Membrane transport in stomatal guard cells: the importance of voltage control.

Potassium uptake and export in the resting conditions and in response to the phytohormone abscisic acid (ABA) were examined under voltage clamp in guard cells of Vicia faba L. In 0.1 mM external K+ (with 5 mM Ca2(+)-HEPES, pH 7.4) two distinct transport states could be identified based on the distribution of the free-running membrane voltage (VM) data in conjunction with the respective I-V and G-V relations. One state was dominated by passive diffusion (mean VM = -143 +/- 4 mV), the other (mean VM = -237 +/- 10 mV) exhibited an appreciable background of primary H+ transport activity. In the presence of pump activity the free-running membrane voltage was negative of the respective K+ equilibrium potential (EK+), in 3 and 10 mM external K+. In these cases VM was also negative of the activation voltage for the inward rectifying K+ current, thus creating a strong bias for passive K+ uptake through inward-rectifying K+ channels. In contrast, when pump activity was absent VM was situated positive of EK+ and cells revealed a bias for K+ efflux. Occasionally spontaneous voltage transitions were observed during which cells switched between the two states. Rapid depolarizations were induced in cells with significant pump activity upon adding 10 microM ABA to the medium. These depolarizations activated current through outward-rectifying K+ channels which was further amplified in ABA by a rise in the ensemble channel conductance. Current-voltage characteristics recorded before and during ABA treatments revealed concerted modulations in current passage through at least four distinct transport processes, results directly comparable to one previous study (Blatt, M.R., 1990, Planta 180:445) carried out with guard cells lacking detectable primary pump activity. Comparative analyses of guard cells in each case are consistent with depolarizations resulting from the activation of an inward-going, as yet unidentified current, rather than an ABA-induced fall in H(+)-ATPase output. Also observed in a number of cells was an inward-directed current which activated in ABA over a narrow range of voltages positive of -150 mV; this and additional features of the current suggest that it may reflect the ABA-dependent activation of an anion channel previously characterized in Vicia guard cell protoplasts, but rule out its function as the primary mechanism for initial depolarization.(ABSTRACT TRUNCATED AT 400 WORDS