Polyploidy is a major driver of evolutionary novelty and speciation in plants. Although we have made significant advances in understanding how plant polyploidy affects genotypes, phenotypes, and abiotic interactions, much less is known about how polyploidy influences plant-biotic interactions. One particularly important biotic interaction is the legume-rhizobia mutualism, in which rhizobia bacteria fix nitrogen in exchange for carbon provided by legume hosts. This mutualism regulates global nutrient cycles and plays a prominent role in the distribution and diversification of the Fabaceae. Despite the widespread importance of this mutualism, we do not fully understand how plant polyploidy affects mutualism traits nor plant benefit from it. Thus, we addressed the following questions: (1) Do polyploid plants nodulate more frequently than diploids across four rhizobial environments? (2) Do polyploid plants produce more nodules than diploids? (3) Do polyploid plants benefit more from the mutualism than diploids? We treated 6 diploid and 6 tetraploid wild accessions of Medicago sativa with four rhizobial environments (inoculated with one of three Sinorhizobium meliloti strains or water). Mutualism traits were quantified as the number, size, and dry biomass of root nodules/plant, whereas host benefit was quantified as leaf number and dry biomass of root and shoot tissue.
Tetraploid Medicago accessions nodulated more frequently (F = 6.49, p = 0.015) and produced significantly more nodules per plant than diploids (F = 5.99, p = 0.014) in three of the four rhizobial environments. Rhizobial environment, but not plant ploidal level, significantly affected host growth response in number of true leaves produced (F = 111.69, p < 0.001). However, tetraploids produced significantly greater shoot and root biomass across all treatments than diploids. Together, these results suggest that plant polyploidy enhances the legume-rhizobia mutualism by increasing nodulation frequency and nodule production, which subsequently increases host benefit from the mutualism. Evaluating the role of plant polyploidy in enhancing the legume-rhizobia mutualism provides critical contributions to ecological and evolutionary theory as well as agricultural practices.