Abstract
Plants possess a blue-light receptor, phototropin, which mediates phototropism, chloroplast relocation, stomatal opening and leaf flattening. Phototropin is a light-regulated kinase whose authentic substrate has remained unknown. The "flippase-kinase" (Fpk) in Saccharomyces cerevisiae (budding yeast), which shows a high similarity to phototropin, phosphorylates and activates "phospholipid translocase" (flippase). Flippase in turn affects cell polarity, membrane transport, and actin remodeling through modifying the tansbilayer lipid asymmetry of cellular membrane.
We hypothesized that phototropin also acts as a flippase-kinase in plants. As the first step, we demonstrated that phototropin complemented multiple phenotypes in the fpk-deficient mutant strains of budding yeast. Hence, phototropin could regulate the flippase activity at least in budding yeast. We then asked whether flippase was involved in the phototropin-mediated responses in plants. Among 12 members of the flippase family (ALA1 - 12), we found that the ala3 mutant of Arabidopsis was defective in stomatal opening induced by blue light. Thus, phototropin may regulate ALA3 directly to trigger stomatal opening.
