COS 38-2 - Evolution of rhizobia populations alter plant adaptation to the moisture environment

Background/Question/Methods

Plants are colonized by diverse microbial communities that influence plant fitness and phenotype through their various activities. Consequently, identifying the drivers of these plant-associated microbial communities may be critical in understanding plant adaptation. Herein, we specifically focused on the contribution of selection from the plant on their associated microbial symbionts and the feedback onto plant adaptation. We hypothesized that plants select microbes that facilitate locally adaptive plant phenotypes. To this end, we conducted a long-term evolution experiment, beginning with rhizobia populations composed of 28 strains mixed in equal abundance, inoculated onto populations of Trifolium repens, as well as just onto soil with no plants. Over the course of 4 plant generations, we imposed wet or dry watering treatments on these populations. Following this selection phase, we evaluated microbe-mediated adaptation by isolating and inoculating the evolved rhizobia strains back onto Trifolium under the original watering conditions. Additionally, following the selection phase, we shotgun sequenced the rhizobia populations to characterize the relative abundance of these strains in the populations. We characterized various rhizobia strain traits, which were linked to these populations’ relative abundances, allowing us to identify the rhizobia traits under selection.

Results/Conclusions

For the original 28 rhizobia strains, we identified significant variation in rhizobia traits between strains, including variation in strain biofilm production, and tolerance to high osmotic conditions. We additionally identified variation in these strains impact on plant phenotype, including their impact plant biomass under both high and low water conditions, as well as their impact on plant germination under both high and low water conditions. Moreover, the various selective treatments, including the watering (wet vs. dry) and plant (plant present vs; plant absent) treatment, were significant drivers on rhizobia population structure and trait distribution. These results suggest that the interaction between the plant and the watering environment mediate selection on these rhizobia populations, and specific rhizobia traits may be the target of selection. We are currently assessing if rhizobia inoculum isolated from the various selective treatments differed in their impact on plants grown under wet vs. dry conditions. Our results thus far highlight the importance of coevolutionary interactions between plants and their associated symbionts play a significant role in driving plant adaptation.