Bacteria related to Bradyrhizobium yuanmingense from Ghana are effective groundnut micro-symbionts

Highlights

• Native rhizobium strains effectively nodulated a popular groundnut variety in Ghana.

• Rhizobium inoculation increased biological N₂ fixation in groundnut grown in soil.

• Ghanaian groundnut strains are genetically related to Bradyrhizobium yuanmingense.

• Native isolates are a potential source of strains for local inoculant production.

Abstract

The identification of locally-adapted rhizobia for effective inoculation of grain legumes in Africa’s semiarid regions is strategic for developing and optimizing cheap nitrogen fixation technologies for smallholder farmers. This study was aimed at selecting and characterising effective native rhizobia, from Ghanaian soils for groundnut (Arachis hypogaea L.) inoculation. From surface-disinfected root nodules of cowpea and groundnut plants grown on farmers’ fields, 150 bacterial isolates were obtained, 30 of which were eventually found to nodulate groundnut plants. After testing the symbiotic potential of these isolates on groundnut on sterilized substrate, seven of them, designated as KNUST 1001–1007, were evaluated in an open field pot experiment using ¹⁵N-labelled soil. Although ¹⁵N dilution analyses did not indicate differences among treatments in the proportion of nitrogen (N) derived from the atmosphere (%Ndfa), all seven strains increased total N derived from N₂ fixation by inoculated groundnut plants as compared to the non-inoculated control. Inoculation with KNUST 1002 led to total N accumulation as high as that of the groundnut reference strain 32H1. Genetic characterisation of the isolates by sequence analysis of 16S rRNA gene, 16S – 23S rRNA intergenic transcribed spacer (ITS) region and nodC gene revealed that isolates KNUST 1003 and 1007 were related to Rhizobium tropici, a common bean symbiont. The other five isolates, including KNUST 1002 belonged to the Bradyrhizobium genus, being closely related to Bradyrhizobium yuanmingense. Therefore, this study revealed novel native Ghanaian rhizobia with potential for the development of groundnut inoculants.