trans‐ 2‐Hexenal exposure enhances disease resistance against Penicillium digitatum by stimulating phenylpropanoid biosynthesis and antioxidant capacity in postharvest citrus fruit

Abstract BACKGROUND trans ‐2‐Hexenal (TH) is a promising alternative to chemical fungicide in controlling postharvest disease in fruits. This research sought to disclose how TH controls citrus green mold and induces fruit resistance through RNA‐Seq, physiochemical and natural decay. RESULTS TH effectively delayed green mold incidence in citrus fruits. RNA‐Seq showed that multiple genes involved in the redox balance, ascorbic acid and aldehyde acid, and phenylpropanoid pathways were greatly influenced. In particular, the expression levels of genes involved in lignin and flavonoid biosynthesis – namely phenylalanine ammonia‐lyase , cinnamate‐4‐hydroxylase , 4‐coumarate‐CoA ligase , peroxidase 31 , cinnamoyl‐CoA reductase 1 , cinnamyl‐alcohol dehydrogenase1 , chalcone synthase2 , flavanone 3‐ hydroxylase , and flavonol synthase – were increased by TH treatment, which thereby promoted flavonoids and lignin accumulation. Meanwhile, the antioxidant capacity of TH‐treated citrus fruit was improved, as demonstrated by elevated levels of ascorbic acid and glutathione, and increased activities of enzymes and gene expression pertaining to the AsA‐GSH cycle, including ascorbate peroxidase 2 , glutathione reductases , monodehydroascorbate reductase 4 , dehydroascorbate reductase 2 , and glutathione s‐transferase , resulting in reduction of H 2 O 2 . Correspondingly, the protective effects of TH on citrus fruits decreased its susceptibility to Penicillium digitatum . CONCLUSION TH exposure at 5 μL L −1 induces disease resistance against P. digitatum by stimulating the biosynthesis of lignin and flavonoid, along with the AsA‐GSH cycle in the peel of citrus fruit, thus improving antioxidant capacity and maintaining redox balance in the citrus fruit. These findings suggest that TH has potential for use as a protective measure against P. digitatum in citrus fruit. © 2026 Society of Chemical Industry.