Postharvest application of melatonin-induced Cryptococcus laurentii suppresses blue mould in grapefruit by regulating cell wall and membrane lipid metabolism

Grapefruits are highly susceptible to postharvest green and blue molds, which accelerate spoilage. This study evaluated how postharvest application of melatonin-induced Cryptococcus laurentii (MT- C. laurentii ) influences cell wall and membrane lipid metabolism in grapefruit, aiming to mitigate or prevent spoilage. Compared to untreated grapefruit, those treated with MT- C. laurentii exhibited reduced disease severity and enhanced expression of genes associated with lipid metabolism and secondary metabolite synthesis. Furthermore, MT- C. laurentii treatment inhibited the degradation of cell wall polysaccharides and reduced the enzymatic activities and gene expression of pectinase, polygalacturonase, β-galactosidase, cellulase, and α-L-arabinofuranosidase, thus preserving the integrity of the cell wall, associated organelles, and overall cellular structure of the treated fruit. Additionally, MT- C. laurentii significantly enhanced the activities of superoxide dismutase, peroxidase, and catalase, while concurrently reducing lipoxygenase activity in grapefruit. This regulation eliminated reactive oxygen species, prevented membrane lipid peroxidation, and inhibited malondialdehyde production. As a result, the application of MT- C. laurentii significantly improved the antioxidant activity and inhibited cell wall degradation in the fruit. Furthermore, treatment with MT- C. laurentii maintained the nutritional quality and firmness of grapefruit, while also delaying its senescence.