Mycorrhiza
What is mycorrhiza?
Mycorrhizal fungi are an inherent part of all ecosystems, including both natural and agricultural ones. The term mycorrhiza which has its origin in Greek words mykes (= fungus) and rhiza (= root) describes a mutualistic association between plant roots and a specific group of soil fungi.
This symbiosis can be characterised by a bi-directional movement of nutrients where carbon flows to the fungus and inorganic nutrients move to the plant. Root colonisation by mycorrhizal fungi can considerably affect the growth and health of host plants that benefit e.g. from improved nutrient uptake, higher resistance to drought, heavy metals or pathogens. Several different types of mycorrhizal associations can be distinguished, involving different groups of fungi and host plants and characterised by distinct morphology patterns:
Arbuscular mycorrhiza
Arbuscular mycorrhiza (AM) formed by obligately symbiotic fungi from the division Glomeromycota represents the most frequent and ubiquitous type of mycorrhizal symbiosis that inhabits the roots of more that 80 % plant species. Spectrum of their host plants ranges from bryophytes and pteridophytes to gymnosperms and angiosperms.
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| Vesicular-arbuscular mycorrhiza: Highly colonized root of maize dyed with trypan blue. Mycorrhizal formations are clearly visible: 1) vesicles; 2) arbuscules Photo: R. Sudová, D. Püschel |
In contrast to ectomycorrhizal fungi, AM presence in soil or plant roots cannot be normally recognised with the naked eye, but only under a microscope. Arbuscular mycorrhizal fungi penetrate, again contrary to ectomycorrhiza, into cortical cells of the host where typical structures are formed - arbuscules and vesicles. Because of the presence of those characteristic structures in root cells, this type of endomycorrhiza was formerly called "vesicular-arbuscular", however, this term was dropped recently since some AM fungi do not form vesicles at all.
Short-life arbuscules with their main trunk and fine branches resembling small trees serve as a site of the intensive nutrient exchange between plant and fungus. In contrast, vesicles, i.e. globose or oval swellings formed terminally or intercalarly on intraradical hyphae, have a storage function.
Besides intraradical structures, AM fungi form also an extensive network of extraradical mycelium in soil that extends into bulk soil beyond the rhizosphere and enables the host plant to absorb nutrients, particularly relatively immobile phosphorus, from a considerably larger volume of the soil. By the differentiation of vegetative hyphae, AM reproductive structures - spores of different size, texture and colour (depending on the species) are formed asexually on extraradical mycelium in soil.
Ectomycorrhiza
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| Ectomycorrhiza: root tip of Pinus nigra colonised by ectomycorrhizal fungus Photo: M. Vohník |
Ectomycorrhiza represents the second widely distributed type of mycorrhiza. This symbioses is developed between fungal species of the phyllum (Bazidiomycota) and roots of trees (mostly conifers).
Intraradical mycelium colonises intercellular space of root cells only. Extraradical mycelium forms dense hyphal sheath on the root surface and prevents formation of root hairs. On the other hand, the mycelium colonises the forest soil to larger extent than roots do. This widely spread mycelial system distinctly improves access of the tree to nutrients and water in soil. Typical structures resulting from the absence of root hairs can be easily observed on root tips. These structures are called ectomycorrhizal tips and their presence or absence on tree roots pose an evidence of ectomycorrhizal symbioses.
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| Ectomycorrhiza: fruit bodies of edible ectomycorrhizal fungi Photo: M. Vohník |
Apart from improvement of tree nutrition, ectomycorrhizal symbioses prevent trees from soil fungal pathogens. The mycelial system connects neighbouring trees and also enables water and nutrient flux among them. Last but not least, ectomycorrhiza is involved in protection of trees from inhibition of alellopathic compounds in soil.
Latest investigations showed that ecto-mycorrhizal mycelium is not limited to the upermost soil layers (organic horizon). Surprisingly high mycelial content and high fungal species diversity was determined in mineral soil horizon as well. No wonder, that another unbelievable fact was discovered - ectomycorrhizal fungi are also involved in rock weathering and mineral transformation. However, they are responsible only for up to 0,5 % of weathering. Therefore, their effect may be omitted compared to abiotic factors.
The list of fungal species gathered in the collection of our department can be seen via the page Links.
Ericoid mycorrhiza
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| Ericoid mycorrhiza: ericoid mykorrhiza occurs especially the members of Ericaceae (e.g. Rhododendron, Azalea, Vaccinium) Photo: M. Vohník |
Ericoid mycorrhiza is a distinguishable type of endomycorrhiza, occurring in the roots of the members of Ericaceae (mostly northern hemisphere distribution) and Epacridaceae (southern hemisphere distribution). Compared to the arbuscular mycorrhiza, it has distinct anatomy, morphology, eco-physiology as well as a spectrum of fungal partners and host plants.
Characteristic anatomical feature of ericoid mycorrhiza is the thin roots of ericoid plants, named hair roots. These anatomically very simple ephemeral organs occur in whole range of ericoid mycorrhizal plants. Ericoid mycorrhizal fungi colonize rhizodermal cells of hair roots forming typical coils and loops inside them, which are the point of nutrient and information exchange between plant and fungus. A dense net of extraradical mycelium spreads from colonized roots into surrounding substrate multiplying by this way the volume of the substrate accessible for the plant to draw nutrients and water from.
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| Ericoid mycorrhiza: a root of Rhododendron sp. colonized by ericoid mycorrhizal fungus Photo: D. Püschel |
Thanks to a wide range of saprotrophic abilities are ericoid mycorrhizal fungi able to survive long periods without host plant, they are cultivable on artificial media with organic sources of carbon. Ericoid mycorrhizal fungi produce a number of special enzymes (proteases, chitinases etc.), which enable them to utilize complex organic compounds as sources of nutrients (peptides, proteins, chitin etc.)
Ericoid plants dominate in nature at particular habitats - peat bogs, moorlands or heathlands - which are characteristic by several factors negatively influencing the dynamics of its plant communities. Among them, low nutrient accessibility, low pH, high C : N ratio and heavy metal toxicity play an important role. It is stated that primarily ericoid mycorrhiza enables ericoid plants to sustain these environmental stresses and successfully compete with other plants.
The list of fungal species gathered in the collection of our department can be seen via the page Links.
Orchid mycorrhiza
Orchid mycorrhiza is an endomycorrhizal association with an extensive intracellular mycelium, occurring in the roots of the members of the Orchidaceae.
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| Orchid mycorrhiza: Dactylorhiza fuchsii. Photo: M. Čuříková, A. Látr |
Orchid mycorrhizal fungi belong to the Basidiomycetes - most frequently to the genus Rhizoctonia (with teleomorphs Tulasnella, Sebacina, Thanatephorus, Ceratobasidium, Armillaria, Fomes etc.).
Holomycotrophic (totally dependent on mycorrhizal fungus) are all achlorophlyllous orchids (eg. Neottia nidus-avis). In general, also all germinating orchids are dependent on their mycobionts, especially in the terms of carbon metabolism, because orchid seeds contain insufficient amount of storage compounds, namely sugars, to support the development of the seedlings.
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| Orchideoid mycorrhiza: Cephalanthera longifolia - cross section, infected cells of primary cortex. Photo: M. Čuříková, A. Látr |
A typical anatomical feature of the orchid mycorrhiza is a coil of hyphal loops called a peloton. Pelotons develop inside the cells of the primary cortex. The fungal hypha penetrates the cortex cell, branches and forms a peloton (the cell in this phase is called a trophocyte). Cell-to-cell infection may occur after the peloton formation. At the end of their lifecycle pelotons collapse to form an aggregate of amorphic fungal tissue. The cell in this phase is called a phagocyte and can be re-colonized by a new fungal hypha. Orchid mycorrhizal fungi usually do not colonize root mezodermis, endodermis and the central stele.
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| Orchid mycorrhiza: Intracellular hyphal peloton. Photo: M. Vohník |
Orchid mycorrhiza supports host plant with nutrients, especially sugars, and water. The transport of carbon compounds from fungus to host plant may be realized by two ways: 1) through the plasmatic membrane of living pelotons (in trophocytic phase), or 2) by phagocytosis of collapsed fungal endophyte (in phagocytic phase). Recent studies support the first way of transport and regard the collapse of the hyphal pelotons rather as a result of defense mechanisms of the host plant against uncontrolled fungal development. In contrast to the general scheme of nutrient exchange in mycorrhizal symbioses, the transport of carbon from the host plant to the fungus is yet under debate in orchid mycorrhiza, even though some experimental evidence exists.
Pseudomycorrhiza - DSE association
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| Pseudomycorrhiza: The root of Vaccinium corymbosum highly colonized by DSE fungus Photo: M. Vohník |
Pseudomycorrhiza, recently called DSE-association (as derived from Dark Septate Endophytes) is a form of coexistence between a special type of filamentous fungi (DSE, typical representative is Phialocephala fortinii Wang & Wilcox) and a wide spectrum of higher plants. It is studied mainly on conifers (species of Pinus) and ericoid plants. It is supposed that the distribution of DSE fungi is limited only by extreme environmental conditions, DSE fungi are not present in waterlogged environments but were present for example in soil samples from Antarctic peninsula.
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| Pseudomycorrhiza: Colony of pseudomycorrhizal fungus Phialocephala fortinii Wang et Wilcox in Petri dish Photo: M. Vohník |
Pseudomycorrhiza is probably the less studied and understood type of the symbiosis between roots of higher plants and fungi. For long period of time it was distinguished only as an example of weak parazitism of DSE fungi on plants. Recent studies however confirmed its bilateral utility, thus its mycorrhizal character, which can be on the other hand turned back to neutralism or weak parasitism by changing of environmental conditions. The plasticity of plants' reactions to DSE colonization makes pseudomycorrhiza an excellent model for the studies on plant-fungus interactions.
DSE fungi have thick, dark brown septate hyphae, which often form a dense sheath around colonized root. Single hyphae can form appressoria, penetrate root tissues and form typical anatomical structures (microsclerotia) inside cells or simply grow along stele.
The list of fungal species gathered in the collection of our department can be seen via the page Links.
