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Cellular and Structural Biology

Cooperative Regulation of Cytoplasmic Streaming and Ca²⁺ Fluxes by Pfr and Photosynthesis in Vallisneria Mesophyll Cells ¹

Department of Biology, College of General Education, Osaka University, Toyonaka, Osaka 560, Japan, Division of Biological Regulation, National Institute for Basic Biology, Okazaki, Aichi 444, Japan, Frontier Research Programs, RIKEN Institute, Wako, Saitama 351-01, Japan

In mesophyll cells of Vallisneria gigantea Graebner, Ca²⁺ regulates the induction and cessation of cytoplasmic streaming. Streaming is induced when the level of calcium in the cytoplasm is lowered through light-accelerated release of Ca²⁺ from the cells (S Takagi, R Nagai [1988] Plant Physiol 88: 228-232). We have now initiated an investigation on the nature of the photoreceptor(s) that are involved in the regulation of Ca²⁺ movements across the cell membrane and of streaming. Streaming is induced only when phytochrome exists in the phytochrome—far redabsorbing form (Pfr)—and photosynthesis is allowed to take place for at least 4 minutes. The former effect is typically photoreversible by red and far-red light, and phytochrome is spectro-photometrically detectable in the crude extract from the leaves. The latter effect is assessed in terms of the wavelength dependency and the effects of diuron and atrazine, two inhibitors of photosynthesis. A similar requirement for Pfr and photosynthesis is found to be associated with the acceleration of Ca²⁺ efflux in the protoplasts. The results suggest that phytochrome and photosynthetic pigment(s) cooperatively regulate cytoplasmic streaming via modulation of the Ca²⁺ transport in the cell membrane.

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