Abstract
Rhizobial bacteria are known for their capacity to fix nitrogen for legume hosts. However ineffective rhizobial genotypes exist and can trigger the formation of nodules but fix little if any nitrogen for hosts. Legumes must employ mechanisms to minimize exploitation by the ineffective rhizobial genotypes to limit fitness costs and stabilize the symbiosis. Here we address two key questions about these host mechanisms. What stages of the interaction are controlled by the host, and can hosts detect subtle differences in nitrogen fixation? We provide the first explicit evidence for adaptive host control in the interaction between Lotus japonicus and Mesorhizobium loti. In both single inoculation and co-inoculation experiments, less effective rhizobial strains exhibited reduced in planta fitness relative to the wildtype M. loti. We uncovered evidence of host control during nodule formation and during post-infection proliferation of symbionts within nodules. We found a linear relationship between rhizobial fitness and symbiotic effectiveness. Our results suggest that L. japonicus can adaptively modulate the fitness of symbionts as a continuous response to symbiotic nitrogen fixation.