Negative Phototropism of Vaucheria terrestris Regulated by Calcium II. Inhibition by Ca^<2+>-Channel Blockers and Mimesis by A23187

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As reported in a previous article [Kataoka (1988a) Plant Cell Physiol. 29: 1323], growing apices of the xanthophycean coenocytic alga, Vaucheria terrestris, bends away from a unilateral blue light, (BL) source, if they are simultaneously irradiated with strong background BL in a solution containing 1-4 mM Ca^<2+>. Since the negative bending is a function of the product of the external Ca^<2+> concentration and the fluence rate of background BL, a BL-induced Ca^<2+>-influx at the apex was hypothesized to be the cause of the phototropic inversion. The present report provides strong evidence for this hypothesis. Addition of the Ca^<2+> channel blockers, La^<3+>, verapamil, nifedipine and nitrendipine to media containing 4 mM Ca^<2+> completely inhibited the phototropic inversion. By contrast, 1 μM A23187 (plus 4 mM Ca^<2+>) not only enhanced the phototropic inversion under background BL, but also mimicked the background BL; i.e. it caused negative bending under safe red light. Inhibition of phototropic inversion by La^<3+> is also observed under conditions where the algae are irradiated with unilateral BL for 1 week. A BL-dependent Ca^<2+> influx and a consequential elevation of the cytoplasmic Ca^<2+>-level in the apical growth region must be involved in the early steps of phototropic response. A BL-controlled opening of L-type Ca^<2+> channels is also suggested.
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