Legume Genomics and Genetics 2026, Vol.17, No.1, 14-31 http://cropscipublisher.com/index.php/lgg 15 The importance of nitrogen-fixing legumes has been recognized empirically for millennia, long before the mechanisms of nodulation and BNF were understood (Lindström and Mousavi, 2019). Systematic research into legume nitrogen fixation began with classical agronomy and microbiology, which documented the yield benefits and soil fertility gains associated with legumes in rotations, green manures, and intercrops, as well as the discovery of rhizobia and root nodules as the sites of symbiotic nitrogen reduction (Lindström and Mousavi, 2019). Over the twentieth century, physiological and ecological studies characterized environmental controls on fixation, including soil mineral nitrogen, water availability, temperature, and cropping system design (Ladha et al., 2022). The advent of molecular biology, genomics, and advanced isotopic methods in the late twentieth and early twenty-first centuries markedly accelerated progress. Quantitative reviews and meta-analyses have now synthesized hundreds of field and pot studies to evaluate Ndfa across legume species, climates, and management regimes, highlighting both the large potential contribution of BNF and its strong dependence on species traits and environmental conditions (Palmero et al., 2022; Yang et al., 2024). In parallel, the genetics of symbiotic nitrogen fixation (SNF) have been intensively dissected, and forward-and reverse-genetic approaches have uncovered nearly 200 plant genes required for nodulation and SNF in model and crop legumes, illuminating processes ranging from early signal perception to nodule senescence (Pankievicz et al., 2019; Roy et al., 2019; Goyal et al., 2021). These advances underpin breeding efforts to improve nitrogen fixation in crops such as soybean, chickpea, pea, and faba bean, and have spurred attempts to transfer nitrogen-fixing capabilities to cereals through synthetic biology and microbiome engineering (Mahmud et al., 2020; Qiao et al., 2024). Despite this progress, the genetic architecture of nitrogen fixation in major legume crops remains incompletely understood, particularly in the context of complex field environments and diverse rhizobial communities. Symbiotic efficiency is determined by coordinated interactions between plant and bacterial genomes, and by the composition of the broader rhizosphere microbiome, which includes “helper” bacteria that can enhance BNF through metabolic support such as vitamin production (Qiao et al., 2024; Yang et al., 2024). On the microbial side, comparative studies have revealed extensive genomic diversity among diazotrophic bacteria, including a wide range of genera capable of nitrogen fixation and large variation in the organization and content of nodulation (nod) and nitrogen fixation (nif, fix) genes (De Lima et al., 2024; Zhong et al., 2024). On the plant side, legume genomes encode elaborate signaling pathways, transcriptional networks, and metabolic systems dedicated to establishing and maintaining effective symbiosis (Pankievicz et al., 2019; Roy et al., 2019). However, many of these components have been characterized in a few model species, and relatively less is known about how nitrogen fixation-related genes are organized, diversified, and functionally constrained across major grain and oilseed legumes cultivated worldwide. Given that natural variation in BNF is substantial within and among legume species, understanding the genomic basis of this variation is essential for targeted improvement through molecular breeding and genome editing (Neda, 2021). In this context, comparative genomics provides a powerful framework for advancing nitrogen fixation research in legume crops. High-quality reference genomes and pan-genomes have now been generated for multiple legumes and their associated rhizobia, enabling systematic comparisons of gene family structures, sequence conservation, and presence–absence variation for key symbiotic functions (De Lima et al., 2024; Zhong et al., 2024). Comparative genomic analyses of Bradyrhizobium, for example, have shown that its pan-genome contains a small set of core housekeeping genes and a vast accessory genome enriched in nodulation, nitrogen fixation, and secretion system genes, which segregate into distinct genetic profiles linked to symbiotic capacity and host range (Zhong et al., 2024). Similar cross-species comparisons in legumes can identify conserved “core” SNF genes as well as lineage-specific expansions and neofunctionalization events in receptors, transcription factors, transporters, and metabolic enzymes that support nodulation and nitrogen assimilation (Pankievicz et al., 2019; De Lima et al., 2024). By integrating evolutionary analyses of gene duplication and divergence, phylogenetic reconstruction of nitrogen fixation-related gene families, and patterns of sequence conservation with emerging functional data, comparative genomics can reveal how nitrogen fixation pathways have diversified among major legume crops, pinpoint candidate genes and regulatory elements underlying superior fixation phenotypes, and guide marker development and genomic selection for enhanced BNF (Roy et al., 2019; De Lima et al., 2024). The present study, “Comparative Genomic Analysis of Nitrogen Fixation Genes in Major Legume Crops,” builds on this foundation
RkJQdWJsaXNoZXIy MjQ4ODYzNA==