Fungi Symbiosis ( Read ) | Biology | CK Foundation
A mycorrhiza is defined as a symbiotic relationship between the roots of plants and fungi. The term mycorrhiza literally means root fungus, but in the broad sense. Because nutrients are often depleted in the soil, most plants form symbiotic relationships called mycorrhizae with fungi that integrate into the plant's root. One important set of survival mechanisms involves creating mutually beneficial ( symbiotic) relationships between plant roots and soil-borne organisms such as.
Mycorrhizae and Plants Make Great Allies Friday, October 5, Ed Bloodnick In undisturbed soils, mycorrhizal fungi are a major component of the soil's natural microflora, but their presence is not guaranteed in all soils when transplanting new plants in a garden bed or in newly landscaped area. Sometimes, nature needs a little help.
Mycorrhizal fungi live in symbiosis with the roots of a large variety of plants, including trees, shrubs, annuals and perennials. These microscopic fungi form a symbiotic association with the plant's root system in which it forms a network of extremely fine filaments, called mycelium, uniting the two partners appears Figure 1.
This union, called "mycorrhiza" "myco" for fungus and "rhiza" for rootallows both partners to form a mutual exchange network at the root zone level to support the plant in a cultural environment that can sometimes be hostile. Picture of roots larger structures with mycorrhizal network smaller threads. Mycorrhizal fungi are microscopic so this picture is enlarged 40 times with a microscope. Premier Tech Horticulture" In this symbiotic relationship, the mycorrhizal network draws nutrients from the soil for plant roots, which would sometimes be inaccessible without the help of this invaluable ally.
Mycelium bring the plant the nutrients it needs to develop properly, elements such as phosphorus copper and zinc, which aren't very mobile in the soil. They also draw water from within the soil's micropores, which would otherwise be inaccessible by the root. Thus, roots colonized by mycorrhizae enables the plant to be better protected and to resist the stress caused by transplanting, drought and heat, while maintaining an optimal growth rate. Mycorrhizal fungi also receive benefit from symbiosis with the plant.
Due to the plants ability to photosynthesize, the plant synthesizes carbon-based substances sugarswhich feeds the fungi. You may say there is an exchange of services between the fungus and the plant. Cattle egret usually associated with cattle or with other large herbivores that while foraging for food, will expose insects that the cattle egret will fit on. The cattle egret benefits from this relationship, but the large herbivore does not, nor is it harmed by the relationship.
A relationship in which one species parasite is obligately dependent upon another organism host for its food and shelter. We have had several examples of this type of relationship, during the course of the semester. For our purpose, symbiosis will be used here in its most common sense, where there is mutual benefit in the relationship.
The two most common example in fungi are mycorrhizae and lichens, which we will cover, today. The subject of symbiosis is usually more scholarly than applicable, but in the case of mycorrhizae, you will see that both scholarly as well as applied research have been carried out.
A mycorrhiza is defined as a 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 Psilotum and bryophytes mosses and liverworts.
A common impression, among non-botanist is if plants are in an area with rich soil and have enough water and sunshine that they will grow well. Although this may be true, this is usually not the case. In fact, this is rarely true in nature. Just as there is a lot happening in the recycling of nutrients, in the soil, there is also a lot going on with respect to interaction of plant roots with other microorganisms.
In the case of mycorrhizal relationships, we are actually talking about a number of different types of relationships. Another words, there are different categories of mycorrhizae. However, in the most common types, the fungus will usually receive carbohydrates of some sort from the plant and there will be enhancement of mineral transport to the plant.
You should recall that in order for plants to grow normally, they require certain essential elements, and I will not review those elements at this time since knowing what they are is really not essential in understanding the concept of mycorrhizae. Generally, in nature, the soil composition is often deficient in one to several essential elements that are required by plants, and it is thought that because the mycelium of the fungus is more extensive than even the roots of the host plant, in the soil, the fungus is able to enhance nutrient uptake for the plant.
Ironically, it is in nutrient rich soil, such as agricultural soil, that plant sometimes do not grow better with a mycorrhizal fungus, but instead the plant may even reject the fungus. In addition to the enhanced nutrient uptake, different categories of mycorrhizae may protect roots against pathogens, produce plant hormones and translocate carbohydrates between plants.
However, there are some generalizations that can be made, concerning mycorrhizae: Mycorrhiza infection area occurs only on the smallest order of secondary roots.
These are the root tips that are still growing, elongating and increasing in girth.
Mycorrhizae and Plants Make Great Allies
So we are talking about just a very small part of the root system of a plant which will be infected by the mycorrhizal fungus. This makes a great deal of sense since this is the only part of the root system that will absorb water and minerals.
However, as I just mentioned, the fungus has a much more extensive growth in the soil. In all mycorrhizae only the cortical cells of the root are invaded by the fungus. This is the area of the root between the epidermis and the vascular tissue of the root. If we look at the cross section of a young root, it would be here where these large somewhat circular cells are. Cross section of a root: Outer most layer is the mantle layer of mycelium.
The round cells are the cortical cells and if you look closely, where the fungus mycelium is growing between the cortical cells, but not penetrating them, is the Hartig net.
All other families form mycorrhizae. It is believed that for many plants that usually form mycorrhizae, they would be unable to survive in their natural habitat without this symbiotic relationship. This has been demonstrated to be true for numerous plants. Types of mycorrhizae recognized can be divided into three categories: Mycelium sheath around root is reduced, or may even be absent, but Hartig Net is usually well developed as in ectomycorrhizae, but the hyphal cells may penetrate the cortical cells as in endomycorrhizae.
However, because of similarities to ectomycorrhizae, they will not specifically be considered here. Description of mycorrhizae types Ectomycorrhizae This category of mycorrhiza is very uniform in appearance, and biologically identical despite having literally thousands of different species fungi, in the Ascomycota and Basidiomycota.
For this reason, it is not subdivided into further subcategories as in endomycorrhizae. It is referred to as "ecto-" because the fungal symbiont does not invade the cell protoplasm.
However, the fungus does form a thick sheath around the root tip and mycelium also grows between the cells of the cortex forming the so-called Hartig net. The infected roots are very distinctive, forming short, paired, branches.
While there are a large number of fungi that are ectomycorrhizae, plants that have ectomycorrhizae are restricted to only a few families of plants, and these plants are always trees.
They are also more common in temperate regions than in the tropics. This type of mycorrhiza is very important in forestry because its association with trees.
In this type of mycorrhiza, the fungal sheath, that forms around the secondary root tips, accumulate minerals from the decomposing litter, before they are able to pass into the deeper mineral layers of the soil where they would be unavailable to the roots. Thus, mycorrhizal fungi are also decomposers as well. Fungus does obtain simple carbohydrates that are produced by the plant, but not used by the plant.
So it appears that these carbohydrates may be produced by the plant specifically for the fungus since they are not utilized by the plant. Fungi involved are members of the Basidiomycota and the Ascomycota.
Also, they are usually species that form large fruitbodies, such as mushrooms, puffballs, truffles, etc. From many years of observations, consistent association could be seen of certain species of trees with certain species of fungi that produce fruitbodies.
This type of mycorrhiza was discovered first for this reason. Although we can grow the mycelium of many ectomycorrhizae fungi in an artificial medium, e. It has been demonstrated that unknown growth factors exuded by the roots seems to stimulate mycelial growth. There is undoubtedly many more factors involved, with regards to growth of the fungi, that are yet unknown.
Formation of fruiting bodies in artificial media also has never been accomplished. This was the reason why "cultivation" of truffles, e. Tuber melanosporum, which form mycorrhizae, requires planting of the host trees that have been inoculated with the fungus in order to obtain fruitbodies. The ectomycorrhizal root that is formed has a morphology that is distinct from that of uninfected roots. One distinctive characteristic of the infected root tips is that they lack root hairs.
This is unusual because root hairs are normally presence, in abundance, in the young root. This morphology is in part due to the fungus secreting auxin, a plant hormone, that acts upon the root development and in the case of gymnosperms, form, thick dichotomous branches. Branching of the root system will differ with different plant families. Ectomycorrhiza of Amanita and Pinus root, from http: Figure of section through root, showing external mantle of hyphae and Hartig net.
Cross section of arbutoid mycorrhiza, showing external mantle of hyphae and Hartig net, from http: 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. Economic Relevance Plants that are involved in ectomycorrhizae are always trees and are found only in a few families.
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.
Mycorrhizal Fungi and Plant Roots: A Symbiotic Relationship
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.
Arbuscular Mycorrhizae 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 called arbuscules that can be seen inside the cells of infected roots. These structures can be recognized by their branched tree-like appearance.
Another structure that can be frequently observed are the rounded vesicles. 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.
There is also extensive mycelium in the soil, but do not appear to be organized in any fashion. Vesicles in roots cells of Sesbania sp. Note some vesicles have been displaced from cells due to preparation of slide. Arbuscule in root cell.
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. Left plant with and right without mycorrhiza. Left plant without and right with mycorrhiza, respectively.
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.Mycorrhiza and the symbiotic relationship between fungi and plants.
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.
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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. 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. Arbutoid Mycorrhiza This group forms associations with plants that are trees and shrubs that belong to the genera Arbutus madroneArctostaphylos 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 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.
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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.