Involvement of Ca2+‐calmodulin in Cd2+ toxicity during the early phases of radish (Raphanus sativus L.) seed germination

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Abstract
The toxicity of Cd2+in vivo during the early phases of radish (Raphanus sativus L.) seed germination and the in vitro Cd2+ effect on radish calmodulin (CaM) were studied. Cd2+ was taken up in the embryo axes of 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 μmol g−1 FW. The reabsorption of K+, which characterizes germination, was inhibited by Cd2+, suggesting that Cd2+ affected metabolic reactivation. The slight effect of Cd2+ on the transmembrane electric potential of the cortical cells of the embryo axes excluded a generalized toxicity of Cd2+ at the plasma membrane level. After 24 h of incubation, Cd2+ induced no increase in total acid‐soluble thiols and Cd2+‐binding peptides able to reduce Cd2+ toxicity. Ca2+ added to the incubation medium partially reversed the Cd2+‐induced inhibition of the increase in fresh weight of embryo axes and concomitantly reduced Cd2+ uptake. Equilibrium dialysis experiments indicated that Cd2+ bound to CaM and competed with Ca2+ in this binding. Cd2+ inhibited the activation of Ca2+‐CaM‐dependent calf‐brain phosphodiesterase, inhibiting 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 radish 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+.