Morphological, genetic and symbiotic characterization of root nodule bacteria isolated from Bambara groundnuts in soils of Lake Victoria basin

Abstract

Bambara groundnut (Vigna subterranea L. Verdc) was used to trap root nodule bacteria in four soils obtained from farmers’ fields in Lake Victoria basin, western Kenya. Sixty-four isolates were obtained and morphological and genetic evaluation resulted in different groups whose distribution was strongly influenced by characteristics of the soil at the site of isolation. Morphologically, majority (70%) of the isolates grew fast and very fast on Yeast Manitol Extract Agar, 26% were intermediate, while 3% were slow growers. The fast, very fast and some intermediates showed positive acid reaction, while the rest showed positive reaction in alkaline conditions. The 16S rRNA gene sequences had above 97% similarities to diverse groups including Rhizobium sp., Bradyrhizobium sp., Burkholderia sp. and Agrobacterium sp. previously isolated from crop plants. Phylogenetic analysis of 18 representative sequences yielded 3 clades with Rhizobium and Agrobacterium type sequences belonging to a single clade, while Bradyrhizobium and Burkholderia type sequences clustering into separate clades. In glasshouse experiments, 7 isolates identified as Bradyrhizobium sp. and Burkholderia sp. produced above 20 effective nodules per plant and corresponding high total biomass values, with one Rhizobium sp. producing similar results. The rest of the isolates showed low levels of nodulation, most of which were ineffective. This study revealed a high diversity of rhizobium bacteria symbiotic to bambara groundnuts in the soils of L. Victoria basin, which should be utilized to enhance its productivity and to improve databases on indigenous rhizobia populations.