Species-specific microsymbiont discrimination mediated by a <i>Medicago</i> receptor kinase.

Abstract

Host range specificity is a prominent feature of the legume-rhizobial symbiosis. <i>Sinorhizobium meliloti</i> and <i>Sinorhizobium medicae</i> are two closely related species that engage in root nodule symbiosis with legume plants of the <i>Medicago</i> genus, but certain <i>Medicago</i> species exhibit selectivity in their interactions with the two rhizobial species. We have identified a <i>Medicago</i> receptor-like kinase, which can discriminate between the two bacterial species, acting as a genetic barrier against infection by most <i>S. medicae</i> strains. Activation of this receptor-mediated nodulation restriction requires a bacterial gene that encodes a glycine-rich octapeptide repeat protein with distinct variants capable of distinguishing <i>S. medicae</i> from <i>S. meliloti</i>. This study sheds light on the coevolution of host plants and rhizobia, shaping symbiotic selectivity in their respective ecological niches.