Legume Genomics and Genetics 2025, Vol.16, No.3, 108-127 http://cropscipublisher.com/index.php/lgg 112 Plants also need to maintain a certain degree of immune vigilance during the symbiotic process. Once the behavior of symbiotic bacteria deviates from mutualism (for example, they no longer fix nitrogen and consume host nutrients), plants will use immune mechanisms to punish or even eliminate the "cheating" bacteria. This phenomenon is called "host checks and balances on symbionts" in symbiotic ecology. It can be seen that legumes actually face a "dual task": they must accept symbiotic microorganisms, but they must not let the symbiotic relationship destroy their own defense balance. In recent years, studies on plant hormones and signaling pathways have revealed that a variety of plant hormones (such as ethylene, jasmonic acid, etc.) are involved in the coordination of symbiosis and immunity. Some factors that positively regulate nodulation are also negative immune regulators, such as Medicago's PUB1 ubiquitin ligase, which not only inhibits excessive infection signals but also promotes moderate nodulation. Some immune-related factors have a negative impact on symbiosis, such as the plant disease resistance gene E3 ubiquitin ligase, which limits rhizobia infection. If it is knocked out, the number of nodules can be increased. Through these complex molecular regulations, legumes achieve a dynamic balance between symbiotic signals and immune responses. Under abiotic stress conditions (such as high salt osmotic stress), this balance is more likely to be broken: stress signals often enhance plant defense pathways, thereby inhibiting the nodulation symbiosis process. Recent studies have revealed that certain GSK3-like protein kinases are induced in large quantities in soybeans under high salt conditions, and after upregulation, they inhibit the symbiotic signaling pathway, resulting in a decrease in nodule number and nitrogen fixation capacity. 3 Legumes in Enhancing Soil Microecology 3.1 Increasing organic matter content in soil Leguminous crops generally have the effect of improving soil fertility, the most direct manifestation of which is in increasing soil organic matter content. Leguminous plant residues (such as roots, stems and leaves) are rich in nitrogen and carbon, and can significantly increase soil organic matter and nutrient levels after litter decomposition. Especially when leguminous crops are turned into soil as green manure, their organic carbon input is large and decomposes quickly, which can increase soil organic carbon storage in a relatively short period of time. Studies have shown that long-term application of leguminous green manure can increase soil organic matter content year by year. For example, in the long-term green manure experiment of irrigated desert soil, soil organic matter increased significantly compared with the control after applying leguminous green manure for more than 10 years, with an average annual increase of more than 5%. For example, planting alfalfa, peas, peanuts and other green manures in the soil of southern orchards can increase soil organic matter content by 5%~27% after one year. The research review of Xu et al. (2021) also pointed out that intercropping leguminous green manure between orchard rows significantly increased soil organic carbon content and improved soil quality. This is consistent with observations in other regions: planting legume cover crops not only increases organic matter and total nitrogen in the soil, but also helps to improve the yield and quality of subsequent crops. Legume crops fix nitrogen in the air as organic nitrogen through symbiotic nitrogen fixation, introduce it into the soil, and make an important contribution to the accumulation of soil organic matter. Compared with the simple application of nitrogen fertilizers, the organic nitrogen produced by biological nitrogen fixation is more conducive to the formation of soil humus and reduces nitrogen leaching. The field experiment of Mahama et al. (2020) showed that planting winter legume cover crops in a no-till sorghum system can reduce the amount of nitrogen fertilizer in the later season, while increasing the content of soil organic nitrogen and organic carbon, thereby reducing nitrogen fertilizer demand and soil N2O emissions. This shows that the application of legume green manure crops has achieved the replacement of part of inorganic nitrogen with biological organic nitrogen, which not only maintains crop yields, but also increases soil organic matter, achieving the dual benefits of fertilizing soil fertility and reducing emissions. Using legume crops to increase soil organic matter is one of the important measures for fertilizing soil in traditional agriculture. For example, in many areas of China, farmers have always had the habit of planting leguminous green manures such as astragalus and vetch to maintain soil fertility and soil fertility. Modern research has more systematically confirmed the scientific nature and long-term benefits of this practice. It can be foreseen that in future sustainable agriculture, making full use of leguminous green manure and returning leguminous crop straw to the field will play a greater role in restoring soil carbon pools and improving soil health.
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