INVOLVEMENT OF CA2-CALMODULIN IN CD2+ TOXICITY DURING THE EARLY PHASES OF RADISH (RAPHANUS-SATIVUS L) SEED-GERMINATION()

The toxicity of Cd2+ in vivo during the early phases of radish (Raphan us sativus L.) seed germination and the in vitro Cd2+ effect on radish calmodulin (CaM) were studied, Cd2+ was taken up in the embryo axes o f radish seeds; the increase in fresh weight of embryo axes after 24 h of incubation was inhibited significantly in the presence of 10 mmol m(-3) Cd2+ in the external medium, when the Cd2+ content in the embryo axes was c. 1.1 mu mol g(-1) FW. The reabsorption of K+, which charac terizes germination, was inhibited by Cd2+, suggesting that Cd2+ affec ted metabolic reactivation. The slight effect of Cd2+ on the transmemb rane electric potential of the cortical cells of the embryo axes exclu ded a generalized toxicity of Cd2+ at the plasma membrane level, After 24 h of incubation, Cd2+ induced no increase in total acid-soluble th iols and Cd2+-binding peptides able to reduce Cd2+ toxicity, Ca2+ adde d to the incubation medium partially reversed the Cd2+-induced inhibit ion of the increase in fresh weight of embryo axes and concomitantly r educed Cd2+ uptake, Equilibrium dialysis experiments indicated that Cd 2+ bound to CaM and competed with Ca2+ in this binding. Cd2+ inhibited the activation of Ca2+-CaM-dependent calf-brain phosphodiesterase, in hibiting the Ca2+-CaM active complex, Cd2+ reduced the binding of CaM to the Ca2+-CaM binding enzymes present in the soluble fraction of the embryo axes of radish seeds. The possibility that Cd2+ toxicity in ra dish seed germination is mediated by the action of Cd2+ on Ca2+-CaM is discussed in relation to the in vivo and in vitro effects of Cd2+.