Legume Genomics and Genetics 2024, Vol.15, No.1, 1-12 http://cropscipublisher.com/index.php/lgg 3 Figure 1 Root nodule development mechanism of leguminous plants (Adopted from Dong et al., 2021) When the SHR-SCR molecular module is overexpressed in the roots of leguminous plants such as alfalfa, it can induce cortical cell division to form a nodule like structure. Ectopic overexpression of SHR-SCR molecular modules in non leguminous plant Arabidopsis and rice roots can also induce root cortex cell division. Therefore, the SHR-SCR molecular module is a sufficient and necessary condition for plant cortex cell division, indicating that the acquisition of SHR-SCR stem cell program modules by leguminous plant cortex cells may be a prerequisite event for symbiotic nodulation and nitrogen fixation in leguminous plants. 2.2 Nitrogen enzyme system and nitrogen fixation process Nitrogen is a large amount of essential nutrients for plant growth and development, and is an important component of molecules such as proteins and nucleic acids in living organisms. Although the Earth's atmosphere contains 78.1% nitrogen, it cannot be directly utilized by the vast majority of plants, making agricultural production highly dependent on industrial nitrogen fertilizers. However, the production of nitrogen fertilizer requires a large amount of fossil fuels, and excessive application of nitrogen fertilizer can cause soil compaction degradation and water pollution, affecting the sustainable development of agriculture. Biological nitrogen fixation is the largest natural source of nitrogen available to organisms in nature. Leguminous plants and rhizobia can interact to form a unique organ, known as symbiotic nodules. Symbiotic nitrogen fixation in root nodules is an important pathway for nitrogen reduction to ammonia that can be utilized by plants in the Earth's ecosystem. It contributes over 60% of terrestrial biological nitrogen fixation annually, affecting primary production and carbon sinks in agriculture and natural ecosystems. To reduce reliance on industrial nitrogen fertilizers and develop green agriculture, it is of great significance (Ke et al., 2022). Symbiotic nitrogen fixation is a high energy consuming enzyme catalytic process, and the carbohydrates fixed by plant photosynthesis are the main carbon and energy sources for symbiotic nitrogen fixation (Figure 2). Therefore, the nitrogen fixation ability of symbiotic nodules needs to be coordinated with the carbon source and energy level of leguminous plants, in order to balance the carbon consumption of symbiotic nitrogen fixation and other life processes and ensure the normal growth of leguminous plants in different environments. However, it is not clear how leguminous plants respond to carbon sources and energy levels to regulate high energy consuming biological nitrogen fixation processes (Ke et al., 2022). The nitrogen fixation ability of leguminous plant nodules is affected by environmental changes. When the external oxygen and phosphorus supply changes, the nitrogen fixation ability of the nodules will be rapidly adjusted, and the energy state of the nodules will also change accordingly, indicating that the change in the energy state of the nodules may be an important inducement for the change in nitrogen fixation ability.
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