Changes in the levels of calmodulin and of a calmodulin inhibitor in the early phases of radish (Raphanus sativus L.) seed germination-effects of ABA and fusicoccin

Abstract

An inhibitor of Ca2+-calmodulin (Cam)-dependent brain phosphodiesterase was present in the soluble fraction of embryo axes from ungermi-nated radish (Raphanus sativus L.) seeds. This inhibitor is a Ca2+-dependent, Cam-binding protein; in fact: (a) its effect was strongly reduced by treatment with proteases; (b) the inhibition was counteracted by Cam but not by Ca2+; (c) on gel filtration in the presence of Ca2+, Cam co-chromatographed with the inhibitor. The inhibitor is heat stable and positively charged at pH 7.5. During early phases of germination, the fresh weight and the levels of DNA and RNA of embryo axes increased, the level of the inhibitor decreased, and the level of Cam increased. Abscisic acid (ABA) inhibited germination, the decrease of inhibitor, and the increase of Cam. Fusicoccin (FC) stimulated the increase in fresh weight but not the increase in the RNA and DNA levels; in this condition, the inhibitor level decreased and the increase in Cam level was higher than in the control. In the presence of both ABA and FC, there was an increase in fresh weight not accompanied by an increase in DNA and RNA levels; Cam increased and, on a fresh weight basis, reached the value of the control. These results indicate that the Ca2+-Cam system was activated in early germination of radish seeds by an increase in Cam and a decrease in the inhibitor levels, that FC, probably through the activation of membrane functions, increased Cam level, and that the ABA inhibition on germination was not mediated by the Ca2+-Cam system.