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
 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.
