Genetic Diversity And Effectiveness Of Elite Indigenous Nodulating Rhizobia On Soybean Productivity In South Kivu Province, Democratic Republic Of Congo

Abstract

Soybean (Glycine max L. Merr.) is an important crop in South Kivu Province of the Democratic Republic of the Congo (DRC), but its productivity has been limited by poor soil fertility. Inoculation of rhizobium is touted as an effective and sustainable way to improve soil fertility and soybean productivity. Despite the introduction of soybean nodulating rhizobia (SNR) through inoculants in South Kivu, biological nitrogen fixation and soybean yield have not increased to the desired level. This study was therefore conducted in South Kivu province to assess the genetic diversity and the effectiveness of indigenous rhizobia in enhancing biological nitrogen fixation and soybean productivity in South Kivu. The genetic diversity was assessed based on 16S rRNA, recA, glnII-2 and glnII-12 genes. Full genomes of 24 selected SNR were obtained on Miseq, libraries prepared using Nextera xt protocols and compared with the published genomes of the commercial strain Bradyrhizobium japonicum USDA 110, accession CP011360. Greenhouse and field experiments were conducted to determine the effectiveness of selected indigenous SNR on soybean’s BNF and yield enhancement. Completely Randomized Design and Randomized Complete Block design were employed in the greenhouse and field experiment respectively. A survey was conducted to assess the perception and determinants of rhizobium inoculants’adoption among soybean smallholder farmers. The 16S rRNA phylogeny showed 70 indigenous rhizobia in two major clusters while two housekeeping genes (recA and glnII) based phylogeny divided them into three clusters showing a high diversity. Six indigenous rhizobia strains and B. japonicum USDA 110 strain clustered together with high bootstrap values (84%) suggesting a high degree of relatedness. Genome features of 24 indigenous rhizobia were determined and varied significantly. The genome size was 8.383 Mb ±0.762 Mb in length with an average Guanine-Cytosine (GC) content of 62%. The chromosomes comprised a mean of 8063±975 genes, 7992±978 potential protein-coding genes, 1.2±0.43 set of rRNA genes and 57±9.8 tRNA genes. Based on genome size, the number of protein-coding genes, C-G content and tRNA, six indigenous rhizobia showed high similarity (mean genetic distance=0.04) with the commercial strains USDA110. The best inoculation treatments across all experiments were the indigenous strains NAC46 and NAC17 which improved nodulation equally or better than the commercial strain USDA 110. In the field NAC46 and NAC17 increased soybean grain yield from 2.4 t ha-1 to 3.3 t ha-1 and 3.4tha-1 respectively; indicating an increase of 68.7% and 70.8% respectively, over the commercial strain

USDA110. The survey results indicated that smallholder farmers perceive rhizobium inoculants as an affordable means (58%) of improving soybeans productivity and strongly agreed that they could use rhizobium inoculants if available in the market (73%), to enhance soybean’s biological nitrogen fixation and productivity. However, the adoption of soybean inoculants was very low (23.9%) and was highly influenced (P<0.01) by the farmer’s location, gender, type of education, the awareness of nodules, the household income and inoculant perception. This study concludes that indigenous rhizobia have higher potential for increasing soybeans yields and BNF in the South Kivu province of DRC. These indigenous SNR could be used for inoculant production for the region.