Rhizobium diversity in the light of evolution

Abstract Rhizobia, nitrogen-fixing symbionts of legumes, are phylogenetically diverse soil bacteria. These bacteria do not share a unique genetic strategy to achieve symbiosis. Instead, they have evolved various mechanisms to induce and invade the root/stem organs where nitrogen fixation occurs. Most rhizobial lineages emerged from horizontal transfer of few essential symbiotic genes located on mobile genetic elements. Besides, rhizobia have extensively recruited their own native functions to optimize symbiosis and refine interaction with host plants. This has likely occurred following symbiotic gene transfer and via genomic adjustments under plant selection pressure. This two-step evolutionary scenario was recently validated by an evolution experiment that progressively turned a plant pathogen into legume symbionts. Acquisition and improvement of the first symbiotic stages, nodulation and infection, were most often gained via the rewiring of the virulence and/or metabolic regulatory network of the ancestral strain.