Nodule branching, size, and symbiosis outcomes are shaped by natural genetic variation in rhizobial bacteria and alfalfa hosts

In symbiosis between hosts and microbes, hosts often create organs that house microbial symbionts. These organs are called nodules in the nitrogen-fixing symbiosis between legume plants and rhizobial bacteria. While many studies have focused on host traits, such as plant growth, nodule count, and their structural changes, these morphological changes could also be associated with rhizobial benefits (e.g., bacterial population size). Here, we investigated how the different morphologies of alfalfa root nodules influence the fitness benefits rhizobia and hosts receive from the symbiosis. We measured how host and strain genetic variation influenced 1) nodule branch number, area, and rhizobial fitness at the individual nodule level and 2) the proportion of branched nodules, nodule number, and plant benefits at the whole plant level. From the rhizobial perspective, we found that host genetics influenced rhizobial fitness more than rhizobial genetics, and that more branched and bigger nodules tend to release more rhizobia. From the host plant perspective, we found that the proportion of branched nodules was strongly negatively correlated with the total nodule number and positively correlated with host benefit across all three alfalfa genotypes. However, the height of the relationship differed by host genotype. Together, these results provide insight into the host and rhizobial genetic contributions to nodule morphology and symbiotic outcomes, suggesting a novel direction for further research in legume-rhizobium mutualism.