Expression of a Cs+-Resistant Guard Cell K+ Channel Confers Cs+-Resistant, Light-lnduced Stomatal Opening in Transgenic Arabidopsis

Inward-rectifying K+ (K+in) channels in the guard cell plasma membrane have been suggested to function as a major pathway for K+ influx into guard cells during stomatal opening. When K+, channels were blocked with external Cs+ in wild-type Arabidopsis guard cells, light-induced stomatal opening was reduced. Transgenic Arabidopsis plants were generated that expressed a mutant of the guard cell K+, channel, KATl, which shows enhanced resistance to the Cs+ block. Stomata in these transgenic lines opened in the presence of external Cs+. Patch-clamp experiments with transgenic guard cells showed that inward K+i, currents were blocked less by Cs+ than were K+ currents in controls. These data provide direct evidence that KATl functions as a plasma membrane K+ channel in vivo and that K+, channels constitute an important mechanism for light-induced stomatal opening. In addition, biophysical properties of K+, channels in guard cells indicate that components in addition to KATl may contribute to the formation of K+in channels in vivo.