Robust biological nitrogen fixation in a model grass–bacterial association

Summary Nitrogen‐fixing rhizobacteria can promote plant growth; however, it is controversial whether biological nitrogen fixation (BNF) from associative interaction contributes to growth promotion. The roots of Setaria viridis, a model C4 grass, were effectively colonized by bacterial inoculants resulting in a significant enhancement of growth. Nitrogen‐13 tracer studies provided direct evidence for tracer uptake by the host plant and incorporation into protein. Indeed, plants showed robust growth under nitrogen‐limiting conditions when inoculated with an ammonium‐excreting strain of Azospirillum brasilense. 11C‐labeling experiments showed that patterns in central carbon metabolism and resource allocation exhibited by nitrogen‐starved plants were largely reversed by bacterial inoculation, such that they resembled plants grown under nitrogen‐sufficient conditions. Adoption of S. viridis as a model should promote research into the mechanisms of associative nitrogen fixation with the ultimate goal of greater adoption of BNF for sustainable crop production. Significance Statement The results indicate that, under the appropriate conditions, the C4 model grass species, Setaria viridis, can obtain 100% of its nitrogen needs from biological nitrogen fixation, as a result of inoculation with plant growth promoting bacteria. The work supports the future use of S. viridis as a model system to explore the mechanistic aspects of associative nitrogen fixation with the goal of transferring this knowledge to important crop species, such as corn.