Effect and Underlying Mechanism of Melatonin Treatment on Rhizopus Rot in Postharvest Peach Fruit

The protective effect and underlying mechanism of melatonin (MT) treatment against Rhizopus stolonifer in postharvest peach fruit (Prunus persica (L.) Batsch cv ‘Xiahui 8’) were studied. Freshly harvested peaches were inoculated with R. stolonifer following pretreatment with different concentrations of MT (0, 0.05, 0.10, 0.50 and 1.00 mmol/L) and then stored at 20 ℃ for up to 60 h. Disease development was measured to determine the most effective MT concentration. Subsequently, the effects of MT treatment on quality parameters, total phenol and lignin contents, disease resistance-related enzyme activity and gene expression in peaches during postharvest storage, and the in vitro growth of R. stolonifer were investigated. The results showed that 0.50 mmol/L MT treatment could most significantly reduce the occurrence of Rhizopus rot and maintain postharvest fruit quality. Furthermore, compared with either of them alone, R. stolonifer inoculation after MT treatment exhibited stronger disease resistance response, resulting in a marked increase in the activities of chitinase (CHI), β-1,3-glucanase (GLU), phenylalanine ammonia lyase (PAL), cinnamate 4-hydroxylase (C4H), 4-coumarate-CoA ligase (4CL), peroxidase (POD) and polyphenol oxidase (PPO), the contents of total phenols and lignin, and the expression levels of defense-related genes such as those encoding CHI (PpCHI), GLU (PpGLU), PAL (PpPAL) and POD (PpPOD). Moreover, MT treatment had no significant inhibitory effect on the growth of R. stolonifer in vitro. Taken together, these results indicate that MT can initiate defense responses to pathogenic infection indirectly by priming defense, thereby reducing the occurrence of peach postharvest diseases.