Blue-Light-Activated Chloroplast Movements: Progress in the Last Decade

Abstract

Chloroplasts accumulate in weakly illuminated regions of the cell and avoid regions exposed to strong light in various plant taxa. These orientation movements of chloroplasts are controlled only by blue light in the mesophyll of vascular land plants. Although efforts to elucidate the mechanisms underlying chloroplast responses to light were undertaken in the whole of the twentieth century, the last decade brought a breakthrough in the knowledge of their molecular basis. The key motive power was the discovery of phototropins, photoreceptors which mediate phototropism, chloroplast relocations, and stomatal movements. In Arabidopsis thaliana, phototropin 1 and phototropin 2 control the accumulation response, with the latter active at higher fluence rates. Phototropin 2 is the only photoreceptor to control chloroplast avoidance movement. Whereas the involvement of actin in the movements is unequivocal, its exact role remains controversial. The search for changes in actin organization, induced specifically by blue light, produced divergent results, which depend on the species, detection method, and fluence rate. Short actin filaments formed at the leading edge of the moving chloroplast under a blue irradiation gradient have been postulated to generate the motive force for chloroplast movement. Alternatively, a blue-light-induced relocation of myosin(s) associated with the chloroplast’s surface has been suggested to determine the direction of movement. Analyses of mutants showing impaired chloroplast responses resulted in the discovery of several components of the blue light signaling pathway and/or the motor apparatus. CHUP1, a protein identified by analyzing a deletion mutant which exhibits a chloroplast unusual positioning phenotype, has been proposed to connect chloroplasts to actin filaments and regulate actin polymerization on the one hand, and to anchor them to the plasma membrane on the other. Two kinesin-like proteins have been reported to mediate chloroplast movement and to anchor the organelles in the cell membrane. THRUMIN1 has been shown to bundle filamentous actin in vitro, and to associate with the plasma membrane and interact with microfilaments in a phototropin-dependent manner in vivo. Other proteins postulated to play important roles in the signaling pathway are JAC1 and PMI1 and 2. Further investigations are necessary to establish the exact functions of proteins believed to take part in the movement mechanism.