Distinct responses of chloroplasts to blue and green laser microbeam irradiations in the centric diatom Pleurosira laevis

The centric diatom Pleurosira laevis is a large unicellular alga, in which ca 200 chloroplasts migrate toward the nuclear cytoplasm through the transvacuolar cytoplasmic strands in response to blue-light irradiation and, on the contrary, toward the cortical cytoplasm in response to green-light irradiation. We analyzed these light-induced chloroplast migrations using a scanning laser microbeam provided by a confocal microscope for intracellular irradiation. Spot irradiation of a blue laser microbeam induced rapid assemblage of chroloplasts into the nuclear cytoplasm regardless of the spot position and spot number. On the other hand, one or two spots of green laser microbeam induced chloroplast accumulation at the spots, although increasing spot numbers suppressed chloroplast accumulation at each spot. In our experimental condition, ca 1 min of blue-light irradiation was sufficient to stimulate movement, whereas green-light irradiation required uninterrupted and longer irradiation time (ca 15 min). Chloroplast assemblage induced by blue-light required extracellular Ca2+, and was inhibited by Ca2+ channel antagonists. Furthermore, higher efficiencies of chloroplast migration were obtained when a single beam spot was fragmented and scattered over wider area of plasma membrane. These observations suggested that blue-light induced a response at the plasma membrane, which subsequently activated Ca2+ permeable channels. This sequence of physiological events is identical to what was previously observed with chloroplast movement in response to mechanical stimulation. Furthermore, experiments with the cytoskeleton-disrupting agents, colchicine and cytochalasin D, indicated that blue-light-induced chloroplast movement required microtubules whereas the green-light-induced response to beam spot required actin filaments.