Legume Genomics and Genetics 2024, Vol.15, No.1, 1-12 http://cropscipublisher.com/index.php/lgg 6 understood, but it has been found that they can be achieved by activating plant hormone signaling pathways and regulating plant gene expression. Overall, signal exchange between rhizobia and plants is a key regulatory mechanism in the process of nodule formation. The Nod factor and other symbiotic effectors activate a series of signaling pathways by binding to plant receptors, thereby regulating the formation of root nodules. The in-depth study of the signal exchange regulation mechanism between rhizobia and plants will help us better understand the molecular mechanism of nodule formation, and provide scientific basis for nodule formation and variety improvement of leguminous crops (Han et al., 2020). 3.3 Plant metabolic regulation induced by rhizobia The symbiotic relationship between rhizobia and plants not only affects nodule formation, but also affects plant growth and development by regulating plant metabolic processes. Rhizobia induce the formation of nodules in plant roots, providing additional nitrogen sources for plants. The increase of this nitrogen source can cause changes in plant metabolism. Firstly, the nodules induced by rhizobia contain a large number of enzymes related to protein synthesis and nitrogen fixation, which increases the protein synthesis and nitrogen metabolism activity of plants; Secondly, rhizobia provide the ability to absorb phosphorus and other nutrients through the formation of symbiotic mycorrhizae, which can alter plant phosphorus metabolism and the utilization of other nutrients. These metabolic changes will affect the growth and development of plants. In addition, rhizobia regulate plant metabolism by activating plant hormone signaling pathways. Rhizobia can induce plant synthesis of the hormone gibberellin and activate the gibberellin signaling pathway, thereby promoting plant growth and root nodule formation. The synthesis and signaling pathways of gibberellin regulate the activity of auxin in plants, affecting the growth and development of plant roots. In addition, rhizobia can also regulate the synthesis and signaling pathways of plant hormones jasmonic acid and ethylene, further affecting plant metabolism and growth and development (Dixon and Kahn, 2004). Rhizobia can initiate the formation of nodules by secreting Nod factors, and can also induce the activation and expression of a series of secondary metabolic pathways in plant root cells. Early studies have shown that the recognition of Nod factors can lead to the accumulation of secondary messengers such as calcium and cGMP in plant cells. These secondary messengers affect the transcription levels of plant hormone related genes such as flavonoid biosynthesis and steroids through transcription factors and their regulatory groups. Meanwhile, studies have also found that Nod factor can upregulate the expression of a unique solanine metabolism pathway enzyme in leguminous plants. This provides an important intracellular regulatory mechanism for promoting nodule formation. In combination with Nod factor, polysaccharides secreted by rhizobia can also have a profound impact on plant metabolism. Polysaccharides, as edible carbon sources, can directly participate in the synthesis and regulation of plant central gluconeogenesis pathways and photosynthetic products after absorption. However, in addition, polysaccharides from rhizobia can also indirectly regulate plant cell division and differentiation through hormone pathways. Research has found that polysaccharides can upregulate the expression levels of cytokinin and auxin in plant cells. These two plant hormones act as secondary messengers and can promote root and branch expansion and thickening, thereby increasing nutrient absorption area. At the same time, some small molecule substances secreted by rhizobia, such as amino inorganic acids and fatty acids, also participate in regulating plant metabolic processes. Amino acids can achieve balanced transformation of inorganic nitrogen and organic nitrogen in plants, which is conducive to the dynamic allocation and utilization of nitrogen. Fatty acids can promote the improvement of photosynthetic efficiency in plant leaves by affecting key enzymes involved in the biosynthesis pathway of biological pigments. In addition, rhizobia can also enter plant cells through RNA molecules packaged outside cells, participate in the coupling of post transcriptional regulatory networks, and affect the rate of key reactions.
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