Molecular Microbiology Research 2024, Vol.14, No.2, 109-118 http://microbescipublisher.com/index.php/mmr 111 2.3 Rhizobia-host specificity Rhizobia exhibit a high degree of host specificity, meaning that specific rhizobial strains can only be compatible with particular leguminous plants. This specificity is primarily determined by the molecular signaling exchanges between the plant and the rhizobia. Flavonoids secreted by the plant roots trigger the production of nodulation factors (Nod factors) in the rhizobia, which are then recognized by the plant, leading to nodule formation (Masson-Boivin and Sachs, 2018; Lepetit and Brouquisse, 2023). This specificity of interaction ensures that rhizobia can only successfully infect and form nodules on compatible leguminous plants. For example, Rhizobium leguminosarum biovar viciae forms nodules on peas and field peas, while Bradyrhizobium japonicum specifically symbiotically associates with soybeans (Kimeklis et al., 2019). The genetic basis of this specificity lies in the symbiotic genes carried by the rhizobia, which determine their ability to interact with specific plant hosts. Understanding the host specificity of rhizobia is crucial for developing effective inoculants to enhance crop production in various agricultural environments. Rhizobia are essential for legume growth and soil fertility due to their unique ability to fix atmospheric nitrogen. The diversity among rhizobial species and their specific interactions with legume hosts highlight the complexity and importance of this symbiotic relationship in agricultural ecosystems. 3 Mechanisms of Symbiotic Nitrogen Fixation 3.1 Infection and nodule formation Root hair curling is the initial step in the infection process where rhizobia attach to the root hairs of legumes. This attachment triggers the root hairs to curl around the bacteria, forming an infection pocket. This process is tightly regulated by the exchange of signaling molecules between the plant and the rhizobia. The plant releases flavonoids that induce the production of Nod factors by rhizobia, which in turn trigger root hair curling (Masson-Boivin and Sachs, 2018; Chakraborty et al., 2022). Following root hair curling, rhizobia enter the plant root through infection threads, which are tubular structures formed by the invagination of the root hair cell wall. The development of infection threads is a critical step that allows rhizobia to travel from the root hair to the root cortex. This process is regulated by various plant hormones and signaling pathways, including cytokinin and auxin, which promote infection thread formation and progression (Liu et al., 2018b; Teulet et al., 2019). Nodule organogenesis involves the differentiation of root cortical cells into a new organ, the nodule, where nitrogen fixation occurs. This process is initiated by the perception of Nod factors and involves complex signaling pathways that coordinate cell division and differentiation. Key transcription factors and signaling molecules, such as NODULE INCEPTION (NIN) and cytokinin, play crucial roles in nodule organogenesis (Velzen et al., 2018; Chakraborty et al., 2022). Additionally, certain rhizobial effectors, like ErnA, can directly trigger nodule formation by modulating host plant signaling pathways (Teulet et al., 2019). 3.2 Nitrogen fixation process Once the nodules form, the rhizobia inside the nodules convert atmospheric nitrogen (N2) into ammonia (NH3) through nitrogenase. The plant then assimilates this ammonia into amino acids, providing an essential nitrogen source for plant growth. The nitrogen fixation process is highly energy-intensive, relying on carbohydrates provided by the plant from photosynthesis to support the activity of nitrogenase (Figure 1) (Lindström and Mousavi, 2019; Lepetit and Brouquisse, 2023). The efficiency of nitrogen fixation can be affected by various factors, including the energy supply for the nitrogenase, interactions with the host plant, and external environmental conditions (Lindström and Mousavi, 2019; Sauviac et al., 2022). Research by Lindström and Mousavi (2019) indicates that the high energy consumption of nitrogenase is a major limiting factor for nitrogen fixation efficiency, and adjusting the oxygen levels and carbon metabolism within the nodules can enhance fixation efficiency. Additionally, differences in nitrogen fixation effectiveness exhibited by rhizobia in specific host plants highlight the need to optimize nitrogen fixation in different plant-microbe symbiotic systems.
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