Melatonin decreases resistance to postharvest green mold on citrus fruit by scavenging defense-related reactive oxygen species

•Melatonin did not increase but decreased citrus resistance to green mould.•ROS was involved in the resistance of citrus fruit to green mould.•ROS level in citrus fruit was declined after melatonin treatment.•Transcriptome analysis identified a lot of citrus DEGs upon melatonin treatment. Melatonin, an important and conserved indoleamine compound, is involved in many plant physiological functions. Postharvest diseases caused by fungal pathogens lead to huge economic losses worldwide, but nothing is currently known about the role of melatonin in fruit resistance to postharvest diseases. Here, we report on the effect of exogenous melatonin treatment of citrus fruit to postharvest green mold caused by Penicillium digitatum (Pd). Melatonin had no obvious effect on Pd germination and growth, but melatonin treatment of citrus fruit significantly promoted disease phenotype of green mold. In addition, the content and associated enzyme activity of hydrogen peroxide (H2O2) in citrus fruit was induced by Pd infection and H2O2 treatment of citrus fruit decreased disease phenotype of green mold. H2O2 content and associated enzyme activity in citrus fruit was declined after melatonin treatment. Transcriptome analysis identified a lot of differentially expressed genes from citrus upon melatonin treatment, which are mainly involved in cell wall, reactive oxygen species metabolism and plant defense. Our results indicate that melatonin decreases resistance to green mold on citrus fruit by scavenging defense-related reactive oxygen species, which provides significant insights into melatonin function in fruit resistance and also molecular mechanisms of fruit resistance to postharvest diseases.