Legume Genomics and Genetics 2025, Vol.16, No.3, 108-127 http://cropscipublisher.com/index.php/lgg 110 In actual function, mycorrhizal fungi significantly expand the effective absorption area of legume roots through their hyphae networks, helping to absorb nutrients (such as phosphorus) and water that are difficult to move in the soil. At the same time, AM bacteria can also change the rhizosphere microbial community. Studies have found that the hyphae of arbuscular mycorrhizal fungi can serve as a "bridge" to help rhizobia spread in the soil and reach the roots of legumes faster, thereby improving nodulation efficiency. For example, a microcosm experiment on peanuts showed that in the presence of an endophytic fungus (Phomopsis liquidambaris) hyphae, the enrichment of rhizobia and the number of nodules in the peanut rhizosphere increased significantly; the fungal hyphae provided a "highway" for rhizobia, making it easier for them to migrate to the roots and induce the formation of more nodules. The study also found that rhizobia exhibited special chemotaxis and proliferation behaviors when moving along the hyphae, and hyphal secretions may promote this process (Figure 1) (Zhang et al., 2020). It can be seen that the presence of AM bacteria can assist rhizobia in colonization and nodulation at the physical and chemical levels. Other studies have shown that the simultaneous inoculation of rhizobia and mycorrhizal fungi has a synergistic effect on the growth of legumes: for example, in alfalfa, the double inoculation treatment significantly increased the plant's nitrogen and phosphorus absorption and biomass accumulation, which was more conducive to nutrient cycling than the single inoculation treatment. This shows that the interaction between arbuscular mycorrhizal fungi and rhizobia can form a more functional microbial consortium in the rhizosphere of legumes, helping plants to fully obtain a variety of nutrients. More profoundly, recent molecular evolutionary studies have revealed that legume-rhizobium symbiosis and AM bacteria symbiosis may have a synergistic relationship in evolution. Some key genes (such as SymRK receptor kinase, etc.) were found to be involved in regulating both symbioses. In their study of the evolutionary mechanism of the legume symbiotic system, Wang and Zhang (2021) proposed that it was precisely because early terrestrial plants already had the ability of mycorrhizal symbiosis that legumes evolved a new symbiosis with nitrogen-fixing rhizobia on this basis, and the two symbioses influenced each other and improved synergistically during the evolution process. This view is supported by symbiotic biogenomics research: the presence of mycorrhizal symbiosis affects the response of legumes to rhizobia and the nodulation characteristics, and legumes without AM bacteria are often more difficult to nodulate efficiently. Therefore, the legume crop symbiotic system can be regarded as a complex tripartite interaction network including plants, fungi, and bacteria, and each component promotes each other, ultimately improving the nutrient utilization and environmental adaptability of plants. 2.3 Recognition and regulation of symbiotic signal molecules The establishment of a symbiotic relationship between legumes and microorganisms requires a fine balance between the exchange of signal molecules and the regulation of the plant's own defense response. In the rhizobium-legume symbiosis, the most critical signal molecule is the aforementioned nodulation factor (Nod factor). After the plant recognizes the Nod factor through the NFR1/NFR5 receptor located on the surface of the root hair, it activates the downstream cascade reaction including calcium ion oscillation and symbiosis-related gene expression, triggering the nodulation process. At the same time, the plant needs to suppress its own immune system to allow the invasion and colonization of symbiotic bacteria, otherwise the rhizobia may be regarded as pathogens and blocked by the plant's defense response. Studies have shown that legumes have functionally differentiated the pathways for identifying symbiotic microorganisms and pathogens: on the one hand, plants trigger symbiotic signals through a unique Nod factor receptor pathway, thereby bypassing the strong immune response induced by typical pathogen-associated molecular patterns (MAMPs); on the other hand, plants still use some immune mechanisms to monitor the symbiotic process to ensure that only the correct strains can successfully nodulate without harming the health of the host. For example, the plant microbial receptor kinase EPR3 can bind to the surface polysaccharide signal on the surface of rhizobia to distinguish whether it is a suitable symbiotic strain. If an "unqualified" signal is detected, a defense response is triggered to limit its infection. For another example, plants produce reactive oxygen and antimicrobial substances to inhibit excessive bacterial growth, but symbiotic rhizobia have evolved tolerance
RkJQdWJsaXNoZXIy MjQ4ODYzNA==