A Raf-like protein kinase BHP mediates blue light-dependent stomatal opening

How do stomata open in response to blue light?  Blue light is perceived by phototropin receptor kinases that activate BLUE LIGHT SIGNALING1 (BLUS1), type 1 protein phosphatase (PP1), and the plasma membrane (PM) H⁺-ATPase.  Hayashi et al. screened a commercial kinase inhibitor library to identify the kinases that phosphorylate PM H⁺-ATPase in response to blue light.  Inhibitors that blocked blue light-stimulated phosphorylation of PM H⁺-ATPase share targets from members of the Raf-like kinase subfamily of MAPKKK enzymes.  A mutant of a Raf-like kinase that is highly-expressed in guard cells could not open stomata in response to blue light.  This gene was named BLUE LIGHT-DEPENDENT H⁺-ATPASE PHOSPHORYLATION (BHP), and its kinase activity is necessary for stomatal opening in response to blue light. BHP localizes to the guard cell cytoplasm.  BHP binds to BLUS1 & PP1 in vitro, but not phototropins or PM H⁺-ATPase.  Hayashi et al. identified BHP as a kinase that interacts with BLUS1 to indirectly regulate the PM H⁺-ATPase and stomatal movement.  (Summary by Daniel Czerny) Sci. Reports 10.1038/srep45586