Field Crop 2024, Vol.7, No.2, 58-69 http://cropscipublisher.com/index.php/fc 61 3.2 Genetic pathways and regulatory mechanisms The genetic pathways and regulatory mechanisms governing SNF are intricate and tightly controlled. The interaction between legumes and rhizobia involves a sophisticated signaling network that ensures successful symbiosis. This includes the perception of rhizobial signals by plant receptors, activation of downstream signaling cascades, and regulation of gene expression to facilitate nodule formation and function (Roy et al., 2020). For instance, the symbiotic microRNA miR172c has been shown to play a role in coupling symbiotic and nutritional signals to promote flowering in soybeans, highlighting the integration of SNF with other physiological processes (Yun et al., 2023). Additionally, the interplay between nitrogen fixation and phosphorus nutrition is regulated through specific signaling crosstalk, which is essential for efficient SNF (Figure 2) (Zhong et al., 2022). Figure 2 Uptake and translocation of phosphorus (P) and nitrogen (N) to legume nodules (Adopted from Zhong et al., 2022) Image caption: Transporters involved in direct and indirect uptake of P to nodules and the acquisition of N in legumes are shown. Nodules have two ways for P acquisition: direct and indirect uptake. Direct uptake is mediated by transporters expressed in the nodules, such as GmPT7 and MtPT6, while indirect uptake includes uptake of P by host plant roots and translocation from roots to the nodules through transporters such as GmPT4, GmPT5, and GmPT10, followed by loading to the fixation zone byMtPHO1;1 and MtPHO1;2. On the contrary, legumes could take up inorganic N in the form of NO3 and NH4 + by nitrate transporters (MtNPF1.7, MtNPF6.8, MtNRT2.1, MtNRT2.1, MtNRT2.3, LjNTR2.1, GmNRT2.1, and GmNRT2.2) and/or ammonium transporters (LjAMT1;2 and LjAMT1;3), respectively. Also, legumes could obtain N from the atmosphere through symbiotic nitrogen fixation with rhizobia in nodules. Pit, inorganic phosphate transporter; Pst, phosphate specific transporter (Adopted from Zhong et al., 2022) 3.3 Advances in genetic modification to enhance nitrogen fixation Recent advances in genetic modification have opened new avenues for enhancing nitrogen fixation in legumes. Techniques such as genome editing and mutagenesis have been employed to improve SNF traits. For example, breeding programs have focused on enhancing symbiotic tolerance to nitrate, inducing supernodulation, and promoting selective nodulation (Herridge and Rose, 2000). Moreover, efforts are underway to transfer nitrogen-fixing capabilities to non-legume crops, which could revolutionize agricultural practices by reducing the reliance on synthetic nitrogen fertilizers (Mahmud et al., 2020). The integration of SNF genetics into mainstream breeding programs and the use of dynamic simulation models to predict and optimize nitrogen fixation under various environmental conditions are also promising strategies for future improvements (Herridge and Rose, 2000; Liu et al., 2011).
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