Legume Genomics and Genetics 2024, Vol.15, No.4, 163-175 http://cropscipublisher.com/index.php/lgg 170 This gene is involved in the shoot-to-root signaling pathway that regulates nodulation and nitrogen nutrition. Variations in the promoter region of GmNN1/FT2a can lead to decreased transcription levels and nitrogen deficiency phenotypes. Manipulating this gene enhances nodulation, plant growth, and nitrogen nutrition (Bender et al., 2022). GmPT7 is a nodule-localized phosphate transporter that plays a significant role in enhancing symbiotic nitrogen fixation and yield. Overexpression of GmPT7 promotes nodulation and increases plant biomass, shoot nitrogen, and phosphorus content, thereby improving soybean yield (Wang et al., 2018). GmHSP17.9 is specifically expressed in the infected regions of nodules and is crucial for nodule development and nitrogen fixation. Overexpression or silencing of this gene significantly affects nodule number, nitrogenase activity, and overall plant growth and seed yield (Yang et al., 2021). 5.4 Practical applications and implications for soybean breeding The molecular mechanisms underlying nitrogen-fixing symbiosis in Fabaceae, particularly in soybean (Glycine max), offer significant potential for practical applications in soybean breeding. Understanding these mechanisms can lead to the development of soybean varieties with enhanced nitrogen fixation capabilities, improved growth, and higher yields, especially in nitrogen-poor soils. The discovery of the symbiotic flowering pathway involving microRNA172c (miR172c) and fixed nitrogen signals provides a crucial insight into how nitrogen fixation can be leveraged to improve soybean flowering and reproductive success. By promoting the expression of florigen-encoding FLOWERING LOCUS T (FT) homologs (GmFT2a/5a), breeders can develop soybean varieties that flower more efficiently under low-nitrogen conditions, thereby enhancing their reproductive success and yield (Yun et al., 2023). The differential performance of symbiotic nitrogen fixation between soybean and common bean highlights the importance of the root and nodule microbiomes. Soybean’s efficient symbiosis with Bradyrhizobium, as opposed to the less effective symbiosis in common bean, suggests that selecting for specific rhizobial strains and optimizing co-inoculation strategies could significantly improve nitrogen fixation and plant growth. This approach can reduce the reliance on chemical fertilizers, promoting more sustainable agricultural practices (Bender et al., 2022). Iron and phosphate are critical for effective nitrogen fixation. The identification of iron transporters such as GmVTL1a and phosphate transporters like GmPT7, which are localized to the symbiosome membrane and nodule cortex respectively, underscores their roles in enhancing nitrogen fixation and overall plant health. Breeding strategies that enhance the expression of these transporters can lead to soybean varieties with improved nutrient uptake, better nodulation, and higher yields (Brear et al., 2020). The metabolic modeling of Sinorhizobium fredii and its interaction with soybean provides a framework for identifying key genes that control nitrogen fixation. By targeting these genes through genetic engineering or selective breeding, it is possible to enhance the nitrogen-fixing efficiency of symbionts, thereby improving the nitrogen availability to the host plant and reducing the need for external nitrogen inputs (Contador et al., 2020). The role of GmNN1/FT2a in shoot-to-root signaling and its impact on nodulation and nitrogen nutrition highlights a critical regulatory pathway that can be manipulated to improve soybean growth. By selecting for alleles that enhance the expression of GmNN1/FT2a, breeders can develop soybean varieties with better nodulation and nitrogen uptake, leading to improved plant health and yield. The phosphorylation of RIN4, a key immune regulator, upon rhizobial inoculation, and its essential role in nodulation, suggests that integrating immune and symbiotic signaling pathways can enhance the establishment of effective symbiosis. Breeding strategies that focus on optimizing RIN4 function could improve the compatibility and efficiency of nitrogen-fixing symbiosis in soybean. 6 Environmental and Agricultural Implications 6.1 Impact of nitrogen-fixing symbiosis on soil fertility Nitrogen-fixing symbiosis in Fabaceae plays a crucial role in sustainable agriculture by enhancing soil fertility through the biological nitrogen fixation (BNF) process. This symbiotic relationship between legumes and rhizobia bacteria allows the conversion of atmospheric nitrogen into a form that plants can utilize, reducing the need for chemical fertilizers. This process not only improves soil health but also supports the growth of subsequent crops
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