EjMYB15 Improves Cold Tolerance of Postharvest Loquat Fruit via Upregulating Antioxidant Enzyme Genes

As cold-sensitive fruits, loquats easily develop chilling injury (CI) during cold storage, which leads to quality deterioration and economic losses. Our prior research indicated that exogenous melatonin (MT) treatment can mitigate CI in postharvest loquats by regulating reactive oxygen species (ROS) metabolism, but the underlying molecular mechanism remains unclear. The primary objective of this study is to decipher the molecular regulatory pathway by which MT alleviates CI in postharvest loquats, focusing on the role of MYB transcription factors (TFs) in modulating antioxidant enzyme genes. Here, MT treatment remarkably reduced CI severity in loquat fruits, as reflected by lower CI index, reduced cell membrane permeability, decreased firmness, lower a* and b* values, and higher L* value, compared with the control group. Moreover, a cold-induced MYB TF, designated EjMYB15, was identified. Compared to non-treated fruits, the expression level of EjMYB15 was maintained at higher levels in MT-treated loquats. Subcellular localization and transactivation assays demonstrated that EjMYB15 is a nuclear-localized transcriptional activator. Electrophoretic mobility shift assay (EMSA) and dual-luciferase reporter (DLR) assays showed that EjMYB15 binds the MYB-binding sites (MBS) in the promoters of four antioxidant enzyme genes (EjCAT1, EjCAT2, EjGST1, and EjGST2), thereby activating their transcription. Taken together, these findings indicate that EjMYB15 positively regulates cold tolerance of loquat fruits by improving ROS scavenging capacity. These results elucidate the regulatory pathway by which MYB TFs mitigate CI and provide new theoretical support for the application of MT in alleviating CI in postharvest fruits.