Microsymbiont discrimination mediated by a host-secreted peptide in Medicago truncatula

Significance The legume–rhizobial symbiosis culminates in the formation of nitrogen-fixing root nodules. This symbiotic relationship plays a critical role in sustainable agriculture because it reduces the need for nitrogen fertilizers. However, nitrogen fixation efficiency varies tremendously between different plant–bacteria combinations, and the molecular mechanisms that regulate this specificity are not well understood. We report that this specificity is regulated by nodule-specific cysteine-rich (NCR) peptides in Medicago truncatula , a model legume closely related to alfalfa ( Medicago sativa ). Our finding provides insights into cross-kingdom signaling in host–bacterial symbioses and makes NCRs attractive agents for engineering legume–rhizobia pairs to optimize nitrogen fixation performance.