Melatonin alleviates cadmium toxicity by regulating root endophytic bacteria community structure and metabolite composition in apple

Abstract The level of Cadmium (Cd) accumulation in orchard soils is increasing, excess Cd will cause serious damage to plants. Melatonin is potent natural antioxidant and has a potential role in alleviating cadmium stress. This study aimed to investigate the effects of exogenous melatonin on a root endophyte bacteria community and metabolite composition under Cd stress. The results showed that melatonin significantly scavenged reactive oxygen species (ROS), and restored the photosynthetic system (manifested by the improved photosynthetic parameters, total chlorophyll content (TCC), and the chlorophyll fluorescence parameters (Fv/Fm)), increased the activity of antioxidant enzymes (the activities of catalase (CAT), superoxide dismutase (SOD), peroxidase (POD), ascorbate oxidase (APX)), and reduced the concentration of Cd in the roots and leaves of apple plants. High-throughput sequencing showed that melatonin increased endophytic bacterial community richness significantly and changed the community structure under Cd stress. The abundance of some potentially beneficial endophytic bacteria (Ohtaekwangia, Streptomyces, Tabrizicola, and Azovibrio) increased significantly, indicating that the plants may absorbe potentially beneficial microorganisms to resist Cd stress. The metabolomics results showed that melatonin significantly changed the composition of root metabolites, the relative abundance of some metabolites decreased, suggesting that melatonin may resist Cd stress by depleting root metabolites. In addition, co-occurrence network analysis indicated that some potentially beneficial endophytes may be influenced by specific metabolites. These results provide a theoretical basis for studying the effects of melatonin on the endophytic bacterial community and metabolic composition in apple plants.