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Background  Leguminous plants are able to form a root nodule symbiosis with nitrogen-fixing soil bacteria called rhizobia. This symbiotic association shows a high level of specificity. Beyond the specificity for the legume family, individual legume species/genotypes can only interact with certain restricted group of bacterial species or strains. Specificity in this system is regulated by complex signal exchange between the two symbiotic partners and thus multiple genetic mechanisms could be involved in the recognition process. Knowledge of the molecular mechanisms controlling symbiotic specificity could enable genetic improvement of legume nitrogen fixation, and may also reveal the possible mechanisms that restrict root nodule symbiosis in non-legumes.    Results  We screened a core collection of  Medicago truncatula  genotypes with several strains of  Sinorhizobium meliloti  and identified a naturally occurring dominant gene that restricts nodulation by  S. meliloti  Rm41. We named this gene as  Mt-NS1 (for   M.  truncatula  n odulation  s pecificity 1). We have mapped the  Mt-NS1  locus within a small genomic region on  M. truncatula  chromosome 8. The data reported here will facilitate positional cloning of the  Mt-NS1  gene.    Conclusions  Evolution of symbiosis specificity involves both rhizobial and host genes. From the bacterial side, specificity determinants include Nod factors, surface polysaccharides, and secreted proteins. However, we know relatively less from the host side. We recently demonstrated that a component of this specificity in soybeans is defined by plant NBS-LRR resistance ( R ) genes that recognize effector proteins delivered by the type III secretion system (T3SS) of the rhizobial symbionts. However, the lack of a T3SS in many sequenced  S. meliloti  strains raises the question of how the specificity is regulated in the  Medicago - Sinorhizobium  system beyond Nod-factor perception. Thus, cloning and characterization of  Mt-NS1  will add a new dimension to our knowledge about the genetic control of nodulation specificity in the legume-rhizobial symbiosis.

Identification of a dominant gene in Medicago truncatula that restricts nodulation by Sinorhizobium meliloti strain Rm41