10.4 Beneficial plant-microbe associations
4 min read•august 7, 2024
Plants form incredible partnerships with microbes in the soil. These tiny allies help plants access nutrients, fight off diseases, and even convert nitrogen from the air into usable forms. It's like having a secret underground support network!
Mycorrhizal fungi and are the superstars of these plant-microbe teams. They boost plant growth, improve , and reduce the need for chemical fertilizers. Nature's own sustainable farming system at work!
Mycorrhizal Associations
Symbiotic Relationships Between Plants and Fungi
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- are symbiotic associations between plant roots and fungi that benefit both partners
- Fungi colonize the plant's roots and extend their hyphae into the surrounding soil, increasing the plant's access to water and nutrients (phosphorus, nitrogen)
- In return, the plant provides the fungus with carbohydrates produced through photosynthesis
- Two main types of mycorrhizae: arbuscular mycorrhizae and
Arbuscular Mycorrhizal Fungi
- are the most common type of mycorrhizae and associate with the majority of land plants
- AMF penetrate the plant's root cells and form highly branched structures called arbuscules within the cells
- Arbuscules are the sites of nutrient exchange between the fungus and the plant
- AMF also produce vesicles, which are storage structures that contain lipids and other nutrients
- Examples of plants that form associations with AMF include legumes (soybeans, alfalfa), grasses (wheat, corn), and many fruit and vegetable crops (tomatoes, potatoes)
Ectomycorrhizae
- Ectomycorrhizae are formed by certain types of fungi, primarily basidiomycetes and ascomycetes, and are associated with woody plants
- Ectomycorrhizal fungi do not penetrate the plant's root cells but instead form a dense network of hyphae around the roots called a mantle
- The fungal hyphae extend into the soil, increasing the plant's access to water and nutrients
- Ectomycorrhizal fungi also form a Hartig net, which is a network of hyphae that grows between the plant's root cells and facilitates nutrient exchange
- Examples of plants that form ectomycorrhizal associations include pine trees, oak trees, and eucalyptus
Nitrogen-Fixing Symbioses
Rhizobia and Legume-Rhizobium Symbiosis
- are soil bacteria that form symbiotic relationships with leguminous plants (beans, peas, alfalfa)
- Rhizobia infect the roots of legumes and stimulate the formation of specialized structures called root nodules
- Within the root nodules, rhizobia convert atmospheric nitrogen (N2) into ammonia (NH3), which the plant can use for growth and development
- In return, the plant provides the rhizobia with carbohydrates and other nutrients
Nodulation Process
- Nodulation is the process by which rhizobia infect the roots of legumes and stimulate the formation of root nodules
- Flavonoids released by the plant roots attract rhizobia and stimulate the expression of nod genes in the bacteria
- Nod factors produced by the rhizobia cause the plant's root hairs to curl and trap the bacteria
- The bacteria enter the root through an infection thread and stimulate the division of root cortical cells, leading to the formation of the nodule
- Inside the nodule, the rhizobia differentiate into bacteroids, which are the nitrogen-fixing form of the bacteria
Nitrogen Fixation
- is the process by which atmospheric nitrogen (N2) is converted into ammonia (NH3), which plants can use for growth and development
- Rhizobia contain the enzyme nitrogenase, which catalyzes the reduction of N2 to NH3
- The plant provides the rhizobia with a low-oxygen environment within the nodule, which is necessary for nitrogenase activity
- The fixed nitrogen is exported from the nodule to the rest of the plant in the form of amino acids and other nitrogen-containing compounds
- Nitrogen fixation by rhizobia is an important source of nitrogen for leguminous crops and reduces the need for synthetic nitrogen fertilizers
Other Beneficial Microbes
Plant Growth-Promoting Rhizobacteria (PGPR)
- PGPR are beneficial bacteria that colonize the rhizosphere (the region of soil surrounding the plant roots) and promote plant growth through various mechanisms
- PGPR can enhance plant growth by producing plant growth regulators such as auxins, cytokinins, and gibberellins
- Some PGPR can solubilize phosphorus and other nutrients, making them more available for plant uptake
- PGPR can also protect plants against pathogens by producing antibiotics, competing for resources, or inducing systemic resistance in the plant
- Examples of PGPR include Pseudomonas, Bacillus, and Azospirillum species
Endophytes
- are microorganisms (bacteria or fungi) that live within plant tissues without causing harm to the plant
- Endophytes can colonize various plant parts, including roots, stems, leaves, and seeds
- Some endophytes can promote plant growth by producing plant growth regulators, fixing nitrogen, or solubilizing nutrients
- Endophytes can also protect plants against biotic and abiotic stresses, such as pathogens, insects, drought, and salinity
- Examples of endophytes include Piriformospora indica, a fungus that promotes growth in various crop plants, and Gluconacetobacter diazotrophicus, a nitrogen-fixing bacterium found in sugarcane
Key Terms to Review (25)
Arbuscular mycorrhizal fungi: Arbuscular mycorrhizal fungi (AMF) are a type of beneficial fungi that form symbiotic relationships with the roots of most land plants. These fungi enhance nutrient uptake, particularly phosphorus, for the plant while receiving carbohydrates in return, fostering a mutually beneficial association that improves plant health and soil structure.
Arbuscular mycorrhizal fungi (AMF): Arbuscular mycorrhizal fungi (AMF) are a group of beneficial fungi that form symbiotic relationships with the roots of most terrestrial plants. These fungi enhance nutrient uptake, particularly phosphorus, while the plant provides carbohydrates and other organic compounds to the fungi, creating a mutually beneficial exchange that supports plant health and growth.
Biocontrol agents: Biocontrol agents are organisms or substances derived from living organisms used to control pest populations and diseases in plants. These agents can include natural predators, parasites, or pathogens that help regulate harmful pests, promoting healthier crop growth and reducing the reliance on chemical pesticides. By fostering beneficial plant-microbe associations, biocontrol agents enhance ecosystem balance and sustainability in agricultural practices.
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.
Biogeochemical cycling: Biogeochemical cycling refers to the movement and transformation of essential elements and compounds through biological, geological, and chemical processes within ecosystems. This cycle ensures that vital nutrients such as carbon, nitrogen, and phosphorus are recycled and made available to living organisms, maintaining ecosystem health and productivity.
Ectomycorrhizae: Ectomycorrhizae are a type of symbiotic association between certain fungi and the roots of vascular plants, where the fungal hyphae form a sheath around the root surface and extend into the soil. This relationship enhances mineral uptake, particularly phosphorus and nitrogen, and improves water absorption for the plant while providing the fungus with carbohydrates produced through photosynthesis. Ectomycorrhizae play a crucial role in nutrient transport and beneficial interactions between plants and microorganisms.
Endophyte colonization: Endophyte colonization refers to the process by which endophytic microorganisms, such as fungi or bacteria, invade and establish themselves within the tissues of a plant without causing any harm. This relationship often benefits the plant by enhancing its growth, resistance to pathogens, and ability to tolerate environmental stresses. Understanding how these microorganisms colonize plants is crucial for appreciating their role in beneficial plant-microbe associations.
Endophytes: Endophytes are microorganisms, mainly fungi and bacteria, that live within the tissues of plants without causing harm. These symbiotic relationships can provide various benefits to the host plant, such as enhanced growth, improved resistance to pathogens, and better tolerance to environmental stress. By residing inside plant tissues, endophytes can also influence plant health and productivity in significant ways.
Exudation: Exudation refers to the process by which plants release various substances, including water, nutrients, and organic compounds, from their roots into the surrounding soil. This process plays a crucial role in enhancing plant growth and establishing beneficial interactions with soil microbes, ultimately contributing to the overall health of the ecosystem.
Legume-rhizobium symbiosis: Legume-rhizobium symbiosis is a mutually beneficial relationship between leguminous plants and nitrogen-fixing bacteria known as rhizobia. In this partnership, rhizobia colonize the root nodules of legumes, where they convert atmospheric nitrogen into a form that the plant can use, while the plant supplies the bacteria with carbohydrates and a protective environment. This relationship plays a crucial role in enhancing soil fertility and is an essential part of sustainable agriculture.
Lynn Margulis: Lynn Margulis was an influential American biologist best known for her work on the symbiotic theory of evolution, which emphasizes the role of symbiosis in the development of complex life forms. Her research highlighted how many organelles in eukaryotic cells, such as mitochondria and chloroplasts, originated from once free-living prokaryotic organisms that entered into a mutualistic relationship with their hosts. This connection between her work and beneficial plant-microbe associations is crucial in understanding how plants interact with various microbes for their survival and evolution.
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.
Mycorrhizae: Mycorrhizae are symbiotic associations between fungi and plant roots that enhance nutrient and water uptake for the plant while providing the fungi with carbohydrates produced through photosynthesis. This relationship is crucial for plant health, particularly in nutrient-poor soils, as it increases the surface area for absorption and facilitates the uptake of minerals such as phosphorus and nitrogen. Mycorrhizae play an essential role in the overall health of ecosystems by promoting plant growth and improving soil structure.
Nitrogen fixation: Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into a form that plants can use, typically ammonia (NH₃), through the action of certain bacteria. This process is crucial for plant growth because nitrogen is an essential component of amino acids, proteins, and nucleic acids. By facilitating the conversion of inert atmospheric nitrogen into biologically available forms, nitrogen fixation plays a significant role in both amino acid biosynthesis and the establishment of beneficial associations between plants and microbes.
Nitrogen-fixing bacteria: Nitrogen-fixing bacteria are microorganisms that convert atmospheric nitrogen ($$N_2$$) into ammonia ($$NH_3$$) or related compounds, making nitrogen available to plants. This process is vital for plant growth because most plants cannot utilize atmospheric nitrogen directly, and instead rely on these bacteria to enrich the soil with bioavailable nitrogen compounds.
Nodulation Process: The nodulation process is the formation of specialized structures called nodules on the roots of certain plants, primarily legumes, in response to the symbiotic relationship with nitrogen-fixing bacteria. This interaction enables plants to convert atmospheric nitrogen into a usable form, enhancing soil fertility and supporting plant growth. The process involves complex signaling between the plant and the bacteria, leading to nodule development and functioning.
Nutrient uptake enhancement: Nutrient uptake enhancement refers to the improved ability of plants to absorb essential nutrients from the soil, facilitated through interactions with beneficial microorganisms. This process not only increases the efficiency of nutrient absorption but also promotes plant health and growth, enabling plants to thrive in various environmental conditions. Such enhancements are vital for sustainable agriculture and ecosystem functioning.
Pathogen resistance: Pathogen resistance refers to the ability of plants to defend themselves against harmful microorganisms, such as bacteria, fungi, and viruses, that can cause diseases. This resistance is crucial for plant health and productivity, as it helps minimize the impact of these pathogens through various physical and biochemical mechanisms.
Plant growth-promoting rhizobacteria (PGPR): Plant growth-promoting rhizobacteria (PGPR) are beneficial bacteria that colonize plant roots and enhance plant growth by various mechanisms. They can improve nutrient availability, promote root development, and induce resistance to diseases, making them crucial players in beneficial plant-microbe associations that support agricultural productivity and sustainability.
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.
Root nodulation: Root nodulation is a biological process where certain plants, particularly legumes, form specialized structures called root nodules that house nitrogen-fixing bacteria. This association allows the plants to convert atmospheric nitrogen into a form they can use for growth, enhancing soil fertility and providing essential nutrients to the plant. Root nodules significantly contribute to the beneficial interactions between plants and microbes, promoting sustainable agricultural practices.
Signal Transduction: Signal transduction is the process by which cells convert external signals into a functional response, often involving a series of molecular events that alter cellular activities. This process is crucial for how cells communicate with their environment, respond to hormones, and interact with other organisms, which are vital for growth, development, and adaptation.
Simone de vos: Simone de Vos refers to the interactions and relationships between plants and beneficial microbes, which enhance plant growth, health, and nutrient acquisition. These associations can include mycorrhizal fungi and nitrogen-fixing bacteria, playing a crucial role in plant physiology by improving nutrient uptake and providing protection against pathogens.
Soil health: Soil health refers to the condition of soil and its capacity to function effectively as a living ecosystem that supports plants, animals, and humans. It encompasses various biological, chemical, and physical properties that determine how well soil can sustain plant growth, retain water, and cycle nutrients. Maintaining soil health is crucial for productive agriculture and is linked to promoting beneficial plant-microbe associations and sustainable farming practices.
Symbiosis: Symbiosis is a biological term that refers to the interaction between two different organisms, which can be beneficial, harmful, or neutral. This interaction can significantly influence the growth, survival, and reproduction of the organisms involved. In the context of plants, symbiosis often involves relationships that enhance nutrient uptake or provide protection against pathogens.