Melatonin enhances the tolerance of apple seedlings to salt stress by regulating the microbial community structure and metabolite composition of the rhizosphere

Purpose The potential roles of melatonin (MT) in alleviating salt stress in apple plants ( Malus hupehensis ) are not understood. Methods We conducted pot experiments in which such plants were exposed to either 0 or 200 mmol/L NaCl together with 100 μmol/L MT. Results MT enhanced salt stress tolerance through activation of the antioxidant system and regulation of Na + and K + balance, which restored plant growth. We used high-throughput sequencing and metabolomics methods to investigate the effects of MT on the microbial community structure and metabolite composition of the rhizosphere. Exogenous MT significantly altered the community structure of bacteria and fungi and increased cooperation among microbial communities. In addition, the communities were enriched with microbes in the genera Thauera , Nocardioides , Desulfuromonas , Amaricoccus , and Microascus , potentially beneficial microorganisms regulated by MT. Exogenous MT also altered the metabolite composition of the soil and increased the relative abundance of beneficial metabolites. Importantly, Spearman correlation analysis revealed that potentially beneficial microorganisms appeared to be recruited by the exudation of (2S)-3-hydroxy-1,2-propanediyl dibutanoate. Conclusions Our results show that MT effectively reduce ROS, regulate Na + and K + balance, and regulate specific microbial communities and metabolites, thus alleviating the inhibitory effects of salt stress. These results improve our understanding of the mechanisms by which MT alleviates salt stress in plants.