RHIZOPINES - THEIR ROLE IN SYMBIOSIS AND COMPETITION

The rhizopine concept is the name given to a recently discovered pheno menon found in some rhizobia-legume symbioses whereby novel compounds are synthesised in bacteroids within nodules and subsequently cataboli sed by the free-living rhizobial strain, but by few other bacteria. In ositol rhizopines, the first found, are present in R. meliloli and R. leguminosarum by, viciae at a frequency of 10 and 14% respectively. In these strains synthesis of the rhizopine is invariably associated wit h the ability to catabolise it. Furthermore, the genes for rhizopine s ynthesis and catabolism are located on the Sym plasmids of these strai ns. We discuss the molecular genetics of the two most studied inositol rhizopines from R. meliloti and describe the coordinated regulation o f rhizopine biosynthesis and nitrogen fixation. Competition studies be tween different rhizobia show that in the presence of a rhizopine prod ucing strain the strain that can catabolize the rhizopine occupies a h igher percentage of the nodules. Possible mechanisms of how this might occur are discussed. Rhizopines show potential for use in improved rh izobial inoculants and possibly for use in the promotion of rhizospher e colonisation by other microorganisms.