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Nitrogen nutrition in the arbuscular mycorrhizal system refers to...
An arbuscular mycorrhiza is a type of mycorrhiza in which the symbiont fungus penetrates the cortical cells of the roots of a vascular plant forming arbuscules.
Nitrogen is a vital macronutrient for plants, necessary for the biosynthesis of many basic cellular components, such as DNA, RNA and proteins. Nitrogen is obtained by plants through roots from inorganic or organic sources, such as amino acids. [1] In agricultural settings, nitrogen may be a limiting factor for plant growth and yield, and in total, as a critical cellular component that a plant deficient in this nitrogen will shunt resources away from its shoot in order to expand its root system so that it can acquire more nitrogen. [2] Arbuscular mycorrhizal fungi are divided into two parts depending on where the mycelium is located. The intra-radical mycelia (IRM) are found within the root itself while the extra-radical mycelium (ERM) are tiny hyphal threads which reach far out into the soil. The IRM is the site of nutrient exchange between the symbionts, while the ERM effectively serves as an extension of the plant's root system by increasing the surface area available for nutrient acquisition, including nitrogen, which can be taken up in the form of ammonium, nitrate or from organic sources. [3] [4] Working with an in vitro system, studies have shown that as much as 29% [5] to 50% [6] of the root nitrogen was taken up via the fungus. This is also true in in planta studies, such as an experiment in which the researchers showed that 75% of the nitrogen in a young maize leaf originated from the ERM. [7]
Nitrogen is a chemical element with symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772. Although Carl Wilhelm Scheele and Henry Cavendish had independently done so at about the same time, Rutherford is generally accorded the credit because his work was published first. The name nitrogène was suggested by French chemist Jean-Antoine-Claude Chaptal in 1790, when it was found that nitrogen was present in nitric acid and nitrates. Antoine Lavoisier suggested instead the name azote, from the Greek ἀζωτικός "no life", as it is an asphyxiant gas; this name is instead used in many languages, such as French, Russian, Romanian and Turkish, and appears in the English names of some nitrogen compounds such as hydrazine, azides and azo compounds.
A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excreted by cells to create non-cellular structures, such as hair, scales, feathers, or exoskeletons. Some nutrients can be metabolically converted to smaller molecules in the process of releasing energy, such as for carbohydrates, lipids, proteins, and fermentation products, leading to end-products of water and carbon dioxide. All organisms require water. Essential nutrients for animals are the energy sources, some of the amino acids that are combined to create proteins, a subset of fatty acids, vitamins and certain minerals. Plants require more diverse minerals absorbed through roots, plus carbon dioxide and oxygen absorbed through leaves. Fungi live on dead or living organic matter and meet nutrient needs from their host.
Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined together to form macromolecules. This process often consists of metabolic pathways. Some of these biosynthetic pathways are located within a single cellular organelle, while others involve enzymes that are located within multiple cellular organelles. Examples of these biosynthetic pathways include the production of lipid membrane components and nucleotides. Biosynthesis is usually synonymous with anabolism.
The precise mechanism(s) by which nitrogen is taken up from the soil by the ERM, transported to the IRM, and then turned over to the plant are still under investigation. Toward elucidating the mechanisms through which nitrogen transfer is completed, the sum of numerous studies have provided the necessary tools to study this process. For example, the detection and measurement of gene expression has enabled researchers to determine which genes are up-regulated in the plant and fungus under various nitrogen conditions. Another important tool is the use of the nitrogen isotope [[<sup>15</sup>N]], which can be distinguished from the more common 14N isotope. Nitrogen-containing compounds thus labeled can be tracked and measured as they move through the fungus and into the plant, as well as how they are incorporated into nitrogen-containing molecules.
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as transfer RNA (tRNA) or small nuclear RNA (snRNA) genes, the product is a functional RNA.
In the biological context of organisms' production of gene products, downregulation is the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external stimulus. The complementary process that involves increases of such components is called upregulation.
Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom.
The current model, first put forth in 2005, proposes that the nitrogen taken up by the fungus is converted in the ERM to arginine, which is then transported to the IRM, where it is released as ammonium into the apoplast for the plant to use. [8] A growing body of data has supported and expanded upon this model. Support has been found primarily in two ways: labeling experiments and the study of gene expression, as demonstrated in a 2010 paper by Tian et al. When labeled nitrogen compounds were added to the ERM compartment of an in vitro bsystem, six fungal genes encoding enzymes involved in the incorporation of inorganic nitrogen into glutamine and its subsequent conversion to arginine were rapidly up-regulated. After a delay, gene expression in the IRM began to show increasing levels of mRNA for genes involved in the breakdown of arginine into urea and the subsequent cleaving of ammonium from the urea molecule. This change in gene expression takes place concurrently with the arrival of 15N labeled arginine from the ERM compartment. [9]
Arginine, also known as L-arginine (symbol Arg or R), is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group, an α-carboxylic acid group, and a side chain consisting of a 3-carbon aliphatic straight chain ending in a guanidino group. At physiological pH, the carboxylic acid is deprotonated (−COO−), the amino group is protonated (−NH3+), and the guanidino group is also protonated to give the guanidinium form (-C-(NH2)2+), making arginine a charged, aliphatic amino acid. It is the precursor for the biosynthesis of nitric oxide. It is encoded by the codons CGU, CGC, CGA, CGG, AGA, and AGG.
Inside a plant, the apoplast is the space outside the plasma membrane within which material can diffuse freely. It is interrupted by the Casparian strip in roots, by air spaces between plant cells and by the plant cuticle.
Enzymes are macromolecular biological catalysts. Enzymes accelerate chemical reactions. The molecules upon which enzymes may act are called substrates and the enzyme converts the substrates into different molecules known as products. Almost all metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life. Metabolic pathways depend upon enzymes to catalyze individual steps. The study of enzymes is called enzymology and a new field of pseudoenzyme analysis has recently grown up, recognising that during evolution, some enzymes have lost the ability to carry out biological catalysis, which is often reflected in their amino acid sequences and unusual 'pseudocatalytic' properties.
Once inside the ERM, the nitrogen molecule may have to travel many centimeters to reach the root. While much progress has been made on either end of the transfer of nitrogen, the mechanism by which the arginine actually moves from the ERM to the IRM remains unresolved. AM fungi are non-septate and lack cell walls between cells, forming one long filament. However, passive flow through the continuous cytoplasm is too slow to explain the transport of nutrients. The mechanism by which the newly manufactured arginine is transported to the plant requires further investigation.
A single plant with its associated fungus is not an isolated entity. It has been shown that mycelia from the roots of one plant actually colonize the roots of nearby plants, creating an underground network of plants of the same or different species. It has been demonstrated that nitrogen is transferred between plants via the hyphal network, sometimes in large amounts. For example, Cheng and Baumgartner found that about 25% of the labeled nitrogen supplied to a source plant, in this case a grass species, was transferred to the sink plant, grapevine. [10] It is widely believed that these hyphal networks are important to local ecosystems and may have agricultural implications.
Some plants, called legumes, can form simultaneous symbiotic relationships with both AM fungi and the nitrogen-fixing bacteria Rhizobia. In fact, both organisms trigger the same pathways in plants during early colonization, indicating that the two very different responses could share a common origin. While the bacteria can supply nitrogen, they cannot provide other benefits of AM fungi; AM actually enhances bacterial colonization, probably by supplying extra phosphorus for the formation of the bacterial habitat within the plant, and thus contributing indirectly to the plant's nitrogen status. [11] It is not known if there is signaling between the two, or only between the plant and each microbe. There is almost certainly competition between the bacterial and fungal partners, whether directly or indirectly, due to the fact that both are dependent on the plant as their sole source of energy. The plant must strive to strike a delicate balance between the maintenance of both partners based on its nutrient status.
A large body of research has shown that AM fungi can, and do, transfer nitrogen to plants and transfer nitrogen between plants, including crop plants. However, it has not been shown conclusively that there is a growth benefit from AM due to nitrogen. Some researchers doubt that AM contribute significantly to plant N status in nature. [12] In one field study, there was negligible transfer between soybeans and corn. [13] Furthermore, AM sometimes appears to be parasitic. This has primarily been seen under conditions of high nitrogen, which is not the usual state in a natural environment. However, it has been shown that in at least one case, colonization by AM fungi under nitrogen-limiting conditions lead to decreased shoot biomass, [14] implying that the relationship does the plant more harm than good. Likewise in a multi-plant system it would be very difficult to find the advantage to the source plants when their nutrients are being shunted to sink plants. These findings are at odds with the observed phenomenon that under conditions of low phosphorus, the degree of AM colonization is inversely proportional to nitrogen availability. [15] Since the plant must supply all of the energy needed to grow and sustain the fungus, it seems counter-intuitive that it would do so without some benefit to itself. Further studies are definitely needed to delineate the details of the relationship between the symbionts, including a gradient of interaction that runs from mutualism to parasitism.
A mycorrhiza is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, its root system. Mycorrhizae play important roles in plant nutrition, soil biology and soil chemistry.
Medicago truncatula, the barrelclover, strong-spined medick, barrel medic, or barrel medick, is a small annual legume native to the Mediterranean region that is used in genomic research. It is a low-growing, clover-like plant 10–60 centimetres (3.9–23.6 in) tall with trifoliate leaves. Each leaflet is rounded, 1–2 centimetres (0.39–0.79 in) long, often with a dark spot in the center. The flowers are yellow, produced singly or in a small inflorescence of two to five together; the fruit is a small, spiny pod.
Nod factors, are signaling molecules produced by soil bacteria known as rhizobia during the initiation of nodules on the root of legumes. A symbiosis is formed when legumes take up the bacteria. The rhizobia produce nitrogen for the plant, and the legumes produce leghemoglobin to carry away any oxygen that would inhibit nitrogenase activity.
Primary succession is one of two types of biological and ecological succession of plant life, occurring in an environment in which new substrate devoid of vegetation and other organisms usually lacking soil, such as a lava flow or area left from retreated glacier, is deposited. In other words, it is the gradual growth of an ecosystem over a longer period of time.
Glomeromycota are one of eight currently recognized divisions within the kingdom Fungi, with approximately 230 described species. Members of the Glomeromycota form arbuscular mycorrhizas (AMs) with the thalli of bryophytes and the roots of vascular land plants. Not all species have been shown to form AMs, and one, Geosiphon pyriformis, is known not to do so. Instead, it forms an endocytobiotic association with Nostoc cyanobacteria. The majority of evidence shows that the Glomeromycota are dependent on land plants for carbon and energy, but there is recent circumstantial evidence that some species may be able to lead an independent existence. The arbuscular mycorrhizal species are terrestrial and widely distributed in soils worldwide where they form symbioses with the roots of the majority of plant species (>80%). They can also be found in wetlands, including salt-marshes, and associated with epiphytic plants.
Glomus is a genus of arbuscular mycorrhizal (AM) fungi, and all species form symbiotic relationships (mycorrhizas) with plant roots. Glomus is the largest genus of AM fungi, with ca. 85 species described, but is currently defined as non-monophyletic.
The ericoid mycorrhiza is a mutualistic relationship formed between members of the plant family Ericaceae and several lineages of mycorrhizal fungi. This symbiosis represents an important adaptation to acidic and nutrient poor soils that species in the Ericaceae typically inhabit, including boreal forests, bogs, and heathlands. Molecular clock estimates suggest that the symbiosis originated approximately 140 million years ago.
Microbial inoculants also known as soil inoculants are agricultural amendments that use beneficial endophytes (microbes) to promote plant health. Many of the microbes involved form symbiotic relationships with the target crops where both parties benefit (mutualism). While microbial inoculants are applied to improve plant nutrition, they can also be used to promote plant growth by stimulating plant hormone production.
The mycorrhizosphere is the region around a mycorrhizal fungus in which nutrients released from the fungus increase the microbial population and its activities. The roots of most terrestrial plants, including most crop plants and almost all woody plants, are colonized by mycorrhiza-forming symbiotic fungi. In this relationship, the plant roots are infected by a fungus, but the rest of the fungal mycelium continues to grow through the soil, digesting and absorbing nutrients and water and sharing these with its plant host. The fungus in turn benefits by receiving photosynthetic sugars from its host. The mycorrhizosphere consists of roots, hyphae of the directly connected mycorrhizal fungi, associated microorganisms, and the soil in their direct influence.
Mycorrhizal networks are underground hyphal networks created by mycorrhizal fungi that connect individual plants together and transfer water, carbon, nitrogen, and other nutrients and minerals. The formation of these networks is context dependent, and can be influenced by factors such as soil fertility, resource availability, host or myco-symbiont genotype, disturbance and seasonal variation. By analogy to the many roles intermediated by the World Wide Web in human communities, the many roles that mycorrhizal networks appear to play in woodland have earned them a colloquial nickname: the Wood Wide Web.. Wood wide web can be described as plants’ way of talking. Plants communicate through the mycorrhizal network in a similar manner to thoughts travelling over the brain’s network of neurons.
Soil carbon storage is an important function of terrestrial ecosystems. Soil contains more carbon than plants and the atmosphere combined. Understanding what maintains the soil carbon pool is important to understand the current distribution of carbon on Earth, and how it will respond to environmental change. While much research has been done on how plants, free-living microbial decomposers, and soil minerals affect this pool of carbon, it is recently coming to light that mycorrhizal fungi—symbiotic fungi that associate with roots of almost all living plants—may play an important role in maintaining this pool as well. Measurements of plant carbon allocation to mycorrhizal fungi have been estimated to be 5-20% of total plant carbon uptake, and in some ecosystems the biomass of mycorrhizal fungi can be comparable to the biomass of fine roots. Recent research has shown that mycorrhizal fungi hold 50 to 70 percent of the total carbon stored in leaf litter and soil on forested islands in Sweden. Turnover of mycorrhizal biomass into the soil carbon pool is thought to be rapid and has been shown in some ecosystems to be the dominant pathway by which living carbon enters the soil carbon pool.
An ectomycorrhiza is a form of symbiotic relationship that occurs between a fungal symbiont and the roots of various plant species. The mycobiont tends to be predominantly from the phyla Basidiomycota and Ascomycota, although a few are represented in the phylum Zygomycota. Ectomycorrhizas form between fungi and the roots of around 2% of plant species. These tend to be composed of woody plants, including species from the birch, dipterocarp, myrtle, beech, willow, pine and rose families.
Rhizophagus is a genus of arbuscular mycorrhizal (AM) fungi that form symbiotic relationships (mycorrhizas) with plant roots. The genome of Rhizophagus irregularis was recently sequenced.
Rhizophagus irregularis is an arbuscular mycorrhizal fungus used as a soil inoculant in agriculture and horticulture. In addition, it is one of the best mycorrhizal varieties of fungi available to mycoforestry, but as it does not produce fruiting bodies it "has virtually no market value as an edible or medicinal mushroom"
Dark septate endophytes (DSE) are a group of endophytic fungi characterized by their morphology of melanized, septate, hyphae. This group is likely paraphyletic, and contain conidial as well as sterile fungi that colonize roots intracellularly or intercellularly. Very little is known about the number of fungal taxa within this group, but all are in the Ascomycota. They are found in over 600 plant species and across 114 families of angiosperms and gymnosperms and co-occur with other types of mycorrhizal fungi. They have a wide global distribution and can be more abundant in stressed environments. Much of their taxonomy, physiology, and ecology are unknown.
Orchid mycorrhizae are symbiotic relationships between the roots of plants of the family Orchidaceae and a variety of fungi. All orchids are myco-heterotrophic at some point in their life cycle. Orchid mycorrhizae are critically important during orchid germination, as an orchid seed has virtually no energy reserve and obtains its carbon from the fungal symbiont.
Mycorrhiza helper bacteria (MHB) are a group of organisms that form symbiotic associations with both ectomycorrhiza and arbuscular mycorrhiza. MHBs are diverse and belong to different bacterial phyla including both gram-negative and gram-positive bacteria. Some of the most common types are Pseudomonas and Streptomyces. MHBs have specific interactions with fungi, but not with the plants. MHB enhance mycorrhizal function, increase mycorrhizal growth, provide nutrients to the fungus and plant, improve soil conductance, select to aid pathogens, and help promote defense mechanisms.
Plant to plant communication via mycorrhizal networks refers to connections through mycorrhizal networks that facilitate communication between plants of the same or different species. Mycorrhizal networks allow for the transfers of signals and cues between plants which influence the behavior of the connected plants by inducing morphological or physiological changes. The chemical substances which act as these signals and cues are referred to as infochemicals. These can be allelochemicals, defensive chemicals or nutrients. Allelochemicals are used by plants to interfere with the growth or development of other plants or organisms, defensive chemicals can help plants in mycorrhizal networks defend themselves against attack by pathogens or herbivores, and transferred nutrients can affect growth and nutrition. Results of studies which demonstrate these modes of communication have led the authors to hypothesize mechanisms by which the transfer of these nutrients can affect the fitness of the connected plants.
Mycorrhizae and changing climate refers to the effects of changing climates on mycorrhizae, the symbiotic association between a fungus and the roots of a vascular host plant.