Ca²⁺-Permeable, Outwardly-Rectifying K⁺ Channels in Mesophyll Cells of Arabidopsis thaliana

Arabidopsis thaliana has become a powerful tool in genetics and molecular biology. In order to use Arabidopsis as a model system for electrophysiological studies on plant cells, a detailed characterization of the transporters present in the plasma and vacuolar membranes of this species is required. We used the patch-clamp technique to study ion channels in the plasma membrane of Arabidopsis mesophyll cells. The most prominent conductance in these cells was a K⁺-selective, voltage-dependent, outwardly-rectifying channel (I_k,out). In the whole-cell configuration, I_K,out was observed in 100% of the cells assayed. In contrast, inward current was observed in less than 50% of the cells which were bathed in 100mM K⁺, and was totally absent from cells bathed in 10 mM K⁺. The activation kinetics of I_k,out were modulated by the external K⁺ concentration with a faster activation at low external K⁺. Tail-current analysis revealed that in addition to K⁺, I_K,out is also permeable to Ca²⁺ and Ba²⁺. Externally applied Ba²⁺ also caused a voltage-dependent decrease in current magnitude, indicating that I_K,out is also partially blocked by this classic K⁺ channel blocker. Single channels studied in outside-out patches showed Ca²⁺ and Ba²⁺ sensitivity, voltage dependence and time activation similar to that of I_K,out in the whole-cell configuration. Given their permeability to Ca²⁺, these channels may function as an avenue for Ca²⁺ influx as well as K⁺ efflux, both of which may affect photosynthesis.