Plasmid biology of Rhizobium leguminosarum

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Rhizobia are bacteria that are notable for their ability to form nitrogen-fixing nodules on the roots of legume plants, such as peas, beans, lentils and alfalfa. Their ability to allow these plants to grow without addition of synthetic nitrogen-containing fertilizers, whose production and application has a high cost in fossil fuel consumption, makes them extremely important in sustainable agriculture. Production of more efficient and more competitive rhizobial strains for use as legume inoculants is a major goal in biotechnology. However, the interactions of introduced rhizobial strains, and the potential for genetic transfer between strains in the environment, are not well understood. Rhizobia are unusual among bacteria in that they have large and complex genomes that usually consist of one chromosome and several very large extrachromosomal genetic elements known as plasmids. The plasmids tend to be very stable, and some of them exhibit properties intermediate between those expected of a plasmid and those of a chromosome. This proposal will focus on the plasmids of Rhizobium leguminosarum biovar viciae strains VF39 and 3841, both of which have six plasmids and can form nodules on peas, lentils, faba beans, vetches, and various Lathyrus species. The emphasis will be on the genes that determine the mechanisms and consequences of transfer by conjugation between bacterial strains, and on the role of plasmid-encoded genes in carbon catabolism and rhizosphere competition. The research builds on previous expertise in the lab studying large plasmids, and the availability of genome sequences for both strains. The two main themes will be 1) detailed analysis of the novel plasmid transfer and type IV secretion systems encoded on plasmids pRleVF39b, pRleVF39a and pRL7JI and 2) analysis of the two distinct, alternate, plasmid-encoded systems for ribitol transport and catabolism that are present in the genomes of both strains. Theme 1 will identify all genes required for transfer of the plasmids, and identify the pathways regulating plasmid transfer. Theme 2 will study the role of ribitol catabolism in rhizosphere colonization and nodule infection, and will determine the contributions each of the two degradation pathways makes in ribitol metabolism. The possible reasons for the existence of two parallel pathways will be investigated.This research should provide important information about diversity, survival and competition, and horizontal gene transfer in an agriculturally important bacterium. This information could be very valuable for the design and application of superior legume inoculant strains for use in sustainable crop production.