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.