Visualizing Changes in Cytosolic-Free Ca2+ during the Response of Stomatal Guard Cells to Abscisic Acid

Protein Carbonylation: Emerging Roles in Plant Redox Biology and Future Prospects

Plants are sessile in nature and they perceive and react to environmental stresses such as abiotic and biotic factors. These induce a change in the cellular homeostasis of reactive oxygen species (ROS). ROS are known to react with cellular components, including DNA, lipids, and proteins, and to interfere with hormone signaling via several post-translational modifications (PTMs). Protein carbonylation (PC) is a non-enzymatic and irreversible PTM induced by ROS. The non-enzymatic feature of the carbonylation reaction has slowed the efforts to identify functions regulated by PC in plants. Yet, in prokaryotic and animal cells, studies have shown the relevance of protein carbonylation as a signal transduction mechanism in physiological processes including hydrogen peroxide sensing, cell proliferation and survival, ferroptosis, and antioxidant response. In this review, we provide a detailed update on the most recent findings pertaining to the role of PC and its implications in various physiological processes in plants. By leveraging the progress made in bacteria and animals, we highlight the main challenges in studying the impacts of carbonylation on protein functions in vivo and the knowledge gap in plants. Inspired by the success stories in animal sciences, we then suggest a few approaches that could be undertaken to overcome these challenges in plant research. Overall, this review describes the state of protein carbonylation research in plants and proposes new research avenues on the link between protein carbonylation and plant redox biology.

An optimized genetically encoded dual reporter for simultaneous ratio imaging of Ca2+ and H+ reveals new insights into ion signaling in plants

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

A role for calcium-dependent protein kinases in differential CO2 - and ABA-controlled stomatal closing and low CO2 -induced stomatal opening in Arabidopsis

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

Intracellular Ca2+ regulation of H+/Ca2+ antiporter YfkE mediated by a Ca2+ mini-sensor

The H⁺/Ca²⁺ (calcium ion) antiporter (CAX) plays an important role in maintaining cellular Ca²⁺ homeostasis in bacteria, yeast, and plants by promoting Ca²⁺ efflux across the cell membranes. However, how CAX facilitates Ca²⁺ balance in response to dynamic cytosolic Ca²⁺ perturbations is unknown. Here, we identified a type of Ca²⁺ "mini-sensor" in YfkE, a bacterial CAX homolog from Bacillus subtilis. The mini-sensor is formed by six tandem carboxylate residues within the transmembrane (TM)5-6 loop on the intracellular membrane surface. Ca²⁺ binding to the mini-sensor triggers the transition of the transport mode of YfkE from a high-affinity to a low-affinity state. Molecular dynamics simulation and fluorescence resonance energy transfer analysis suggest that Ca²⁺ binding to the mini-sensor causes an adjacent segment, namely, the exchanger inhibitory peptide (XIP), to move toward the Ca²⁺ translocation pathway to interact with TM2a in an inward-open cavity. The specific interaction was demonstrated with a synthetic peptide of the XIP, which inhibits YfkE transport and interrupts conformational changes mediated by the mini-sensor. By comparing the apo and Ca²⁺-bound CAX structures, we propose the following Ca²⁺ transport regulatory mechanism of YfkE: Ca²⁺ binding to the mini-sensor induces allosteric conformational changes in the Ca²⁺ translocation pathway via the XIP, resulting in a rearrangement of the Ca²⁺-binding transport site in the midmembrane. Since the Ca²⁺ mini-sensor and XIP sequences are also identified in other CAX homologs and/or Ca²⁺ transporters, including the mammalian Na⁺/Ca²⁺ exchanger (NCX), our study provides a regulatory mechanism for the Ca²⁺/cation transporter superfamily.

Autophagy-Like Cell Death Regulates Hydrogen Peroxide and Calcium Ion Distribution in Xa3/Xa26-Mediated Resistance to Xanthomonas oryzae pv. oryzae

The broad-spectrum and durable resistance gene Xa3/Xa26 against Xanthomonas oryzae pv. oryzae (Xoo) has been widely exploited in rice production in China. But the cytological features of the Xa3/Xa26-mediated resistance reaction have been rarely reported. This study reveals the cytological characteristics of the Xa3/Xa26-mediated resistance reaction against Xoo to uncover the functions of hypersensitive response programmed cell death (HR-PCD) in rice. Autophagy-like cell death, which was characterized by double-membrane bodies appearance in xylem parenchyma cell and mesophyll cell, was inhibited by autophagy inhibitor 3-methyladenin (3-MA). The autophagy-related genes were induced to reach a high level in resistance reaction. The hydrogen peroxide (H₂O₂) maintained a low concentration on the plasma membrane. The calcium ions localized on the apoplast were transferred into the vacuole. The autophagy inhibitor (3-MA) impaired Xa3/Xa26-mediated resistance by promoting the accumulation of H₂O₂, and inhibited the transfer of extracellular calcium ions into the vacuole in the xylem parenchyma cells and mesophyll cells. Therefore, the HR-PCD belongs to autophagy-like cell death in the Xa3/Xa26-mediated resistance reaction. These results suggest that the autophagy-like cell death participates in the Xa3/Xa26-mediated resistance by negatively regulating H₂O₂ accumulation, in order to abolish oxidative stress and possibly activate calcium ion signals in xylem parenchyma cells of the rice leaf.

Quantitative phosphoproteomics of lectin receptor-like kinase VI.4 dependent abscisic acid response in Arabidopsis thaliana

Lectin receptor-like kinases (LecRKs) play important roles in the responses to adverse environment stress. Abscisic acid (ABA) is a plant hormone involved in plant growth, development and adverse environmental stress responses. Although some studies of ABA response LecRK genes have been reported, the molecular mechanisms of LecRKs regulation of downstream pathways under ABA induction are not well understood. The present study showed that LecRK-VI.4 responded to ABA and negatively regulated stomatal closure. Here, a quantitative phosphoproteomics approach based on mass spectrometry was employed to study the roles of LecRK-VI.4 in the ABA signaling pathway. Metal oxide affinity beads and C18 chromatography were used for phosphopeptide enrichment and separation. The isobaric tags for relative and absolute quantitation were used for profiling the phosphoproteome of mutant lecrk-vi.4-1 and wild-type Col-0 Arabidopsis under normal growth conditions or ABA treatments. In total, 475 unique phosphopeptides were quantified, including 81 phosphopeptides related to LecRK-VI.4 regulation. Gene ontology, protein-protein interaction and motif analysis were performed. The bioinformatics data showed that phosphorylated proteins regulated by LecRK-VI.4 had close relations with factors of stomatal function, which included aquaporin activity, H⁺ pump activity and the Ca²⁺ concentration in the cytoplasm. These data have expanded our understanding of how LecRK-VI.4 regulates ABA-mediated stomatal movements.

Calcium Signalling in Plant Biotic Interactions

Calcium (Ca2+) is a universal second messenger involved in various cellular processes, leading to plant development and to biotic and abiotic stress responses. Intracellular variation in free Ca2+ concentration is among the earliest events following the plant perception of environmental change. These Ca2+ variations differ in their spatio-temporal properties according to the nature, strength and duration of the stimulus. However, their conversion into biological responses requires Ca2+ sensors for decoding and relaying. The occurrence in plants of calmodulin (CaM) but also of other sets of plant-specific Ca2+ sensors such as calmodulin-like proteins (CMLs), Ca2+-dependent protein kinases (CDPKs) and calcineurin B-like proteins (CBLs) indicate that plants possess specific tools and machineries to convert Ca2+ signals into appropriate responses. Here, we focus on recent progress made in monitoring the generation of Ca2+ signals at the whole plant or cell level and their long distance propagation during biotic interactions. The contribution of CaM/CMLs and CDPKs in plant immune responses mounted against bacteria, fungi, viruses and insects are also presented.

Direct phosphorylation and activation of a mitogen-activated protein kinase by a calcium-dependent protein kinase in rice

The mitogen-activated protein kinase (MAPK) is a pivotal point of convergence for many signaling pathways in eukaryotes. In the classical MAPK cascade, a signal is transmitted via sequential phosphorylation and activation of MAPK kinase kinase, MAPK kinase (MKK), and MAPK. The activation of MAPK is dependent on dual phosphorylation of a TXY motif by an MKK, which is considered the sole kinase to phosphorylate and activate MAPK. Here, we report a novel regulatory mechanism of MAPK phosphorylation and activation besides the canonical MAPK cascade. A rice (Oryza sativa) calcium-dependent protein kinase (CDPK), CPK18, was identified as an upstream kinase of MAPK (MPK5) in vitro and in vivo. Curiously, CPK18 was shown to phosphorylate and activate MPK5 without affecting the phosphorylation of its TXY motif. Instead, CPK18 was found to predominantly phosphorylate two Thr residues (Thr-14 and Thr-32) that are widely conserved in MAPKs from land plants. Further analyses reveal that the newly identified CPK18-MPK5 pathway represses defense gene expression and negatively regulates rice blast resistance. Our results suggest that land plants have evolved an MKK-independent phosphorylation pathway that directly connects calcium signaling to the MAPK machinery.

A Receptor-Like Kinase Mediates Ammonium Homeostasis and Is Important for the Polar Growth of Root Hairs in Arabidopsis

Ammonium (NH4+) is both a necessary nutrient and an important signal in plants, but can be toxic in excess. Ammonium sensing and regulatory mechanisms in plant cells have not been fully elucidated. To decipher the complex network of NH4+ signaling, we analyzed [Ca2+]cyt-associated protein kinase (CAP) genes, which encode signaling components that undergo marked changes in transcription levels in response to various stressors. We demonstrated that CAP1, a tonoplast-localized receptor-like kinase, regulates root hair tip growth by maintaining cytoplasmic Ca2+ gradients. A CAP1 knockout mutant (cap1-1) produced elevated levels of cytoplasmic NH4+. Furthermore, root hair growth of cap1-1 was inhibited on Murashige and Skoog medium, but NH4+ depletion reestablished the Ca2+ gradient necessary for normal growth. The lower net NH4+ influx across the vacuolar membrane and relatively alkaline cytosolic pH of cap1-1 root hairs implied that mutation of CAP1 increased NH4+ accumulation in the cytoplasm. Furthermore, CAP1 functionally complemented the npr1 (nitrogen permease reactivator protein) kinase yeast mutant, which is defective in high-affinity NH4+ uptake via MEP2 (methylammonium permease 2), distinguishing CAP1 as a cytosolic modulator of NH4+ levels that participates in NH4+ homeostasis-regulated root hair growth by modulating tip-focused cytoplasmic Ca2+ gradients.

Calcium Signals from the Vacuole

The vacuole is by far the largest intracellular Ca(2+) store in most plant cells. Here, the current knowledge about the molecular mechanisms of vacuolar Ca(2+) release and Ca(2+) uptake is summarized, and how different vacuolar Ca(2+) channels and Ca(2+) pumps may contribute to Ca(2+) signaling in plant cells is discussed. To provide a phylogenetic perspective, the distribution of potential vacuolar Ca(2+) transporters is compared for different clades of photosynthetic eukaryotes. There are several candidates for vacuolar Ca(2+) channels that could elicit cytosolic [Ca(2+)] transients. Typical second messengers, such as InsP₃ and cADPR, seem to trigger vacuolar Ca(2+) release, but the molecular mechanism of this Ca(2+) release still awaits elucidation. Some vacuolar Ca(2+) channels have been identified on a molecular level, the voltage-dependent SV/TPC1 channel, and recently two cyclic-nucleotide-gated cation channels. However, their function in Ca(2+) signaling still has to be demonstrated. Ca(2+) pumps in addition to establishing long-term Ca(2+) homeostasis can shape cytosolic [Ca(2+)] transients by limiting their amplitude and duration, and may thus affect Ca(2+) signaling.

Identification of cyclic GMP-activated nonselective Ca2+-permeable cation channels and associated CNGC5 and CNGC6 genes in Arabidopsis guard cells

Cytosolic Ca(2+) in guard cells plays an important role in stomatal movement responses to environmental stimuli. These cytosolic Ca(2+) increases result from Ca(2+) influx through Ca(2+)-permeable channels in the plasma membrane and Ca(2+) release from intracellular organelles in guard cells. However, the genes encoding defined plasma membrane Ca(2+)-permeable channel activity remain unknown in guard cells and, with some exceptions, largely unknown in higher plant cells. Here, we report the identification of two Arabidopsis (Arabidopsis thaliana) cation channel genes, CNGC5 and CNGC6, that are highly expressed in guard cells. Cytosolic application of cyclic GMP (cGMP) and extracellularly applied membrane-permeable 8-Bromoguanosine 3',5'-cyclic monophosphate-cGMP both activated hyperpolarization-induced inward-conducting currents in wild-type guard cells using Mg(2+) as the main charge carrier. The cGMP-activated currents were strongly blocked by lanthanum and gadolinium and also conducted Ba(2+), Ca(2+), and Na(+) ions. cngc5 cngc6 double mutant guard cells exhibited dramatically impaired cGMP-activated currents. In contrast, mutations in CNGC1, CNGC2, and CNGC20 did not disrupt these cGMP-activated currents. The yellow fluorescent protein-CNGC5 and yellow fluorescent protein-CNGC6 proteins localize in the cell periphery. Cyclic AMP activated modest inward currents in both wild-type and cngc5cngc6 mutant guard cells. Moreover, cngc5 cngc6 double mutant guard cells exhibited functional abscisic acid (ABA)-activated hyperpolarization-dependent Ca(2+)-permeable cation channel currents, intact ABA-induced stomatal closing responses, and whole-plant stomatal conductance responses to darkness and changes in CO2 concentration. Furthermore, cGMP-activated currents remained intact in the growth controlled by abscisic acid2 and abscisic acid insensitive1 mutants. This research demonstrates that the CNGC5 and CNGC6 genes encode unique cGMP-activated nonselective Ca(2+)-permeable cation channels in the plasma membrane of Arabidopsis guard cells.

Nox enzymes and new thinking on reactive oxygen: a double-edged sword revisited

Reactive oxygen species (ROS) are a chemical class of molecules that have generally been conceptualized as deleterious entities, albeit ones whose destructive properties could be harnessed as antimicrobial effector functions to benefit the whole organism. This appealingly simplistic notion has been turned on its head in recent years with the discovery of the NADPH oxidases, or Noxes, a family of enzymes dedicated to the production of ROS in a variety of cells and tissues. The Nox-dependent, physiological generation of ROS is highly conserved across virtually all multicellular life, often as a generalized response to microbes and/or other exogenous stressors. This review discusses the current knowledge of the role of physiologically generated ROS and the enzymes that form them in both normal biology and disease.

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.

FRET-based genetically encoded sensors allow high-resolution live cell imaging of Ca²⁺ dynamics

Temporally and spatially defined calcium signatures are integral parts of numerous signalling pathways. Monitoring calcium dynamics with high spatial and temporal resolution is therefore critically important to understand how this ubiquitous second messenger can control diverse cellular responses. Yellow cameleons (YCs) are fluorescence resonance energy transfer (FRET)-based genetically encoded Ca(2+) -sensors that provide a powerful tool to monitor the spatio-temporal dynamics of Ca(2+) fluxes. Here we present an advanced set of vectors and transgenic lines for live cell Ca(2+) imaging in plants. Transgene silencing mediated by the cauliflower mosaic virus (CaMV) 35S promoter has severely limited the application of nanosensors for ions and metabolites and we have thus used the UBQ10 promoter from Arabidopsis and show here that this results in constitutive and stable expression of YCs in transgenic plants. To improve the spatial resolution, our vector repertoire includes versions of YCs that can be targeted to defined locations. Using this toolkit, we identified temporally distinct responses to external ATP at the plasma membrane, in the cytosol and in the nucleus of neighbouring root cells. Moreover analysis of Ca(2+) dynamics in Lotus japonicus revealed distinct Nod factor induced Ca(2+) spiking patterns in the nucleus and the cytosol. Consequently, the constructs and transgenic lines introduced here enable a detailed analysis of Ca(2+) dynamics in different cellular compartments and in different plant species and will foster novel approaches to decipher the temporal and spatial characteristics of calcium signatures.

Abscisic acid and CO2 signalling via calcium sensitivity priming in guard cells, new CDPK mutant phenotypes and a method for improved resolution of stomatal stimulus-response analyses

BACKGROUND

Stomatal guard cells are the regulators of gas exchange between plants and the atmosphere. Ca(2+)-dependent and Ca(2+)-independent mechanisms function in these responses. Key stomatal regulation mechanisms, including plasma membrane and vacuolar ion channels have been identified and are regulated by the free cytosolic Ca(2+) concentration ([Ca(2+)](cyt)).

SCOPE

Here we show that CO(2)-induced stomatal closing is strongly impaired under conditions that prevent intracellular Ca(2+) elevations. Moreover, Ca(2+) oscillation-induced stomatal closing is partially impaired in knock-out mutations in several guard cell-expressed Ca(2+)-dependent protein kinases (CDPKs) here, including the cpk4cpk11 double and cpk10 mutants; however, abscisic acid-regulated stomatal movements remain relatively intact in the cpk4cpk11 and cpk10 mutants. We further discuss diverse studies of Ca(2+) signalling in guard cells, discuss apparent peculiarities, and pose novel open questions. The recently proposed Ca(2+) sensitivity priming model could account for many of the findings in the field. Recent research shows that the stomatal closing stimuli abscisic acid and CO(2) enhance the sensitivity of stomatal closing mechanisms to intracellular Ca(2+), which has been termed 'calcium sensitivity priming'. The genome of the reference plant Arabidopsis thaliana encodes for over 250 Ca(2+)-sensing proteins, giving rise to the question, how can specificity in Ca(2+) responses be achieved? Calcium sensitivity priming could provide a key mechanism contributing to specificity in eukaryotic Ca(2+) signal transduction, a topic of central interest in cell signalling research. In this article we further propose an individual stomatal tracking method for improved analyses of stimulus-regulated stomatal movements in Arabidopsis guard cells that reduces noise and increases fidelity in stimulus-regulated stomatal aperture responses ( Box 1). This method is recommended for stomatal response research, in parallel to previously adopted blind analyses, due to the relatively small and diverse sizes of stomatal apertures in the reference plant Arabidopsis thaliana.

ABA signaling in stomatal guard cells: lessons from Commelina and Vicia

Abscisic acid (ABA) signaling mechanisms have been studied in a broad variety of plant species using complementary analyses, taking advantage of different methodologies suitable for each plant species. Early studies on ABA biosynthesis using Solanum lycopersicum mutants suggested an importance of ABA synthesis in stomatal closure. To understand ABA signaling in guard cells, cellular, biochemical and electrophysiological studies in Vicia faba and Commelina communis have been conducted, providing fundamental knowledge that was further reconfirmed by molecular genetic studies of Arabidopsis. In this article, examples of stomatal studies in several plants and prospects in ABA research are discussed.

Hypoxia-inducible genes encoding small EF-hand proteins in rice and tomato

Rice has evolved metabolic and morphological adaptations to low-oxygen stress to grow in submerged paddy fields. To characterize the molecular components that mediate the response to hypoxia in rice, we identified low-oxygen stress early response genes by microarray analysis. Among the highly responsive genes, five genes, OsHREF1 to OsHREF5, shared strong homology. They encoded small proteins harboring two EF-hands, typical Ca(2+)-binding motifs. Homologous genes were found in many land plants, including SlHREF in tomato, which is also strongly induced by hypoxia. SlHREF induction was detected in both roots and shoots of tomato plants under hypoxia. With the exception of OsHREF5, OsHREF expression was unaffected by drought, salinity, cold, or osmotic stress. Fluorescent signals of green fluorescent protein-fused OsHREFs were detected in the cytosol and nucleus. Ruthenium red, an inhibitor of intracellular Ca(2+) release, repressed induction of OsHREF1-4 under hypoxia. The HREFs may be related to the Ca(2+) response to hypoxia.

Nox enzymes from fungus to fly to fish and what they tell us about Nox function in mammals

The production of reactive oxygen species (ROS) in a highly regulated fashion is a hallmark of members of the NADPH oxidase (Nox) family of enzymes. Nox enzymes are present in most eukaryotic groups such as the amebozoid, fungi, algae and plants, and animals, in which they are involved in seemingly diverse biological processes. However, a comprehensive survey of Nox functions throughout biology reveals common functional themes. Noxes are often activated in response to stressful conditions such as nutrient starvation, physical damage, or pathogen attack. Although the end result varies depending on the organism and tissue, Nox-produced ROS mediate the response to the adverse stimuli, such as innate immunity responses in plants and animals or cell differentiation in Dictyostelium, fungi, and plants. These responses involve ROS-mediated signaling mechanisms occurring at intracellular or cell-to-cell levels and sometimes involve cell wall or extracellular matrix cross-linking. Indeed, Noxes are involved in local and systemic signaling from plants to fish and in cross-linking of the plant hair-cell wall, synthesis of the nematode cuticle, and formation of the sea urchin fertilization envelope. The extensive use of Nox enzymes in biology to regulate cell-to-cell signaling and morphogenesis suggests that additional functions in mammalian signaling and development remain to be discovered.

Signal percolation through plants and the shape of the calcium signature

Plants respond to almost any kind of external stimulus with transients in their cytoplasmic free calcium concentration ([Ca(2+)](c)). A huge variety of kinetics recorded by optical techniques has been reported in the past. This variety has been credited the specificity needed to explain how information about incoming stimuli is evaluated by the organism and turned into the right physiological responses which provide advantages for survival and reproduction. A physiological response often takes place away from the site of stimulation. This requires cell-to-cell communication. Hence, responding cells are not necessarily directly stimulated but rather receive an indirect stimulus via cell-to-cell communication. It appears unlikely that the '[Ca(2+)](c) signature' in the primarily stimulated cell is conveyed over long distances via cell-to-cell communication from the 'receptor cells' to the 'effector cells'. Here, a novel aspect is highlighted to explain the variety of [Ca(2+)] kinetics seen by integrating methods of [Ca(2+)](c) recording. Plants can generally be seen as cellular automata with specific morphology and capable for cell-to-cell communication. Just a few rules are needed to demonstrate how waves of [Ca(2+)](c)-increases percolate through the organism and thereby deliver a broad variety of 'signatures'. Modelling intercellular signalling may be a possible way to find explanations for different kinds of signal transmission, signal amplification, wave formation, oscillations and stimulus-response coupling. The basic examples presented here show that care has to be taken when interpreting cellular '[Ca(2+)](c) signatures' recorded by optical techniques which integrate over a big number of cells or even whole plants.

Calcium elevation-dependent and attenuated resting calcium-dependent abscisic acid induction of stomatal closure and abscisic acid-induced enhancement of calcium sensitivities of S-type anion and inward-rectifying K channels in Arabidopsis guard cells

Stomatal closure in response to abscisic acid depends on mechanisms that are mediated by intracellular [Ca2+] ([Ca2+]i), and also on mechanisms that are independent of [Ca2+]i in guard cells. In this study, we addressed three important questions with respect to these two predicted pathways in Arabidopsis thaliana. (i) How large is the relative abscisic acid (ABA)-induced stomatal closure response in the [Ca2+]i-elevation-independent pathway? (ii) How do ABA-insensitive mutants affect the [Ca2+]i-elevation-independent pathway? (iii) Does ABA enhance (prime) the Ca2+ sensitivity of anion and inward-rectifying K+ channel regulation? We monitored stomatal responses to ABA while experimentally inhibiting [Ca2+]i elevations and clamping [Ca2+]i to resting levels. The absence of [Ca2+]i elevations was confirmed by ratiometric [Ca2+]i imaging experiments. ABA-induced stomatal closure in the absence of [Ca2+]i elevations above the physiological resting [Ca2+]i showed only approximately 30% of the normal stomatal closure response, and was greatly slowed compared to the response in the presence of [Ca2+]i elevations. The ABA-insensitive mutants ost1-2, abi2-1 and gca2 showed partial stomatal closure responses that correlate with [Ca2+]i-dependent ABA signaling. Interestingly, patch-clamp experiments showed that exposure of guard cells to ABA greatly enhances the ability of cytosolic Ca2+ to activate S-type anion channels and down-regulate inward-rectifying K+ channels, providing strong evidence for a Ca2+ sensitivity priming hypothesis. The present study demonstrates and quantifies an attenuated and slowed ABA response when [Ca2+]i elevations are directly inhibited in guard cells. A minimal model is discussed, in which ABA enhances (primes) the [Ca2+]i sensitivity of stomatal closure mechanisms.

The Clickable Guard Cell, Version II: Interactive Model of Guard Cell Signal Transduction Mechanisms and Pathways

Guard cells are located in the leaf epidermis and pairs of guard cells surround and form stomatal pores, which regulate CO(2) influx from the atmosphere into leaves for photosynthetic carbon fixation. Stomatal guard cells also regulate water loss of plants via transpiration to the atmosphere. Signal transduction mechanisms in guard cells integrate a multitude of different stimuli to modulate stomatal apertures. Stomata open in response to light. Stomata close in response to drought stress, elevated CO(2), ozone and low humidity. In response to drought, plants synthesize the hormone abscisic acid (ABA) that triggers closing of stomatal pores. Guard cells have become a highly developed model system for dissecting signal transduction mechanisms in plants and for elucidating how individual signaling mechanisms can interact within a network in a single cell. Many new findings have been made in the last few years. This chapter is an update of an electronic interactive chapter in the previous edition of The Arabidopsis Book (Mäser et al. 2003). Here we focus on mechanisms for which genes and mutations have been characterized, including signaling components for which there is substantial signaling, biochemical and genetic evidence. Ion channels have been shown to represent targets of early signal transduction mechanisms and provide functional signaling and quantitative analysis points to determine where and how mutations affect branches within the guard cell signaling network. Although a substantial number of genes and proteins that function in guard cell signaling have been identified in recent years, there are many more left to be identified and the protein-protein interactions within this network will be an important subject of future research. A fully interactive clickable electronic version of this publication can be accessed at the following web site: http://www-biology.ucsd.edu/labs/schroeder/clickablegc2/. The interactive clickable version includes the following features: Figure 1. Model for the roles of ion channels in ABA signaling.Figure 2. Blue light signaling pathways in guard cells.Figure 3. ABA signaling pathways in guard cells.Figure 1 is linked to explanations that appear upon mouse-over. Figure 2 and Figure 3 are clickable and linked to info boxes, which in turn are linked to TAIR, to relevant abstracts in PubMed, and to updated background explanations from Schroeder et al (2001), used with permission of Annual Reviews of Plant Biology.

The grateful dead: calcium and cell death in plant innate immunity

Plant cells sensing pathogenic microorganisms evoke defence systems that can confer resistance to infection. This innate immune reaction can include triggering of basal defence responses as well as programmed cell death, or hypersensitive response (HR). In both cases (basal defence and HR), pathogen perception is translated into elevated cytosolic Ca(2+) (mediated by plasma membrane and intracellular channels) as an early step in a signalling cascade. Cyclic nucleotide-gated channels contribute to this influx of Ca(2+) into the cell. The molecular nature of other transport proteins contributing to the Ca(2+) elevation is unclear. Pathogen recognition occurs at two levels: the perception of pathogen-associated molecular pattern (PAMP) molecules widely present in microorganisms, and an interaction between pathogen avirulence gene products (if present) and corresponding plant R (resistance) gene products. The Ca(2+) elevation occurring in response to PAMP perception or R gene interactions could occur due to phosphorylation events, G-protein signalling and/or an increase in cyclic nucleotides. Downstream from the initial Ca(2+) rise, the signalling cascade includes: activation of calmodulin and protein kinases, and nitric oxide and reactive oxygen species generation. Some of these downstream events amplify the Ca(2+) signal by further activation of Ca(2+) transporters.

Over-expression of calcium-dependent protein kinase 13 and calreticulin interacting protein 1 confers cold tolerance on rice plants

Calcium is a ubiquitous signaling molecule and changes in cytosolic calcium concentration are involved in plant responses to various stimuli. The rice calcium-dependent protein kinase 13 (CDPK13) and calreticulin interacting protein 1 (CRTintP1) have previously been reported to be involved in cold stress response in rice. In this study, rice lines transformed with sense CDPK13 or CRTintP1 constructs were produced and used to investigate the function of these proteins. When the plants were incubated at 5 degrees C for 3 days, leaf blades of both the sense transgenic and vector control rice plants became wilted and curled. When the plants were transferred back to non-stress conditions after cold treatment, the leaf blades died, but the sheaths remained green in the sense transgenic rice plants. Expression of CDPK13 or CRTintP1 was further examined in several rice varieties including cold-tolerant rice varieties. Accumulation of these proteins in the cold-tolerant rice variety was higher than that in rice varieties that are intermediate in their cold tolerance. To examine whether over-expression of CDPK13 and CRTintP1 would have any effect on the proteins or not, sense transgenic rice plants were analyzed using proteomics. The 2D-PAGE profiles of proteins from the vector control were compared with those of the sense transgenic rice plants. Two of the proteins that differed between these lines were calreticulins. The results suggest that CDPK13, calreticulin and CRTintP1 might be important signaling components for response to cold stress in rice.

CDPKs CPK6 and CPK3 function in ABA regulation of guard cell S-type anion- and Ca(2+)-permeable channels and stomatal closure

Abscisic acid (ABA) signal transduction has been proposed to utilize cytosolic Ca(2+) in guard cell ion channel regulation. However, genetic mutants in Ca(2+) sensors that impair guard cell or plant ion channel signaling responses have not been identified, and whether Ca(2+)-independent ABA signaling mechanisms suffice for a full response remains unclear. Calcium-dependent protein kinases (CDPKs) have been proposed to contribute to central signal transduction responses in plants. However, no Arabidopsis CDPK gene disruption mutant phenotype has been reported to date, likely due to overlapping redundancies in CDPKs. Two Arabidopsis guard cell-expressed CDPK genes, CPK3 and CPK6, showed gene disruption phenotypes. ABA and Ca(2+) activation of slow-type anion channels and, interestingly, ABA activation of plasma membrane Ca(2+)-permeable channels were impaired in independent alleles of single and double cpk3cpk6 mutant guard cells. Furthermore, ABA- and Ca(2+)-induced stomatal closing were partially impaired in these cpk3cpk6 mutant alleles. However, rapid-type anion channel current activity was not affected, consistent with the partial stomatal closing response in double mutants via a proposed branched signaling network. Imposed Ca(2+) oscillation experiments revealed that Ca(2+)-reactive stomatal closure was reduced in CDPK double mutant plants. However, long-lasting Ca(2+)-programmed stomatal closure was not impaired, providing genetic evidence for a functional separation of these two modes of Ca(2+)-induced stomatal closing. Our findings show important functions of the CPK6 and CPK3 CDPKs in guard cell ion channel regulation and provide genetic evidence for calcium sensors that transduce stomatal ABA signaling.

Abscisic acid stimulates the expression of two isoforms of plasma membrane Ca2+-ATPase in Arabidopsis thaliana seedlings

AT-ACA8 and AT-ACA9 are two plasma membrane (PM) Ca (2+)-ATPases of ARABIDOPSIS THALIANA. In this article the expression of AT-ACA8, AT-ACA9, and of AT-ACA10, a third isoform of Ca (2+)-ATPase closely related to PM Ca (2+)-ATPases, was analysed and the effect of the hormone abscisic acid (ABA) on the expression level of PM Ca (2+)-ATPase specific transcripts was investigated. In adult plants of A. THALIANA, AT-ACA8 and AT-ACA10 are expressed in all organs considered whereas AT-ACA9 is expressed only in flowers. All isoforms of PM Ca (2+)-ATPases can be detected in young seedlings but the amount of AT-ACA9 mRNA is much lower than those of AT-ACA8 and AT-ACA10. ABA markedly and rapidly stimulates the expression of both AT-ACA8 and AT-ACA9 genes in young seedlings but not that of AT-ACA10. ABA also increases the level of AT-ACA8 protein at the PM, suggesting a role for PM Ca (2+)-ATPases in ABA signalling.

Root-to-shoot signalling: apoplastic alkalinization, a general stress response and defence factor in barley (Hordeum vulgare)

We used a noninvasive microprobe technique to record in substomatal cavities of barley leaves the apoplastic pH response to different stress situations. When K+ (or Na+) activity at the roots of intact plants was increased from 1 to 50 mM, the leaf apoplastic pH increased by 0.4 to 0.6 units within 8 to 12 min when stomata were open, and within 15 to 20 min when stomata were closed. This reaction was accompanied by a correlative increase in K+ activity. Addition of 1 microM abscisic acid caused an apoplastic alkalinization of 0.5 to 0.8 units, and low temperatures (4 degrees C) increased pH by 0.2 to 0.3 units. Addition of 100 mM sorbitol or pH changes in the range 4.0 to 7.9 had no effect, ruling out that osmotic potential and/or pH is the carried signal. On detached leaves, the same treatments yielded qualitatively similar results, suggesting that the xylem is the most likely signal path. Following the attack of powdery mildew, the apoplastic pH of barley leaves substantially increases. We demonstrate that in susceptible barley, pretreatment (soil drench) with the resistance-inducing chemical benzo- (1,2,3)thiadiazole-7-carbothioic acid S-methyl ester markedly enhances this pH response. This is consistent with previous finding that apoplastic alkalinization is related to the degree of resistance towards this fungus.

SKS6, a multicopper oxidase-like gene, participates in cotyledon vascular patterning during Arabidopsis thaliana development

SKU5-Similar 6 (SKS6) is a one of a large gene family of 19 members in Arabidopsis thaliana (L.) Heynh that encode multicopper oxidase-like proteins that are related to ferroxidases, ascorbate oxidases and laccases. Only one member of the family has been previously studied; Skewed5 (SKU5) is involved in the control of root growth. The encoded SKS6 protein, like SKU5 appears to lack a functional copper-binding site and is most closely related to Bp10 from Brassica napus and Ntp303 from Nicotiana tobacum. The SKS6 promoter contains many putative regulatory sites and differential expression of an SKS6::GUS reporter gene revealed selective induction in several seedling tissues including guard cells, root cortex cells, and leaf margin hydathodes. It was also expressed later in flower development in flower primordia, ovules, and the abscission zones of seeds and siliques. Furthermore, SKS6 was upregulated in roots in response to treatment of seedlings with the hormones abscisic acid, indole-3 acetic acid, 2,4-dichlorophenoxyacetic acid and aminocyclopropane-1-carboxylate. A loss-of function sks6-1 T-DNA insertion allele revealed that cotyledon vascular patterning is affected in the mutant, suggesting a role for the protein in metabolism of nutrients or hormones in the hydathodes, the sites of auxin synthesis and chemical recycling.

Gene-specific expression and calcium activation of Arabidopsis thaliana phospholipase C isoforms

•  PI-PLCs synthesise the calcium releasing second messenger IP₃ . We investigated the expression patterns of the Arabidopsis PI-PLC gene family and measured in vitro activity of encoded enzymes. •  Gene specific RT-PCR and promoter-GUS fusions were used to analyse AtPLC gene expression patterns. The five available AtPLC cDNAs were expressed as fusion proteins in Escherichia coli. •  All members of the AtPLC gene family were expressed in multiple organs of the plant. AtPLC1, and AtPLC5 expression was localized to the vascular cells of roots and leaves with AtPLC5::GUS also detected in the guard cells. AtPLC4::GUS was detected in pollen and cells of the stigma surface. In seedlings, AtPLC2 and AtPLC3 were constitutively expressed, while AtPLCs 1, 4 and 5 were induced by abiotic stresses. AtPLC1-5 were all shown to have phospholipase C activity in the presence of calcium ions. •  AtPLCs showed limited tissue specific expression and expression of at least three genes was increased by abiotic stress. The differing calcium sensitivities of recombinant AtPLC protein activities may provide a mechanism for generating calcium signatures.

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.

Calcium in plants

Calcium is an essential plant nutrient. It is required for various structural roles in the cell wall and membranes, it is a counter-cation for inorganic and organic anions in the vacuole, and the cytosolic Ca2+ concentration ([Ca2+]cyt) is an obligate intracellular messenger coordinating responses to numerous developmental cues and environmental challenges. This article provides an overview of the nutritional requirements of different plants for Ca, and how this impacts on natural flora and the Ca content of crops. It also reviews recent work on (a) the mechanisms of Ca2+ transport across cellular membranes, (b) understanding the origins and specificity of [Ca2+]cyt signals and (c) characterizing the cellular [Ca2+]cyt-sensors (such as calmodulin, calcineurin B-like proteins and calcium-dependent protein kinases) that allow plant cells to respond appropriately to [Ca2+]cyt signals.

Calcium: just a chemical switch?

The calcium-signature hypothesis has evolved as a concept to explain specificity in signaling pathways that utilise calcium as a second messenger. In plant biology, this hypothesis was purely conceptual and based only upon correlative observations until recently. In the past few years, however, empirical data have emerged from experiments that were specifically designed to tackle the question of how specificity is encoded by calcium. In light of the attractive calcium-signature hypothesis, other potential explanations for signalling specificity have been overshadowed and ignored: it has been assumed that the calcium-signature dogma will explain all plant calcium signaling. However, there is a good deal of evidence supporting a counter-hypothesis in which calcium does not itself encode specificity but is merely an essential 'switch' in signaling. At the very least, both hypotheses are likely to be true in different situations, and it may well be that the calcium-signature hypothesis describes the exception rather than the rule.

Encoding specificity in plant calcium signalling: hot-spotting the ups and downs and waves

Calcium ions function as intracellular second messengers in regulating a plethora of cellular processes from acclimative stress responses to survival and programmed cell death. The generation of specificity in Ca2+ signals is dependent on influx and efflux from the extracellular milieu, cytosol and intracellular organelles. One aspect of plant Ca2+ signalling that is currently attracting a great deal of interest is how 'Ca2+-signatures', specific spatio-temporal changes in cytosolic-free Ca2+, encode the necessary information to bring about this range of physiological responses. Here, current information is reviewed on how Ca2+-signatures are generated in plant cells and how stimulus-specific information can be encoded in the form of Ca2+-signatures.

Convergence of calcium signaling pathways of pathogenic elicitors and abscisic acid in Arabidopsis guard cells

A variety of stimuli, such as abscisic acid (ABA), reactive oxygen species (ROS), and elicitors of plant defense reactions, have been shown to induce stomatal closure. Our study addresses commonalities in the signaling pathways that these stimuli trigger. A recent report showed that both ABA and ROS stimulate an NADPH-dependent, hyperpolarization-activated Ca(2+) influx current in Arabidopsis guard cells termed "I(Ca)" (Z.M. Pei, Y. Murata, G. Benning, S. Thomine, B. Klüsener, G.J. Allen, E. Grill, J.I. Schroeder, Nature [2002] 406: 731-734). We found that yeast (Saccharomyces cerevisiae) elicitor and chitosan, both elicitors of plant defense responses, also activate this current and activation requires cytosolic NAD(P)H. These elicitors also induced elevations in the concentration of free cytosolic calcium ([Ca(2+)](cyt)) and stomatal closure in guard cells. ABA and ROS elicited [Ca(2+)](cyt) oscillations in guard cells only when extracellular Ca(2+) was present. In a 5 mM KCl extracellular buffer, 45% of guard cells exhibited spontaneous [Ca(2+)](cyt) oscillations that differed in their kinetic properties from ABA-induced Ca(2+) increases. These spontaneous [Ca(2+)](cyt) oscillations also required the availability of extracellular Ca(2+) and depended on the extracellular potassium concentration. Interestingly, when ABA was applied to spontaneously oscillating cells, ABA caused cessation of [Ca(2+)](cyt) elevations in 62 of 101 cells, revealing a new mode of ABA signaling. These data show that fungal elicitors activate a shared branch with ABA in the stress signal transduction pathway in guard cells that activates plasma membrane I(Ca) channels and support a requirement for extracellular Ca(2+) for elicitor and ABA signaling, as well as for cellular [Ca(2+)](cyt) oscillation maintenance.

Analysis and effects of cytosolic free calcium increases in response to elicitors in Nicotiana plumbaginifolia cells

Cell suspensions obtained from Nicotiana plumbaginifolia plants stably expressing the apoaequorin gene were used to analyze changes in cytosolic free calcium concentrations ([Ca(2+)](cyt)) in response to elicitors of plant defenses, particularly cryptogein and oligogalacturonides. The calcium signatures differ in lag time, peak time, intensity, and duration. The intensities of both signatures depend on elicitor concentration and extracellular calcium concentration. Cryptogein signature is characterized by a long-sustained [Ca(2+)](cyt) increase that should be responsible for sustained mitogen-activated protein kinase activation, microtubule depolymerization, defense gene activation, and cell death. The [Ca(2+)](cyt) increase in elicitor-treated cells first results from a calcium influx, which in turns leads to calcium release from internal stores and additional Ca(2+) influx. H(2)O(2) resulting from the calcium-dependent activation of the NADPH oxidase also participates in [Ca(2+)](cyt) increase and may activate calcium channels from the plasma membrane. Competition assays with different elicitins demonstrate that [Ca(2+)](cyt) increase is mediated by cryptogein-receptor interaction.

Hypersensitivity of abscisic acid-induced cytosolic calcium increases in the Arabidopsis farnesyltransferase mutant era1-2

Cytosolic calcium increases were analyzed in guard cells of the Arabidopsis farnesyltransferase deletion mutant era1-2 (enhanced response to abscisic acid). At low abscisic acid (ABA) concentrations (0.1 microM), increases of guard cell cytosolic calcium and stomatal closure were activated to a greater extent in the era1-2 mutant compared with the wild type. Patch clamping of era1-2 guard cells showed enhanced ABA sensitivity of plasma membrane calcium channel currents. These data indicate that the ERA1 farnesyltransferase targets a negative regulator of ABA signaling that acts between the points of ABA perception and the activation of plasma membrane calcium influx channels. Experimental increases of cytosolic calcium showed that the activation of S-type anion currents downstream of cytosolic calcium and extracellular calcium-induced stomatal closure were unaffected in era1-2, further supporting the positioning of era1-2 upstream of cytosolic calcium in the guard cell ABA signaling cascade. Moreover, the suppression of ABA-induced calcium increases in guard cells by the dominant protein phosphatase 2C mutant abi2-1 was rescued partially in era1-2 abi2-1 double mutant guard cells, further reinforcing the notion that ERA1 functions upstream of cytosolic calcium and indicating the genetic interaction of these two mutations upstream of ABA-induced calcium increases.

GUARD CELL SIGNAL TRANSDUCTION

Guard cells surround stomatal pores in the epidermis of plant leaves and stems. Stomatal pore opening is essential for CO2 influx into leaves for photosynthetic carbon fixation. In exchange, plants lose over 95% of their water via transpiration to the atmosphere. Signal transduction mechanisms in guard cells integrate hormonal stimuli, light signals, water status, CO2, temperature, and other environmental conditions to modulate stomatal apertures for regulation of gas exchange and plant survival under diverse conditions. Stomatal guard cells have become a highly developed model system for characterizing early signal transduction mechanisms in plants and for elucidating how individual signaling mechanisms can interact within a network in a single cell. In this review we focus on recent advances in understanding signal transduction mechanisms in guard cells.

Cellular signaling and volume control in stomatal movements in plants

Stomatal guard cells are unique as a plant cell model and, because of the depth of present knowledge on ion transport and its regulation, offer a first look at signal integration in higher plants. A large body of data indicates that Ca(2+) and H(+) act independently, integrating with protein kinases and phosphatases, to control the gating of the K(+) and Cl(-) channels that mediate solute flux for stomatal movements. Oscillations in the cytosolic-free concentration of Ca(2+) contribute to a signaling cassette, integrated within these events through an unusual coupling with membrane voltage for solute homeostasis. Similar cassettes are anticipated to include control pathways linked to cytosolic pH. Additional developments during the last two years point to events in membrane traffic that play equally important roles in stomatal control. Research in these areas is now adding entirely new dimensions to our understanding of guard cell signaling.

Calcium: silver bullet in signaling

Accumulating evidence suggests that Ca(2+) serves as a messenger in many normal growth and developmental process and in plant responses to biotic and abiotic stresses. Numerous signals have been shown to induce transient elevation of [Ca(2+)](cyt) in plants. Genetic, biochemical, molecular and cell biological approaches in recent years have resulted in significant progress in identifying several Ca(2+)-sensing proteins in plants and in understanding the function of some of these Ca(2+)-regulated proteins at the cellular and whole plant level. As more and more Ca(2+)-sensing proteins are identified it is becoming apparent that plants have several unique Ca(2+)-sensing proteins and that the downstream components of Ca(2+) signaling in plants have novel features and regulatory mechanisms. Although the mechanisms by which Ca(2+) regulates diverse biochemical and molecular processes and eventually physiological processes in response to diverse signals are beginning to be understood, recent studies have raised many interesting questions. Despite the fact that Ca(2+) sensing proteins are being identified at a rapid pace, progress on the function(s) of many of them is limited. Studies on plant 'signalome' - the identification of all signaling components in all messengers mediated transduction pathways, analysis of their function and regulation, and cross talk among these components - should help in understanding the inner workings of plant cell responses to diverse signals. New functional genomics approaches such as reverse genetics, microarray analyses coupled with in vivo protein-protein interaction studies and proteomics should not only permit functional analysis of various components in Ca(2+) signaling but also enable identification of a complex network of interactions.

Ca2+ channels at the plasma membrane of stomatal guard cells are activated by hyperpolarization and abscisic acid

In stomatal guard cells of higher-plant leaves, abscisic acid (ABA) evokes increases in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) by means of Ca(2+) entry from outside and release from intracellular stores. The mechanism(s) for Ca(2+) flux across the plasma membrane is poorly understood. Because [Ca(2+)](i) increases are voltage-sensitive, we suspected a Ca(2+) channel at the guard cell plasma membrane that activates on hyperpolarization and is regulated by ABA. We recorded single-channel currents across the Vicia guard cell plasma membrane using Ba(2+) as a charge-carrying ion. Both cell-attached and excised-patch measurements uncovered single-channel events with a maximum conductance of 12.8 +/- 0.4 pS and a high selectivity for Ba(2+) (and Ca(2+)) over K(+) and Cl(-). Unlike other Ca(2+) channels characterized to date, these channels rectified strongly toward negative voltages with an open probability (P(o)) that increased with [Ba(2+)] outside and decreased roughly 10-fold when [Ca(2+)](i) was raised from 200 nM to 2 microM. Adding 20 microM ABA increased P(o), initially by 63- to 260-fold; in both cell-attached and excised patches, it shifted the voltage sensitivity for channel activation, and evoked damped oscillations in P(o) with periods near 50 s. A similar, but delayed response was observed in 0.1 microM ABA. These results identify a Ca(2+)-selective channel that can account for Ca(2+) influx and increases in [Ca(2+)](i) triggered by voltage and ABA, and they imply a close physical coupling at the plasma membrane between ABA perception and Ca(2+) channel control.

Abscisic acid signal transduction in guard cells is mediated by phospholipase D activity

In guard cells, the plant hormone abscisic acid (ABA) inhibits stomatal opening and induces stomatal closure through the coordinated regulation of ion transport. Despite this central role of ABA in regulating stomatal function, the signal transduction events leading to altered ion fluxes remain incompletely understood. We report that the activity of the enzyme phospholipase D (PLD) transiently increased in guard cell protoplasts at 2.5 and 25 min after ABA application. Treatment of guard cell protoplasts with phosphatidic acid (PtdOH), one of the products of PLD activity, led to an inhibition of the activity of the inward K+ channel. PtdOH also induced stomatal closure and inhibited stomatal opening when added to epidermal peels. Application of 1-butanol (1-buOH), a selective inhibitor of PtdOH production by PLD, inhibited the increase in PtdOH production elicited by ABA. 1-BuOH treatment also partially prevented ABA-induced stomatal closure and ABA-induced inhibition of stomatal opening. This inhibitory effect of buOH was enhanced by simultaneous application of nicotinamide, an inhibitor of cADP ribose action. These results suggest that in the guard cell, ABA activates the enzyme PLD, which leads to the production of PtdOH. This PtdOH is then involved in triggering subsequent ABA responses of the cell via a pathway operating in parallel to cADP ribose-mediated events.

Arabidopsis abi1-1 and abi2-1 phosphatase mutations reduce abscisic acid-induced cytoplasmic calcium rises in guard cells

Elevations in cytoplasmic calcium ([Ca(2)+](cyt)) are an important component of early abscisic acid (ABA) signal transduction. To determine whether defined mutations in ABA signal transduction affect [Ca(2)+](cyt) signaling, the Ca(2)+-sensitive fluorescent dye fura 2 was loaded into the cytoplasm of Arabidopsis guard cells. Oscillations in [Ca(2)+](cyt) could be induced when the external calcium concentration was increased, showing viable Ca(2)+ homeostasis in these dye-loaded cells. ABA-induced [Ca(2)+](cyt) elevations in wild-type stomata were either transient or sustained, with a mean increase of approximately 300 nM. Interestingly, ABA-induced [Ca(2)+](cyt) increases were significantly reduced but not abolished in guard cells of the ABA-insensitive protein phosphatase mutants abi1 and abi2. Plasma membrane slow anion currents were activated in wild-type, abi1, and abi2 guard cell protoplasts by increasing [Ca(2)+](cyt), demonstrating that the impairment in ABA activation of anion currents in the abi1 and abi2 mutants was bypassed by increasing [Ca(2)+](cyt). Furthermore, increases in external calcium alone (which elevate [Ca(2)+](cyt)) resulted in stomatal closing to the same extent in the abi1 and abi2 mutants as in the wild type. Conversely, stomatal opening assays indicated different interactions of abi1 and abi2, with Ca(2)+-dependent signal transduction pathways controlling stomatal closing versus stomatal opening. Together, [Ca(2)+](cyt) recordings, anion current activation, and stomatal closing assays demonstrate that the abi1 and abi2 mutations impair early ABA signaling events in guard cells upstream or close to ABA-induced [Ca(2)+](cyt) elevations. These results further demonstrate that the mutations can be bypassed during anion channel activation and stomatal closing by experimental elevation of [Ca(2)+](cyt).

Arabidopsis abi1-1 and abi2-1 phosphatase mutations reduce abscisic acid-induced cytoplasmic calcium rises in guard cells

Elevations in cytoplasmic calcium ([Ca(2)+](cyt)) are an important component of early abscisic acid (ABA) signal transduction. To determine whether defined mutations in ABA signal transduction affect [Ca(2)+](cyt) signaling, the Ca(2)+-sensitive fluorescent dye fura 2 was loaded into the cytoplasm of Arabidopsis guard cells. Oscillations in [Ca(2)+](cyt) could be induced when the external calcium concentration was increased, showing viable Ca(2)+ homeostasis in these dye-loaded cells. ABA-induced [Ca(2)+](cyt) elevations in wild-type stomata were either transient or sustained, with a mean increase of approximately 300 nM. Interestingly, ABA-induced [Ca(2)+](cyt) increases were significantly reduced but not abolished in guard cells of the ABA-insensitive protein phosphatase mutants abi1 and abi2. Plasma membrane slow anion currents were activated in wild-type, abi1, and abi2 guard cell protoplasts by increasing [Ca(2)+](cyt), demonstrating that the impairment in ABA activation of anion currents in the abi1 and abi2 mutants was bypassed by increasing [Ca(2)+](cyt). Furthermore, increases in external calcium alone (which elevate [Ca(2)+](cyt)) resulted in stomatal closing to the same extent in the abi1 and abi2 mutants as in the wild type. Conversely, stomatal opening assays indicated different interactions of abi1 and abi2, with Ca(2)+-dependent signal transduction pathways controlling stomatal closing versus stomatal opening. Together, [Ca(2)+](cyt) recordings, anion current activation, and stomatal closing assays demonstrate that the abi1 and abi2 mutations impair early ABA signaling events in guard cells upstream or close to ABA-induced [Ca(2)+](cyt) elevations. These results further demonstrate that the mutations can be bypassed during anion channel activation and stomatal closing by experimental elevation of [Ca(2)+](cyt).

Cameleon calcium indicator reports cytoplasmic calcium dynamics in Arabidopsis guard cells

Cytoplasmic free calcium ([Ca2+]cyt) acts as a stimulus-induced second messenger in plant cells and multiple signal transduction pathways regulate [Ca2+]cyt in stomatal guard cells. Measuring [Ca2+]cyt in guard cells has previously required loading of calcium-sensitive dyes using invasive and technically difficult micro-injection techniques. To circumvent these problems, we have constitutively expressed the pH-independent, green fluorescent protein-based calcium indicator yellow cameleon 2.1 in Arabidopsis thaliana (Miyawaki et al. 1999; Proc. Natl. Acad. Sci. USA 96, 2135-2140). This yellow cameleon calcium indicator was expressed in guard cells and accumulated predominantly in the cytoplasm. Fluorescence ratio imaging of yellow cameleon 2.1 allowed time-dependent measurements of [Ca2+]cyt in Arabidopsis guard cells. Application of extracellular calcium or the hormone abscisic acid (ABA) induced repetitive [Ca2+]cyt transients in guard cells. [Ca2+]cyt changes could be semi-quantitatively determined following correction of the calibration procedure for chloroplast autofluorescence. Extracellular calcium induced repetitive [Ca2+]cyt transients with peak values of up to approximately 1.5 microM, whereas ABA-induced [Ca2+]cyt transients had peak values up to approximately 0.6 microM. These values are similar to stimulus-induced [Ca2+]cyt changes previously reported in plant cells using ratiometric dyes or aequorin. In some guard cells perfused with low extracellular KCl concentrations, spontaneous calcium transients were observed. As yellow cameleon 2.1 was expressed in all guard cells, [Ca2+]cyt was measured independently in the two guard cells of single stomates for the first time. ABA-induced, calcium-induced or spontaneous [Ca2+]cyt increases were not necessarily synchronized in the two guard cells. Overall, these data demonstrate that that GFP-based cameleon calcium indicators are suitable to measure [Ca2+]cyt changes in guard cells and enable the pattern of [Ca2+]cyt dynamics to be measured with a high level of reproducibility in Arabidopsis cells. This technical advance in combination with cell biological and molecular genetic approaches will become an invaluable tool in the dissection of plant cell signal transduction pathways.

Abscisic acid induces oscillations in guard-cell cytosolic free calcium that involve phosphoinositide-specific phospholipase C

Oscillations in cytosolic free Ca2+ concentration ([Ca2+]cyt) are an important component of Ca2+-based signal transduction pathways. This fact has led us to investigate whether oscillations in [Ca2+]cyt are involved in the response of stomatal guard cells to the plant hormone abscisic acid (ABA). We show that ABA induces oscillations in guard-cell [Ca2+]cyt. The pattern of the oscillations depended on the ABA concentration and correlated with the final stomatal aperture. We examined the mechanism by which ABA generates oscillations in guard-cell [Ca2+]cyt by using 1-(6-[17beta-3-methoxyestra-1,3, 5(10)-trien-17-yl]aminohexyl)-1H-pyrrole-2,5-dione (U-73122), an inhibitor of phosphoinositide-specific phospholipase C (PI-PLC)-dependent processes in animals. U-73122 inhibited the hydrolysis of phosphatidylinositol 4,5-bisphosphate by a recombinant PI-PLC, isolated from a guard-cell-enriched cDNA library, in a dose-dependent manner. This result confirms that U-73122 is an inhibitor of plant PI-PLC activity. U-73122 inhibited both ABA-induced oscillations in [Ca2+]cyt and stomatal closure. In contrast, U-73122 did not inhibit external Ca2+-induced oscillations in guard-cell [Ca2+]cyt and stomatal closure. Furthermore, there was no effect of the inactive analogue 1-(6-[17beta-3-methoxyestra-1,3, 5(10)-trien-17-yl]aminohexyl)-2,5-pyrrolidinedione on recombinant PI-PLC activity or ABA-induced and external Ca2+-induced oscillations in [Ca2+]cyt and stomatal closure. This lack of effect suggests that the effects of U-73122 in guard cells are the result of inhibition of PI-PLC and not a consequence of nonspecific effects. Taken together, our data suggest a role for PI-PLC in the generation of ABA-induced oscillations in [Ca2+]cyt and point toward the involvement of oscillations in [Ca2+]cyt in the maintenance of stomatal aperture by ABA.

Abscisic acid-induced stomatal closure mediated by cyclic ADP-ribose

Abscisic acid (ABA) is a plant hormone involved in the response of plants to reduced water availability. Reduction of guard cell turgor by ABA diminishes the aperture of the stomatal pore and thereby contributes to the ability of the plant to conserve water during periods of drought. Previous work has demonstrated that cytosolic Ca2+ is involved in the signal transduction pathway that mediates the reduction in guard cell turgor elicited by ABA. Here we report that ABA uses a Ca2+-mobilization pathway that involves cyclic adenosine 5'-diphosphoribose (cADPR). Microinjection of cADPR into guard cells caused reductions in turgor that were preceded by increases in the concentration of free Ca2+ in the cytosol. Patch clamp measurements of isolated guard cell vacuoles revealed the presence of a cADPR-elicited Ca2+-selective current that was inhibited at cytosolic Ca2+ >/= 600 nM. Furthermore, microinjection of the cADPR antagonist 8-NH2-cADPR caused a reduction in the rate of turgor loss in response to ABA in 54% of cells tested, and nicotinamide, an antagonist of cADPR production, elicited a dose-dependent block of ABA-induced stomatal closure. Our data provide definitive evidence for a physiological role for cADPR and illustrate one mechanism of stimulus-specific Ca2+ mobilization in higher plants. Taken together with other recent data [Wu, Y., Kuzma, J., Marechal, E., Graeff, R., Lee, H. C., Foster, R. & Chua, N.-H. (1997) Science 278, 2126-2130], these results establish cADPR as a key player in ABA signal transduction pathways in plants.

Differential responses of abaxial and adaxial guard cells of broad bean to abscisic acid and calcium

Regulation by abscisic acid (ABA) and Ca2+ of broad bean (Vicia faba) abaxial and adaxial guard cell movements and inward K+ currents were compared. One millimolar Ca2+ in the bathing medium inhibited abaxial stomatal opening by 60% but only inhibited adaxial stomatal opening by 15%. The addition of 1 &mgr;M ABA in the bathing medium resulted in 80% inhibition of abaxial but only 45% inhibition of adaxial stomatal opening. Similarly, ABA and Ca2+ each stimulated greater abaxial stomatal closure than adaxial stomatal closure. Whole-cell patch-clamp results showed that the inward K+ currents of abaxial guard cells were inhibited by 60% (-180 mV) in the presence of 1.5 &mgr;M Ca2+ in the cytoplasm, whereas the inward K+ currents of adaxial guard cells were not affected at all by the same treatment. Although 1 &mgr;M ABA in the cytoplasm inhibited the inward K+ currents to a similar extent for both abaxial and adaxial guard cells, the former were more sensitive to ABA applied externally. These results suggest that the abaxial stomata are more sensitive to Ca2+ and ABA than adaxial stomata in regard to stomatal opening and closing processes and that the regulation of the inward K+ currents by ABA may not proceed via a Ca2+-signaling pathway in adaxial guard cells. Therefore, there may be different pathways for ABA- and Ca2+-mediated signal transduction in abaxial and adaxial guard cells.

The control of specificity in guard cell signal transduction

Stomatal guard cells have proven to be an attractive system for dissecting the mechanisms of stimulus–response coupling in plants. In this review we focus on the intracellular signal transduction pathways by which extracellular signals bring about closure and opening of the stomatal pore. It is proposed that guard cell signal transduction pathways may be organized into functional arrays or signalling cassettes that contain elements common to a number of converging signalling pathways. The purpose of these signalling cassettes may be to funnel extracellular signals down onto the ion transporters that control the fluxes of ions that underlie stomatal movements. Evidence is emerging that specificity in guard cell signalling may be, in part, encoded in complex spatio–temporal patterns of increases in the concentration of cytosolic–free calcium ([Ca2+]cyt). It is suggested that oscillations in [Ca2+]cytmay generate calcium signatures that encode information concerning the stimulus type and strength. New evidence is presented that suggests that these calcium signatures may integrate information when many stimuli are present.

Signal transduction and ion channels in guard cells

Our understanding of the signalling mechanisms involved in the process of stomatal closure is reviewed. Work has concentrated on the mechanisms by which abscisic acid (ABA) induces changes in specific ion channels at both the plasmalemma and the tonoplast, leading to efflux of both K+ and anions at both membranes, requiring four essential changes. For each we need to identify the specific channels concerned, and the detailed signalling chains by which each is linked through signalling intermediates to ABA. There are two global changes that are identified following ABA treatment: an increase in cytoplasmic pH and an increase in cytoplasmic Ca2+, although stomata can close without any measurable global increase in cytoplasmic Ca2+. There is also evidence for the importance of several protein phosphatases and protein kinases in the regulation of channel activity. At the plasmalemma, loss of K+ requires depolarization of the membrane potential into the range at which the outward K+ channel is open. ABA-induced activation of a non-specific cation channel, permeable to Ca2+, may contribute to the necessary depolarization, together with ABA-induced activation of S-type anion channels in the plasmalemma, which are then responsible for the necessary anion efflux. The anion channels are activated by Ca2+ and by phosphorylation, but the precise mechanism of their activation by ABA is not yet clear. ABA also up-regulates the outward K+ current at any given membrane potential; this activation is Ca(2+)-independent and is attributed to the increase in cytoplasmic pH, perhaps through the marked pH-sensitivity of protein phosphatase type 2C. Our understanding of mechanisms at the tonoplast is much less complete. A total of two channels, both Ca(2+)-activated, have been identified which are capable of K+ efflux; these are the voltage-independent VK channel specific to K+, and the slow vacuolar (SV) channel which opens only at non-physiological tonoplast potentials (cytoplasm positive). The SV channel is permeable to K+ and Ca2+, and although it has been argued that it could be responsible for Ca(2+)-induced Ca2+ release, it now seems likely that it opens only under conditions where Ca2+ will flow from cytoplasm to vacuole. Although tracer measurements show unequivocally that ABA does activate efflux of Cl- from vacuole to cytoplasm, no vacuolar anion channel has yet been identified. There is clear evidence that ABA activates release of Ca2+ from internal stores, but the source and trigger for ABA-induced increase in cytoplasmic Ca2+ are uncertain. The tonoplast and another membrane, probably ER, have IP3-sensitive Ca2+ release channels, and the tonoplast has also cADPR-activated Ca2+ channels. Their relative contributions to ABA-induced release of Ca2+ from internal stores remain to be established. There is some evidence for activation of phospholipase C by ABA, by an unknown mechanism; plant phospholipase C may be activated by Ca2+ rather than by the G-proteins used in many animal cell signalling systems. A further ABA-induced channel modulation is the inhibition of the inward K+ channel, which is not essential for closing but will prevent opening. It is suggested that this is mediated through the Ca(2+)-activated protein phosphatase, calcineurin. The question of Ca(2+)-independent stomatal closure remains controversial. At the plasmalemma the stimulation of K+ efflux is Ca(2+)-independent and, at least in Arabidopsis, activation of anion efflux by ABA may also be Ca(2+)-independent. But there are no indications of Ca(2+)-independent mechanisms for K+ efflux at the tonoplast, and the appropriate anion channel at the tonoplast is still to be found. There is also evidence that ABA interferes with a control system in the guard cell, resetting its set-point to lower contents, suggesting that stretch-activated channels also feature in the regulation of guard cell ion channels, perhaps through interactions with cytoskeletal proteins. (ABSTRACT TRUN