Abstract

Pseudomonas chlororaphis H2Q4 can induce systemic resistance in plants; however, the molecular mechanisms remain unclear. In this study, we found that the N-hexanoyl-L-homoserine lactone (C6-HSL) secreted by H2Q4 would upregulate the expression of phenylpropanoid biosynthesis genes and enhance the resistance of apples to Cytospora mali. Furthermore, histochemical staining revealed a significant increase in lignin deposition in the C6-HSL-treated apple leaves. Biochemical assays revealed that C6-HSL can bind to MdWRKY22, a negative regulator of lignin biosynthesis and C. mali resistance, and promote its degradation in vivo. DNA affinity purification sequencing (DAP-seq) and luciferase (LUC) assay revealed that MdWRKY22 binds to the promoter of apple mannitol dehydrogenase (MdMTD) and activates its transcription. The apple transient transformation experiment indicates that MdMTD has a negative regulatory effect on lignin biosynthesis and disease resistance. Collectively, C6-HSL reduces the transcription of MdMTD by degrading MdWRKY22, thereby enhancing the accumulation of lignin and disease resistance in apple.