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Chemotaxis towards root exudates and subsequent biofilm formation are very important for root colonization and for providing the beneficial functions of plant growth-promoting rhizobacteria (PGPRs). In this study, in comparison with other root-secreted compounds, d-galactose in the root exudates of cucumber was found to be a strong chemoattractant at the concentration of 1 μM for Bacillus velezensis SQR9. Chemotaxis assays with methyl-accepting chemotaxis proteins (MCPs) deletion strains demonstrated that McpA was solely responsible for chemotaxis towards d-galactose. Interestingly, d-galactose significantly enhanced the biofilm formation of SQR9 in an McpA-dependent manner. Further experiment showed that d-galactose also enhanced root colonization by SQR9. In addition, the secretion of d-galactose by cucumber roots could be induced by inoculation with SQR9, indicating that d-galactose may be an important signal in the interaction between plant and SQR9. These findings suggested that the root-secreted d-galactose was a signal, the secretion of which was induced by the beneficial bacteria, and which in turn induced colonization of the bacteria. [ABSTRACT FROM AUTHOR]