Root exudates play important roles in root-soil microbe interactions,but the function of root secreted signals involved in beneficial microbe-plant-pathogen associations has not been studied in detail.In this study,it was found that infection of Fusarium oxysporum f.sp.cucumerinum J.H.Owen (FOC) on cucumber had a positive effect (3.3-fold increase) on the root colonization of the plant growth-promoting rhizobacteria (PGPR) strain Bacillus amyloliquefaciens SQR9 compared with the uninfected control.Chemical analysis of root exudates showed that the secretion of citric acid (2.3 μM) and fumaric acid (5.7 μM) was enhanced in FOC infected cucumber plants while in the non-infected control those compounds were under the detection limit.Chemotaxis analysis revealed that B.amyloliquefaciens SQR9 showed enhanced chemotaxis response to the root exudates of FOC infected cucumber seedlings.Further experiments demonstrated that citric acid acts as a chemoattactant and fumaric acid as a biofilm formation stimulator for B.amyloliquefaciens SQR9.These results suggest that plants use root exudates as signals to attract and promote colonization of PGPR when infected by pathogens.