The article discusses the mutualistic relationship between legumes and rhizobia, focusing on how host plants impose sanctions on non-cooperative rhizobia. The authors manipulate the atmosphere in soybean plants to prevent rhizobia from fixing nitrogen, a process that is beneficial to the plant but costly for the bacteria. They find that this manipulation significantly reduces the fitness of non-fixing rhizobia, leading to a decrease in their reproductive success and survival. The sanctions are likely mediated by a decrease in oxygen supply to non-fixing nodules, as evidenced by lower nodule interior oxygen concentrations and permeability. The study supports the hypothesis that host sanctions play a crucial role in stabilizing the legume-rhizobium mutualism by rewarding cooperative behavior and penalizing cheating. This mechanism could be important in stabilizing other mutualistic symbioses, especially when multiple symbiont strains coexist in a host.The article discusses the mutualistic relationship between legumes and rhizobia, focusing on how host plants impose sanctions on non-cooperative rhizobia. The authors manipulate the atmosphere in soybean plants to prevent rhizobia from fixing nitrogen, a process that is beneficial to the plant but costly for the bacteria. They find that this manipulation significantly reduces the fitness of non-fixing rhizobia, leading to a decrease in their reproductive success and survival. The sanctions are likely mediated by a decrease in oxygen supply to non-fixing nodules, as evidenced by lower nodule interior oxygen concentrations and permeability. The study supports the hypothesis that host sanctions play a crucial role in stabilizing the legume-rhizobium mutualism by rewarding cooperative behavior and penalizing cheating. This mechanism could be important in stabilizing other mutualistic symbioses, especially when multiple symbiont strains coexist in a host.