The Role of a 14-3-3 Protein in Stomatal Opening Mediated by PHOT2 in Arabidopsis

The 14-3-3 lambda isoform is required for normal stomatal opening mediated by PHOT2 in Arabidopsis thaliana. Arabidopsis PHOTOTROPIN2 (PHOT2) interacts with the lambda-isoform 14-3-3 protein both in yeast two-hybrid screening and in an in vitro pull-down assay. Further yeast two-hybrid analysis also showed that the PHOT2 C-terminal kinase domain was required for the interaction. Site-directed mutagenesis indicated that PHOT2 Ser-747 is essential for the yeast interaction. Phenotypic characterization of a loss-of-function 14-3-3 lambda mutant in a phot1 mutant background showed that the 14-3-3 lambda protein was necessary for normal PHOT2-mediated blue light-induced stomatal opening. PHOT2 Ser-747 was necessary for complementation of the blue light-activated stomatal response in a phot1 phot2 double mutant. The 14-3-3 lambda mutant in the phot1 mutant background allowed normal phototropism and normal chloroplast accumulation and avoidance responses. It also showed normal stomatal opening mediated by PHOT1 in a phot2 mutant background. The 14-3-3 kappa mutant had no effect on stomatal opening in response to blue light. Although the 14-3-3 lambda mutant had no chloroplast movement phenotype, the 14-3-3 kappa mutation caused a weaker avoidance response at an intermediate blue light intensity by altering the balance between the avoidance and accumulation responses. The results highlight the strict specificity of phototropin-mediated signal transduction pathways.