Effects of abscisic acid on K+ channels in Vicia faba guard cell protoplasts

The effect of water potential on accumulation of some essential elements in sugarbeet leaves, Beta vulgaris ssp. vulgaris

An investigation has been conducted on the effect of reduced water potential in nutrient solution on the accumulation of some essential macro- and micro nutrients in the aboveground pails of young sugarbeet plants. Plants of 8 different sugarbeet genotypes were exposed for 21 days to a nutrient solution whose water potential of 0.1 MPa was regulated by PEG. Contents of N, P, K Ca, Mg, Fe, Mn, Cu and Zn declined in all genotypes under water deficiency but the intensity of reduction varied among the genotypes. The results indicated that some harmful effects of water deficiency could be attributed to disturbances in plant mineral nutrition, especially the lack of N, P, and Mg, as well as to impaired ratios between the contents of particular elements, especially K/Ca.

pH, abscisic acid and the integration of metabolism in plants under stressed and non-stressed conditions: cellular responses to stress and their implication for plant water relations

A paradigm for the response of plants to stress is presented which suggests that plants move towards a state of minimal metabolic activity as a stress intensifies and remain in that state until that stress is relieved. The paradigm is based on the proposition that cells that interface with the transpiration stream employ variations on the following theme to move towards that state. Tension on the apoplastic water opens a mechanosensitive Ca2+ channel, a response that is augmented by apoplastic ABA. The resulting elevated cytoplasmic Ca2+ deactivates a plasmalemma H+/ATPase and also activates a K(+)-H+ symport. The inflow of K+ and H+ depolarizes the membrane and renders the apoplast less acidic, the protons being removed to the vacuole and the K+ ions being re-exported via the K+ outward rectifying channel. The onset of darkness in guard and mesophyll cells deactivates the plasmalemma H+/ATPase and then the events outlined above ensue except that these cells do not appear to utilize either Ca2+ or ABA during these changes. In stressed cells it is proposed that elevated cytoplasmic Ca2+ activates the release of an ABA precursor from a stored form. ABA is then released in the apoplast after export of the precursor if the activity of the K(+)-H+ symport has brought the apoplastic pH close to 7.0. It is proposed that aquaporins in the xylem parenchyma and mesophyll cells are opened by elevated cytoplasmic Ca2+ when the water potential of the transpiration stream is high so that water can be stored in the 'xylem parenchyma reservoir'. The water in this reservoir is then used to increase the water potential in the transpiration stream when the water column is under tension and to help repair embolisms by a mechanism that resembles stomatal closure.

External pH effects on the depolarization-activated K channels in guard cell protoplasts of Vicia faba

Previous studies reveal that the pH of the apoplastic solution in the guard cell walls may vary between 7.2 and 5.1 in closed and open stomata, respectively. During these aperture and pH changes, massive K+ fluxes cross the cellular plasma membrane driving the osmotic turgor and volume changes of guard cells. Therefore, we examined the effect of extracellular pH on the depolarization-activated K channels (KD channels), which constitute the K+ efflux pathway, in the plasma membrane of Vicia faba guard cell protoplasts. We used patch clamp, both in whole cells as well as in excised outside-out membrane patches. Approximately 500 KD channels, at least, could be activated by depolarization in one protoplast (density: approximately 0.6 micron-2). Acidification from ph 8.1 to 4.4 decreased markedly the whole-cell conductance, GK, of the KD channels, shifted its voltage dependence, GK-EM, to the right on the voltage axis, slowed the rate of activation and increased the rate of deactivation, whereas the single channel conductance was not affected significantly. Based on the GK-EM shifts, the estimated average negative surface charge spacing near the KD channel is 39 A. To quantify the effects of protons on the rates of transitions between the hypothesized conformational states of the channels, we fitted the experimental macroscopic steady state conductance-voltage relationship and the voltage dependence of time constants of activation and deactivation, simultaneously, with a sequential three-state model CCO. In terms of this model, protonation affects the voltage-dependent properties via a decrease in localized, rather than homogeneous, surface charge sensed by the gating moieties. In terms of either the CO or CCO model, the protonation of a site with a pKa of 4.8 decreases the voltage-independent number of channels, N, that are available for activation by depolarization.

Changes in cytosolic pH and calcium of guard cells precede stomatal movements

Stomatal opening is induced by indoleacetic acid (IAA), cytokinins, and fusicoccin (FC), whereas stomatal closure is induced by abscisic acid (ABA). To test the effect of these growth regulators on guard cell cytosolic Ca2+ ([Ca2+]cyt) and pH (pHcyt), epidermal strips were taken from the lower side of leaves of the orchid Paphiopedilum tonsum and were loaded with acetomethoxy-esterified forms of the Ca2+ indicator fluo-3 or the pH indicator 2',7'-bis(2-carboxyethyl)-5(6)carboxyfluorescein. Basal [Ca2+]cyt ranged from 0.05 to 0.3 M and was 0.22 +/- 0.015 (n = 21). Increases in both [Ca2+]cyt and pHcyt were observed in guard cells after application of 10-100 M ABA to open stomata, and these preceded stomatal closure. The increase in [Ca2+]cyt ranged from 1.5- to 3-fold and was seen in 7 of 10 experiments. Guard cell alkalinization began within 2 min of ABA treatment and continued for the next 8 min. The increase ranged from 0.04 to 0.3 pH unit and was seen in 13 of 14 experiments. Guard cell [Ca2+]cyt increased, whereas pHcyt decreased after treatment of closed stomata with IAA, kinetin, or FC. In response to 50-100 M IAA, [Ca2+]cyt increased 1.5- to 2-fold in all cases, and pHcyt decreased 0.2-0.4 unit within 5 min in 7 experiments. Within 12 min, 10-100 M kinetin caused [Ca2+]cyt to increase in 28 of 34 experiments (1.3- to 2.5-fold) and pHcyt fell 0.1-0.4 unit in 15 of 17 treatments. The response to 10-50 M FC was similar in both time and magnitude. These results show that stomatal opening is accompanied by an increase in [Ca2+]cyt and cytosolic acidification in the guard cells, whereas stomatal closure is preceded by an increase in [Ca2+]cyt and cytosolic alkalinization in the guard cells. The order of these events is still uncertain, but changes in pHcyt are correlated with stomatal movement, and these changes may be an important factor in the regulation of guard cell movement.

Mechanisms of action of abscisic acid at the cellular level

Abscisic acid (ABA) has been implicated in the control of a diverse range of physiological processes in higher plants. In this review, we focus on the events which constitute the cellular responses to ABA. Current evidence suggests that it is possible to classify the responses to ABA on the basis of whether they are rapid, involving ion fluxes (typified by the stomatal response), or slower and requiring alterations to gene expression (for example the response of cereal embryos to ABA). In our consideration of ABA stimulus response coupling pathways, we have chosen to highlight the role of the calcium ion in the rapid responses, while we have concentrated on the contribution of as-acting elements and trans-acting factors in the regulation of ABA-responsive genes. We also draw attention to the possibility that interaction may exist between these pathways. Additionally, we discuss the controls of ABA concentrations during development and in response to environmental stimuli. Factors which contribute to the controls of ABA sensitivity are also reviewed. In our conclusions, we suggest that a general role for ABA may be to prepare tissue for entry into a new and different physiological state, perhaps by resetting the direction of cellular metabolism. CONTENTS Summary 9 I. Introduction 10 II. Stimulus response coupling 17 Synopsis 27 Acknowledgements 28 References 28.

Whole-cell and single-channel currents across the plasmalemma of corn shoot suspension cells

Whole-cell sealed-on pipettes have been used to measure electrical properties of the plasmalemma surrounding protoplasts isolated from Black Mexican sweet corn shoot cells from suspension culture. In these protoplasts the membrane resting potential (Vm) was found to be -59 +/- 23 mV (n = 23) in 1 mM Ko+. The mean Vm became more negative as [K+]o decreased, but was more positive than the K+ equilibrium potential. There was no evidence of electrogenic pump activity. We describe four features of the current-voltage characteristic of the plasmalemma of these protoplasts which show voltage-gated channel activity. Depolarization of the whole-cell membrane from the resting potential activates time- and voltage-dependent outward current through K(+)-selective channels. A local minimum in the outward current-voltage curve near Vm = 150 mV suggests that these currents are mediated by two populations of K(+)-selective channels. The absence of this minimum in the presence of verapamil suggests that the activation of one channel population depends on the influx of Ca2+ into the cytoplasm. We identify unitary currents from two K(+)-selective channel populations (40 and 125 pS) which open when the membrane is depolarized; it is possible that these mediate the outward whole-cell current. Hyperpolarization of the membrane from the resting potential produces time- and voltage-dependent inward whole-cell current. Current activation is fast and follows an exponential time course. The current saturates and in some cases decreases at membrane potentials more negative than -175 mV. This current is conducted by poorly selective K+ channels, where PCl/PK = 0.43 +/- 0.15. We describe a low conductance (20 pS) channel population of unknown selectivity which opens when the membrane is hyperpolarized. It is possible that these channels mediate inward whole-cell current. When the membrane is hyperpolarized to potentials more negative than -250 mV large, irregular inward current is activated. A third type of inward whole-cell current is briefly described. This activates slowly and with a U-shaped current-voltage curve over the range of membrane potentials -90 less than Vm less than 0 mV.

Cytosolic calcium regulates a potassium current in corn (Zea mays) protoplasts

The voltage- and time-dependent K+ current, IK+ out, elicited by depolarization of corn protoplasts, was inhibited by the addition of calcium channel antagonists (nitrendipine, nifedipine, verapamil, methoxyverapamil, bepridil, but not La3+) to the extracellular medium. These results suggested that the influx of external Ca2+ was necessary for K+ current activation. The IC50, concentration of inhibitor that caused 50% reduction of the current, for nitrendipine was 1 microM at a test potential of +60 mV following a 20-min incubation period. In order to test whether intracellular Ca2+ actuated the K+ current, we altered either the Ca2+ buffering capacity or the free Ca2+ concentration of the intracellular medium (pipette filling solution). By these means, IK+out could be varied over a 10-fold range. Increasing the free Ca2+ concentration from 40 to 400 nM also shifted the activation of the K+ current toward more negative potentials. Maintaining cytoplasmic Ca2+ at 500 nM with 40 nM EGTA resulted in a more rapid activation of the K+ current. Thus the normal rate of activation of this current may reflect changes in cytoplasmic Ca2+ on depolarization. Increasing intracellular Ca2+ to 500 nM or 1 microM also led to inactivation of the K+ current within a few minutes. It is concluded that IK+out is regulated by cytosolic Ca2+, which is in turn controlled by Ca2+ influx through dihydropyridine-, and phenylalkylamine-sensitive channels.

Pharmacology of the Ca2(+)-dependent K+ channel in corn protoplasts

We investigated the sensitivity of the Ca2(+)-dependent K+ current, IK(Ca), present in corn protoplasts, to different K+ channel blockers. IK(Ca) was inhibited by external Cs+ (10 mM), Ba2+ (10 mM), and quinine (0.5 mM): reagents which block many types of outward-rectifying K+ channels. In contrast 4-aminopyridine (5 mM), an inhibitor of delayed rectifier or inactivating K+ currents, had no effect. Neither of the peptide toxins, apamin or charybdotoxin, specific for Ca2(+)-dependent K+ channels in animal cells, inhibited currents when used in the nanomolar concentration range. However, higher levels of charybdotoxin (10 microM) caused marked reduction of IK(Ca).

Cytoplasmic calcium affects the gating of potassium channels in the plasma membrane ofChara corallina: a whole-cell study using calcium-channel effectors

The action of a wide range of drugs effective on Ca(2+) channels in animal tissues has been measured on Ca(2+) channels open during the action potential of the giant-celled green alga,Chara corallina. Of the organic effectors used, only the 1,4-dihydropyridines were found to inhibit reversibly Ca(2+) influx, including, unexpectedly, Bay K 8644 and both isomers of 202-791. Methoxyverapamil (D-600), diltiazem, and the diphenylbutylpiperidines, fluspirilene and pimozide were found not to affect the Ca(2+) influx. Conversely, bepridil greatly and irreversibly stimulated Ca(2+) influx, and with time, stopped cytoplasmic streaming (which is sensitive to increases in cytoplasmic Ca(2+)). By apparently altering the cytoplasmic Ca(2+) levels with various drugs, it was found that (with the exception of the inorganic cation, La(3+)) treatments likely to lead to an increase in cytoplasmic Ca(2+) levels caused an increase in the rate of closure of the K(+) channels. Similarly, treatments likely to lead to a decrease in cytoplasmic Ca(2+) decreased the rate of K(+) channel closure. The main effect of bepridil on the K(+) channels was to increase the rate of voltage-dependent channel closure. The same effect was obtained upon increasing the external concentration of Ca(2+), but it is likely that this was due to effects on the external face of the K(+) channel. Addition of any of the 1,4-dihydropyridines had the opposite effect on the K(+) channels, slowing the rate of channel closure. They sometimes also reduced K(+) conductance, but this could well be a direct effect on the K(+) channel; high concentrations (50 to 100 μM) of bepridil also reduced K(+) conductance. No effect of photon irradiance or of abscisic acid could be consistently shown on the K(+) channels. These results indicate a control of the gating of K(+) channels by cytoplasmic Ca(2+), with increased free Ca(2+) levels leading to an increased rate of K(+)-channel closure. As well as inhibiting Ca(2+) channels, it is suggested that La(3+) acts on a Ca(2+)-binding site of the K(+) channel, mimicking the effect of Ca(2+) and increasing the rate of channel closure.

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.

Calcium influx at the plasmalemma of isolated guard cells of Commelina communis : Effects of abscisic acid

The influx of (45)Ca into isolated guard cells of Commelina communis L. has been measured, using short uptake times, and washing in ice-cold La(3+)-containing solutions to remove extracellular tracer after the loading period. Over 0.5-4 min the uptake was linear with time, through the origin. Over 20-200μM external Ca(2+) the influx measured with 10-20 mM external KCl was in the range 0.3-2.3 pmol·cm(-2)·s(-1) (on the basis of estimated guard-cell area); with only 1 mM KCl externally the (45)Ca influx was significantly reduced, in the range 0.3-1.1 pmol·cm(-2)·s(-1) for external Ca(2+) of 50-100 μM. The results indicate that the Ca-channel is voltage-sensitive, opening with depolarisation. No consistent effect of the addition of abscisic acid could be found. In different experiments, on the addition of 0.1 mM abscisic acid the Ca(2+) influx was sometimes stimulated by 28-79%, was sometimes unaffected, and was sometimes inhibited by 16-29%. The results rule out a long-lasting stimulation of (45)Ca influx by ABA, but they do not rule out a transient stimulation followed by inhibition, perphaps as a consequence of down-regulation of Ca(2+) influx by increasing cytoplasmic Ca(2+). The hypothesis that ABA may act via an action on Ca(2+) influx, increasing cytoplasmic Ca(2+), with consequent effects on voltage-dependent and Ca(2+)-dependent ion channels in both plasmalemma and tonoplast, is neither proved nor disproved by these results.

Channel-mediated K(+) flux in barley aleurone protoplasts

Gibberellic acid (GA3) stimulates K(+) efflux from the barley (Hordeum vulgare L. cv. Himalaya) aleurone. We investigated the mechanism of K(+) flux across the plasma membrane of aleurone protoplasts using patch-clamp techniques. Potassium-ion currents, measured over the entire surface of the protoplast plasma membrane, were induced when the electrochemical gradient for K(+) was inward (into the cytoplasm). The magnitude and voltage-dependence of this inward current were the same in protoplasts treated with GA3 and in control protoplasts (no GA3). Inward currents activated by negative shifts in the membrane potential (EM) from the Nernst potential for K(+) (EK) showed membrane conductance to be a function of the electrochemical gradient (i.e. EM-EK). Single-channel influx currents of K(+) were recorded in small patches of the plasma membrane. These channels had a single-channel conductance of 5-10 pS with 100 mM K(+) on the inside and 10 mM K(+) on the outside of the plasma membrane. Single-channel currents, like whole-cell currents, were the same in protoplasts treated with GA3 and control protoplasts. Voltage-gated efflux currents were found only in protoplasts tha thad been incubated without GA3. We conclude that K(+) influx in the aleurone is mediated by channels and these membrane proteins are not greatly effected by GA3.