Symbiotic functioning and bradyrhizobial biodiversity of cowpea (Vigna unguiculataL. Walp.) in Africa

Abstract

BackgroundCowpea is the most important food grain legume in Sub-Saharan Africa. However, no study has so far assessed rhizobial biodiversity and/or nodule functioning in relation to strain IGS types at the continent level. In this study, 9 cowpea genotypes were planted in field experiments in Botswana, South Africa and Ghana with the aim of i) trapping indigenous cowpea root-nodule bacteria (cowpea "rhizobia") in the 3 countries for isolation, molecular characterisation using PCR-RFLP analysis, and sequencing of the 16S - 23S rDNA IGS gene, ii) quantifying N-fixed in the cowpea genotypes using the 15N natural abundance technique, and iii) relating the levels of nodule functioning (i.e. N-fixed) to the IGS types found inside nodules.ResultsField measurements of N2 fixation revealed significant differences in plant growth, δ15N values, %Ndfa and amounts of N-fixed between and among the 9 cowpea genotypes in Ghana and South Africa. Following DNA analysis of 270 nodules from the 9 genotypes, 18 strain IGS types were found. Relating nodule function to the 18 IGS types revealed significant differences in IGS type N2-fixing efficiencies. Sequencing the 16S - 23S rDNA gene also revealed 4 clusters, with cluster 2 forming a distinct group that may be a new Bradyrhizobium species. Taken together, our data indicated greater biodiversity of cowpea bradyrhizobia in South Africa relative to Botswana and Ghana.ConclusionsWe have shown that cowpea is strongly dependant on N2 fixation for its N nutrition in both South Africa and Ghana. Strain IGS type symbiotic efficiency was assessed for the first time in this study, and a positive correlation was discernible where there was sole nodule occupancy. The differences in IGS type diversity and symbiotic efficiency probably accounts for the genotype × environment interaction that makes it difficult to select superior genotypes for use across Africa. The root-nodule bacteria nodulating cowpea in this study all belonged to the genus Bradyrhizobium. Some strains from Southern Africa were phylogenetically very distinct, suggesting a new Bradyrhizobium species.

The Nodule Microbiome: N2-Fixing Rhizobia Do Not Live Alone

“…An exception is V. unguiculata (cowpea), where early studies showed that nine different cowpea genotypes collected from fields in different parts of Africa had diverse bradyrhizobial strains residing in nodules based on intergenic-spacer restriction length polymorphism analysis, and the nodules also accumulated different amounts of fixed N. However, greater Bradyrhizobium diversity was observed in plants growing in a South African soil compared with soils from Ghana and Botswana (Pule-Meulenberg et al 2010). A study of the nodule microbiomes of two different cowpea genotypes growing in Brazil reported that bacterial communities were influenced more by soil type than plant genotype (Leite et al 2017).…”

Cowpea yield under the influence of cropping system, location and nitrogen fertilization

“…The ARC-IIC experimental station (Rustenburg) has clay percentage of 49.5 and receives an annual mean rainfall of 661 mm. Potchefstroom (ARC-GCI) has plinthic catena soil, eutrophic, red soil widespread (Pule-Meulenberg et al, 2010). The soil at Taung is described as Hutton, deep, fine sandy dominated red freely drained, eutrophic with parent material that originated from Aeolian deposits (Staff, 1999).…”

A large diversity of non-rhizobial endophytes found in legume root nodules in Flanders (Belgium)

“…While many studies in the past have focussed on plant species of economic importance, including soybean, common bean, cowpea, chickpea and red clover (Delorme et al, 2003;Laranjo et al, 2004;Kuklinsky-Sobral et al, 2005;Duodu et al, 2007;Hung et al, 2007;Laranjo et al, 2008;Ogutcu et al, 2008;Appunu et al, 2009;Chagas et al, 2010;Li et al, 2010;Pule-Meulenberg et al, 2010), native legume species have generally received less attention. In Belgium, legumes are restricted to the Faboideae subfamily and contain 102 plant species in 30 different genera (Lambinon et al, 1998).…”