Studies of Phylogeny, Symbiotic Functioning and Ecological Traits of Indigenous Microsymbionts Nodulating Bambara Groundnut (Vigna subterranea L. Verdc) in Eswatini.

Rhizobial microsymbionts of grain legumes are ubiquitous in soils and exhibit a wide range of diversity with respect to colony morphology, genetic variability, biochemical characteristics, and phylogenetic relationships. This study assessed the phylogenetic positions of rhizobial microsymbionts of Bambara groundnut from Eswatini exhibiting variations in morpho-physiology, adaptive characteristics, and N2-fixing efficiency. The isolates' ERIC-PCR profiles revealed the presence of high genetic variation among them. These test isolates also exhibited differences in pH tolerance and IAA production. Multilocus sequence analysis based on the 16S rRNA, atpD, glnII, gyrB, and recA gene sequences of representative test isolates closely aligned them to the type strains of Bradyrhizobium arachidis, B. manausense, B. guangdongense, B. elkanii, and B. pachyrhizi. However, some isolates showed a high divergence from the known reference type strains, indicating that they may represent species yet to be properly characterized and described. Functional characterization in the glasshouse revealed that most of the isolates from the contrasting Agro-ecologies of Eswatini were efficient in N2 fixation, and therefore elicited greater stomatal conductance and photosynthetic rates in the homologous Bambara groundnut. Of the 75 isolates tested, 51% were more effective than the commercial Bradyrhizobium sp. strain CB756, with relative symbiotic effectiveness ranging from 138 to 308%. The findings of this study indicated that the analysis of housekeeping genes and functional traits of Bambara-nodulating microsymbionts can provide a clear view for understanding and predicting rhizobial community structure across environmental gradients.