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 and greenhouse gas emissions, 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 relatively 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 strain VF39, a symbiont of 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 other roles of plasmid-encoded genes in the ability of the bacterium to survive and thrive in the soil, rhizosphere, and nodule environments. The research builds on previous expertise in the lab studying large plasmids, and the availability of genome sequences of the bacterium and multiple bacteriophages that infect it. The main themes will be 1) detailed analysis of the novel plasmid transfer and type IV secretion systems encoded on plasmids pRleVF39b and pRleVF39a and 2) identification of factors regulating plasmid transfer genes 3) identification of the protein-protein interactions involved in plasmid transfer 4) continued exploration of the role of plasmid-encoded catabolic genes in the ecological success of rhizobia, and 5) identification of plasmid-encoded loci that are involved in limiting phage infections. Information on horizontal gene transfer in rhizobia is vital to understanding the consequences of release of any future optimized strains (included genetically enhanced strains) as legume inoculants.  Understanding the attributes, including phage resistance and catabolic capacity, that allow individual strains to persist in soil, successfully colonize the root region, and compete with other strains, is of great importance in selecting and designing better inoculant strains.