A membrane-delimited pathway of G-protein regulation of the guard-cell inward K+ channel

Stomatal responses to VPD utilize guard cell intracellular signaling components

Stomatal pores, vital for CO2 uptake and water loss regulation in plants, are formed by two specialized guard cells. Despite their importance, there is limited understanding of how guard cells sense and respond to changes in vapor pressure difference (VPD). This study leverages a selection of CO2 hyposensitive and abscisic acid (ABA) signaling mutants in Arabidopsis, including heterotrimeric G protein mutants and RLK (receptor-like kinase) mutants, along with a variety of canola cultivars to delve into the intracellular signaling mechanisms prompting stomatal closure in response to high VPD. Stomatal conductance response to step changes in VPD was measured using the LI-6800F gas exchange system. Our findings highlight that stomatal responses to VPD utilize intracellular signaling components. VPD hyposensitivity was particularly evident in mutants of the ht1 (HIGH LEAF TEMPERATURE1) gene, which encodes a protein kinase expressed mainly in guard cells, and in gpa1-3, a null mutant of the sole canonical heterotrimeric Gα subunit, previously implicated in stomatal signaling. Consequently, this research identifies a nexus in the intricate relationships between guard cell signal perception, stomatal conductance, environmental humidity, and CO2 levels.

Stomatal closure induced by hydrogen-rich water is dependent on GPA1 in Arabidopsis thaliana

Hydrogen (H₂) is a new signaling molecule that regulates stomatal closure via stimulating the generation of reactive oxygen species (ROS) and nitric oxide (NO) in Arabidopsis thaliana. GPA1 is the sole heterotrimeric G protein canonical α subunit found in Arabidopsis genome and functions in stomatal closure. Here, we estimated a possible role of Arabidopsis GPA1 in hydrogen-rich water (HRW)-induced stomatal closure. Our data indicated that HRW induced significant stomatal closure as well as the generation of ROS and NO in the Col-0 guard cells. However, the production of ROS and NO and stomatal closure induced by HRW were absent in the gpa1-4 mutant lacking the expression of AtGPA1. By contrast, overexpression of AtGPA1 in gpa1-4 (AtGPA1-HA/gpa1-4) restored stomatal closure and the generation of NO and ROS in the presence of HRW. Taken together, our results suggest that GPA1 is necessary for HRW-induced stomatal closure in Arabidopsis.

Electrophysiological Identification and Activity Analyses of Plasma Membrane K+ Channels in Maize Guard Cells

Stomatal movement, which plays an essential role in plant transpiration and photosynthesis, is controlled by ion channels that mediate K+ and anion fluxes across the plasma membrane (PM) of guard cells. These channels in dicots are accurately regulated by various physiological factors, such as pH, abscisic acid (ABA) and Ca2+; however, the data in monocots are limited. Here the whole-cell patch-clamping technique was applied to analyze the properties and regulations of PM K+ channels in maize guard cells. The results indicated that the hyperpolarization-activated inward-rectifying channels were highly K+-selective. These inward K+ (Kin) channels were sensitive to extracellular K+. Their slope factor (S) decreased when the apoplastic K+ concentration decline, causing a positive shift of the half-activation potential (V1/2). Their activities were promoted by apoplastic acidification but inhibited by apoplastic and cytosolic alkalization. Nevertheless, the outward K+ (Kout) channel activities were uniquely promoted by cytosolic alkalization. Both apoplastic and cytosolic ABA inhibited Kin channels independent of cytosolic Ca2+ ([Ca2+]cyt). And two Ca2+-dependent mechanisms with different Ca2+ affinities may mediate resting- and high-[Ca2+]cyt-induced inhibition on Kin channels, respectively. However, resting [Ca2+]cyt impaired the inhibition of Kin channels induced by apoplastic ABA, not cytosolic ABA. Furthermore, the result that high [Ca2+]cyt attenuated ABA-induced inhibition highlighted the importance of [Ca2+]cyt for Kin channel regulation. There may exist a Ca2+-dependent regulation of the Ca2+-independent ABA signaling pathways for Kin channel inhibition. These results provided an electrophysiological view of the multiple level regulations of PM K+ channel activities and kinetics in maize guard cells.

Phytohormone-Mediated Stomatal Response, Escape and Quiescence Strategies in Plants under Flooding Stress

Generally, flooding causes waterlogging or submergence stress which is considered as one of the most important abiotic factors that severely hinders plant growth and development. Plants might not complete their life cycle even in short duration of flooding. As biologically intelligent organisms, plants always try to resist or survive under such adverse circumstances by adapting a wide array of mechanisms including hormonal homeostasis. Under this mechanism, plants try to adapt through diverse morphological, physiological and molecular changes, including the closing of stomata, elongating of petioles, hollow stems or internodes, or maintaining minimum physiological activity to store energy to combat post-flooding stress and to continue normal growth and development. Mainly, ethylene, gibberellins (GA) and abscisic acid (ABA) are directly and/or indirectly involved in hormonal homeostasis mechanisms. Responses of specific genes or transcription factors or reactive oxygen species (ROS) maintain the equilibrium between stomatal opening and closing, which is one of the fastest responses in plants when encountering flooding stress conditions. In this review paper, the sequential steps of some of the hormone-dependent survival mechanisms of plants under flooding stress conditions have been critically discussed.

Melatonin activates BKCa channels in cerebral artery myocytes via both direct and MT receptor/PKC-mediated pathway

The pineal hormone melatonin is a neuroendocrine hormone with high membrane permeability that is involved in regulation of circadian rhythm of several biological functions. Large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels are abundantly expressed in vascular smooth muscle cells and play an important role in vascular tone regulation. We investigated the mechanisms through which myocyte BK_Ca channels mediate effects of melatonin on cerebral arteries (CAs). Arterial contractility measurements showed that melatonin alone did not change vascular tone in CAs; however, it induced concentration-dependent vasodilation of phenylephrine-induced contraction in CAs. In the presence of the potent endothelial oxide synthase inhibitor, N^ω-nitro-L-arginine methyl ester, melatonin-elicited relaxation was significantly inhibited by iberiotoxin (BK_Ca channel blocker). Melatonin significantly increased BK_Ca currents but not voltage-gated K⁺ (K_V) currents in whole-cell recordings. Melatonin decreased the amplitude of Ca²⁺ sparks and spontaneous transient outward currents (STOCs), however, a significant increase in open probability of BK_Ca channels was observed in both inside-out and cell-attached patch-clamp recordings. This melatonin-induced enhancement of BK_Ca channel activity was significantly suppressed by luzindole (melatonin MT₁/MT₂ receptor inhibitor), U73122 (phospholipase C (PLC) inhibitor), and Ro31-8220 (protein kinase C (PKC) inhibitor). Melatonin had no significant effects on sarcoplasmic reticulum release of Ca²⁺. These findings indicate that melatonin-induced vasorelaxation of CAs is partially attributable to direct (passing through the cell membrane) and indirect (via melatonin MT₁/MT₂ receptors-PLC-PKC pathway) activation of BK_Ca channels on CA myocytes.

The role of G-proteins in plant immunity

Heterotrimeric G-proteins play an important regulatory role in multiple physiological processes, including the plant immune response, and substantial progress has been made in elucidating the G-protein-mediated defense-signaling network. This mini-review discusses the importance of G-proteins in plant immunity. We also provide an overview of how G-proteins affect plant cell death and stomatal movement. Our recent studies demonstrated that G-proteins are involved in signal transduction and induction of stomatal closure and defense responses. We also discuss future directions for G-protein signaling studies involving plant immunity.

Roles of heterotrimeric G proteins in guard cell ion channel regulation

Stomata are formed by pairs of surrounding guard cells and perform important roles in photosynthesis, transpiration and innate immunity of terrestrial plants. Ionic solutes in the cytosol of guard cells are important for cell turgor and volume change. Consequently, transmembrane flux of ions such as K+, Cl-, and malate2- through K+ channels and anion channels of guard cells are a direct driving force for turgor change, while the opening of calcium permeable channels can serve as a trigger of cytosolic free calcium concentration elevations or oscillations, which play second messenger roles. In plants, heterotrimeric G proteins have fewer members than in animals, but they are well investigated and found to regulate these channels and to play fundamental roles in guard cell function. This mini-review focuses on the recent understanding of G-protein regulation of ion channels on the plasma membrane of guard cells and their participation in stomatal movements.

The guard cell as a single-cell model towards understanding drought tolerance and abscisic acid action

Stomatal guard cells are functionally specialized epidermal cells usually arranged in pairs surrounding a pore. Changes in ion fluxes, and more specifically osmolytes, within the guard cells drive opening/closing of the pore, allowing gas exchange while limiting water loss through evapo-transpiration. Adjustments of the pore aperture to optimize these conflicting needs are thus centrally important for land plants to survive, especially with the rise in CO(2) associated with global warming and increasing water scarcity this century. The basic biophysical events in modulating membrane transport have been gradually delineated over two decades. Genetics and molecular approaches in recent years have complemented and extended these earlier studies to identify major regulatory nodes. In Arabidopsis, the reference for guard cell genetics, stomatal opening driven by K(+) entry is mainly through KAT1 and KAT2, two voltage-gated K(+) inward-rectifying channels that are activated on hyperpolarization of the plasma membrane principally by the OST2 H(+)-ATPase (proton pump coupled to ATP hydrolysis). By contrast, stomatal closing is caused by K(+) efflux mainly through GORK, the outward-rectifying channel activated by membrane depolarization. The depolarization is most likely initiated by SLAC1, an anion channel distantly related to the dicarboxylate/malic acid transport protein found in fungi and bacteria. Beyond this established framework, there is also burgeoning evidence for the involvement of additional transporters, such as homologues to the multi-drug resistance proteins (or ABC transporters) as intimated by several pharmacological and reverse genetics studies. General inhibitors of protein kinases and protein phosphatases have been shown to profoundly affect guard cell membrane transport properties. Indeed, the first regulatory enzymes underpinning these transport processes revealed genetically were several protein phosphatases of the 2C class and the OST1 kinase, a member of the SnRK2 family. Taken together, these results are providing the first glimpses of an emerging signalling complex critical for modulating the stomatal aperture in response to environmental stimuli.

Abscisic acid regulation of guard-cell K+ and anion channels in Gbeta- and RGS-deficient Arabidopsis lines

In mammals, basal currents through G protein-coupled inwardly rectifying K(+) (GIRK) channels are repressed by Galpha(i/o)GDP, and the channels are activated by direct binding of free Gbetagamma subunits released upon stimulation of Galpha(i/o)-coupled receptors. However, essentially all information on G protein regulation of GIRK electrophysiology has been gained on the basis of coexpression studies in heterologous systems. A major advantage of the model organism, Arabidopsis thaliana, is the ease with which knockout mutants can be obtained. We evaluated plants harboring mutations in the sole Arabidopsis Galpha (AtGPA1), Gbeta (AGB1), and Regulator of G protein Signaling (AtRGS1) genes for impacts on ion channel regulation. In guard cells, where K(+) fluxes are integral to cellular regulation of stomatal apertures, inhibition of inward K(+) (K(in)) currents and stomatal opening by the phytohormone abscisic acid (ABA) was equally impaired in Atgpa1 and agb1 single mutants and the Atgpa1 agb1 double mutant. AGB1 overexpressing lines maintained a wild-type phenotype. The Atrgs1 mutation did not affect K(in) current magnitude or ABA sensitivity, but K(in) voltage-activation kinetics were altered. Thus, Arabidopsis cells differ from mammalian cells in that they uniquely use the Galpha subunit or regulation of the heterotrimer to mediate K(in) channel modulation after ligand perception. In contrast, outwardly rectifying (K(out)) currents were unaltered in the mutants, and ABA activation of slow anion currents was conditionally disrupted in conjunction with cytosolic pH clamp. Our studies highlight unique aspects of ion channel regulation by heterotrimeric G proteins and relate these aspects to stomatal aperture control, a key determinant of plant biomass acquisition and drought tolerance.

Isolation and whole-cell patch clamping of Arabidopsis guard cell protoplasts

INTRODUCTIONThe flux of ions across membranes via ion channels is vital to cellular responses to internal and external stimuli, and therefore to cellular survival in changing circumstances. Patch clamping is a powerful technique for ion channel investigation, because it enables measurement of both net ion fluxes across the entire surface area of a cell and ion currents flowing through a single open channel. However, unlike animal cells, plant cells are surrounded by cell walls that prevent the physical contact between the patch pipette and the plasma membrane necessary for the patch clamp technique. To demonstrate how patch clamping can be applied to plant physiology research, we describe a protocol used to record potassium ion (K(+)) channel currents in Arabidopsis guard cell protoplasts (a widely studied model cell type in plant biology). The protocol requires a two-step cellulase and pectinase digestion to isolate high quality Arabidopsis guard cell protoplasts (i.e., plant cells lacking their cell walls), preparation of suitable glass capillary microelectrodes, and formation of the whole-cell configuration with a gigaohm (GΩ) seal. We also describe the history of the protocol and list other types of plant cells from which successful patch clamp recordings have been obtained.

Cloning and expression of Dcga gene encoding alpha subunit of GTP-binding protein in carrot seedlings

A homology-based cloning strategy yielded a cDNA clone designated Dcga, presumably encoding alpha subunit of GTP-binding protein, from carrot (Daucus carota) seedlings. Molecular phylogenetic tree analysis of G protein alpha subunits from various biological sources suggested that, unlike in animal cells, classification of Galpha proteins into specific subfamilies could not be applicable to the proteins from higher plants. The restriction digests prepared from genomic DNA of carrot showed one or two hybridized signals in Southern blot analyses, and the expression level of Dcga was appreciably decreased upon the exposure of carrot to high temperature or the prolonged treatment with salt. These results suggest that Dcga occurs as single or double copy genes in carrot genome, and its transcript might play specific roles in heat- and salt-induced responses of the plant.

Auxin-dependent cell division and cell elongation. 1-Naphthaleneacetic acid and 2,4-dichlorophenoxyacetic acid activate different pathways

During exponential phase, the tobacco (Nicotiana tabacum) cell line cv Virginia Bright Italia-0 divides axially to produce linear cell files of distinct polarity. This axial division is controlled by exogenous auxin. We used exponential tobacco cv Virginia Bright Italia-0 cells to dissect early auxin signaling, with cell division and cell elongation as physiological markers. Experiments with 1-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) demonstrated that these 2 auxin species affect cell division and cell elongation differentially; NAA stimulates cell elongation at concentrations that are much lower than those required to stimulate cell division. In contrast, 2,4-D promotes cell division but not cell elongation. Pertussis toxin, a blocker of heterotrimeric G-proteins, inhibits the stimulation of cell division by 2,4-D but does not affect cell elongation. Aluminum tetrafluoride, an activator of the G-proteins, can induce cell division at NAA concentrations that are not permissive for division and even in the absence of any exogenous auxin. The data are discussed in a model where the two different auxins activate two different pathways for the control of cell division and cell elongation.

Extracellular calmodulin-induced stomatal closure is mediated by heterotrimeric G protein and H2O2

Extracellular calmodulin (ExtCaM) exerts multiple functions in animals and plants, but the mode of ExtCaM action is not well understood. In this paper, we provide evidence that ExtCaM stimulates a cascade of intracellular signaling events to regulate stomatal movement. Analysis of the changes of cytosolic free Ca2+ ([Ca2+]cyt) and H2O2 in Vicia faba guard cells combined with epidermal strip bioassay suggests that ExtCaM induces an increase in both H2O2 levels and [Ca2+]cyt, leading to a reduction in stomatal aperture. Pharmacological studies implicate heterotrimeric G protein in transmitting the ExtCaM signal, acting upstream of [Ca2+]cyt elevation, and generating H2O2 in guard cell responses. To further test the role of heterotrimeric G protein in ExtCaM signaling in stomatal closure, we checked guard cell responses in the Arabidopsis (Arabidopsis thaliana) Galpha-subunit-null gpa1 mutants and cGalpha overexpression lines. We found that gpa1 mutants were insensitive to ExtCaM stimulation of stomatal closure, whereas cGalpha overexpression enhanced the guard cell response to ExtCaM. Furthermore, gpa1 mutants are impaired in ExtCaM induction of H2O2 generation in guard cells. Taken together, our results strongly suggest that ExtCaM activates an intracellular signaling pathway involving activation of a heterotrimeric G protein, H2O2 generation, and changes in [Ca2+]cyt in the regulation of stomatal movements.

The Arabidopsis putative G protein-coupled receptor GCR1 interacts with the G protein alpha subunit GPA1 and regulates abscisic acid signaling

Heterotrimeric G proteins composed of alpha, beta, and gamma subunits link ligand perception by G protein-coupled receptors (GPCRs) with downstream effectors, providing a ubiquitous signaling mechanism in eukaryotes. The Arabidopsis thaliana genome encodes single prototypical Galpha (GPA1) and Gbeta (AGB1) subunits, and two probable Ggamma subunits (AGG1 and AGG2). One Arabidopsis gene, GCR1, encodes a protein with significant sequence similarity to nonplant GPCRs and a predicted 7-transmembrane domain structure characteristic of GPCRs. However, whether GCR1 actually interacts with GPA1 was unknown. We demonstrate by in vitro pull-down assays, by yeast split-ubiquitin assays, and by coimmunoprecipitation from plant tissue that GCR1 and GPA1 are indeed physically coupled. GCR1-GPA1 interaction depends on intracellular domains of GCR1. gcr1 T-DNA insertional mutants exhibit hypersensitivity to abscisic acid (ABA) in assays of root growth, gene regulation, and stomatal response. gcr1 guard cells are also hypersensitive to the lipid metabolite, sphingosine-1-phosphate (S1P), which is a transducer of the ABA signal upstream of GPA1. Because gpa1 mutants exhibit insensitivity in aspects of guard cell ABA and S1P responses, whereas gcr1 mutants exhibit hypersensitivity, GCR1 may act as a negative regulator of GPA1-mediated ABA responses in guard cells.

Regulation of K+ channel activities in plants: from physiological to molecular aspects

Plant voltage-gated channels belonging to the Shaker family participate in sustained K+ transport processes at the cell and whole plant levels, such as K+ uptake from the soil solution, long-distance K+ transport in the xylem and phloem, and K+ fluxes in guard cells during stomatal movements. The attention here is focused on the regulation of these transport systems by protein-protein interactions. Clues to the identity of the regulatory mechanisms have been provided by electrophysiological approaches in planta or in heterologous systems, and through analogies with their animal counterparts. It has been shown that, like their animal homologues, plant voltage-gated channels can assemble as homo- or heterotetramers associating polypeptides encoded by different Shaker genes, and that they can bind auxiliary subunits homologous to those identified in mammals. Furthermore, several regulatory processes (involving, for example, protein kinases and phosphatases, G proteins, 14-3-3s, or syntaxins) might be common to plant and animal Shakers. However, the molecular identification of plant channel partners is still at its beginning. This paper reviews current knowledge on plant K+ channel regulation at the physiological and molecular levels, in the light of the corresponding knowledge in animal cells, and discusses perspectives for the deciphering of regulatory networks in the future.

Cloning and characterization of two full-length cDNAs, TaGA1 and TaGA2, encoding G-protein alpha subunits expressed differentially in wheat genome

In the present study, we identified and characterized two cDNAs, named TaGA1 and TaGA2, encoding alpha subunits of heterotrimeric G proteins synthesized from one-week-old seedling mRNAs of common wheat cv. S615 using RACE PCR and RT-PCR methods. The clone TaGA1 contained an open reading frame that encoded a protein consisting of 383 amino acid residues with a molecular mass of 51.3 kDa, whereas the clone TaGA2 contained an open reading frame encoding 390 amino acids with a molecular mass of 52.5 kDa. At the amino acid level, both cDNAs (TaGA1 and TaGA2) showed 70-96% and 30-40% homologies to plant and animal G-protein alpha (G alpha) subunits, respectively, and 97.7% homology to each other. The regions essential for binding to GTP were conserved among all G alpha subunits in higher plants and mammals examined. However, the C-terminal amino acid sequences of TaGA1 and TaGA2 were similar to those of cereal G alpha subunits (rice and barley) but were different from the analogous sequences of mammalian G alpha subunits as well as from those of the leguminous and Solanaeceous G alpha subunits. Southern analysis revealed that the hexaploid wheat genome contained three major copies of G alpha subunit gene with a few less homologous copies. The analysis of the expression for G alpha subunit genes in wheat showed that both TaGA1 and TaGA2 mRNAs were abundant in one-week-old seedlings, immature seeds harvested one-week after anthesis, young spikes and internodes, indicating constitutive expression patterns in all of the organs tested. Especially, young spikes and internodes exhibited increased levels of mRNA accumulation, suggesting that G alpha subunit gene is highly expressed in actively elongating and fast growing tissues. Moreover, both TaGA1 and TaGA2 showed genome-specific expressions in wheat and may participate in the light-regulated growth and development of the seedlings.

Secondary messengers and phospholipase A2 in auxin signal transduction

Despite recent progress auxin signal transduction remains largely scetchy and enigmatic. A good body of evidence supports the notion that the ABP1 could be a functional receptor or part of a receptor, respectively, but this is not generally accepted. Evidence for other functional receptors is lacking, as is any clearcut evidence for a function of G proteins. Protons may serve as second messengers in guard cells but the existing evidence for a role of calcium remains to be clearified. Phospholipases C and D seem not to have a function in auxin signal transduction whereas the indications for a role of phospholipase A2 in auxin signal transduction accumulated recently. Mitogen-activated protein kinase (MAPK) is modulated by auxin and the protein kinase PINOID has a role in auxin transport modulation even though their functional linkage to other signalling molecules is ill-defined. It is hypothesized that signal transduction precedes activation of early genes such as IAA genes and that ubiquitination and the proteasome are a mechanism to integrate signal duration and signal strength in plants and act as major regulators of hormone sensitivity.

Physical and functional interaction of the Arabidopsis K(+) channel AKT2 and phosphatase AtPP2CA

The AKT2 K(+) channel is endowed with unique functional properties, being the only weak inward rectifier characterized to date in Arabidopsis. The gene is expressed widely, mainly in the phloem but also at lower levels in leaf epiderm, mesophyll, and guard cells. The AKT2 mRNA level is upregulated by abscisic acid. By screening a two-hybrid cDNA library, we isolated a protein phosphatase 2C (AtPP2CA) involved in abscisic acid signaling as a putative partner of AKT2. We further confirmed the interaction by in vitro binding studies. The expression of AtPP2CA (beta-glucuronidase reporter gene) displayed a pattern largely overlapping that of AKT2 and was upregulated by abscisic acid. Coexpression of AtPP2CA with AKT2 in COS cells and Xenopus laevis oocytes was found to induce both an inhibition of the AKT2 current and an increase of the channel inward rectification. Site-directed mutagenesis and pharmacological analysis revealed that this functional interaction involves AtPP2CA phosphatase activity. Regulation of AKT2 activity by AtPP2CA in planta could allow the control of K(+) transport and membrane polarization during stress situations.

A mutant Arabidopsis heterotrimeric G-protein beta subunit affects leaf, flower, and fruit development

A genetic screen was performed to find new mutants with an erecta (er) phenotype and to identify genes that may function with ER, a receptor-like kinase. These mutants were named elk (for erecta-like) and were placed into five complementation groups. We positionally cloned ELK4 and determined that it encodes AGB1, a putative heterotrimeric G-protein beta subunit. Therefore, elk4 was renamed agb1. agb1-1 plants express similar fruit phenotypes, as seen in er plants, but differ from er in that the stem is only slightly shorter than that in the wild type, the pedicel is slightly longer than that in the wild type, and the leaves are rounder than those in er mutants. Molecular analysis of agb1-1 indicates that it is likely a null allele. AGB1 mRNA is expressed in all tissues tested but is highest in the silique. Analysis of agb1-1 er double mutants suggests that AGB1 may function in an ER developmental pathway regulating silique width but that it functions in parallel pathways affecting silique length as well as leaf and stem development. The finding that AGB1 is involved in the control of organ shape suggests that heterotrimeric G-protein signaling is a developmental regulator in Arabidopsis.

A mutant Arabidopsis heterotrimeric G-protein beta subunit affects leaf, flower, and fruit development

A genetic screen was performed to find new mutants with an erecta (er) phenotype and to identify genes that may function with ER, a receptor-like kinase. These mutants were named elk (for erecta-like) and were placed into five complementation groups. We positionally cloned ELK4 and determined that it encodes AGB1, a putative heterotrimeric G-protein beta subunit. Therefore, elk4 was renamed agb1. agb1-1 plants express similar fruit phenotypes, as seen in er plants, but differ from er in that the stem is only slightly shorter than that in the wild type, the pedicel is slightly longer than that in the wild type, and the leaves are rounder than those in er mutants. Molecular analysis of agb1-1 indicates that it is likely a null allele. AGB1 mRNA is expressed in all tissues tested but is highest in the silique. Analysis of agb1-1 er double mutants suggests that AGB1 may function in an ER developmental pathway regulating silique width but that it functions in parallel pathways affecting silique length as well as leaf and stem development. The finding that AGB1 is involved in the control of organ shape suggests that heterotrimeric G-protein signaling is a developmental regulator in Arabidopsis.

Extracellular protons inhibit the activity of inward-rectifying potassium channels in the motor cells of Samanea saman pulvini

The intermittent influx of K+ into motor cells in motor organs (pulvini) is essential to the rhythmic movement of leaves and leaflets in various plants, but in contrast to the K+ influx channels in guard cells, those in pulvinar motor cells have not yet been characterized. We analyzed these channels in the plasma membrane of pulvinar cell protoplasts of the nyctinastic legume Samanea saman using the patch-clamp technique. Inward, hyperpolarization-activated currents were separated into two types: time dependent and instantaneous. These were attributed, respectively, to K+ -selective and distinctly voltage-dependent K(H) channels and to cation-selective voltage-independent leak channels. The pulvinar K(H) channels were inhibited by external acidification (pH 7.8-5), in contrast to their acidification-promoted counterparts in guard cells. The inhibitory pH effect was resolved into a reversible decline of the maximum conductance and an irreversible shift of the voltage dependence of K(H) channel gating. The leak appeared acidification insensitive. External Cs (10 mM in 200 mM external K+) blocked both current types almost completely, but external tetraethylammonium (10 mM in 200 mM external K+) did not. Although these results do not link these two channel types unequivocally, both likely serve as K+ influx pathways into swelling pulvinar motor cells. Our results emphasize the importance of studying multiple model systems.

Plant G proteins: the different faces of GPA1

Biochemical studies suggest that G proteins mediate a variety of signaling processes in plants, yet Arabidopsis has only one gene, GPA1, for a canonical G protein alpha subunit. Recent studies indicate that the GPA1 protein is involved in a number of very different cellular processes.

Structure and function of heterotrimeric G proteins in plants

Heterotrimeric G proteins are mediators that transmit the external signals via receptor molecules to effector molecules. The G proteins consist of three different subunits: alpha, beta, and gamma subunits. The cDNAs or genes for all the alpha, beta, and gamma subunits have been isolated from many plant species, which has contributed to great progress in the study of the structure and function of the G proteins in plants. In addition, rice plants lacking the alpha subunit were generated by the antisense method and a rice mutant, Daikoku d1, was found to have mutation in the alpha-subunit gene. Both plants show abnormal morphology such as dwarfism, dark green leaf, and small round seed. The findings revealed that the G proteins are functional molecules regulating some body plans in plants. There is evidence that the plant G proteins participate at least in signaling of gibberellin at low concentrations. In this review, we summarize the currently known information on the structure of plant heterotrimeric G proteins and discuss the possible functions of the G proteins in plants.

G protein regulation of ion channels and abscisic acid signaling in Arabidopsis guard cells

The phytohormone abscisic acid (ABA) promotes plant water conservation by decreasing the apertures of stomatal pores in the epidermis through which water loss occurs. We found that Arabidopsis thaliana plants harboring transferred DNA insertional mutations in the sole prototypical heterotrimeric GTP-binding (G) protein alpha subunit gene, GPA1, lack both ABA inhibition of guard cell inward K(+) channels and pH-independent ABA activation of anion channels. Stomatal opening in gpa1 plants is insensitive to inhibition by ABA, and the rate of water loss from gpa1 mutants is greater than that from wild-type plants. Manipulation of G protein status in guard cells may provide a mechanism for controlling plant water balance.

The effects of ABA on channel-mediated K(+) transport across higher plant roots

The transport and accumulation of K(+) in higher plant roots is regulated by ABA. Molecular and electrophysiological techniques have identified a number of discrete transporters which are involved in the translocation of K(+) from the soil solution to the shoots of higher plants. Furthermore, recent reports have shown that ABA regulates K(+) channel activity in maize and Arabidopsis roots which suggests that ABA regulation of K(+) transport in roots is, at least in part, ion channel-mediated. The signalling processes which underlie the ABA regulation of K(+) channels have been investigated. The effects of ABA on the membrane potential of intact maize root cells were also studied. It was found that ABA regulated the membrane potential of root cells and that this regulation is consistent with the hypothesis that ABA-induced K(+) accumulation in roots is mediated by K(+) channels.

Distinct abscisic acid signaling pathways for modulation of guard cell versus mesophyll cell potassium channels revealed by expression studies in Xenopus laevis oocytes

Regulation of guard cell ion transport by abscisic acid (ABA) and in particular ABA inhibition of a guard cell inward K(+) current (I(Kin)) is well documented. However, little is known concerning ABA effects on ion transport in other plant cell types. Here we applied patch clamp techniques to mesophyll cell protoplasts of fava bean (Vicia faba cv Long Pod) plants and demonstrated ABA inhibition of an outward K(+) current (I(Kout)). When mesophyll cell protoplast mRNA (mesophyll mRNA) was expressed in Xenopus laevis oocytes, I(Kout) was generated that displayed similar properties to I(Kout) observed from direct analysis of mesophyll cell protoplasts. I(Kout) expressed by mesophyll mRNA-injected oocytes was inhibited by ABA, indicating that the ABA signal transduction pathway observed in mesophyll cells was preserved in the frog oocytes. Co-injection of oocytes with guard cell protoplast mRNA and cRNA for KAT1, an inward K(+) channel expressed in guard cells, resulted in I(Kin) that was similarly inhibited by ABA. However, oocytes co-injected with mesophyll mRNA and KAT1 cRNA produced I(Kin) that was not inhibited by ABA. These results demonstrate that the mesophyll-encoded signaling mechanism could not substitute for the guard cell pathway. These findings indicate that mesophyll cells and guard cells use distinct and different receptor types and/or signal transduction pathways in ABA regulation of K(+) channels.

The presence of a heterotrimeric G protein and its role in signal transduction of extracellular calmodulin in pollen germination and tube growth

The role of heterotrimeric G proteins in pollen germination, tube growth, and signal transduction of extracellular calmodulin (CaM) was examined in lily pollen. Two kinds of antibodies raised against animal Gzalpha, one against an internal sequence and the other against its N terminus, cross-reacted with the same 41-kD protein from lily pollen plasma membrane. This 41-kD protein was also specifically ADP ribosylated by pertussis toxin. Microinjection of the membrane-impermeable G protein agonist GTP-gamma-S into a pollen tube increased its growth rate, whereas microinjection of the membrane-impermeable G protein antagonist GDP-beta-S and the anti-Galpha antibody decreased pollen tube growth. The membrane-permeable G protein agonist cholera toxin stimulated pollen germination and tube growth. Anti-CaM antiserum inhibited pollen germination and tube growth, and this inhibitory effect was completely reversed by cholera toxin. The membrane-permeable heterotrimeric G protein antagonist pertussis toxin completely stopped pollen germination and tube growth. Purified CaM, when added directly to the medium of plasma membrane vesicles, significantly activated GTPase activity in plasma membrane vesicles, and this increase in GTPase activity was completely inhibited by pertussis toxin and the nonhydrolyzable GTP analogs GTP-gamma-S and guanylyl-5'-imidodiphosphate. The GTPase activity in plasma membrane vesicles was also stimulated by cholera toxin. These data suggest that heterotrimeric G proteins may be present in the pollen system where they may be involved in the signal transduction of extracellular CaM and in pollen germination and tube growth.

Mu-opioid agonist-induced activation of G-protein-coupled inwardly rectifying potassium current in rat periaqueductal gray neurons

The characteristics of the inwardly rectifying K+ current activated by a mu-type opioid agonist, D-Ala2,N-MePhe4,Gly5-ol-enkephalin (DAMGO), were examined in the acutely dissociated rat periaqueductal gray neurons using the nystatin-perforated and the conventional whole-cell recording modes under voltage-clamp conditions. DAMGO activated inward currents in a concentration- and voltage-dependent manner. The DAMGO-induced current was an inwardly rectifying K+ current (I(DAMGO)) which was sensitive to K+ channel blockers, quinine and Ba2+ but insensitive to Cs+ and tetraethylammonium. In the conventional whole-cell clamp mode, guanosine 5'-O-(2-thiodiphosphate) trilithium salt (GDPbetas, 0.4 mM) inhibited the amplitude of I(DAMGO) to 28% of that of the initial current. After the intracellular perfusion with guanosine 5'-O-(3-thiotriphosphate) tetralithium salt (GTPgammas, 0.4 mM) for 1 min, the first application of DAMGO irreversibly activated I(DAMGO). By the extracellular application of N-ethylmaleimide at a concentration of 50 microM for 2 min, I(DAMGO) was completely abolished. When a conventional whole-cell patch was made with a patch-pipette containing 1 microg/ml of pertussis toxin together with 1 mM of beta-nicotinamide adenine dinucleotide, I(DAMGO) gradually declined to about 41% of its initial amplitude. The extracellular application of second messenger modulators including protein kinase inhibitor (staurosporin), protein kinase A activators (forskolin, 3-isobutyl-l-methyl-xanthine and dibutyryladenosine 3'5'-cyclic monophosphate) and protein kinase C activators (phorbol-12-myristate-13-acetate and 1-oleoyl-2-acetyl-sn-glycerol) had no effect on I(DAMGO). These results suggest that (i) DAMGO-activated inwardly rectifying K+ current is mediated by pertussis toxin-sensitive guanine nucleotide binding proteins (G-proteins); (ii) the types of G protein involved in I(DAMGO) are Gi and/or Go; and (iii) the G-proteins exert their roles in I(DAMGO) without any mediation of the second messenger systems.

A laser microsurgical method of cell wall removal allows detection of large-conductance ion channels in the guard cell plasma membrane

Application of patch clamp techniques to higher-plant cells has been subject to the limitation that the requisite contact of the patch electrode with the cell membrane necessitates prior enzymatic removal of the plant cell wall. Because the wall is an integral component of plant cells, and because cell-wall-degrading enzymes can disrupt membrane properties, such enzymatic treatments may alter ion channel behavior. We compared ion channel activity in enzymatically isolated protoplasts of Vicia faba guard cells with that found in membranes exposed by a laser microsurgical technique in which only a tiny portion of the cell wall is removed while the rest of the cell remains intact within its tissue environment. "Laser-assisted" patch clamping reveals a new category of high-conductance (130 to 361 pS) ion channels not previously reported in patch clamp studies on plant plasma membranes. These data indicate that ion channels are present in plant membranes that are not detected by conventional patch clamp techniques involving the production of individual plant protoplasts isolated from their tissue environment by enzymatic digestion of the cell wall. Given the large conductances of the channels revealed by laser-assisted patch clamping, we hypothesize that these channels play a significant role in the regulation of ion content and electrical signalling in guard cells.

Plant hormone perception and action: a role for G-protein signal transduction?

Plants perceive and respond to a profusion of environmental and endogenous signals that influence their growth and development. The G-protein signalling pathway is a mechanism for transducing extracellular signals that is highly conserved in a range of eukaryotes and prokaryotes. Evidence for the existence of G-protein signalling pathways in higher plants is reviewed, and their potential involvement in plant hormone signal transduction evaluated. A range of biochemical and molecular studies have identified potential components of G-protein signalling in plants, most notably a homologue of the G-protein coupled receptor superfamily (GCR1) and the G alpha and G beta subunits of heterotrimeric G-proteins. G-protein agonists and antagonists are known to influence a variety of signalling events in plants and have been used to implicate heterotrimeric G-proteins in gibberellin and possibly auxin signalling. Antisense suppression of GCR1 in Arabidopsis leads to a phenotype which supports a role for this receptor in cytokinin signalling. These observations suggest that higher plants have at least some of the components of G-protein signalling pathways and that these might be involved in the action of certain plant hormones.

A higher plant seven-transmembrane receptor that influences sensitivity to cytokinins

BACKGROUND

All organisms perceive and respond to a profusion of environmental and endogenous signals that influence growth, development and behavior. The G-protein signalling pathway is a highly conserved mechanism for transducing extracellular signals, and the superfamily of receptors that have seven transmembrane (7TM) domains is a primary element of this pathway. Evidence that heterotrimeric G proteins are involved in signal transduction in plants is accumulating, prompting speculation that plant 7TM receptors might exist.

RESULTS

Using information in the dbEST database of expressed sequence tags, we isolated an Arabidopsis thaliana gene (GCR1) that encodes a protein with seven predicted membrane-spanning domains and other features characteristic of 7TM receptors. The protein shows 18-23% amino-acid identity (46-53% similarity) to, and good colinear alignment with, 7TM receptors from three different families. Its highest sequence identity is with the Dictyostelium cAMP receptors. GCR1 is expressed at very low levels in the roots, stems and leaves of Arabidopsis; it is a single-copy gene which maps close to the restriction fragment length polymorphism marker m291 on chromosome 5. Transgenic Arabidopsis expressing antisense GCR1 under the control of the constitutive cauliflower mosaic virus 35S promoter have reduced sensitivity to cytokinins in roots and shoots, yet respond normally to all other plant hormones. This suggests a functional role for GCR1 in cytokinin signal transduction.

CONCLUSIONS

GCR1 encodes the first 7TM receptor homologue identified in higher plants and is involved in cytokinin signal transduction. This discovery suggests that 7TM receptors are ancient and predate the divergence of plants and animals.

Heterotrimeric G proteins are implicated in gibberellin induction of a-amylase gene expression in wild oat aleurone

The role of heterotrimeric G proteins in gibberellin (GA) induction of a-amylase gene expression was examined in wild oat aleurone protoplasts. Mas7, a cationic amphiphilic tetradecapeptide that stimulates GDP/GTP exchange by heterotrimeric G proteins, specifically induced alpha-amylase gene expression and enzyme secretion in a very similar manner to GA1. In addition, Mas7 stimulated expression of an alpha-Amy2/54:GUS promoter and reporter construct in transformed protoplasts. Both Mas7 and GA1 induction of alpha-amylase mRNA were insensitive to pertussis toxin. Hydrolysis-resistant nucleotides were introduced into aleurone protoplasts during transfection with reporter gene constructs. GDP-beta-S, which inhibits GDP/GTP exchange by heterotrimeric G proteins, completely prevented GA1 induction of alpha-Amy2/54:GUS expression, whereas GTP-gamma-S, which activates heterotrimeric G proteins, stimulated expression very slightly. Novel cDNA sequences from Galpha and Gbeta subunits were cloned from wild oat aleurone cells. By using RNA gel blot analysis, we found that the transcripts were expressed at a low level. Heterotrimeric G proteins have been implicated in several events during plant growth and development, and these data suggest that they may be involved in GA regulation of alpha-amylase gene expression in aleurone.

Actin Filaments Modulate Both Stomatal Opening and Inward K+-Channel Activities in Guard Cells of Vicia faba L

Actin antagonists have previously been shown to alter responses of Commelina communis stomata to physiological stimuli, implicating actin filaments in the control of guard cell volume changes (M. Kim, P.K. Hepler, S.-O. Eun, K.S. Ha, Y. Lee [1995] Plant Physiol 109: 1077-1084). Since K+ channels in the guard cell play an important role in stomatal movements, we examined the possible regulation of K+-channel activities by the state of actin polymerization. Agents affecting actin polymerization altered light-induced stomatal opening and inward K+-channel activities measured by patch clamping in Vicia faba. Cytochalasin D, which induces depolymerization of actin filaments, promoted light-induced stomatal opening and potentiated the inward K+ current in guard cell protoplasts. Phalloidin, a stabilizer of filamentous actin, inhibited both light-induced stomatal opening and inward K+ current. Inward K+-channel activities in outside-out membrane patches showed responses to these agents that support results at the whole-cell current level, suggesting that cytochalasin D facilitates and phalloidin inhibits K+ influx in intact guard cells, thus resulting in enhancement and inhibition of stomatal opening, respectively. To our knowledge, this is the first report that provides evidence that actin filaments may regulate an important physiological process by modulating the activities of ion channels in plant cells.

Biochemical characteristics of a rice (Oryza sativa L., IR36) G-protein alpha-subunit expressed in Escherichia coli

A cDNA encoding the alpha-subunit of the heterotrimeric G-protein in rice (RGA1) was overexpressed in Escherichia coli and then isolated by Ni2+-nitrilotriacetic acid affinity chromatography. The molecular mass of RGA1 bearing a His tag was approx. 49 kDa. Immunoblot analysis using anti-RGA1 revealed that the RGA1 protein is most abundant in seedling leaves and least abundant in mature roots. It exists at particularly high levels in the immature embryo after pellicle extrusion. In addition, the RGA1 antiserum exhibited a difference in binding affinity for Galpha proteins from monocots (maize and rice) and dicots (Arabidopsis, pea, soya bean and tomato); whereas it cross-reacted with Galpha proteins of monocots, it did not with those of dicot plants. When bound to guanosine 5'-(gamma-thio)triphosphate (GTP[S]), the RGA1 protein was partially protected from tryptic proteolysis. In the presence of GTP[S], trypsin cleaved the RGA1 protein into four fragments 24, 14, 11 and 5 kDa in size. When RGA1 was bound to GDP, only the 5 kDa polypeptide was seen on SDS/PAGE after trypsin digestion. Photoaffinity labelling with [alpha-32P]GTP and a GTP[S]-binding assay revealed that RGA1 incorporated 32P and showed specific binding to a guanine nucleotide. Guanidine binding of RGA1 was affected by the concentration of MgCl2 (maximum at 2 mM). The rate of guanine nucleotide binding of RGA1 (kon,GTP[S]=0.0141+/-0.0014 min-1) and, at steady state, the kcat value for GTP hydrolysis (0.0075+/-0.0001 min-1) were very low even at 2 mM MgCl2. The binding affinity for the nucleotides examined was in the order GTP-S- >/= GTP > GDP > CTP > ATP >/= dTTP.

Expression of human muscarinic cholinergic receptors in tobacco

We expressed human m1, m2 and chimeric muscarinic cholinergic receptors (MAChR) in tobacco plants and in cultured BY2 tobacco cells using Agrobacterium-mediated transformation. The membranes of most transgenic plants and calli bound muscarinic ligands with appropriate affinities, kinetics and pharmacologic specificity, as determined by direct and competitive binding measurements using the muscarinic ligand [3H]quinuclidinyl benzylate (QNB). Membranes of untransformed plants and calli or those transformed with vector alone did not bind [3H]QNB. Preliminary experiments did not suggest regulation of endogenous plant G protein signalling pathways by the recombinant receptors. Membranes from one callus clone expressed m1 MAChR at the level of 2.0-2.5 pmol [3H]QNB bound per mg membrane protein, more than the number of m1 MAChR in mammalian brain and comparable to that expressed in Sf9 insect cells using baculovirus vectors. This work demonstrates high level expression of active G protein-coupled receptors in plants, such that signaling might be genetically reconstituted by co-expression of appropriate G proteins and effectors.

Cloned potassium channels from eukaryotes and prokaryotes

Potassium channels contribute to the excitability of neurons and signaling in the nervous system. They arise from multiple gene families including one for voltage-gated potassium channels and one for inwardly rectifying potassium channels. Features of potassium permeation, channel gating and regulation, and subunit interaction have been analyzed. Potassium channels of similar design have been found in animals ranging from jellyfish to humans, as well as in plants, yeast, and bacteria. Structural similarities are evident for the pore-forming alpha subunits and for the beta subunits, which could potentially regulate channel activity according to the level of energy and/or reducing power of the cell.

Inhibition of an inward rectifier potassium channel (Kir2.3) by G-protein betagamma subunits

The molecular basis of G-protein inhibition of inward rectifier K+ currents was examined by co-expression of G-proteins and cloned Kir2 channel subunits in Xenopus oocytes. Channels encoded by Kir2.3 (HRK1/HIR/BIRK2/BIR11) were completely suppressed by co-expression with G-protein betagamma subunits, whereas channels encoded by Kir2. 1 (IRK1), which shares 60% amino acid identity with Kir2.3, were unaffected. Co-expression of Galphai1 and Galphaq subunits also partially suppressed Kir2.3 currents, but Galphat, Galphas, and a constitutively active mutant of Galphail (Q204L) were ineffective. Gbetagamma and Kir2.3 subunits were co-immunoprecipitated using an anti-Kir2.3 antibody. Direct binding of G-protein betagamma subunits to fusion proteins containing Kir2.3 N terminus, but not to fusion proteins containing Kir2.1 N terminus, was also demonstrated. The results are consistent with suppression of Kir2.3 currents resulting from a direct protein-protein interaction between the channel and G-protein betagamma subunits. When Kir2.1 and Kir2.3 subunits were coexpressed, the G-protein inhibitory phenotype of Kir2.3 was dominant, suggesting that co-expression of Kir2.3 with other Kir subunits might give rise to novel G-protein-inhibitable inward rectifier currents.

G proteins regulate dihydropyridine binding to moss plasma membranes

The role of calcium as an activator and regulator of many biological processes is linked to the ability of the cell to rapidly change its cytoplasmic calcium levels. Calcium acts as an intracellular messenger in hormone-induced bud formation during the development of the moss Physcomitrella patens. Calcium transport and ligand binding studies have implicated plasma membrane-localized 1, 4-dihydropyridine (DHP)-sensitive calcium channels in this increase in cellular calcium. To understand the regulation of the moss calcium channel, we investigated the involvement of GTP binding regulatory proteins (G proteins). Guanosine 5'-(gamma-thio)triphosphate (GTPgammaS), a nonhydrolyzable GTP analog that locks G proteins into their active state, stimulated DHP binding to high affinity receptors in the moss plasma membrane. DHP binding was measured as the ability of the DHP agonist Bay K8644 or the DHP antagonist nifedipine to compete with the DHP arylazide [3H]azidopine for binding to moss plasma membranes. G protein stimulation of binding was seen when competition was carried out with either nifedipine or Bay K8644. G proteins regulated the rates of association and dissociation of bound [3H]azidopine, and stimulation was dependent on GTPgammaS concentration. Guanosine 5'-(beta-thio)diphosphate, a GDP analog that locks G proteins into their inactivated state, did not affect the dose dependence of either the agonist or the antagonist. These results suggest that G proteins may act via a membrane-delimited pathway to regulate calcium channels in the moss plasma membrane.

Laser microsurgery of higher plant cell walls permits patch-clamp access

Plasma membranes of guard cells in epidermal peels of Vicia faba and Commelina communis can be made accessible to a patch-clamp pipet by removing a small portion (1-3 micrometers in diameter) of the guard cell wall using a microbeam of ultraviolet light generated by a nitrogen laser. Using this laser microsurgical technique, we have measured channel activity across plasma membranes of V. faba guard cells in both cell-attached and isolated patch configurations. Measurements made in the inside-out patch configuration revealed two distinct K(+)-selective channels. Major advantages of the laser microsurgical technique include the avoidance of enzymatic protoplast isolation, the ability to study cell types that have been difficult to isolate as protoplasts or for which enzymatic isolation protocols result in protoplasts not amenable to patch-clamp studies, the maintenance of positional information in single-channel measurements, reduced disruption of cell-wall-mediated signaling pathways, and the ability to investigate intercellular signaling through studies of cells remaining situated within tissue.

Modulation of the serotonin-activated K+ channel by G protein subunits and nucleotides in rat hippocampal neurons

In hippocampal neurons, 5-hydroxytryptamine (5-HT) activates an inwardly rectifying K+ current via G protein. We identified the K+ channel activated by 5-HT (K5-HT channel) and studied the effects of G protein subunits and nucleotides on the K+ channel kinetics in adult rat hippocampal neurons. In inside-out patches with 10 microM 5-HT in the pipette, application of GTP (100 microM) to the cytoplasmic side of the membrane activated an inwardly rectifying K+ channel with a slope conductance of 36 +/- 1 pS (symmetrical 140 mM K+) at -60 mV and a mean open time of 1.1 +/- 0.1 msec (n = 5). Transducin beta gamma activated the K5-HT channels and this was reversed by alpha-GDP. Whether the K5-HT channel was activated endogenously (GTP, GTP gamma S) or exogenously (beta gamma), the presence of 1 mM ATP resulted in a approximately 4-fold increase in channel activity due in large part to the prolongation of the open time duration. These effects of ATP were irreversible and not mimicked by AMPPMP, suggesting that phosphorylation might be involved. However, inhibitors of protein kinases A and C (H-7, staurosporine) and tyrosine kinase (tyrphostin 25) failed to block the effect of ATP. These results show that G beta gamma activates the G protein-gated K+ channel in hippocampal neurons, and that ATP modifies the gating kinetics of the channel, resulting in increased open probability via as yet unknown pathways.

The Role of Potassium Channels in the Temperature Control of Stomatal Aperture

We used the patch-clamp technique to examine the effect of temperature (13-36[deg]C) on the depolarization-activated K channels (KD channels) and on the hyperpolarization-activated channels (KH channels) in the plasma membrane of Vicia faba guard-cell protoplasts. The steady-state whole-cell conductance of both K channel types increased with temperature up to 20[deg]C. However, whereas the whole-cell conductance of the KH channels increased further and saturated at 28[deg]C, that of KD channels decreased at higher temperatures. The unitary conductance of both channel types increased with temperature like the rate of diffusion in water (temperature quotient of approximately 1.5), constituting the major contribution to the conductance increase in the whole cells. The mean number of available KH channels was not affected significantly by temperature, but the mean number of available KD channels increased significantly between 13 and 20[deg]C and declined drastically above 20[deg]C. This decrease and the reduced steady-state voltage-dependent probability of opening of the KD channels above 28[deg]C (because of a shift of voltage dependence by +21 mV) account for the depression of the whole-cell KD conductance at the higher temperatures. This may be a basic mechanism by which leaves of well-watered plants keep their stomata open during heat stress to promote cooling by transpiration.

Abscisic acid signaling

The plant hormone abscisic acid (ABA) regulates the development and germination of seeds, as well as the adaptation of vegetative tissues to conditions of environmental stress. During the past year, considerable insights have been gained into the molecular nature of the complex signaling network that mediates the actions of ABA. Biophysical studies indicate that at least some of the effects of ABA in stomatal guard cells involve intracellular receptors. Also, increasing evidence supports the view that guard cells contain redundant ABA transduction pathways, and that cytoplasmic Ca2+ acts as a second messenger in at least one of these pathways. Finally, mutational analysis in Arabidopsis indicates that the multiple effects of ABA at the whole plant level are mediated by overlapping branches of a highly ramified signaling network. Two Arabidopsis loci that determine ABA sensitivity have already been cloned and found to encode a protein phosphatase and a transcriptional activator.