• blue light;
  • gas exchange;
  • H+ pump;
  • protein kinase;
  • stomata

The opening of stomata, which is driven by the accumulation of K+ salt in guard cells, is induced by blue light (BL). The BL activates the H+ pump; however, the mechanism by which the perception of BL is transduced into the pump activation remains unknown. We present evidence that the pump is the plasma membrane H+-ATPase and that BL activates the H+-ATPase via phosphorylation. A pulse of BL (30 s, 100 μmol/m2/s) increased ATP hydrolysis by the plasma membrane H+-ATPase and H+ pumping in Vicia guard cell protoplasts with a similar time course. The H+-ATPase was phosphorylated reversibly by BL, and the phosphorylation levels paralleled the ATP hydrolytic activity. The phosphorylation occurred exclusively in the C-termini of H+-ATPases on both serine and threonine residues in two isoproteins of H+-ATPase in guard cells. An endogenous 14-3-3 protein was co-precipitated with H+-ATPase, and the recombinant 14-3-3 protein bound to the phosphorylated C-termini of H+-ATPases. These findings demonstrate that BL activates the plasma membrane H+-ATPase via phosphorylation of the C-terminus by a serine/threonine protein kinase, and that the 14-3-3 protein has a key role in the activation.