Symbiosis: Mycorrhizae and Lichens
Some elements, such as nitrogen and phosphorus, are required in large quantities In lichen, fungi live in close proximity with photosynthetic cyanobateria; the algae mycorrhiza: a symbiotic association between a fungus and the roots of a. Discusses parasitic and mutualistic relationships of fungi. relationships fungi have with other organisms, including mycorrhizae and lichen. Different symbiotic mycorrhizal associations between plants and fungi occur, almost in the soil and a large surface area for interaction with other micro- organisms. . However, it is fairly well accepted that lichens participate in mycelia of 32P-labelled phosphate sequestered from the mycelium of the.
Ericoid mycorrhizas also occur in warm Mediterranean climate zones in chaparral vegetation systems throughout the world, suggesting that nutritional, rather than climatic, factors determine their distribution. Around plant species form this type of mycorrhizal association with various fungi from the Ascomycota but only one species, Rhizoscyphus ericae earlier known as Hymenoscyphus ericae has been studied in detail.
This fungus penetrates the cell walls of roots and forms coiled structures within each cell without penetrating the host plasmalemma. Recent evidence suggests that basidiomycetes belonging to clade B Sebacinales are also common ericoid mycorrhizal fungi Selosse et al.
Many long-lived perennial plants and trees form a third type of mycorrhiza, ectomycorrhiza. The fungi are predominantly from the Basidiomycota and Ascomycota and as many as 10 fungal species and plant species may be involved, globally Taylor and Alexander, Although this represents only a small fraction of the total number of terrestrial plants, these species often form the dominant components of forest ecosystems occupying a disproportionately large area.
The fungi do not penetrate the host cells, and the symbiosis is characterized by the presence of a fungal mantle or sheath around each of the short roots, as well as a network of intercellular hyphae penetrating between the epidermal and cortical cells, the so-called Hartig net.
Like the arbuscules in arbuscular mycorrhiza, this interface is an effective way of increasing the surface area of contact between the fungus and its plant host. The mantle is usually connected to a more or less well-developed extraradical mycelium which may extend for many centimetres from the root into the soil. In most types of mycorrhizal symbiosis the fungal symbionts depend upon their autotrophic plant hosts to supply carbon, however in the orchid mycorrhiza formed by orchids this dependency may be reversed.
The family Orchidaceae is the largest in the plant kingdom and estimated to contain 30 species. Although most orchids have green leaves and are autotrophic when fully established, about species are achlorophyllous as adults and all pass through a germination and early development phase when they are dependent on an external supply of nutrients and organic carbon.
They have minute dust like seeds with no reserves, and are initially entirely dependent upon the supply of carbon and nitrogen from fungi. The fungi colonizing these plants were originally thought to be effective saprotrophs or parasites, but DNA-based studies of the fungi have now shown that most of them are mycorrhizal fungi simultaneously colonizing other autotrophic plants Leake, The carbon supplies of green orchids are normally met by association with basidiomycetous Rhizoctonia species characterized as saprophytes or plant parasites, but in fully myco-heterotrophic orchids there is evidence that this ancestral trait has been abandoned in favour of association with other basidiomycete species that are obligate ectomycorrhizal symbionts of autotrophic trees and shrubs Leake, The Monotropaceae are mycoheterotrophic plants that form a fifth type of mycorrhizal association, monotropoid mycorrhiza.
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!
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. 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. 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.