Research Assistant The Pennsylvania State University State College, Pennsylvania
Body of Abstract: The perennial forage legume, Alfalfa (Medicago sativa), provides a nutrient-dense feed for livestock, increases plant-available nitrogen, and improves soil health. In collaboration with the facultative symbiont rhizobia, chimeric organs called nodules can develop within the hosts’ roots, facilitating the conversion of atmospheric nitrogen in exchange for photosynthates. These nodules exhibit variation in size and shape, with the potential for branching if the nodule meristem bifurcates. We hypothesized that the prolonged interaction between rhizobia and specific alfalfa varieties influences nodule morphology. First, to assess the factors affecting these traits in agricultural fields, we collected 276 pools of ~75 nodules from five alfalfa varieties across two Pennsylvania sites spanning multiple seasons and planting years. Within a site, we found that the planting year strongly influenced the proportion of branched nodules. Moreover, we observed consistent host variety and collection season effects across locations. Second, in our greenhouse experiment, we inoculated three alfalfa varieties with 117 individual rhizobial isolates exposed to their respective hosts for five years and measured the effect of isolate identity on the number of branched nodules, total nodule count, and chlorophyll content. We observed that the isolate identity was significantly influencing the prevalence of branched nodules, instead of the respective host identity, while the proportion of branched nodules correlated with the hosts’ chlorophyll content. Moving forward, we will identify candidate genes in the symbiont Sinorhizobium meliloti correlated with nodule branching by analyzing allele frequencies in our pooled sequence data.