Erica Houser

Legumes form a symbiosis with bacteria called rhizobia, which form nodules on their roots and fix atmospheric nitrogen for the plant in exchange for sugars. The legume-rhizobia symbiosis is globally widespread and integral to global nitrogen cycles and plant productivity, with applications in soil restoration and in agriculture as an alternative to environmentally damaging synthetic fertilizers. The development of effective rhizobial inoculants in these settings requires a deeper understanding of the legume-rhizobia symbiosis establishment. Genetic studies have indicated that recognition and utilization of compounds secreted by host roots and rhizobial motility are consistent determinants of strain success in establishing nodules, or competitiveness. Therefore, these traits could be targeted to improve the efficacy of rhizobial inoculants in agricultural settings. Here, we assessed motility of several Sinorhizobium meliloti and S. medicae rhizobia strains via 1) standard semisolid agar motility assays for swimming, swarming, and spreading motility and 2) motility assays performed in soil. We then tested for correlations among types of motility, and to previously collected plant-associated rhizobial fitness. We found that high spreading and swarming capabilities on semisolid agar did not predict dispersal rates in soil, nor nodule occupancy success, suggesting that these assays may not reflect strain competitiveness in ecologically realistic settings.