Plant polyploidy can immediately alter nodule size, N fixation rate and the identity of rhizobial symbionts hosted by polyploid legumes, but many of the mechanistic hypotheses proposed here, such as bacteroid number and enhancements of the nodule environment, remain unexplored. Although current evidence supports a role of plant polyploidy in enhancing key aspects of the legume-rhizobia mutualism, the underlying mechanisms and effects on host benefit from the mutualism remain unresolved.