Actin-based photo-orientation movement of chloroplasts in plant cells.

In photosynthesizing plant cells, chloroplasts change their arrangements and/or positions in response to light irradiation. These photo-orientation movements of chloroplasts are believed to play important roles in optimizing the photosynthetic activity of plant cells. We have been investigating the roles of the actin cytoskeleton in the intracellular movement and positioning of chloroplasts using the aquatic monocot Vallisneria gigantea Graebner and the terrestrial dicot Spinacia oleracea L. (spinach). In Vallisneria epidermal cells, chloroplasts accumulate on the cytoplasmic layer facing the top surface (outer periclinal layer) under dim red light, whereas they move to the cytoplasmic layer perpendicular to the outer periclinal layer (anticlinal layer) under strong blue light. Concomitant with these responses, actin filaments exhibit dramatic changes in their configurations. The possible modes of action of the actin cytoskeleton to regulate the movement and positioning of chloroplasts are briefly summarized, together with our recent analysis of the association of actin filaments with chloroplasts isolated from spinach leaves.