Slight Water Loss Combined with Methyl Jasmonate Treatment Improves Actinidia arguta Resistance to Gray Mold by Modulating Reactive Oxygen Species and Phenylpropanoid Metabolism

In this study, we aimed to elucidate the mechanism through which treatment with slight water loss combined with methyl jasmonate (MeJA) regulates gray mold development in <i>Actinidia arguta</i>, focusing on reactive oxygen species (ROS) and phenylpropanoid metabolism. The results showed that water loss alone, MeJA alone, and their combination each reduced the incidence of disease, with the combined treatment showing the greatest efficacy. At the end of the storage period, the combined treatment enhanced the activities of superoxide dismutase (SOD), polyphenol oxidase (PPO), peroxidase (POD), phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H), and 4-coumarate-CoA ligase (4CL). It also increased the accumulation of defense-related substances (total phenol and lignin contents) and up-regulated <i>AaPAL</i>, <i>Aa4CL</i>, <i>AaC4H</i>, and <i>AaC3'H</i> gene expression. Furthermore, the combined treatment reduced the disease severity index from 60% to 16% and delayed onset by 2 d. In conclusion, slight water loss combined with MeJA treatment effectively suppressed gray mold. This effect may be attributed to activation of ROS metabolism, induction of phenylpropanoid metabolism, and up-regulation of related genes, which enhanced the resistance of the fruit to gray mold.