The study explores how host plants enforce sanctions against rhizobia that fail to fix nitrogen in root nodules, thereby stabilizing the legume-rhizobium mutualism. Soybean plants were shown to penalize rhizobia that do not fix nitrogen by reducing their reproductive success by about 50% in whole-plant, split-root, and single-nodule experiments. Non-invasive monitoring suggested that decreased oxygen supply might be the mechanism for these sanctions. The findings support the hypothesis that legumes select for more cooperative rhizobia by imposing sanctions based on the amount of nitrogen fixed by rhizobia. This mechanism helps stabilize mutualistic symbioses by ensuring that symbionts provide costly services to the host, indirectly benefiting competitors. The study also highlights the importance of host sanctions in maintaining mutualistic cooperation, as seen in other systems like the yucca-moth mutualism. The results demonstrate that differences in resource allocation at the nodule level are linked to differences in rhizobial fitness. The mechanism by which sanctions are carried out is likely related to oxygen supply, as non-fixing nodules had lower oxygen concentrations and permeability. The study also discusses the evolutionary implications of these findings, suggesting that host sanctions can lead to the evolutionary stabilization of mutualistic symbioses. The research provides insights into the evolutionary dynamics of mutualistic relationships and the role of host sanctions in maintaining cooperation.The study explores how host plants enforce sanctions against rhizobia that fail to fix nitrogen in root nodules, thereby stabilizing the legume-rhizobium mutualism. Soybean plants were shown to penalize rhizobia that do not fix nitrogen by reducing their reproductive success by about 50% in whole-plant, split-root, and single-nodule experiments. Non-invasive monitoring suggested that decreased oxygen supply might be the mechanism for these sanctions. The findings support the hypothesis that legumes select for more cooperative rhizobia by imposing sanctions based on the amount of nitrogen fixed by rhizobia. This mechanism helps stabilize mutualistic symbioses by ensuring that symbionts provide costly services to the host, indirectly benefiting competitors. The study also highlights the importance of host sanctions in maintaining mutualistic cooperation, as seen in other systems like the yucca-moth mutualism. The results demonstrate that differences in resource allocation at the nodule level are linked to differences in rhizobial fitness. The mechanism by which sanctions are carried out is likely related to oxygen supply, as non-fixing nodules had lower oxygen concentrations and permeability. The study also discusses the evolutionary implications of these findings, suggesting that host sanctions can lead to the evolutionary stabilization of mutualistic symbioses. The research provides insights into the evolutionary dynamics of mutualistic relationships and the role of host sanctions in maintaining cooperation.