As mycorrhizal ( Alt Gr. Μύκης Mykes , fungal 'and ῥίζα rhiza , root'; plurality mycorrhizae or mycorrhiza ) is a form of symbiosis of fungi and plants referred to, wherein a fungus with the fine root system of a plant is in contact.
The mycorrhizal fungi supply the plant with nutrient salts and water and in turn receive part of the assimilates produced by photosynthesis in (green) plants . In a beech forest, about a third of the photosynthesis products are consumed by the mycorrhizal fungi. In contrast to other soil fungi, many mycorrhizal fungi lack the enzymes necessary to break down complex carbohydrates . That is why they are dependent on the supply from the plant. Compared to plants, mycorrhizal fungi have a significantly greater ability to dissolve minerals and water from the soil . The water, nitrogen and phosphate supply of the " infected " plants is often improved. Furthermore, the mycorrhization offers a certain protection against root pathogenic and aboveground pests, such as aphids or harmful fungal infections . It also increases the drought resistance of the plants, which can be particularly beneficial in extreme locations.
Because of their ability to deliver nutrients directly to the roots of crops, research is currently being conducted into whether the mushrooms can be used as a substitute for mineral fertilizers . The static range for the inorganic fertilizer supplies, v. a. of phosphorus is only 50 to 100 years old ( peak phosphorus ). Mycorrhiza may be an alternative.
Many plant species rely on specific mycorrhizal fungi for optimal growth. From an evolutionary point of view, however, it is largely unclear why the mycorrhizal partner should always appear as a mutualist . Symbioses are generally prone to exploiters and deceivers, because it is always cheaper for the deceiving partner to use the advantages of the partnership (e.g. easily digestible nutrients from the plant) without providing anything in return (e.g. minerals) . Newer concepts in the mycorrhizal-plant symbiosis are therefore based on a gradient of the relationships that ranges from mutualism to strict parasitism . Plants also try to benefit from mycorrhizal fungi without giving anything in return. Mycorrhizal parasites among plants are found among orchids (e.g. coral root and bird nest root ) and chlorophyll-free parasite plants (e.g. Corsia , Monotropa hypopytis ). Research into the mechanisms of mutual manipulation and deception between symbiotic partners is a research area of modern ecology.
The symbiosis of land plants and fungi occurred in the Devonian , 400 million years ago. About 90 percent of the land plants are capable of mycorrhizal formation, whereby about 6,000 types of fungi can associate with plants. It is believed that the arbuscular mycorrhiza (AM) made it possible for the first terrestrial plants to colonize land in the first place. This is supported by the phylogenetic finding that the common ancestor of all land plants had genes that are necessary for the development of arbuscular mycorrhiza. Around 200 species of arbuscular mycorrhizal fungi (see below) have been described worldwide, which are in symbiosis with around 80 percent of all land plant species. Such an unspecific symbiosis can hardly have developed afterwards. Begon, Harper and Townsend even write in their textbook on ecology (1986): "Most higher plants do not have roots, they have mycorrhizae" (in the original: "Most higher plants do not have roots, they have mycorrhizae." )
Due to their specific properties, mycorrhizae are traditionally divided into three different groups. Another division distinguishes between five mutualistic (ecto-, ect-endo-, arbutoid, ericoid and arbuscular) and two antagonistic (orchid and monotropoid) mycorrhizae (after Smith & Read 1997, modified).
What all forms have in common is that fungal hyphae permeate the soil and transport nutrients to the plants.
This connection represents the most common root symbiosis in Central European forests . The mycelium (the entirety of the branching hyphae) forms a dense coat (sheath) around the young, uncorked root ends. As a reaction, the root ends swell to a clump and no longer develop root hairs. The fungal hyphae also grow into the root bark, but do not penetrate the root cells, but rather form a network in the extracellular spaces that facilitates the exchange of nutrients between the fungus and the plant ( Hartig's network ). The hyphae of the fungus take over the task of the missing root hairs. They reach far into the soil matrix, so that a good and extensive nutrient and water absorption is ensured. In addition, the mycorrhizae protect the tree roots from infections caused by the penetration of bacteria or other fungi. This form of mycorrhiza is typical of trees from the birch , beech , pine , willow and rose families . Mushroom partners are mostly mushrooms from the order Boletales and Agaricales , in rare cases sac mushrooms such as the truffle and special cuplets such as the cedar sand bristle. While most of the plant partners can thrive in suitable locations without fungi, there are some of them that are absolutely dependent on fungi as partners. It is assumed that a large number of large mushrooms are capable of ectomycorrhiza - in Central Europe over 1,000 species from the genera veils , pigeons and milklings , knightlings , snails , amanita and bulbous mushrooms , chanterelles .
Here part of the hyphae of the fungus penetrates the cells of the root bark of the plant partner. The latter are predominantly herbaceous plants , only in rare cases trees. The hyphae that surrounds the root of the ectomycorrhiza is missing here. Inside the cell, the mushrooms form a kind of haustorium . This allows nutrients and water to be released and carbohydrates to be absorbed. Plant species from the following families are almost always in symbiosis with a mushroom partner: heather , wintergreen plants and orchids . The symbiotic mushrooms are mostly mushrooms from the order Cantharellales , as well as their anamorphic forms Rhizoctonia and Ceratorhiza . At least with orchids, this form of endomycorrhiza is mandatory for their development.
Arbuscular or outdated also vesicular-arbuscular mycorrhiza (short: VA mycorrhiza) is a special form of endomycorrhiza: typical for this most common type of mycorrhiza is the formation of arbuscles - these are branched, delicate hyphae in the shape of a tree within the root cells. Some taxa also form vesicles - thick-walled fungal cells form in the plant's root tissue. The number of plants that can benefit from AM is very large. These include many crops whose symbiosis increased phosphate supply can have a positive effect on the yield. The fungi involved are assigned to the arbuscular mycorrhizal fungi in the newly created division (phylum) Glomeromycota .
Importance for the phosphate supply of plants
Of the three main inorganic nutrients ( NPK ) in plants, the phosphate in particular is often the limiting factor because, unlike potassium , it is not always freely available in dissolved form, and neither can it be actively absorbed like nitrate , but only passively through diffusion . In order to be able to supply itself effectively with phosphate, the plant root would have to penetrate further and further into as yet undeveloped areas of the soil if it were not in most cases to remove the VA mycorrhizal fungi, which are much better suited for this. In humus soils such as forest soils, the phosphate is mostly organically bound in the form of phytates ( inositol phosphates). This is where the special ability of the ectomycorrhizal fungi to release phosphate from such organic compounds comes into play: Forest trees form mycorrhizal plexuses directly under the leaf layer on the ground and in this way cover most of their phosphate requirements.
The discovery of mycorrhiza was a multi-step process. In the years 1840 to 1880, various individual observations appeared that had to do with the phenomenon. It was not until Franz Kamieński (1881) and Albert Bernhard Frank (1885) recognized the overall picture correctly and published works with clearly running experiments. The word mycorrhiza was first used by Frank.
Today there are systematic collections of mycorrhiza in several European countries. One of these is operated by the agricultural research institute Agroscope in Zurich, where half of the 100 mycorrhizal mushroom species known in Switzerland are already represented. They grow in pots filled with an oil binding agent. When moist, the porous granulate is an ideal substrate to keep the mushrooms alive. Using such collections, the researchers can examine each individual species for its properties. One of the goals is to find out which types of fungi form a nutrient network with which plants. This knowledge could be useful in organic farming, weed control or the greening of flat roofs.
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