Exogenous melatonin enhanced cadmium stress tolerance of cucumber seedlings (Cucumis sativus L.)

Cadmium (Cd) stress seriously affects cucumber growth, yield, and quality. Melatonin (MT) can enhance plant resistance to abiotic stresses. However, studies on the mechanism of MT in enhancing plant resistance are limited. To better understand the underlying physiological and molecular mechanisms, the antioxidant defenses, photosynthesis, and transcriptome profiles of cucumber were analyzed under different growth conditions. The results demonstrated that exogenous MT significantly alleviated the Cd-induced damages to cucumber seedlings. Compared with Cd treatment, MT + Cd treatment enhanced antioxidant enzyme activities, suppressed ROS production, and improved photosynthesis in cucumber seedlings. Intriguingly, the scavenging or inhibition of hydrogen peroxide and nitric oxide reversed the positive effects of melatonin described above. RNA-seq identified 1299 differentially expressed genes in MD (MT + Cd) _vs_ D (Cd). Further detailed analyses suggested that MT-regulated genes are mainly related to photosynthesis, membrane lipid peroxidation, and plant hormone metabolism. In addition, some transcription factors and heavy metal transporters were involved in MT-induced Cd tolerance in cucumber seedlings. The results laid the foundation for further elucidation of the mechanism of exogenous MT-mediated Cd tolerance in cucumber and provided a theoretical reference for the future utilization of melatonin to improve Cd tolerance in cucumber.