Membrane transport in stomatal guard cells: The importance of voltage control

Summary

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(V _M ) data in conjunction with the respective I-V and G-V relations. One state was dominated by passive diffusion (mean V _M = −143± 4 mV), the other (mean V _M = −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 (E ⁺_K ), in 3 and 10 mm external K⁺. In these cases V _M 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 V _M was situated positive of E ⁺_K 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 μm 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. The analyses also yield indirect evidence for a rise in cytoplasmic Ca²⁺ activity in ABA, as well as for a K⁺ current distinct from the dominant inward and outward-rectifying K⁺ channels, but neither support nor discount a role for Ca²⁺ influx in depolarizing the membrane. A striking similarity was found for the modulation of inward currents either in response to ABA or after spontaneous depolarizations. This renders the possibility of an agonist (i.e., ABA) activated Ca²⁺ current across the plasma membrane as trigger for the voltage transitions unlikely.