3.6 Identification and functional enrichment analysis of DAMs
The samples at 24 hpi were further selected for widely targeted metabolome analysis. A total of 910 metabolites were identified, which could be divided into 11 classes (Figure 4A and Supplementary Table 5). Among them, flavonoids were the largest group, accounting for 22.53% of the total identified metabolites. Less variations were observed among biological replicates for each sample, but there were great variations between samples of Payzawat and PI511890 based on PCA analysis (Figure 4B).
A total of 132 DAMs (83 up-regulated and 49 down-regulated) were induced in PI511890 by GSB pathogen inoculation (Figure 4C), and 153 DAMs (73 up-regulated and 80 down-regulated) were detected in Payzawat. There were 36 DAMs commonly detected in PI511890 and Payzawat after GSB pathogen infection (Figure 4D). Interestingly, there were six DAMs showing completely opposite accumulation patterns in Payzawat and PI511890 (Figure 5A). Among them, oxalic acid and methyl caffeate were reported to be involved in disease resistance. The accumulation of oxalic acid was increased in resistant PI5111890 while decreased in susceptible Payzawat after GSB pathogen inoculation. An opposite accumulation pattern was observed for methyl caffeate. These results suggested that methyl caffeate is probably not directly involved in the GSB resistance. However, oxalic acid has the potential to be used as a marker metabolite for GSB resistance in melon, especially at the early stage.