Ca(2+)-dependent activation of guard cell anion channels, triggered by hyperpolarization, is promoted by prolonged depolarization.

Rapid stomatal closure is driven by the activation of S-type anion channels in the plasma membrane of guard cells. This response has been linked to Ca(2+) signalling, but the impact of transient Ca(2+) signals on S-type anion channel activity remains unknown. In this study, transient elevation of the cytosolic Ca(2+) level was provoked by voltage steps in guard cells of intact Nicotiana tabacum plants. Changes in the activity of S-type anion channels were monitored using intracellular triple-barrelled micro-electrodes. In cells kept at a holding potential of -100 mV, voltage steps to -180 mV triggered elevation of the cytosolic free Ca(2+) concentration. The increase in the cytosolic Ca(2+) level was accompanied by activation of S-type anion channels. Guard cell anion channels were activated by Ca(2+) with a half maximum concentration of 515 nm (SE = 235) and a mean saturation value of -349 pA (SE = 107) at -100 mV. Ca(2+) signals could also be evoked by prolonged (100 sec) depolarization of the plasma membrane to 0 mV. Upon returning to -100 mV, a transient increase in the cytosolic Ca(2+) level was observed, activating S-type channels without measurable delay. These data show that cytosolic Ca(2+) elevation can activate S-type anion channels in intact guard cells through a fast signalling pathway. Furthermore, prolonged depolarization to 0 mV alters the activity of Ca(2+) transport proteins, resulting in an overshoot of the cytosolic Ca(2+) level after returning the membrane potential to -100 mV.