Molecular Microbiology Research 2024, Vol.14, No.4, 188-197 http://microbescipublisher.com/index.php/mmr 189 and stress responses. By synthesizing the results of recent studies, it aims to highlight key aspects of leguminous-rhizobia symbiosis and identify future research directions that can further optimize this beneficial relationship for agricultural applications. 2 Soil Microbiology of Rhizobium 2.1 Characteristics and diversity of rhizobia in soil Rhizobia are a diverse group of soil bacteria that can form symbiotic relationships with leguminous plants, leading to the formation of nitrogen-fixing nodules on the roots. The diversity of rhizobia is extensive, encompassing several genera such as Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium, and Azorhizobium. These bacteria exhibit a wide range of host interactions, with some species showing specificity towards particular legume hosts, while others are more promiscuous (Andrews and Andrews, 2016). For instance, in the Core Cape Subregion of South Africa, rhizobia from the genera Mesorhizobiumand Burkholderia were found to dominate, with Mesorhizobium showing a preference for the tribe Psoraleeae and Burkholderia for the Podalyrieae. Additionally, the genetic diversity of rhizobia can be influenced by ecological factors such as soil acidity and site elevation (Lemaire, 2015). 2.2 Soil Conditions favoring rhizobium survival The survival and distribution of rhizobia in soil are influenced by various soil conditions, including nutrient availability, pH, and organic matter content. For example, Mesorhizobium septentrionale was predominantly found in barren soils with low nitrogen and organic carbon content, while Mesorhizobium temperatumwas more common in nitrogen-rich fields (Yan et al., 2016). Soil fertility, therefore, plays a crucial role in determining the distribution of rhizobia associated with different legume plants. Additionally, the presence of arbuscular mycorrhizal (AM) fungi can significantly impact rhizobial populations. AM fungi promote the accumulation of rhizobia in the rhizosphere, enhancing nodulation and overall plant health (Figure 1) (Wang et al., 2020). This symbiotic relationship between AM fungi and rhizobia is essential for the establishment and maintenance of effective legume-rhizobium symbioses, particularly under stress conditions such as drought (Sharma et al., 2020). 2.3 Interaction with other soil microorganisms Rhizobia do not exist in isolation but interact with a myriad of other soil microorganisms, including beneficial fungi and bacteria. One notable interaction is with AM fungi, which can modulate the rhizosphere microbiota to favor rhizobial symbiosis. This tripartite relationship between legumes, rhizobia, and AM fungi enhances nutrient uptake and stress tolerance in plants. Additionally, the presence of other soil microorganisms, such as pathogenic microbes, necessitates that legumes discriminate between beneficial symbionts and potential pathogens. This discrimination is crucial for plant survival and effective symbiosis (Yang et al., 2021). Furthermore, the type 3 secretion system (T3SS) in some rhizobia can either promote or inhibit nodulation, depending on the host plant species, highlighting the complex interactions between rhizobia and their legume hosts (Jiménez-Guerrero et al., 2022). 3 Molecular Mechanisms of Symbiosis 3.1 Rhizobium infection process The rhizobium infection process begins with the recognition of rhizobial Nod factors by the legume root hairs, which triggers a series of cellular responses. This recognition leads to root hair curling and the formation of infection threads that facilitate the entry of rhizobia into the root cortex. The plant's control over this process is stringent, ensuring that only compatible rhizobia are allowed entry. This involves a complex interplay of signaling molecules and transcription factors that regulate the infection process and subsequent nodule formation (Oldroyd et al., 2011; Yang et al., 2021).
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