Effects of calcium availability on calmodulin level and on metabolic reactivation in early germination phases of radish seeds : ninth international workshop on plant membrane biology

EFFECT' OF CALCIUM AVAILABILITY ON CALMODULIN LEVEL AND ON METABOLIC REACTIVATION IN EARLY GERMINATION PHASES OF RADISH SEEDS. Cocucci. M. and Negrini. N. Dipartimento di Fisiologia delle Piante coltivate e Chimica agraria. Facolta di agraria. Universita di Milano. via Celoria 2. 20133 Milano. Italy. Calcium and Calmodulin (CaM) control many physiological processes in plants. The level of Ca2+-CaM complex can be modulated by the level of intracellular Ca2+ and also by the level of CaM. CaM level appears to play an important role in the metabolic reactivation which characterizes seed germination (1. 2. 3). In radish seeds Ca2+-CaM active complex is also modulated by the presence of a proteinaceous inhibitor (1). Seed germination is also characterized by the reactivation of membrane functions suggesting a precocious recovery of the control of ionic homeostasis. The fact that Ca2+ in early germination was released into the medium (1) sugge~ts a high level of cytosolic Ca2+. The reduction of Ca2+ availability during radish seed germination was obtained by incubating the seeds in the presence of the Ca2+ chelator EGTA. The presence of EGTA inhibited germination (measured as increase in fr. wt.) at concentrations higher than 1 mM. When Ca2+ was administered together with EGTA. the inhibition was removed suggesting that the EGTA effect is due to the reduction of Ca2+ availability. EGTA treatment decreased the level of free Ca2+ in the medium. but the cation bound to EGTA increased; parallelly Ca2+ in the embryo axes decreased. At 1 mM EGTA. the reduction of Ca2+ availability only slightly affected metabolic reactivation (measured as RNA and DNA increases) and had no effect on membrane reactivation. The level and specific activity of CaM in the soluble fraction of radish embryos strongly increased in early germination phases (ca. +450%). 1 mM EGTA greatly reduced (ca.-50%) the increases in level and specific activity of CaM. When the soluble fraction was analyzed on Sephadex G75 column. CaM was eluted in several peaks at MW higher than 30 ill and in particular. at 36 kD. CaM was co-eluted with the proteinaceus CaM inhibitor present in ungerminated seeds. CaM eluted in this zone decreased in the seeds germinated in the presence of EGT A. The decrease in ]eve] and specific activity of CaM was not accompanied by a decrease in germination and metabolic reactivation when the seed germinate at Jow Ca2+ suggesting that the level of the cation might control CaM increase and level. that the level of CaM could contribute to control the level of cytosolic Ca2+ and that the Ca2+-CaM dependent activities could also be also controlled by the presence of the CaM inhibitor. 1. 2. 3. Cocucci. M. and Negrini. N. (1988) Plallt Physinl. 88.910- 914. Cocucci. M. and Negrini. N. (1989) Plalll Membrane trallsport. Dainty 1. et al. eds. pp 355- 360. Cocucci, M. and Negrini. N. (1991)Physiol. Plalll. 82.143- 149. I I I