Biological nitrogen fixation has a global importance on par with photosynthesis and contributes one of the most widely limiting elements for terrestrial plant growth. Leguminous plants have been studied for centuries due to their symbiotic (‘living together’) and typically mutualistic associations with specific soil-dwelling bacteria collectively known as rhizobia. These rhizobia reduce atmospheric nitrogen gas to ammonia within specialized root organs called nodules and engage in complex signaling and metabolic interactions with their host plants. However, these interactions are known to be strongly context-dependent with varying functionality in different abiotic conditions. Growing evidence demonstrates that the biotic context can also dramatically influence the symbiosis. This work uses controlled inoculation experiments to test the effects of both specific non-rhizobial bacteria and complex soil rhizosphere communities on the rhizobium-legume symbiosis in Medicago.
Results/Conclusions
We find that a rhizosphere associated bacterium enhances plant growth in an additive manner with nitrogen-fixing rhizobia. This strain enhances the performance of rhizobia in nodules, while rhizobia depress rhizosphere populations. We also find that while total soil communities do not alter plant performance, they can reduce the symbiotic interaction, i.e., number of nodules formed by rhizobia, in a host plant genotype dependent manner. Ongoing work is investigating the impact of a broader collection of rhizosphere and endosphere isolates on symbiosis. This work suggests the complex communities of microbes that plants associate with in nature can interact with one another via mechanisms operating through the host plant.