Legume Genomics and Genetics 2024, Vol.15, No.4, 163-175 http://cropscipublisher.com/index.php/lgg 164 of the molecular basis of nitrogen-fixing symbiosis in Fabaceae, highlighting its significance for both ecological and agricultural systems. 2 Historical Perspective and Evolution of Nitrogen-Fixing Symbiosis 2.1 Evolutionary origins of nitrogen fixation in Fabaceae The evolutionary origins of nitrogen fixation in Fabaceae are deeply rooted in the plant's ability to form symbiotic relationships with nitrogen-fixing bacteria. This capability is believed to have evolved in response to nitrogen-limited environments that existed after the advent of phototrophy but before the widespread presence of oxygen in the atmosphere. The Fabaceae family, which includes legumes, has developed a highly specialized symbiotic relationship with rhizobia, a group of soil bacteria capable of fixing atmospheric nitrogen. This relationship is facilitated by the formation of root nodules, specialized structures that house the nitrogen-fixing bacteria (Liu et al., 2018). Phylogenomic studies have revealed that the ability to form these symbiotic relationships likely evolved from ancient gene duplication events that predate the origin of nodulation (Rutten et al., 2020). These events provided the genetic foundation necessary for the development of the complex signaling pathways and structural adaptations required for effective nitrogen fixation (Svistoonoff et al., 2013). 2.2 Historical advancements in the study of nitrogen fixation The study of nitrogen fixation has seen significant advancements over the years, particularly with the advent of molecular and genomic technologies. Early research focused on the morphological and physiological aspects of nitrogen-fixing symbioses, but recent studies have delved into the genetic and molecular mechanisms underlying these interactions. For instance, the sequencing of plant genomes has provided insights into the genetic basis of nitrogen-fixing symbiosis and revealed the presence of conserved gene families involved in this process. Additionally, the discovery of multiple independent losses of nitrogen-fixing symbiosis in various plant lineages has highlighted the evolutionary fragility and complexity of this trait (Griesmann et al., 2018). Advances in model legume species, such as Medicago truncatula, have also uncovered the genetic components that allow rhizobia to establish chronic intracellular infections without causing harm to the host plant. These findings have paved the way for a deeper understanding of the co-evolutionary dynamics between plants and their symbiotic partners. 2.3 Comparative analysis of nitrogen-fixing and non-nitrogen-fixing Fabaceae Comparative analyses between nitrogen-fixing and non-nitrogen-fixing Fabaceae have revealed significant differences in their genetic and physiological traits. Nitrogen-fixing species possess a suite of genes that are essential for the establishment and maintenance of symbiotic relationships with rhizobia. These genes are often conserved across different nitrogen-fixing species, indicating a common evolutionary origin (Battenberg et al., 2018). In contrast, non-nitrogen-fixing species lack these critical genes, and their genomes often show traces of lost pathways that could have supported nitrogen-fixing symbiosis. This suggests that the ability to fix nitrogen has been lost multiple times throughout the evolutionary history of Fabaceae, likely due to adverse selection pressures (Griesmann et al., 2018). Furthermore, the presence of specialized structures such as root nodules in nitrogen-fixing species contrasts with the absence of such structures in non-nitrogen-fixing species, highlighting the significant morphological adaptations associated with nitrogen fixation (Liu et al., 2018). These comparative studies underscore the evolutionary and ecological significance of nitrogen-fixing symbiosis in Fabaceae and provide valuable insights into the genetic and functional diversity within this plant family. 3 Molecular Mechanisms of Symbiotic Nitrogen Fixation 3.1 Signal exchange between host plants and rhizobium The initiation of symbiotic nitrogen fixation in Fabaceae involves a sophisticated exchange of chemical signals between the host plant and rhizobium bacteria. Plant roots secrete flavonoids, which act as signaling molecules to activate the expression of nodulation (nod) genes in rhizobium. These genes are responsible for the synthesis of lipochitooligosaccharides known as Nod factors (NFs) (Lepetit and Brouquisse, 2023). Nod factors are crucial for inducing root hair deformation, cortical cell division, and the initiation of nodule organogenesis at very low concentrations. The structural specificity of Nod factors, determined by various substitutions on their
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