Plant-Microbe Interactions

Investigating soybean nodulation by Bradyrhizobium

Although the metabolism of legume plants and bacteria within plant nodules is relatively well known, the metabolic pathways used by free-living Rhizobia have received less attention. Rhizobia encounter diverse environments as they migrate through bulk soil, to the carbon-rich rhizosphere surrounding the roots, to the interior of the root nodules. As the bacteria survive and reproduce in these habitats, they must adjust their metabolism based on the available carbon and energy sources. Certain metals and small molecules present in the environment can act as enzyme cofactors to facilitate flux through various pathways.

Ln-dependent metabolism was first observed in Bradyrhizobia in 2019 and is attributed to the activities of the Ln-dependent methanol dehydrogenase XoxF. Methanol is a common substrate in plant environments, since it is a waste product generated during the maturation of plant cell walls. Therefore, the ability to utilize methanol as a source of carbon and energy may confer a selective advantage to Bradyrhizobia that express both XoxF and downstream steps in the methanol assimilation pathway. Bioinformatic data indicate that the Bradyrhizobia use a different methanol assimilation pathway than common model methylotrophs like Methylobacterium. Based on bioinformatics and enhanced growth in low oxygen conditions, Bradyrhizobia may be oxidizing methanol to CO₂ and assimilating this CO₂ using the Calvin Benson Cycle. We are currently testing this hypothesis by investigating the ways in which Ln might influence the growth and competition of Bradyrhizobia in the plant environment by understanding the effects of Ln on Bradyrhizobia metabolism. These findings will suggest ways to improve the symbiotic relationship between Bradyrhizobia and their symbiotic plant counterparts, either by the addition of Ln to the soil, or the genetic modification of commercial Bradyrhizobia strains.