Sanctions, Cooperation, and the Stability of Plant-Rhizosphere Mutualisms

There are both costs and benefits for host plants that associate with microbes in the rhizosphere. Typically, an individual plant associates with multiple microbial genotypes varying in mutualistic benefit. This creates a potential tragedy of the commons where less-mutualistic strains potentially share in the collective benefits, while paying less of the costs. Therefore, maintaining cooperation over the course of evolution requires specific mechanisms that reduce the fitness benefits from “cheating.” Sanctions that discriminate among partners based on actual symbiotic performance are a key mechanism in rhizobia and may exist in many rhizosphere mutualisms, including rhizobia, mycorrhizal fungi, root endophytes, and perhaps free-living rhizosphere microbes. Where they exist, sanctions may take different forms depending on the system. Despite sanctions, less-effective symbionts still persist. We suggest this is because of mixed infection at spatial scales that limit the effects of sanctions, variation among plants in the strength of sanctions, and conflicting selection regimes.