allelic. Darkness induced changes in the extent of actin filament bundling in WT. These were abolished in opal5. Disruption of filamentous actin using latrunculin B or cytochalasin D restored wild-type stomatal sensitivity to darkness in opal5. Our findings suggest that the stomatal response to darkness is mediated by reorganisation of guard cell actin filaments, a process that is finely tuned by the conserved SCAR/WAVE-Arp2/3 actin regulatory module.

H2O2 and Ca2+-based signaling and associated ion accumulation, antioxidant systems and secondary metabolism orchestrate the response to NaCl stress in perennial ryegrass

Little is known about the interplay between Ca2+ and H2O2 signaling in stressed cool-season turfgrass. To understand better how Ca2+ and H2O2 signals are integrated to enhance grass acclimation to stress conditions, we analyzed the rearrangements of endogenous ion accumulation, antioxidant systems and secondary metabolism in roots, stems and leaves of perennial ryegrass (Lolium perenne L.) treated with exogenous Ca2+ and H2O2 under salinity. Ca2+ signaling remarkably enhanced the physiological response to salt conditions. Ca2+ signaling could maintain ROS homeostasis in stressed grass by increasing the responses of antioxidant genes, proteins and enzymes. H2O2 signaling could activate ROS homeostasis by inducing antioxidant genes but weakened Ca2+ signaling in leaves. Furthermore, the metabolic profiles revealed that sugars and sugar alcohol accounted for 49.5-88.2% of all metabolites accumulation in all treated leaves and roots. However, the accumulation of these sugars and sugar alcohols displayed opposing trends between Ca2+ and H2O2 application in salt-stressed plants, which suggests that these metabolites are the common regulatory factor for Ca2+ and H2O2 signals. These findings assist in understanding better the integrated network in Ca2+ and H2O2 of cool-season turfgrass' response to salinity.

Peroxisomal NADP-isocitrate dehydrogenase is required for Arabidopsis stomatal movement

Peroxisomes are subcellular organelles characterized by a simple morphological structure but have a complex biochemical machinery involved in signaling processes through molecules such as hydrogen peroxide (H2O2) and nitric oxide (NO). Nicotinamide adenine dinucleotide phosphate (NADPH) is an essential component in cell redox homeostasis, and its regeneration is critical for reductive biosynthesis and detoxification pathways. Plants have several NADPH-generating dehydrogenases, with NADP-isocitrate dehydrogenase (NADP-ICDH) being one of these enzymes. Arabidopsis contains three genes that encode for cytosolic, mitochondrial/chloroplastic, and peroxisomal NADP-ICDH isozymes although the specific function of each of these remains largely unknown. Using two T-DNA insertion lines of the peroxisomal NADP-ICDH designated as picdh-1 and picdh-2, the data show that the peroxisomal NADP-ICDH is involved in stomatal movements, suggesting that peroxisomes are a new element in the signaling network of guard cells.

Arabidopsis thaliana CML25 mediates the Ca(2+) regulation of K(+) transmembrane trafficking during pollen germination and tube elongation

The concentration alteration of cytosolic-free calcium ([Ca(2+) ]cyt ) is a well-known secondary messenger in plants and plays important roles during pollen grain germination and tube elongation. Here we demonstrate that CML25, a member of calmodulin-like proteins, has Ca(2+) -binding activity and plays a role in pollen grain germination, tube elongation and seed setting. CML25 transcript was abundant in mature pollen grains and pollen tubes, and its product CML25 protein was primarily directed to the cytoplasm. Two independent CML25 loss-of-function T-DNA insertion mutants suffered a major reduction in both the rate of pollen germination and the elongation of the pollen tube. Also, pollen grains of cml25 mutants were less sensitive to the external K(+) and Ca(2+) concentration than wild-type pollen. The disruption of CML25 increased the [Ca(2+) ]cyt in both the pollen grain and the pollen tube, which in turn impaired the Ca(2+) -dependent inhibition of whole-cell inward K(+) currents in protoplasts prepared from these materials (pollen grain and pollen tube). Complementation of cml25-1 mutant resulted in the recovery of wild-type phenotype. Our findings indicate that CML25 is an important transducer in the Ca(2+) -mediated regulation of K(+) influx during pollen germination and tube elongation.

Mechanisms underlying turgor regulation in the estuarine alga Vaucheria erythrospora (Xanthophyceae) exposed to hyperosmotic shock

Aquatic organisms are often exposed to dramatic changes in salinity in the environment. Despite decades of research, many questions related to molecular and physiological mechanisms mediating sensing and adaptation to salinity stress remain unanswered. Here, responses of Vaucheria erythrospora, a turgor-regulating xanthophycean alga from an estuarine habitat, have been investigated. The role of ion uptake in turgor regulation was studied using a single cell pressure probe, microelectrode ion flux estimation (MIFE) technique and membrane potential (Em ) measurements. Turgor recovery was inhibited by Gd(3+) , tetraethylammonium chloride (TEA), verapamil and orthovanadate. A NaCl-induced shock rapidly depolarized the plasma membrane while an isotonic sorbitol treatment hyperpolarized it. Turgor recovery was critically dependent on the presence of Na(+) but not K(+) and Cl(-) in the incubation media. Na(+) uptake was strongly decreased by amiloride and changes in net Na(+) and H(+) fluxes were oppositely directed. This suggests active uptake of Na(+) in V. erythrospora mediated by an antiport Na(+) /H(+) system, functioning in the direction opposite to that of the SOS1 exchanger in higher plants. The alga also retains K(+) efficiently when exposed to high NaCl concentrations. Overall, this study provides insights into mechanisms enabling V. erythrospora to regulate turgor via ion movements during hyperosmotic stress.

United in diversity: mechanosensitive ion channels in plants

Mechanosensitive (MS) ion channels are a common mechanism for perceiving and responding to mechanical force. This class of mechanoreceptors is capable of transducing membrane tension directly into ion flux. In plant systems, MS ion channels have been proposed to play a wide array of roles, from the perception of touch and gravity to the osmotic homeostasis of intracellular organelles. Three families of plant MS ion channels have been identified: the MscS-like (MSL), Mid1-complementing activity (MCA), and two-pore potassium (TPK) families. Channels from these families vary widely in structure and function, localize to multiple cellular compartments, and conduct chloride, calcium, and/or potassium ions. However, they are still likely to represent only a fraction of the MS ion channel diversity in plant systems.

Arabidopsis thaliana calmodulin-like protein CML24 regulates pollen tube growth by modulating the actin cytoskeleton and controlling the cytosolic Ca(2+) concentration

Cytosolic free calcium ([Ca(2+)]cyt), which is essential during pollen germination and pollen tube growth, can be sensed by calmodulin-like proteins (CMLs). The Arabidopsis thaliana genome encodes over 50 CMLs, the physiological role(s) of most of which are unknown. Here we show that the gene AtCML24 acts as a regulator of pollen germination and pollen tube extension, since the pollen produced by loss-of-function mutants germinated less rapidly than that of wild-type (WT) plants, the rate of pollen tube extension was slower, and the final length of the pollen tube was shorter. The [Ca(2+)]cyt within germinated pollen and extending pollen tubes produced by the cml24 mutant were higher than their equivalents in WT plants, and pollen tube extension was less sensitive to changes in external [K(+)] and [Ca(2+)]. The pollen and pollen tubes produced by cml24 mutants were characterized by a disorganized actin cytoskeleton and lowered sensitivity to the action of latrunculin B. The observations support an interaction between CML24 and [Ca(2+)]cyt and an involvement of CML24 in actin organization, thereby affecting pollen germination and pollen tube elongation.

Gate control: guard cell regulation by microbial stress

Terrestrial plants rely on stomata, small pores in the leaf surface, for photosynthetic gas exchange and transpiration of water. The stomata, formed by a pair of guard cells, dynamically increase and decrease their volume to control the pore size in response to environmental cues. Stresses can trigger similar or opposing movements: for example, drought induces closure of stomata, whereas many pathogens exploit stomata and cause them to open to facilitate entry into plant tissues. The latter is an active process as stomatal closure is part of the plant's immune response. Stomatal research has contributed much to clarify the signalling pathways of abiotic stress, but guard cell signalling in response to microbes is a relatively new area of research. In this article, we discuss present knowledge of stomatal regulation in response to microbes and highlight common points of convergence, and differences, compared to stomatal regulation by abiotic stresses. We also expand on the mechanisms by which pathogens manipulate these processes to promote disease, for example by delivering effectors to inhibit closure or trigger opening of stomata. The study of pathogen effectors in stomatal manipulation will aid our understanding of guard cell signalling.

ARP2/3 complex-mediated actin dynamics is required for hydrogen peroxide-induced stomatal closure in Arabidopsis

Multiple cellular events like dynamic actin reorganization and hydrogen peroxide (H(2)O(2)) production were demonstrated to be involved in abscisic acid (ABA)-induced stomatal closure. However, the relationship between them as well as the underlying mechanisms remains poorly understood. Here, we showed that H(2)O(2) generation is indispensable for ABA induction of actin reorganization in guard cells of Arabidopsis that requires the presence of ARP2/3 complex. H(2)O(2) -induced stomatal closure was delayed in the mutants of arpc4 and arpc5, and the rate of actin reorganization was slowed down in arpc4 and arpc5 in response to H(2)O(2), suggesting that ARP2/3-mediated actin nucleation is required for H(2)O(2) -induced actin cytoskeleton remodelling. Furthermore, the expression of H(2)O(2) biosynthetic related gene AtrbohD and the accumulation of H(2)O(2) was delayed in response to ABA in arpc4 and arpc5, demonstrating that misregulated actin dynamics affects H(2)O(2) production upon ABA treatment. These results support a possible causal relation between the production of H(2)O(2) and actin dynamics in ABA-mediated guard cell signalling: ABA triggers H(2)O(2) generation that causes the reorganization of the actin cytoskeleton partially mediated by ARP2/3 complex, and ARP2/3 complex-mediated actin dynamics may feedback regulate H(2)O(2) production.

Spread the news: systemic dissemination and local impact of Ca²⁺ signals along the phloem pathway

We explored the idea of whether electropotential waves (EPWs) primarily act as vehicles for systemic spread of Ca(2+) signals. EPW-associated Ca(2+) influx may trigger generation and amplification of countless long-distance signals along the phloem pathway given the fact that gating of Ca(2+)-permeable channels is a universal response to biotic and abiotic challenges. Despite fundamental differences, both action and variation potentials are associated with a sudden Ca(2+) influx. Both EPWs probably disperse in the lateral direction, which could be of essential functional significance. A vast set of Ca(2+)-permeable channels, some of which have been localized, is required for Ca(2+)-modulated events in sieve elements. There, Ca(2+)-permeable channels are clustered and create so-called Ca(2+) hotspots, which play a pivotal role in sieve element occlusion. Occlusion mechanisms play a central part in the interaction between plants and phytopathogens (e.g. aphids or phytoplasmas) and in transient re-organization of the vascular symplasm. It is argued that Ca(2+)-triggered systemic signalling occurs in partly overlapping waves. The forefront of EPWs may be accompanied by a burst of free Ca(2+) ions and Ca(2+)-binding proteins in the sieve tube sap, with a far-reaching impact on target cells. Lateral dispersion of EPWs may induce diverse Ca(2+) influx and handling patterns (Ca(2+) signatures) in various cell types lining the sieve tubes. As a result, a variety of cascades may trigger the fabrication of signals such as phytohormones, proteins, or RNA species released into the sap stream after product-related lag times. Moreover, transient reorganization of the vascular symplasm could modify cascades in disjunct vascular cells.

Glycine betaine-mediated potentiation of HSP gene expression involves calcium signaling pathways in tobacco exposed to NaCl stress

Glycine betaine (GB) can enhance heat tolerance and the accumulation of heat-shock protein (HSP) in plants, but the effects of GB on HSP accumulation during salt stress were not previously known. To investigate the mechanism of how GB influences the expression of HSP, wild-type tobacco (Nicotiana tabacum) seedlings pretreated with exogenous GB and BADH-transgenic tobacco plants that accumulated GB in vivo were studied during NaCl stress. A transient Ca(2+) efflux was observed in the epidermal cells of the elongation zone of tobacco roots after NaCl treatment for 1-2 min. After 24 h of NaCl treatment, an influx of Ca(2+) was observed; a low concentration of GB significantly increased NaCl-induced Ca(2+) influx. GB increased the intracellular free calcium ion concentration and enhanced the expression of the calmodulin (CaM) and heat-shock transcription factor (HSF) genes resulting in potentiated levels of HSPs. Pharmacological experiments confirmed that Ca(2+) and CaM increased HSFs and HSPs gene expression, which coincided with increased the levels of HSP70 accumulation. These results suggest a mechanism by which GB acted as a cofactor in the NaCl induction of a Ca(2+) -permeable current. A possible regulatory model of Ca(2+) -CaM in the signal transduction pathway for induction of transcription and translation of the active HSPs is described.

Mechanosensitive channels: feeling tension in a world under pressure

Plants, like other organisms, are facing multiple mechanical constraints generated both in their tissues and by the surrounding environments. They need to sense and adapt to these forces throughout their lifetimes. To do so, different mechanisms devoted to force transduction have emerged. Here we focus on fascinating proteins: the mechanosensitive (MS) channels. Mechanosensing in plants has been described for centuries but the molecular identification of MS channels occurred only recently. This review is aimed at plant biologists and plant biomechanists who want to be introduced to MS channel identity, how they work and what they might do in planta? In this review, electrophysiological properties, regulations, and functions of well-characterized MS channels belonging to bacteria and animals are compared with those of plants. Common and specific properties are discussed. We deduce which tools and concepts from animal and bacterial fields could be helpful for improving our understanding of plant mechanotransduction. MS channels embedded in their plasma membrane are sandwiched between the cell wall and the cytoskeleton. The consequences of this peculiar situation are analyzed and discussed. We also stress how important it is to probe mechanical forces at cellular and subcellular levels in planta in order to reveal the intimate relationship linking the membrane with MS channel activity. Finally we will propose new tracks to help to reveal their physiological functions at tissue and plant levels.

A force of nature: molecular mechanisms of mechanoperception in plants

The ability to sense and respond to a wide variety of mechanical stimuli-gravity, touch, osmotic pressure, or the resistance of the cell wall-is a critical feature of every plant cell, whether or not it is specialized for mechanotransduction. Mechanoperceptive events are an essential part of plant life, required for normal growth and development at the cell, tissue, and whole-plant level and for the proper response to an array of biotic and abiotic stresses. One current challenge for plant mechanobiologists is to link these physiological responses to specific mechanoreceptors and signal transduction pathways. Here, we describe recent progress in the identification and characterization of two classes of putative mechanoreceptors, ion channels and receptor-like kinases. We also discuss how the secondary messenger Ca(2+) operates at the centre of many of these mechanical signal transduction pathways.

Interaction between Calcium and Actin in Guard Cell and Pollen Signaling Networks

Calcium (Ca(2+)) plays important roles in plant growth, development, and signal transduction. It is a vital nutrient for plant physical design, such as cell wall and membrane, and also serves as a counter-cation for biochemical, inorganic, and organic anions, and more particularly, its concentration change in cytosol is a ubiquitous second messenger in plant physiological signaling in responses to developmental and environmental stimuli. Actin cytoskeleton is well known for its importance in cellular architecture maintenance and its significance in cytoplasmic streaming and cell division. In plant cell system, the actin dynamics is a process of polymerization and de-polymerization of globular actin and filamentous actin and that acts as an active regulator for calcium signaling by controlling calcium evoked physiological responses. The elucidation of the interaction between calcium and actin dynamics will be helpful for further investigation of plant cell signaling networks at molecular level. This review mainly focuses on the recent advances in understanding the interaction between the two aforementioned signaling components in two well-established model systems of plant, guard cell, and pollen.

Involvement of the sieve element cytoskeleton in electrical responses to cold shocks

This study dealt with the visualization of the sieve element (SE) cytoskeleton and its involvement in electrical responses to local cold shocks, exemplifying the role of the cytoskeleton in Ca(2+)-triggered signal cascades in SEs. High-affinity fluorescent phalloidin as well as immunocytochemistry using anti-actin antibodies demonstrated a fully developed parietal actin meshwork in SEs. The involvement of the cytoskeleton in electrical responses and forisome conformation changes as indicators of Ca(2+) influx was investigated by the application of cold shocks in the presence of diverse actin disruptors (latrunculin A and cytochalasin D). Under control conditions, cold shocks elicited a graded initial voltage transient, ΔV1, reduced by external La(3+) in keeping with the involvement of Ca(2+) channels, and a second voltage transient, ΔV2. Cytochalasin D had no effect on ΔV1, while ΔV1 was significantly reduced with 500 nm latrunculin A. Forisome dispersion was triggered by cold shocks of 4°C or greater, which was indicative of an all-or-none behavior. Forisome dispersion was suppressed by incubation with latrunculin A. In conclusion, the cytoskeleton controls cold shock-induced Ca(2+) influx into SEs, leading to forisome dispersion and sieve plate occlusion in fava bean (Vicia faba).

Role and interrelationship of Gα protein, hydrogen peroxide, and nitric oxide in ultraviolet B-induced stomatal closure in Arabidopsis leaves

Heterotrimeric G proteins have been shown to transmit ultraviolet B (UV-B) signals in mammalian cells, but whether they also transmit UV-B signals in plant cells is not clear. In this paper, we report that 0.5 W m(-2) UV-B induces stomatal closure in Arabidopsis (Arabidopsis thaliana) by eliciting a cascade of intracellular signaling events including Gα protein, hydrogen peroxide (H2O2), and nitric oxide (NO). UV-B triggered a significant increase in H2O2 or NO levels associated with stomatal closure in the wild type, but these effects were abolished in the single and double mutants of AtrbohD and AtrbohF or in the Nia1 mutants, respectively. Furthermore, we found that UV-B-mediated H2O2 and NO generation are regulated by GPA1, the Gα-subunit of heterotrimeric G proteins. UV-B-dependent H2O2 and NO accumulation were nullified in gpa1 knockout mutants but enhanced by overexpression of a constitutively active form of GPA1 (cGα). In addition, exogenously applied H2O2 or NO rescued the defect in UV-B-mediated stomatal closure in gpa1 mutants, whereas cGα AtrbohD/AtrbohF and cGα nia1 constructs exhibited a similar response to AtrbohD/AtrbohF and Nia1, respectively. Finally, we demonstrated that Gα activation of NO production depends on H2O2. The mutants of AtrbohD and AtrbohF had impaired NO generation in response to UV-B, but UV-B-induced H2O2 accumulation was not impaired in Nia1. Moreover, exogenously applied NO rescued the defect in UV-B-mediated stomatal closure in the mutants of AtrbohD and AtrbohF. These findings establish a signaling pathway leading to UV-B-induced stomatal closure that involves GPA1-dependent activation of H2O2 production and subsequent Nia1-dependent NO accumulation.

The reorganization of actin filaments is required for vacuolar fusion of guard cells during stomatal opening in Arabidopsis

The reorganization of actin filaments (AFs) and vacuoles in guard cells is involved in the regulation of stomatal movement. However, it remains unclear whether there is any interaction between the reorganization of AFs and vacuolar changes during stomatal movement. Here, we report the relationship between the reorganization of AFs and vacuolar fusion revealed in pharmacological experiments, and characterizing stomatal opening in actin-related protein 2 (arp2) and arp3 mutants. Our results show that cytochalasin-D-induced depolymerization or phalloidin-induced stabilization of AFs leads to an increase in small unfused vacuoles during stomatal opening in wild-type (WT) Arabidopsis plants. Light-induced stomatal opening is retarded and vacuolar fusion in guard cells is impaired in the mutants, in which the reorganization and the dynamic parameters of AFs are aberrant compared with those of the WT. In WT, AFs tightly surround the small separated vacuoles, forming a ring that encircles the boundary membranes of vacuoles partly fused during stomatal opening. In contrast, in the mutants, most AFs and actin patches accumulate abnormally around the nuclei of the guard cells, which probably further impair vacuolar fusion and retard stomatal opening. Our results suggest that the reorganization of AFs regulates vacuolar fusion in guard cells during stomatal opening.

Gravitropism and mechanical signaling in plants

Mechanical stress is a critical signal affecting morphogenesis and growth and is caused by a large variety of environmental stimuli such as touch, wind, and gravity in addition to endogenous forces generated by growth. On the basis of studies dating from the early 19th century, the plant mechanical sensors and response components related to gravity can be divided into two types in terms of their temporal character: sensors of the transient stress of reorientation (phasic signaling) and sensors capable of monitoring and responding to the extended, continuous gravitropic signal for the duration of the tropic growth response (tonic signaling). In the case of transient stress, changes in the concentrations of ions in the cytoplasm play a central role in mechanosensing and are likely a key component of initial gravisensing. Potential candidates for mechanosensitive channels have been identified in Arabidopsis thaliana and may provide clues to these rapid, ionic gravisensing mechanisms. Continuous mechanical stress, on the other hand, may be sensed by other mechanisms in addition to the rapidly adapting mechnaosensitive channels of the phasic system. Sustaining such long-term responses may be through a network of biochemical signaling cascades that would therefore need to be maintained for the many hours of the growth response once they are triggered. However, classical physiological analyses and recent simulation studies also suggest involvement of the cytoskeleton in sensing/responding to long-term mechanoresponse independently of the biochemical signaling cascades triggered by initial graviperception events.

Abscisic Acid synthesis and response

Abscisic acid (ABA) is one of the "classical" plant hormones, i.e. discovered at least 50 years ago, that regulates many aspects of plant growth and development. This chapter reviews our current understanding of ABA synthesis, metabolism, transport, and signal transduction, emphasizing knowledge gained from studies of Arabidopsis. A combination of genetic, molecular and biochemical studies has identified nearly all of the enzymes involved in ABA metabolism, almost 200 loci regulating ABA response, and thousands of genes regulated by ABA in various contexts. Some of these regulators are implicated in cross-talk with other developmental, environmental or hormonal signals. Specific details of the ABA signaling mechanisms vary among tissues or developmental stages; these are discussed in the context of ABA effects on seed maturation, germination, seedling growth, vegetative stress responses, stomatal regulation, pathogen response, flowering, and senescence.

Cadmium impairs ion homeostasis by altering K+ and Ca2+ channel activities in rice root hair cells

Cadmium (Cd2+) interferes with the uptake, transport and utilization of several macro- and micronutrients, which accounts, at least in part, for Cd2+ toxicity in plants. However, the mechanisms underlying Cd2+ interference of ionic homeostasis is not understood. Using biophysical techniques including membrane potential measurements, scanning ion-selective electrode technique for non-invasive ion flux assays and patch clamp, we monitored the effect of Cd2+ on calcium (Ca2+) and potassium (K+) transport in root hair cells of rice. Our results showed that K+ and Ca2+ contents in both roots and shoots were significantly reduced when treated with exogenous Cd2+. Further studies revealed that three cellular processes may be affected by Cd2+, leading to changes in ionic homeostasis. First, Cd2+ -induced depolarization of the membrane potential was observed in root hair cells, attenuating the driving force for cation uptake. Second, the inward conductance of Ca2+ and K+ was partially blocked by Cd2+, decreasing uptake of K+ and Ca2+ . Third, the outward K+ conductance was Cd2+ -inducible, decreasing the net content of K+ in roots. These results provide direct evidence that Cd2+ impairs uptake of Ca2+ and K+, thereby disturbing ion homeostasis in plants.

Disrupted actin dynamics trigger an increment in the reactive oxygen species levels in the Arabidopsis root under salt stress

Changes in actin dynamics represent the primary response of the plant cell to extracellular signaling. Recent studies have now revealed that actin remodeling is involved in abiotic stress tolerance in plants. In our current study, the relationship between the changes in actin dynamics and the reactive oxygen species (ROS) level at the initial stages of salt stress was investigated in the elongation zone of the Arabidopsis root tip. We found that a 200 mM NaCl treatment disrupted the dynamics of the actin filaments within 10 min and increased the ROS levels in the elongation zone cells of the Arabidopsis root tip. We further found that the NADPH oxidase activity inhibitor, diphenyleneiodonium, treatment blocked this ROS increase under salt stress conditions. The roles of actin dynamics and the NADPH oxidases in ROS generation were further analyzed using the actin-specific agents, latrunculin B (Lat-B) and jasplakinolide (Jasp), and mutants of Arabidopsis NADPH oxidase AtrbohC. Lat-B and Jasp promote actin depolymerization and polymerization, respectively, and both were found to enhance the ROS levels following NaCl treatment. However, this response was abolished in the atrbohC mutants. Our present results thus demonstrate that actin dynamics are involved in regulating the ROS level in Arabidopsis root under salt stress conditions.

KEY MESSAGE

Salt stress disrupts the dynamics of the actin filaments in Arabidopsis in the short term which are involved in regulating the ROS levels that arise under salt stress conditions via the actions of the AtrbohC.

A type of voltage-dependent Ca2+ channel on Vicia faba guard cell plasma membrane outwardly permeates K+

The fine regulation of stomatal aperture is important for both plant photosynthesis and transpiration, while stomatal closing is an essential plant response to biotic and abiotic stresses such as drought, salinity, wounding, and pathogens. Quick stomatal closing is primarily due to rapid solute loss. Cytosolic free calcium ([Ca(2+)](cyt)) is a ubiquitous second messenger, and its elevation or oscillation plays important roles in stomatal movements, which can be triggered by the opening of Ca(2+)-permeable channels on the plasma membrane. For Ca(2+)-permeable channel recordings, Ba(2+) is preferred as a charge-carrying ion because it has higher permeability to Ca(2+) channels and blocks K(+) channel activities to facilitate current recordings; however, it prevents visualization of Ca(2+) channels' K(+) permeability. Here, we employed Ca(2+) instead of Ba(2+) in recording Ca(2+)-permeable channels on Vicia faba guard cell plasma membrane to mimic physiological solute conditions inside guard cells more accurately. Inward Ca(2+) currents could be recorded at the single-channel level, and these currents could be inhibited by micromolar Gd(3+), but their reversal potential is far away from the theoretical equilibrium potential for Ca(2+). Further experiments showed that the discrepancy of the reversal potential of the recorded Ca(2+) currents is influenced by cytosolic K(+). This suggests that voltage-dependent Ca(2+) channels also mediate K(+) efflux at depolarization voltages. In addition, a new kind of high-conductance channels with fivefold to normal Ca(2+) channel and 18-fold to normal outward K(+) conductance was found. Our data presented here suggest that plants have their own saving strategies in their rapid response to stress stimuli, and multiple kinds of hyperpolarization-activated Ca(2+)-permeable channels coexist on plasma membranes.

Targeting of Cameleons to various subcellular compartments reveals a strict cytoplasmic/mitochondrial Ca²⁺ handling relationship in plant cells

Here we describe use of a mitochondrial targeted Cameleon to produce stably transformed Arabidopsis plants that enable analyses of mitochondrial Ca²⁺ dynamics in planta and allow monitoring of the intra-mitochondrial Ca²⁺ concentration in response to physiological or environmental stimuli. Transgenic plants co-expressing nuclear and mitochondrial targeted Cameleons were also generated and analyzed. Here we show that mitochondrial Ca²⁺ accumulation is strictly related to the intensity of the cytoplasmic Ca²⁺ increase, demonstrating a tight association between mitochondrial and cytoplasmic Ca²⁺ dynamics. However, under all experimental conditions, mitochondrial Ca²⁺ dynamics were substantially different from those monitored in the cytoplasm, demonstrating that mitochondria do not passively sense cytosolic Ca²⁺, but actively modulate the intra-mitochondrial level of the cation. In particular, our analyses show that the kinetics of Ca²⁺ release from mitochondria are much slower than in the cytoplasm and nucleus. The mechanisms and functional implications of these differences are discussed.

The ARP2/3 complex mediates guard cell actin reorganization and stomatal movement in Arabidopsis

Guard cell actin reorganization has been observed in stomatal responses to a wide array of stimuli. However, how the guard cell signaling machinery regulates actin dynamics is poorly understood. Here, we report the identification of an allele of the Arabidopsis thaliana ACTIN-RELATED PROTEIN C2/DISTORTED TRICHOMES2 (ARPC2) locus (encoding the ARPC2 subunit of the ARP2/3 complex) designated high sugar response3 (hsr3). The hsr3 mutant showed increased transpirational water loss that was mainly due to a lesion in stomatal regulation. Stomatal bioassay analyses revealed that guard cell sensitivity to external stimuli, such as abscisic acid (ABA), CaCl(2), and light/dark transition, was reduced or abolished in hsr3. Analysis of a nonallelic mutant of the ARP2/3 complex suggested no pleiotropic effect of ARPC2 beyond its function in the complex in regard to stomatal regulation. When treated with ABA, guard cell actin filaments underwent fast disruption in wild-type plants, whereas those in hsr3 remained largely bundled. The ABA insensitivity phenotype of hsr3 was rescued by cytochalasin D treatment, suggesting that the aberrant stomatal response was a consequence of bundled actin filaments. Our work indicates that regulation of actin reassembly through ARP2/3 complex activity is crucial for stomatal regulation.

Stochastic dynamics of actin filaments in guard cells regulating chloroplast localization during stomatal movement

Actin filaments and chloroplasts in guard cells play roles in stomatal function. However, detailed actin dynamics vary, and the roles that they play in chloroplast localization during stomatal movement remain to be determined. We examined the dynamics of actin filaments and chloroplast localization in transgenic tobacco expressing green fluorescent protein (GFP)-mouse talin in guard cells by time-lapse imaging. Actin filaments showed sliding, bundling and branching dynamics in moving guard cells. During stomatal movement, long filaments can be severed into small fragments, which can form longer filaments by end-joining activities. With chloroplast movement, actin filaments near chloroplasts showed severing and elongation activity in guard cells during stomatal movement. Cytochalasin B treatment abolished elongation, bundling and branching activities of actin filaments in guard cells, and these changes of actin filaments, and as a result, more chloroplasts were localized at the centre of guard cells. However, chloroplast turning to avoid high light, and sliding of actin fragments near the chloroplast, was unaffected following cytochalasin B treatment in guard cells. We suggest that the sliding dynamics of actin may play roles in chloroplast turning in guard cells. Our results indicate that the stochastic dynamics of actin filaments in guard cells regulate chloroplast localization during stomatal movement.

Circadian rhythm leaf movement of Phaseolus vulgaris and the role of calcium ions

Legume plants, due to their distinctive botanical characteristics, such as leaf movements, physiological characteristics, such as nitrogen fixation, and their abilities to endure environmental stresses, have important roles in sustainable pastures development. Leaf movement of legume plants is turgor regulated and osmotically active fluxes of ions between extensor and flexor of pulvinus cause this movement. To determine the role of calcium ions in circadian leaf movements of Phaseolus vulgaris L., a radiotracer technique experiment using 45Ca ions were employed. Measurements were taken during circadian leaf movements, and samples were taken from different parts of the leaflet. The 45Ca beta-particle activity reduced from leaflet base pulvinus to leaf tip. The pulvinus had the highest activity, while the leaf tip had the lowest. By increase of the ratio of 45Ca beta-particle activity within flexor to extensor (Fl/Ex) the midrib-petiole angle, as an indicator of leaf movement, increased linearly during circadian leaf movement (r = 0.86). The 45Ca beta-particle activity of Flex/Ext ratio reduced linearly (r = -0.88) toward midnight. In conclusion, it was found that calcium ions accumulation is opposite to the fluxes of osmatically active ions and water movement. Calcium ions accumulate at less negative water potential side of the pulivnus.

Vacuolar two-pore K+ channels act as vacuolar osmosensors

• Plant two-pore K(+) channels (TPKs) have been shown previously to play a role in vacuolar K(+) homeostasis. TPK activity is insensitive to membrane voltage, but regulated by cytoplasmic calcium and 14-3-3 proteins. This study reports that membrane stretch and osmotic gradients also alter the activity of TPKs from Arabidopsis, rice and barley, and that this may have a physiological relevance for osmotic homeostasis. • Mechanosensitivity was studied using patch clamp experiments and TPKs from Arabidopsis, rice and barley. In addition, the capability of TPKs to act as osmosensors was determined. By using protoplast disruption assays and intact plant survival assays, in genotypes that differed in TPK expression, the physiological relevance of TPK-based osmosensing was tested. • TPKs from all three species showed varying degrees of mechanosensitivity. TPK activity in channels from all three species was sensitive to trans-tonoplast osmotic gradients. TPK osmosensing is likely to proceed via the detection of small perturbations in membrane tension. Intact plant and protoplast assays showed that TPK-based osmosensing is important during exposure to rapid changes in external osmolarity. • Vacuolar TPK channels can act as intracellular osmosensors and rapidly increase channel activity during hypo-osmotic shock to release vacuolar K(+) .

Extracellular Ca²+ alleviates NaCl-induced stomatal opening through a pathway involving H₂O₂-blocked Na+ influx in Vicia guard cells

To gain further insights into the function of extracellular Ca²+ in alleviating salt stress, Vicia faba guard cell protoplasts (GCPs) were patch-clamped in a whole-cell configuration. The results showed that 100 mM NaCl clearly induced Na+ influx across the plasma membrane in GCPs and promoted stomatal opening. Extracellular Ca²+ at 10 mM efficiently blocked Na+ influx and inhibited stomatal opening, which was partially abolished by La³+ (an inhibitor of plasma membrane Ca²+ channel) or catalase (CAT, a H₂O₂ scavenger), respectively. These results suggest that the plasma membrane Ca²+ channels and H₂O₂ possibly mediate extracellular Ca²+-blocked Na+ influx in GCPs. Furthermore, extracellular Ca²+ activated the plasma membrane Ca²+ channels under NaCl stress, which was partially abolished by CAT. These results, taken together, indicate that hydrogen peroxide (H₂O₂) likely regulates Na+ uptake by activating plasma membrane Ca²+ channels in GCPs. In accordance with this hypothesis, H₂O₂ could mimic extracellular Ca²+ to activate Ca²+ channels and block Na+ influx in guard cells. A single-cell analysis of cytosolic free Ca²+ ([Ca²+](cyt)) using Fluo 3-AM revealed that extracellular Ca²+ induced the accumulation of cytosolic Ca²+ under NaCl stress, but had few effects on the accumulation of cytosolic Ca²+ under non-NaCl conditions. All of these results, together with our previous studies showing that extracellular Ca²+ induced the generation of H₂O₂ in GCPs during NaCl stress, indicate that extracellular Ca²+ alleviates salt stress, likely by activating the H₂O₂-dependent plasma membrane Ca²+ channels, and the increase in cytosolic Ca²+ appears to block Na+ influx across the plasma membrane in Vicia guard cells, leading to stomatal closure and reduction of water loss.

The plant-specific actin binding protein SCAB1 stabilizes actin filaments and regulates stomatal movement in Arabidopsis

Microfilament dynamics play a critical role in regulating stomatal movement; however, the molecular mechanism underlying this process is not well understood. We report here the identification and characterization of STOMATAL CLOSURE-RELATED ACTIN BINDING PROTEIN1 (SCAB1), an Arabidopsis thaliana actin binding protein. Plants lacking SCAB1 were hypersensitive to drought stress and exhibited reduced abscisic acid-, H(2)O(2)-, and CaCl(2)-regulated stomatal movement. In vitro and in vivo analyses revealed that SCAB1 binds, stabilizes, and bundles actin filaments. SCAB1 shares sequence similarity only with plant proteins and contains a previously undiscovered actin binding domain. During stomatal closure, actin filaments switched from a radial orientation in open stomata to a longitudinal orientation in closed stomata. This switch took longer in scab1 plants than in wild-type plants and was correlated with the delay in stomatal closure seen in scab1 mutants in response to drought stress. Our results suggest that SCAB1 is required for the precise regulation of actin filament reorganization during stomatal closure.

Actin microfilaments and vacuoles are downstream targets of H2O2 signalling pathways in hyperosmotic stress-induced stomatal closure

Changes in osmotic pressure can induce stomatal closure to reduce transpirational water loss from plants. In the present work, we investigated the mechanism underlying the perception and transduction of extracellular changes in osmotic pressure in Vicia faba L. guard cells. Using an epidermal strip bioassay and laser-scanning confocal microscopy, we provide evidence that hyperosmotic stress treatment led to stomatal closure and the rapid promotion of hydrogen peroxide (H2O2) production in V. faba guard cells. The effects were largely reduced by H2O2 scavengers ASA, CAT, NADPH oxidase inhibitor DPI and cell wall peroxidase inhibitor SHAM. These results indicate that hyperosmotic stress induces stomatal closure by promoting H2O2 production. Cytochalasin B (CB), latrunculin B (Lat B) and jasplakinolide (JK) inhibited stomatal closure induced by hyperosmotic stress but didn't prevent the increase of endogenous H2O2 levels, suggesting that microfilaments reorganisation participates in stomatal closure induced by hyperosmotic stress, and may act downstream of H2O2 signalling processes. In addition, we observed splitting of big vacuoles into many small vacuoles in response to hyperosmotic stress and H2O2 treatment, and CB inhibited these changes of vacuoles; stomatal closure was also inhibited. Taken together these results indicate that the stomatal closure in response to hyperosmotic stress may initiate H2O2 generation, and that reorganisation of microfilaments and the changing of vacuoles occurs downstream of H2O2 signalling processes.