Figure 4 Classification of metabolites and identification of DAMs. A : Classification and composition of identified metabolites. B : Principal component analysis (PCA) for the samples based on the identified metabolites.C : Numbers of DAMs identified by pairwise comparisons.D : Venn diagram of DAMs identified in PI511890 and Payzawat after GSB infection. MRT and MST represent the samples of GSB pathogen inoculated PI511890 and Payzawat, respectively. MRC and MSC represent the controls of PI511890 and Payzawat, respectively.
Several flavonoids were found to be specifically accumulated in Payzawat and PI511890 after GSB infection, respectively (Figure 5B). A total of 20 flavonoids (15 up-regulated and 5 down-regulated) were specifically induced in Payzawat, among which apigenin and its derivatives were the most abundant. Moreover, there were 23 flavonoids (17 up-regulated and 6 down-regulated) specifically induced in PI511890. Among them, chrysoeriol and its derivatives were the most abundant. These results suggested that GSB pathogen infection induces significant changes in the species and content of flavonoids in Payzawat and PI511890. Moreover, chrysoeriol and its derivatives also have the potential to be used as marker metabolites for GSB resistance in melon.
KEGG enrichment analysis of DAMs showed that seven pathways were enriched in both PI511890 and Payzawat after infection, including amino acid metabolism (alanine, aspartate and glutamate metabolism, phenylalanine metabolism), carbohydrate metabolism (C5-Branched dibasic acid metabolism, glyoxylate and dicarboxylate metabolism), biosynthesis of other secondary metabolites (flavonoid biosynthesis), chemical structure transformation maps (biosynthesis of plant hormones, and biosynthesis of phenylpropanoids) (Figure 5C). The results indicated that these pathways are involved in the common defense response of melon to GSB. Moreover, there were nine pathways specifically enriched in PI511890 after infection, including amino acid metabolism (tryptophan metabolism, phenylalanine, tyrosine and tryptophan biosynthesis), membrane transport (phosphotransferase system, ABC transporters), carbohydrate metabolism (ascorbate and aldarate metabolism), biosynthesis of secondary metabolites, metabolism of cofactors and vitamins. On the other hand, 11 pathways were specifically enriched in Payzawat after infection, such as biosynthesis of other secondary metabolites (flavone and flavonol biosynthesis), carbohydrate metabolism (pentose phosphate pathway, butanoate metabolism), lipid metabolism (linoleic acid metabolism), amino acid metabolism (lysine biosynthesis, glutathione metabolism), metabolism of cofactors and vitamins (vitamin B6 metabolism), and PPAR signaling pathway. These results demonstrated that PI511890 and Payzawat exhibit contrasting defense responses to GSB by regulating different metabolic pathways.