The <i>Agrobacterium fabrum</i> efflux pump PecM is produced in response to the plant exudate 4-hydroxybenzaldehyde to avoid disruption of central metabolism

<i>Agrobacterium fabrum</i> is a phytopathogen that causes crown gall disease. In the rhizosphere, it encounters plant exudates, some of which are toxic, such as 4-hydroxybenzaldehyde (4HBA). Others, including 4-hydroxybenzoate (4HB), participate in the induction of virulence genes. <i>A. fabrum</i> encodes the transcription factor PecS, which has been reported to enhance bacterial fitness in the rhizosphere. The gene encoding PecS is divergent from <i>pecM</i>, which encodes an efflux pump. PecS represses both <i>pecS</i> and <i>pecM</i>, as evidenced by increased expression in the presence of the PecS ligand urate and by elevated <i>pecM</i> expression in a <i>pecS</i> disruption strain. We report here that the expression of <i>pecM</i> is induced selectively by 4HBA. Expression of genes encoding enzymes involved in the degradation of 4HB is induced by both 4HBA and 4HB, as expected; however, overexpression of <i>pecM</i> attenuates the induction by 4HBA, suggesting that 4HBA is a substrate for PecM. Consistent with this inference, untargeted metabolomics shows that 4HBA accumulates intracellularly when <i>pecM</i> is disrupted. Analysis of PecS by thermal stability assay and DNase I footprinting suggests that 4HBA is not a ligand for PecS. Taken together, our data suggest that 4HBA is a substrate for PecM.IMPORTANCEPlant roots secrete a number of compounds that may be toxic to bacteria residing in the surrounding soil. One such bacterium is <i>Agrobacterium fabrum</i>, which infects plants and induces tumor formation. We show here that an <i>A. fabrum</i> strain in which the efflux pump PecM has been disrupted accumulates 4-hydroxybenzaldehyde, and that this plant root exudate induces the expression of <i>pecM</i>. Our data suggest that PecM and PecS, a transcription factor that regulates <i>pecM</i> expression, both function to promote <i>A. fabrum</i> fitness in the rhizosphere. As a competitive advantage in the rhizosphere is a prerequisite for subsequent plant infection, our data contribute to a more complete understanding of the <i>A. fabrum</i> infection process.