ThenodHandnodQHost Range Genes ofRhizobium melilotiBehave as Avirulence Genes inR. leguminosarumbv.viciaeand Determine Changes in the Production of Plant-Specific Extracellular Signals
Transfer of a plasmid carrying the Rhizobium meliloti host range nod genes to R. leguminosarum bv. viciae 1) enabled R. l. bv. viciae to infect and nodulate alfalfa, the normal host of R. meliloti, and 2) strongly inhibited the ability of R. l. bv. viciae to infect and nodulate its normal host, common vetch. Using transposon Tn5-linked mutations, we showed that R. meliloti nodH and nodQ genes were responsible for the changes in the symbiotic specificity of R. l. bv. viciae. Using root hair deformation (Had) and root deformation (Tsr) bioassays on alfalfa and vetch, respectively we demonstrated that the sterile supernatant solutions of cultures of the various R. l. bv. viciae derivatives, in which the nod genes had been induced, contained specific extracellular factors active on alfalfa or vetch. A correlation was observed between the specificity of the symbiotic behavior of bacterial cells in the nodulation tests and the specificity of their sterile filtrates in the bioassays, which indicates that in the R. l. bv. viciae hybrids the R. meliloti nodH and nodQ genes determine the changes in host range by helping to convert the vetch-specific signal(s) into an alfalfa-specific one(s). Both types of signals active on alfalfa or vetch are heat-stable, have a molecular mass less than 5,000 Da, and have some hydrophobic properties.