Background: Diazotrophic Bradyrhizobium spp. are well known for their ability to trigger nodule formation on a variety of legume species. In nodules, Bradyrhizobium utilizes plant-derived carbohydrates in exchange for fixed nitrogen. The genes essential for the nodulation and nitrogen-fixation trait are clustered in a genomic region, which is known as the 'symbiotic island'. Recently, novel non-diazotrophic Bradyrhizobium spp. have been found to be highly abundant in soils, suggesting that these species can also have a 'free-living' life history. However, whether non-diazotrophic Bradyrhizobium spp. can live in association with plants remains elusive.
Results: In this study, we show that Bradyrhizobium spp. are common root endophytes of non-legume plant species - including Arabidopsis thaliana (Arabidopsis) - grown in an ecological setting. From a single Arabidopsis root, four Bradyrhizobium sp. strains (designated MOS001 to MOS004) were isolated. Comparative genome analysis revealed that these strains were genetically and functionally highly diverse, but did not harbour the nodulation and the nitrogen fixation gene clusters. Comparative colonization experiments, with MOS strains and nitrogen-fixing symbiotic strains, revealed that all tested Bradyrhizobium spp. can colonize the root endophytic compartment of Arabidopsis.
Conclusion: This study provides evidence that both diazotrophic and non-diazotrophic Bradyrhizobium spp. colonize the root endophytic compartment of a wide variety of plant species, including the model species Arabidopsis. This demonstrates that plant roots form a major ecological niche for Bradyrhizobium spp., which might be ancestral to the evolution of the nodulation and nitrogen-fixation trait in this genus.
Keywords: Arabidopsis; Bradyrhizobium; Endophytic compartment; Root colonization.