Rhizobial isolates that were obtained from both surface and deep soil samples in the Sahelian and Sudano-Guinean areas of Senegal (West Africa) under Acacia albida trees were compared with representative strains of known rhizobial species and genera. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of proteins was used to determine the taxonomic positions of these organisms and the relationships between isolates obtained from the surface and isolates obtained from deep soil. Most of the isolates belonged to eight electrophoretic clusters containing representative strains of Bradyrhizobium japonicum, Bradyrhizobium elkanii, and Bradyrhizobium sp. Isolates were also characterized by the Biolog system, and the results were compared with the results obtained by SDS-PAGE of total proteins; the level of correlation was very low. DNA-rRNA hybridizations with 16s or 23s rRNA from Bradyrhizobium japonicum LMG 6138T (T = type strain) confirmed that most of the protein electrophoretic clusters belong in the Bradyrhizobium-Rhodopseudomonas rRNA complex. Sequencing of 16s rRNA genes showed that some of the A. ulbida-nodulating isolates belong to a separate lineage together with representatives of other protein electrophoretic clusters. Other isolates that belong to the same electrophoretic cluster as the type strain of Bradyrhizobium japonicum are considered members of the lineage represented by this type strain. The first lineage is as far removed from Bradyrhizobium japonicum as it is from the genus Afipia, Bhstobacter denitnpcans, and the genus Rhodopseudomonas. The possible relationship among electrophoretic group, geographic origin, and depth of isolation at a particular site is discussed.Jordan (15, 16) divided the rhizobia into two genera, Rhizobium and Bradyrhizobium, containing fast-growing and slowly growing strains, respectively.At one time the genus Rhizobium was divided into three species, Rhizobium leguminosarum (with Rhizobium leguminosarum bv. viciae, Rhizobium leguminosarum bv. phaseoli, and Rhizobium leguminosarum bv. trifolii), Rhizobium meliloti, and Rhizobium loti. In 1984 Scholla and Elkan (27) proposed a new species, Rhizobium fredii; later, Chen et al. (3) transferred this taxon on phenotypic grounds to the new genus Sinorhizobium and described a second species, Sinorhizobium xinjiangensis. Recently, however, on the basis of the results of partial 16s rRNA sequence analyses Jarvis et al. (13) concluded that the name Sinorhizobium is a synonym of the name Rhizobium, that all fast-growing soybean-nodulating strains belong to a single species, Rhizobium fredii, and that additional studies are needed to confirm the status of Sinorhizobium xinjiangensis as a separate taxon. Since that time the following four new species have been proposed: Rhizobium galegae (21), Rhizobium huakuii (2), Rhizobium tropici (for the former Rhizobium leguminomrum bv. phaseoli type 11) (22), and Rhizobium etli (for Rhizobium leguminosarum bv. phaseoli type I strains) (28).According to Jordan (16), the genus B...

No abstract

No abstract

Soil cores were drilled under the leguminous tree Acacia albida growing in two different ecoclimatic zones of West Africa: the Sahelian area (100 to 500 mm of annual rainfall) and the Sudano-Guinean area (1,000 to 1,500 mm of annual rainfall). Soil samples were collected at different depths from the surface down to the water table level and analyzed for the presence of rhizobia able to nodulate A. albida. In both areas, population densities of rhizobia were substantially greater near the water table than near the surface. In the Sahelian area, rhizobia were present as deep as 34 m at a concentration of 1.3 × 10 3 /g of soil. In the Sudano-Guinean area, population densities at 0.5 to 4.5 m depth were higher than in the Sahelian area and, at several depths, comparable to that of temperate soils supporting legume crops (10 4 rhizobia per g of soil). Surface and deep soil isolates from all four sites were found to be slow-growing rhizobia ( Bradyrhizobium sp.). The proportion of effective isolates was almost the same within surface and deep soils.

No abstract