DOI: 10.1073/pnas.1700460114">

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

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Proceedings of the National Academy of Sciences


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. The legume–rhizobial symbiosis results in the formation of root nodules that provide an ecological niche for nitrogen-fixing bacteria. However, plant–bacteria genotypic interactions can lead to wide variation in nitrogen fixation efficiency, and it is not uncommon that a bacterial strain forms functional (Fix+ ) nodules on one plant genotype but nonfunctional (Fix−) nodules on another. Host genetic control of this specificity is unknown. We herein report the cloning of the Medicago truncatula NFS1 gene that regulates the fixation-level incompatibility with the microsymbiont Sinorhizobium meliloti Rm41. We show that NFS1 encodes a nodule-specific cysteine-rich (NCR) peptide. In contrast to the known role of NCR peptides as effectors of endosymbionts’ differentiation to nitrogen-fixing bacteroids, we demonstrate that specific NCRs control discrimination against incompatible microsymbionts. NFS1 provokes bacterial cell death and early nodule senescence in an allele-specific and rhizobial strain-specific manner, and its function is dependent on host genetic background.

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DOI: 10.1073/pnas.1700460114