Legume-Rhizobia Symbiosis
Introduction
Legumes, which include many common foods such as soybeans and peas, are well known as the only plant family with the capability to fix nitrogen. Most environmental nitrogen is in the form of N₂, or atmospheric nitrogen. However, for plants to uptake and use this vital nutrient, it must be in an organic form such as ammonia (NH₃), nitrate (NO₃-), or nitrite (NO₂-). Legumes possess the ability to fix atmospheric nitrogen into organic compounds through their symbiotic relationship with Rhizobia bacteria. During rhizobial symbiosis, rhizobia infects plant roots to form root nodules. The bacteria facilitate nitrogen uptake for the plants (S, J. 2023). The rhizobia provides the legume with a source of fixed nitrogen, while in turn, the host legume provides the rhizobia with key nutrients for metabolism (U, M. 2013). The availability of organic nitrogen in the soil is a major limiting factor for plant growth (S, J. 2023). Because of this symbiosis with rhizobia is crucial for productivity in legumes (P, N. A. 2003). Research on this symbiotic interaction has the potential to enhance food production and make sustainable changes in the field of agriculture (P, N. A. 2003). This page will discuss the process for nodulation and key genes of this symbiosis.
Initiating Legume-Rhizobia Symbiosis
The first step in initiating the symbiotic relationship is the release of flavonoids and isoflavonoids from the plant roots (S, J. 2023). Flavanoids are released by the legume plants during periods of nitrogen deficiency and act as chemical signals that attract rhizobia bacteria. Rhizobia is a type of gram-negative proteobacteria found in soil. They belong to a family of nitrogen-fixing bacteria (S, J. 2023).
Upon receiving these chemical signals, rhizobia colonizes the surface of the plant roots and releases reciprocal signaling molecules known as Nod factors (D, J. J. 2015). Nod factors are synthesized by the enzymes NodM, NodC, NodB, and NodA (S, J. 2023). These signaling molecules induce root hair deformation, cortical cell proliferation, and infection thread formation in the host plant. These are precursory steps to the formation of nodules. (S, J. 2023). The deformed root hairs envelop the rhizobia. The rhizobia then degrade the cell wall to penetrate the plant root and the plant cell’s plasma membrane envelopes the bacteria, forming the infection thread (G, N. 2009, S, J. 2023). Infection thread formation requires continuous generation of the plant's cell membrane and wall (S, J. 2023). The infection thread allows the bacteria to move deeper into the plant tissue to colonize root cells (D, J. J. 2015; U, M 2013).
The deformed root hairs envelop the rhizobia. The rhizobia then degrade the cell wall to penetrate the plant root and the plant cell’s plasma membrane envelopes the bacteria, forming the infection thread (G, N. 2009, S, J. 2023). Infection thread formation requires continuous generation of the plant's cell membrane and wall (S, J. 2023). The infection thread allows the bacteria to move deeper into the plant tissue to colonize root cells (D, J. J. 2015; U, M 2013). The rhizobia are transported into the root cell cytoplasm by endocytosis (U, M. 2013).
Inside the root cell, the rhizobia are contained in a plant-derived membrane. This unit is called a symbiosome and serves as the functional unit of nitrogen fixation in the legume root nodules (D, J. J. 2015; U, M. 2013). Symbiosomes may contain any number of bacteria cells (D, J. J. 2015).
Once the root nodule has developed, Rhizobia begins to differentiate to carry out the necessary functions for symbiotic nitrogen fixation (S, J. 2023). These differentiated bacteria are known as bacteroids. Differentiation is initiated by a peptide released by the host legume which causes the rhizobia to differentiate according to the plant’s specific needs (S, J. 2023). Differentiated bacteroids form either determinate or indeterminate nodules which each have varying functions in nitrogen fixation (S, J. 2023; U, M 2013).
Following these steps, the nitrogen-fixing process can now occur. Rhizobia will produce nitrogenase enzymes which convert inorganic N₂ into organic NH₃ (P, N. A. 2003). Once the nitrogen is fixed, it is transported and incorporated into amino acids and ureides for plant growth (D, J. J. 2015).
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Include some current research, with at least one image.
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Here we cite April Murphy's paper on microbiomes of the Kokosing river. [5]
Key Genes in Symbiotic Nitrogen Fixation
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CFuture Prospects
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References
- ↑ 1.0 1.1 1.2 Zigli DD, Brew L, Obeng-Denteh W, Kwofie S. On the Application of Homeomorphism on Amoeba Proteus. Ghana Journal of Technology. 2021 Mar 31;5(2):43-7.
- ↑ Bartlett et al.: Oncolytic viruses as therapeutic cancer vaccines. Molecular Cancer 2013 12:103.
- ↑ Lee G, Low RI, Amsterdam EA, Demaria AN, Huber PW, Mason DT. Hemodynamic effects of morphine and nalbuphine in acute myocardial infarction. Clinical Pharmacology & Therapeutics. 1981 May;29(5):576-81.
- ↑ 4.0 4.1 text of the citation
- ↑ 5.0 5.1 Murphy A, Barich D, Fennessy MS, Slonczewski JL. An Ohio State Scenic River Shows Elevated Antibiotic Resistance Genes, Including Acinetobacter Tetracycline and Macrolide Resistance, Downstream of Wastewater Treatment Plant Effluent. Microbiology Spectrum. 2021 Sep 1;9(2):e00941-21.
Edited by Rebecca Holland, student of Joan Slonczewski for BIOL 116, 2024, Kenyon College.
