Mycorrhizosphere

Last updated April 26, 2019

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.^[1]

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.

A crop is a plant or animal product that can be grown and harvested extensively for profit or subsistence. Crop may refer either to the harvested parts or to the harvest in a more refined state. Most crops are cultivated in agriculture or aquaculture. A crop is usually expanded to include macroscopic fungus, or alga (algaculture).

A woody plant is a plant that produces wood as its structural tissue. Woody plants are usually either trees, shrubs, or lianas. These are usually perennial plants whose stems and larger roots are reinforced with wood produced from secondary xylem. The main stem, larger branches, and roots of these plants are usually covered by a layer of bark. Wood is a structural cellular adaptation that allows woody plants to grow from above ground stems year after year, thus making some woody plants the largest and tallest terrestrial plants.

Organisms involved

The mycorrhizosphere involves a community of microorganisms. There are three divisions of fungi that can form mycorrhizae, the Glomeromycota, Ascomycota, and Basidiomycota. Glomeromycota can form arbuscular mycorrhizae with angiosperms (flowering plants), gymnosperms (seed-producing plants), pteridophytes, mosses, lycopods, and Psilotales. Ascomycota fungi form ericoid mycorrhizas with plants of the order Ericales, and ectomycorrhizas with trees. Basidiomycota fungi form ecto-, orchid, monotropoid, arbutoid, and some ericoid mycorrhizae. Fungal hyphae are thinner than plant roots, which allows them to penetrate areas in the soil with moisture and nutrient that are inaccessible to roots. In some cases, mycorrhizal fungi in the mycorrhizosphere may ward off fungal plant pathogens. For example, the arbuscular mycorrhizal fungus Glomus intraradices reduces germination of the pathogen Fusarium oxysporum , but stimulates germination of a non-pathogenic fungus Trichoderma harzianum .^[1]

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.

Ascomycota division or phylum of the kingdom Fungi that, together with the Basidiomycota, form the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defining feature of this fungal group is the "ascus", a microscopic sexual structure in which nonmotile spores, called ascospores, are formed. However, some species of the Ascomycota are asexual, meaning that they do not have a sexual cycle and thus do not form asci or ascospores. Familiar examples of sac fungi include morels, truffles, brewer's yeast and baker's yeast, dead man's fingers, and cup fungi. The fungal symbionts in the majority of lichens such as Cladonia belong to the Ascomycota.

Basidiomycota is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya within the kingdom Fungi.

Soil bacteria in the mycorrhizosphere influence plant growth in several ways: by affecting the uptake of nutrients, providing protection against pathogens, contributing to nitrogen fixation, and contributing to mineral weathering. Archaea are also known to exist in mycorrhizosphere. They may contribute to nitrogen fixation and produce antibiotic compounds, but the extent of their interactions with other organisms and their overall function is not well known.^[1] Soil protozoa feed on bacteria, and, in some cases, hyphae. The type of mycorrhizae greatly influences the protozoal population in a mycorrhizosphere. For example, a Paxillus involutus –conifer mycorrhiza seems to reduce the density of protozoa in the resulting mycorrhizosphere, but mycorrhizae formed with Lactarius rufus and Suillus bovinus have the opposite effect. Other soil organisms that influence the mycorrhizosphere community include soil animals such as nematodes, mites, and earthworms, which forage on roots, hyphae and their associated microorganisms.^[1]

Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about half of the bacterial phyla have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

Nitrogen fixation is a process by which nitrogen in the air is converted into ammonia (NH₃) or related nitrogenous compounds. Atmospheric nitrogen, is molecular dinitrogen (N₂), a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation converts N₂ into ammonia, which is metabolized by most organisms.

Archaea A domain of single-celled prokaryotic microorganisms

Archaea constitute a domain of single-celled microorganisms. These microbes are prokaryotes, meaning they have no cell nucleus. Archaea were initially classified as bacteria, receiving the name archaebacteria, but this classification is outdated.

Related Research Articles

Mycelium is the vegetative part of a fungus or fungus-like bacterial colony, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates into a homokaryotic mycelium, which cannot reproduce sexually; when two compatible homokaryotic mycelia join and form a dikaryotic mycelium, that mycelium may form fruiting bodies such as mushrooms. A mycelium may be minute, forming a colony that is too small to see, or it may be extensive, as in Armillaria ostoyae:

Is this the largest organism in the world? This 2,400-acre [970-hectare] site in eastern Oregon had a contiguous growth of mycelium before logging roads cut through it. ... Mushroom-forming forest fungi are unique in that their mycelial mats can achieve such massive proportions.

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.

Soil biology is the study of microbial and faunal activity and ecology in soil. Soil life, soil biota, soil fauna, or edaphon is a collective term that encompasses all organisms that spend a significant portion of their life cycle within a soil profile, or at the soil-litter interface. These organisms include earthworms, nematodes, protozoa, fungi, bacteria, different arthropods, as well as some reptiles, and species of burrowing mammals like gophers, moles and prairie dogs. Soil biology plays a vital role in determining many soil characteristics. The decomposition of organic matter by soil organisms has an immense influence on soil fertility, plant growth, soil structure, and carbon storage. As a relatively new science, much remains unknown about soil biology and its effect on soil ecosystems.

Glomus aggregatum is an arbuscular mycorrhizal fungus used as a soil inoculant in agriculture and horticulture. Like other species in this phylum it forms obligate symbioses with plant roots, where it obtains carbon (photosynthate) from the host plant in exchange for nutrients and other benefits.

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.

Nitrogen nutrition in the arbuscular mycorrhizal system refers to...

Soil microbiology is the study of microorganisms in soil, their functions, and how they affect soil properties. It is believed that between two and four billion years ago, the first ancient bacteria and microorganisms came about on Earth's oceans. These bacteria could fix nitrogen, in time multiplied, and as a result released oxygen into the atmosphere. This led to more advanced microorganisms, which are important because they affect soil structure and fertility. Soil microorganisms can be classified as bacteria, actinomycetes, fungi, algae and protozoa. Each of these groups has characteristics that define them and their functions in soil.

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 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"

The root microbiome is the dynamic community of microorganisms associated with plant roots. Because they are rich in a variety of carbon compounds, plant roots provide unique environments for a diverse assemblage of soil microorganisms, including bacteria, fungi and archaea. The microbial communities inside the root and in the rhizosphere are distinct from each other, and from the microbial communities of bulk soil, although there is some overlap in species composition. Root microbiota affect plant host fitness and productivity in a variety of ways. Members of the root microbiome benefit from plant sugars or other carbon rich molecules. Individual members of the root microbiome may behave differently in association with different plant hosts, or may change the nature of their interaction within a single host as environmental conditions or host health change.

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.

Mycorrhizal amelioration of heavy metals or pollutants is a process by which mycorrhizal fungi in a mutualistic relationship with plants can sequester toxic compounds from the environment, as a form of bioremediation.

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.

References

1 2 3 4 Timonen S, Marschner P (2006). "Mycorrhizosphere concept". In Mukerji JG, Manoharachary C, Singh J. Microbial Activity in the Rhizoshere. Soil Biology. 7. Berlin; New York: Springer. pp. 155–72. ISBN 978-3-540-29182-4.

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[Timonen_2006-1] 1 2 3 4 Timonen S, Marschner P (2006). "Mycorrhizosphere concept". In Mukerji JG, Manoharachary C, Singh J. Microbial Activity in the Rhizoshere. Soil Biology. 7. Berlin; New York: Springer. pp. 155–72. ISBN 978-3-540-29182-4.