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

New Fava Bean Guard Cell Signaling Mutant Impaired in ABA-Induced Stomatal Closure

We isolated a mutant from Vicia faba L. cv. House Ryousai. It wilts easily under strong light and high temperature conditions, suggesting that its stomatal movement may be disturbed. We determined responses of mutant guard cells to some environmental stimuli. Mutant guard cells demonstrated an impaired ability to respond to ABA in 0.1 mM CaCl(2) and stomata did not close in the presence of up to 1 mM ABA, whereas wild-type stomata closed when exposed to 10 micro M ABA. Elevating external Ca(2+) caused a similar degree of stomatal closure in the wild type and the mutant. A high concentration of CO(2) (700 micro l liter(-1)) induced stomatal closure in the wild type, but not in the mutant. On the basis of these results, we propose the working hypothesis that the mutation occurs in the region downstream of CO(2) and ABA sensing and in the region upstream of Ca(2+) elevation. The mutant is named fia (fava bean impaired in ABA-induced stomatal closure).

Calcium-based signalling systems in guard cells

Calcium is a ubiquitous intracellular signal responsible for controlling numerous cellular processes in both plants and animals. As an example, Ca²⁺ has been shown to be a second messenger in the signal transduction pathways by which stomatal guard cells respond to external stimuli. Regulated increases in the cytosolic concentration of free calcium ions ([Ca²⁺ ]_cyt ) in guard cells have been observed to be a common intermediate in many of the pathways leading to either opening or closing of the stomatal pore. This observation has prompted investigations into how specificity is encoded in the Ca²⁺ signal. It has been suggested that the key to generating stimulus-specific calcium signatures lies in the ability to access differentially the cellular machinery controlling calcium influx and release from intracellular stores. Several important components of the calcium-based signalling pathways have been identified in guard cells including cADPR, phospholipase C-InsP₃ , InsP₆ and H₂ O₂ . These data suggest that the pathways for intracellular mobilization of Ca²⁺ are evolutionarily conserved between plants and animals. ABBREVIATIONS: ABA, abscisic acid; [Ca²⁺ ]_cyt , cytosolic free calcium concentration; [Ca²⁺ ]_ext , external calcium concentration; I_K,in ; inward-rectifying K⁺ currents; InsP₃ , inositol-1,4,5-trisphosphate; InsP₆ , inositol hexakisphosphate; PLC, phospholipase C; PLD, phospholipase D; PA, phosphatidic acid; H₂ O₂ , hydrogen peroxide; AAPK, ABA-activated serine-threonine protein kinase; cADPR, cyclic adenosine 5'-diphosphoribose; U73122, 1-(6-{[17â-3-methoxyestra-1,3,5(10)-trien-17-yl]amino}hexyl)-1H-pyrrole-2, 5-dione; RyR; ryanodine receptor; CICR; calcium-induced calcium-release; I_Ca , inward calcium current.

The characterization of differential calcium signalling in tobacco guard cells

Two novel approaches for the study of Ca2+-mediated signal transduction in stomatal guard cells are described. Stimulus-induced changes in guard-cell cytosolic Ca2+ ([Ca2+]cyt) were monitored using viable stomata in epidermal strips of a transgenic line of Nicotiana plumbaginifolia expressing aequorin (the proteinous luminescent reporter of Ca2+) and in a new transgenic line in which aequorin expression was targeted specifically to the guard cells. The results indicated that abscisic acid (ABA)-induced stomatal closure was accompanied by increases in [Ca2+]cyt in epidermal strips. In addition to ABA, mechanical and low-temperature signals directly affected stomatal behaviour, promoting rapid closure. Elevations of guard-cell [Ca2+]cyt play a key role in the transduction of all three stimuli. However, there were striking differences in the magnitude and kinetics of the three responses. Studies using Ca2+ channel blockers and the Ca2+ chelator EGTA further suggested that mechanical and ABA signals primarily mobilize Ca2+ from intracellular store(s), whereas the influx of extracellular Ca2+ is a key component in the transduction of low-temperature signals. These results illustrate an aspect of Ca2+ signalling whereby the specificity of the response is encoded by different spatial or kinetic Ca2+ elevations.

Ca2+ channels at the plasma membrane of stomatal guard cells are activated by hyperpolarization and abscisic acid

In stomatal guard cells of higher-plant leaves, abscisic acid (ABA) evokes increases in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) by means of Ca(2+) entry from outside and release from intracellular stores. The mechanism(s) for Ca(2+) flux across the plasma membrane is poorly understood. Because [Ca(2+)](i) increases are voltage-sensitive, we suspected a Ca(2+) channel at the guard cell plasma membrane that activates on hyperpolarization and is regulated by ABA. We recorded single-channel currents across the Vicia guard cell plasma membrane using Ba(2+) as a charge-carrying ion. Both cell-attached and excised-patch measurements uncovered single-channel events with a maximum conductance of 12.8 +/- 0.4 pS and a high selectivity for Ba(2+) (and Ca(2+)) over K(+) and Cl(-). Unlike other Ca(2+) channels characterized to date, these channels rectified strongly toward negative voltages with an open probability (P(o)) that increased with [Ba(2+)] outside and decreased roughly 10-fold when [Ca(2+)](i) was raised from 200 nM to 2 microM. Adding 20 microM ABA increased P(o), initially by 63- to 260-fold; in both cell-attached and excised patches, it shifted the voltage sensitivity for channel activation, and evoked damped oscillations in P(o) with periods near 50 s. A similar, but delayed response was observed in 0.1 microM ABA. These results identify a Ca(2+)-selective channel that can account for Ca(2+) influx and increases in [Ca(2+)](i) triggered by voltage and ABA, and they imply a close physical coupling at the plasma membrane between ABA perception and Ca(2+) channel control.

Measurements of Ca 2+ fluxes in intact plant cells

Owing to the central role of Ca 2+ in signal transduction processes, it is important to measure membrane fluxes of Ca 2+ in cells which are as undisturbed as possible, particularly when studying the control of these fluxes. To this end, techniques have been developed to measure Ca 2+ fluxes in intact, turgid plant cells. The measurements are principally of influx across the plasma membrane where Ca 2+ transport is likely to occur through cation-selective channels. The most direct method measures tracer fluxes of Ca 2+ , but special procedures are required to distinguish between influx and extracellular binding of Ca 2+ . Unfortunately, such techniques are currently only applicable to giant cells where surgical separation of the intracellular contents from the cell wall is possible. The influx of Ca 2+ into normal, resting cells of the green alga Chara corallina is usually about 0.3 nmol m -2 s -1 (at an external Ca 2+ concentration of 0.5 mol m -3 ). This flux is up to 5 times higher in actively growing cells, 20 times higher in cells depolarized by 20 mol m -3 K + and 1000 times higher during an action potential. Reducing cell turgor by a wide range of solutes increases Ca 2+ influx, especially near plasmolysis. Ca 2+ influx is sensitive to alterations in both external and cytosolic pH, but is inhibited by complete darkness and by low concentrations of La 3+ . Various organic Ca 2+ channel antagonists had mixed effects on Ca 2+ influx into Chara . The work described in this paper should enable further study of the control of Ca 2+ fluxes into intact, turgid plant cells, and their role in signal transduction and the control of cellular activities.

Two genes that encode Ca(2+)-dependent protein kinases are induced by drought and high-salt stresses in Arabidopsis thaliana

Two cDNA clones, cATCDPK1 and cATCDPK2, encoding Ca(2+)-dependent, calmodulin-independent protein kinases (CDPK) were cloned from Arabidopsis thaliana and their nucleotide sequences were determined. Northern blot analysis indicated that the mRNAs corresponding to the ATCDPK1 and ATCDPK2 genes are rapidly induced by drought and high-salt stress but not by low-temperature stress or heat stress. Treatment of Arabidopsis plants with exogenous abscisic acid (ABA) had no effect on the induction of ATCDPK1 or ATCDPK2. These findings suggest that a change in the osmotic potential of the environment can serve as a trigger for the induction of ATCDPK1 and ATCDPK2. Putative proteins encoded by ATCDPK1 and ATCDPK2 which contain open reading frames of 1479 and 1488 bp, respectively, are designated ATCDPK1 and ATCDPK2 and show 52% identity at the amino acid sequence level. ATCDPK1 and ATCDPK2 exhibit significant similarity to a soybean CDPK (51% and 73%, respectively). Both proteins contain a catalytic domain that is typical of serine/threonine protein kinases and a regulatory domain that is homologous to the Ca(2+)-binding sites of calmodulin. Genomic Southern blot analysis suggests the existence of a few additional genes that are related to ATCDPK1 and ATCDPK2 in the Arabidopsis genome. The ATCDPK2 protein expressed in Escherichia coli was found to phosphorylate casein and myelin basic protein preferentially, relative to a histone substrate, and required Ca2+ for activation.

Blockers of Ca2+ channels in the plasmalemma of perfused Characeae cells

Ionic currents in the plasmalemma of perfused Nitella syncarpa cells identified as currents through Ca2+ channels were registered for the first time. The effect of 1,4-dihydropyridine derivatives (nifedipine, nitrendipine, riodipine) and phenylalkylamines (verapamil, D600) as well as the agonist CGP-28392 on the Ca2+ channels in the plasmalemma of perfused cells of Nitellopsis obtusa and Nitella syncarpa have been studied. A blocking effect of 1,4-dihydropyridine derivatives and phenylalkylamines on the plasmalemma Ca2+ channels has been detected. Phenylalkylamines have been found to block both inward and outward Ca2+ currents. The activating effect of the agonist CGP-28392 on the Ca2+ channels of plasmalemma has been shown.

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.