6.1 Nitrogen fixation and symbiotic relationships
3 min read•august 7, 2024
Nitrogen fixation is a vital process in , turning atmospheric nitrogen into usable forms. Symbiotic relationships between plants and bacteria, like Rhizobium with , make this possible in nature. These partnerships form specialized root structures called nodules.
, the key enzyme in this process, needs lots of energy and a low-oxygen environment to work. Plants provide this setup in nodules, along with to control oxygen levels. In return, bacteria supply plants with fixed nitrogen for growth.
Nitrogen Fixation Process
Converting Atmospheric Nitrogen to Ammonia
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- Nitrogen fixation converts atmospheric nitrogen gas (N2) into ammonia (NH3) which can be used by plants
- Occurs through by bacteria or through industrial processes like the Haber-Bosch process
- Biological nitrogen fixation accounts for about 60% of the total nitrogen fixed on Earth each year
- Crucial step in the allows nitrogen to be incorporated into organic compounds and made available to plants and other organisms
Nitrogenase Enzyme and Bacteroids
- Nitrogenase is the enzyme responsible for catalyzing the reduction of atmospheric nitrogen to ammonia
- Consists of two protein components: dinitrogenase reductase (Fe protein) and dinitrogenase (MoFe protein)
- Requires a large amount of energy in the form of ATP (16 ATP per N2 reduced) and a source of electrons (ferredoxin)
- are differentiated bacterial cells within root nodules that contain nitrogenase and carry out nitrogen fixation
- Bacteroids have a larger size, different shape, and altered metabolism compared to free-living bacteria
- Surrounded by a plant-derived membrane called the peribacteroid membrane which controls nutrient exchange
Symbiotic Bacteria
Rhizobium and Legume Symbiosis
- Rhizobium is a genus of gram-negative soil bacteria that forms a symbiotic relationship with legumes
- Infects the roots of leguminous plants, leading to the formation of root nodules
- Within the nodules, Rhizobium differentiates into bacteroids and fixes atmospheric nitrogen for the plant
- In return, the plant provides the bacteria with carbohydrates and a protected environment
- Examples of legumes that form symbiosis with Rhizobium include soybeans, alfalfa, and peas
Frankia and Actinorhizal Symbiosis
- is a genus of actinobacteria that forms symbiotic relationships with
- Actinorhizal plants are a diverse group of woody plants that form root nodules with Frankia, including alders (Alnus), bayberry (Myrica), and sweet fern (Comptonia)
- Frankia infects the roots of actinorhizal plants and induces the formation of root nodules, similar to Rhizobium in legumes
- Within the nodules, Frankia differentiates into vesicles, which are specialized nitrogen-fixing structures
Nod Factors and Symbiosis Establishment
- are signaling molecules produced by in response to flavonoids secreted by legume roots
- Nod factors are lipochitooligosaccharides (LCOs) that vary in their structure depending on the rhizobial species
- Nod factors trigger a series of responses in the plant root, including root hair curling, infection thread formation, and nodule organogenesis
- Perception of Nod factors by the plant is mediated by LysM receptor-like kinases, which initiate a signaling cascade leading to symbiosis establishment
Plant Structures
Legume Root Nodules
- Legume nodules are specialized plant organs that develop on the roots of leguminous plants in response to infection by rhizobia
- Nodules provide a low-oxygen environment necessary for nitrogenase activity and nitrogen fixation
- Consist of a central zone containing infected plant cells filled with bacteroids, surrounded by uninfected plant cells
- Nodules are connected to the plant's vascular system, allowing for efficient exchange of nutrients between the plant and the bacteria
- Examples of legumes that form nodules include soybeans, peas, and
Actinorhizal Root Nodules
- Actinorhizal nodules are formed on the roots of actinorhizal plants in response to infection by Frankia
- Actinorhizal nodules have a coralloid structure, with multiple lobes arising from a common base
- Each lobe contains a central vascular bundle surrounded by cortical cells infected with Frankia
- Actinorhizal nodules have a higher oxygen concentration compared to legume nodules, and Frankia forms specialized vesicles for nitrogen fixation
Leghemoglobin and Oxygen Regulation
- Leghemoglobin is an oxygen-binding protein found in legume nodules, similar to hemoglobin in animal blood
- Produced by the plant in response to rhizobial infection and nodule development
- Leghemoglobin binds to oxygen, maintaining a low oxygen concentration in the nodule to protect nitrogenase from inactivation
- Gives legume nodules their characteristic pink or red color when active
- Oxygen regulation by leghemoglobin is crucial for efficient nitrogen fixation in legume-rhizobia symbiosis
Key Terms to Review (23)
Actinorhizal plants: Actinorhizal plants are a group of flowering plants that form symbiotic relationships with nitrogen-fixing actinobacteria, specifically the genus Frankia. This mutualistic association allows these plants to convert atmospheric nitrogen into a usable form, enhancing soil fertility and supporting plant growth. This type of nitrogen fixation is crucial for ecosystems, particularly in nitrogen-poor environments, as it enables these plants to thrive while enriching the surrounding soil.
Ammonification: Ammonification is the process by which organic nitrogen from decomposed matter is converted into ammonia (NH₃) by microorganisms in the soil. This process plays a crucial role in the nitrogen cycle, as it transforms nitrogen from dead organisms and waste into a form that can be utilized by plants, bridging the gap between organic matter decomposition and nitrogen availability for plant uptake.
Bacteroids: Bacteroids are specialized, differentiated forms of nitrogen-fixing bacteria found within the root nodules of leguminous plants. They play a crucial role in establishing a symbiotic relationship with their host plants, converting atmospheric nitrogen into ammonia, which is then used by the plant for growth and development.
Biofertilizers: Biofertilizers are natural substances that enhance soil fertility and promote plant growth by providing essential nutrients through biological processes. These products often contain live microorganisms, such as bacteria, fungi, or algae, that improve nutrient availability and uptake by plants. By fostering symbiotic relationships with plants and enhancing soil health, biofertilizers play a crucial role in sustainable agriculture and can reduce the reliance on chemical fertilizers.
Biological nitrogen fixation: Biological nitrogen fixation is the process by which certain microorganisms convert atmospheric nitrogen gas ($$N_2$$) into ammonia ($$NH_3$$), making it available for use by plants. This process is crucial for the nitrogen cycle and supports plant growth, as most plants cannot directly utilize atmospheric nitrogen. Nitrogen-fixing bacteria often form symbiotic relationships with plants, enhancing nutrient availability in soil.
Clover: Clover is a genus of plants in the legume family, known scientifically as Trifolium, that plays a significant role in nitrogen fixation and forming symbiotic relationships with soil bacteria. These plants are commonly found in grasslands and agricultural settings and contribute to soil health by improving nitrogen levels through their unique partnership with Rhizobium bacteria. This not only supports clover growth but also benefits surrounding plants and ecosystems.
Crop yield: Crop yield refers to the amount of crop produced per unit area of land, typically measured in tons per hectare or bushels per acre. It is a crucial indicator of agricultural productivity and efficiency, influencing food supply and economic stability.
Cyanobacteria: Cyanobacteria are a group of photosynthetic bacteria, often referred to as blue-green algae, that can perform nitrogen fixation. They are crucial for ecosystems as they contribute to nutrient cycling and have symbiotic relationships with various plants and organisms.
Frankia: Frankia is a genus of nitrogen-fixing bacteria that forms symbiotic relationships with certain plants, particularly actinorhizal plants. These bacteria have the unique ability to convert atmospheric nitrogen into ammonia, which can be utilized by the host plant, enhancing soil fertility and promoting plant growth. The relationship between Frankia and its host plants is mutualistic, benefiting both organisms involved.
Isotope labeling: Isotope labeling is a technique used in biological and environmental studies where isotopes, which are variants of elements with different atomic masses, are introduced into a system to trace and study the movement and transformation of substances. This method is particularly useful for understanding processes like nitrogen fixation, as it allows researchers to track the uptake and utilization of nitrogen by plants and their symbiotic relationships with microbes.
Leghemoglobin: Leghemoglobin is a globular protein found in the root nodules of leguminous plants, primarily responsible for transporting oxygen to nitrogen-fixing bacteria. It is structurally similar to hemoglobin and plays a crucial role in maintaining a low oxygen concentration in the root nodule, allowing nitrogen-fixing bacteria to thrive while protecting them from oxygen toxicity.
Legumes: Legumes are a group of plants belonging to the family Fabaceae, known for their ability to form symbiotic relationships with nitrogen-fixing bacteria. This unique feature allows legumes to convert atmospheric nitrogen into a form that is usable by plants, playing a crucial role in soil fertility and nutrient cycling.
Molecular techniques: Molecular techniques refer to a set of methods used to study and manipulate DNA, RNA, and proteins at the molecular level. These techniques allow scientists to analyze genetic material, assess gene expression, and understand the interactions between organisms in symbiotic relationships, particularly in the context of nitrogen fixation and plant-microbe interactions.
Mutualism: Mutualism is a type of symbiotic relationship where both organisms involved benefit from the interaction. This relationship can enhance the survival and reproduction of both species, leading to various adaptations that facilitate cooperation. Such interactions can be crucial for ecosystem functioning, influencing nutrient cycles, plant growth, and species diversity.
Nitrate reductase: Nitrate reductase is an enzyme responsible for the reduction of nitrate (NO₃⁻) to nitrite (NO₂⁻) in plants, fungi, and certain bacteria. This process is a crucial step in nitrogen assimilation, allowing organisms to convert inorganic nitrogen sources into organic forms that can be used for amino acid and protein synthesis.
Nitrogen Cycle: The nitrogen cycle is a natural process through which nitrogen is converted between its various chemical forms, including atmospheric nitrogen (N₂), ammonia (NH₃), nitrites (NO₂⁻), and nitrates (NO₃⁻). This cycle is essential for the sustainability of ecosystems as it enables the transformation of nitrogen into forms that can be utilized by living organisms, particularly through processes like nitrogen fixation and interactions within symbiotic relationships.
Nitrogenase: Nitrogenase is an enzyme complex responsible for the biological process of nitrogen fixation, which converts atmospheric nitrogen ($$N_2$$) into ammonia ($$NH_3$$). This enzyme plays a crucial role in the nitrogen cycle by enabling certain microorganisms, especially bacteria, to convert inert atmospheric nitrogen into a form that can be assimilated by plants, ultimately supporting plant growth and agricultural productivity.
Nod factors: Nod factors are signaling molecules produced by certain soil bacteria, particularly Rhizobia, that initiate the process of root nodule formation in leguminous plants. These molecules play a crucial role in establishing a symbiotic relationship, allowing plants to fix atmospheric nitrogen, which is essential for their growth and overall health.
Plant nutrition: Plant nutrition refers to the process by which plants absorb essential nutrients from the soil and use them to grow, develop, and carry out physiological functions. This process is crucial for maintaining plant health and productivity, as it directly impacts photosynthesis, metabolism, and overall growth. Key elements in plant nutrition include macronutrients like nitrogen, phosphorus, and potassium, as well as micronutrients that support various biochemical processes.
Rhizobia: Rhizobia are a group of soil bacteria known for their ability to fix nitrogen in symbiotic relationships with leguminous plants. These bacteria infect the root hairs of their host plants, leading to the formation of root nodules where nitrogen fixation occurs, providing essential nutrients for plant growth and enhancing soil fertility. This interaction is crucial for sustainable agriculture and improving crop yields, as well as maintaining ecosystem health.
Soil fertility: Soil fertility refers to the ability of soil to provide essential nutrients and support healthy plant growth. It encompasses a range of factors including nutrient availability, soil structure, pH levels, and the presence of beneficial microorganisms. High soil fertility is crucial for sustainable agriculture and ecosystem health, as it directly affects plant productivity and resilience.
Sustainable agriculture: Sustainable agriculture is a farming approach that seeks to balance the need for food production with the preservation of environmental quality and resources for future generations. This practice focuses on maintaining healthy ecosystems, promoting biodiversity, and fostering symbiotic relationships between plants, animals, and soil to create resilient agricultural systems that can withstand challenges like climate change.
Symbiotic nitrogen fixation: Symbiotic nitrogen fixation is a biological process where certain plants, particularly legumes, form mutualistic relationships with specific bacteria called rhizobia. These bacteria reside in the root nodules of the plants and convert atmospheric nitrogen into ammonia, a form that plants can readily use for growth. This relationship not only enhances the nutrient availability for the plants but also enriches the soil, making it beneficial for surrounding flora.