Dominant Negative Guard Cell K⁺ Channel Mutants Reduce Inward-Rectifying K⁺ Currents and Light-Induced Stomatal Opening in Arabidopsis

Inward-rectifying potassium ($\text{K}^{+}{}_{\text{in}}$) channels in guard cells have been suggested to provide a pathway for K+ uptake into guard cells during stomatal opening. To test the proposed role of guard cell $\text{K}^{+}{}_{\text{in}}$ channels in light-induced stomatal opening, transgenic Arabidopsis plants were generated that expressed dominant negative point mutations in the $\text{K}^{+}{}_{\text{in}}$ channel subunit KAT1. Patch-clamp analyses with transgenic guard cells from independent lines showed that $\text{K}^{+}{}_{\text{in}}$ current magnitudes were reduced by approximately 75% compared with vector-transformed controls at -180 mV, which resulted in reduction in light-induced stomatal opening by 38% to 45% compared with vector-transformed controls. Analyses of intracellular K+ content using both sodium hexanitrocobaltate (III) and elemental x-ray microanalyses showed that light-induced K+ uptake was also significantly reduced in guard cells of $\text{K}^{+}{}_{\text{in}}$ channel depressor lines. These findings support the model that $\text{K}^{+}{}_{\text{in}}$ channels contribute to K+ uptake during light-induced stomatal opening. Furthermore, transpirational water loss from leaves was reduced in the $\text{K}^{+}{}_{\text{in}}$ channel depressor lines. Comparisons of guard cell $\text{K}^{+}{}_{\text{in}}$ current magnitudes among four different transgenic lines with different $\text{K}^{+}{}_{\text{in}}$ current magnitudes show the range of activities of $\text{K}^{+}{}_{\text{in}}$ channels required for guard cell K+ uptake during light-induced stomatal opening.