Blue Light-induced Chloroplast Avoidance and Phototropic Responses Exhibit Distinct Dose Dependency of PHOTOTROPIN2 in Arabidopsis thaliana

PHOTOTROPIN2 (PHOT2) is a unique photoreceptor involved in chloroplast avoidance movement and also regulates blue light (BL) responses, such as phototropism and leaf flattening, together with PHOTOTROPIN1 (PHOT1) in Arabidopsis thaliana. Previous work showed that the defect of the phot2-1 mutant in chloroplast avoidance movement was a semidominant trait. In the present study, we examined PHOT2 dose dependency of BL responses using the phot1-5 phot2-1 double mutant expressing an AtPHOT2-GFP (P2G) fusion protein. Chloroplast avoidance and phototropic responses of P2G transgenic lines were enhanced in a manner dependent on the P2G levels, whereas the leaf flattening phenotype was simply complemented by P2G equivalent to the wild type (WT) PHOT2 level. The chloroplast avoidance velocity of P2G transgenic lines exhibited enhanced sensitivity to BL in comparison with WT. In contrast, the defect of the phototropic response was rescued by P2G expression equivalent only to the response of the phot1 mutant. These results collectively indicate that each BL response has distinct threshold levels of PHOT2 requirement.