Transcriptome analysis and physiological indicators reveal the role of sulfur in cadmium accumulation and transportation in water spinach (Ipomoea aquatica Forsk.).

Cadmium (Cd) contamination of croplands has become a threat to crop food safety and human health. In this study, we investigated the effect of sulfur on the growth of water spinach under Cd stress and the amount of Cd accumulation by increasing the soil sulfate content. We found that the biomass of water spinach significantly increased after the application of sulfur while the shoot Cd concentration was considerably reduced (by 31%). The results revealed that sulfur could promote the expression of PME and LAC genes, accompanied by an increase in PME activity and lignin content. Also, the cell wall Cd content of water spinach roots was significantly increased under sulfur treatment. This finding suggests that sulfur could enhance the adsorption capacity of Cd by promoting the generation of cell wall components, thereby inhibiting the transportation of Cd via the apoplastic pathway. In addition, the higher expression of Nramp5 under the Cd1S0 (concentration of Cd and sulfur are 2.58 and 101.31 mg/kg respectively) treatment led to increased Cd uptake. The CAX3 and ABC transporters and GST were expressed at higher levels along with a higher cysteine content and GSH/GSSR value under Cd1S1 (concentration of Cd and sulfur are 2.60 and 198.36 mg/kg respectively) treatment, which contribute to the Cd detoxification and promotion of Cd compartmentalization in root vacuoles, thereby reducing the translocation of Cd to the shoot via the symplastic pathway.