Orchid mycorrhiza - Wikipedia
Mycorrhizae: The Symbiotic Relationship between Fungi and Roots. Many plants form Mycorrhizae function as a physical barrier to pathogens. They also. Mycorrhizae are considered to be a mutualistic relationship because both In this association the fungus occurs inside the cells of the plant root as a highly . roots first with a shovel or trowel you will leave all of the hair roots in the ground. The two most common example in fungi are mycorrhizae and lichens, which we is defined as a symbiotic relationship between the roots of plants and fungi. . One distinctive characteristic of the infected root tips is that they lack root hairs.
Branching of the root system will differ with different plant families. The ectendomycorrhizae morphology is like that of the ectomycorrhizae, i. The only real morphological difference is that the host roots cells are penetrated by hyphal cell of fungus. Also, the fungi involved have not been identified.
Most of these are utilized as a source of lumber, and in the case of the Pine family, millions of trees are used annually, this time of year, as Christmas trees.
When planting these trees, it is a routine practice, in forestry, to inoculate the seedling with a mycorrhizal fungus. This group of mycorrhiza have also been tested as a means of resisting fungal, root pathogens. It was reasoned that if the fungal sheath of the ectomycorrhizal fungus is covering the root tips, fungal root pathogens would be unable to gain entry into the root system of the host.
Endomycorrhizae Although far less conspicuous because they do not produce large fruiting bodies, such as mushrooms, this category of mycorrhiza is far more common than the ectomycorrhizal type. Generally, it can be said that plants that do not form ectomycorrhizae will be the ones that form endomycorrhizae. 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. Note some vesicles have been displaced from cells due to preparation of slide.
Arbuscule in root cell.
Plant Production and Protection Division: Microsymbionts
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. 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.
Mycorrhizal Fungi and Plant Roots: A Symbiotic Relationship
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. 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.Mycorrhizal Fungi Animation
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.
Below, we consider three of the commonest types of mycorrhiza. Each of them represents a distinctive type of association.
Orchid mycorrhizas Some types of orchid are non-photosynthetic; others only produce chlorophyll when they have grown past the seedling stage. In all cases, the plant depends on sugars derived from a fungal partner for at least part of its life.
The minute orchid seeds, with negligible nutrient reserves, will not germinate unless a fungus infects them, although the seeds can germinate aseptically if supplied with the 'fungal sugar' trehalose.
- Orchid mycorrhiza
These mycorrhizas are unusual because, in effect, the plant parasitises the fungus that invades it. The fungi in these associations resemble the common plant pathogen Rhizoctonia solani, but recent taxonomic studies have assigned them to several related genera. They are mainly saprotrophic - they grow by degrading organic matter in soil - but they might obtain trace elements or some other factor from the plant.
An interesting variation on this theme is shown by some orchids and other non-photosynthetic plants Monotropa species that have wood-rotting species of Armillaria as their mycorrhizal symbionts. In some of these cases the fungus can even be a pathogen of tree roots, so that the non-photosynthetic plant gains its nutrients by indirect parasitism of a living tree see Armillaria.
Figure A above shows a cross-section of the outer part of the protocorm of an orchid, Neottia, stained to reveal the masses of fungal hyphae arrowheads. Figure B shows part of a section at much higher magnification.
The cells of the orchid are filled with coils of fungal hyphae but, significantly, the plant cells are still alive and they contain nuclei n. The fungal coils will only last a few days or weeks before they are digested those in the nucleate cell on the right of Figure B appear to be degenerate and the process of invasion and digestion will begin again.
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.
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.