Chloroplast assemblage by mechanical stimulation and its intercellular transmission in diatom cells

Summary

By touching a cell ofPleurosira laevis, a centric diatom, the chloroplasts in the cortical cytoplasm immediately migrate to the nuclear cytoplasm which is located at the center of the cell. The time required for migration, is about 10 s and that for restoration is about 15 min. The touch could be replaced by an electric stimulation. A 7 V pulse for 1 ms was required for the induction of chloroplast assemblage. To obtain chloroplast assemblage in 50% of the cells, the amount of time and voltage required were inversely proportional. Higher concentrations of KCl also induced chloroplast assemblage. Neither contact nor electric stimulation in Ca²⁺ free medium induced chloroplast assemblage. Verapamil, a Ca²⁺ channel blocker, in medium suppressed the chloroplast assemblage. Ca²⁺ ionophore caused the chloroplast assemblage. These results suggested that the depolarization of the plasma membrane was a trigger of the induction of chloroplast assemblage and the influx of Ca²⁺ to the cytoplasm through the channel was essential for this. The chloroplast assemblage was transmitted successively to the other cells of the filamentous colony, though these cells were linked only by a mucilaginous substance. Furthermore, the transmission occurred in cells which were placed separately. The maximum distance possible for this jumping transmission was 200 μm in the culture medium. To search for the release of any chemical transmitter, contact stimulation was applied to one cell at the middle of a filamentous colony which was placed in water streaming along the axis of the filament. Chloroplast assemblage was induced more in downstream cells than in upstream ones. This result suggested the existence of some substance which mediated the transmission of the stimulation.