Comparative transcriptomic analysis of powdery mildew resistant and susceptible melon inbred lines to identify the genes involved in the response to Podosphaera xanthii infection

Powdery mildew (PM) is a major fungal disease caused by Podosphaera xanthii or Golovinomyces cichoracearum in melons worldwide. However, the genes and mechanism involved in the resistance to PM in melon have not yet been elucidated. In this study, RNA-seq analysis of melon between a resistant (wm-6) inbred line and a susceptible (12D-1) inbred line was conducted under infected and control conditions. A total of 3000 significant differentially expressed genes (DEGs) were identified, with 1526 DEGs between the two lines under the controlled conditions, suggesting a significant difference in the basal gene expressions between them. Additionally, our results indicated that: (1) the active response of 12D-1 to P. xanthii infection significantly differed from that of wm-6; (2) the regulatory network of 12D-1 in response to P. xanthii infection was more complex and diverse than that of wm-6; (3) the pathways related to phenylpropanoid biosynthesis and the DEGs that encode glucan endo-1,3-beta- glucosidases and cytochrome P450 proteins may play important roles in the resistance of P. xanthii infection; (4) the peroxidase (POD) activities were induced in the two lines by upregulating the expression of different POD genes during PM infection. These results will not only provide new insights into molecular resistance mechanisms against PM infection but may also provide us with a new gene pool for breeding PM-resistant melon varieties.