Mechanisms of fusicoccin action: A dominant role for secondary transport in a higher-plant cell

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

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

The mechanism of SO2 -induced stomatal closure differs from O3 and CO2 responses and is mediated by nonapoptotic cell death in guard cells

Plants closing stomata in the presence of harmful gases is believed to be a stress avoidance mechanism. SO₂ , one of the major airborne pollutants, has long been reported to induce stomatal closure, yet the mechanism remains unknown. Little is known about the stomatal response to airborne pollutants besides O₃ . SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1) and OPEN STOMATA 1 (OST1) were identified as genes mediating O₃ -induced closure. SLAC1 and OST1 are also known to mediate stomatal closure in response to CO₂ , together with RESPIRATORY BURST OXIDASE HOMOLOGs (RBOHs). The overlaying roles of these genes in response to O₃ and CO₂ suggested that plants share their molecular regulators for airborne stimuli. Here, we investigated and compared stomatal closure event induced by a wide concentration range of SO₂ in Arabidopsis through molecular genetic approaches. O₃ - and CO₂ -insensitive stomata mutants did not show significant differences from the wild type in stomatal sensitivity, guard cell viability, and chlorophyll content revealing that SO₂ -induced closure is not regulated by the same molecular mechanisms as for O₃ and CO₂ . Nonapoptotic cell death is shown as the reason for SO₂ -induced closure, which proposed the closure as a physicochemical process resulted from SO₂ distress, instead of a biological protection mechanism.

Biology of SLAC1-type anion channels - from nutrient uptake to stomatal closure

Contents 46 I. 46 II. 47 III. 50 IV. 53 V. 56 VI. 57 58 58 References 58 SUMMARY: Stomatal guard cells control leaf CO₂ intake and concomitant water loss to the atmosphere. When photosynthetic CO₂ assimilation is limited and the ratio of CO₂ intake to transpiration becomes suboptimal, guard cells, sensing the rise in CO₂ concentration in the substomatal cavity, deflate and the stomata close. Screens for mutants that do not close in response to experimentally imposed high CO₂ atmospheres identified the guard cell-expressed Slowly activating anion channel, SLAC1, as the key player in the regulation of stomatal closure. SLAC1 evolved, though, before the emergence of guard cells. In Arabidopsis, SLAC1 is the founder member of a family of anion channels, which comprises four homologues. SLAC1 and SLAH3 mediate chloride and nitrate transport in guard cells, while SLAH1, SLAH2 and SLAH3 are engaged in root nitrate and chloride acquisition, and anion translocation to the shoot. The signal transduction pathways involved in CO₂ , water stress and nutrient-sensing activate SLAC/SLAH via distinct protein kinase/phosphatase pairs. In this review, we discuss the role that SLAC/SLAH channels play in guard cell closure, on the one hand, and in the root-shoot continuum on the other, along with the molecular basis of the channels' anion selectivity and gating.

Stomatal size, speed, and responsiveness impact on photosynthesis and water use efficiency

The control of gaseous exchange between the leaf and bulk atmosphere by stomata governs CO₂ uptake for photosynthesis and transpiration, determining plant productivity and water use efficiency. The balance between these two processes depends on stomatal responses to environmental and internal cues and the synchrony of stomatal behavior relative to mesophyll demands for CO₂. Here we examine the rapidity of stomatal responses with attention to their relationship to photosynthetic CO₂ uptake and the consequences for water use. We discuss the influence of anatomical characteristics on the velocity of changes in stomatal conductance and explore the potential for manipulating the physical as well as physiological characteristics of stomatal guard cells in order to accelerate stomatal movements in synchrony with mesophyll CO₂ demand and to improve water use efficiency without substantial cost to photosynthetic carbon fixation. We conclude that manipulating guard cell transport and metabolism is just as, if not more likely to yield useful benefits as manipulations of their physical and anatomical characteristics. Achieving these benefits should be greatly facilitated by quantitative systems analysis that connects directly the molecular properties of the guard cells to their function in the field.

Systems analysis of guard cell membrane transport for enhanced stomatal dynamics and water use efficiency

Stomatal transpiration is at the center of a crisis in water availability and crop production that is expected to unfold over the next 20 to 30 years. Global water usage has increased 6-fold in the past 100 years, twice as fast as the human population, and is expected to double again before 2030, driven mainly by irrigation and agriculture. Guard cell membrane transport is integral to controlling stomatal aperture and offers important targets for genetic manipulation to improve crop performance. However, its complexity presents a formidable barrier to exploring such possibilities. With few exceptions, mutations that increase water use efficiency commonly have been found to do so with substantial costs to the rate of carbon assimilation, reflecting the trade-off in CO₂ availability with suppressed stomatal transpiration. One approach yet to be explored in detail relies on quantitative systems analysis of the guard cell. Our deep knowledge of transport and homeostasis in these cells gives real substance to the prospect for reverse engineering of stomatal responses, using in silico design in directing genetic manipulation for improved water use and crop yields. Here we address this problem with a focus on stomatal kinetics, taking advantage of the OnGuard software and models of the stomatal guard cell recently developed for exploring stomatal physiology. Our analysis suggests that manipulations of single transporter populations are likely to have unforeseen consequences. Channel gating, especially of the dominant K⁺ channels, appears the most favorable target for experimental manipulation.

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

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

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

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

Plasma membrane depolarization induced by abscisic acid in Arabidopsis suspension cells involves reduction of proton pumping in addition to anion channel activation, which are both Ca2+ dependent

In Arabidopsis suspension cells a rapid plasma membrane depolarization is triggered by abscisic acid (ABA). Activation of anion channels was shown to be a component leading to this ABA-induced plasma membrane depolarization. Using experiments employing combined voltage clamping, continuous measurement of extracellular pH, we examined whether plasma membrane H(+)-ATPases could also be involved in the depolarization. We found that ABA causes simultaneously cell depolarization and medium alkalinization, the second effect being abolished when ABA is added in the presence of H+ pump inhibitors. Inhibition of the proton pump by ABA is thus a second component leading to the plasma membrane depolarization. The ABA-induced depolarization is therefore the result of two different processes: activation of anion channels and inhibition of H(+)-ATPases. These two processes are independent because impairing one did not suppress the depolarization. Both processes are however dependent on the [Ca2+]cyt increase induced by ABA since increase in [Ca(2+)](cyt) enhanced anion channels and impaired H(+)-ATPases.

Parallel recordings of photosynthetic electron transport and K+-channel activity in single guard cells

Stomata open in response to red and blue light. Red light-induced stomatal movement is mediated by guard cell chloroplasts and related to K+-uptake into these motor cells. We have combined a new type of microchlorophyll fluorometer with the patch-clamp technique for parallel studies of the photosynthetic electron transport and activity of plasma membrane K+ channels in single guard cell protoplast. In the whole-cell configuration and presence of ATP in the patch-pipette, the activity of the K+-uptake channels remained constant throughout the course of an experiment (up to 30 min) while photosynthetic activity declined to about 50%. In the absence of ATP inward K+ currents declined in a time-dependent manner. Under these ATP-free conditions, photosynthetic electron transport was completely blocked within 8 min. ADP together with orthophosphate was able to prevent inhibition of photosynthetic electron transport and run-down of K+-channel activity. The results demonstrate that the combination of these two techniques is suited to directly study cytosolic factors as common regulators of photosynthesis and plasma membrane transport within a single-cell.

The role of ion channels in light-dependent stomatal opening

Stomatal opening represents a major determinant of plant productivity and stress management. Because plants lose water essentially through open stomata, volume control of the pore-forming guard cells represents a key step in the regulation of plant water status. These sensory cells are able to integrate various signals such as light, auxin, abscisic acid, and CO(2). Following signal perception, changes in membrane potential and activity of ion transporters finally lead to the accumulation of potassium salts and turgor pressure formation. This review analyses recent progress in molecular aspects of ion channel regulation and suggests how these developments impact on our understanding of light- and auxin-dependent stomatal action.

Calcium and ABA-induced stomatal closure

Water loss from leaves is regulated by the state of stomatal pores, whose aperture is controlled by the level of potassium salt accumulation in guard cells. In water stress conditions abscisic acid (ABA), produced or imported into leaves, and acting on the outside of the guard cell induces net loss of potassium salts, and hence stomatal closure. The mechanism of ABA-induced closure and the role of calcium in the process are discussed. There are two questions at issue, whether Ca 2+ -regulated fluxes of specific ions are an obligatory part of the signal cascade, and if this is the case, w hether the necessary ABA-induced increase in cytoplasmic Ca 2+ arises from Ca 2+ influx at the plasmalemma, or by Ca 2+ release from internal stores, or both. Tracer flux studies establish that ABA-induced closure involves transient stimulation of both anion and cation fluxes at the plasmalemma, and stimulation of the transfer of both anions and cations from vacuole to cytoplasm. ABA-induced efflux transients can occur in very low external Ca 2+ , but their reduction in the presence of La 3+ suggests that Ca 2+ influx is required for the response. The flux work can only be interpreted in terms of defined ion channels identified by electrical work, either whole-cell voltage clamping or patch clamp studies, and of the responses of these channels to Ca 2+ and to ABA. Electrical work identifies a number of ion channels in the plasmalemma; these include an inward K + channel open at negative membrane potentials, and inhibited by increase in cytoplasmic Ca 2+ , an outward K + channel open at more positive membrane potentials, which is insensitive to Ca 2+ but is more active at higher pH, a voltage-sensitive, Ca 2+ -dependent anion channel, active only over a restricted range of potentials (about — 100 mV to —50 mV), and some ill-defined conductances lumped together as the ‘leak’ or background conductance, which may include channels (selective or nonselective) allowing Ca 2+ influx. The leak conductance is increased by increase in cytoplasmic Ca 2+ . Guard cells are capable of responding to inositol 1,4,5-trisphosphate released in the cytoplasm, by increasing cytoplasmic Ca 2+ , by inhibition of the inward K + channel and by stimulation of the leak conductance (but w ithout effect on the outw ard K + channel), and by stomatal closure. Recent work suggests that there is considerable turnover in the phosphoinositide cycle in guard cells, within 30 s of treatment with ABA. Measurements by fluorescence techniques of cytoplasmic Ca 2+ in guard cells following treatment with ABA give conflicting results. Some work shows increase in cytoplasmic Ca 2+ in response to ABA, other studies show variable behaviour, with most cells closing in response to ABA, but without detectable changes in cytoplasmic Ca 2+ . Nevertheless it seems likely that increases in cytoplasmic Ca 2+ , at least locally, are a universal feature of the ABA response, but that they may be difficult to detect with present techniques. Fluorescence studies also show alkalinization of guard cell cytoplasm in response to ABA. Whole cell electrical studies identify a number of ABA-induced changes. They show (i) depolarization of cells with very negative membrane potentials to potentials which are positive to E K , and thus out of the activation range for the inward K + channel, and within the range for the outward K + channel, (ii) activation of an inward current, a voltage-insensitive component of the leak conductance, which is responsible for the depolarization, (iii) deactivation of the inward K + channel, (iv) activation of a voltage-sensitive channel carrying inward current, probably the Ca 2+ -sensitive anion channel, and (v) the slower activation of the outward K + channel. The activation of the inward leak current seems to be the primary response, but its nature is not clearly established; a non-selective cation channel, which may allow Ca 2+ influx, is perhaps most likely. Thus the early events in the ABA-response include stimulation of tracer efflux, activation of an ill-defined component of the leak conductance, producing an inward current, and turnover in the phosphoinositide cycle. These occur within the first minute, but their time sequence and causal relationships are not yet clear. A plausible scheme for ABA-induced closure can be devised, involving Ca 2+ influx through a non-selective cation channel as the first event, producing depolarization and increase (possibly local) in cytoplasmic Ca 2+ . This may then be supplemented by release of Ca 2+ from internal stores, triggered by inositol 1,4,5-trisphosphate produced by activation of phospholipase C. Increase in cytoplasmic Ca 2+ will give deactivation of the inward K + channel, and activation of the Ca 2+ -dependent anion channel, but some other trigger is required to explain the activation of the outward K + channel; increase in cytoplasmic pH (observed, but of mechanism unknown) is the most likely candidate. This is one scheme, but others can also be devised, and with the gaps still existing in our description of the events involved, and their time sequence, a definitive hypothesis is not yet available.

Characterization of the plasma-membrane H(+)-ATPase from Vicia faba guard cells : Modulation by extracellular factors and seasonal changes

Stomatal movement is controlled by external and internal signals such as light, phytohormones or cytoplasmic Ca(2+). Using Vicia faba L., we have studied the dose-dependent effect of auxins on the modulation of stomatal opening, mediated through the activity of the plasma-membrane H(+)-ATPase. The patch-clamp technique was used to elucidate the electrical properties of the H(+)-ATPase as effected by growth regulators and seasonal changes. The solute composition of cytoplasmic and extracellular media was selected to record pump currents directly with high resolution. Proton currents through the ATPase were characterized by a voltage-dependent increase in amplitude, positive to the resting potential, reaching a plateau at more depolarized values. Upon changes in extracellular pH, the resting potential of the cell shifted with a non-Nernst potential response (±21 mV), indicating the contribution of a depolarizing ionic conductance other than protons to the permeability of the plasma membrane. The use of selective inhibitors enabled us to identify the currents superimposing the H(+)-pump as carried by Ca(2+). Auxinstimulation of this electroenzyme resulted in a rise in the outwardly directed H(+) current and membrane hyperpolarization, indicating that modulation of the ATPase by the hormone may precede salt accumulation as well as volume and turgor increase. Annual cycles in pump activity (1.5-3.8 μA · cm(-2)) were expressed by a minimum in pump current during January and February. Resting potentials of up to -260 mV and plasmamembrane surface area, on the other hand, did not exhibit seasonal changes. The pump activity per unit surface area was approximately 2- to 3-fold higher in guard cells than in mesophyll cells and thus correlates with their physiological demands.

Voltage dependence of the Chara proton pump revealed by current-voltage measurement during rapid metabolic blockade with cyanide

It is generally agreed that solute transport across the Chara plasma membrane is energized by a proton electrochemical gradient maintained by an H(+)-extruding ATPase. Nonetheless, as deduced from steady-state current-voltage (I-V) measurements, the kinetic and thermodynamic constraints on H(+)-ATPase function remain in dispute. Uncertainties necessarily surround long-term effects of the relatively nonspecific antagonists used in the past; but a second, and potentially more serious problem has sprung from the custom of subtracting, across the voltage spectrum, currents recorded following pump inhibition from currents measured in the control. This practice must fail to yield the true I-V profile for the pump when treatments alter the thermodynamic pressure on transport. We have reviewed these issues, using rapid metabolic blockade with cyanide and fitting the resultant whole-cell I-V and difference-current-voltage (dI-V) relations to a reaction kinetic model for the pump and parallel, ensemble leak. Measurements were carried out after blocking excitation with LaCl3, so that steady-state currents could be recorded under voltage clamp between -400 and +100 mV. Exposures to 1 mM NaCN (CN) and 0.4 mM salicylhydroxamic acid (SHAM) depolarized (positive-going) Chara membrane potentials by 44-112 mV with a mean half time of 5.4 +/- 0.8 sec (n = 13). ATP contents, which were followed in parallel experiments, decayed coincidently with a mean half time of 5.3 +/- 0.9 sec [( ATP]t = 0, 0.74 +/- 0.3 mM; [ATP]t = infinity, 0.23 +/- 0.02 mM). Current-voltage response to metabolic blockade was described quantitatively in context of these changes in ATP content and the consequent reduction in pump turnover rate accompanied by variable declines in ensemble leak conductance. Analyses of dI-V curves (+/- CN + SHAM) as well as of families of I-V curves taken at times during CN + SHAM exposures indicated a stoichiometry for the pump of one charge (H+) transported per ATP hydrolyzed and an equilibrium potential near -420 mV at neutral external pH; under these conditions, the pump accounted for approximately 60-75% of the total membrane conductance near Vm. Complementary results were obtained also in fitting previously published I-V data gathered over the external pH range 4.5-7.5. Kinetic features deduced for the pump were dominated by a slow step preceding H+ unloading outside, and by recycling and loading steps on the inside which were in rapid equilibrium.(ABSTRACT TRUNCATED AT 400 WORDS)

Tansley Review No. 21 Plant ion channels: whole-cell and single channel studies

Ion channels are proteins which catalyse rapid, passive, electrogenic uniport of ions through pores spanning an otherwise poorly permeable lipid bilayer. Among other processes, fluxes through ion channels are responsible for action potentials - large, transient changes in membrane potential which have been known of in plants for over 100 years. Much disparate information on ion channels in plant cells has accumulated over the past few years. In an attempt to synthesize these data, the properties of at least 18 different ion channels are collated in this review. Channels are initially classified according to ion selectivity (Ca²⁺ , Cl^- , K⁺ and H⁺ ); then gating characteristics (i.e. control of opening and closing), unitary conductance and pharmacology are used to distinguish further different sub-types of channels. To provide a background for this overview, the fundamental properties which define ion channels in animal cells, namely conduction, selectivity and gating, are described. Appropriate techniques for the study of ion channels are also assessed. The review concludes with a discussion on the role of ion channels in plant cells, although any comment on functions beyond turgor regulation and general statements about signalling remains largely speculative. The study of ion channels in plant cells is still at an early stage and it is hoped that this review will provide a framework upon which further work in both algae and vascular plants can be based. CONTENTS Summary 305 I. Introduction: plant electrophysiology 306 II. A general description of ion channels 306 III. Ion channels in plants 310 IV. Ca²⁺ channels 313 V. Cl^- channels 315 VI. K⁺ channels in the plasma membrane 318 VII. K⁺ channels in the tonoplast 322 VIII. Channels in thylakoids 324 IX. H⁺ channels 324 X. Functions of channels 325 XI. Conclusions 328 Acknowledgements 328 References 329.

Potassium channel currents in intact stomatal guard cells: rapid enhancement by abscisic acid

Evidence of a role for abscisic acid (ABA) in signalling conditions of water stress and promoting stomatal closure is convincing, but past studies have left few clues as to its molecular mechanism(s) of action; arguments centred on changes in H(+)-pump activity and membrane potential, especially, remain ambiguous without the fundamental support of a rigorous electrophysiological analysis. The present study explores the response to ABA of K(+) channels at the membrane of intact guard cells of Vicia faba L. Membrane potentials were recorded before and during exposures to ABA, and whole-cell currents were measured at intervals throughout to quantitate the steady-state and time-dependent characteristics of the K(+) channels. On adding 10 μM ABA in the presence of 0.1, 3 or 10 mM extracellular K(+), the free-running membrane potential (V m) shifted negative-going (-)4-7 mV in the first 5 min of exposure, with no consistent effect thereafter. Voltage-clamp measurements, however, revealed that the K(+)-channel current rose to between 1.84- and 3.41-fold of the controls in the steady-state with a mean halftime of 1.1 ± 0.1 min. Comparable changes in current return via the leak were also evident and accounted for the minimal response in V m. Calculated at V m, the K(+) currents translated to an average 2.65-fold rise in K(+) efflux with ABA. Abscisic acid was not observed to alter either K(+)-current activation or deactivation.These results are consistent with an ABA-evoked mobilization of K(+) channels or channel conductance, rather than a direct effect of the phytohormone on K(+)-channel gating. The data discount notions that large swings in membrane voltage are a prerequisite to controlling guard-cell K(+) flux. Instead, thev highlight a rise in membrane capacity for K(+) flux, dependent on concerted modulations of K(+)-channel and leak currents, and sufficiently rapid to account generally for the onset of K(+) loss from guard cells and stomatal closure in ABA.

Mechanisms of fusicoccin action: kinetic modification and inactivation of K(+) channels in guard cells

Fusicoccin commonly is thought to promote secondary solute transport via an increase in electrical driving force which follows the enhancement of primary, "electrogenic" H(+) extrusion by the plant plasma membrane H(+)-ATPase. However, previous electrical studies ofVicia faba L. guard cells in FC (Blatt, 1988, Planta174, 187) demonstrated, in addition to a limited rise in pump current, appreciable declines in membrane conductance near and positive to the free-running membrane potential (V m). Much of the current at these potentials could have been carried by outward-rectifying K(+) channels which were progressively inactivated in FC. We have examined this possibility in electrical studies, using whole-cell currents measured under voltage clamp to quantitate steadystate and kinetic characteristics of the K(+) channels both before and during exposure to FC; channels block in tetraethylammonium chloride was exploited to assess changes in background 'leak' currents. The cells showed little evidence of primary pump activity, a fact which further simplified analyses. Under these conditions, outward-directed K(+) channel current contributed to charge balance maintainingV m, and adding 10 μM FC on average depolarized (positive-going)V m. Steady-state current-voltage relations revealed changes both in K(+) channel and in leak currents underlying the voltage response. Changes in the leak were variable, but on average the leak equilibrium potential was shifted (+)19 mV and leak conductance declined by 21% over 30-40 min in FC. Potassium currents were inactivated irreversibly and with halftimes (current maxima) of 6.2-10.7 min. Inactivation was voltage-dependent, so that the activation ("gating") potential for the current was shifted, positive-going, with time in FC. Channel gating kinetics, inferred from the macroscopic currents, were also affected; current rise at positive potentials accelerated 4.5-fold and more, but in a manner apparently independent of voltage and extracellular potassium concentration. Current decay at negative potentials was quickened, also. These results identify the outward-rectifying K(+) channels as one site of action for FC at a higher plant cell membrane; they complete the link introduced in the preceding paper between K(+) channel current, K(+)((86)Rb(+)) flux and irreversible cation uptake in the toxin. The data also offer some insights toward a kinetic description of channel gating. Finally, they provide a vehicle for interpreting FC-induced changes in K(+) and net H(+) flux, and in membrane potential without the necessity for postulating gross changes in H(+) pumping.

Mechanisms of fusicoccin action: evidence for concerted modulations of secondary K(+) transport in a higher plant cell

Fusicoccin (FC) has long been known to promote K(+) uptake in higher plant cells, including stomatal guard cells, yet the precise mechanism behind this enhancement remains uncertain. Membrane hyperpolarization, thought to arise from primary H(+) pumping stimulated in FC, could help drive K(+) uptake, but the extent to which FC stimulates influx and uptake frequently exceeds any reasonable estimates from Constant Field Theory based on changes in the free-running membrane potential (V m) alone; furthermore, unidirectional flux analyses have shown that in the toxin K(+) ((86)Rb(+)) exchange plummets to 10% of the control (G.M. Clint and E.A.C. MacRobbie 1984, J. Exp. Bot.35 180-192). Thus, the activities of specific pathways for K(+) movement across the membrane could be modified in FC. We have explored a role for K(+) channels in mediating these fluxes in guard cells ofVicia faba L. The correspondence between FC-induced changes in chemical ((86)Rb(+)) flux and in electrical current under voltage clamp was followed, using the K(+) channel blocker tetraethylammonium chloride (TEA) to probe tracer and charge movement through K(+)-selective channels. Parallel flux and electrical measurements were carried out when cells showed little evidence of primary pump activity, thus simplifying analyses. Under these conditions, outward-directed K(+) channel current contributed appreciably to charge balance maintainingV m, and adding 10 mM TEA to block the current depolarized (positive-going)V m; TEA also reduced(86)Rb(+) efflux by 68-80%. Following treatments with 10 μM FC, both K(+) channel current and(86)Rb(+) efflux decayed, irreversbly and without apparent lag, to 10%-15% of the controls and with equivalent half-times (approx. 4 min). Fusicoccin also enhanced(86)Rb(+) influx by 13.9-fold, but the influx proved largely insensitive to TEA. Overall, FC promotednet cation uptake in 0.1 mM K(+) (Rb(+)), despite membrane potentials which were 30-60 mVpositive of the K(+) equilibrium potential. These results tentatively link (chemical) cation efflux to charge movement through the K(+) channels. They offer evidence of an energy-coupled mechanism for K(+) uptake in guard cells. Finally, the data reaffirm early suspicions that FC alters profoundly the K(+) transport capacity of the cells, independent of any changes in membrane potential.

Photoaffinity labeling and partial purification of the putative plant receptor for the fungal wilt-inducing toxin, fusicoccin

The high-affinity fusicoccin-binding protein (FCBP) was solubilized from plasma-membrane vesicles prepared from leaves of Vicia faba L. by aqueous two-phase partitioning. Conditions for the solubilization of intact FCBP-radioligand complexes were worked out. About 60-70% of the complexes can be solubilized with 50-60 mM nonanoyl-N-methylglucamide in the presence of 1 mg· ml(-1) soybean phosphatidylcholine, type IV S, and 20% (v/v) glycerol at pH 5.5. The slow dissociation of the radioligand, 9'-nor-fusicoccin-8'-alcohol-[(3)H] from the FCBP at low temperatures permits the purification of FCBP-radioligand complexes at 4-10° C by fast protein liquid chromatography on anion-exchange and gel permeation columns. The FCBP, extracted from plasma membranes with cholate and chromatographed in the presence of this detergent, gave an apparent molecular mass (Mr) of 80±20 kDa on gel permeation columns under the conditions used. By comparison of the elution profiles of the fraction most enriched in FCBP-radioligand complexes with polypeptide patterns obtained on sodium dodecyl sulfate-polyacrylamide gels, a polypeptide with an Mr of approx. 34kDa co-separated with the radioactivity profile. A second, faint band of approx. 31 kDa was sometimes also observed co-electrophoresing. Photoaffinity labeling of plasma-membrane vesicles with the new compound 9'-nor-8'[(3,5-[(3)H]-4-azidobenzoy)ethylenediamine]-fusicoccin ([(3)H]ABE-FC) and subsequent separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis labeled a single band with an Mr of 35±1 kDa. Labeling in this band was strongly reduced when the membranes were incubated with [(3)H]ABE-FC in the presence of 0.1-1 μM fusicoccin. From our data, we conclude (i) that the 34-35-kDa polypeptide represents the FCBP and (ii) that in detergent extracts of plasma membranes this polypeptide is probably present as a di- or trimeric structure.