ABA activates multiple Ca(2+) fluxes in stomatal guard cells, triggering vacuolar K(+)(Rb(+)) release

The mechanisms by which abscisic acid (ABA) activates the release of K(+)(Rb(+)) from the vacuole of stomatal guard cells, a process essential for ABA-induced stomatal closure, have been investigated by tracer flux measurements. The form and timing of the ABA-induced efflux transient could be manipulated by treatments that alter three potential Ca(2+) fluxes into the cytoplasm, the influx from the outside and two pathways of internal release, those dependent on phospholipase C (inhibited by U73122) and cyclic ADP-ribose (inhibited by nicotinamide). Ba(2+), acting as a competitive inhibitor of Ca(2+) influx but also as an inhibitor of internal release, was an effective inhibitor of the transient. The results suggest that a threshold level of cytoplasmic Ca(2+) is required for the initiation of the minimal efflux transient after a lag period and with a low rate of rise. As conditions improve for the generation of an efflux transient (higher ABA or reduced Ba(2+)), a second threshold is crossed, generating a transient with zero lag and rapid rate of rise. This may reflect different Ca(2+) levels required for activation of different tonoplast K(+) channels. In this state, at high ABA, the transient is inhibited by removal of external Ca(2+), suggesting Ca(2+) influx makes a major contribution to increase in cytoplasmic Ca(2+). By contrast, at low ABA, the transient is not inhibited by removal of external Ca(2+) but is sensitive to either U73122 or nicotinamide, suggesting internal release makes the major contribution, involving both pathways. ABA appears to activate all three processes, and their relative importance depends on conditions.