Characterization of Azorhizobium caulinodans gen. nov., sp. nov., a Stem-Nodulating Nitrogen-Fixing Bacterium Isolated from Sesbania rostrata

Twenty stem- and root-nodulating bacterial strains isolated from stem nodules of Sesbania rostrata were compared by numerical analysis of 221 phenotypic features with nine strains which effectively nodulate only the roots of this plant and with representative strains from the genera Rhizobium and Bradyrhizobium. Representative organisms from the different clusters were investigated further, together with possibly related organisms, by performing comparative gel electrophoresis of whole-cell proteins and by performing deoxyribonucleic acid (DNA)-DNA and DNA-ribosomal ribonucleic acid (rRNA) hybridizations. 3H-labeled rRNA was prepared from Sesbania stem- and root-nodulating bacterial strain ORS 571T (T = type strain); [14C]rRNA from Bradyrhizobium japonicum NZP 5549T was also used. The following conclusions were drawn: (i) the Sesbania root-nodulating bacterial strains are genuine rhizobia; (ii) the Sesbania stem- and root-nodulating strains are quite different from Rhizobium and Bradyrhizobium, and thus they constitute a separate rRNA subbranch on the Rhodopseudomonas palustris rRNA branch in rRNA superfamily IV; and (iii) the closest relative of these organisms is Xanthobacter, but they are phenotypically and genotypically sufficiently different from the latter genus to deserve a separate generic rank. Because the feature of free-living nitrogen fixation is quite discriminative, a new genus, Azorhizobium, is proposed, with one species, Azorhizobium caulinodans. The type strain is strain ORS 571 (= LMG 6465).