Abstract
BackgroundDiazotrophic 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.ResultsIn 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.ConclusionThis 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.
Highlights
Diazotrophic Bradyrhizobium spp. are well known for their ability to trigger nodule formation on a variety of legume species
The bacterial communities of the root endophytic compartments of these species have been determined by 16S rDNA amplicon sequencing, after growing for 8 weeks in the summer of 2016 on an experimental plot in the Mossel (Schneijderberg et al in preparation)
The widespread abundance of Bradyrhizobium spp. in the root endophytic compartment of all studied plant species indicates that root colonization by strains of this genus is a generic phenomenon in the Mossel area
Summary
Diazotrophic Bradyrhizobium spp. are well known for their ability to trigger nodule formation on a variety of legume species. Bradyrhizobium utilizes plant-derived carbohydrates in exchange for fixed nitrogen. Whether non-diazotrophic Bradyrhizobium spp. can live in association with plants remains elusive. An eminent and well-studied example of an intimate interaction is the relationship between legumes and a paraphyletic group of nitrogen-fixing α- and β-proteobacteria, collectively known as rhizobia [1]. Defined as “slow-growing, non-acidproducing root nodule bacteria of leguminous plants” [4], this genus encompasses 37 species [5]. The genomes of Bradyrhizobium spp. are relatively large (7–10 Mb), and the nodulation (nod/nol) and nitrogen fixation (nif/ fix) gene clusters are located on a symbiotic island within the genome [8]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
