Abstract: Regulation of the inward K+ -channels in the guard cell plasma membranes plays impotant roles in regulation of stomatal movement in responses to exogenous and endogenous signals. It is well-known that elevation of cytosolic Ca2+ in guard cells inactivates these inward K + channels, and consequently inhibits stomatal opening or induces stomatal closing, yet the downstream molecular mechanism for the Ca2 + -mediated inhibition of the inward K+ channels remains unknown. The calmodulin-like domain protein kinases (CDPKs) have been identified as an unique group of protein kinases in higher plant cells. As a downstream regulator, CDPK may play roles in mediating Ca2+ regulation on the inward K+ -channels in stomatal guard cells. The authors have applied the patchclamp technique to investigate if CDPK be involved in the regulation of the inward K+ -channels in Vicia faba guard cells by cytosolic Ca2+ . The presence of the 1.5 μmol/L intracellular Ca2 + result-ed in inhibition of the inward K+ channel activity by 60%, while the addition of purified CDPK from the cytoplasmic side resulted in greater inhibition than Ca2+ alone. Histone Ⅲ-S and protamine, which is the substrate and substrate competitive inhibitor of CDPKs respectively, completely reversed the Ca2+ -induced inhibition of the inward K+ channel activities. These results are the first reported evidences for that CDPKs are involved in the Ca2+ -mediated inward K+ -channel regulation in guard cells.

Key words: Calmodulin-like domain protein kinases (CDPKs), Guard cells, K+ channels, Signal transduction, Phosphorylation, Dephosphorylation