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 (IK,out) in the plasma membrane of Arabidopsis thaliana mesophyll cells in terms of the Hodgkin-Huxley formalism. We compared the kinetic characteristics of IK,out in Arabidopsis with IK,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 IK,out shows a clear voltage dependence, the time constant of half-activation (tau 1/2) and the elementary rate constant of channel opening (a) were calculated at the potential of half-activation (V1/2). The Arabidopsis IK,out activates relatively slowly and this is reflected in a tau 1/2 of approximately 1 s. The reason for this slow activation is twofold. Firstly, the value of a is 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, IK,out in Arabidopsis has four closed states, while tobacco and Vicia have only two. As observed in other plant species, the activation kinetics of IK,out in Arabidopsis are sensitive to external K+: V1/2 shifts with EK but remains approximately 50 mV more positive than EK.