Molecular Microbiology Research 2024, Vol.14, No.2, 109-118 http://microbescipublisher.com/index.php/mmr 110 This study will explore the mechanisms and efficiency of symbiotic nitrogen fixation in different legume-rhizobia associations and assess their impact on plant growth and soil fertility. The focus is on investigating the genetic, physiological, and environmental factors that influence this process and determining strategies to enhance the effectiveness of rhizobial inoculants in agricultural practices. A deeper understanding of the interactions between rhizobia and leguminous plants is expected to improve crop yields and reduce reliance on chemical fertilizers. This study aspires to develop innovative approaches to enhance the efficiency of biological nitrogen fixation, achieve more sustainable agricultural systems, and contribute to global food security and environmental protection. 2 Overview of Rhizobia 2.1 Definition and characteristics of rhizobia Rhizobia are soil bacteria that form a close symbiotic relationship with leguminous plants, belonging to the family Rhizobiaceae or related taxa. These microorganisms infect the root tissues of host plants and induce the formation of structures known as nodules, which are the primary sites for nitrogen fixation. Inside the nodules, rhizobia convert atmospheric nitrogen (N2) into ammonia (NH3) using specialized enzyme systems such as nitrogenase (Masson-Boivin and Sachs, 2018; Lindström and Mousavi, 2019). This process is crucial for plants, as nitrogen is an essential element for the synthesis of amino acids, proteins, and nucleic acids. The ability of rhizobia to fix nitrogen not only improves the growth efficiency of leguminous plants but also enhances soil fertility, providing long-term agricultural benefits for crop rotation. The nitrogen-fixing capability of rhizobia plays a significant role in sustainable agriculture by substantially reducing the reliance on synthetic nitrogen fertilizers. The extensive use of synthetic nitrogen fertilizers can lead to environmental issues such as water eutrophication and greenhouse gas emissions, whereas rhizobia offer a natural and environmentally friendly alternative. By forming symbiotic relationships with leguminous plants, rhizobia not only meet the nitrogen needs of the plants but also enhance the nitrogen cycling in ecosystems, contributing to soil health and productivity. This natural nitrogen source shows great potential in organic and low-input agricultural systems, promising to advance the sustainability of agricultural practices (Pankievicz et al., 2019). 2.2 Types of rhizobia Rhizobia are diverse and can be classified into several genera, including Rhizobium, Bradyrhizobium, Sinorhizobium, and Mesorhizobium, among others. Each genus contains various species, which differ in their ability to form nodules with different leguminous host plants and their nitrogen-fixing efficiency. For example, Bradyrhizobium diazoefficiens, from the genus Bradyrhizobium, is well-known for its symbiotic relationship with soybeans, while Rhizobium leguminosarumfrom the genus Rhizobiumcommonly forms symbioses with peas and lentils (Nguyen et al., 2019; Dupin et al., 2020). Sinorhizobiumtypically forms nodules with leguminous plants like alfalfa, similar to those formed by Rhizobium. Mesorhizobium is associated with plants like chickpeas and lentils, with nodules that exhibit characteristics intermediate between determinate and indeterminate types. Each type of rhizobia has evolved to adapt to specific environmental conditions and form symbiotic relationships with particular legume populations. Recent research has revealed the adaptability of rhizobia under different environmental conditions, especially their nodulation and nitrogen-fixing abilities in high nitrate environments. These newly discovered rhizobial strains demonstrate the potential to maintain efficient nitrogen fixation even in conditions traditionally considered unfavorable for nodule formation and nitrogen fixation. The emergence of these strains offers new hope for improving the nitrogen-fixing efficiency of leguminous plants under adverse soil conditions, particularly in the high nitrate environments commonly found in agricultural production, and they may have significant application prospects (Nguyen et al., 2019).
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