Microsymbiont discrimination mediated by a host-secreted peptide in <i>Medicago truncatula</i>.

Abstract

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 <i>Medicago truncatula NFS1</i> gene that regulates the fixation-level incompatibility with the microsymbiont <i>Sinorhizobium meliloti</i> Rm41. We show that <i>NFS1</i> 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. <i>NFS1</i> 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.