MYCORRHIZAE | MYKE
(a) Differential relationships between fungal and plant structures for each of the main types They are formed by interactions between plants and fungi in the phylum These mycorrhizal associations occurred in all early-diverging lineages of .. help newly-arrived plants to become established by nurturing their seedlings. New research has shed light on how Earth's first plants began to that soil fungi formed mutually beneficial relationships with early land plants Mutualistic mycorrhiza-like symbiosis in the most ancient group of land plants. It can take several months for a plant to become fully mycorrhizal if only Mycorrhizal fungi colonize roots forming a biological link between the root and the soil. Can mycorrhizal fungus inoculum be applied to established plants like trees.
Ericoid Mycorrhizae Plants having this group of mycorrhiza are commonly found in acidic, peatland soils and include members of genera Calluna heatherRhododendron, Azaleas and Vaccinium blueberriesof the family Ericaceae. Ericoid mycorrhizae have evolved in association with plants that are continually stressed by factors within the soil. The soil is typically extremely acid, peatland soil, low in available minerals because mineralization is inhibited. Plants with ericoid mycorrhizae seem to have a high tolerance to these stresses and there is good reason to believe that this is related to the presence of the mycorrhizal fungus and that the survival of the host is dependent upon the fungus.
The mycorrhizal association is most similar to that of an endomycorrhiza because fungus growth is extensive in the root cortex. The fungus penetrates the cell wall and invaginates plasmalemma and is filled with coiled hyphae, like those in orchid mycorrhizae. No mantle is formed.
Infected cells are fully packed with fungal hyphae. Fungus species are mostly members of the Ascomycota, in the genus Hymenoscyphus.
Cross section of ericoid root, showing coiled hyphae. The host cell dies as the association disintegrates, thereby restricting the functional life, i. Monotropoid Mycorrhizae One of the characteristics that we normally attribute to plants is that they have chlorophyll and can produce their own food through the process of photosynthesis.
However, this is not true of all plants. The Monotropaceae and Pyrolaceae are two families of plants that are achlorophyllous.
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Thus, plants in these families are more dependent upon their mycorrhizal partners than plants which can carry out photosynthesis. Monotropa uniflora left from http: The means by which food is obtained by these plants is the same as in achlorophyllous orchids. However, morphologically, they are very different. The achlorophyllous host has mycorrhizae roots that appear to be formed by an ectomycorrhizal fungus, but the epidermal and outer cortical cells are penetrated by the fungus, as in endomycorrhizal plants.
The fungus also forms an ectomycorrhizal relationship with a tree which is capable of photosynthesis. So, as in the case of the epiphytic orchids, the photosynthetic tree indirectly provides carbohydrates to these achlorophyllous plants, as well as to the fungus.
Both hosts probably obtain their mineral requirements through the fungus. Lichens The most well known example of a symbiosis between fungi and plants is the lichen, if you will allow me to include algae as plants. The concept of what constitutes a lichen has broaden significantly in the last 25 years to include some species of mushrooms, slime molds, and some members of the Zygomycota.
However, we will discuss lichens in the traditional sense, as an association between a fungus and an alga that develops into a unique morphological form that is distinct from either partner. The fungus component of the lichen is referred to as the mycobiont and the alga is the phycobiont. Because the morphology of lichen species was so distinct, they were once thought to be genetically autonomous until the Swiss Botanist Simon Schwendener described their dual nature in Prior to that time, because of the morphology of many of the "leafy" species of lichens, they were considered to be related to bryophytes, i.
Although, lichens are now known to be composite organisms, they are still named for the fungus part of the association since that is the prominent part of the lichen thallus.
A thallus is an old botanical term used to describe "plants" that do not have leaves, stems and roots, and its origin goes back to a time when only two kingdoms were recognized in classifying organisms, i. Prior toorganisms such as algae, bacteria and fungi, were included in the plant kingdom. InWhitaker, proposed a five kingdom system that was used for many years, but may soon also become outdated. Although, this term is antiquated, it is still used to describe the "bodies" of algae, fungi and of course lichens.
The only group of plants, in which we still use the term thallus, to refer to the plant body, are the bryophytes. Although the lichen thallus is composed of an algal and fungal component, lichens are not studied in mycology or phycology that part of botany that studies algae. Instead, they are studied in their own discipline, lichenology. There are relatively few lichen researchers.
Of these most are systematists. As a result, there are still some basic questions concerning this symbiosis that are unanswered or at least up for debate. One of the most basic questions, that has been asked since the discovery of the lichen symbiosis, concerns whether lichens represent a true mutualistic symbiosis or nothing more than a variation of a host-parasite relationship.
There is evidence supporting both sides. That it represented a mutualistic symbiosis, in which the alga was believed to contribute the food supply through photosynthesis, and the fungus protected the alga from desiccation, harmful solar radiation and provided the alga with water and inorganic nutrients, was postulated by Beatrix Potter, the writer and illustrator of Peter Rabbit, soon after Schwendener had determined the true nature of the lichen thallus.
In order to understand both sides of the issue, lets look at the morphology and anatomy of lichens. The Lichen Thallus In the traditional sense of lichens, their thallus can be artificially divided into four forms: Foliose Lichens Lichen thallus which is generally "leaf-like", in appearance and attached to the substrate at various points by root-like structures called rhizines.
Because of their loose attachment, they can easily be removed. These are the lichens which can generally be mistaken for bryophytes, specifically liverworts. It is possible, or even probable, that herbaria still contain lichens that have been mistakenly identified as liverworts. If we look at these a foliose lichen in longitudinal section, from top to bottom, we would be able to distinguished the following layers: Often composed of tightly interwoven mycelium, which gives it a cellular appearance.
This cellular appearance is referred to as pseudoparenchymatous. Composed of interwoven hyphae with the host algal cells. This is the ideal location for the algal cells. Beneath the upper cortex so that it receives the optimal amount of solar radiation, for photosynthesis, but not direct solar radiation which would be harmful.
Composed of loosely interwoven mycelium. Layer is entirely fungal. Usually same composition as the upper cortex and attached to the substrate by root-like structures called rhizines. The rhizines are entirely fungal, in origin, and serve to anchor it to the substrate. Thus, the foliose lichens also have what is referred to as a dorsiventral thallus, i. Sectional views, illustrating how the three thallus types of lichens differ.
The entire lower surface is attached to the substrate. These lichens are so thin that they often appear to be part of the substrate on which they are growing. The following link shows an image of several lichen thalli. Crustose species that are brightly colored often give the substrate a "spray-painted" appearance. The thallus has the upper cortex, algal and medullary layers in common with the foliose lichens, but does not have a lower cortex. The medullary layer attached directly to the substrate and the margins are attached by the upper cortex.
This type of lichen is tightly flattened to its substrate and the entire lower surface medulla is attached, making it impossible to remove the thallus from its substrate.
Fruticose Lichens The thallus is often composed of pendulous "hair-like or less commonly upright branches finger-like. The thallus is attached at a single point by a holdfast. In cross section, the thallus can usually be seen to be radially symmetrical, i.
The layers that can be recognized are the cortex, algal layer, medullary layer, and in some species the center has a "cord" which is composed of tightly interwoven mycelium. Other species have a hollow center that lack this central cord. Fructicose lichen thallus is attached to its substrate at a single point, but finding that point is not that easy!
Mycorrhiza - Wikipedia
Biology of Lichens In looking at the anatomy of the lichen, it is obvious that there is interaction between the phycobiont and mycobiont, but what kind of interaction is occurring. One school of thou0ght is that the alga produces the food material and that the fungus protects alga from desiccation, high light intensities, mechanical injuries and provides it with water and minerals.
This is the reasoning that many introductory text books have adopted and they define a lichen as a mutualistic symbiosis. However, in studies that have been done that examines the alga-fungus interface, it can be clearly seen that haustoria, specialized feeding structures present in parasitic fungi, penetrate the alga cells. Thus, many lichenologist have defined this relationship as a controlled form of parasitism.
There is more evidence and I would like to go over some of these. Illustration of haustoria penetrating algal cells give evidence that the lichen symbiosis is really a controlled form of parasitism. Conditions outside these parameters will usually be fatal for most species of fungi and algae.
However, lichens occur all over the world. They even occur in arctic and hot, dry desert areas where few organisms can live or even survive.
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Thus, the lichen is able to exploit habitats that few other organisms are able to utilize that seem likely to be the result of their mutualistic, symbiotic relationship. Another experiment that demonstrates that lichens represent a mutualistic symbiotic relationship was carried out in the laboratory by Vernon Ahmadjian. Although, it is not difficult to separate the myco- and phycobiont components of the lichen, and grow them separately in the laboratory, putting the component back together is another story.
For many years it was not possible to put the two together to reform the lichen thallus. The reason for this was the method that was used in attempting to reform the lichen thallus. These types of media did not work. Ahmadjian reasoned that if the lichen represents a symbiosis, the reason that the relationship formed was because, in nature, neither one could obtain all the nutrients necessary for survival and that only after the two organisms interacted was it possible.
Thus, Ahmadjian created a minimal medium, which would not support the growth of either the myco- or phycobiont, and inoculated them into that medium. This method successfully reformed the lichen thallus, in the laboratory, for the first time. Although, it would appear that there is a great deal more evidence supporting the lichen thallus as a product of mutualistic symbiosis, there are still many who believe that the relationship is that of a balance parasitism that favors the mycobiont.
A Few Words on The Lichen Component Although there are approximately 13, species of lichens recognized, the number of taxonomic groups of fungi and algae that produce the lichen thallus are few. Mycobionts In the traditional sense of lichens, which is how we are defining lichens, the fungal components are always in the Ascomycota, specifically in those groups that form their asci and ascospores in fruiting bodies. The fungi involved in the lichen symbiosis are never found to be free-living in nature.
Phycobiont Regardless of whether we are using the traditional or expanded definition of lichens, the algae involved in the association are the same. Of all the different species of algae that are known, only the divisions Chlorophyta "green" algae and Cyanophyta "blue-green" algae or Cyanobacteria are involved in lichen formation. The latter are actually bacteria rather than algae although they were classified as such once upon a time.
Furthermore, within these divisions, only a few genera are involved in the lichen symbiosis. Finally, mycorrhizae develop the root system through a more efficient uptake of water and nutrients by the plant. Mycorrhizae is the symbiotic association of mycorrhizal fungi with plant roots. Through this symbiosis, they naturally develop and mutually reinforce themselves without hindering or cannibalizing each other.
The symbiosis is beneficial to both as the fungi bring water and nutrients to the plant whereas the plant provides the fungi with the sugars they need to proliferate. The root system's extended network provides several benefits to the plants colonized by mycorrhizae. Plus, mycorrhizae provide to the plant greater tolerance to drought, salinity, soil compaction, and other environmental stresses. This results in the reduction of the transplant shock, a faster rooting development, an increase of the survival rate and the maintenance of soil structure.
Mycorrhizae "transform" or solubilize the phosphorus contained in the soil to the benefit of the plant. Therefore, plant development and growth are improved, just as flowering and fructification. Mycorrhizal fungi have existed since the first plants appeared on dry land more than million years ago. One form of such immobilization occurs in soil with high clay content, or soils with a strongly basic pH.
The mycelium of the mycorrhizal fungus can, however, access many such nutrient sources, and make them available to the plants they colonize. 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 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.
These structures have been shown to host nitrogen fixing bacteria which contribute a significant amount of nitrogen and allow the pines to colonize nutrient-poor sites.
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 acid that dissolve or chelate many ions, or release them from minerals by ion exchange.
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.
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. 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[ edit ] Recent research has shown that plants connected by mycorrihzal fungi can use these underground connections to produce and receive warning signals.