Search for: All records

Host range specificity is a prominent feature of the legume-rhizobial symbiosis.Sinorhizobium melilotiandSinorhizobium medicaeare two closely related species that engage in root nodule symbiosis with legume plants of theMedicagogenus, but certainMedicagospecies exhibit selectivity in their interactions with the two rhizobial species. We have identified aMedicagoreceptor–like kinase, which can discriminate between the two bacterial species, acting as a genetic barrier against infection by mostS. medicaestrains. Activation of this receptor-mediated nodulation restriction requires a bacterial gene that encodes a glycine-rich octapeptide repeat protein with distinct variants capable of distinguishingS. medicaefromS. meliloti. This study sheds light on the coevolution of host plants and rhizobia, shaping symbiotic selectivity in their respective ecological niches. 

Plants have evolved the ability to distinguish between symbiotic and pathogenic microbial signals. However, potentially cooperative plant–microbe interactions often abort due to incompatible signaling. The Nodulation Specificity 1 ( NS1 ) locus in the legume Medicago truncatula blocks tissue invasion and root nodule induction by many strains of the nitrogen-fixing symbiont Sinorhizobium meliloti . Controlling this strain-specific nodulation blockade are two genes at the NS1 locus, designated NS1a and NS1b , which encode malectin-like leucine-rich repeat receptor kinases. Expression of NS1a and NS1b is induced upon inoculation by both compatible and incompatible Sinorhizobium strains and is dependent on host perception of bacterial nodulation (Nod) factors. Both presence/absence and sequence polymorphisms of the paired receptors contribute to the evolution and functional diversification of the NS1 locus. A bacterial gene, designated rns1 , is required for activation of NS1 -mediated nodulation restriction. rns1 encodes a type I-secreted protein and is present in approximately 50% of the nearly 250 sequenced S. meliloti strains but not found in over 60 sequenced strains from the closely related species Sinorhizobium medicae . S. meliloti strains lacking functional rns1 are able to evade NS1 -mediated nodulation blockade.