Research highlights

Symbiotic evolution

Rhizobia are bacteria that are mutually symbiotic with leguminous plants: upon infecting their hosts, they induce the formation of nodules and fix nitrogen. The rhizobia are phylogenetically diverse and are thought to have arisen through recurrent, independent horizontal gene transfers of key symbiotic genes. However, gene transfer is not sufficient to induce symbiosis in all bacteria. Catherine Masson-Boivin and colleagues performed experimental evolution studies (PLoS Biol. 8, e1000280, 2010) of a nonsymbiotic bacteria, Ralstonia solanacearum, carrying the symbiotic plasmid of the rhizobium Cupriavidus taiwanensis. The authors used the resulting chimeric strain to repeatedly inoculate host plants. They recovered three clones of Ralstonia that were able to induce host nodulation. They then resequenced the genomes of the three experimentally evolved clones, as well as their immediate ancestors, and identified three SNPs in the hrpG and hrcV genes, which are involved in the HrpG-controlled virulence pathway. Inactivation of either gene was sufficient to confer nodulation ability on Ralstonia. Although nitrogen fixation was not achieved, the work shows that a recipient bacteria can rapidly evolve the ability to nodulate upon transfer of a symbiotic plasmid. PC