Mycorrhizal Fungi and Plant Roots | MOTHER EARTH NEWS
Mycorrhizal fungi are a heterogeneous group of diverse fungal taxa, has revealed some of the cellular processes that underlie symbiosis. Mycorrhizal fungi have existed since the first plants appeared on dry land more than million years ago. They form a close symbiotic relationship with plant. A mycorrhiza is a symbiotic association between a fungus and the roots of a.
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. 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.
WHAT ARE MYCORRHIZAE?
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.Mycorrhizal Fungi Animation
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.
Arbuscular mycorrhizal fungi in terms of symbiosis-parasitism continuum.
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.
Mycorrhizal Fungi and Plant Roots: A Symbiotic Relationship
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.
- 31.3B: Mycorrhizae: The Symbiotic Relationship between Fungi and Roots
- Arbuscular mycorrhizal fungi in terms of symbiosis-parasitism continuum.
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. 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. 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.
All about Mycorrhizae, its benefits, application and research and development
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 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.
There is no periradical phase and the extraradical phase consists of sparse hyphae that don't extend very far into the surrounding soil. They might form sporocarps probably in the form of small cupsbut their reproductive biology is little understood. It is however different from ericoid mycorrhiza and resembles ectomycorrhiza, both functionally and in terms of the fungi involved. Myco-heterotrophy This type of mycorrhiza occurs in the subfamily Monotropoideae of the Ericaceaeas well as several genera in the Orchidaceae.
These plants are heterotrophic or mixotrophic and derive their carbon from the fungus partner. This is thus a non-mutualistic, parasitic type of mycorrhizal symbiosis. Orchid mycorrhiza All orchids are myco-heterotrophic at some stage during their lifecycle and form orchid mycorrhizas with a range of basidiomycete fungi.
In such a relationship, both the plants themselves and those parts of the roots that host the fungi, are said to be mycorrhizal. The Orchidaceae are notorious as a family in which the absence of the correct mycorrhizae is fatal even to germinating seeds. This relationship was noted when mycorrhizal fungi were unexpectedly found to be hoarding nitrogen from plant roots in times of nitrogen scarcity. Researchers argue that some mycorrhizae distribute nutrients based upon the environment with surrounding plants and other mycorrhizae.
They go on to explain how this updated model could explain why mycorrhizae do not alleviate plant nitrogen limitation, and why plants can switch abruptly from a mixed strategy with both mycorrhizal and nonmycorrhizal roots to a purely mycorrhizal strategy as soil nitrogen availability declines.
On the right side of this diagram, the arbuscular mycorrhiza pathway, which branches off from the plant root, which is the brown cylinder-like figure in the image, provides the plant with nutrients, including, most importantly, phosphate and nitrogen. My reference source for this information is: In return, the plant gains the benefits of the mycelium 's higher absorptive capacity for water and mineral nutrients, partly because of the large surface area of fungal hyphae, which are much longer and finer than plant root hairsand partly because some such fungi can mobilize soil minerals unavailable to the plants' roots.
The effect is thus to improve the plant's mineral absorption capabilities. 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.