Abstract
Chloroplasts move to weak light (accumulation response), but escape from strong light (avoidance response). In Arabidopsis thaliana, it was shown that blue light receptor phototropins control the direction and the velocity of chloroplast movement by regulating chloroplast actin filaments (cp-actin filaments). Recently, we identified two coiled-coil proteins, WEB1 (weak chloroplast movement under blue light 1) and PMI2 (plastid movement impaired 2), as the factors controlling the velocity of chloroplast movement. WEB1 and PMI2 interacted in yeast cells and BiFC analyses in plant cells revealed that their interaction could occur in cytoplasm. Both web1 and pmi2 mutant plants were impaired especially in the avoidance response and their chloroplasts moved more slowly compared to those in wild-type plants. Cp-actin filaments transiently disappeared immediately after strong blue light irradiation and reappeared at the front region of moving chloroplasts in wild type plants. These changes of cp-actin filaments did not occur in both web1 and pmi2 mutant plants. These results indicate that WEB1-PMI2 determines chloroplast velocity by regulating cp-actin filament dynamics.
