Ion antagonism between calcium and magnesium in phytochrome-mediated degradation of storage starch in Spirodela polyrhiza

Abstract

Light and calcium regulation were studied in two processes running parallel in turions of Spirodela polyrhiza (L.) Schleiden: in germination and starch degradation. When germination of turions is induced by a single red light (R) pulse the response depends on exogenously applied Ca 2+. This Ca 2+ dependence is absent in continuous and intermittent R (hourly applied short R pulses that can substitute for continuous R). Phytochrome-mediated degradation of storage starch in the turions starts after the onset of germination. The degradation proceeds only in continuous or intermittent R. Contrary to germination, a single R pulse is ineffective in inducing starch degradation. The process, categorized as a special low-fluence phytochrome response, requires Ca 2+. Moreover, a Ca 2+/Mg 2+ ion antagonism was detected: significant starch degradation proceeds in the absence of both Ca 2+ and Mg 2+. Inhibitors of Ca 2+-uptake (La 3+, verapamil) and of calmodulin (chlorpromazine, trifluoperazine) diminish the light-induced starch degradation. This suggests that the antagonistic activity of Ca 2+ and Mg 2+ in this process is associated with the signal transduction chain, similar to that in the R pulse-induced turion germination. Because the Ca 2+ requirement is different in germination and starch degradation, the Ca 2+-sensitive step cannot be the same in the two responses. This Ca 2+/Mg 2+ antagonism can be used to clarify the relationship between biochemical reactions of the composed response of starch degradation and the mode of their regulation.