The distribution of inositol rhizopine genes in Rhizobium populations

The rhizopine concept states that rhizobia possessing rhizopine genes redirect nodule metabolites in the bacteroid to form selective growth substrates for the nodule-eliciting strain. This would suggest rhizopine strains are able to proliferate in the rhizosphere at the expense of other rhizobia unable to take advantage of the nutrient source. To determine if rhizopine strains are widely distributed in nature 332 strains of Rhizobium were surveyed for rhizopine catabolism. A total of 11% of R. meliloti and 12% of R. leguminosarum bv. viciae were able to catabolize the rhizopine l-3-O-methyl-scyllo-inosamine (3-O-MSI). These strains also could direct the synthesis of 3-O-MSI or scyllo-inosamine (SI) inside nodules. In addition, 2 of 9 Rhizobium sp. (Phaseolus) could catabolize 3-O-MSI, although these ineffective strains could not direct the synthesis of any rhizopine. No R. leguminosarum bv. trifolii, R. leguminosarum bv. phaseoli, R. etli or R. Tropici tested catabolized 3-O-MSI. All 3-O-MSI-catabolizing strains also catabolized SI. Rhizopine strains were found in both annual and perennial plants and were geographically widespread.