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DNA Homologies among Members of the Genus Azorhizobium and Other Stem- and Root-Nodulating Bacteria Isolated from the Tropical Legume Sesbania rostrata

¹Laboratoire d'Ecologie Microbienne, UniversitÉ Claude Bernard Lyon 1, F-69622 Villeurbanne Cedex, F-69392 Lyon Cedex 8, France
²Office de Recherche Scientifique et Technique Outre Mer, F-69392 Lyon Cedex 8, France
³UnitÉ de Recherche AssociÉe au Centre National de la Recherche Scientifique 697, F-69392 Lyon Cedex 8, France
⁴Laboratoire de BactÉriologie, FacultÉ de MÉdecine Alexis Carrel, F-69392 Lyon Cedex 8, France

^* Corresponding author.

ABSTRACT

The diversity among 191 bacterial strains isolated from stem and root nodules (151 and 40 strains, respectively) of Sesbania rostrata grown in different geographical areas in Senegal and in The Philippines was studied by using DNA-DNA hybridization techniques (S1 nuclease method), by determining DNA base compositions, by performing legume nodulation tests, and by determining nitrogenase activity. The following conclusions were drawn. (i) All of the strains produced stem and root nodules on S. rostrata. (ii) Most of the organisms (184 strains) belonged to the genus Azorhizobium; their guanine-plus-cytosine contents ranged from 66 to 68 mol%, they fixed N₂ under free-living conditions, and they produced effective nodules on the stems and roots of S. rostrata. (iii) The seven other strains probably belonged to the genus Rhizobium, since guanine-plus-cytosine contents ranged from 59 to 63 mol% and they did not fix N₂ under free-living conditions; three strains produced effective root nodules, but their stem nodules exhibited very low activity or were ineffective, and the four remaining strains produced ineffective nodules on both stems and roots. (iv) The genetic diversity among the 184 Azorhizobium strains allowed us to divide them into two genomic species; genomic species 1 constituted the major group (175 strains) and corresponded to Azorhizobium caulinodans since all of the strains were more than 79% related to type strain ORS 571, and genomic species 2 contained nine strains that were only 44 to 53% related to type strain ORS 571 (difference between the denaturation temperatures of homologous and heterologous hybrids, more than 6°C) and more than 76% related to reference strains SD02 and SG28 (difference between the denaturation temperatures of homologous and heterologous hybrids, less than 3°C). The species that were distinct from A. caulinodans cannot be named until they can be differentiated by phenotypic tests.