The affect of cadmium on sulfate assimilation enzymes in Brassica juncea

The expression of mRNA and enzyme activity for the sulfur assimilation enzymes ATP sulfurylase (AS) and APS reductase (AR) was measured over 24 h in Brassica juncea exposed to 50 mu M CdSO4. AS mRNA increased approximate to 3-fold only in roots and there was no change in enzyme activity. AR mRNA level was unaffected in leaves but enzyme activity declined by approximate to 7-fold. In roots AR mRNA and enzyme increased asynchronously; mRNA increased approximate to 5-fold after 6 h while enzyme activity increased approximate to 5-fold in 24 h. In plants exposed for 3 h to Cd and then transferred to Cd-free medium AR activity increased approximate to 5-fold more rapidly than in plants that were continuously exposed. Feeding of cysteine or glutathione abolished the Cd-induced increases in mRNA and enzyme activity. In plants that were exposed to varying Cd doses up to 200 mu M the level of AR mRNA increased linearly with the dose but the activity lagged behind the mRNA and was inhibited above 50 mu M. These results indicate that the lag in enzyme expression is likely due to an effect of Cd. However, not all heavy metals had the same effect. When treated with Zn, Pb, Cu, and Hg AR mRNA and enzyme activity increased synchronously. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.