Orchid Mycorrhizae. Orchid mycorrhiza is endomycorrhizal and have fungal partners that are saprotrophic or pathogenic species of Basidiomycota, but a some are ectomycorrhizae, e.
All orchids must form mycorrhizae. 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, the fungal symbiont colonizes the plant shortly after seed germination and form characteristic, coiled hyphae within the cortical cells of the root.
The hyphae in the host cells collapse or are digested by the host that will supply the embryo with its carbon source and vitamins until it is able to photosynthesize. Unlike other mycorrhizal fungi, orchid mycorrhizal fungi can also 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 growth.
The relationships that orchid species have with the mycorrhizal fungi are variable and is dependent on their nutritional needs. Those orchids that are photosynthetic still retain their fungal partners, but it is not clear as to what role it is playing.
However, the achlorophyllous orchids will require it even as adult plants. In these species the associate fungus forms a tripartate relationship , where the fungus also forms a relationship with a photosynthetic plant and channel its nutrient to the orchid. The fungus will also supply both plants with inorganic nutrients.
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. This group forms associations with plants that are trees and shrubs that belong to the genera Arbutus madrone , Arctostaphylos manzanita and Arctous alpinus mountain bearberry. They have characteristics that are both ecto- and endo-mycorrhizae: There is a formation of an external mantle of mycelium that forms a hartig's net, as in ectomycorrhiza, but intracelllar penetration of cortical cells occurs as in endomycorrhiza.
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 heather , Rhododendron , Azaleas and Vaccinium blueberries , of 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.
The host cell dies as the association disintegrates, thereby restricting the functional life, i. 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.
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. 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 to , organisms such as algae, bacteria and fungi, were included in the plant kingdom. In , Whitaker, 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. In the traditional sense of lichens, their thallus can be artificially divided into four forms: foliose , crustose and fruticose. 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:.
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. 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. 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. 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.
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. Fructicose lichens : Forming much branched shrub like bodies and remaining attached to the substratum by their narrow basal portion. In some lichens the algal body remains scattered in the thallus, while in other they occur in one or two layers.
For examples, Cladonia, Evernia, Usnea, etc. Reproduction in Lichens Lichens reproduce by following three methods: Vegetative reproduction Asexual reproduction Sexual reproduction Vegetative reproduction : Vegetative reproduction is a process in which new offspring produces through the body of single organism. The different types of vegetative reproduction takes place in lichens are by soredia, isidia and cephalodia.
Asexual reproduction : Asexual reproduction takes place by the formation of spores formed by the fungal partner. Each spore on germination sends hyphae in all directions and spore comes in contact with the requisite alga, spore branches and covers up the algal cell.
This combined body forms a lichen thallus. In many lichens, a flask shaped cavity called pycnidium is formed, from where spore like pycnidiospores develop and when they come in contact with the requisite alga, they form new thallus.
Sexual reproduction Sexual reproduction takes place in certain ascolichens. The fruiting body is either a cup shaped apothecium or a flask shaped perithecium. Sex organs are well differentiated, the male is flask shaped chamber called spermogonium, within numerous, minute, non motile cellsa are formed. Female organ is called as carpogonium.
Carpogonium consists of basal swollen portion called the ascogonium, and a upper tubular portion called the trichogyne. The male and female sex organs on maturation undergo the process of fusion. As a result of the fertilization and further growth the final product, i. Useful activities of Lichens The uses of Lichens are given below: Some species of lichen are used as food by human beings and animals, E. The lichens etch the rocks, through the both chemical and physical actions, there after the plants like mosses and grasses come in succession and utilize the first soil formed by lichens.
So, are called soil builders. Many useful medicines are obtained from lichen. Some lichens are used in making perfume. Some are very sensitive to pollution and have been used as environmental indicators. Lichens have a body called a thallus, an outer, tightly packed fungal layer called a cortex , and an inner, loosely packed fungal layer called a medulla Figure 1. Lichens use hyphal bundles called rhizines to attach to the substrate.
Lichens are classified as fungi and the fungal partners belong to the Ascomycota and Basidiomycota. Lichens can also be grouped into types based on their morphology. There are three major types of lichens, although other types exist as well.
Lichens that are tightly attached to the substrate, giving them a crusty appearance, are called crustose lichens. Those that have leaf-like lobes are foliose lichens ; they may only be attached at one point in the growth form, and they also have a second cortex below the medulla.
Finally, fruticose lichens have rounded structures and an overall branched appearance. Figure 2 shows an example of each of the forms of lichens. Figure 2. Examples of the three types of lichens are shown here. The doctor explains that ringworm is a general term for a condition caused by multiple species. The first step is to take a scraping for examination under the microscope, which the doctor has already done.
He explains that he has identified the infection as a fungus, and that the antifungal cream works against the most common fungi associated with ringworm. However, the cream may not work against some species of fungus. If the cream is not working after a couple of weeks, Anthony should come in for another visit, at which time the doctor will take steps to identify the species of the fungus.
Positive identification of dermatophytes requires culturing. These features suggest that the fungus is Trichophyton rubrum , a common cause of ringworm.
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