Exogenous cysteine alleviates cadmium stress better than sulfate by enhancing nonenzymatic antioxidant system in Brassica juncea

The contamination of soil by cadmium (Cd) is a widely concerned environmental problem threatening plant growth and human health. The essential nutrient sulfur plays a crucial role in plant responses to Cd stress. Cysteine (Cys) is the first product of sulfur assimilation and plays a cardinal role for detoxicating heavy metals in plants. Brown mustard (Brassica juncea (L.) Czern) has good application prospect in phytoremediation of heavy metals-polluted soil due to its high enrichment capacity for multiple heavy metals. However, the effect of exogenous Cys in mitigating Cd toxicity in B. juncea remains uncertain. This study explored the effect of exogenous Cys on improving the tolerance of B. juncea to Cd, and compared it with equimolar concentration of sulfate. The findings revealed that the optimum concentration of exogenous Cys to improve Cd tolerance was 1 mM. Exogenous application of Cys or sulfate elevated contents of endogenous Cys, glutathione (GSH), phytochelatin (PC), ascorbic acid (AsA), dehydroascorbic acid (DHA), and oxidized glutathione (GSSG), as well as the ratios of AsA/DHA and GSH/GSSG, and enhanced the activities of glutathione reductase (GR), thus effectively attenuating Cd toxicity and significantly improving the photosynthetic rate and growth rate of Cd-stressed B. juncea seedlings. In summary, the scavenging of reactive oxygen species (ROS) by Cys or sulfate is mainly dependent on enhancing nonenzymatic antioxidant system. Additionally, the effect of Cys on enhancing the tolerance of B. juncea to Cd is better than that of equimolar concentration of sulfate.