Photoorientation of chloroplasts in protonemal cells of the fernAdiantum as analyzed by use of a video-tracking system

Photoorientation of chloroplasts mediated by phytochrome and blue light-absorbing pigment in protonemal cells of the fernAdiantum was studied by use of inhibitors of the cytoskeleton and was analyzed with a video-tracking system. The photoorientation responses were inhibited by cytochalasin B and by N-ethylmaleimide (NEM) but not by colchicine, suggesting that the photomovement depends on the actomyosin system. In the dark, chloroplasts moved randomly, being independent of one another. After induction of photoorientation by polarized red light, most chloroplasts that had been located at the margin of cells moved almost perpendicularly to the cell axis toward the site of photoorientation. This type of movement was hardly ever observed in the dark. Under polarized blue light, such specific movements were less evident but were still observed in the case of a few chloroplasts. After photoorientation was complete, chloroplasts still moved in random directions but their mobility was lower than that in the dark, indicating the presence of some anchoring mechanism. When EGTA was applied, photoorientation was inhibited but this inhibition was overcome by the addition of CaCl₂. Video-tracking of chloroplasts in the dark revealed that the mobility of chloroplasts was higher in medium with EGTA than in medium with EGTA plus CaCl₂ and that many of the chloroplasts moved jerkily in the medium with EGTA. This change in the nature of movements was also seen under polarized light, resulting in the disturbance of photoorientation. These results indicate that the inhibition of photoorientation at low concentrations of Ca²⁺ ions may be due to change in the nature of chloroplast movement.