Abscisic Acid-Induced Phosphoinositide Turnover in Guard Cell Protoplasts of Vicia faba

Guard cell protoplasts of Vicia faba treated with 10 μM (+)abscisic acid (ABA) in the light exhibited a 20% decrease in diameter within 1.5 h, from 24.1 to 19.6 μm. Within 10 s of administration of ABA, a 90% increase in levels of inositol 1,4,5-trisphosphate was observed, provided that cells were treated with Li+, an inhibitor of inositol phosphatase activity, prior to incubation. Concomitantly, levels of 32P-labeled phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate decreased 20% compared to levels in control cells; levels of label in the membrane lipids phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol did not change significantly in response to ABA treatment. These results show that phosphoinositide turnover is activated in response to ABA in guard cells. We conclude that phosphoinositide signaling is likely to be a step in the biochemical cascade that couples ABA to guard cell shrinking and stomatal closure.