Rhizopines—Their role in symbiosis and competition

The rhizopine concept is the name given to a recently discovered phenomenon found in some rhizobia-legume symbioses whereby novel compounds are synthesised in bacteroids within nodules and subsequently catabolised by the free-living rhizobial strain, but by few other bacteria. Inositol rhizopines, the first found, are present in R. meliloti and R. leguminosarum bv. viciae at a frequency of 10 and 14% respectively. In these strains synthesis of the rhizopine is invariably associated with the ability to catabolise it. Furthermore, the genes for rhizopine synthesis and catabolism are located on the Sym plasmids of these strains. We discuss the molecular genetics of the two most studied inositol rhizopines from R. meliloti and describe the coordinated regulation of rhizopine biosynthesis and nitrogen fixation. Competition studies between different rhizobia show that in the presence of a rhizopine producing strain the strain that can catabolize the rhizopine occupies a higher percentage of the nodules. Possible mechanisms of how this might occur are discussed. Rhizopines show potential for use in improved rhizobial inoculants and possibly for use in the promotion of rhizosphere colonisation by other microorganisms.