The role of microbial resource mutualists in plant adaptation to abiotic environments

Despite more than eighty years of study, the selective agents driving local adaptation largely remain unknown, in part because populations exist in complex environments where they experience both abiotic and biotic conditions that can exert strong selection. We used a replicated reciprocal transplant experiment combined with a greenhouse inoculation experiment to investigate plant adaptation to soil moisture and the role of nitrogen-fixing rhizobium mutualists in plant local adaptation. We find that the annual legume hog peanut (Amphicarpaea bracteata) is locally adapted to soil moisture conditions and that interactions with nitrogen-fixing rhizobia likely contribute to the observed local adaptation. Specifically, plant populations from wet sites transplanted into wet habitats were more likely than those from dry sites to associate with rhizobium mutualists and formed more nodules and had higher nitrogen fixation rates when inoculated with rhizobium strains isolated from wet compared to dry habitats. As a result, local adaptation to wet environments was most apparent when plants successfully associated with rhizobia in the field. In sum, our results suggest that: 1) soil moisture is a strong cause of local adaptation in this system and 2) divergence in how plant populations interact with rhizobia likely contributes to plant local adaptation to soil moisture. These findings illustrate how biotic interactions can influence plant adaptation to a strong abiotic gradient and highlight the need to consider microbial mutualists in studies of plant local adaptation.