Abstract
Cowpea (Vigna unguiculata L. Walp.) is indigenous to Africa, and highly valued for its N2-fixing trait and the nutritional attributes of its grain and leaves. The species’ ability to establish effective symbiosis with diverse rhizobial populations gives it survival and growth advantage in N-limited environments. To explore the functional diversity and phylogenetic positions of rhizobia nodulating cowpea in Africa, nodules were collected from various cowpea varieties grown in soils from the Guinea savanna and Sudano-sahelian agroecologies of Northern Ghana, and from the lowveld and middleveld areas of Mpumalanga Province in South Africa. Box-PCR profiling and multilocus sequence analysis revealed the presence of diverse microsymbionts responsible for cowpea nodulation across the study sites. BOX-PCR amplifications yielded variable band sizes, ranging from 618 bp to 5354 bp, which placed the isolates in six major clusters (Cluster A–F). Phylogenetic analysis based on 16S rRNA, atpD, glnII, gyrB, rpoB, nifH and nodC genes revealed the presence of diverse Bradyrhizobium sp. closely related to Bradyrhizobium daqingense, Bradyrhizobium subterraneum, Bradyrhizobium yuanmingense, Bradyrhizobium embrapense, Bradyrhizobium pachyrhizi, Bradyrhizobium elkanii and novel Bradyrhizobium species in the soils studied, a finding that could be attributed to the unique edapho-climatic conditions of the contrasting environments. The test isolates exhibited distinct symbiotic efficiencies, and also induced variable (p ≤ 0.001) photosynthetic rates, leaf transpiration, total chlorophyll and shoot biomass accumulation on cowpea (their homologous host). Canonical correspondence analysis showed that the distribution of these microsymbionts was influenced by the concentrations of macro- and micronutrients in soils. The pairwise genetic distances derived from phylogenies and nodule functioning showed significant (p < 0.05) correlation, which suggests that local environmental factors played a major role in the cowpea-Bradyrhizobium symbiosis.
Highlights
Legumes constitute an important component of sustainable agriculture and ecosystem functioning as a result of their contribution to soil health from symbiosis with nodule microsymbionts[1,2]
Cowpea can establish effective symbiosis with diverse rhizobial populations belonging to the genera Bradyrhizobium and Rhizobium[10,13], which gives this legume an ability to grow in diverse environments where other legumes may fail to survive
A total of 98 bacterial isolates were obtained from the root nodules collected from cowpea
Summary
Legumes constitute an important component of sustainable agriculture and ecosystem functioning as a result of their contribution to soil health from symbiosis with nodule microsymbionts[1,2]. Despite the importance of cowpea to the livelihoods of rural communities, its average grain yield on farmers’ fields is low (typically < 1 t.ha−1) due to a range of factors which include soil nutrient deficiencies, sub-optimal symbiosis, and insect pests diseases. The cowpea-rhizobia symbiosis can account for up to 96% of the plant’s N requirement, as well as contribute to the N needs of subsequent cereal crops in Sub-Saharan Africa, where the soils are nutrient-poor[17,18]. The identification of novel root-nodule bacteria and their evaluation for symbiotic effectiveness forms the basis for their use in inoculant formulation aimed at tapping the agronomic and ecological benefits of the legume-rhizobia symbiosis[20]
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