Possible involvement of calmodulin in the regulation of ATPase activity in guard cells

Calcium ions play an important role in the regulation of stomatal movement and the mechanism underlying this action is yet to be determined. It is suggested that guard cell plasma membrane ATPase is a target for calcium action and that this effect is mediated by calmodulin. In this study, the effects of calcium and two calmodulin antagonists on ATPase activity in a crude homogenate of Commelina communis L. guard cell protoplasts were examined. The homogenate contained Mg²⁺-dependent, K⁺-simulated ATPase activity, which was inhibited by CaCl² while stimulated by the calmodulin antagonists, compound 48/80 and chlorpromazine. The calmodulin antagonists partially reversed the inhibitory effect of calcium ions. The results support the possibility of calmodulin involvement in the regulation of guard cell ATPase activity by calcium ions.