Cation sensitivity and kinetics of guard-cell potassium channels differ among species
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hannel, GCKC1in, 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+ channels of the same cell type differ among species. The channels shared basic features including voltage-dependence, selectivity and single-channel conductance. They activated at hyperpolarization (V1/2 ≈ −164 mV) with single channels of 7 pS underlying the whole-cell current. The channel density in S. tuberosum was higher than in V. faba and N. tabacum while the activation and deactivation kinetics were faster in the latter two species. Among different monovalent cations the K+ channels discriminated strongly against Na+, Li+, and Cs+. The sensitivity to Cs+ was similar for the three species. Extracellular Ca2+ blocked the V.␣faba K+ channel at concentrations ≥1 mM but only affected its functional homologs in S. tuberosum and N.␣tabacum at higher concentrations and more-negative membrane potentials. Like the differences in Ca2+-sensitivity, protoplasts from the three species differed remarkably in their response towards extracellular pH changes. Whereas protons neither altered the open probability nor the kinetic parameters of the V. faba GCKC1in, in S. tuberosum and N. tabacum this cation affected the voltage-dependent properties strongly. An increase in proton concentration from pH 8.5 to 4.5 shifted the potential of half-maximal open probability to less-negative values with a maximum effect around pH 6.2. The pH modulation of the K+ channels 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 species-specific variations in the control of stomatal aperture.
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