Symbiosis: Mycorrhizae and Lichens
The fungi then colonises the plant root network, when the AMF hyphae The symbiotic relationship between the fungi and plant can assist in. Mycorrhizal Fungi and Plant Roots: A Symbiotic Relationship Observations of hyphae bound together with root hairs weren't reported until the 19th century. Plants cannot extract the necessary nitrogen from soil, so they form symbiotic relationships with Mycorrhizae: The Symbiotic Relationship between Fungi and Roots . There are three sensitive hairs in the center of each half of each leaf.
Fungi forming this association are members of the Basidiomycota. 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. 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 Role of Arbuscular Mycorrhizal Fungi in Plant Nutrition - Greenhouse Product News
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.
Mycorrhiza - Wikipedia
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! 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.
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. Some genera, such as Trebouxia, are known to only occur in lichens and are not free-living, but there are also examples that are free-living.
Economic Relevance Economically, lichens have little significance. Perhaps this is why there is so little interest in this group of organisms.
- Mycorrhizal Fungi and Plant Roots: A Symbiotic Relationship
One way that they have been utilized is in the extraction of blue, red, brown or yellow dyes in the garment industry. Also, the indicator pigments used in litmus paper was also derived from lichens.
Previously, we briefly mentioned lichens as a source of pharmaceutical compounds. You can include some "folk" remedies in this category as well. They are also used in the cosmetic industry, in the making of perfumes and essential. Finally, some species have been used as food.
One species, Lecanora esculenta, is a species that grows in the mountains near Israel and are typically blown free from their substrate. Desert tribes grind up the lichen, dry it and mix it with dry meal to form a flour. It is postulated that this is the species lichen that is referred to as "Manna from Heaven" when Moses led the Hebrews across the desert during biblical time.
One species, Cladonia rangiferina reindeer mossis fed upon by reindeers and cattle. This has led to the discovery that lichens readily absorb radioactive elements. After open-air, atomic testing, both Alaskan Eskimos and Scandinavian Laplanders were found to have high levels of radioactive contamination, which they had absorbed from eating reindeer, which in turn ate lichens.
Other Significant Uses for Lichens Lichens are conspicuously absent in and surrounding cities because many species are sensitive to pollution, especially to sulfur dioxide and flourine, which are common pollutants.
For this reason, they have been commonly used as indicators of pollutants. In urban areas, where lichen surveys have been carried out, the absence of certain indicator species is used as early warnings of decrease in air quality.
Lichens also play a very significant role in nature. They are the pioneers in rocky substrates, where there is no soil. Lichens break down the rocky substrate into soil and their decomposing thallus fertilize the newly produced soil, making it possible for the plant habitation. Reproduction Reproduction of the lichen is entirely asexual. It may occur by soredia sing.: This is best seen in a sectioned lichen. The other means of asexual reproduction is by isidia sing.: Ascospores and conidia also form, but these will only reproduce the fungus.
It is assumed that these structures will come in contact with a suitable algal host and resynthesis the lichen thallus. However, the latter are not thought to be significant in lichen reproduction. From left to right: Clusters of soralia, two soredia, as seen through the microscope, isidia and section through soredium.
The part of the lichen that is composed of interwoven hyphae with the host algal cells. A category of Endomycorrhizae characterized by the production of globose structures, called vesicles, and branched, tree-like structures called arbuscules, in the cortex of the root cells.
The root cells lyse these structures and receive the minerals from the fungus, in this matter. Such relationships are usually not obligate, and neither species will be adversely affected if the relationship does not occur. Lichen that is very thin and flattened against the substrate. This type of lichen lacks a lower cortex and is attached to its substrate by the medullary layer. Thus, these lichens are very thin and often appear to be part of the substrate on which they are growing.
Lichens that have a leafy appearance and are attached to their substrate by structures called rhizines. Lichens that are often composed of pendulous "hair-like or less commonly upright branches finger-like and attached at a single point.
In cross section, the thallus can usually be seen to be rounded, i. Thus, there is a single cortex layer. Asexual reproductive structures found on lichens that are upright, cylindrical to swollen in appearance. Structures break off and can form another lichen. The symbiotic relationship between a fungus and an alga that develops into a unique morphological form that is distinct from either partner.
The fungal component of the lichen. In the traditional sense, the fungus is a member of the Ascomycota. The symbiotic relationship between the roots of plants and fungi. The term mycorrhiza literally means root fungus, but in the broad sense of the term, the interaction does not always occur only with the roots of plants, a mycorrhizal relationship also includes plants that do not have roots, such as and bryophytes mosses and liverworts.
The algal component of the lichen. The alga is usually a member of the Chlorophyta or Cyanobacteria. Click here for further images of orchid mycorrhizas 2.
Arbuscular mycorrhizas Arbuscular mycorrhizas are found on the vast majority of wild and crop plants, with an important role in mineral nutrient uptake and sometimes in protecting against drought or pathogenic attack.
Structures resembling those of the present-day AM fungi have been found in fossils of primitive pteridophytes of the Devonian period. It is thought that these fungi colonised the earliest land plants and that mycorrhizal associations could have been essential for development of the land flora. The fungi involved are members of the zygomycota related to Mucor.
They are classified currently in six genera Acaulospora, Entrophospora, Gigaspora, Glomus, Sclerocystis and Scutellospora and they seem to be obligate symbionts: The image above shows part of a clover root from the Pentland Hills near Edinburgh, naturally infected by an AM fungus. There was no evidence of fungal infection until the root tissues were cleared with strong alkali and then stained with trypan blue to reveal the fungus. The site of penetration is shown at top right, where the fungus produced a pre-penetration swelling appressorium, apthen it grew between the root cells and formed finely branched arbuscules arb and swollen vesicles v.
The arbuscules are thought to be sites of nutrient exchange - the fungus obtains sugars from the plant, and the plant obtains mineral nutrients e. Vesicles are thought to be used for storage. Root hairs rh are also labelled. The image below shows a single arbuscule with its repeated dichotomous branching inside a root cell. The plant cell remains alive, because its membrane extends to encase all the branches of the fungus.Mycorrhiza and the symbiotic relationship between fungi and plants.
Strictly speaking, therefore, the fungus is always outside of the cell, surrounded by the cell membrane. Feeding relationships of this type, in which a fungus produces special nutrient-absorbing structures within the host cells, are termed biotrophic.
For further details see Biotrophic plant pathogens. The hyphae of AM fungi extend into soil, where their large surface area and efficient absorption enable them to obtain mineral nutrients, even if these are in short supply or are relatively immobile. AM fungi seem to be particularly important for absorption of phosphorus, a poorly mobile element, and a proportion of the phosphate that they absorb has been shown to be passed to the plant.
Click here for further images of arbuscular mycorrhizas 3. Ectomycorrhizas Ectomycorrhizas sometimes termed ectotrophic mycorrhizas are characteristic of many trees in the cooler parts of the world - for example pines, spruces, firs, oaks, birches in the Northern Hemisphere and eucalypts in Australia.
However, some trees e. The fungi involved are mainly Ascomycota and Basidiomycotaincluding many that produce the characteristic toadstools of the forest floor Figures A-C below. Most of these fungi can be grown in laboratory culture but, unlike the wood-rotting fungi, they are poor degraders of cellulose and other plant wall materials.
So they gain most of their sugars from the living plant roots in natural conditions. Rings of toadstools of mycorrhizal fungi Hebeloma and Lactarius species around the base of a birch tree. The gills of this toadstool were cut to show that the fungus exudes a milky fluid when the tissues are damaged - hence the name 'Lactarius'.
Fruitbody of Amanita muscaria the 'fly agaric' which is a typical mycorrhizal fungus of birches but also can be found on some other trees. In ectomycorrhizas the terminal branches of the root system are highly modified - the roots are short and stumpy, covered with a mantle sheath of fungal tissue the creamy-white root surface in Dand there are few or no root hairs.
The fungus takes over the normal nutrient-absorbing role of the root hairs. In E the fungal mantle is less conspicuous, but the fuzzy appearance of the roots is due to many fungal hyphae growing from the mantle into the soil.
Such roots are seen easily if the undecomposed, surface litter is scaped away from the forest floor to reveal the decomposing litter containing a mass of mycorrhizas and their fungal networks. Cross-section of a pine mycorrhiza, showing the substantial fungal sheath that encases the root labelled 's'.