Genetic variation in responses to mutualists and drought in an invasive plant
As the global climate changes, abiotic stresses become more frequent and severe for plants. While the pace of climate change appears to be increasing, rates of evolution may not keep pace with the rate of climate change. Individuals can instead mitigate the effects of abiotic stress by acclimating to their changing environments through phenotypic plasticity. Plasticity in drought-resistant traits in plants has been well documented, and mutualistic bacteria have been shown to increase the fitness of many plants in the face of stress. Genetic variation in plasticity and the ability to associate with rhizobia may affect the invasion success of some legumes. Here we address: 1) Is there genetic variability in the phenotypic plasticity of drought-resistant traits in plants? 2) Do rhizobia affect genotype responses to drought?, and 3) Do rhizobia affect plant plasticity in response to drought? We used 40 genotypes of an invasive legume, Medicago polymorpha, to manipulate drought conditions and the presence of rhizobia.
We found lower biomass in plants exposed to drought. However, the response of Medicago genotypes to drought varied with genotype. Genotypes also expressed variation in response to rhizobia and the number of rhizobia nodules affected the response to drought. These results suggest that mutualistic rhizobia may play a role beyond a simple resource mutualism, but may also protect plants from stressful abiotic conditions. Moreover, the variation in genotypes that we observed in this species that has successfully invaded many parts of the world suggests that genetically determined traits, such as drought tolerance or rhizobia association, may play a role in invasive species success.