Legume Genomics and Genetics 2024, Vol.15, No.1, 1-12 http://cropscipublisher.com/index.php/lgg 5 The formation of root nodules provides additional nitrogen sources for plants, and through the formation of symbiotic mycorrhizal fungi, it provides the ability to absorb other nutrients, thereby promoting plant growth and development. The formation of root nodules enables leguminous plants to absorb nitrogen from the atmosphere through symbiotic relationships with rhizobia and convert it into ammonia nitrogen that can be utilized by plants. This provides additional nitrogen sources for plants to meet their nitrogen needs for growth and development. Due to nitrogen being a key nutrient element in plant growth and development, the formation of root nodules can increase the biomass accumulation and yield of plants. In addition to providing additional sources of nitrogen, the formation of root nodules also promotes the absorption of other nutrients by plants through the formation of symbiotic mycorrhizae. Symbiotic mycorrhizal fungi can increase the absorption surface area of plant roots and provide other nutrients such as phosphorus and potassium required by plants through symbiosis with rhizobia. This symbiotic relationship enables leguminous plants to more effectively absorb and utilize nutrients in the soil, thereby improving plant growth and development (Dixon and Kahn, 2004). In addition, the formation of root nodules also affects the root structure and morphological development of plants. The formation of root nodules can lead to changes in the morphology of plant roots, such as thickening and thickening of the root system. Such changes help to increase the contact area between the rhizosphere and soil of plants, enhance the absorption capacity of roots for water and nutrients, and promote plant growth and development. Overall, the formation of root nodules has a significant impact on the growth and development of leguminous plants. The formation of nodules provides an additional source of nitrogen to meet the nitrogen requirements for plant growth and development. Meanwhile, the formation of root nodules promotes the absorption of other nutrients by plants through the formation of symbiotic mycorrhizae. In addition, the formation of root nodules also affects the root structure and morphological development of plants. The in-depth study of the relationship between root nodule formation and plant growth and development will help us better understand the growth and development regulation mechanisms of leguminous plants, and provide scientific basis for the optimization cultivation and variety improvement of leguminous crops. 3.2 Regulatory mechanisms of signal exchange between rhizobia and plants The signal exchange between rhizobia and plants is a crucial step in the formation of nodules, which is regulated through a series of signaling molecules and pathways. During the formation of nodules, rhizobia induce the formation of nodules in plant roots by releasing a special type of signaling molecule called Nod factors. Nod factors are a diverse class of signaling molecules with highly diverse structures and biological activities. The receptors in plant roots can recognize and respond to specific Nod factors, thereby initiating signaling pathways for root nodule formation. The perception and response process of Nod factor is achieved through a series of receptors and signaling molecules. In leguminous plants, receptor proteins NFR1 and NFR5 play a crucial role in the formation of root nodules. These receptors are able to recognize and bind Nod factors, thereby activating downstream signaling pathways. Once receptors bind to Nod factors, they activate a series of signaling molecules, such as kinases and phosphatidylinositol kinases. These signaling molecules further regulate a series of gene expression, thereby initiating the process of root nodule formation (People et al., 1994). In addition to the signal transduction of Nod factors, rhizobia also regulate the formation of nodules by releasing other signaling molecules. For example, some rhizobia can release a class of small molecule compounds called symbiotic effectors. These symbiotic effectors can regulate the morphology and physiological status of plant roots, further promoting the formation of root nodules. The regulatory mechanism of symbiotic effectors is not fully
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