Genetic diversity of root-nodulating Rhizobia associated with cowpea genotypes in different agro-ecological regions: a review

Cowpea (Vigna unguiculata (L.) Walp) is an essential legume crop that plays a significant role in sustainable agricultural systems, especially in the dry and semi-arid areas of Sub-Saharan Africa. It belongs to the group of tropical and subtropical legume species that readily form symbiotic relationships with rhizobial bacteria which enhance soil fertility, improve agricultural yields, and boost overall productivity. Current agricultural practices emphasize the adoption of sustainable, eco-friendly farming techniques that are practical and economically beneficial for smallholder farmers who often face resource limitations. The identification, selection, and characterization of indigenous rhizobia for their symbiotic capabilities are critical steps in developing inoculants that promote sustainable legume production during the challenges of climate change. Moreover, these inoculants can enhance nitrogen fixation even in challenging environmental conditions. A majority of rhizobial strains used as bio-fertilizers can improve soil health and stimulate nitrogen fixation in leguminous plants. Many native rhizobial strains were recognized for their ability to effectively nodulate cowpea roots, including prominent symbiotic bacteria such as Rhizobium spp., Bradyrhizobium spp., Enterobacter spp., Paraburkholderia phenoliruptrix, Pseudomonas sp., Stenotrophomonas sp., and Mesorhizobium spp. This review offers a comprehensive assessment of the biodiversity of indigenous symbiotic rhizobia associated with cowpea cultivation in various agro-ecological regions. It highlights their genetic diversity and offers valuable insights for the development of effective agricultural inoculants.