Kinetic analysis of the K⁺-selective outward rectifier in Arabidopsis mesophyll cells: A comparison with other plant species

This paper gives a kinetic analysis of the K⁺-selective outward-rectifier (I(K,out)) in the plasma membrane of Arabidopsis thaliana mesophyll cells in terms of the Hodgkin-Huxley formalism. We compared the kinetic characteristics of I(K,out) in Arabidopsis with I(K,out) channels in three other plant species that were subjected to a similar analysis: tobacco suspension cells, Vicia faba guard cells and Plantago media root cells. Because the activation kinetics of I(K,out) shows a clear voltage dependence, the time constant of half-activation (τ(1/2)) and the elementary rate constant of channel opening (α) were calculated at the potential of half-activation (V(1/2)). The Arabidopsis I(K,out) activates relatively slowly and this is reflected in a τ(1/2) of approximately 1 s. The reason for this slow activation is twofold. Firstly, the value of α of 1.5 s^-1 falls at the lower end of the range of values obtained for tobacco, Vicia and Plantago: 1.1 to 3.0 s^-1. Secondly, I(K,out) in Arabidopsis has four closed states, while tobacco and Vicia have only two. As observed in other plant species, the activation kinetics of I(K,out) in Arabidopsis are sensitive to external K⁺: V(1/2) shifts with E(K) but remains approximately 50 mV more positive than E(K).