Legume Genomics and Genetics 2024, Vol.15, No.4, 163-175 http://cropscipublisher.com/index.php/lgg 166 Figure 2 Overview of the N and C fluxes within the rhizobium-legume holobiont using symbiotic nitrogen fixation as the sole source of nitrogen (Adopted from Lepetit and Brouquisse, 2023) Image caption: Atmospheric N2 is fixed into NH3/NH4 + by nitrogenase in the bacteroids of the root nodule. The resulting NH4 + is delivered to the plant cells where it is assimilated into amino acids (AAs). In temperate legumes, asparagine (Asn) is the main form of transport of nitrogen from the root to the shoot through the xylem flux, whereas in some tropical legumes, this transport involves ureides (Adapted from Lepetit and Brouquisse, 2023) Nodule organogenesis is a complex process that involves the differentiation of root cortical cells into a new organ capable of housing nitrogen-fixing bacteria. This process is tightly regulated by both plant and bacterial signals, ensuring the formation of a functional nodule. The plant’s systemic signaling mechanisms, which integrate nutritional demands and environmental conditions, play a crucial role in regulating nodule formation and development. These mechanisms ensure that nodule formation is synchronized with the plant's overall nutritional status and photosynthetic capacity (Oldroyd et al., 2011). 3.3 Nitrogenase enzyme complex The nitrogenase enzyme complex is responsible for the reduction of atmospheric nitrogen (N2) to ammonia (NH3), a form that can be assimilated by the plant. This complex consists of two main components: the dinitrogenase reductase and the dinitrogenase, which work together to catalyze the reduction process under low oxygen conditions (diCenzo et al., 2020). The activity of nitrogenase is highly sensitive to oxygen, necessitating a microaerobic environment within the nodule for optimal function. The expression and activity of nitrogenase are tightly regulated by both bacterial and plant genes. In rhizobium, the key regulator NifA controls the expression of nitrogenase genes, ensuring that the enzyme is produced only under appropriate conditions. Additionally, the plant exerts control over nitrogenase activity through systemic signaling pathways that respond to the plant's nitrogen and carbon status, further fine-tuning the symbiotic relationship (Oldroyd et al., 2011; Lindström and Mousavi, 2019).
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