Plasmids of Rhizobium and Their Role in Symbiotic Nitrogen Fixation

Abstract

Publisher Summary This chapter discusses the plasmids of Rhizobium and their role in symbiotic nitrogen fixation. Rhizobia are gram-negative soil bacteria which fix nitrogen in a symbiotic association with plants of the family Leguminosae; however, this classical definition must be extended now to include nodulation and nitrogen fixation on a nonlegume, Parasponia . Rhizobium parasponia can infect this tropical tree and fix nitrogen in a manner very similar to that observed in legumes. In both the cases, establishment of the symbiosis starts with invasion of plant root or stem by free-living rhizobia followed by a series of steps that result in the formation of a nodule. It is in these nodules that nitrogen fixation takes place. Both the plant and the bacteria undergo differentiation that is regulated by gene expression. Members of the genus Rhizobium are of great economic importance because of their ability to fix nitrogen. The genus has somewhat informally been separated into those species that are fast growers and those that are slow growers. An understanding of the genes of Rhizobium involved in plant symbiosis and nitrogen fixation has moved very rapidly with the fast-growing strains ( Rhizobium leguminosarum , Rhizobium trifolii , Rhizobium meliloti , and R. parasponia and R.fredii ).