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 Ca2+ fluxes into the cytoplasm, the influx from the outside and two pathways of internal release, those dependent on phospholipase C (inhibited byU73122 ) and cyclic ADP-ribose (inhibited by nicotinamide). Ba2+ , acting as a competitive inhibitor of Ca2+ 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 Ca2+ 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 Ba2+ ), a second threshold is crossed, generating a transient with zero lag and rapid rate of rise. This may reflect different Ca2+ levels required for activation of different tonoplast K+ channels. In this state, at high ABA, the transient is inhibited by removal of external Ca2+ , suggesting Ca2+ influx makes a major contribution to increase in cytoplasmic Ca2+ . By contrast, at low ABA, the transient is not inhibited by removal of external Ca2+ but is sensitive to eitherU73122 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.