Molecular Pathogens, 2025, Vol.16, No.5, 226-235 http://microbescipublisher.com/index.php/mp 229 The above-mentioned signaling molecules and genes interact to form a complex network, which provides the molecular basis for root nodule formation. 3.3 Molecular mechanisms of rhizobia invasion and infection line formation When rhizobia induce root hair curling and attach to it, they begin to invade the host tissue. The root hair cell wall is partially degraded, forming an infection line channel extending inward, and the rhizobia gradually advance toward the root cortex along the infection line. The growth of the infection line is actively controlled by plant cells, and its channel walls are deposited and continuously extended by plant cell wall components (Zhang et al., 2020). Plant cytoskeletal reorganization, such as the directional aggregation of microfilaments, is critical for the directed growth of infection threads. Eventually, the infection line reaches the nodule primordium, and rhizobia are released into the newly differentiated nodule cells through the end of the infection line. During the formation of infection lines and the release of bacterial cells, a variety of microorganisms and plant factors are involved in regulation. Rhizobium needs to synthesize surface polysaccharides and secreted proteins with appropriate structures to ensure smooth infection. Exopolysaccharides (EPS) and lipopolysaccharides (LPS) help rhizobia avoid immune recognition and maintain the stable extension of the infection line (Zhang et al., 2023). In the absence of critical surface polysaccharides, infection often aborts resulting in nodulation failure. 4 Regulation of Host Plant Interactions with Rhizobia 4.1 The role of root exudates in the identification and attraction of bacterial flora Legume roots secrete a variety of compounds that attract and screen soil microorganisms. Among them, flavonoids are key components in soybean root exudates and can specifically induce the expression of nod genes of corresponding rhizobia and promote their synthesis of nodulation factor signals, thus starting symbiotic dialogue (Qiu et al., 2024). At the same time, the amino acids, sugars, organic acids, etc. secreted by the roots provide nutrients for rhizosphere microorganisms and enhance the chemotaxis and colonization capabilities of beneficial bacteria. The composition and quantity of root exudates will vary under different soybean varieties and environmental conditions, which will affect the recruitment efficiency of rhizobia. For example, under stress conditions such as phosphorus deficiency, plants may increase the organic acids secreted by roots to attract microorganisms to assist in nutrient acquisition (Dardanelli et al., 2010; Hiremath et al., 2024); and certain secondary metabolites can selectively attract symbiotic rhizobia and inhibit pathogenic microorganisms. Root exudates thus serve as both "microbial feed" and "chemical signal". On the one hand, they provide nutrients to attract suitable strains, and on the other hand, they use signals to guide rhizobia to gather in the roots and initiate the symbiosis process. 4.2 Balance of plant immune responses and symbiotic signals The plant innate immune system can recognize pathogenic microorganisms and trigger defense responses, but when establishing symbiosis with rhizobia, the host must balance immune defense and symbiotic acceptance. Typically, the perception of microbial-associated molecular patterns (MAMPs) by plant pattern recognition receptors (PRRs) induces a classical immune response (PTI). However, symbiotic signals such as nodulation factors secreted by rhizobia can locally inhibit or circumvent part of the host's immune response to prevent excessive defense from interfering with the symbiotic process. Legumes balance immune and symbiotic signaling through multiple mechanisms. For example, some LysM-type receptor kinases are involved in recognizing friendly signals (Nod factors) and sensing pathogenic signals (such as fungal chitin fragments), but their downstream pathways are differentiated, resulting in different responses to commensal and pathogenic bacteria (Yang et al., 2021). Even if some rhizobia invade the roots, the host will monitor them. If their nitrogen fixation is ineffective or their numbers are abnormal, the plant can activate defenses to limit nodulation or promote premature degeneration of already formed root nodules (Sarrette et al., 2024). This dynamic game ensures that plants protect against pathogens while allowing beneficial symbiosis to proceed smoothly.
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