A mycorrhiza (from
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a
symbiotic association between a
fungus
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...
and a
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
. The term mycorrhiza refers to the role of the fungus in the plant's
rhizosphere
The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microo ...
, its
root
In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...
system. Mycorrhizae play important roles in
plant nutrition
Plant nutrition is the study of the chemical elements and compounds necessary for plant growth and reproduction, plant metabolism and their external supply. In its absence the plant is unable to complete a normal life cycle, or that the element i ...
,
soil biology
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 ...
, and
soil chemistry
Soil chemistry is the study of the chemical characteristics of soil. Soil chemistry is affected by mineral composition, organic matter and environmental factors. In the early 1850s a consulting chemist to the Royal Agricultural Society in England ...
.
In a mycorrhizal association, the fungus colonizes the host plant's root tissues, either
intracellular
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
ly as in
arbuscular mycorrhizal fungi
An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'', a.k.a. ''endomycorrhiza'') is a type of mycorrhiza in which the symbiont fungus (''AM fungi'', or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. ...
(AMF or AM), or
extracellular
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
ly as in
ectomycorrhizal fungi. The association is sometimes
mutualistic. In particular species or in particular circumstances, mycorrhizae may have a
parasitic
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
association with host plants.
Definition
A mycorrhiza is a symbiotic association between a green plant and a fungus. The plant makes organic molecules such as sugars by
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
and supplies them to the fungus, and the fungus supplies to the plant water and mineral nutrients, such as
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
, taken from the soil. Mycorrhizas are located in the roots of vascular plants, but mycorrhiza-like associations also occur in
bryophytes and there is fossil evidence that early land plants that lacked roots formed arbuscular mycorrhizal associations.
Most plant species form mycorrhizal associations, though some families like
Brassicaceae
Brassicaceae () or (the older) Cruciferae () is a medium-sized and economically important family of flowering plants commonly known as the mustards, the crucifers, or the cabbage family. Most are herbaceous plants, while some are shrubs. The leav ...
and
Chenopodiaceae
Amaranthaceae is a family of flowering plants commonly known as the amaranth family, in reference to its type genus '' Amaranthus''. It includes the former goosefoot family Chenopodiaceae and contains about 165 genera and 2,040 species, making i ...
cannot. Different forms for the association are detailed in the next section. The most common is the
arbuscular type that is present in 70% of plant species, including many crop plants such as wheat and rice.
Evolution
Fossil and genetic evidence indicate that mycorrhizae are ancient, potentially as old as the
terrestrialization of plants. Genetic evidence indicates that all land plants share a single common ancestor, which appears to have quickly adopted mycorrhizal symbiosis, and research suggests that proto-mycorrhizal fungi were a key factor enabling plant terrestrialization. The 400 million year old
Rhynie chert
The Rhynie chert is a Lower Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness (a Lagerstätte). It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is locate ...
contains an assemblage of fossil plants preserved in sufficient detail that arbuscular mycorrhizae have been observed in the stems of
''Aglaophyton major'', giving a lower bound for how late mycorrhizal symbiosis may have developed.
Ectomycorrhizae developed substantially later, during the
Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
period, while most other modern mycorrhizal families, including orchid and erchoid mycorrhizae, date to the period of
angiosperm radiation in the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
period. There is genetic evidence that the symbiosis between
legumes
A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock fo ...
and
nitrogen-fixing bacteria
Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...
is an extension of mycorrhizal symbiosis. The modern distribution of mycorrhizal fungi appears to reflect an increasing complexity and competition in root morphology associated with the dominance of angiosperms in the
Cenozoic Era
The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...
, characterized by complex ecological dynamics between species.
Types
Mycorrhizas are commonly divided into ''ectomycorrhizas'' and ''endomycorrhizas''. The two types are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate individual
cells within the root, while the
hyphae of endomycorrhizal fungi penetrate the cell wall and
invaginate
Invagination is the process of a surface folding in on itself to form a cavity, pouch or tube. In developmental biology, invagination is a mechanism that takes place during gastrulation. This mechanism or cell movement happens mostly in the vegeta ...
the
cell membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
.
Endomycorrhiza includes ''arbuscular'', ''ericoid'', and ''orchid mycorrhiza'', while ''arbutoid mycorrhizas'' can be classified as ''ectoendomycorrhizas''. ''Monotropoid'' mycorrhizas form a special category.
Ectomycorrhiza
![Raudonvirsis1-vi](https://upload.wikimedia.org/wikipedia/commons/8/8e/Raudonvirsis1-vi.jpg)
Ectomycorrhizas, or EcM, are symbiotic associations between the roots of around 10% of plant families, mostly woody plants including the
birch
A birch is a thin-leaved deciduous hardwood tree of the genus ''Betula'' (), in the family Betulaceae, which also includes alders, hazels, and hornbeams. It is closely related to the beech- oak family Fagaceae. The genus ''Betula'' contains ...
,
dipterocarp
Dipterocarpaceae is a family of 16 genera and about 695 known species of mainly tropical lowland rainforest trees. The family name, from the type genus ''Dipterocarpus'', is derived from Greek (''di'' = two, ''pteron'' = wing and ''karpos'' = f ...
,
eucalyptus
''Eucalyptus'' () is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with several other genera in the tribe Eucalypteae, including '' Corymbia'', they are commonly known as e ...
,
oak
An oak is a tree or shrub in the genus ''Quercus'' (; Latin "oak tree") of the beech family, Fagaceae. There are approximately 500 extant species of oaks. The common name "oak" also appears in the names of species in related genera, notably ''L ...
,
pine
A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanical Garden accep ...
, and
rose
A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...
[ families, ]orchids
Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant.
Along with the Asteraceae, they are one of the two largest families of flowering ...
, and fungi belonging to the Basidiomycota, Ascomycota, and Zygomycota
Zygomycota, or zygote fungi, is a former division or phylum of the kingdom Fungi. The members are now part of two phyla: the Mucoromycota and Zoopagomycota. Approximately 1060 species are known. They are mostly terrestrial in habitat, living ...
. Some EcM fungi, such as many ''Leccinum
''Leccinum'' is a genus of fungi in the family Boletaceae. It was the name given first to a series of fungi within the genus ''Boletus'', then erected as a new genus last century. Their main distinguishing feature is the small, rigid projections ...
'' and ''Suillus
''Suillus'' is a genus of basidiomycete fungi in the family Suillaceae and order Boletales. Species in the genus are associated with trees in the pine family (Pinaceae), and are mostly distributed in temperate locations in the Northern Hemisphe ...
'', are symbiotic with only one particular genus of plant, while other fungi, such as the ''Amanita
The genus ''Amanita'' contains about 600 species of agarics, including some of the most toxic known mushrooms found worldwide, as well as some well-regarded edible species. This genus is responsible for approximately 95% of the fatalities resul ...
'', are generalists that form mycorrhizas with many different plants.[ An individual tree may have 15 or more different fungal EcM partners at one time.][ Thousands of ectomycorrhizal fungal species exist, hosted in over 200 genera. A recent study has conservatively estimated global ectomycorrhizal fungal species richness at approximately 7750 species, although, on the basis of estimates of knowns and unknowns in macromycete diversity, a final estimate of ECM species richness would probably be between 20,000 and 25,000.
Ectomycorrhizas consist of a hyphal sheath, or mantle, covering the root tip and a ]Hartig net
Hartig net is a network of inward-growing hyphae, that extends into the root, penetrating between the epidermis and cortex of ectomycorrhizal plants. This network is a site of nutrient exchange between the fungus and the host plant. The Hartig net ...
of hyphae surrounding the plant cells within the root cortex
Cortex or cortical may refer to:
Biology
* Cortex (anatomy), the outermost layer of an organ
** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain''
*** Motor cortex, the regions of the cerebral cortex i ...
. In some cases the hyphae may also penetrate the plant cells, in which case the mycorrhiza is called an ectendomycorrhiza. Outside the root, ectomycorrhizal extramatrical mycelium
Ectomycorrhizal extramatrical mycelium (also known as extraradical mycelium) is the collection of filamentous fungal hyphae emanating from ectomycorrhizas. It may be composed of fine, hydrophilic hypha which branches frequently to explore and expl ...
forms an extensive network within the soil and leaf litter.
Nutrients can be shown to move between different plants through the fungal network. Carbon has been shown to move from paper birch
''Betula papyrifera'' (paper birch, also known as (American) white birch and canoe birch) is a short-lived species of birch native to northern North America. Paper birch is named for the tree's thin white bark, which often peels in paper like ...
trees into Douglas-fir
The Douglas fir (''Pseudotsuga menziesii'') is an evergreen conifer species in the pine family, Pinaceae. It is native to western North America and is also known as Douglas-fir, Douglas spruce, Oregon pine, and Columbian pine. There are three va ...
trees thereby promoting succession in ecosystem
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
s. The ectomycorrhizal fungus ''Laccaria bicolor
''Laccaria bicolor'' is a small tan-colored mushroom with lilac gills. It is edible but not choice, and grows in mixed birch and pine woods. It is found in the temperate zones of the globe, in late summer and autumn. ''L. bicolor'' is an e ...
'' has been found to lure and kill springtail
Springtails (Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects (the other two are the Protura and Diplura). Although the three orders are sometimes grouped together in a class called Ento ...
s to obtain nitrogen, some of which may then be transferred to the mycorrhizal host plant. In a study by Klironomos and Hart, Eastern White Pine inoculated with ''L. bicolor'' was able to derive up to 25% of its nitrogen from springtails. When compared with non-mycorrhizal fine roots, ectomycorrhizae may contain very high concentrations of trace elements, including toxic metals (cadmium, silver) or chlorine.
The first genomic sequence for a representative of symbiotic fungi, the ectomycorrhizal basidiomycete ''L. bicolor'', was published in 2008. An expansion of several multigene families occurred in this fungus, suggesting that adaptation to symbiosis proceeded by gene duplication. Within lineage-specific genes those coding for symbiosis-regulated secreted proteins showed an up-regulated expression in ectomycorrhizal root tips suggesting a role in the partner communication. ''L. bicolor'' is lacking enzymes involved in the degradation of plant cell wall components (cellulose, hemicellulose, pectins and pectates), preventing the symbiont from degrading host cells during the root colonisation. By contrast, ''L. bicolor'' possesses expanded multigene families associated with hydrolysis of bacterial and microfauna polysaccharides and proteins. This genome analysis revealed the dual saprotrophic
Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ( ...
and biotrophic
Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...
lifestyle of the mycorrhizal fungus that enables it to grow within both soil and living plant roots.
Arbutoid mycorrhiza
This type of mycorrhiza involves plants of the Ericaceae subfamily Arbutoideae
The Arbutoideae are a subfamily in the plant family Ericaceae. Phylogenetic analysis supported all genera of the subfamily as monophyletic, except ''Arbutus''. Moreover, it was suggested that the non-sister relationship between Mediterranean and N ...
. It is however different from ericoid mycorrhiza and resembles ectomycorrhiza, both functionally and in terms of the fungi involved. It differs from ectomycorrhiza in that some hyphae actually penetrate into the root cells, making this type of mycorrhiza an ''ectendomycorrhiza''.
Endomycorrhiza
Endomycorrhizas are variable and have been further classified as arbuscular, ericoid, arbutoid, monotropoid, and orchid mycorrhizas.
Arbuscular mycorrhiza
Arbuscular mycorrhiza
An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'', a.k.a. ''endomycorrhiza'') is a type of mycorrhiza in which the symbiont fungus (''AM fungi'', or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. ( ...
s, or AM (formerly known as vesicular-arbuscular mycorrhizas, or VAM), are mycorrhizas whose hyphae penetrate plant cells, producing structures that are either balloon-like (vesicles) or dichotomously branching invaginations (arbuscules) as a means of nutrient exchange. The fungal hyphae do not in fact penetrate the protoplast (i.e. the interior of the cell), but invaginate the cell membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
. The structure of the arbuscules greatly increases the contact surface area between the hypha and the cell cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
to facilitate the transfer of nutrients between them.
Arbuscular mycorrhizas are formed only by fungi in the division
Division or divider may refer to:
Mathematics
*Division (mathematics), the inverse of multiplication
*Division algorithm, a method for computing the result of mathematical division
Military
*Division (military), a formation typically consisting ...
Glomeromycota. Fossil evidence and DNA sequence analysis suggest that this mutualism appeared 400–460 million years ago, when the first plants were colonizing land. Arbuscular mycorrhizas are found in 85% of all plant families, and occur in many crop species.[ The hyphae of arbuscular mycorrhizal fungi produce the glycoprotein ]glomalin
Glomalin is a glycoprotein produced abundantly on hyphae and spores of arbuscular mycorrhizal (AM) fungi in soil and in roots. Glomalin was discovered in 1996 by Sara F. Wright, a scientist at the USDA Agricultural Research Service. The name com ...
, which may be one of the major stores of carbon in the soil. Arbuscular mycorrhizal fungi have (possibly) been asexual for many millions of years and, unusually, individuals can contain many genetically different nuclei (a phenomenon called heterokaryosis
A heterokaryon is a multinucleate cell that contains genetically different nuclei. Heterokaryotic and heterokaryosis are derived terms. This is a special type of syncytium. This can occur naturally, such as in the mycelium of fungi during sexual ...
).
Ericoid mycorrhiza
Ericoid mycorrhiza
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 specie ...
s are the third of the three more ecologically important types. They have a simple intraradical (growth in cells) phase, consisting of dense coils of hyphae in the outermost layer of root cells. There is no periradical phase and the extraradical phase consists of sparse hyphae that don't extend very far into the surrounding soil. They might form sporocarps (probably in the form of small cups), but their reproductive biology is poorly understood.
Ericoid mycorrhizas have also been shown to have considerable saprotrophic
Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ( ...
capabilities, which would enable plants to receive nutrients from not-yet-decomposed materials via the decomposing actions of their ericoid partners.
Orchid mycorrhiza
All orchids
Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant.
Along with the Asteraceae, they are one of the two largest families of flowering ...
are myco-heterotrophic
Myco-heterotrophy (from Greek μύκης , "fungus", ἕτερος ', "another", "different" and τροφή ', "nutrition") is a symbiotic relationship between certain kinds of plants and fungi, in which the plant gets all or part of its food fro ...
at some stage during their lifecycle and form orchid mycorrhiza Orchid mycorrhizae are endomycorrhizal fungi which develop symbiotic relationships with the roots and seeds of plants of the family Orchidaceae. Nearly all orchids are myco-heterotrophic at some point in their life cycle. Orchid mycorrhizae are cr ...
s with a range of basidiomycete fungi. Their hyphae penetrate into the root cells and form pelotons (coils) for nutrient exchange.
Monotropoid mycorrhiza
This type of mycorrhiza occurs in the subfamily Monotropoideae
Monotropoideae, sometimes referred to as monotropes, are a flowering plant subfamily in the family Ericaceae. Members of this subfamily are notable for their mycoheterotrophic and non-photosynthesizing or achlorophyllous characteristics.
Descrip ...
of the Ericaceae
The Ericaceae are a family of flowering plants, commonly known as the heath or heather family, found most commonly in acidic and infertile growing conditions. The family is large, with c.4250 known species spread across 124 genera, making it th ...
, as well as several genera in the Orchidaceae
Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant.
Along with the Asteraceae, they are one of the two largest families of flowering ...
. These plants are heterotrophic
A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
or mixotrophic A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comp ...
and derive their carbon from the fungus partner. This is thus a non-mutualistic, parasitic
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
type of mycorrhizal symbiosis.
Mutualist dynamics
Mycorrhizal fungi form a mutualistic relationship with the roots of most plant species. In such a relationship, both the plants themselves and those parts of the roots that host the fungi, are said to be mycorrhizal. Relatively few of the mycorrhizal relationships between plant species and fungi have been examined to date, but 95% of the plant families investigated are predominantly mycorrhizal either in the sense that most of their species associate beneficially with mycorrhizae, or are absolutely dependent on mycorrhizae. The Orchidaceae
Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant.
Along with the Asteraceae, they are one of the two largest families of flowering ...
are notorious as a family in which the absence of the correct mycorrhizae is fatal even to germinating seeds.
Recent research into ectomycorrhizal plants in boreal forests
Taiga (; rus, тайга́, p=tɐjˈɡa; relates to Mongolic and Turkic languages), generally referred to in North America as a boreal forest or snow forest, is a biome characterized by coniferous forests consisting mostly of pines, spruces, ...
has indicated that mycorrhizal fungi and plants have a relationship that may be more complex than simply mutualistic. This relationship was noted when mycorrhizal fungi were unexpectedly found to be hoarding nitrogen from plant roots in times of nitrogen scarcity. Researchers argue that some mycorrhizae distribute nutrients based upon the environment with surrounding plants and other mycorrhizae. They go on to explain how this updated model could explain why mycorrhizae do not alleviate plant nitrogen limitation, and why plants can switch abruptly from a mixed strategy with both mycorrhizal and nonmycorrhizal roots to a purely mycorrhizal strategy as soil nitrogen availability declines. It has also been suggested that evolutionary and phylogenetic relationships can explain much more variation in the strength of mycorrhizal mutualisms than ecological factors.
Sugar-water/mineral exchange
The mycorrhizal mutualistic association provides the fungus with relatively constant and direct access to carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...
s, such as glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
and sucrose. The carbohydrates are translocated from their source (usually leaves) to root tissue and on to the plant's fungal partners. In return, the plant gains the benefits of the mycelium
Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates ...
's higher absorptive capacity for water and mineral nutrients, partly because of the large surface area of fungal hyphae, which are much longer and finer than plant root hair
Root hair, or absorbent hairs, are outgrowths of epidermal cells, specialized cells at the tip of a plant root. They are lateral extensions of a single cell and are only rarely branched. They are found in the region of maturation, of the root. Root ...
s, and partly because some such fungi can mobilize soil minerals unavailable to the plants' roots. The effect is thus to improve the plant's mineral absorption capabilities.
Unaided plant roots may be unable to take up nutrients that are chemically or physically immobilised; examples include phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phospho ...
ions
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
and micronutrients such as iron. One form of such immobilization occurs in soil with high clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
content, or soils with a strongly basic pH. The mycelium
Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates ...
of the mycorrhizal fungus can, however, access many such nutrient sources, and make them available to the plants they colonize. Thus, many plants are able to obtain phosphate, without using soil as a source. Another form of immobilisation is when nutrients are locked up in organic matter that is slow to decay, such as wood, and some mycorrhizal fungi act directly as decay organisms, mobilising the nutrients and passing some onto the host plants; for example, in some dystrophic
Dystrophic lakes, also known as humic lakes, are lakes that contain high amounts of humic substances and organic acids. The presence of these substances causes the water to be brown in colour and have a generally low pH of around 4.0-6.0. Due to ...
forests, large amounts of phosphate and other nutrients are taken up by mycorrhizal hyphae acting directly on leaf litter, bypassing the need for soil uptake. ''Inga alley cropping Inga alley cropping is the planting agricultural crops between rows of Inga trees. It has been promoted by Mike Hands.
Using the Inga tree for alley cropping has been proposed as an alternative to the much more ecologically destructive slash an ...
'', proposed as an alternative to slash and burn
Slash-and-burn agriculture is a farming method that involves the cutting and burning of plants in a forest or woodland to create a field called a swidden. The method begins by cutting down the trees and woody plants in an area. The downed veget ...
rainforest destruction,[Elkan, Daniel. ''Slash-and-burn farming has become a major threat to the world's rainforest'' ]The Guardian
''The Guardian'' is a British daily newspaper. It was founded in 1821 as ''The Manchester Guardian'', and changed its name in 1959. Along with its sister papers ''The Observer'' and ''The Guardian Weekly'', ''The Guardian'' is part of the Gu ...
21 April 2004 relies upon mycorrhiza within the root system of species of ''Inga
''Inga'' is a genus of small tropical, tough-leaved, nitrogen-fixing treesElkan, Daniel. "Slash-and-burn farming has become a major threat to the world's rainforest" ''The Guardian'' 21 April 2004 and shrubs, subfamily Mimosoideae. ''Inga''s ...
'' to prevent the rain from washing phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
out of the soil.
In some more complex relationships, mycorrhizal fungi do not just collect immobilised soil nutrients, but connect individual plants together by mycorrhizal networks
A Mycorrhizal network (also known as a common mycorrhizal network or CMN) is an underground network found in forests and other plant communities, created by the hyphae of mycorrhizal fungi joining with plant roots. This network connects individu ...
that transport water, carbon, and other nutrients directly from plant to plant through underground hyphal networks.
''Suillus tomentosus
''Suillus tomentosus'' is a species of mushroom. The common names of the species are blue-staining slippery Jack, poor man's slippery Jack, and woolly-capped suillus.
Description
The cap is wide, pale to orange-yellow with grayish brownish or r ...
'', a basidiomycete
Basidiomycota () is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. More specifically, Ba ...
fungus, produces specialized structures known as tuberculate ectomycorrhizae with its plant host lodgepole pine
''Pinus contorta'', with the common names lodgepole pine and shore pine, and also known as twisted pine, and contorta pine, is a common tree in western North America. It is common near the ocean shore and in dry montane forests to the subalpine, ...
(''Pinus contorta'' var. ''latifolia''). These structures have been shown to host nitrogen fixing
Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
which contribute a significant amount of nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and allow the pines to colonize nutrient-poor sites.[
]
Mechanisms
The mechanisms by which mycorrhizae increase absorption include some that are physical and some that are chemical. Physically, most mycorrhizal mycelia are much smaller in diameter than the smallest root or root hair, and thus can explore soil material that roots and root hairs cannot reach, and provide a larger surface area for absorption. Chemically, the cell membrane chemistry of fungi differs from that of plants. For example, they may secrete organic acids that dissolve or chelate
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...
many ions, or release them from minerals by ion exchange
Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...
. Mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils.
Disease, drought and salinity resistance and its correlation to mycorrhizae
Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
s. These associations have been found to assist in plant defense both above and belowground. Mycorrhizas have been found to excrete enzymes that are toxic to soil borne organisms such as nematodes. More recent studies have shown that mycorrhizal associations result in a priming effect of plants that essentially acts as a primary immune response. When this association is formed a defense response is activated similarly to the response that occurs when the plant is under attack. As a result of this inoculation, defense responses are stronger in plants with mycorrhizal associations.
AMF was also significantly correlated with soil biological fertility variables such as soil microbial communities and associated disease suppressiveness. Thus, ecosystem services
Ecosystem services are the many and varied benefits to humans provided by the natural environment and healthy ecosystems. Such ecosystems include, for example, agroecosystems, forest ecosystem, grassland ecosystems, and aquatic ecosystems. ...
provided by AMF may depend on the soil microbiome. Furthermore, AMF was significantly correlated with soil physical variable, but only with water level and not with aggregate stability. and are also more resistant to the effects of drought.
The significance of arbuscular mycorrhizal fungi includes alleviation of salt stress and its beneficial effects on plant growth and productivity. Although salinity can negatively affect arbuscular mycorrhizal fungi, many reports show improved growth and performance of mycorrhizal plants under salt stress conditions.
Resistance to insects
Research has shown that plants connected by mycorrhizal fungi can use these underground connections to produce and receive warning signals. Specifically, when a host plant is attacked by an aphid
Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A t ...
, the plant signals surrounding connected plants of its condition. The host plant releases volatile organic compound
Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...
s (VOCs) that attract the insect's predators. The plants connected by mycorrhizal fungi are also prompted to produce identical VOCs that protect the uninfected plants from being targeted by the insect. Additionally, this assists the mycorrhizal fungi by preventing the plant's carbon relocation which negatively affects the fungi's growth and occurs when the plant is attacked by herbivores.
Colonization of barren soil
Plants grown in sterile soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
s and growth media often perform poorly without the addition of spore
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
s or hyphae of mycorrhizal fungi to colonise the plant roots and aid in the uptake of soil mineral nutrients. The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes. The introduction of alien mycorrhizal plants to nutrient-deficient ecosystems puts indigenous non-mycorrhizal plants at a competitive disadvantage. This aptitude to colonize barren soil is defined by the category Oligotroph
An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates o ...
.
Resistance to toxicity
Fungi have been found to have a protective role for plants rooted in soils with high metal concentrations, such as acidic
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...
and contaminated soils. Pine
A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanical Garden accep ...
trees inoculated with ''Pisolithus tinctorius
''Pisolithus arhizus'' is a widespread Sclerodermataceae, earth-ball like fungus, which may in fact be several closely related species. Common names include dead man's foot and dyeball. It is known in Australia as the horse dung fungus, in Sout ...
'' planted in several contaminated sites displayed high tolerance to the prevailing contaminant, survivorship and growth.[ One study discovered the existence of '']Suillus luteus
''Suillus luteus'' is a bolete fungus, and the type species of the genus ''Suillus''. A common fungus native all across Eurasia from Ireland to Korea, it has been introduced widely elsewhere, including North and South America, southern Africa ...
'' strains with varying tolerance of zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
. Another study discovered that zinc-tolerant strains of ''Suillus bovinus
''Suillus bovinus'', also known as the Jersey cow mushroom or bovine bolete, is a pored mushroom of the genus '' Suillus'' in the family Suillaceae. A common fungus native to Europe and Asia, it has been introduced to North America and Aust ...
'' conferred resistance to plants of ''Pinus sylvestris
''Pinus sylvestris'', the Scots pine (UK), Scotch pine (US) or Baltic pine, is a species of tree in the pine family Pinaceae that is native to Eurasia. It can readily be identified by its combination of fairly short, blue-green leaves and or ...
''. This was probably due to binding of the metal to the extramatricial mycelium
Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates ...
of the fungus, without affecting the exchange of beneficial substances.[
]
Climate change
Occurrence of mycorrhizal associations
Mycorrhizas are present in 92% of plant families studied (80% of species), with arbuscular mycorrhiza
An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'', a.k.a. ''endomycorrhiza'') is a type of mycorrhiza in which the symbiont fungus (''AM fungi'', or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. ( ...
s being the ancestral and predominant form, and the most prevalent symbiotic association found in the plant kingdom. The structure of arbuscular mycorrhizas has been highly conserved since their first appearance in the fossil record, with both the development of ectomycorrhizas, and the loss of mycorrhizas, evolving convergently on multiple occasions.
Discovery
Associations of fungi with the roots of plants have been known since at least the mid-19th century. However early observers simply recorded the fact without investigating the relationships between the two organisms. This symbiosis was studied and described by Franciszek Kamieński in 1879–1882.[ From p. 129: ''"Der ganze Körper ist also weder Baumwurzel noch Pilz allein, sondern ähnlich wie der Thallus der Flechten, eine Vereinigung zweier verschiedener Wesen zu einem einheitlichen morphologischen Organ, welches vielleicht passend als ''Pilzwurzel'', ''Mycorhiza'' bezeichnet werden kann."'' (The whole body is thus neither tree root nor fungus alone, but similar to the thallus of lichens, a union of two different organisms into a single morphological organ, which can be aptly designated as a "fungus root", a ''mycorrhiza''.)]
See also
*Effect of climate change on plant biodiversity
Climate change is a long-term change in the average weather patterns that have come to define Earth’s local, regional and global climates. These changes have a broad range of observed effects that are synonymous with the term. Climate change is ...
*Endosymbiont
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
*Epibiont
An epibiont (from the Ancient Greek meaning "living on top of") is an organism that lives on the surface of another living organism, called the basibiont ("living underneath"). The interaction between the two organisms is called epibiosis. An ep ...
, an organism that grows on another life form
*Endophyte
An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease. Endophytes are ubiquitous and have been found in all species of plants studied to date; h ...
* Epiphyte
*Epiphytic fungus
An epiphytic fungus is a fungus that grows upon, or attached to, a living plant. The term epiphytic derives from the Greek ''epi-'' (meaning 'upon') and ''phyton'' (meaning 'plant').
Examples
Many examples of epiphytic microorganisms exist. The e ...
*Mucigel
Mucigel is a slimy substance that covers the root cap of the roots of plants. It is a highly hydrated polysaccharide, most likely a pectin, which is secreted from the outermost (epidermal) cells of the rootcap. Mucigel is formed in the Golgi bo ...
*Mycorrhizae and changing climate
Mycorrhizae and climate change refers to the effects of climate change on mycorrhizae, a fungus which forms an Endosymbiont, endosymbiotic relationship between with a Vascular plant, vascular host plant by colonizing its roots, and the effects brou ...
*Mycorrhizal fungi and soil carbon storage
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 ca ...
*Mycorrhizal network
A Mycorrhizal network (also known as a common mycorrhizal network or CMN) is an underground network found in forests and other plant communities, created by the hyphae of mycorrhizal fungi joining with plant roots. This network connects indivi ...
*Plant to plant communication via mycorrhizal networks
A Mycorrhizal network (also known as a common mycorrhizal network or CMN) is an underground network found in forests and other plant communities, created by the hyphae of mycorrhizal fungi joining with plant roots. This network connects individu ...
* Rhizobia
*Suzanne Simard
Suzanne Simard (born 1960)Cori Vanchierim'An ecologist’s new book gets at the root of trees’ social lives,' Science News 28 June 2021 is a Canadian scientist who is a professor in the Department of Forest and Conservation Sciences at the Uni ...
References
External links
International Mycorrhiza Society
International Mycorrhiza Society
Mohamed Hijri: A simple solution to the coming phosphorus crisis
video recommending agricultural mycorrhiza use to conserve phosphorus reserves & 85% waste problem @Ted.com
Comprehensive illustrations and lists of mycorrhizal and nonmycorrhizal plants and fungi
Biosafety research into genetically modified barley
MycorWiki
a portal concerned with the biology and ecology of ectomycorrhizal fungi and other forest fungi.
{{Authority control
Plant roots
Soil biology
Symbiosis
Oligotrophs
Fungus ecology