Paired Medicago receptors mediate broad-spectrum resistance to nodulation by Sinorhizobium meliloti carrying a species-specific gene.

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 () locus in the legume blocks tissue invasion and root nodule induction by many strains of the nitrogen-fixing symbiont . Controlling this strain-specific nodulation blockade are two genes at the locus, designated and , which encode malectin-like leucine-rich repeat receptor kinases. Expression of and is induced upon inoculation by both compatible and incompatible 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 locus. A bacterial gene, designated , is required for activation of -mediated nodulation restriction. encodes a type I-secreted protein and is present in approximately 50% of the nearly 250 sequenced strains but not found in over 60 sequenced strains from the closely related species . strains lacking functional are able to evade -mediated nodulation blockade.