The affect of cadmium on sulfate assimilation enzymes in Brassica juncea

Abstract

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 μM CdSO 4. AS mRNA increased ≈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 ≈7-fold. In roots AR mRNA and enzyme increased asynchronously; mRNA increased ≈5-fold after 6 h while enzyme activity increased ≈5-fold in 24 h. In plants exposed for 3 h to Cd and then transferred to Cd-free medium AR activity increased ≈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 μM the level of AR mRNA increased linearly with the dose but the activity lagged behind the mRNA and was inhibited above 50 μM. 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.