Legume Genomics and Genetics 2024, Vol.15, No.3, 140-151 http://cropscipublisher.com/index.php/lgg 141 applications, this study aims to provide a holistic approach to leveraging the benefits of rhizobium-legume symbiosis for sustainable agriculture. 2 Overview of Rhizobium-Legume Symbiosis 2.1 Biological basis of symbiosis The symbiotic relationship between rhizobia and legumes is a classic example of mutualism, where both partners derive significant benefits. Legumes provide rhizobia with carbohydrates and a niche within root nodules, while rhizobia fix atmospheric nitrogen (N2) into ammonia, which is assimilated by the plant for growth and development. This biological nitrogen fixation is crucial for improving soil fertility and reducing the need for chemical fertilizers, making it an environmentally friendly alternative (Mendoza-Suárez et al., 2021). The process of nitrogen fixation in rhizobium-legume symbiosis involves several stages, starting with the exchange of chemical signals between the plant and the bacteria. Rhizobia produce Nod factors that are recognized by specific receptors on the legume roots, initiating nodule formation. Within these nodules, rhizobia convert atmospheric nitrogen into ammonia through the action of the nitrogenase enzyme complex. This ammonia is then assimilated by the plant, providing a vital source of nitrogen for its growth (Clúa et al., 2018; Wang et al., 2018; Yang et al., 2021). 2.2 Rhizobiumspecies and their specificity Rhizobia are a diverse group of bacteria, primarily classified into alpha- and beta-proteobacteria. Key genera include Rhizobium, Bradyrhizobium, Ensifer (formerly Sinorhizobium), and Mesorhizobium, among others. Each genus contains multiple species and strains, each with varying abilities to form symbiotic relationships with different legume hosts (Masson-Boivin et al., 2009; Andrews and Andrews, 2016). The specificity of rhizobium-legume interactions is a complex trait influenced by both the bacterial and plant genomes. Specificity can occur at multiple stages of the symbiotic process, from initial bacterial attachment to nodule development and nitrogen fixation. For instance, Bradyrhizobiumspp. are typically associated with legumes in the Caesalpinioideae sub-family, while Rhizobium and Ensifer species are more common in the Papilionoideae sub-family. Some legumes exhibit promiscuity, nodulating with multiple rhizobial species, whereas others show strict specificity (Figure 1) (Wang et al., 2012; Andrews and Andrews, 2016; Wang et al., 2018). Host specificity is also influenced by environmental factors and the presence of native rhizobial populations in the soil. For example, Mimosa species in Brazil show specificity towards Burkholderia, while in Mexico, they prefer Rhizobium/Ensifer, and in Uruguay, Cupriavidus is the preferred symbiont. This specificity is often related to the relative abundance of these rhizobia in different soils (Andrews and Andrews, 2016; Mendoza-Suárez et al., 2021; Chakraborty et al., 2022). In summary, the rhizobium-legume symbiosis is a finely tuned interaction with significant implications for agricultural productivity and sustainability. Understanding the genetic and environmental factors that influence this symbiosis can lead to the development of more effective biofertilizers and improved legume crop yields. 3 Genetic Insights into Rhizobium-Legume Interaction 3.1 Molecular genetics of rhizobium The genomic structure of Rhizobiumspecies is complex and includes both chromosomal and plasmid-borne genes essential for symbiosis and nitrogen fixation. For instance, Rhizobiumsp. NGR234 possesses two dicarboxylate transport systems, one located on the chromosome and the other on a symbiotic plasmid, which are crucial for nitrogen fixation in tropical legumes. Additionally, the genomes of several Rhizobium species have been sequenced, revealing key genes involved in symbiotic nitrogen fixation (SNF) and providing insights into their functional roles (Dall’Agnol et al., 2014; Dwivedi et al., 2015). The regulation of nitrogen fixation genes in Rhizobium is tightly controlled and involves various genetic and environmental factors. For example, the dctAgene in Rhizobiumsp. NGR234, which is essential for dicarboxylate transport and nitrogen fixation, is regulated by unique promoter sequences distinct from those in other Rhizobium species. Moreover, proteomic studies have identified host factors, such as bioactive peptides, that control gene
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