Abstract
The rhizobia-legume symbiosis is a highly important source of nitrogen (N) in both natural and agricultural systems. Nodulated legumes are found in nearly all terrestrial and even some aquatic ecosystems. Rhizobial microsymbionts are phylogenetically and genetically diverse. Their symbiotic ability is conferred by a group of approximately 400 genes, which enable nodulation and N2-fixation with the legume host and can be acquired by horizontal gene transfer. While certain rhizobial species are known to be specifically associated with particular legume hosts, recent studies also provide evidence of emerging trends in rhizobial biogeography, in which edaphic and climatic factors strongly influence rhizobial distribution patterns. Research at the Centre for Rhizobium Studies (CRS) has shown that an understanding of these distribution patterns is critical in assessing the suitability of novel legumes and their associated rhizobia for introduction into agricultural systems. Rhizobial biogeography also appears to play a role in determining which species of microsymbionts are nodule occupants of several invasive legume weeds. We present here an overview of the adaptation of different rhizobial species and genera to specific environments, with case studies of species of Burkholderia and Sinorhizobium (Ensifer). The sequenced genomes of more than 142 strains of rhizobia from 9 different genera are now available, largely through joint ventures between the CRS, the DoE Joint Genome Institute (JGI) and an international consortium of scientists (http://genome.jgi.doe.gov/programs/bacteria-archaea/GEBA-RNB.jsf). These can be used to determine connections between rhizobial genetic backgrounds and patterns of biogeographic distribution.