Molecular Pathogens, 2025, Vol.16, No.5, 226-235 http://microbescipublisher.com/index.php/mp 227 nitrogen fixation efficiency. Therefore, it is necessary to systematically review these developments to clarify future research directions for improving soybean nitrogen fixation efficiency. 2 Biological Basis of Nitrogen Fixation in Soybean Root Nodules 2.1 Physiological and molecular mechanisms of root nodule formation Nodule formation is a critical first step in the soybean-rhizobia symbiosis. Rhizobium bacteria usually colonize the surface of root hairs and induce root hair curling, enveloping the bacteria within them. Subsequently, rhizobia invade the interior of the root through the infection line. The infection thread is a tube structure formed by the ingrowth of root hair cell walls that guides the bacteria through the cortex. Synchronously with this process, root cortex cells are stimulated by signals and begin to divide, forming root nodule primordia (Zhang et al., 2020). Within the root nodule primordium, the invading rhizobia are released from the infection line into the plant cells and are wrapped in a symbiont (like symbiont capsule) formed by the plant cell membrane, and the root nodule gradually develops into a new organ. During nodulation, phytohormone levels are redistributed and cooperate with symbiotic signals. 2.2 Structure and catalytic function of nitrogenase system Nitrogenase is composed of ferritin and molybdenum ferritin. MoFe protein contains molybdenum-iron cofactor (FeMo-co), which serves as the active center of the nitrogen fixation reaction. With the energy provided by ATP hydrolysis, Fe protein transfers electrons to MoFe protein to drive nitrogen reduction (Djurdjević et al., 2020). Nitrogenase is extremely sensitive to oxygen and is rapidly inactivated under aerobic conditions. Nitrogenase catalyzes the reduction of inert nitrogen (N₂) in the atmosphere to ammonia (NH₃). This process consumes huge amounts of energy. Approximately 16 moles of ATP are consumed for every mole of N₂ fixed (Ipata and Pesi, 2015). The ammonia produced by nitrogenase is rapidly assimilated into ammonium salts by rhizobia and released for plant uptake. Since the nitrogen fixation process continues to require a large amount of ATP and reducing power, plants must provide sufficient carbon sources to rhizobia for their respiratory metabolism to obtain energy. Finally, rhizobia efficiently convert nitrogen in the air into plant-available nitrogen through nitrogenase to meet the nitrogen needs of the host (Sun and Shahrajabian, 2024). 2.3 Energy consumption and metabolic regulation of nitrogen fixation reaction Biological nitrogen fixation is a highly energy-consuming process, and the host plant must provide a large amount of energy and carbon skeleton to the rhizobia to maintain the work of nitrogenase. Sucrose and other substances produced by photosynthesis are transported to the roots through the phloem and decomposed into organic acids (such as malic acid) in the root nodules for respiratory metabolism by rhizobia, thus providing ATP and reducing power (Poole and Allaway, 2000). There are dicarboxylic acid transporters in plants to transport carbon sources into root nodule cells to support the energy needs of symbiotic bacteria. In order to coordinate the nitrogen fixation process with the overall metabolism of the plant, a delicate metabolic regulatory network has been established between the host and symbiotic bacteria. The ammonia produced by rhizobia is quickly assimilated into organic nitrogen such as glutamine, and the toxicity of excessive accumulation of ammonia is avoided through plant-microbial division of labor metabolism. At the same time, plants adjust resource input to root nodules based on their own nitrogen nutritional status. When external nitrogen is sufficient, the carbon supply to root nodules is reduced and nitrogen fixation is inhibited. When nitrogen is deficient, plants increase support for root nodules (Ohyama et al., 2017). 3 Rhizobium Recognition and Symbiosis Signaling Pathways 3.1 Signal recognition mediated by Nod factors and plant receptors Rhizobium establishes dialogue with the host by secreting specific signaling molecules, one of the core signals is nodulation factor (Nod factor). Nod factor is a class of lipopeptide-oligosaccharide molecules synthesized by the rhizobia nod gene cluster. Nod factors produced by different strains have subtle differences in the structure of the sugar backbone and fatty acid side chains. This diversity determines the specificity of the symbiosis between the
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