Symbiosis: Mycorrhizae and Lichens
This may be the best studied ecological group of fungi because of the agricultural significance. Among the less devastating crop plant pathogens is Ustilago. Two important symbioses involve fungi: the mycorrhizae that occur on the roots of almost all vascular plants and the lichens that have evolved entirely differen. Mycorrhizas are symbiotic relationships between certain fungi and the roots of plants. The fine fungal threads (called hyphae) either ensheath or penetrate the.
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.
Another form of immobilisation is when nutrients are locked up in organic matter that is slow to decay, such as wood, and some mycorrhizal fungi act directly as decay organisms, mobilising the nutrients and passing some onto the host plants; for example, in some dystrophic forests, large amounts of phosphate and other nutrients are taken up by mycorrhizal hyphae acting directly on leaf litter, bypassing the need for soil uptake.
These structures have been shown to host nitrogen fixing bacteria which contribute a significant amount of nitrogen and allow the pines to colonize nutrient-poor sites. Physically, most mycorrhizal mycelia are much smaller in diameter than the smallest root or root hair, and thus can explore soil material that roots and root hairs cannot reach, and provide a larger surface area for absorption. Chemically, the cell membrane chemistry of fungi differs from that of plants.
For example, they may secrete organic acid that dissolve or chelate many ions, or release them from minerals by ion exchange. These associations have been found to assist in plant defense both above and belowground. Mycorrhizas have been found to excrete enzymes that are toxic to soil borne organisms such as nematodes. When this association is formed a defense response is activated similarly to the response that occurs when the plant is under attack.
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. 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 rootit would be here where these large somewhat circular cells are. 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: 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. The infected roots are very distinctive, forming short, paired, branches Fig. Infected roots from pine.
Note the distinctive, short, pairs of 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.
Fungi Symbiosis ( Read ) | Biology | CK Foundation
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 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 ectomycorrhizal 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 is mycorrhizal, 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.
Mycorrhiza - Wikipedia
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 causing it to have branching seen here.
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.
Mycorrhizal Fungi and Plant Roots: A Symbiotic Relationship
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. 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.