Molecular Soil Biology 2024, Vol.15, No.2, 74-86 http://bioscipublisher.com/index.php/msb 76 and attachment of rhizobia to the root hairs of the host plant, followed by the infection process where rhizobia enter the root cells and induce nodule formation. Inside the nodules, rhizobia differentiate into bacteroids, which are capable of fixing atmospheric nitrogen into ammonia, a form that the plant can utilize for growth and development (Masson-Boivin and Sachs, 2018; Schwember et al., 2019). Figure 1 Examples of rhizosphere chemical signaling that could influence the interaction of roots with both rhizobia and nematodes (Adopted from Costa et al., 2021) Image caption: Root-knot nematodes(RKN) produce ascarosides in the rhizosphere. Rhizobia produce Nod factors, quorum-sensing signals, and other signals perceived by the host. Both rhizobiaand nematodes can induce the production and exudation of (iso)flavonoids that can have functions in chemotaxis of rhizobia toward the root or attraction orrepulsion of nematodes to and from the root. They can also inhibit nematode motility, further enhance or repress Nod gene expression in rhizobia, and alterproduction of rhizobial quorum-sensing signaling. Ascarosides as well as Nod factors, quorum-sensing signals, and surface polysaccharides of rhizobia canalso modulate defense responses, which may have indirect effects on the tolerance of further infection with rhizobia or RKN. These can include induction ofethylene, methyl jasmonate (MeJA), methyl salicylate (MeSA), and salicylic acid (SA). Isoflavonoids can also influence other organisms in the rhizosphere thatcould indirectly interact with RKN or rhizobia or the host. For example, isoflavonoids can be inhibitors of fungal pathogens that may form secondary infectionsin roots infected by RKN (Adopted from Costa et al., 2021) 3.2 Biochemical pathways involved in nitrogen fixation The nitrogen fixation process in legume nodules involves several biochemical pathways that facilitate the conversion of atmospheric nitrogen (N2) into ammonia (NH3). One of the key pathways is the carbonic anhydrase (CA)-phosphoenolpyruvate carboxylase (PEPC)-malate dehydrogenase (MDH) pathway, which plays a crucial role in regulating the nitrogen fixation process. This pathway helps in maintaining the balance of carbon and nitrogen metabolism within the nodules, ensuring efficient nitrogen fixation. Additionally, the uptake hydrogenase activity in certain bacteroid strains recycles hydrogen produced during nitrogen fixation, thereby enhancing the overall efficiency of the process by recouping ATP used for hydrogen production (Ciccolella et al., 2010; Schwember et al., 2019). 3.3 Role of nitrogenase enzyme complex The nitrogenase enzyme complex is central to the nitrogen fixation process. This enzyme complex, found in the bacteroids within the root nodules, catalyzes the reduction of atmospheric nitrogen (N2) to ammonia (NH3). The nitrogenase complex is highly sensitive to oxygen, which necessitates the microaerobic conditions provided by the
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