Predicting essential components of signal transduction networks: a dynamic model of guard cell abscisic acid signaling

Plants both lose water and take in carbon dioxide through microscopic stomatal pores, each of which is regulated by a surrounding pair of guard cells. During drought, the plant hormone abscisic acid (ABA) inhibits stomatal opening and promotes stomatal closure, thereby promoting water conservation. Dozens of cellular components have been identified to function in ABA regulation of guard cell volume and thus of stomatal aperture, but a dynamic description is still not available for this complex process. Here we synthesize experimental results into a consistent guard cell signal transduction network for ABA-induced stomatal closure, and develop a dynamic model of this process. Our model captures the regulation of more than 40 identified network components, and accords well with previous experimental results at both the pathway and whole-cell physiological level. By simulating gene disruptions and pharmacological interventions we find that the network is robust against a significant fraction of possible perturbations. Our analysis reveals the novel predictions that the disruption of membrane depolarizability, anion efflux, actin cytoskeleton reorganization, cytosolic pH increase, the phosphatidic acid pathway, or K(+) efflux through slowly activating K(+) channels at the plasma membrane lead to the strongest reduction in ABA responsiveness. Initial experimental analysis assessing ABA-induced stomatal closure in the presence of cytosolic pH clamp imposed by the weak acid butyrate is consistent with model prediction. Simulations of stomatal response as derived from our model provide an efficient tool for the identification of candidate manipulations that have the best chance of conferring increased drought stress tolerance and for the prioritization of future wet bench analyses. Our method can be readily applied to other biological signaling networks to identify key regulatory components in systems where quantitative information is limited.

Light regulation of stomatal movement

Stomatal pores, each surrounded by a pair of guard cells, regulate CO2 uptake and water loss from leaves. Stomatal opening is driven by the accumulation of K+ salts and sugars in guard cells, which is mediated by electrogenic proton pumps in the plasma membrane and/or metabolic activity. Opening responses are achieved by coordination of light signaling, light-energy conversion, membrane ion transport, and metabolic activity in guard cells. In this review, we focus on recent progress in blue- and red-light-dependent stomatal opening. Because the blue-light response of stomata appears to be strongly affected by red light, we discuss underlying mechanisms in the interaction between blue-light signaling and guard cell chloroplasts.

G-protein complex mutants are hypersensitive to abscisic acid regulation of germination and postgermination development

Abscisic acid (ABA) plays regulatory roles in a host of physiological processes throughout plant growth and development. Seed germination, early seedling development, stomatal guard cell functions, and acclimation to adverse environmental conditions are key processes regulated by ABA. Recent evidence suggests that signaling processes in both seeds and guard cells involve heterotrimeric G proteins. To assess new roles for the Arabidopsis (Arabidopsis thaliana) Galpha subunit (GPA1), the Gbeta subunit (AGB1), and the candidate G-protein-coupled receptor (GCR1) in ABA signaling during germination and early seedling development, we utilized knockout mutants lacking one or more of these components. Our data show that GPA1, AGB1, and GCR1 each negatively regulates ABA signaling in seed germination and early seedling development. Plants lacking AGB1 have greater ABA hypersensitivity than plants lacking GPA1, suggesting that AGB1 is the predominant regulator of ABA signaling and that GPA1 affects the efficacy of AGB1 execution. GCR1 acts upstream of GPA1 and AGB1 for ABA signaling pathways during germination and early seedling development: gcr1 gpa1 double mutants exhibit a gpa1 phenotype and agb1 gcr1 and agb1 gcr1 gpa1 mutants exhibit an agb1 phenotype. Contrary to the scenario in guard cells, where GCR1 and GPA1 have opposite effects on ABA signaling during stomatal opening, GCR1 acts in concert with GPA1 and AGB1 in ABA signaling during germination and early seedling development. Thus, cell- and tissue-specific functional interaction in response to a given signal such as ABA may determine the distinct pathways regulated by the individual members of the G-protein complex.

Brassinosteroids and plant function: some clues, more puzzles

The role of brassinosteroids (BRs) in plant function has been intensively studied in the last few years. Mutant analysis has demonstrated that the ability to synthesize, perceive and respond to BRs is essential to normal plant growth and development. Several key elements of BR response have been identified using both genetic and biochemical approaches, and molecular models that parallel Wingless (Wnt), transforming growth factor beta (TGF beta) and receptor tyrosine kinase (RTK) signalling in animals have been proposed. Many studies have demonstrated the role of BRs, alone and in interaction with other plant hormones, in processes such as cell elongation and seed germination. In contrast, little is known about how the sensing of BRs is connected to specific physiological responses such as stress resistance. There remain many open questions about how these connections are made.

G protein regulation of disease resistance during infection of rice with rice blast fungus

When the effects of specific pathogen-produced elicitor compounds on plant cells are recognized by specific plant resistance (R) gene products, a local defense response called the hypersensitive response is triggered. Production of reactive oxygen species (ROS) and induction of defense genes are components of this response. In rice cultivars harboring null mutations in the G protein alpha subunit, RGA1, ROS production and defense gene induction by the rice blast fungus Magnaporthe grisea and its sphingolipid elicitors are reduced or delayed. These results implicate heterotrimeric guanine nucleotide-binding proteins (G proteins) as important players in plant pathogen response.

G protein signaling in the regulation of rice seed germination

Seed germination and early seedling growth are regulated by an intricate network of hormonal signaling pathways. The plant hormones gibberellic acid (GA) and abscisic acid (ABA) are particularly important positive and negative regulators of these processes, respectively. In the cereal rice, mutant analysis has implicated heterotrimeric guanine nucleotide-binding protein (G protein) signaling in GA-stimulated expression of several genes, including expression of the gene encoding the hydrolytic enzyme alpha-amylase, which breaks down carbohydrate reserves and thus mobilizes nutrients that nourish the young seedling.

G proteins Go green: a plant G protein signaling FAQ sheet

Plants, like animals, use signal transduction pathways based on heterotrimeric guanine nucleotide-binding proteins (G proteins) to regulate many aspects of development and cell signaling. Some components of G protein signaling are highly conserved between plants and animals and some are not. This Viewpoint compares key aspects of G protein signal transduction in plants and animals and describes the current knowledge of this system in plants, the questions that still await exploration, and the value of research on plant G proteins to scientists who do not study plants. Pathways in Science's Signal Transduction Knowledge Environment Connections Maps database provide details about the emerging roles of G proteins in several cellular processes of plants.

G protein signaling in the regulation of Arabidopsis seed germination

Seed germination and early seedling growth are regulated by an intricate network of hormonal signaling pathways. The plant hormones gibberellic acid (GA) and brassinosteroids (BRs) are important positive regulators of these processes, whereas the hormone abscisic acid (ABA) is a potent negative regulator. In Arabidopsis, analysis of mutant plants has implicated heterotrimeric guanine nucleotide-binding protein (G protein) components in regulation of seed germination by all three of these hormones.

Arabidopsis sphingosine kinase and the effects of phytosphingosine-1-phosphate on stomatal aperture

Sphingolipids are a major component of membrane lipids and their metabolite sphingosine-1-phosphate (S1P) is a potent lipid mediator in animal cells. Recently, we have shown that the enzyme responsible for S1P production, sphingosine kinase (SphK), is stimulated by the phytohormone abscisic acid in guard cells of Arabidopsis (Arabidopsis thaliana) and that S1P is effective in regulating guard cell turgor. We have now characterized SphK from Arabidopsis leaves. SphK activity was mainly associated with the membrane fraction and phosphorylated predominantly the Delta4-unsaturated long-chain sphingoid bases sphingosine (Sph) and 4,8-sphingadienine, and to a lesser extent, the saturated long-chain sphingoid bases dihydrosphingosine and phytosphingosine (Phyto-Sph). 4-Hydroxy-8-sphingenine, which is a major sphingoid base in complex glycosphingolipids from Arabidopsis leaves, was a relatively poor substrate compared with the corresponding saturated Phyto-Sph. In contrast, mammalian SphK1 efficiently phosphorylated Sph, dihydrosphingosine, and 4,8-sphingadienine, but not the 4-hydroxylated long-chain bases Phyto-Sph and 4-hydroxy-8-sphingenine. Surface dilution kinetic analysis of Arabidopsis SphK with Sph presented in mixed Triton X-100 micelles indicated that SphK associates with the micellar surface and then with the substrate presented on the surface. In addition, measurements of SphK activity under different assay conditions combined with phylogenetic analysis suggest that multiple isoforms of SphK may be expressed in Arabidopsis. Importantly, we found that phytosphingosine-1-phosphate, similar to S1P, regulates stomatal apertures and that its action is impaired in guard cells of Arabidopsis plants harboring T-DNA null mutations in the sole prototypical G-protein alpha-subunit gene, GPA1.

Plants: the latest model system for G-protein research

In humans, heterotrimeric G proteins couple stimulus perception by G-protein-coupled receptors (GPCRs) with numerous downstream effectors. By contrast, despite great complexity in their signal-transduction attributes, plants have a simpler repertoire of G-signalling components. Nonetheless, recent studies on Arabidopsis thaliana have shown the importance of plant G-protein signalling in such fundamental processes as cell proliferation, hormone perception and ion-channel regulation.

Plant heterotrimeric G protein function: insights from Arabidopsis and rice mutants

Heterotrimeric G proteins have been implicated in a wide range of plant processes. These include responses to hormones, drought, and pathogens, and developmental events such as lateral root formation, hypocotyl elongation, hook opening, leaf expansion, and silique development. Results and concepts emerging from recent phenotypic analyses of G-protein component mutants in Arabidopsis and rice are adding to our understanding of G-protein mechanisms and functions in higher plants.

Guard cells: a dynamic signaling model

The year 2003 has provided a continuing accretion of knowledge concerning the diverse ways in which guard cells sense and respond to abscisic acid. A deeper understanding of the biochemical mechanisms governing the response of guard cells to blue light has been gained, and new insights have been garnered regarding roles of the extracellular matrix in stomatal regulation.

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.

GCR1 can act independently of heterotrimeric G-protein in response to brassinosteroids and gibberellins in Arabidopsis seed germination

Signal recognition by seven-transmembrane (7TM) cell-surface receptors is typically coupled by heterotrimeric G-proteins to downstream effectors in metazoan, fungal, and amoeboid cells. Some responses perceived by 7TM receptors in amoeboid cells and possibly in human cells can initiate downstream action independently of heterotrimeric G-proteins. Plants use heterotrimeric G-protein signaling in the regulation of growth and development, particularly in hormonal control of seed germination, but it is not yet clear which of these responses utilize a 7TM receptor. Arabidopsis GCR1 has a predicted 7TM-spanning domain and other features characteristic of 7TM receptors. Loss-of-function gcr1 mutants indicate that GCR1 plays a positive role in gibberellin- (GA) and brassinosteroid- (BR) regulated seed germination. The null mutants of GCR1 are less sensitive to GA and BR in seed germination. This phenotype is similar to that previously observed for transcript null mutants in the Galpha-subunit, gpa1. However, the reduced sensitivities toward GA and BR in the single gcr1, gpa1, and agb1 (heterotrimeric G-protein beta-subunit) mutants are additive or synergistic in the double and triple mutants. Thus, GCR1, unlike a typical 7TM receptor, apparently acts independently of the heterotrimeric G-protein in at least some aspects of seed germination, suggesting that this alternative mode of 7TM receptor action also functions in the plant kingdom.

Sphingolipid signalling in Arabidopsis guard cells involves heterotrimeric G proteins

In animals, the sphingolipid metabolite sphingosine-1-phosphate (S1P) functions as both an intracellular messenger and an extracellular ligand for G-protein-coupled receptors of the S1P receptor family, regulating diverse biological processes ranging from cell proliferation to apoptosis. Recently, it was discovered in plants that S1P is a signalling molecule involved in abscisic acid (ABA) regulation of guard cell turgor. Here we report that the enzyme responsible for S1P production, sphingosine kinase (SphK), is activated by ABA in Arabidopsis thaliana, and is involved in both ABA inhibition of stomatal opening and promotion of stomatal closure. Consistent with this observation, inhibition of SphK attenuates ABA regulation of guard cell inward K(+) channels and slow anion channels, which are involved in the regulation of stomatal pore size. Surprisingly, S1P regulates stomatal apertures and guard cell ion channel activities in wild-type plants, but not in knockout lines of the sole prototypical heterotrimeric G-protein alpha-subunit gene, GPA1 (refs 5, 6, 7-8). Our results implicate heterotrimeric G proteins as downstream elements in the S1P signalling pathway that mediates ABA regulation of stomatal function, and suggest that the interplay between S1P and heterotrimeric G proteins represents an evolutionarily conserved signalling mechanism.

OPEN STOMATA1 opens the door to ABA signaling in Arabidopsis guard cells

The plant hormone abscisic acid plays a crucial role in plant responses to drought, salinity and cold. A recent report shows that mutations in the OST1 gene, encoding a serine-threonine protein kinase, render Arabidopsis thaliana guard cells insensitive to abscisic acid, such that stomata remain open in the presence of this phytohormone.

Modulation of an RNA-binding protein by abscisic-acid-activated protein kinase

Protein kinases are involved in stress signalling in both plant and animal systems. The hormone abscisic acid mediates the responses of plants to stresses such as drought, salinity and cold. Abscisic-acid-activated protein kinase (AAPK -- found in guard cells, which control stomatal pores -- has been shown to regulate plasma membrane ion channels. Here we show that AAPK-interacting protein 1 (AKIP1), with sequence homology to heterogeneous nuclear RNA-binding protein A/B, is a substrate of AAPK. AAPK-dependent phosphorylation is required for the interaction of AKIP1 with messenger RNA that encodes dehydrin, a protein implicated in cell protection under stress conditions. AAPK and AKIP1 are present in the guard-cell nucleus, and in vivo treatment of such cells with abscisic acid enhances the partitioning of AKIP1 into subnuclear foci which are reminiscent of nuclear speckles. These results show that phosphorylation-regulated RNA target discrimination by heterogeneous nuclear RNA-binding proteins may be a general phenomenon in eukaryotes, and implicate a plant hormone in the regulation of protein dynamics during rapid subnuclear reorganization.

Preparation and applications of Arabidopsis thaliana guard cell protoplasts

•  Guard cells play an important role in the physiology and development of plants. The genetic resources available for Arabidopsis thaliana make it the most favorable plant species for the study of guard cell processes, but it is not easy to isolate highly purified preparations of large numbers of guard cells from this species. Here, we describe methods for isolation of both guard cell and mesophyll cell protoplasts from A. thaliana and their use in the study of unique biochemical and cellular properties of these cell types. •  Protocols developed for large- and small-scale preparation of guard cell protoplasts and mesophyll cell protoplasts are described, followed by specific examples of their use in electrophysiological, biochemical and molecular approaches such as patch clamping, enzyme assays, and reverse-transcription polymerase chain reaction. •  The protocols described yield millions of highly purified, viable guard cell protoplasts and mesophyll cell protoplasts from A. thaliana. These protoplasts have been used successfully in the study of ion channel properties, assay of ABA activation in phospholipase D activity and comparisons of gene and protein expression levels. •  These techniques make it possible to elucidate electrophysiological, biochemical and molecular genetic pathways of guard cell function.

From milliseconds to millions of years: guard cells and environmental responses

During the past year, significant advances have been made in our understanding of stomatal development and its response to climate change, and in our knowledge of how guard cell Ca(2+) oscillations encode environmental signals. Recent studies on (de)phosphorylation mechanisms have provided new information on how guard cells respond to abscisic acid and blue light.

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.

Apparent absence of a redox requirement for blue light activation of pump current in broad bean guard cells

In guard cells, membrane hyperpolarization in response to a blue light (BL) stimulus is achieved by the activation of a plasma membrane H(+)-ATPase. Using the patch clamp technique on broad bean (Vicia faba) guard cells we demonstrate that both steady-state- and BL-induced pump currents require ATP and are blocked by vanadate perfused into the guard cell during patch clamp recording. Background-pump current and BL-activated currents are voltage independent over a wide range of membrane potentials. During BL-activated responses significant hyperpolarization is achieved that is sufficient to promote K(+) uptake. BL activation of pump current becomes desensitized by three or four pulses of 30 s x 100 micromol m(-2) s(-1) BL. This desensitization is not a result of pump inhibition as maximal responses to fusicoccin are observed after full BL desensitization. BL treatments prior to whole cell recording show that BL desensitization is not due to washout of a secondary messenger by whole cell perfusion, but appears to be an important feature of the BL-stimulated pump response. We found no evidence for an electrogenic BL-stimulated redox chain in the plasma membrane of guard cells as no steady-state- or BL-activated currents are detected with NADH or NADPH added to the cytosol in the absence of ATP. Steady-state- nor BL-activated currents are affected by the inclusion along with ATP of 1 mM NADH in the pipette under saturating red light or by including NADPH in the pipette under darkness or saturating red light. These data suggest that reduced products of photosynthesis do not significantly modulate plasma membrane pump currents and are unlikely to be critical regulators in BL-stimulation of the plasma membrane H(+)-ATPase in guard cells.

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.

Increases in cytosolic Ca2+ are not required for abscisic acid-inhibition of inward K+ currents in guard cells of Vicia faba L

The inward K+ channels (IKin) of guard cells are inhibited upon application of abscisic acid (ABA). It has been postulated that I(Kin) inhibition requires an elevation in cytosolic free Ca2+ levels ([Ca2+]c) because: (i) experimental increases in [Ca2+]c can mimic the ABA effect, and; (ii) ABA can trigger an elevation of [Ca2+]c in guard cells. However, not all guard cells respond to ABA with a [Ca2+]c increase, and the magnitude of the increases that do occur is variable. Therefore, an obligate role for Ca2+ in the regulation of downstream effectors of ABA response, such as the I(Kin) channels, remains in question. In this study, we developed a methodology for simultaneous patch clamping and confocal ratiometric Ca2+ imaging of Vicia faba L. guard-cell protoplasts. This allowed us to directly assess the relationship between ABA-induced changes in [Ca2+]c and I(Kin) inhibition. In the presence of extracellular Ca2+, the extent of [Ca2+]c elevation correlated with the extent of I(Kin) inhibition. However, upon chelation of either extracellular Ca2+, [Ca2+]c or both, extracellular Ca2+ and [Ca2+]c, [Ca2+]c elevation did not occur in response to ABA yet I(Kin) currents were still strongly inhibited. These data illustrate that Ca2+-independent regulation is involved in ABA-inhibition of stomatal opening processes.

Kinetic analysis of the K(+)-selective outward rectifier in Arabidopsis mesophyll cells: a comparison with other plant species

This paper gives a kinetic analysis of the K(+)-selective outward-rectifier (IK,out) in the plasma membrane of Arabidopsis thaliana mesophyll cells in terms of the Hodgkin-Huxley formalism. We compared the kinetic characteristics of IK,out in Arabidopsis with IK,out channels in three other plant species that were subjected to a similar analysis: tobacco suspension cells, Vicia faba guard cells and Plantago media root cells. Because the activation kinetics of IK,out shows a clear voltage dependence, the time constant of half-activation (tau 1/2) and the elementary rate constant of channel opening (a) were calculated at the potential of half-activation (V1/2). The Arabidopsis IK,out activates relatively slowly and this is reflected in a tau 1/2 of approximately 1 s. The reason for this slow activation is twofold. Firstly, the value of a is 1.5 s-1 falls at the lower end of the range of values obtained for tobacco, Vicia and Plantago: 1.1 to 3.0 s-1. Secondly, IK,out in Arabidopsis has four closed states, while tobacco and Vicia have only two. As observed in other plant species, the activation kinetics of IK,out in Arabidopsis are sensitive to external K+: V1/2 shifts with EK but remains approximately 50 mV more positive than EK.

Regulation of abscisic acid-induced stomatal closure and anion channels by guard cell AAPK kinase

Abscisic acid (ABA) stimulates stomatal closure and thus supports water conservation by plants during drought. Mass spectrometry-generated peptide sequence information was used to clone a Vicia faba complementary DNA, AAPK, encoding a guard cell-specific ABA-activated serine-threonine protein kinase (AAPK). Expression in transformed guard cells of AAPK altered by one amino acid (lysine 43 to alanine 43) renders stomata insensitive to ABA-induced closure by eliminating ABA activation of plasma membrane anion channels. This information should allow cell-specific, targeted biotechnological manipulation of crop water status.

Regulation of abscisic acid-induced stomatal closure and anion channels by guard cell AAPK kinase

Abscisic acid (ABA) stimulates stomatal closure and thus supports water conservation by plants during drought. Mass spectrometry-generated peptide sequence information was used to clone a Vicia faba complementary DNA, AAPK, encoding a guard cell-specific ABA-activated serine-threonine protein kinase (AAPK). Expression in transformed guard cells of AAPK altered by one amino acid (lysine 43 to alanine 43) renders stomata insensitive to ABA-induced closure by eliminating ABA activation of plasma membrane anion channels. This information should allow cell-specific, targeted biotechnological manipulation of crop water status.

Ozone inhibits guard cell K+ channels implicated in stomatal opening

Ozone (O3) deleteriously affects organisms ranging from humans to crop plants, yet little is understood regarding the underlying mechanisms. In plants, O3 decreases CO2 assimilation, but whether this could result from direct O3 action on guard cells remained unknown. Potassium flux causes osmotically driven changes in guard cell volume that regulate apertures of associated microscopic pores through which CO2 is supplied to the photosynthetic mesophyll tissue. We show in Vicia faba that O3 inhibits (i) guard cell K+ channels that mediate K+ uptake that drives stomatal opening; (ii) stomatal opening in isolated epidermes; and (iii) stomatal opening in leaves, such that CO2 assimilation is reduced without direct effects of O3 on photosynthetic capacity. Direct O3 effects on guard cells may have ecological and agronomic implications for plant productivity and for response to other environmental stressors including drought.

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 thaliana 'extra-large GTP-binding protein' (AtXLG1): a new class of G-protein

Heterotrimeric GTP-binding proteins, composed of alpha, beta, and gamma subunits, are involved in signal transduction pathways in animal and plant systems. In plants, physiological analyses implicate heterotrimeric G-proteins in ion channel regulation, light signaling, and hormone and pathogen responses. However, only one class of plant G alpha genes has been identified to date. We have cloned a novel gene, 'Arabidopsis thaliana extra-large GTP-binding protein' (AtXLG1). AtXLG1 appears to be a member of a small gene family and is transcribed in all tissues assayed: roots, leaves, stems, flowers, and fruits. The conceptually translated protein from AtXLG1 is 99 kDa, twice as large as typical G alpha proteins. The carboxy-terminal half of the AtXLG1 protein has significant homology to animal and plant G alpha proteins. This region includes a GTP-binding domain, a predicted helical domain, and an aspartate/glutamate-rich loop, which are characteristics of G alpha's. Despite the absence of some of the amino acids implicated in GTP binding and hydrolysis by crystallographic and mutational analyses of mammalian G alpha's, recombinant AtXLG1 binds GTP with specificity. The amino-terminal region of AtXLG1 contains domains homologous to the bacterial TonB-box, which is involved in energy transduction between the inner and outer bacterial membranes, and to zinc-finger proteins. Given the unique structure of AtXLG1, it will be of interest to uncover its physiological functions.

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.

The calculation of intracellular ion concentrations and membrane potential from cell-attached and excised patch measurements. Cytosolic K+ concentration and membrane potential in Vicia faba guard cells

Ion channel activity in cell-attached patch recordings shows channel behavior under more physiological conditions than whole-cell and excised patch measurements. Yet the analysis of cell-attached patch measurements is complicated by the fact that the system is ill defined with respect to the intracellular ion activities and the electrical potential actually experienced by the membrane patch. Therefore, of the several patch-clamp configurations, the information that is obtained from cell-attached patch measurements is the most ambiguous. The present study aims to achieve a better understanding of cell-attached patch measurements. Here we describe a method to calculate the intracellular ion concentration and membrane potential prevailing during cell-attached patch recording. The first step is an analysis of the importance of the input resistance of the intact cell on the cell-attached patch measurement. The second step, and actual calculation, is based on comparison of the single channel conductance and reversal potential in the cell-attached patch and excised patch configurations. The method is demonstrated with measurements of membrane potential and cytosolic K+ concentrations in Vicia faba guard cells. The approach described here provides an attractive alternative to the measurement of cytosolic ion concentrations with fluorescent probes or microelectrodes.

Ca(2+)-permeable, outwardly-rectifying K+ channels in mesophyll cells of Arabidopsis thaliana

Arabidopsis thaliana has become a powerful tool in genetics and molecular biology. In order to use Arabidopsis as a model system for electrophysiological studies on plant cells, a detailed characterization of the transporters present in the plasma and vacuolar membranes of this species is required. We used the patch-clamp technique to study ion channels in the plasma membrane of Arabidopsis mesophyll cells. The most prominent conductance in these cells was a K(+)-selective, voltage-dependent, outwardly-rectifying channel (IK,out). In the whole-cell configuration, IK,out was observed in 100% of the cells assayed. In contrast, inward current was observed in less than 50% of the cells which were bathed in 100 mM K+, and was totally absent from cells bathed in 10 mM K+. The activation kinetics of IK,out were modulated by the external K+ concentration with a faster activation at low external K+. Tail-current analysis revealed that in addition to K+, IK,out is also permeable to Ca2+ and Ba2+. Externally applied Ba2+ also caused a voltage-dependent decrease in current magnitude, indicating that IK,out is also partially blocked by this classic K+ channel blocker. Single channels studied in outside-out patches showed Ca2+ and Ba2+ sensitivity, voltage dependence and time activation similar to that of IK,out in the whole-cell configuration. Given their permeability to Ca2+, these channels may function as an avenue for Ca2+ influx as well as K+ efflux, both of which may affect photosynthesis.

Guard cells possess a calcium-dependent protein kinase that phosphorylates the KAT1 potassium channel

Increasing evidence suggests that changes in cytosolic Ca2+ levels and phosphorylation play important roles in the regulation of stomatal aperture and as ion transporters of guard cells. However, protein kinases responsible for Ca2+ signaling in guard cells remain to be identified. Using biochemical approaches, we have identified a Ca(2+)-dependent protein kinase with a calmodulin-like domain (CDPK) in guard cell protoplasts of Vicia faba. Both autophosphorylation and catalytic activity of CDPK are Ca2+ dependent. CDPK exhibits a Ca(2+)-induced electrophoretic mobility shift and its Ca(2+)-dependent catalytic activity can be inhibited by the calmodulin antagonists trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide. Antibodies to soybean CDPK alpha cross-react with CDPK. Micromolar Ca2+ concentrations stimulate phosphorylation of several proteins from guard cells; cyclosporin A, a specific inhibitor of the Ca(2+)-dependent protein phosphatase calcineurin enhances the Ca(2+)-dependent phosphorylation of several soluble proteins. CDPK from guard cells phosphorylates the K+ channel KAT1 protein in a Ca(2+)-dependent manner. These results suggest that CDPK may be an important component of Ca2+ signaling in guard cells.

Seal-promoting solutions and pipette perfusion for patch clamping plant cells

Patch-clamp technology has greatly increased our knowledge of plant membrane transport. However, the success of patch clamping crucially relies on establishing a high resistance (G omega) seal between the membrane and the patch-clamp pipette. This can prove problematic in many plant-cell preparations. It is therefore of great importance to develop protocols for protoplast isolation, maintenance and seal formation that improve seal rate. This study investigated whether the pH and the K+ and the Cl(-)concentration of the pipette solution had an effect on the seal formation. High pH and absence of K+ significantly promoted membrane sealing, whereas the concentration of Cl- had no effect. To reap the benefit of seal-promoting pipette solutions and yet retain the option to adjust this solution to experimental requirements, a pipette perfusion apparatus was implemented. The perfusion system was successfully applied in cell-attached patch, excised-patch and whole-cell configurations, using plasma membrane and tonoplast of three different species. The system enables complete solution exchange within minutes and is potentially of great benefit in the study of channel selectivity, the application of (cytoplasmic) channel blockers and the study of primary and secondary transport.

Laser-assisted patch clamping: a methodology

Laser microsurgery can be used to perform both cell biological manipulations, such as targeted cell ablation, and molecular genetic manipulations, such as genetic transformation and chromosome dissection. In this report, we describe a laser microsurgical method that can be used either to ablate single cells or to ablate a small area (1-3 microns diameter) of the extracellular matrix. In plants and microorganisms, the extracellular matrix consists of the cell wall. While conventional patch clamping of these cells, as well as of many animal cells, requires enzymatic digestion of the extracellular matrix, we illustrate that laser microsurgery of a portion of the wall enables patch clamp access to the plasma membrane of higher plant cells remaining situated in their tissue environment. What follows is a detailed description of the construction and use of an economical laser microsurgery system, including procedures for single cell and targeted cell wall ablation. This methodology will be of interest to scientists wishing to perform cellular or subcellular ablation with a high degree of accuracy, or wishing to study how the extracellular matrix affects ion channel function.

A membrane-delimited effect of internal pH on the K+ outward rectifier of Vicia faba guard cells

ABA stimulation of outward K+ current (IK,out) in Vicia faba guard cells has been correlated with a rise in cytosolic pH (pHi). However, the underlying mechanism by which IK,out is affected by pHi has remained unknown. Here, we demonstrate that pHi regulates outward K+ current in isolated membrane patches from Vicia faba guard cells. The stimulatory effect of alkalinizing pHi was voltage insensitive and independent of the two free calcium levels tested, 50 nM and 1 microM. The single-channel conductance was only slightly affected by pHi. Based on single-channel measurements, the kinetics of time-activated whole-cell current, and the analysis of current noise in whole-cell recordings, we conclude that alkaline pHi enhances the magnitude of IK,out by increasing the number of channels available for activation. The fact that the pHi effect is seen in excised patches indicates that signal transduction pathways involved in the regulation of IK,out by pHi, and by implication, components of hormonal signal transduction pathways that are downstream of pHi, are membrane-delimited.

Transport proteins of the plant plasma membrane

Recently developed molecular and genetic approaches have enabled the identification and functional characterization of novel genes encoding ion channels, ion carriers, and water channels of the plant 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.

Cyclic AMP stimulates K+ channel activity in mesophyll cells of Vicia faba L

Whole-cell patch-clamp recordings from Vicia faba mesophyll protoplasts reveal that outward K+ current is increased in a dose-dependent fashion by intracellular application of cAMP. The enhancement of the outward current by cAMP is specific and it cannot be mimicked by a series of nucleotides that includes AMP, cGMP, and GMP. The enhancement is evoked by micromolar concentrations of cAMP in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine. PKI or Walsh inhibitor, a specific peptide inhibitor of cAMP-dependent protein kinase (PKA), inhibits the outward K+ current. Adenosine 3',5'-phosphothioate, a competitive inhibitor of PKA, has a similar effect. Conversely, the catalytic subunit of PKA (cAMP independent) from bovine brain enhances the magnitude of the outward K+ current in the absence of added cAMP. Our results indicate that cAMP modulates K+ channel activity in mesophyll cells and suggest that this modulation occurs through a cAMP-regulated protein kinase.

Evidence for protein phosphatase 1 and 2A regulation of K+ channels in two types of leaf cells

Ion channels control ion fluxes across membranes, membrane potential, and signal transduction between and within cells. Protein kinases and phosphatases are important regulators involved in stimulus-response coupling in eukaryotic organisms. We have identified in extracts of Vicia faba leaf cells protein phosphatase activities inhibited by okadaic acid (OA) and calyculin A (CA), two inhibitors of protein phosphatases 1 and 2A. Using whole-cell patch-clamp techniques, we have demonstrated that inward K+ currents in guard cells are inhibited by nanomolar concentrations of OA or CA, whereas outward K+ currents are not affected. However, the same inhibitors enhance the magnitude of outward K+ currents in mesophyll cells. A phosphatase antagonist, adenosine-5'-O-(3-thiotriphosphate), has an effect similar to OA and CA on outward K+ currents in mesophyll cells. Our findings suggest that protein phosphatases 1 and/or 2A play different physiological roles in modulating the activity of K+ channels in mesophyll cells and guard cells.

Analysis of a soluble calmodulin binding protein from fava bean roots: identification of glutamate decarboxylase as a calmodulin-activated enzyme

The identity of a soluble 62-kD Ca(2+)-dependent calmodulin binding protein (CaM-BP) from fava bean seedlings was determined. Using 125I-CaM overlay assays, a class of soluble CaM-BPs was detected in extracts of tissues comprising the axis of 1.5-week-old seedlings, excluding the root tip and emergent leaves. The size of these CaM-BPs was not uniform within all parts of the plant; the apparent molecular masses were 62 kD in roots, 60 kD in stems, and 64 kD in nodules. The root 62-kD CaM-BP was purified, and internal microsequence analysis was performed on the protein. A tryptic peptide derived from the CaM-BP consisted of a 13-residue sequence corresponding to a highly conserved region of glutamate decarboxylase (GAD), an enzyme that catalyzes the alpha-decarboxylation of glutamate to form the stress-related metabolite gamma-aminobutyrate. Activity assays of partially purified, desalted, root GAD revealed a 50% stimulation by the addition of 100 microM Ca2+, a 100% stimulation by the addition of 100 microM Ca2+ plus 100 nM CaM, and no appreciable stimulation by CaM in the absence of added Ca2+. The demonstration that plant GAD is a Ca(2+)-CaM-stimulated enzyme provides a model in which stress-linked metabolism is modulated by a Ca(2+)-mediated signal transduction pathway.