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/m 2 /s) increased ATP hydrolysis by the plasma membrane H ϩ -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 coprecipitated 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.