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

Abstract

ABSTRACT The toxicity of Cd 2+ in vivo during the early phases of radish ( Raphanus sativus L.) seed germination and the in vitro Cd 2+ effect on radish calmodulin (CaM) were studied. Cd 2+ 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 Cd 2+ in the external medium, when the Cd 2+ content in the embryo axes was c . 1.1 μmol g −1 FW. The reabsorption of K + , which characterizes germination, was inhibited by Cd 2+ , suggesting that Cd 2+ affected metabolic reactivation. The slight effect of Cd 2+ on the transmembrane electric potential of the cortical cells of the embryo axes excluded a generalized toxicity of Cd 2+ at the plasma membrane level. After 24 h of incubation, Cd 2+ induced no increase in total acid‐soluble thiols and Cd 2+ ‐binding peptides able to reduce Cd 2+ toxicity. Ca 2+ added to the incubation medium partially reversed the Cd 2+ ‐induced inhibition of the increase in fresh weight of embryo axes and concomitantly reduced Cd 2+ uptake. Equilibrium dialysis experiments indicated that Cd 2+ bound to CaM and competed with Ca 2+ in this binding. Cd 2+ inhibited the activation of Ca 2+ ‐CaM‐dependent calf‐brain phosphodiesterase, inhibiting the Ca 2+ ‐CaM active complex. Cd 2+ reduced the binding of CaM to the Ca 2+ ‐CaM binding enzymes present in the soluble fraction of the embryo axes of radish seeds. The possibility that Cd 2+ toxicity in radish seed germination is mediated by the action of Cd 2+ on Ca 2+ ‐CaM is discussed in relation to the in vivo and in vitro effects of Cd 2+ .