Negative Phototropism of Vaucheria terrestris Regulated by Calcium II. Inhibition by Ca2+-Channel Blockers and Mimesis by A23187

As reported in a previous article [Kataoka (1988a) Plant CellPhysiol. 29: 1323], growing apices of the xanthophycean coenocyticalga, Vaucheria terrestris, bends away from a unilateral bluelight (BL) source, if they are simultaneously irradiated withstrong background BL in a solution containing 1–4 mM Ca²⁺.Since the negative bending is a function of the product of theexternal Ca²⁺ concentration and the fluence rate of backgroundBL, a BL-induced Ca²⁺-influx at the apex was hypothesized tobe the cause of the phototropic inversion. The present reportprovides strong evidence for this hypothesis. Addition of theCa²⁺ channel blockers, La³⁺, verapamil, nifedipine and nitrendipineto media containing 4 mM Ca²⁺ completely inhibited the phototropicinversion. By contrast, 1 µM A23187[GenBank] (plus 4 mM Ca²⁺) notonly enhanced the phototropic inversion under background BL,but also mimicked the background BL; i.e. it caused negativebending under safe red light. Inhibition of phototropic inversionby La³⁺ is also observed under conditions where the algae areirradiated with unilateral BL for 1 week. A BL-dependent Ca²⁺influx and a consequential elevation of the cytoplasmic Ca²⁺-levelin the apical growth region must be involved in the early stepsof phototropic response. A BL-controlled opening of L-type Ca²⁺channels is also suggested. ¹A part of this study was reported at the 3rd Phycological Congressat Melbourne 1988 (Kataoka 1988b) and XXII Yamada Conferenceon Plant Water Relations and Growth Under Stress at Osaka 1989(Kataoka 1989)²Dedicated to Prof. em. Dr. Noburo Kamiya on the occasion ofhis 77th birthday (Received May 23, 1990; Accepted July 13, 1990)