Ca2+ channels at the plasma membrane of stomatal guard cells are activatedby hyperpolarization and abscisic acid

In stomatal guard cells of higher-plant leaves, abscisic: acid (ABA) evokes increases in cytosolic: free Ca2+ concentration ([Ca2+](i)) by means of Ca 2+ entry from outside and release from intracellular stores. The mechanism( s) for Ca2+ flux across the plasma membrane is poorly understood. Because [ Ca2+](i) increases are voltage-sensitive, we suspected a Ca2+ channel at th e guard cell plasma membrane that activates on hyperpolarization and is reg ulated by ABA. We recorded single-channel currents across the Vicia guard c ell plasma membrane using Ba2+ as a charge-carrying ion. Both cell-attached and excised-patch measurements uncovered single-channel events with a maxi mum conductance of 12.8 +/- 0.4 pS and a high selectivity for Ba2+ (and Ca2 +) over K+ and Cl-. Unlike other Ca2+ channels characterized to date, these channels rectified strongly toward negative voltages with an open probabil ity (P-o) that increased with [Ba2+] outside and decreased roughly 10-fold when [Ca2+](i) was raised from 200 nM to 2 mu M. Adding 20 mu M ABA increas ed P-o, initially by 63- to 260-fold; in both cell-attached and excised pat ches, it shifted the voltage sensitivity for channel activation, and evoked damped oscillations in P-o with periods near 50 s. A similar, but delayed response was observed in 0.1 mu M ABA. These results identify a Ca2+-select ive channel that can account for Ca2+ influx and increases in [Ca2+](i) tri ggered by voltage and ABA, and they imply a close physical coupling at the plasma membrane between ABA perception and Ca2+ channel control.