LRS1 phosphorylated by blue light plays important role for stomatal opening

Blue light receptor phototropin is a protein kinase and initiates blue light signaling pathway including phosphorylation and activation of plasma membrane (PM) H+-ATPase for stomatal opening. Activation of the PM H+-ATPase provides driving force for K+ uptake via K+in channels. The influx of K+ is electrically balanced by intracellular anions such as Cl− and malate produced through starch degradation, increasing cellular osmotic potential and thereby driving water influx and stomatal opening. Our phosphoproteimic analysis revealed that blue light induces phosphorylation of Ser395 in VfLRS in guard cell protoplasts from Vicia faba. VfLRS shows high similarity of LATERAL ROOT STIMULATOR 1 (LRS1, AT3G05090) in Arabidopsis thaliana and the phosphorylation site is conserved as Ser393 in LRS1. Therefore, we investigated stomatal phenotype using a T-DNA-inserted knockout mutant of LRS1, lrs1. The lrs1 mutant exhibited normal blue light-induced stomatal opening and normal blue light-induced phosphorylation of Thr948 of PM H+-ATPase (numbering according to AHA1) in guard cells. Interestingly, we found that red light-induced starch accumulation in guard cells was severely suppressed in the lrt1 mutant. The reduced starch accumulation in lrs1 guard cells suggest a lower content of starch metabolites, such as malate. We then examined blue light-induced stomatal opening in KGlu buffer, which lacks anions. As expected, the lrs1 mutant showed significantly reduced blue light-induced stomatal opening under this condition. Taken together, these results indicate that LRS1 plays important roles in starch accumulation and metabolism and contribute to stomatal opening.