Citral enhances disease resistance in postharvest citrus fruit through inducing jasmonic acid pathway and accumulating phenylpropanoid compounds

Citral, a monoterpene naturally present in volatile oils of Litsea cubeba, Cymbopogon flexuosus and citrus peel, can enhance disease resistance in postharvest citrus fruit and reduce fruit decay due to Penicillium digitatum. To clarify the defense mechanism involved in the enhancement of disease resistance, RNA-seq, metabolomics and biochemical analyses were integrated to global change in citral-treated fruit. Results showed that citral fumigation was effective at controlling citrus green mold. Citral increased the activities of phenylalanine ammonialyase, peroxidase, and polyphenol oxidase, followed by elevated contents of total phenolics, flavonoids, and lignin. Metabolomics revealed that citral treatment triggered accumulation of the plant hormones methyl jasmonate, abscisic acid, and indoleacetic acid, and many phenylpropanoid metabolites including curcumin, cinnamyl alcohol, hesperidin, syringin, 7-methoxyflavonol, vitexin-2-O-rhamnoside, coniferin, naringin, kaempferol-7-neohesperidoside, and trans-cinnamaldehyde. RNA-seq results revealed that the expression levels of multiple genes involved in jasmonic acid (JA) profiles and phenylpropanoid biosynthesis were markedly upregulated by citral. These findings suggest that the JA pathway is positively involved in the induced disease resistance of citrus fruit by citral, and provide a novel theoretical support by which citral fumigation manages postharvest citrus green mold.