Symbiotic interactions between microbes and plants are ubiquitous in nature. These symbioses can facilitate a plant’s ability to tolerate biotic and abiotic stress. For example, resource mutualists, such as arbuscular mycorrhizal fungi and nitrogen-fixing bacteria can not only aid nutrient acquisition but also confer tolerance to drought and pH stress. Using an annual legume, Amphicarpaea bracteata, and nitrogen-fixing bacteria, Bradyrhizobium sp., as a model system, I investigated whether symbiotic microbes mediate plant fitness responses and adaptation to abiotic stressors, including soil moisture, limited light availability, and nitrogen limitations.First, using a large reciprocal transplant experiment, I demonstrated that soil moisture is likely an important selective agent driving plant adaptation. Additionally, I found that symbiotic rhizobia influence patterns of plant adaption to soil moisture. Given the intimate relationship between plants and symbiotic microbes, such as mycorrhizae, endophytes and rhizobia, such patterns may be prevalent in nature. My results also highlight the importance of examining both biotic and abiotic factors in adaptation studies.Second, because rhizobia are notoriously difficult to manipulate in the field, and to further identify soil moisture as the selective agent driving plant local adaptation, I conducted a multi-factorial greenhouse experiment manipulating soil moisture, plant genotype and rhizobia genotype (both collected from the same wet or dry sites). While I found weak evidence of plant adaptation to soil moisture, I found that rhizobia performance was strongly affected by the match between rhizobium origin and plant origin (wet or dry sites), suggesting that plant divergence across wet and dry sites results in traits that differentially benefit rhizobium genotypes isolated from wet versus dry sites.Finally I tested for plant population variation in plant response to other key selective agents on the legume-rhizobium mutualism (light and nitrogen availability). I found that plants and rhizobia responded differently to changes in resource availability. Symbiosis was most beneficial for rhizobia under high light and low nitrogen conditions, as predicted by resource mutualisms theory. For plants, however, symbiosis was beneficial in low nitrogen treatments regardless of light conditions. These asymmetric effects of both traded resources are, in part, driven by plants’ ability to control nodulation under unfavorable conditions.
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