Molecular Pathogens, 2025, Vol.16, No.5, 226-235 http://microbescipublisher.com/index.php/mp 230 4.3 The role of host gene expression and epigenetic regulation in nitrogen fixation The host plant controls the root-nodule symbiosis process by regulating gene expression. At the local level, the expression of root nodulation-related genes directly determines the formation and function of root nodules; at the overall level, the plant regulates the number of nodules and the intensity of nitrogen fixation through long-distance signals. The autonomic regulation (AON) mechanism can prevent excessive nodulation. When the number of root nodules reaches a threshold, the CLE signal peptide produced in the roots is transported to the stems and leaves, and is sensed by the stem receptor kinase and sends an inhibitory signal to prevent the formation of new root nodules and avoid excessive consumption of nutrients (Figure 2) (Yang et al., 2021). Research in recent years has shown that small RNAs and epigenetics also play an important role in root nodule symbiosis. miR172c is an important small RNA that can promote nodule formation; NARK signal down-regulates miR172c, thereby feedback controlling the number of nodules. At the same time, some studies have found that root nodule development is accompanied by changes in host DNA methylation and histone modification status. Some genes that promote symbiosis exhibit DNA demethylation and histone hyperacetylation in root nodules, which is conducive to their high-level expression (Basu and Kumar, 2020). This shows that the host coordinates its response to rhizobia symbiosis through gene expression and epigenetic mechanisms, providing a multi-level regulatory means to improve nitrogen fixation efficiency. Figure 2 Stages and regulatory networks of indeterminate nodule organogenesis in Medicago truncatula (Adopted from Yang et al., 2021) 5 Genetic Diversity and Functional Differences of Rhizobia 5.1 Classification of rhizobia populations and differences in nitrogen-fixing abilities Soybean symbiotic rhizobia are taxonomically diverse and mainly include groups such as Bradyrhizobium and Sinorhizobium. Different strains have significant differences in genetic characteristics and symbiotic performance. For example, slow-growing strains such as Bradyrhizobium japonicum and Bradyrhizobium diazoefficiens are widely used in soybean inoculants and have strong nitrogen-fixing abilities; while fast-growing strains such as Sinorhizobium fredii are more advantageous in some soils and have different host ranges and environmental adaptability (Temprano-Vera et al., 2018; Barros-Carvalho et al., 2019).
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