Extracellular Ba2+ and voltage interact to gate Ca2+ channels at the plasma membrane of stomatal guard cells

The CDPK-related protein kinase HvCRK2 and HvCRK4 interact with HvCML32 to negatively regulate drought tolerance in transgenic Arabidopsis thaliana

Calcium-dependent protein kinase (CDPK) as one of calcium sensors were play important roles in stress responses. CDPK-related protein kinase (CRK) was identified as subgroup III of CDPK has been characterized in many plants, but the members and functions of CRK genes in hulless barley (Hordeum vulgare L.) has not been clarified. Here, we identified four HvCRK genes and named HvCRK1-4 according to chromosomes localization. Moreover, the physiological function of highly induced genes of HvCRK2 and HvCRK4 were investigated in drought stress tolerance by examining their overexpression transgenic lines functions generated in Arabidopsis thaliana. Under drought stress, both overexpression HvCRK2 and HvCRK4 displayed reduced drought resistance, and accompanied by higher accumulation levels of ROS. Notably, overexpression of HvCRK2 and HvCRK4 reduced sensitivity to exogenous ABA, meanwhile the expression of ABA-responsive genes in transgenic plants were down-regulated compared to the wild type in response to drought stress. Furthermore, the physically interaction of HvCRK2 and HvCRK4 with calmodulin (CaM) and calmodulin-like (CML) proteins were determined in vivo, the further results showed that HvCML32 binds to HvCRK2/4 S_TKC structural domains and negatively regulates drought tolerance. In summary, this study identified HvCRK members and indicated that HvCRK2 and HvCRK4 genes play negative roles in drought tolerance, and provide insight into potential molecular mechanism of HvCRK2 and HvCRK4 in response to drought stress.

Overexpression of tonoplast Ca2+-ATPase in guard cells synergistically enhances stomatal opening and drought tolerance

Stomata play a crucial role in plants by controlling water status and responding to drought stress. However, simultaneously improving stomatal opening and drought tolerance has proven to be a significant challenge. To address this issue, we employed the OnGuard quantitative model, which accurately represents the mechanics and coordination of ion transporters in guard cells. With the guidance of OnGuard, we successfully engineered plants that overexpressed the main tonoplast Ca2+-ATPase gene, ACA11, which promotes stomatal opening and enhances plant growth. Surprisingly, these transgenic plants also exhibited improved drought tolerance due to reduced water loss through their stomata. Again, OnGuard assisted us in understanding the mechanism behind the unexpected stomatal behaviors observed in the ACA11 overexpressing plants. Our study revealed that the overexpression of ACA11 facilitated the accumulation of Ca2+ in the vacuole, thereby influencing Ca2+ storage and leading to an enhanced Ca2+ elevation in response to abscisic acid. This regulatory cascade finely tunes stomatal responses, ultimately leading to enhanced drought tolerance. Our findings underscore the importance of tonoplast Ca2+-ATPase in manipulating stomatal behavior and improving drought tolerance. Furthermore, these results highlight the diverse functions of tonoplast-localized ACA11 in response to different conditions, emphasizing its potential for future applications in plant enhancement.

A charged existence: A century of transmembrane ion transport in plants

If the past century marked the birth of membrane transport as a focus for research in plants, the past 50 years has seen the field mature from arcane interest to a central pillar of plant physiology. Ion transport across plant membranes accounts for roughly 30% of the metabolic energy consumed by a plant cell, and it underpins virtually every aspect of plant biology, from mineral nutrition, cell expansion, and development to auxin polarity, fertilization, plant pathogen defense, and senescence. The means to quantify ion flux through individual transporters, even single channel proteins, became widely available as voltage clamp methods expanded from giant algal cells to the fungus Neurospora crassa in the 1970s and the cells of angiosperms in the 1980s. Here, I touch briefly on some key aspects of the development of modern electrophysiology with a focus on the guard cells of stomata, now without dispute the premier plant cell model for ion transport and its regulation. Guard cells have proven to be a crucible for many technical and conceptual developments that have since emerged into the mainstream of plant science. Their study continues to provide fundamental insights and carries much importance for the global challenges that face us today.

Stomatal Responses of Two Drought-Tolerant Barley Varieties with Different ROS Regulation Strategies under Drought Conditions

Drought stress is a major obstacle to agricultural production. Stomata are central to efforts to improve photosynthesis and water use. They are targets for manipulation to improve both processes and the balance between them. An in-depth understanding of stomatal behavior and kinetics is important for improving photosynthesis and the WUE of crops. In this study, a drought stress pot experiment was performed, and a transcriptome analysis of the leaves of three contrasting, cultivated barley genotypes Lumley (Lum, drought-tolerant), Golden Promise (GP, drought-sensitive), and Tadmor (Tad, drought-tolerant), generated by high-throughput sequencing, were compared. Lum exhibited a different WUE at the leaf and whole-plant levels and had greater CO₂ assimilation, with a higher g_s under drought stress. Interestingly, Lum showed a slower stomatal closure in response to a light-dark transition and significant differences compared to Tad in stomatal response to the exogenous application of ABA, H₂O₂, and CaCl₂. A transcriptome analysis revealed that 24 ROS-related genes were indeed involved in drought response regulation, and impaired ABA-induced ROS accumulation in Lum was identified using ROS and antioxidant capacity measurements. We conclude that different stomatal ROS responses affect stomatal closure in barley, demonstrating different drought regulation strategies. These results provide valuable insight into the physiological and molecular basis of stomatal behavior and drought tolerance in barley.

Membrane voltage as a dynamic platform for spatiotemporal signaling, physiological, and developmental regulation

Membrane voltage arises from the transport of ions through ion-translocating ATPases, ion-coupled transport of solutes, and ion channels, and is an integral part of the bioenergetic "currency" of the membrane. The dynamics of membrane voltage-so-called action, systemic, and variation potentials-have also led to a recognition of their contributions to signal transduction, both within cells and across tissues. Here, we review the origins of our understanding of membrane voltage and its place as a central element in regulating transport and signal transmission. We stress the importance of understanding voltage as a common intermediate that acts both as a driving force for transport-an electrical "substrate"-and as a product of charge flux across the membrane, thereby interconnecting all charge-carrying transport across the membrane. The voltage interconnection is vital to signaling via second messengers that rely on ion flux, including cytosolic free Ca2+, H+, and the synthesis of reactive oxygen species generated by integral membrane, respiratory burst oxidases. These characteristics inform on the ways in which long-distance voltage signals and voltage oscillations give rise to unique gene expression patterns and influence physiological, developmental, and adaptive responses such as systemic acquired resistance to pathogens and to insect herbivory.

Stomatal immunity against fungal invasion comprises not only chitin-induced stomatal closure but also chitosan-induced guard cell death

Many pathogenic fungi exploit stomata as invasion routes, causing destructive diseases of major cereal crops. Intensive interaction is expected to occur between guard cells and fungi. In the present study, we took advantage of well-conserved molecules derived from the fungal cell wall, chitin oligosaccharide (CTOS), and chitosan oligosaccharide (CSOS) to study how guard cells respond to fungal invasion. In Arabidopsis, CTOS induced stomatal closure through a signaling mediated by its receptor CERK1, Ca2+, and a major S-type anion channel, SLAC1. CSOS, which is converted from CTOS by chitin deacetylases from invading fungi, did not induce stomatal closure, suggesting that this conversion is a fungal strategy to evade stomatal closure. At higher concentrations, CSOS but not CTOS induced guard cell death in a manner dependent on Ca2+ but not CERK1. These results suggest that stomatal immunity against fungal invasion comprises not only CTOS-induced stomatal closure but also CSOS-induced guard cell death.

Mg2+ Is a Missing Link in Plant Cell Ca2+ Signalling and Homeostasis-A Study on Vicia faba Guard Cells

Hyperpolarization-activated calcium channels (HACCs) are found in the plasma membrane and tonoplast of many plant cell types, where they have an important role in Ca²⁺-dependent signalling. The unusual gating properties of HACCs in plants, i.e., activation by membrane hyperpolarization rather than depolarization, dictates that HACCs are normally open in the physiological hyperpolarized resting membrane potential state (the so-called pump or P-state); thus, if not regulated, they would continuously leak Ca²⁺ into cells. HACCs are permeable to Ca²⁺, Ba²⁺, and Mg²⁺; activated by H₂O₂ and the plant hormone abscisic acid (ABA); and their activity in guard cells is greatly reduced by increasing amounts of free cytosolic Ca²⁺ ([Ca²⁺]_Cyt), and hence closes during [Ca²⁺]_Cyt surges. Here, we demonstrate that the presence of the commonly used Mg-ATP inside the guard cell greatly reduces HACC activity, especially at voltages ≤ −200 mV, and that Mg²⁺ causes this block. Therefore, we firstly conclude that physiological cytosolic Mg²⁺ levels affect HACC gating and that channel opening requires either high negative voltages (≥−200 mV) or displacement of Mg²⁺ away from the immediate vicinity of the channel. Secondly, based on structural comparisons with a Mg²⁺-sensitive animal inward-rectifying K⁺ channel, we propose that the likely candidate HACCs described here are cyclic nucleotide gated channels (CNGCs), many of which also contain a conserved diacidic Mg²⁺ binding motif within their pores. This conclusion is consistent with the electrophysiological data. Finally, we propose that Mg²⁺, much like in animal cells, is an important component in Ca²⁺ signalling and homeostasis in plants.

Plant abiotic stress response and nutrient use efficiency

Abiotic stresses and soil nutrient limitations are major environmental conditions that reduce plant growth, productivity and quality. Plants have evolved mechanisms to perceive these environmental challenges, transmit the stress signals within cells as well as between cells and tissues, and make appropriate adjustments in their growth and development in order to survive and reproduce. In recent years, significant progress has been made on many fronts of the stress signaling research, particularly in understanding the downstream signaling events that culminate at the activation of stress- and nutrient limitation-responsive genes, cellular ion homeostasis, and growth adjustment. However, the revelation of the early events of stress signaling, particularly the identification of primary stress sensors, still lags behind. In this review, we summarize recent work on the genetic and molecular mechanisms of plant abiotic stress and nutrient limitation sensing and signaling and discuss new directions for future studies.

Traversing the Links between Heavy Metal Stress and Plant Signaling

Plants confront multifarious environmental stresses widely divided into abiotic and biotic stresses, of which heavy metal stress represents one of the most damaging abiotic stresses. Heavy metals cause toxicity by targeting crucial molecules and vital processes in the plant cell. One of the approaches by which heavy metals act in plants is by over production of reactive oxygen species (ROS) either directly or indirectly. Plants act against such overdose of metal in the environment by boosting the defense responses like metal chelation, sequestration into vacuole, regulation of metal intake by transporters, and intensification of antioxidative mechanisms. This response shown by plants is the result of intricate signaling networks functioning in the cell in order to transmit the extracellular stimuli into an intracellular response. The crucial signaling components involved are calcium signaling, hormone signaling, and mitogen activated protein kinase (MAPK) signaling that are discussed in this review. Apart from signaling components other regulators like microRNAs and transcription factors also have a major contribution in regulating heavy metal stress. This review demonstrates the key role of MAPKs in synchronously controlling the other signaling components and regulators in metal stress. Further, attempts have been made to focus on metal transporters and chelators that are regulated by MAPK signaling.

PYR/PYL/RCAR abscisic acid receptors regulate K+ and Cl- channels through reactive oxygen species-mediated activation of Ca2+ channels at the plasma membrane of intact Arabidopsis guard cells

The discovery of the START family of abscisic acid (ABA) receptors places these proteins at the front of a protein kinase/phosphatase signal cascade that promotes stomatal closure. The connection of these receptors to Ca(2+) signals evoked by ABA has proven more difficult to resolve, although it has been implicated by studies of the pyrbactin-insensitive pyr1/pyl1/pyl2/pyl4 quadruple mutant. One difficulty is that flux through plasma membrane Ca(2+) channels and Ca(2+) release from endomembrane stores coordinately elevate cytosolic free Ca(2+) concentration ([Ca(2+)]i) in guard cells, and both processes are facilitated by ABA. Here, we describe a method for recording Ca(2+) channels at the plasma membrane of intact guard cells of Arabidopsis (Arabidopsis thaliana). We have used this method to resolve the loss of ABA-evoked Ca(2+) channel activity at the plasma membrane in the pyr1/pyl1/pyl2/pyl4 mutant and show the consequent suppression of [Ca(2+)]i increases in vivo. The basal activity of Ca(2+) channels was not affected in the mutant; raising the concentration of Ca(2+) outside was sufficient to promote Ca(2+) entry, to inactivate current carried by inward-rectifying K(+) channels and to activate current carried by the anion channels, both of which are sensitive to [Ca(2+)]i elevations. However, the ABA-dependent increase in reactive oxygen species (ROS) was impaired. Adding the ROS hydrogen peroxide was sufficient to activate the Ca(2+) channels and trigger stomatal closure in the mutant. These results offer direct evidence of PYR/PYL/RCAR receptor coupling to the activation by ABA of plasma membrane Ca(2+) channels through ROS, thus affecting [Ca(2+)]i and its regulation of stomatal closure.

OnGuard, a computational platform for quantitative kinetic modeling of guard cell physiology

Stomatal guard cells play a key role in gas exchange for photosynthesis while minimizing transpirational water loss from plants by opening and closing the stomatal pore. Foliar gas exchange has long been incorporated into mathematical models, several of which are robust enough to recapitulate transpirational characteristics at the whole-plant and community levels. Few models of stomata have been developed from the bottom up, however, and none are sufficiently generalized to be widely applicable in predicting stomatal behavior at a cellular level. We describe here the construction of computational models for the guard cell, building on the wealth of biophysical and kinetic knowledge available for guard cell transport, signaling, and homeostasis. The OnGuard software was constructed with the HoTSig library to incorporate explicitly all of the fundamental properties for transporters at the plasma membrane and tonoplast, the salient features of osmolite metabolism, and the major controls of cytosolic-free Ca²⁺ concentration and pH. The library engenders a structured approach to tier and interrelate computational elements, and the OnGuard software allows ready access to parameters and equations 'on the fly' while enabling the network of components within each model to interact computationally. We show that an OnGuard model readily achieves stability in a set of physiologically sensible baseline or Reference States; we also show the robustness of these Reference States in adjusting to changes in environmental parameters and the activities of major groups of transporters both at the tonoplast and plasma membrane. The following article addresses the predictive power of the OnGuard model to generate unexpected and counterintuitive outputs.

Systems dynamic modeling of the stomatal guard cell predicts emergent behaviors in transport, signaling, and volume control

The dynamics of stomatal movements and their consequences for photosynthesis and transpirational water loss have long been incorporated into mathematical models, but none have been developed from the bottom up that are widely applicable in predicting stomatal behavior at a cellular level. We previously established a systems dynamic model incorporating explicitly the wealth of biophysical and kinetic knowledge available for guard cell transport, signaling, and homeostasis. Here we describe the behavior of the model in response to experimentally documented changes in primary pump activities and malate (Mal) synthesis imposed over a diurnal cycle. We show that the model successfully recapitulates the cyclic variations in H⁺, K⁺, Cl⁻, and Mal concentrations in the cytosol and vacuole known for guard cells. It also yields a number of unexpected and counterintuitive outputs. Among these, we report a diurnal elevation in cytosolic-free Ca²⁺ concentration and an exchange of vacuolar Cl⁻ with Mal, both of which find substantiation in the literature but had previously been suggested to require additional and complex levels of regulation. These findings highlight the true predictive power of the OnGuard model in providing a framework for systems analysis of stomatal guard cells, and they demonstrate the utility of the OnGuard software and HoTSig library in exploring fundamental problems in cellular physiology and homeostasis.

Dynamic regulation of guard cell anion channels by cytosolic free Ca2+ concentration and protein phosphorylation

In guard cells, activation of anion channels (I(anion)) is an early event leading to stomatal closure. Activation of I(anion) has been associated with abscisic acid (ABA) and its elevation of the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). However, the dynamics of the action of [Ca(2+)](i) on I(anion) has never been established, despite its importance for understanding the mechanics of stomatal adaptation to stress. We have quantified the [Ca(2+)](i) dynamics of I(anion) in Vicia faba guard cells, measuring channel current under a voltage clamp while manipulating and recording [Ca(2+)](i) using Fura-2 fluorescence imaging. We found that I(anion) rises with [Ca(2+)](i) only at concentrations substantially above the mean resting value of 125 +/- 13 nm, yielding an apparent K(d) of 720 +/- 65 nm and a Hill coefficient consistent with the binding of three to four Ca(2+) ions to activate the channels. Approximately 30% of guard cells exhibited a baseline of I(anion) activity, but without a dependence of the current on [Ca(2+)](i). The protein phosphatase antagonist okadaic acid increased this current baseline over twofold. Additionally, okadaic acid altered the [Ca(2+)](i) sensitivity of I(anion), displacing the apparent K(d) for [Ca(2+)](i) to 573 +/- 38 nm. These findings support previous evidence for different modes of regulation for I(anion), only one of which depends on [Ca(2+)](i), and they underscore an independence of [Ca(2+)](i) from protein (de-)phosphorylation in controlling I(anion). Most importantly, our results demonstrate a significant displacement of I(anion) sensitivity to higher [Ca(2+)](i) compared with that of the guard cell K(+) channels, implying a capacity for variable dynamics between net osmotic solute uptake and loss.

Functional interaction of the SNARE protein NtSyp121 in Ca2+ channel gating, Ca2+ transients and ABA signalling of stomatal guard cells

There is now growing evidence that membrane vesicle trafficking proteins, especially of the superfamily of SNAREs, are critical for cellular signalling in plants. Work from this laboratory first demonstrated that a soluble, inhibitory (dominant-negative) fragment of the SNARE NtSyp121 blocked K+ and Cl- channel responses to the stress-related hormone abscisic acid (ABA), but left open a question about functional impacts on signal intermediates, especially on Ca2+-mediated signalling events. Here, we report one mode of action for the SNARE mediated directly through alterations in Ca2+ channel gating and its consequent effects on cytosolic-free [Ca2+] ([Ca2+]i) elevation. We find that expressing the same inhibitory fragment of NtSyp121 blocks ABA-evoked stomatal closure, but only partially suppresses stomatal closure in the presence of the NO donor, SNAP, which promotes [Ca2+]i elevation independently of the plasma membrane Ca2+ channels. Consistent with these observations, Ca2+ channel gating at the plasma membrane is altered by the SNARE fragment in a manner effective in reducing the potential for triggering a rise in [Ca2+]i, and we show directly that its expression in vivo leads to a pronounced suppression of evoked [Ca2+]i transients. These observations offer primary evidence for the functional coupling of the SNARE with Ca2+ channels at the plant cell plasma membrane and, because [Ca2+]i plays a key role in the control of K+ and Cl- channel currents in guard cells, they underscore an important mechanism for SNARE integration with ion channel regulation during stomatal closure.

The generation of Ca(2+) signals in plants

The calcium ion is firmly established as a ubiquitous intracellular second messenger in plants. At their simplest, Ca(2+)-based signaling systems are composed of a receptor, a system for generating the increase in [Ca(2+)]cyt, downstream components that are capable of reacting to the increase in [Ca(2+)]cyt, and other cellular systems responsible for returning [Ca(2+)]cyt to its prestimulus level. Here we review the various mechanisms responsible for generating the stimulus-induced increases in [Ca(2+)]cyt known as Ca(2+) signals. We focus particularly on the mechanisms responsible for generating [Ca(2+)]cyt oscillations and transients and use Nod Factor signaling in legume root hairs and stimulus-response coupling in guard cells to assess the physiological significance of these classes of Ca(2+) signals.

Heavy metal toxicity: cadmium permeates through calcium channels and disturbs the plant water status

Because plant wilting has been described as a consequence of cadmium (Cd2+) toxicity, we investigate Cd2+ effects on plant water losses, gas exchanges and stomatal behaviour in Arabidopsis thaliana L. Effects of 1-week Cd2+ application in hydroponic condition (CdCl2 10-100 micro m) were analyzed. A 10- micro m Cd2+ concentration had no significant effect on the plant-water relationship and carbon assimilation. At higher Cd2+ concentrations, a Cd2+ -dependent decrease in leaf conductance and CO2 uptake was observed despite the photosynthetic apparatus appeared not to be affected as probed by fluorescence measurements. In epidermal strip bioassays, nanomolar Cd2+ concentrations reduced stomatal opening under light in A. thaliana, Vicia faba and Commelina communis. Application of 5 micro m ABA limited the root-to-shoot translocation of cadmium. However, the Cd2+-induced stomatal closure was likely ABA-independent, since a 5-day treatment with 50 micro m Cd2+ did not affect the plant relative water content. Additionally, a similar Cd2+-induced stomatal closure was observed in the ABA insensitive mutant abi1-1. Interestingly, this mutant displayed a higher transpiration rate than the wild type but did not accumulate more Cd2+, arguing that Cd2+ uptake is not dependent only on the transpiration flow. Application of putative calcium channels inhibitors suppressed the inhibitory effect of Cd2+ in epidermal strip experiments, suggesting that Cd2+ could enter the guard cell through calcium channels. Patch-clamp studies with V. faba guard cell protoplasts showed that plasma membrane K+ channels were insensitive to external Cd2+ application whereas Ca2+ channels were found permeable to Cd2+. In conclusion, we propose that Cd2+ affects guard cell regulation in an ABA-independent manner by entering the cytosol via Ca2+ channels.

Protein phosphorylation activates the guard cell Ca2+ channel and is a prerequisite for gating by abscisic acid

Protein phosphorylation and cytosolic-free [Ca2+] ([Ca2+]i) contribute to signalling cascades evoked by the water-stress hormone abscisic acid (ABA) that lead to stomatal closure in higher-plant leaves. ABA activates an inward-rectifying Ca2+ channel at the plasma membrane of stomatal guard cells, promoting Ca2+ entry by shifting the voltage-sensitivity of the channels. Because many of these effects could be mediated by kinase/phosphatase action at the membrane, we examined a role for protein (de-)phosphorylation in plasma membrane patches from Vicia guard cells. Ca2+ channel activity decayed rapidly in excised patches, and recovered on adding ATP (K1/2, 1.3 +/- 0.7 mm) but not the non-hydrolyzable analog ATPgammaS. ABA activation of the channel required the presence of ATP and like ABA, the 1/2 A-type protein phosphatase antagonists okadaic acid (OA) and calyculin A (CA) enhanced Ca2+ channel activity by increasing the open probability and number of active channels. Neither ATP nor the antagonists affected the mean open lifetime of the channel, suggesting an action through changes in closed lifetime distributions. Like ABA, OA and CA shifted the voltage-sensitivities of the Ca2+ current and [Ca2+]i increases in intact guard cells towards positive voltages. OA and CA also augmented the [Ca2+]i rise evoked by hyperpolarization and delayed its recovery. These results demonstrate a membrane-delimited interaction between 1/2 A-type protein phosphatase(s) and the Ca2+ channel or associated proteins, and they are consistent with a role for protein (de-)phosphorylation in ABA signalling mediated directly through Ca2+ channel gating that leads to [Ca2+]i increases in the guard cells.

Genes for calcium-permeable channels in the plasma membrane of plant root cells

In plant cells, Ca(2+) is required for both structural and biophysical roles. In addition, changes in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) orchestrate responses to developmental and environmental signals. In many instances, [Ca(2+)](cyt) is increased by Ca(2+) influx across the plasma membrane through ion channels. Although the electrophysiological and biochemical characteristics of Ca(2+)-permeable channels in the plasma membrane of plant cells are well known, genes encoding putative Ca(2+)-permeable channels have only recently been identified. By comparing the tissue expression patterns and electrophysiology of Ca(2+)-permeable channels in the plasma membrane of root cells with those of genes encoding candidate plasma membrane Ca(2+) channels, the genetic counterparts of specific Ca(2+)-permeable channels can be deduced. Sequence homologies and the physiology of transgenic antisense plants suggest that the Arabidopsis AtTPC1 gene encodes a depolarisation-activated Ca(2+) channel. Members of the annexin gene family are likely to encode hyperpolarisation-activated Ca(2+) channels, based on their corresponding occurrence in secretory or elongating root cells, their inhibition by La(3+) and nifedipine, and their increased activity as [Ca(2+)](cyt) is raised. Based on their electrophysiology and tissue expression patterns, AtSKOR encodes a depolarisation-activated outward-rectifying (Ca(2+)-permeable) K(+) channel (KORC) in stelar cells and AtGORK is likely to encode a KORC in the plasma membrane of other Arabidopsis root cells. Two candidate gene families, of cyclic-nucleotide gated channels (CNGC) and ionotropic glutamate receptor (GLR) homologues, are proposed as the genetic correlates of voltage-independent cation (VIC) channels.