Antibacterial Activity and Mechanism of Three Root Exudates from Mulberry Seedlings against Ralstonia pseudosolanacearum

Bacterial wilt is a significant soil-borne disease that poses a threat to mulberry production yield and quality of agricultural production worldwide. However, the disease resistance mechanisms dependent on root exudates are not well understood. In this present study, we investigated the antibacterial mechanisms of the main active substances (erucamide, oleamide, and camphor bromide) present in mulberry root exudates (MRE) againstRalstonia pseudosolanacearum(Rp), the causal agent of bacterial wilt. Our findings revealed that these three active substances inhibited the growth activity ofRpby affecting the cell morphology and extracellular polysaccharide content, as well as triggering a burst of reactive oxygen species. The active substances induced oxidative stress, leading to a decrease inRpgrowth. Additionally, the expression levels of key genes in the hrp gene cluster (hrpB,hrpX, andhrpF) and other virulence-related genes (such asripAW,ripAE,Rs5-4819,Rs5-4374,ace,egl3, andpehB) were significantly reduced upon treatment with the active substances. Further pathogenicity experiments demonstrated that root exudates (at a concentration of 1.5 mg·mL−1) delayed or slowed down the occurrence of bacterial wilt in mulberry. These findings provide valuable insight into the antimicrobial mechanisms of MRE againstRpand lay a theoretical foundation for the development and application of biocontrol agents to control mulberry bacterial wilt.