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 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 strains VF39 and 3841, both of which have six plasmids, with an emphasis on the genes that determine plasmid replication, stability, function in cells, and potential for transfer by conjugation between bacterial strains. The research builds on previous expertise in the lab for dealing with plasmids, and the availability of genome sequences for both strains. Two of the main themes will be 1) examining how plasmids ensure, by partitioning and other mechanisms, that they are stably maintained from one generation to the next, and 2) how a novel plasmid transfer system in VF39 functions to move other plasmids between strains. In addition, we plan to continue our ongoing work on the role of plasmid-encoded genes in survival of rhizobia in the soil and in competition for colonization and infection of host plants.