A Theoretical Framework for the Genetic Basis of Nitrogen Fixation Efficiency in Symbiotic Systems of Tropical Legumes in Nigeria

Biological nitrogen fixation through legume-Rhizobium symbiosis is a critical and renewable nitrogen input into Nigerian farming systems, but the genetic basis of variation in symbiotic nitrogen fixation efficiency among both the legume host and Rhizobium partners under tropical Nigerian soil conditions has been insufficiently theorised and empirically characterised. This study developed an original theoretical framework for the genetic basis of nitrogen fixation efficiency in tropical legume symbiotic systems, with empirical grounding in soybean-Bradyrhizobium and cowpea-Bradyrhizobium systems in the Guinea savanna of Kwara and Niger States. A genetic theory-building methodology combining host genome-wide association study, Rhizobium population genomics, and symbiotic trait QTL analysis was employed. Host GWAS using 250 diverse soybean and 200 cowpea accessions with 100k and 50k SNP arrays respectively identified 12 and 8 genomic regions associated with variation in acetylene reduction assay-measured nitrogen fixation efficiency and plant nitrogen content. Rhizobium population genomics of 80 isolates from the two states identified three phylogenetically distinct Bradyrhizobium populations with significant variation in nitrogen fixation gene cluster architecture (nifH, fixNOQP). The original Symbiotic NF Efficiency Genetic Framework (SNFEGF) proposes that NF efficiency is codetermined by plant loci controlling nodule vascular architecture, plant carbon supply quality, oxygen protection mechanisms, and Rhizobium nif gene expression under nodule-specific oxygen conditions, with each component having distinct genetic control. Expert review confirmed significant original theoretical contribution.

Keywords: nitrogen fixation, symbiotic efficiency, legume genetics, Bradyrhizobium, Guinea savanna Nigeria