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

Abstract

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 <i>Nodulation Specificity 1</i> (<i>NS1</i>) locus in the legume <i>Medicago truncatula</i> blocks tissue invasion and root nodule induction by many strains of the nitrogen-fixing symbiont <i>Sinorhizobium meliloti</i>. Controlling this strain-specific nodulation blockade are two genes at the <i>NS1</i> locus, designated <i>NS1a</i> and <i>NS1b</i>, which encode malectin-like leucine-rich repeat receptor kinases. Expression of <i>NS1a</i> and <i>NS1b</i> is induced upon inoculation by both compatible and incompatible <i>Sinorhizobium</i> 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 <i>NS1</i> locus. A bacterial gene, designated <i>rns1</i>, is required for activation of <i>NS1</i>-mediated nodulation restriction. <i>rns1</i> encodes a type I-secreted protein and is present in approximately 50% of the nearly 250 sequenced <i>S. meliloti</i> strains but not found in over 60 sequenced strains from the closely related species <i>Sinorhizobium medicae</i>. <i>S. meliloti</i> strains lacking functional <i>rns1</i> are able to evade <i>NS1</i>-mediated nodulation blockade.