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.

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.

An Abscisic Acid-Activated and Calcium-Independent Protein Kinase from Guard Cells of Fava Bean

Abscisic acid (ABA) regulation of stomatal aperture is known to involve both Ca2+-dependent and Ca2+-independent signal transduction pathways. Electrophysiological studies suggest that protein phosphorylation is involved in ABA action in guard cells. Using biochemical approaches, we identified an ABA-activated and Ca2+- independent protein kinase (AAPK) from guard cell protoplasts of fava bean. Autophosphorylation of AAPK was rapidly (~1 min) activated by ABA in a Ca2+- independent manner. ABA-activated autophosphorylation of AAPK occurred on serine but not on tyrosine residues and appeared to be guard cell specific. AAPK phosphorylated histone type III-S on serine and threonine residues, and its activity toward histone type III-S was markedly stimulated in ABA-treated guard cell protoplasts. Our results suggest that AAPK may play an important role in the Ca2+-independent ABA signaling pathways of guard cells.

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.

Abscisic Acid-Induced Phosphoinositide Turnover in Guard Cell Protoplasts of Vicia faba

Guard cell protoplasts of Vicia faba treated with 10 [mu]M (+)abscisic acid (ABA) in the light exhibited a 20% decrease in diameter within 1.5 h, from 24.1 to 19.6 [mu]m. Within 10 s of administration of ABA, a 90% increase in levels of inositol 1,4,5-trisphosphate was observed, provided that cells were treated with Li+, an inhibitor of inositol phosphatase activity, prior to incubation. Concomitantly, levels of 32P-labeled phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate decreased 20% compared to levels in control cells; levels of label in the membrane lipids phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol did not change significantly in response to ABA treatment. These results show that phosphoinositide turnover is activated in response to ABA in guard cells. We conclude that phosphoinositide signaling is likely to be a step in the biochemical cascade that couples ABA to guard cell shrinking and stomatal closure.

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.

Anion-Channel Blockers Inhibit S-Type Anion Channels and Abscisic Acid Responses in Guard Cells

The effects of anion-channel blockers on light-mediated stomatal opening, on the potassium dependence of stomatal opening, on stomatal responses to abscisic acid (ABA), and on current through slow anion channels in the plasma membrane of guard cells were investigated. The anion-channel blockers anthracene-9-carboxylic acid (9-AC) and niflumic acid blocked current through slow anion channels of Vicia faba L. guard cells. Both 9-AC and niflumic acid reversed ABA inhibition of stomatal opening in V. faba L. and Commelina communis L. The anion-channel blocker probenecid also abolished ABA inhibition of stomatal opening in both species. Additional tests of 9-AC effects on stomatal aperture in Commelina revealed that application of this anion-channel blocker allowed wide stomatal opening under low (1 mM) KCI conditions and increased the rate of stomatal opening under both low and high (100 mM) KCI conditions. These results indicate that anion channels can function as a negative regulator of stomatal opening, presumably by allowing anion efflux and depolarization, which prohibits ion up-take in guard cells. Furthermore, 9-AC prevented ABA induction of stomatal closure. A model in which ABA activation of anion channels contributes a rate-limiting mechanism during ABA-induced stomatal closure and inhibition of stomatal opening is discussed.

Characterization of the Red Beet Plasma Membrane H+-ATPase Reconstituted in a Planar Bilayer System

The transport activity of the red beet (Beta vulgaris L.) plasma membrane H+-ATPase was examined following reconstitution into a planar bilayer membrane. Fusion of partially purified plasma membrane H+-ATPase with the bilayer membrane was accomplished by perfusion of proteoliposomes against the bilayer under hypoosmotic conditions. Following incorporation into the bilayer, an ATP-dependent current was measured that demonstrated properties consistent with those of the plasma membrane H+-ATPase. Current production was substrate specific for ATP, inhibited by orthovanadate, and insensitive to 200 nM erythrosin B but inhibited by 100 [mu]M erythrosin B. When current production was measured as a function of Mg:ATP concentration, a simple Michaelis-Menten relationship was observed and a Km of 0.62 mM was estimated. Current-voltage analysis of ATP-dependent current in the presence of 0.5 mM ATP, 20 mM ADP, 40 mM orthophosphate, and an opposing 2.5-unit [delta]pH revealed a reversal potential of about -149 mV. Based on the free energy available from ATP hydrolysis, this reversal potential is consistent with an H+/ATP stoichiometry of 1. This study demonstrates the usefulness of a planar bilayer system for investigation of energy coupling to H+ transport by the plasma membrane H+-ATPase.

Is ATP Required for K+ Channel Activation in Vicia Guard Cells?

In vivo, K+ entry into guard cells via inward-rectifying K+ channels is indirectly driven by ATP via an H+-ATPase that hyperpolarizes the membrane potential. However, whether activation of the K+ channels of guard cells requires ATP remains unknown. In the present study, both whole-cell and single-channel patch-clamp techniques were used to address this question. Exogenous ATP, ADP, and adenosine-5[prime]-O-(3-thiotriphosphate) applied to the cytoplasm had no effect on whole-cell K+ currents of Vicia faba L. guard cells. Azide, an inhibitor of oxidative phosphorylation, also had no effect. However, an ATP-scavenging system, glucose plus hexokinase, inhibited whole-cell inward K+ currents by 30 to 40%. Single-channel results acquired from cytoplasm-free inside-out membrane patches showed definite activation of inward K+ channels by ATP. Other nucleotides, such as ADP, adenosine-5[prime]-O(3-thiotriphosphate), and GTP, did not increase channel activity in the membrane patches. Inward K+ channel activity in membrane patches preactivated by exogenous ATP was inhibited by glucose plus hexokinase. These results suggest that a low concentration of ATP is required for activation of the inward K+ channels of the guard-cell plasma membrane. The issue of how ATP as a signal regulates these K+ channels is discussed.

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.

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

GTP-binding protein (G-protein) regulation of inward rectifying K+ channels in the plasma membrane of Vicia (Vicia faba L.) guard cells has previously been demonstrated at the whole-cell level. However, whether a cytosolic signal transduction chain is required for G-protein regulation of K+ channels in Vicia guard cells, or in any plant cell type, remains unknown. In the present study, we assayed effects of several G-protein regulators on inward K+ channels in isolated inside-out membrane patches from Vicia guard cell protoplasts. Guanosine 5'-[gamma-thio]triphosphate, a nonhydrolyzable GTP analog that locks G proteins into their activated state, decreased the open state probability (Po) of single inward K+ channels. This decrease in Po was accompanied by an increase in one of the closed time constants of the K+ channel. Guanosine 5'-[beta-thio]diphosphate, a GDP analog that locks G proteins into their inactivated state, slightly increased the Po of the inward K+ channel and shortened the closed time constants. Pertussis toxin and cholera toxin, which ADP-ribosylate G proteins at different sites, decreased the Po of the inward K+ channel. Our data indicate that G proteins can act via a membrane-delimited pathway to regulate inward K+ channels in the guard-cell plasma membrane.

Inhibition of inward K+ channels and stomatal response by abscisic acid: an intracellular locus of phytohormone action

Abscisic acid (ABA), a plant hormone whose production is stimulated by water stress, reduces the apertures of stomatal pores in the leaf surface, thereby lessening transpirational water loss. It has been thought that inhibition of stomatal opening and promotion of stomatal closure by ABA are initiated by the binding of extracellular ABA to a receptor located in the guard-cell plasma membrane. However, in the present research, we employ three distinct experimental approaches to demonstrate that ABA can act from within guard cells to regulate stomatal apertures. (i) The extent to which ABA inhibits stomatal opening and promotes stomatal closure in Commelina communis L. is proportional to the extent of ABA uptake, as assayed with [3H]ABA. (ii) Direct microinjection of ABA into the cytoplasm of Commelina guard cells precipitates stomatal closure. (iii) Application of ABA to the cytosol of Vicia faba L. guard-cell protoplasts via patch-clamp techniques inhibits inward K+ currents, an effect sufficient to inhibit stomatal opening. These results demonstrate an intracellular locus of phytohormone action and imply that the search for hormone receptor proteins should be extended to include intracellular compartments.

Inhibition of guard-cell inward K+ channels by abscisic acid: links and gaps in the signal transduction chain

Abscisic acid (ABA) affects a number of ion transport mechanisms in guard cells. One effect of ABA is to inhibit inward K+ channels, an effect that can contribute to the inhibition of stomatal opening. One model of the signal transduction cascade mediating this response involves ABA-activation of a G-protein, which in turn activates phospholipase C, resulting in production of inositol 1,4,5-trisphosphate (IP3), elevation of cytosolic free Ca2+ levels (Cai), and activation of a Ca(2+)-dependent phosphatase (protein phosphatase 2B; PP2B) which mediates channel inhibition. A review of the literature reveals that several of the links in this putative signal transduction chain have been established. The G-protein activator, GTP gamma S, inhibits inward K+ currents. Exogenous IP3 inhibits inward K+ currents. Exogenous IP3 elevates Cai. Elevated Cai inhibits inward K+ currents, and the inhibition by ABA of inward K+ currents does require Ca2+. Exogenous PP2B inhibits inward K+ currents, and an endogenous Ca(2+)-dependent phosphatase activity is present in guard cells. However, significant gaps in the chain remain. There is no direct evidence that ABA activates a G-protein. The presence or absence of a G-protein-activated phospholipase C in guard cells has not been investigated. Elevation of Cai by ABA is variable, and the reasons for this variability remain to be established. It is not known whether or not activation of a guard-cell PP2B homolog is the exclusive mechanism by which Ca2+ inhibits the channels.

Circadian Rhythms in Stomatal Responsiveness to Red and Blue Light

Stomata of many plants have circadian rhythms in responsiveness to environmental cues as well as circadian rhythms in aperture. Stomatal responses to red light and blue light are mediated by photosynthetic photoreceptors; responses to blue light are additionally controlled by a specific blue-light photoreceptor. This paper describes circadian rhythmic aspects of stomatal responsiveness to red and blue light in Vicia faba. Plants were exposed to a repeated light:dark regime of 1.5:2.5 h for a total of 48 h, and because the plants could not entrain to this short light:dark cycle, circadian rhythms were able to "free run" as if in continuous light. The rhythm in the stomatal conductance established during the 1.5-h light periods was caused both by a rhythm in sensitivity to light and by a rhythm in the stomatal conductance established during the preceding 2.5-h dark periods. Both rhythms peaked during the middle of the subjective day. Although the stomatal response to blue light is greater than the response to red light at all times of day, there was no discernible difference in period, phase, or amplitude of the rhythm in sensitivity to the two light qualities. We observed no circadian rhythmicity in net carbon assimilation with the 1.5:2.5 h light regime for either red or blue light. In continuous white light, small rhythmic changes in photosynthetic assimilation were observed, but at relatively high light levels, and these appeared to be attributable largely to changes in internal CO2 availability governed by stomatal conductance.

Immunosuppressants implicate protein phosphatase regulation of K+ channels in guard cells

The elevation of Ca2+ levels in the cytoplasm inactivates inward-rectifying K+ channels that play a central role in regulating the apertures of stomatal pores in higher plants. However, the mechanism for the Ca(2+)-mediated inhibition of K(+)-channel function is unknown. Using patch-clamp techniques, we show that cyclophilin-cyclosporin A and FK506-binding protein-FK506 complexes, which are highly specific inhibitors of protein phosphatase 2B (calcineurin), block Ca(2+)-induced inactivation of K+ channels in Vicia faba guard cells. A constitutively active calcineurin fragment that is Ca(2+)-independent inhibits K(+)-channel activity in the absence of Ca2+. We have also identified an endogenous Ca(2+)-dependent phosphatase activity from V. faba that is inhibited by the cyclophilin-cyclosporin A and FK506-binding protein-FK506 complexes. Our findings implicate a Ca(2+)-dependent, calcineurin-like protein phosphatase in a Ca2+ signal-transduction pathway of higher plants.

Comparison of K(+)-channel activation and deactivation in guard cells from a dicotyledon (Vicia faba L.) and a graminaceous monocotyledon (Zea mays)

We describe and compare inward and outward whole-cell K(+) currents across the plasma membrane surrounding guard-cell protoplasts from the dicotyledon, Vicia faba, and the graminaceous monocotyledon, Zea mays. Macrosopic whole-cell current is considered in terms of microscopic single-channel activity, which involves discrete steps between conducting (open) and nonconducting (closed) states of the channel protein. Kinetic equations are used to model the number of open and closed states for channels conducting K(+) influx (K(in)) and K(+) efflux (K(out)) in the two species, and to calculate the rate at which open-closed transitions occur. The opening and closure of K(in) channels in both Vicia and Zea follow single-exponential timecourses, indicating that K(in)-channel proteins in each species simply fluctuate between one open and one closed state. In both species, opening of K(in) channels is voltage-independent, but closure of K(in) channels is faster at more positive membrane potentials. In response to identical voltage stimuli, K(in) channels in Zea open and close approximately three times as fast as in Vicia. In contrast to K(in), K(out) channels in Zea open and close more slowly than in Vicia. The closure of K(out) channels follows a single-exponential timecourse in each species, indicating one open state. The kinetics of K(out)-channel opening are more complicated and indicate the presence of at least two (Vicia) or three (Zea) closed states.

Characterization of a G-protein-regulated outward K+ current in mesophyll cells of vicia faba L

Whole-cell voltage-dependent currents in isolated mesophyll protoplasts of Vicia faba were investigated by patch-clamp techniques. With 104 mM K+ in the cytosol and 13 mM K+ in the external solution, depolarization of the plasma membrane from -47 mV to potentials between -15 and +85 mV activated a voltage- and time-dependent outward current (Iout). The average magnitude of Iout at +85 mV was 28.5 +/- 3.3 pA.pF-1. No inward voltage-dependent current was observed upon hyperpolarization of the plasma membrane from -55 mV to potentials as negative as -175 mV. Time-activated outward current was blocked by Ba2+ (1 mM BaCl2) and was not observed when K+ was eliminated from the external and internal solutions, indicating that this outward current was carried primarily by K+ ions. The voltage dependency of outward K+ current revealed a possible mechanism for K+ efflux from mesophyll cells. A GDP analogue guanosine 5'-[beta-thio]diphosphate (500 microM) significantly enhanced outward K+ current. The outward K+ current was inhibited by the GTP analogue guanosine 5'-[gamma-thio]triphosphate (500 microM) and by an increase in cytoplasmic free Ca2+ concentrations. Cholera toxin, which ADP-ribosylates guanine nucleotide-binding regulatory proteins, also inhibited outward K+ current. These findings illustrate the presence in mesophyll cells of outward-rectifying K+ channels that are regulated by GTP-binding proteins and calcium.

Permeation of Ca2+ through K+ channels in the plasma membrane of Vicia faba guard cells

The whole-cell patch-clamp method has been used to measure Ca2+ influx through otherwise K(+)-selective channels in the plasma membrane surrounding protoplasts from guard cells of Vicia faba. These channels are activated by membrane hyperpolarization. The resulting K+ influx contributes to the increase in guard cell turgor which causes stomatal opening during the regulation of leaf-air gas exchange. We find that after opening the K+ channels by hyperpolarization, depolarization of the membrane results in tail current at voltages where there is no electrochemical force to drive K+ inward through the channels. Tail current remains when the reversal potential for permeant ions other than Ca2+ is more negative than or equal to the K+ equilibrium potential (-47 mV), indicating that the current is due to Ca2+ influx through the K+ channels prior to their closure. Decreasing internal [Ca2+] (Cai) from 200 to 2 nM or increasing the external [Ca2+] (Cao) from 1 to 10 mM increases the amplitude of tail current and shifts the observed reversal potential to more positive values. Such increases in the electrochemical force driving Ca2+ influx also decrease the amplitude of time-activated current, indicating that Ca2+ permeation is slower than K+ permeation, and so causes a partial block. Increasing Cao also (i) causes a positive shift in the voltage dependence of current, presumably by decreasing the membrane surface potential, and (ii) results in a U-shaped current-voltage relationship with peak inward current ca. -160 mV, indicating that the Ca2+ block is voltage dependent and suggesting that the cation binding site is within the electric field of the membrane. K+ channels in Zea mays guard cells also appear to have a Cai- and Cao-dependent ability to mediate Ca2+ influx. We suggest that the inwardly rectifying K+ channels are part of a regulatory mechanism for Cai. Changes in Cao and (associated) changes in Cai regulate a variety of intracellular processes and ion fluxes, including the K+ and anion fluxes associated with stomatal aperture change.

Synergistic effect of light and fusicoccin on stomatal opening : epidermal peel and patch clamp experiments

Upon incubation of epidermal peels of Commelina communis in 1 millimolar KCl, a synergistic effect of light and low fusicoccin (FC) concentrations on stomatal opening is observed. In 1 millimolar KCl, stomata remain closed even in the light. However, addition of 0.1 micromolar FC results in opening up to 12 micrometers. The same FC concentration stimulates less than 5 micrometers of opening in darkness. The synergistic effect (a) decreases with increasing FC or KCl concentrations; (b) is dark-reversible; (c) like stomatal opening in high KCl concentrations (120 millimolar) is partially inhibited by the K(+) channel blocker, tetraethyl-ammonium(+) (20 millimolar). In whole-cell patch-clamp experiments with guard cell protoplasts of Vicia faba, FC (1 or 10 micromolar) stimulates an increase in outward current that is essentially voltage independent between - 100 and +60 millivolts, and occurs even when the membrane potential is held at a voltage (-60 millivolts) at which K(+) channels are inactivated. These results are indicative of FC activation of a H(+) pump. FC effects on the magnitude of inward and outward K(+) currents are not observed. Epidermal peel and patch clamp data are both consistent with the hypothesis that the plasma membrane H(+) ATPase of guard cells is a primary locus for the FC effect on stomatal apertures.

Whole-cell K(+) current across the plasma membrane of guard cells from a grass: Zea mays

Knowledge of ion fluxes in the dumbell-shaped guard cells of grass species has been limited by the difficulty of obtaining isolated epidermes or guard-cell protoplasts for use in radioactive-tracer or electrophysiological studies. We describe here a method for isolating guard-cell protoplasts from Zea mays L. Whole-cell patch clamp has been used to measure K(+)-channel current across the plasma membrane surrounding these protoplasts. Two populations of K(+)-permeable channels have been identified. Hyperpolarization of the membrane to potentials (Vm) more negative than -100 mV results in inward K(+) current through one population of channels. Inward current activation is faster than in the dicotyledon, Vicia faba L. (mean activation half-time 26 ms (Z. mays) versus 123 ms (V. faba) at Vm=-180 mV). Steady-state current density is less than in V. faba (-22 μA · cm(-2) (Z. mays) versus -40 μA · cm(-2) (V. faba) at Vm=- 180 mV in 12 mM external K(+)). Depolarization of the membrane to potentials more positive than -20 mV results in outward K(+) current through a second population of channels; these channels activate and (upon repolarization of the membrane) deactivate more slowly than in V. faba (mean activation half-time 375 ms (Z. mays) versus 187 ms (V. faba) at Vm=+ 80 mV) but result in a similar steady-state current density (23.8 μA · cm(-2) (Z. mays) versus 28.7 μA · cm(-2) (V. faba) at Vm= + 80 mV with 105 mM internal K(+)). Omission of K(+) eliminates the current. The K(+) current is sensitive to both internal and external Ca(2+) concentration: increasing internal Ca(2+) from 2 nM to 0.2 μM or increasing external Ca(2+) from 1 mM to 8.5 mM reduces the magnitude of both inward and outward current.

Cellular distribution of calmodulin and calmodulin-binding proteins in Vicia faba L

The distribution of calmodulin (CaM) and CaM-binding proteins within Vicia faba was investigated. Both CaM and CaM-binding proteins were found to be differentially distributed among organs, tissues, and protoplast types. CaM levels, on a per protein basis, were found to be the highest in leaf epidermis, containing 3-fold higher levels of CaM than in total leaf. Similarly, guard cell and epidermal cell protoplasts were also found to have higher levels of CaM than mesophyll cell protoplasts. 125I-CaM blot overlay assays were performed to qualitatively examine CaM-binding proteins in these protoplast types as well as in whole tissues and organs. CaM-binding proteins with Mr 52,000, 78,000, and 115,000 were common in all metabolically active plant parts. Unique CaM-binding protein bands were detected in guard cell protoplasts (Mr 39,000, 88,000), stems (Mr 45,000, 60,000, 64,000), and roots (Mr 62,000), suggesting the presence of specialized CaM-dependent processes in these cells and organs.

Evidence for G-Protein Regulation of Inward K+ Channel Current in Guard Cells of Fava Bean

Recent reports have shown that GTP-binding proteins (G-proteins) are present in plants but have given limited indication as to their site of action. G-proteins in animal cells transduce extracellular signals into intracellular or membrane-mediated events, including the regulation of ion channels. Using whole-cell patch clamp, we provide evidence that a G-protein in guard cells of fava bean regulates the magnitude (and not the kinetics) of inward current through K+-selective ion channels in the plasma membrane. GDP[beta]S (100 to 500 [mu]M) increases inward K+ current, whereas GTP[gamma]S (500 [mu]M) has the opposite effect. The control nucleotides ADP[beta]S and ATP[gamma]S (500 [mu]M) do not affect K+ current. Reduction of inward current by GTP[gamma]S is eliminated in the presence of the Ca2+ chelator, BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N[prime],N[prime],-tetraacetic acid) (5 mM). When applied intracellularly, the G-protein regulators, cholera toxin and pertussis toxin, both decrease inward K+ current. The entry of K+ (and anions) into guard cells increases their turgor, opening stomatal pores in the leaf epidermis that allow gas exchange with the environment. Our data suggest the involvement of a G-protein in the inhibition of K+ uptake and stomatal opening. Changes in stomatal aperture, vital to both photosynthesis and plant water status, reflect guard-cell responsiveness to a variety of known environmental signals. The results presented here indicate that, in plants as well as animals, ion channel regulation by environmental stimuli may be mediated by G-proteins.