Molecular Soil Biology 2024, Vol.15, No.2, 74-86 http://bioscipublisher.com/index.php/msb 77 root nodules. The enzyme complex consists of two main components: the dinitrogenase reductase and the dinitrogenase. The dinitrogenase reductase transfers electrons to the dinitrogenase, which then reduces N2 toNH3 in an ATP-dependent reaction. The efficiency and regulation of the nitrogenase enzyme complex are critical for the effectiveness of the nitrogen fixation process, and understanding these mechanisms is key to improving the symbiotic relationship between legumes and rhizobia (Masson-Boivin and Sachs, 2018; Lindström and Mousavi, 2019). By understanding these biological mechanisms, researchers can better appreciate the intricate symbiotic relationship between legumes and rhizobia, which has significant implications for sustainable agriculture and food production. 4 Molecular and Genetic Basis of Symbiosis 4.1 Genetic signals and molecular interactions between legumes and rhizobia The symbiotic relationship between legumes and rhizobia is a complex process that involves a series of genetic signals and molecular interactions. Legumes release flavonoids into the soil, which are recognized by rhizobia, triggering the production of Nod factors. These Nod factors are essential signaling molecules that initiate the symbiotic process by inducing root hair curling and the formation of infection threads, which allow rhizobia to enter the plant root cells (Wang et al., 2018). The specificity of this interaction is governed by a variety of host and bacterial genes, ensuring compatibility between the symbiotic partners (Wang et al., 2018). Additionally, systemic signaling mechanisms within the plant adjust the symbiotic relationship based on the plant's nutritional status, ensuring that nodule formation and function are tightly regulated to meet the plant's nitrogen demands (Figure 2) (Lepetit and Brouquisse, 2023). 4.2 Nodulation genes and signaling pathways Nodulation in legumes is controlled by a complex network of genes and signaling pathways. Nearly 200 genes have been identified as essential for symbiotic nitrogen fixation (SNF) in legumes, including those involved in the control of microbial infection, nodule development, and nodule function. Key genes such as NIN (Nodule Inception) and NSP1/NSP2 (Nodulation Signaling Pathway) play crucial roles in the early stages of nodule formation by regulating the expression of other nodulation-related genes (Roy et al., 2020). The Nod factor-independent symbiotic signaling pathway, observed in some legume species like Aeschynomene indica, represents an alternative mechanism where nodulation occurs without the typical Nod factors, highlighting the diversity of nodulation strategies among legumes (Bonaldi et al., 2010). 4.3 Recent advancements in genetic engineering to enhance symbiosis Recent advancements in genetic engineering have opened new avenues for enhancing the symbiotic relationship between legumes and rhizobia. Techniques such as CRISPR/Cas9 and other genome editing tools have been employed to modify key genes involved in nodulation and nitrogen fixation, aiming to improve the efficiency and effectiveness of these processes (Roy et al., 2020). For instance, the development of Agrobacterium rhizogenes-mediated transformation protocols for non-model legumes like Aeschynomene indica has facilitated the study and manipulation of symbiotic genes in these species (Bonaldi et al., 2010). Additionally, the horizontal transfer of symbiosis islands, which contain essential symbiotic genes, has been shown to rapidly evolve new, competitive strains of rhizobia, although these strains may sometimes be suboptimal in nitrogen fixation (Nandasena et al., 2007). These advancements hold promise for developing legume varieties with enhanced symbiotic capabilities, potentially leading to increased agricultural productivity and sustainability. 5 Diversity of Rhizobia and Host Specificity 5.1 Different genera and species of rhizobia Rhizobia, the symbiotic bacteria responsible for nitrogen fixation in legumes, belong to several genera within the Alphaproteobacteria and Betaproteobacteria classes. Key genera include Rhizobium, Bradyrhizobium, Mesorhizobium, Ensifer (formerly Sinorhizobium), Azorhizobium, and Burkholderia (Gage, 2004; Remigi et al., 2016). Each genus encompasses multiple species, which exhibit varying degrees of host specificity and symbiotic
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