Melatonin reprograms sulfur metabolism and reestablishes redox homeostasis to enhance cadmium tolerance in Brassica juncea

Cadmium (Cd) is a toxic heavy metal that inhibits plant growth and compromises key physiological and metabolic processes, including photosynthesis, antioxidant defense, and nutrient homeostasis. Melatonin (MT) is a multifunctional signaling molecule that is widely distributed in different parts of plants and can alleviate the inhibitory effect of stress on plant growth and development. Brassica juncea exhibits high sulfur demand, a complex sulfur metabolic network, and pronounced Cd accumulation capacity. How MT application affects sulfur resource allocation in B. juncea under Cd stress, thereby influencing redox balance, remains unclear. In this paper, we investigated the effects of foliar application of MT on the growth, Cd tolerance, and physiological and molecular characteristics of B. juncea exposed to 50 mg kg −1 CdCl 2 in soil. These findings revealed that exogenous MT treatment significantly increased the activities of antioxidant enzymes (superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase), synthesis of non-enzymatic antioxidants (glutathione, glutathione disulfide, ascorbic acid, and dehydroascorbic acid), maintained the levels of photosynthetic pigments (chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid) and net photosynthetic rate, decreased the contents of reactive oxygen species and malondialdehyde in seedlings, as well as affected glutathione S-transferase activity, cysteine and glucosinolate contents and expression of genes BCAT4 , CYP83A1 , and ST5a - b - c related to sulfur metabolism, thus protecting cells from injury and improving the ability of B. juncea to resist Cd stress. Taken together, the alleviating of Cd toxicity in B. juncea by MT application is closely correlated with redox homeostasis and sulfur metabolism.