[The role of actomyosin in blue light-induced chloroplast movements]

Chloroplast redistribution in the cell depends on direction, fluence-rate and spectral composition of the incident light. Two photoreceptors, phototropin 1 and 2, control the chloroplast responses in higher land plants. Actin and myosin form the motor system. Although numerous results point to calcium and phosphoinositides as the secondary messengers, the signal transduction pathways remain unidentified. Two possible roles of the acto-myosin system in the mechanism of chloroplast redistribution have been discussed. Reorganization of the actin cytoskeleton has been observed in water plants where the chloroplast movements depend on red light. This reorganization appears to be associated with cytoplasmic streaming. In higher land plants, e.g. in Arabidopsis thaliana, the chloroplast responses depend only on blue light. Neither specific light-induced reorganization of actin nor cytoplasmic streaming have been observed in the mesophyll of these species. A blue light-specific relocalization of myosins accompanies the chloroplast responses in Arabidopsis. Thus myosins might be potential targets of light signaling in higher land plants.