Inositol 1,4,5-trisphosphate (InsP3) is an important component of calcium-based signal transduction pathways in eukaryotic cells. Hydrolysis of phosphatidylinositol 4,5-bisphosphate by phosphoinositide-specific phospholipase C (PI-PLC) generates InsP3, which mobilizes calcium from endomembrane stores, and diacylglycerol. We have used a combined molecular and cell physiological approach to examined the role of the PI-PLC/InsP3-mediated calcium mobilizing pathway in the signal transduction pathway(s) by which stomatal guard cells respond to environmental stimuli such as abscisic acid (ABA) and drought. We have identified a guard cell PI-PLC that is specifically inhibited by an aminosteroid, U-73122; ABA-induced stomatal closure and ABA-induced oscillations in guard cell cytosolic free calcium are both inhibited by U-73122 (Staxen et al., 1999, Proc. Natl. Acad. Sci. USA 96 1779-1784). In addition, microinjection of the competitive InsP3 inhibitor heparin directly into the cytosol of guard cells also inhibits the ABA-induced reduction in guard cell turgor. These data, together with the results of our studies using Nicotiana rustica plants transformed with sense and antisense constructs of the PI-PLC, suggest a role for the PI-PLC/InsP3-mediated calcium-mobilizing pathway in the response of stomata to ABA and drought.