Metabolomic study of stress responses leading to plant resistance in mandarin fruit mediated by preventive applications of Bacillus subtilis cyclic lipopeptides

Although green mold rot caused by Penicillium digitatum is a major postharvest disease in mandarin fruit, the fruit's defense mechanism at the metabolomic level is largely unknown. Here, the expressed metabolome network leading to plant resistance to stresses induced by exposing of different agents was analyzed. Inoculation of mandarin fruits with eight individual agents, including four Bacillus cyclic lipopeptides (CLPs) produced by B. subtilis ABS-S14, three phytohormones and P. digitatum, resulted in different wound appearance on flavedo (mandarin peel) tissues. Subsequent metabolomic analysis using dansylation isotope labeling LC–MS detected and quantified 4717 metabolites, including 77 metabolites positively identified belonging to 39 metabolic pathways. The preventive applications of CLPs showed the greatest effect with many up-regulated metabolite changes in fruit tissues, including two important secondary metabolites, serotonin and tyramine, which were reported to stimulate plant defensive system during stress. Further analysis indicated that CLPs triggered the metabolism of glycine, serine and threonine, a major pathway to induce serotonin production, and activated tyrosine metabolism resulting in an increase of tyramine production. These findings provide the new insights for fruit protection manipulation from green mold pathogen invasion during postharvest storage.