Rhizobia protect their legume hosts against soil-borne microbial antagonists in a host-genotype-dependent manner

Abstract

Microbial inhabitants of the rhizosphere can have substantial impacts on the fitness of their associated host plants, in both beneficial and detrimental ways. Soil-borne pathogens can impose severe fitness costs that can be mitigated or eliminated in many cases by co-occurring beneficial bacteria that directly or indirectly temper pathogens’ antagonistic effects. Rhizobial bacteria are best known for their role as nitrogen fixing symbionts in the rhizobia-legume mutualism but there is growing evidence that they can also act as protective agents against microbial pathogens. This study examined the role of rhizobial bacterium Ensifer medicae in protecting the burclover, Medicago polymorpha, against antagonistic soil microbes in complex soil communities. Exposing plants to concentrated slurries of soil inoculum had an adverse effect on all aspects of plant fitness in M. polymorpha. However, inoculating plants with rhizobia increased plant survival in the presence of live soil inoculum from 18% to over 80%. In addition, the soil microbiome changed the symbiotic relationship between plants and rhizobia; plants that were co-inoculated with dilute soil inoculum and a beneficial rhizobium produced more nodules with a higher nodule biomass than plants inoculated with rhizobium alone. Finally, we found that the effects of soil microbes and rhizobia on root biomass, root:shoot ratio, and nodule number differed between host genotypes, indicating there is potential for complex plant-bacterial interactions to respond to selection and potentially contribute to the maintenance of both plant genetic variation and bacterial diversity.