In several recent studies, MDR efflux pumps of phytopathogenic bacteria were shown to be involved in the extrusion of plant-derived antimicrobial metabolites, which promotes host colonization and enhances virulence (Martinez et al., 2009, and references therein). Plant-associated soil bacteria

are challenged in several ways, for example by abiotic environmental stresses or competing organisms and their metabolic products. At least conceptually, symbiotic and phytopathogenic bacteria appear to initiate similar programs for invasion and colonization (Soto et al., 2006; Deakin & Broughton, 2009). Therefore, the expression of efflux proteins seems to be a useful common trait of these bacteria that allows them to cope with the toxic selleck screening library compounds that they may encounter click here during infection. In this work, we have characterized an RND-type multidrug efflux system, termed BdeAB, in the legume symbiont B. japonicum. Another putative efflux pump, RagCD, was described previously in B. japonicum (Krummenacher & Narberhaus, 2000). However,

ragCD mutants did not differ from the wild type in their antibiotic susceptibility profile and in their symbiotic phenotype. By contrast, we have shown here that the loss of the BdeAB proteins increases the susceptibility toward aminoglycoside antibiotics, supporting the idea that these proteins principally function as a drug efflux pump. Unlike the RmrAB efflux pump of the bean symbiont R. etli, which was shown to be required for nodulation (Gonzalez-Pasayo

& Martinez-Romero, 2000), the B. japonicum bdeAB mutant was not affected in nodule formation. However, soybean nodules elicited by this strain contained fewer bacteroids as compared with nodules formed by the wild type. The impaired colonization by the ΔbdeAB strain might account for the decreased nitrogen-fixation activity in these nodules. It is known that legumes synthesize phytoalexins not only in response to a pathogenic attack but also in the presence of rhizobia Liothyronine Sodium (see the review by Baron & Zambryski, 1995, and references therein). In fact, the RmrAB efflux pump confers tolerance to plant-derived antimicrobial compounds (Gonzalez-Pasayo & Martinez-Romero, 2000). Recently, another example of the importance of export proteins in plant–microorganisms interactions was reported. In Mesorhizobium tianshanense, a LysE-family exporter for the antimetabolite canavanine was identified, which helps those rhizobia to survive in a canavanine-rich legume rhizosphere (Cai et al., 2009). It is tempting to speculate that the BdeAB system provides a similar advantage to B. japonicum, perhaps coping with an as yet unidentified soybean-derived compound. The observation that symbiosis of the B.