Foliar application of melatonin reduces cadmium uptake and Cd-induced oxidative stress in tobacco under Cd stress

Melatonin (MT) is a well-known signaling molecule in animals and plants. It significantly enhances the antioxidant capacity and resistance of plants to abiotic stress. This study revealed that the application of exogenous melatonin improved resistance to cadmium (Cd) stress in tobacco. This improvement was evident through reductions in malondialdehyde (MDA) and Cd contents and inhibition of tobacco growth. Specifically, in the 200 mu mol/L MT treatment, MDA and Cd content were reduced by 34.30 and 64.63%, respectively, compared with the single Cd stress. Furthermore, root length, fresh weight, the SPAD index, maximal leaf length, maximal leaf width, and plant height increased by 147.92%, 245.39%, 35.56%, 71.91%, 37.33%, and 307.41%, respectively. Additionally, MT pretreatment controlled Cd-induced oxidative damage to tobacco by directly improving water absorption capacity and enhancing antioxidant enzyme activities in cells. In the 200 mu mol/L MT treatment, soluble protein content increased by 151.32%, compared with the single Cd stress. Furthermore, the activities of peroxidase, superoxide dismutase, and glutathione increased by 334.01%, 140.39%, and 414.11%, respectively. Molecular analysis demonstrated that exogenous MT modulates the expression of genes related to MT synthesis, antioxidant enzyme synthesis, and Cd transport, thereby reducing Cd toxicity. The expression levels of the six selected genes were consistent with transcriptome data. This study provides important insights into the molecular mechanisms underlying the role of MT in regulating Cd stress in plants.