Enhanced rhizosphere colonization of beneficial Bacillus amyloliquefaciens SQR9 by pathogen infection

Root exudates play important roles in root–soil microorganism interactions and can mediate tripartite interactions of beneficial microorganisms–plant– pathogen in the rhizosphere. However, the roles of organic acid components in this process have not been well studied. In this study the colonization of a plant growth-promoting rhizobacterium, Bacillus amyloliquefaciens SQR9, on cucumber root infected by Fusarium oxysporum f. sp. cucumerinum J. H. Owen (FOC) was investigated. Chemotaxis and biofilm formation response of SQR9 to root exudates and their organic acid components were analysed. Infection of FOC on cucumber had a positive effect (3.30-fold increase) on the root colonization of SQR9 compared with controls. Root secretion of citric acid (2.3 0.2 lM) and fumaric acid (5.7 0.5 lM) was enhanced in FOCinfected cucumber plants. Bacillus amyloliquefaciens SQR9 exhibited enhanced chemotaxis to root exudates of FOC-infected cucumber seedlings. Further experiments demonstrated that citric acid acts as a chemoattractant and fumaric acid as a stimulator of biofilm formation in this process. These results suggest that root exudates mediate the interaction of cucumber root and rhizosphere strain B. amyloliquefaciens SQR9 and enhance its root colonization.