XVI International Botanical Congess

Blue light (BL) activates H+ pump in guard cells and induces stomatal opening, however, the activation mechanism of H+ pump by BL remains unknown. In this study, we investigated ATP hydrolytic activity in response to BL in guard cell protoplasts (GCPs). Illumination of GCPs with a pulse of BL increased vanadate-sensitive ATP hydrolytic activity by two-fold in the guard-cell extracts. The BL-stimulated ATP hydrolysis reached maximal at 2.5 min after the start of the BL pulse, when the rate of BL-dependent H+ pumping in GCPs showed maximum, and decreased to basal level within 20 min after the BL pulse. BL decreased Km for ATP from 0.29 to 0.17 mM and increased Vmax from 1.96 to 3.23 nmol Pi per hr per ug protein. These results strongly suggest that BL-dependent H+ pumping in GCPs is mediated by the plasma membrane H+-ATPase. Effect of BL on phosphorylation status of the H+-ATPase will be reported.