Effects of Cadmium Treatment on the Uptake and Translocation of Sulfate in Arabidopsis thaliana

Cadmium (Cd) is a highly toxic and non-essential element for plants, whereas phytochelatins and glutathione are low-molecular-weight sulfur compounds that function as chelators and play important roles in detoxification. Cadmium exposure is known to induce the expression of sulfur-assimilating enzymes and sulfate uptake by roots. However, the molecular mechanism underlying Cd-induced changes remains largely unknown. Accordingly, we analyzed the effects of Cd treatment on the uptake and translocation of sulfate and accumulation of thiols in Arabidopsis thaliana. Both wild type (WT) and null mutant (sel1-10 and sel1-18) plants of the sulfate transporter SULTR1; 2 exhibited growth inhibition when treated with CdCl2. However, the mutant plants exhibited a lower growth rate and lower Cd accumulation. Cadmium treatment also upregulated the transcription of SULTR1; 2 and sulfate uptake activity in WT plants, but not in mutant plants. In addition, the sulfate, phytochelatin and total sulfur contents were preferentially accumulated in the shoots of both WT and mutant plants treated with CdCl2, and sulfur K-edge XANES spectra suggested that sulfate was the main compound responsible for the increased sulfur content in the shoots of CdCl2-treated plants. Our results demonstrate that Cd-induced sulfate uptake depends on SULTR1; 2 activity, and that CdCl2 treatment greatly shifts the distribution of sulfate to shoots, increases the sulfate concentration of xylem sap and upregulates the expression of SULTRs involved in root-toshoot sulfate transport. Therefore, we conclude that root-toshoot sulfate transport is stimulated by Cd and suggest that the uptake and translocation of sulfate in CdC(l)2-treated plants are enhanced by demand-driven regulatory networks.