Molecular Soil Biology 2024, Vol.15, No.3, 129-139 http://bioscipublisher.com/index.php/msb 130 reduce the environmental impact of agriculture by decreasing the reliance on chemical fertilizers (Yates et al., 2021b). Moreover, certain Rhizobium strains have been shown to tolerate abiotic stresses, such as extreme temperatures and soil acidity, making them valuable for improving crop resilience in challenging environments (Yates et al., 2021a). This study aims to provide a comprehensive review of the role of Rhizobiumin legume nitrogen fixation and its impact on soil health and sustainable agriculture. It will explore the mechanisms underlying Rhizobium-legume symbiosis, the benefits of this symbiotic relationship for soil fertility and crop productivity, and the potential of Rhizobiumas a biofertilizer to reduce the need for synthetic nitrogen inputs. Additionally, the study will discuss the challenges and opportunities in optimizing Rhizobium-legume interactions to enhance agricultural sustainability and resilience in the face of climate change. By synthesizing current research findings, this study seeks to highlight the importance of Rhizobiumin promoting soil health and sustainable agricultural practices. 2The Rhizobium-Legume Symbiosis 2.1 Detailed mechanism of Rhizobiuminfection in legume roots The infection process of Rhizobiumin legume roots is a highly coordinated sequence of events that begins with the recognition of rhizobial Nod factors by the plant (Wang et al., 2018a). This recognition triggers a cascade of responses, including the growth of root hairs and the formation of infection threads through which the bacteria enter the root cells. (Chakraborty et al., 2022) The rhizobia then induce cell division in the root cortex, leading to the formation of nodule primordia (Figure 1) (Yang et al., 2021). The bacteria are eventually enclosed in intracellular compartments called symbiosomes, where they differentiate into bacteroids capable of nitrogen fixation. The research of Yang et al. (2021) illustrates the stages of nodule organogenesis in Medicago truncatula, highlighting the role of various gene regulatory networks. The process begins with the priming of root cells, especially cortical cells, facilitated by the SHR–SCR module. Key transcriptional complexes activate cell divisions in response to symbiotic signals, leading to the initiation of nodule formation. This is followed by outgrowth, where extensive cell divisions result in the formation of the nodule primordium. Finally, the nodule matures, developing a vascular system and an infection zone, primarily derived from cortical cells. This organized process is crucial for effective symbiosis in legumes. 2.2 Formation and structure of root nodules Root nodule formation is a complex process that involves several stages. Initially, rhizobia attach to the root hairs and produce Nod factors, which are recognized by the plant, leading to root hair curling and the formation of infection threads (Lindström et al., 2022). These threads guide the bacteria into the root cortex, where they induce cell division and form nodule primordia. The developing nodule then differentiates into a mature structure housing the nitrogen-fixing bacteroids within symbiosomes (Oldroyd et al., 2011). The nodule structure is specialized to facilitate efficient nitrogen fixation, with a well-organized vascular system to transport nutrients and fixed nitrogen between the plant and the bacteria (Andrews and Andrews, 2016). 2.3 Biochemical pathways involved in nitrogen fixation The biochemical pathways involved in nitrogen fixation are intricate and tightly regulated. Within the symbiosomes, bacteroids convert atmospheric nitrogen (N2) into ammonia (NH3) using the nitrogenase enzyme complex (Lepetit and Brouquisse, 2023). This process is energy-intensive and requires a continuous supply of ATP and reducing power, which are provided by the plant in the form of dicarboxylates and other metabolites (Schulte et al., 2021). The plant also regulates oxygen levels within the nodule to maintain the microaerobic conditions necessary for nitrogenase activity. Key metabolic pathways, such as the carbonic anhydrase-phosphoenolpyruvate carboxylase-malate dehydrogenase (CA-PEPC-MDH) pathway, play crucial roles in maintaining the redox balance and facilitating the efficient exchange of nutrients between the plant and the bacteroids (Schwember et al., 2022).
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