Symbiosis: Mycorrhizae and Lichens
Symbiosis refers to the close relationship between two or more than two The symbiosis theory is not only widely used in animal sciences and. Phoresy: A loose association where a usually, smaller organism is using a A mycorrhiza is defined as a symbiotic relationship between the roots of It is referred to as "ecto-" because the fungal symbiont does not invade the cell protoplasm. One category of interactions describes the different ways organisms obtain their food and . Symbiotic relationships are not always positive for both participants.
Examples of metabiosis are hermit crabs using gastropod shells to protect their bodies, and spiders building their webs on plants.
Parasitism Head scolex of tapeworm Taenia solium is adapted to parasitism with hooks and suckers to attach to its host. In a parasitic relationshipthe parasite benefits while the host is harmed. Parasitism is an extremely successful mode of life; as many as half of all animals have at least one parasitic phase in their life cycles, and it is also frequent in plants and fungi.
Moreover, almost all free-living animal species are hosts to parasites, often of more than one species. Mimicry Mimicry is a form of symbiosis in which a species adopts distinct characteristics of another species to alter its relationship dynamic with the species being mimicked, to its own advantage. Batesian mimicry is an exploitative three-party interaction where one species, the mimic, has evolved to mimic another, the model, to deceive a third, the dupe.
In terms of signalling theorythe mimic and model have evolved to send a signal; the dupe has evolved to receive it from the model. This is to the advantage of the mimic but to the detriment of both the model, whose protective signals are effectively weakened, and of the dupe, which is deprived of an edible prey.
For example, a wasp is a strongly-defended model, which signals with its conspicuous black and yellow coloration that it is an unprofitable prey to predators such as birds which hunt by sight; many hoverflies are Batesian mimics of wasps, and any bird that avoids these hoverflies is a dupe.
Amensalism is an asymmetric interaction where one species is harmed or killed by the other, and one is unaffected by the other. Competition is where a larger or stronger organism deprives a smaller or weaker one from a resource. However, because of the absence of a macroscopic of macroscopic fruitbodies, the presence of endomycorrhizae is more difficult to demonstrate.
Because of the lack of visibility, this group was considered to be rare until a method was devised that could readily detect such fungi in the soil and demonstrate that they are in fact very common. There are several categories of endomycorrhizae. The only common feature that they all share is that the mycelium of the fungal symbiont will gain entry into the host, root cells by cellulolytic enzymes.
Unlike the ectomycorrhizae, roots which are infected with mycorrhizal fungi do not differ morphologically from those that are not infected, i. However, the type of association that is formed between the host and fungus vary a great deal in the different categories of endomycorrhizae.
Vesicular-Arbuscular Mycorrhizae VAM This category of mycorrhiza can be found throughout the world, but more abundant in the tropics than in temperate regions, and is associated with more plants than any of the other categories of mycorrhizae.
The name of this type of mycorrhizae comes from the distinct structures that can be seen inside the cells of the infected roots, the rounded vesicles Fig. There is also extensive mycelium in the soil, but none of it is organized in any fashion.
The vesicles and arbuscules contain the stored minerals that are needed by the plant. These structures lyse in the root cells and in this way the minerals become available to the plant.
Vesicles in roots cells of Sesbania sp.
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Note some vesicles have been displaced from cells due to preparation of slide. Arbuscule in root cell.15 INCREDIBLE Mutual Animal Relationships
Arbuscules are characterized by their tree-like appearance. The group of fungi involved is always a member of the Zygomycota. There are only a few genera of fungi involved, but because of the lack of specificity of these genera to specific host plants, they have been found to have largest host range of any mycorrhizal group. The VAM fungi normally produce assorted types of spores which can be used in the identification of these fungi, i.
It was once thought that these fungi were nothing more than a rare curiosity. However, this was only because a technique was needed, which could more efficiently find VAM spores, than by simply sifting through the soil. Once this technique was found, this type of mycorrhiza was found to be the most common in nature. It is because VAM have a broad host range they were once considered to be a future tool in agriculture, i.
However, because these fungi cannot be grown in the absence of a host plant, individual inoculations would have to be done for each plant. This would be impractical for any grains grown as well as for most crops, but have been utilized in planting of fruit trees which are planted individually.
There are a number of native plants which are endangered, in which attempts at growing them from seeds and cuttings at NTBG have not been very good. A few years ago, while Drs.
Symbiosis - Wikipedia
While inoculation of VAM fungi did greatly improve the survival of the young plants, it would not be the whole answer to their problems. Some species of native Hawaiian plants that were given inoculated with and without VAM fungi are shown on Figs. Orchid Mycorrhizae This category of endomycorrhizae are mostly members of the Basidiomycota.
All orchids are infected with this type of mycorrhizal fungus. Orchid mycorrhizae are functionally different than in the above two types because of the unique nutritional needs of orchid plants. In most plants, the seed contains a food supply that will feed the embryo, until germination occurs, at which time the plant becomes photosynthetic and can produce its own food.
However, orchid seeds are very minute and contain a very small food reserve for the embryo. This food supply is usually depleted by the time that the first few cell divisions of the embryo has occurred. During this critical period of time between the end of their stored food supply until they become photosynthetic if they are photosynthetic orchids, many are notthey are dependent upon the mycorrhizae for survival.
Most orchid seeds will not even germinate until the fungal symbiont penetrates seed coat of the seed. Because of the lack of food in the embryo of the orchid, the fungus not only supplies minerals, but also organic compounds to the orchid such as carbohydrates and possibly other metabolites such as vitamins.
Thus, it is the orchid that is deriving the carbohydrate from the fungus rather than the other way around. Unlike the other mycorrhizal fungi, these fungi digest organic materials, from the surrounding environment of the orchid, into glucose, ribose and other simple carbohydrate and these nutrients are translocated into the orchid to support their own growth.
The relationships that orchid species have with the mycorrhizal fungi are variable and is dependent on their nutritional needs. Some orchids become photosynthetic when their leaves develop while others are achlorophyllous. So those that are photosynthetic do not require the mycorrhizae at that time, but often still retains the fungal symbiont as a partner. However, the achlorophyllous species will require it even as adult plants.
Some relationship are unique and very interesting. Many orchids are epiphytes, that is they live on other plants rather than in soil, and achlorophyllous. In experiments with orchid epiphytes, it has been demonstrated that the mycorrhizal fungus on the orchid roots also acts as a parasite upon the plant which the orchid is growing. In this type of relationship, food is being transferred, by the fungus, from the tree, on which the orchid is growing, to the orchid.
This brings up another interesting point concerning orchid mycorrhizal fungi. The fungus involved is often known to be a serious pathogen to most plants, but for some reason seems to be a benefactor to the orchid. Commercially, orchids are grown with an external source of organic carbon compounds and sometimes vitamins. However, this does not work with all orchid species. Ericaceous Mycorrhizae The mycorrhiza formed in this group is between fungi in the Ascomycota, and more rarely in the Deuteromycota, and species in the families Epacridaceae, Ericaceae and Pyrolaceae.
Three subcategories are recognized, arbutoid, ericoid and monotropoid. We will briefly cover the latter two groups. Ericoid Mycorrhizae This group is probably the most important, with respect, to its potential applications. Ericoid mycorrhizae have evolved in association with plants that are continually stressed by factors within the soil. The soil is typically extremely acid, 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. Monotropoid Mycorrhizae One of the characteristics that we always 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 which are achlorophyllous. Thus, plants in these families are more dependent upon their mycorrhizal partners than plants which can carry out photosynthesis. The means by which food is obtained by these plants is similar to that of the epiphytic orchids described above.
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.
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.
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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 which we still presently use.
Although, this term is now dated, it is still used to describe the "bodies" algae, fungi and of course lichens. The only group of organisms that are still considered to be 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 taxonomist. 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 dessication, 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.
Two examples of this thallus type is Pseudocyphellaria anthraspsis and Hypogymnia imshaugii. 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. Crustose Lichens Lichen thallus which is very thin and flattened against the substrate. 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. The most prominent is Buellia aeruginascenswhich has a light, olive-green thallus with small, dark, black fruiting bodies. 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. 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.