Explaining coexistence of nitrogen fixing and non-fixing rhizobia in legume-rhizobia mutualism using mathematical modeling

Abstract

In the mutualism established between legumes and soil bacteria known as rhizobia, bacteria from soil infect plants roots and reproduce inside root nodules where they fix atmospheric N 2 for plant nutri- tion, receiving carbohydrates in exchange. Host-plant sanctions against non N 2 fixing, cheating bacterial symbionts have been proposed to act in the legume- Rhizobium symbiosis, to preserve the mutualistic re- lationship. Sanctions include decreased rhizobial survival in nodules occupied by cheating rhizobia. Previ- ously, a simple population model experimentally based showed that the coexistence of fixing and cheat- ing rhizobia strains commonly found in field conditions is possible, and that the inclusion of sanctions leads to the extinction of cheating strains in soil. Here, we extend the previous model to include other factors that could complicate the sanction scenario, like horizontal transmission of symbiotic plasmids, turning non-nodulating strains into nodulating rhizobia, and competition between fixing and cheating strains for nodulation. In agreement with previous results, we show that plant populations persist even in the presence of cheating rhizobia without incorporating any sanction against the cheater populations in the model, under the realistic assumption that plants can at least get some amount of fixed N 2 from the effectively mutualistic rhizobia occupying some nodules. Inclusion of plant sanctions leads to the un- realistic extinction of cheater strains in soil. Our results agree with increasing experimental evidence and theoretical work showing that mutualisms can persist in presence of cheating partners.