CATION SENSITIVITY AND KINETICS OF GUARD-CELL POTASSIUM CHANNELS DIFFER AMONG SPECIES

In patch-clamp studies we compared the electrical properties of an inw ard rectifying guard cell K+ channel, GCKCl(in), from three major crop plants Solanum tuberosum L., Nicotiana tabacum L., and Vicia faba L. Selecting guard cells for our analyses we aimed to test whether K+ cha nnels of the same cell type differ among species. The channels shared basic features including voltage-dependence, selectivity and single-ch annel conductance. They activated at hyperpolarization (V-1/2 approxim ate to -164 mV) with single channels of 7 pS underlying the whole-cell current. The channel density in S. tuberosum was higher than in V. fa ba and N. tabacum while the activation and deactivation kinetics were faster in the latter two species. Among different monovalent cations t he K+ channels discriminated strongly against Na+. Li+, and Cs+. The s ensitivity to Cs+ was similar for the three species. Extracellular Ca2 + blocked the V. faba K+ channel at concentrations greater than or equ al to 1 mM but only affected its functional homologs in S. tuberosum a nd N. tabacum at higher concentrations and more-negative membrane pote ntials. Like the differences in Ca2+-sensitivity, protoplasts from the three species differed remarkably in their response towards extracell ular pH changes. Whereas protons neither altered the open probability nor the kinetic parameters of the V. faba GCKCl(in), in S. tuberosum a nd N. tabacum this cation affected the voltage-dependent properties st rongly. An increase in proton concentration from pH 8.5 to 4.5 shifted the potential of half-maximal open probability to less-negative value s with a maximum effect around pH 6.2. The pH modulation of the K+ cha nnels could be described assuming a two-state model where the open and closed channel can be protonated. The observed differences in cation- sensitivity and voltage-dependent kinetics between K+ channels reflect the diversification of guard-cell channels that may contribute to spe cies-specific variations in the control of stomatal aperture.