Integrative transcriptomic and metabolomic alterations unravel the effect of melatonin on mitigating postharvest chilling injury upon plum (cv.Friar) fruit

Plum fruit often suffer severe chilling injury (CI) during storage. Plum (Prunus salicina Lindl. cv. Friar) fruit were treated with melatonin at various concentrations and stored at 0 ℃ for 6 w and additional 3 d of shelf-life at 23 ℃. Observations revealed that the treatment with 1.0 mmol L−1 melatonin mitigated CI of the fruit by reducing flesh-reddening and 'ethylene burst'. The melatonin treatment also suppressed fruit softening and maintained energy status. Results from UPLC-MS assay showed that several individual phenolic compounds involved in the biosynthesis of flavonoids and anthocyanins, especially, the red pigment of cyanidin-3-O-glucoside, were stimulated by cold stress and its removal, but the stimulation was effectively inhibited by melatonin treatment, and thereby flesh-reddening was alleviated. Results from RNA-sequencing unraveled that cold stress and its removal altered transcriptomic profiling of the plum fruit, but the melatonin treatment hindered the transcription of genes related to the anthocyanin biosynthesis. Correlation analysis indicated that seven MYBs were positively correlated with anthocyanin biosynthesis, which were generally down-regulated in the melatonin-treated plums. The integrative analysis of transcriptomic and metabolic alterations revealed differentially but delicately coordinated inhibition of postharvest physiological metabolisms of 'Friar' plums by melatonin in response to cold stress.