The reverse mutation of CsMLO8 results in susceptibility to powdery mildew via inhibiting cell wall apposition formation and cell death in cucumber (Cucumis sativus L.)

Powdery mildew (PM) is an obligate biotrophic pathogen that requires living cells to grow and reproduce, causing huge losses in cucumbers. However, the resistance mechanism against PM in cucumber remains poorly understood. In this study, a susceptible mutant m6326 was found from the EMS-mutagenized population of No.26. Genetic analysis of the F2 population and sequence analysis showed that No.26 was a natural mlo mutant, and m6326 was a CsMLO8 revertant. The phytopathological investigation and transcriptome analysis of the resistant line (No.26) and susceptible revertant (m6326) after PM infection were carried out. The higher rate of spore penetration, longer hypha and more conidia in m6326 were observed than that in No.26. Histological analysis showed that the formation of cell wall apposition and cell death in No.26 leaves was faster and more than that in m6326 after powdery mildew infection. Transcriptome analysis identified differentially expressed genes between the resistant and susceptible lines. The phenylpropanoid biosynthesis pathway and ROS scavenging genes were significantly up-regulated in No.26 compared with that in m6326. Our findings suggested that the reverse mutation of CsMLO8 results in susceptibility to powdery mildew, which was related to cell wall apposition formation and cell death.