Mutualistic rhizobia harbor genetic variation for traits related to parasite infection

Microbial mutualists partially determine many host traits, including traits related to infection by parasites. However, while microbial effects on host trait plasticity is fairly well established, whether microbial mutualists contribute to genetic variation in infection-related remains an open question. Here we paired 10 mutualistic Sinorhizobium meliloti rhizobacteria strains with 20 Medicago truncatula plant genotypes in an incomplete factorial design, and experimentally infected the plants with parasitic root-knot nematodes. We used this design to estimate rhizobia contributions to genetic variation in four infection-related traits: host resistance, parasite virulence, host tolerance, and mutualism robustness. We find that rhizobia contribute to genetic variation in host resistance and mutualism robustness, and to genetic variation in parasite virulence via genotype-by-genotype interactions with the host. Rhizobia did not contribute to variation in host tolerance. The influence of rhizobia strains on parasite resistance was partially explained by their effect on host root growth. These results underscores the influence that microbial mutualists have on their hosts response to parasite infection, and suggests that resource mutualists may impact host-parasite evolution. Teaser TextMicrobial mutualists like nitrogen-fixing rhizobacteria influence their host traits. Past work indicates that different strains of rhizobia may influence their host plants interactions with nematode parasites. But how does this influence compare to the genetic variation present in hosts? We explore the contribution of genetic variation across rhizobia strains to infection-related traits in their host and find that rhizobia contribute to genetic variation for parasite resistance and virulence in their host.