Legume Genomics and Genetics 2025, Vol.16, No.3, 108-127 http://cropscipublisher.com/index.php/lgg 109 planting and making good use of leguminous crops in the ecological agricultural model has an important role in promoting the realization of weight loss and efficiency, soil fertility, and sustainable agricultural development. This study will systematically explore the symbiotic relationship between leguminous crops and soil microorganisms (mainly nitrogen-fixing rhizobia and arbuscular mycorrhizal fungi) and the ecological benefits they bring, and explain the role of leguminous crops in improving soil microecology, such as increasing soil organic matter, enzyme activity, and improving soil physical and chemical properties. Then, the supporting role of soil microorganisms on nutrient absorption and stress resistance of legumes is analyzed, including nitrogen fixation to improve nitrogen utilization, high drought and salt tolerance, and induction of plant immunity and biological control potential. On this basis, from the perspective of ecological and environmental benefits, the role of legume-microorganism symbiosis in reducing fertilizer use and greenhouse gas emissions, repairing degraded soil functions, and enhancing the stability of agricultural ecosystems is discussed. The above viewpoints are confirmed through specific case studies (such as soybean-rhizobium long-term positioning experiments, peanut microbial inoculation yield increase experiments, and the role of legumes in crop rotation). This study summarizes the role of symbiotic relationships in sustainable agriculture and proposes directions for future research and practice to promote its better application in agriculture. 2 Symbiotic Basis between Legumes and Soil Microbes 2.1 Classification and symbiotic mechanism of rhizobia Nitrogen-fixing rhizobia are a type of soil bacteria that can infect the roots of legumes and form nodules. They mainly belong to the α- and β-subgroups of the Proteobacteria, including multiple taxa such as Rhizobium, Bradyrhizobium, and Bacillus thuringiensis (now Ensifer/Sintoferria). Different legumes tend to form symbiotic relationships with specific types of rhizobia. For example, soybeans usually coexist with Bradyrhizobium strains, while alfalfa prefers Rhizobia strains. The process of symbiotic nitrogen fixation between rhizobia and legumes is very complex, involving strict species or strain matching and a series of exquisite "molecular dialogues". At the beginning of the symbiosis, the roots of legumes secrete flavonoids to attract rhizobia in the soil to swim toward the roots. Matching rhizobia regulate the nodulation (nod) gene through NodD, synthesize and release specific "nodulation factor" (Nod factor), which is a modified oligosaccharide signal molecule. After the receptor kinase complex on the epidermal cells of the plant roots (such as NFR1/NFR5 of Lotus japonicus, LYK3/NFP of Medicago truncatula, etc.) senses the nodulation factor, it activates the downstream symbiotic signaling pathway, induces the root hair to bend and be invaded by bacteria. Subsequently, the root hair cell wall is partially degraded to form an infection line for the rhizobia to enter. The bacteria elongate and divide continuously along the infection line, and finally invade the cortical cells. At the same time, the cortical cells divide again to form the initial flowering meristem and develop into nodules. In the nodule, the rhizobia are wrapped by the plant cell membrane to become bacteroids, and begin to efficiently fix atmospheric nitrogen and reduce it to ammonium nitrogen for plant use. The entire symbiotic process requires strict coordination between the two parties: the plant provides a carbon source and a shelter environment, the microorganisms provide nitrogen, and through complex signal exchanges to ensure the establishment and maintenance of a mutually beneficial symbiotic relationship. 2.2 Role of mycorrhizal fungi in synergistic interaction In addition to rhizobia, many legumes also establish symbiosis (i.e., mycorrhizal symbiosis) with arbuscular mycorrhizal fungi (AM fungi) in the soil, which is of great significance to the nutrient acquisition and environmental adaptability of plants. Interestingly, there is a high degree of commonality in the signal transduction pathway between mycorrhizal symbiosis and nodule symbiosis: the receptors and downstream signals of mycorrhizal factors (Myc factors) secreted by AM fungi that plants recognize partially overlap with the pathways for recognizing nodulation factors, which is considered to be the result of legumes using evolutionary commonalities to establish two symbiotic relationships. Studies have shown that arbuscular mycorrhizal symbiosis appeared in terrestrial plants more than 400 million years ago, and legumes later "borrowed" the signal mechanism of this ancient symbiosis to develop a new symbiosis with rhizobia. Therefore, there is synergy and a common signal network between AM fungi symbiosis and nodule symbiosis.
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