Synergistic effect of light and fusicoccin on stomatal opening: Epidermal peel and patch clamp experiments

Upon incubation of epidermal peels of Commelina communis in 1 millimolar KCl, a synergistic effect of light and low fusicoccin (FC) concentrations on stomatal opening is observed. In 1 millimolar KCl, stomata remain closed even in the light. However, addition of 0.1 micromolar FC results in opening up to 12 micrometers. The same FC concentration stimulates less than 5 micrometers of opening in darkness. The synergistic effect (a) decreases with increasing FC or KCl concentrations; (b) is dark-reversible; (c) like stomatal opening in high KCl concentrations (120 millimolar) is partially inhibited by the K⁺ channel blocker, tetraethylammonium⁺ (20 millimolar). In whole-cell patch-clamp experiments with guard cell protoplasts of Vicia faba, FC (1 or 10 micromolar) stimulates an increase in outward current that is essentially voltage independent between -100 and +60 millivolts, and occurs even when the membrane potential is held at a voltage (-60 millivolts) at which K⁺ channels are inactivated. These results are indicative of FC activation of a H⁺ pump. FC effects on the magnitude of inward and outward K⁺ currents are not observed. Epidermal peel and patch clamp data are both consistent with the hypothesis that the plasma membrane H⁺ ATPase of guard cells is a primary locus for the FC effect on stomatal apertures.