Abstract
Chloroplast movement is essential for proper functions of chloroplasts and can be induced by various external stimuli. Chloroplast photorelocation movement is mediated by the photoreceptor phototropin (phot) family proteins. In most plant species tested, chloroplast movement depends on actin filaments but the underlying molecular mechanism is unknown. In Arabidopsis thaliana, we identified a new system that utilizes short actin filaments (cp-actins) at the chloroplast periphery of the plasma membrane side for chloroplast photorelocation and anchoring to the plasma membrane. Upon irradiation with blue light inducing chloroplast movement, cp-actins relocalized to the leading edge of chloroplasts prior to and during photorelocation. Under weak light cp-actins increase on stable chloroplasts, whereas when strong blue light was given cp-actins disappears transiently before biased cp-actin formation. These cp-actin regulations are mediated by phototropin and are impaired in other mutants defective in chloroplast movement. Thus, cp-actins were essential for phot-mediated chloroplast photorelocation movement and anchoring.
