Figure 5 KEGG enrichment analysis of DAMs and in-depth analysis of key metabolites. A : Six DAMs (a-f) with opposite accumulation patterns in Payzawat and PI511890. The horizontal axis indicates the value of log2FoldChange. Blue bars represent the down-regulated DAMs. Orange bars represent up-regulated DAMs. B : Flavonoids specifically accumulated in Payzawat and PI511890 after GSB pathogen infection. C : KEGG enrichment analysis for DAMs. 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.
3.7 Integration of transcriptome and metabolome profiles
To systematically understand the defense response of melon to GSB at both transcriptomic and metabolic levels, an integrated transcriptome and metabolome analysis was performed. A total of eight and 18 KEGG pathways were significantly enriched in transcriptome and metabolome in PI511890 after infection, respectively (Figure 6A). However, only phenylpropanoid biosynthesis and flavonoid biosynthesis were commonly enriched for both DEGs and DAMs in PI511890 after infection. Similarly, a total of 21 and 20 KEGG pathways were enriched for DEGs and DAMs in Payzawat after infection, respectively (Figure 6A). Several pathways, including amino acid metabolism (alanine, aspartate and glutamate metabolism, phenylalanine metabolism, butanoate metabolism, glutathione metabolism), carbohydrate metabolism (pentose phosphate pathway, glyoxylate and dicarboxylate metabolism), and biosynthesis of other secondary metabolites (phenylpropanoid biosynthesis, phenylalanine, tyrosine and tryptophan biosynthesis, flavone and flavonol biosynthesis), were commonly enriched in transcriptome and metabolome in Payzawat after infection.
Since phenylpropanoid biosynthesis was the only pathway commonly enriched for DEGs and DAMs in both PI511890 and Payzawat after GSB pathogen infection, the profiles of genes and metabolites in this pathway were further analyzed (Figure 6). Except for the gene encoding chalcone isomerase (MELO3C016680.2 ) that was down regulated in PI511890, the other genes were mainly upregulated in both PI511890 and Payzawat after GSB pathogen infection. Accumulation of lignins decreased in PI511890 but not changed in Payzawat. Accumulation of eriodictyol specifically increased in PI511890, while apigenin specifically increased in Payzawat. Furthermore, glyoxylate and dicarboxylate metabolism and metabolic pathways were also enriched in transcriptome and metabolome, in which the accumulation of oxalic acid in PI511890 was significantly upregulated in glyoxylate cycle and citric acid cycle (TCA cycle) (Supplementary Figure 5, Supplementary Figure 6). These results demonstrated that eriodictyol and oxalic acid have the potential to be used as marker metabolites for GSB resistance in melon. Moreover, the inconsistency of gene expression patterns and the related metabolite accumulation patterns suggested that post-transcription regulation is widely involved in the defense response of melon to GSB.