Fig. 6: Schematic model for blue light and photosynthesis-mediated activation of H+-ATPase in the guard cells.

Blue light-activated phototropins phosphorylate BLUS1 protein kinase, which mediates the phosphorylation of Thr-881 and Thr-948. Thr-948 phosphorylation is a prerequisite for Thr-881 phosphorylation in response to blue light. Simultaneous phosphorylation of Thr-881 and Thr-948 promotes H+-ATPase activity and stomatal opening. Guard cell photosynthesis also mediates Thr-881 phosphorylation.