· Molecular Microbiology Research, 2025, Vol.15, No.1, 37-44 http://microbescipublisher.com/index.php/mmr 40 provides a "target" for genetic engineering, which can be adjusted to these genes to improve its coordination efficiency with soybeans (Contador et al., 2020). The purpose of these methods is very simple - to make rhizobia more "capable" and help soybeans grow well in different environments. 4.2 Exogenous activators and promoters In addition to changing the strain, you can also add some foreign "helpers" to the soil, such as nutrients or signal molecules, to enhance the interaction between soybeans and rhizobia. For example, some studies have found that if soybeans are inoculated with rhizobia, especially when there is less sulfur, the growth and nitrogen fixation ability of soybeans will be better (Hu et al., 2023). There are also small molecules such as hydrogen sulfide (H₂S), which can also be used as a "signal". It can help soybeans absorb and utilize nitrogen better, so that even if the nitrogen is not enough, soybeans can grow well and yields will also increase (Zhang et al., 2020). Therefore, using these exogenous substances can help soybeans and rhizobia cooperate more smoothly. 4.3 Molecular breeding and soybean variety improvement In addition to modifying bacteria and adding substances, we can also start with the soybean itself. Molecular breeding is a method used to improve soybean varieties, with the purpose of making soybeans easier to pair with rhizobia successfully. Now scientists have found many genes and loci (called QTL) related to nitrogen fixation. This information can be used to select soybean varieties that are highly yielded and effective in fixing nitrogen (Dwivedi et al., 2015). In addition, the SNP markers extracted from nodules-related genes can also help us screen out those soybean varieties with stronger "cooperation capabilities". In general, these technologies are designed to make soybeans better after inoculation with rhizobia, not only increase yield, but also use less fertilizers and are more environmentally friendly. 5 Case Studies 5.1 Screening and application of high-efficiency Rhizobium strains Selecting and using good rhizobia strains is an important way to improve the nitrogen fixation ability of soybeans. Some Rhizobium (such as Rhizobium sp. R1 and R. cellularilyticum R3) have been found to help soybeans germinate faster under drought conditions and also allow plants to grow better (Igiehon et al., 2019). In addition, there is a strain called Sinorhizobium fredii CCBAU45436 that has better nitrogen fixation effect on cultivated soybeans than on wild soybeans (Contador et al., 2020). These examples tell us that it is really important to choose a strain that suits a specific environment and a specific soy variety. 5.2 Nitrogen fixation optimization strategies under environmental stresses Environmental pressures such as saline or drought will reduce the nitrogen fixation efficiency of soybeans. Research has found that a protein called GmSK2-8 in soybeans "starts" during salt stress, which affects some key transcription factors, preventing them from working properly, and eventually causing the nodule to fail to grow (Shome et al., 2022). However, there are ways to alleviate these problems. For example, adding hydrogen sulfide (H₂S) can allow soybeans to absorb nitrogen better when nitrogen is lacking, and can also help reuse nitrogen in the body. In this way, soybeans can not only survive, but also grow well and produce won’t lose too much (Santachiara et al., 2019). These studies show that we can mitigate the impact of the environment through genetic means or adding some specific substances. 5.3 Molecular mechanisms of the symbiotic system 5.3.1 Signal molecules and genetic regulation of nodule formation Signal molecules are critical in the formation of nodules. For example, in a salt environment, GmSK2-8 in soybeans will interact with a transcription factor called GmNSP1. This effect makes GmNSP1 unable to bind to the symbiotic gene promoter, and the result is that the nodule cannot grow. If the effect of GmSK2-8 is "turned off" with RNA interference, soybeans can grow more nodules even in salt environments. This suggests that GmSK2-8 inhibits the formation of nodules (He et al., 2020). The study also found that the two mutants, Gmnsp1-a and nsp1-b1, also had more nodules under salt stress than normal varieties, indicating that GmNSP1 is also very important here. Further experiments (such as ChIP experiments) also provide evidence that in salt
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