Ion Channels in Arabidopsis Plasma Membrane : Transport Characteristics and Involvement in Light-Induced Voltage Changes

Abstract

White light (25 watts per square meter) induced an increase in plasma membrane K+-channel activity and a 30- to 70-millivolt transient membrane depolarization (completed in 2-3 minutes) in Arabidopsis thaliana leaf mesophyll cells. Transport characteristics of three types of ion channels in the plasma membrane were determined using inside-out patches. With 220 millimolar K+ on the cytoplasmic side of the patch and 50 millimolar K+ in the pipette, (220/50 K), the open-channel current-voltage curves of these channels were sigmoidal and consistent with an enzyme kinetic model. Two channel types were selective for K+ over Na+ and Cl−. One (named PKC1) had a maximum conductance (Gmax) of 44 picosiemens at a membrane voltage (Vm) of −65 mV in (220/50 K) and is stimulated by light. The other (PKC2) had Gmax = 66 picosiemens at Vm = 60 millivolts in (220/50 K). The third channel type (PCC1) transported K+ and Na+ about equally well but not Cl−. It had Gmax = 109 picosiemens at Vm = 55 millivolts in (250/50 K) with 10 millimolar Ca2+ on the cytoplasmic side. Reducing Ca2+ to 0.1 millimolar increased PCC1 open-channel currents by approximately 50% in a voltage-independent manner. Averaged over time, PKC2 and PCC1 currents strongly outward rectified and PKC1 currents did so weakly. Reductants (1 millimolar dithiothreitol or 10 millimolar β-mercaptoethanol) added to the cytoplasmic side of an excised patch increased the open probability of all three channel types.