Figure 1.

Acid activation of a guard cell inward rectifying K⁺ channel in vivo and in vitro. (A) In the whole-cell configuration guard cell protoplasts from Solanum tuberosum were clamped at −63 mV to elicit inward K⁺ currents during subsequent 3-s pulses to −180 mV. (B) Two-electrode voltage-clamp recordings of inward K⁺ currents through kst1 expressed in Xenopus oocytes. With the oocyte clamped at −20 mV KST1-specific inward K⁺ currents were induced by membrane hyperpolarization to −170 mV. In contrast to KAT1 an artificial acidification of the cytoplasm of oocytes using acetate did not affect the gating behavior of KST1 (data not shown). In both systems channel activity of the inward rectifier increased with the extracellular proton concentration (pH 7.4–5.6). (C) Open–closed transitions of single inward K⁺ channels in cell-free outside–out membrane patches from isolated protoplasts exposed to pH 5.6 and 7.4. Single channels were recorded between t = 100 and 300 ms during steps to voltages indicated from a holding potential of −46 mV. Note that open-channel amplitudes (o₁) were identical for both proton concentrations. (D) pH- and voltage-dependent open probability of the K⁺ uptake channel from one representative guard cell protoplast. Data points were fitted by Boltzmann functions (po = N⋅{1 + exp[(V − V½)/V_s]}⁻¹) and scaled to the value N obtained in pH 4.5. The activation threshold shifts to less negative potentials with a decrease in pH (♦, pH 7.4; ▾, pH 6.5; •, pH 5.6; ▴, pH 4.5).