Molecular Microbiology Research 2024, Vol.14, No.4, 188-197 http://microbescipublisher.com/index.php/mmr 191 3.2 Nodule formation and development 3.2.1 Initiation of nodule organogenesis Nodule organogenesis is initiated by the perception of rhizobial Nod factors, which activate signaling pathways in the root epidermis. This leads to localized cell division in the root cortex, forming a nodule primordium. The coordination between rhizobial infection and nodule organogenesis is crucial, as it ensures that the developing nodule is properly colonized by rhizobia (Hoang et al., 2020). Plant hormones such as cytokinins and auxins play significant roles in this process, promoting cell division and differentiation required for nodule formation (He et al., 2020). 3.2.2 Signal exchange between host and rhizobium The signal exchange between the host plant and rhizobium is a critical step in establishing a successful symbiosis. Rhizobia produce Nod factors that are recognized by specific receptors on the legume roots, initiating a signaling cascade that leads to nodule formation. This process is tightly regulated by both partners to ensure compatibility and effective symbiosis. Additionally, environmental factors such as nutrient availability and abiotic stresses can influence this signaling exchange, adding another layer of complexity to the symbiotic relationship (Wang et al., 2018; Chakraborty et al., 2022). 3.2.3 Genetic regulation of nodule development The genetic regulation of nodule development involves a network of transcription factors and signaling molecules that coordinate the various stages of nodule formation and function. Key transcription factors such as NSP1 and NSP2 are essential for the activation of symbiotic genes and the initiation of nodule organogenesis. Recent studies have identified several other transcription factors that integrate symbiotic signaling with responses to environmental stresses, highlighting the dynamic nature of this regulatory network. Additionally, microRNAs have been shown to play crucial roles in fine-tuning gene expression during nodule development, impacting processes such as hormone signaling and spatial regulation of gene expression (Hoang et al., 2020). 3.3 Nitrogen fixation pathways Nitrogen fixation within the nodules is facilitated by the enzyme nitrogenase, which converts atmospheric nitrogen into ammonia. This process is highly energy-intensive and requires a constant supply of photosynthates from the plant. The nodule environment is optimized for nitrogen fixation, with leghemoglobins playing a crucial role in maintaining low oxygen levels to protect nitrogenase activity. Recent research has also highlighted the importance of systemic signaling mechanisms that adjust nodule function based on the plant's nitrogen demand and environmental conditions (Larrainzar et al., 2020; Lepetit and Brouquisse, 2023). Understanding these pathways and their regulation is essential for improving the efficiency of biological nitrogen fixation in agricultural systems. 4 Environmental and Ecological Influences 4.1 Effects of abiotic stress on symbiosis Abiotic stresses such as salinity, drought, and heavy metal contamination significantly impact the legume-rhizobium symbiosis. These stress factors can suppress the growth and symbiotic characteristics of most rhizobia, although some strains exhibit tolerance to these conditions. For instance, certain rhizobia strains can form effective nitrogen-fixing symbioses under salt, heat, and acid stresses, and sometimes even in the presence of heavy metals. The resilience of the symbiosis to low to moderate stress is notable, but severe stress generally inhibits both legume dry matter and the proportional dependence on biological nitrogen fixation (BNF) (Chalk et al., 2010). Additionally, transcription factors (TFs) play a crucial role in integrating the response to abiotic stress with the symbiotic process, adding another layer of complexity to the regulation of this interaction (Chakraborty et al., 2022). 4.2 Role of soil nutrients in symbiotic efficiency Soil nutrients are critical for the efficiency of the legume-rhizobium symbiosis. The presence of essential nutrients like phosphorus, zinc, and copper, which are often limited in diffusion, can be enhanced through the symbiotic
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