Relevance of tolC and expression of acrAB and emrAB in Erwinia chrysanthemi

Plants produce a repertoire of antimicrobial chemicals, some of which are produced in response to an infection. Although much is understood about the virulence capabilities of many bacterial plant pathogens, little is known about the mechanisms employed by phytopathogens to survive die onslaught of the plant chemical environment. Here, die first report of the role of TolC in phytopathogenesis is presented. TolC is the outer membrane component of several multi-drug resistance (MDR) efflux pumps, such as AcrAB and EmrAB in E. coli, and plays an important role in the survival and virulence of many bacterial animal pathogens. A tolC mutant of E. chrysanthemi was found to be extremely sensitive to antimicrobial agents including several plant-derived chemicals. This mutant was unable to grow in planta and its ability to cause plant tissue maceration was severely compromised. The tolC mutant was shown to be defective m the efflux of berberine, a model antimicrobial plant chemical. These results suggest that the E. chrysanthemi to/C plays an important role in the survival and colonization of the pathogen in plant tissue by conferring resistance to the antimicrobial compounds produced by plants. Therefore, in order to assess the plausible implication of MDR during plant disease, the expression of the E. chrysanthemi acrAB and emrAB homologs was investigated using their respective promoter-fusions to reporter genes. Both pumps appear to be expressed in planta and in vitro in the presence of several plant-derived molecules. Plant-derived molecules, such as salicylic acid, hydrogen peroxide, paraquat, and genistein have previously been shown to stimulate antibiotic resistance in other bacteria. Interestingly, jasmonate, which is produced in plants as part of the plant defense response to pathogen invasion, was found to stimulate expression of both the pumps in E. chrysanthemi. This is the first report of jasmonate-dependent expression of bacterial efflux pump genes. Avirulence of the E. chrysanthemi tolC mutant and expression of acrAB and emrAB pumps in planta as well as in vitro in the presence of plant-defense related molecules suggest that MDR efflux pumps may play an important role in pathogenesis of this bacterium.