Legume Genomics and Genetics 2024, Vol.15, No.4, 163-175 http://cropscipublisher.com/index.php/lgg 168 rates (Soyano et al., 2013). Additionally, engineering plants to express more efficient or novel nodulation genes from other species can potentially broaden the range of rhizobia that can establish symbiosis, thus improving nitrogen fixation under diverse environmental conditions (Rogers and Oldroyd, 2014). Several case studies highlight the success of genetic enhancements in improving symbiotic efficiency. In one study, the overexpression of a modified version of the TF NIN in Lotus japonicus resulted in a significant increase in nodule number and enhanced nitrogen fixation, demonstrating the potential of targeted gene overexpression (Soyano et al., 2013). Another example involves the transfer of the symbiosis island from a highly efficient nitrogen-fixing bacterium to a less efficient strain, resulting in improved symbiotic performance and plant growth (Gonzalez et al., 2010). These case studies underscore the potential of genetic engineering to optimize symbiotic interactions and enhance agricultural productivity. 5 Case Study: Rhizobium-Legume Symbiosis in Soybean (Glycine max) 5.1 Overview of soybean’s agricultural importance Soybean (Glycine max) is a critical crop globally, valued for its high protein and oil content, which makes it a staple in both human and animal diets. The agricultural significance of soybean is further enhanced by its ability to engage in symbiotic nitrogen fixation with rhizobium bacteria, reducing the need for nitrogenous fertilizers and promoting sustainable farming practices. Soybean’s role in agriculture is multifaceted. It is a major source of protein and oil, with applications ranging from food products to industrial uses. The crop's ability to fix atmospheric nitrogen through symbiosis with rhizobium bacteria is particularly important in regions with nitrogen-deficient soils. This symbiotic relationship not only improves soil fertility but also enhances crop yields and reduces the environmental impact of synthetic fertilizers (Dall’Agnol et al., 2014; Zhao et al., 2021; Veličković et al., 2022). Research has shown that inoculation with Rhizobium japonicum can significantly increase soybean yield, particularly in nutrient-poor soils. For instance, an experiment conducted in the southern savanna region of Ghana demonstrated that rhizobium inoculation, along with nitrogen and phosphorus supplementation, significantly improved various growth parameters and yield components of soybean (Dall’Agnol et al., 2014). This highlights the potential of rhizobium inoculation to enhance soybean productivity in challenging agricultural environments. The interaction between soybean and rhizobium is influenced by various factors, including the presence of other symbiotic organisms such as mycorrhizal fungi. Studies have indicated that the tripartite symbiosis involving soybean, rhizobium, and mycorrhizal fungi can lead to improved photosynthetic efficiency and nutrient uptake, further boosting plant growth and yield (Veličković et al., 2022). 5.2 Molecular mechanisms specific to soybean-rhizobium symbiosis The molecular mechanisms underlying the soybean-rhizobium symbiosis are complex and involve a series of genetic and biochemical interactions that facilitate the establishment and maintenance of this mutualistic relationship. Several key molecular players and pathways have been identified that are specific to the soybean (Glycine max) and its interaction with rhizobium. One critical aspect of this symbiosis is the role of the Glycogen Synthase Kinase 3 (GSK3)-like kinase, GmSK2-8, which has been shown to inhibit symbiotic signaling and nodule formation under salt stress conditions. GmSK2-8 interacts with Glycine max Nodulation Signaling Pathway 1 (GmNSP1) transcription factors, phosphorylating them and thereby inhibiting their ability to bind to the promoters of symbiotic genes. This phosphorylation reduces nodule formation under stress conditions, highlighting a regulatory mechanism that modulates symbiosis in response to environmental stress (He et al., 2020). Another significant molecular component is the Yellow Stripe-like 7 (GmYSL7) transporter, which is localized on the symbiosome membrane in soybean nodules. GmYSL7 is responsible for transporting oligopeptides, which are crucial for proper nodule development and nitrogenase activity. Silencing GmYSL7 results in reduced nitrogenase activity and impaired nodule development, indicating its essential role in the symbiotic process (Figure 3) (Gavrin et al., 2021). Hydrogen sulfide (H2S) has also been identified as a positive signaling molecule in the soybean-rhizobium symbiotic system. Exogenous application of H2S has been shown to promote soybean growth,
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