Symbiotic dimensions of clover diversity

Background/Question/Methods

The biodiversity and ecosystem functions of natural plant communities is tied to their shared soil microbial communities. Diverse North American clover (Trifolium)-Rhizobium communities are an exemplar system in which positive and negative feedbacks can be dissected at molecular, functional, and taxonomic levels. Rhizobia are a primary bacterial symbiont of legumes such as Trifoliumand infect roots to form nodules, where the bacteria fix atmospheric nitrogen into biologically available forms. The overarching goal of this work is to bridge organismal and community scales to predict coexistence and inform variation in biological nitrogen fixation. We use manipulative greenhouse experiments to link symbiotic traits with performance across competitive environments along with sequence-based approaches to identify symbiont genotypes and delineate the broader nodule microbiome.

Results/Conclusions

We find that Trifolium plants make more large nodules in their home soils and that nodulation correlates with performance, demonstrating that symbiotic nitrogen-fixing rhizobia are an important part of the niche. Competition experiments show that plants allocate more to nodules when competing against members of their own species, suggesting that symbiotic interactions play a role in species coexistence. Nodule genotyping shows widespread sharing of rhizobium strains across Trifolium species with the majority of structuring occuring between different soils rather than species. Using 16S amplicon sequencing, we find that nodules harbor a small but diverse community of bacteria that is largely conserved; however, species structure this community to a small extent as does the soil from which they presumably originate, with the majority of variation in community structure explained by a soil x species interaction.