Molecular Soil Biology 2024, Vol.15, No.2, 74-86 http://bioscipublisher.com/index.php/msb 75 The aim of this study was to systematically investigate the symbiotic relationship between legumes and rhizobia, focusing on the mechanism of nitrogen fixation and its impact on sustainable agriculture. By examining the regulatory pathways, environmental interactions, and evolutionary aspects of this symbiotic relationship, this study seeks to provide a comprehensive understanding of how legumes and rhizobia collaborate to improve nitrogen availability, and hopes to highlight the critical role of natural nitrogen boosters in promoting agricultural sustainability and environmental health. 2 Historical Background and Discovery 2.1 Early observations and scientific discoveries The symbiotic relationship between legumes and rhizobia has been a subject of scientific curiosity for centuries. Early agricultural practices recognized the beneficial effects of legumes on soil fertility long before the underlying biological mechanisms were understood. The positive impact of legumes on soil health and crop yields was noted in ancient agricultural texts, highlighting the empirical knowledge of farmers who observed improved plant growth following legume cultivation (Lindström and Mousavi, 2019). The scientific exploration of this symbiosis began in earnest in the late 19th and early 20th centuries. Researchers discovered that legumes formed unique root structures called nodules, which were later identified as the sites of nitrogen fixation. This discovery was pivotal, as it linked the presence of these nodules to the enhanced nitrogen content in the soil, providing a biological explanation for the observed agricultural benefits (Lindström and Mousavi, 2019). The identification of rhizobia as the bacterial partners responsible for nitrogen fixation within these nodules marked a significant milestone in microbiology and plant sciences (Masson-Boivin et al., 2009). 2.2 Milestones in the study of legume-rhizobia interactions The study of legume-rhizobia interactions has progressed through several key milestones, each contributing to researchers’ current understanding of this complex symbiosis. One of the early breakthroughs was the recognition of the specificity between legume species and their compatible rhizobial strains. This specificity is crucial for the successful establishment of the symbiotic relationship and effective nitrogen fixation (Clúa et al., 2018). Advancements in molecular biology and genetics have further elucidated the mechanisms underlying this specificity. The discovery of Nod factors, signaling molecules produced by rhizobia, was a major milestone. These molecules are recognized by specific receptors on the legume roots, initiating the formation of nodules and the symbiotic process (Figure 1) (Costa et al., 2021). The identification and characterization of these signaling pathways have provided insights into how legumes discriminate between beneficial rhizobia and other soil microbes, including pathogens (Clúa et al., 2018; Wang et al., 2018). Recent metagenomic studies have expanded researchers’ understanding of the soil microbiome and its influence on legume-rhizobia interactions. These studies have revealed the complexity of microbial communities in the soil and their impact on plant health and symbiosis efficiency (Clúa et al., 2018). Additionally, research has shown that environmental factors, such as soil nitrate levels and herbivory, can significantly alter the dynamics of nitrogen fixation and resource allocation within the plant (Thompson and Lamp, 2021). The ongoing exploration of genetic and molecular mechanisms continues to uncover new aspects of this symbiosis. For instance, the horizontal transfer of symbiotic genes among rhizobial species has been identified as a factor contributing to the widespread distribution and evolutionary success of these bacteria (Masson-Boivin and Sachs, 2018). This horizontal gene transfer has allowed rhizobia to adapt to diverse legume hosts and environmental conditions, further enhancing their ecological and agricultural significance. 3 Biological Mechanism of Nitrogen Fixation 3.1 Structure and function of root nodules Root nodules are specialized structures formed on the roots of leguminous plants as a result of symbiotic interactions with rhizobia. These nodules house the nitrogen-fixing bacteria, providing a microaerobic environment essential for the nitrogen fixation process. The formation of root nodules begins with the recognition
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