Symbiosis: Mycorrhizae and Lichens
There are many species of lichen within the Caledonian Forest. Among Mycorrhizas are symbiotic relationships between certain fungi and the roots of plants. Symbiosis in lichens is the mutually helpful symbiotic relationship of green algae and/or blue-green algae (cyanobacteria) living among filaments of a fungus. Lichens, on the other hand, are composite organisms consisting of a symbiotic relationship between an alga and a fungus. Lichens are epiphytic, which means .
Other lichen fungi occur in only five orders in which all members are engaged in this habit Orders GraphidalesGyalectalesPeltigeralesPertusarialesand Teloschistales.
Lichenized and nonlichenized fungi can even be found in the same genus or species. TrebouxiophyceaePhaeophyceaeChlorophyceae have been found to associate with the lichen-forming fungi. One fungus, for example, can form lichens with a variety of different algae.
The thalli produced by a given fungal symbiont with its differing partners will be similar, and the secondary metabolites identical, indicating that the fungus has the dominant role in determining the morphology of the lichen. Further, the same algal species can occur in association with different fungal partners. Lichens are known in which there is one fungus associated with two or even three algal species. Rarely, the reverse can occur, and two or more fungal species can interact to form the same lichen.
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. 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.Lichens - Millie Davenport
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. 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.
Symbiosis in lichens
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.
Lichens and trees
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.
Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by an haustorium. Beneath this algal layer is a third layer of loosely interwoven fungal hyphae without algal cells. This layer is called the medulla.
Beneath the medulla, the bottom surface resembles the upper surface and is called the lower cortex, again consisting of densely packed fungal hyphae. The lower cortex often bears root-like fungal structures known as rhizines, which serve to attach the thallus to the substrate on which it grows. Lichens sometimes also contain structures made from fungal metabolites, for example crustose lichens sometimes have a polysaccharide layer in the cortex. Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the fungal component may consist of more than one genetic individual of that species.
This seems to also be true of the photobiont species involved. Reproduction Thalli and apothecia on a foliose lichen Many lichens reproduce asexuallyeither by vegetative reproduction or through the dispersal of diaspores containing algal and fungal cells.
Soredia singular soredium are small groups of algal cells surrounded by fungal filaments that form in structures called soralia, from which the soredia can be dispersed by wind. Another form of diaspore are isidia, elongated outgrowths from the thallus that break off for mechanical dispersal.
Fruticose lichens in particular can easily fragment. Due to the relative lack of differentiation in the thallus, the line between diaspore formation and vegetative reproduction is often blurred. Many lichens break up into fragments when they dry, dispersing themselves by wind action, to resume growth when moisture returns.
Many lichen fungi appear to reproduce sexually in a manner typical of fungiproducing spores that are presumably the result of sexual fusion and meiosis. Following dispersal, such fungal spores must meet with a compatible algal partner before a functional lichen can form.
Fungi Symbiosis ( Read ) | Biology | CK Foundation
This may be a common form of reproduction in basidiolichens, which form fruitbodies resembling their nonlichenized relatives. Among the ascolichens, spores are produced in spore-producing bodies, the three most common spore body types are the apothecia, perithecia, and the pycnidia.
Ecology Crustose and foliose lichens on a wall Lichens are often the first to settle in places lacking soilconstituting the sole vegetation in some extreme environments, such as those found at high mountain elevations and at high latitudes.
Some survive in the tough conditions of deserts, and others on frozen soil of the arctic regions. Recent ESA research shows that lichen can even endure extended exposure to space. Lichens must compete with plants for access to sunlight, but because of their small size and slow growth, they thrive in places where higher plants have difficulty growing.
A major ecophysiological advantage of lichens is that they are poikilohydric poikilo—variable, hydric—relating to watermeaning that though they have little control over the status of their hydration, they can tolerate irregular and extended periods of severe desiccation.
Like some mossesliverwortsfernsand a few "resurrection plants," upon desiccation, lichens enter a metabolic suspension or stasis known as cryptobiosis in which the cells of the lichen symbionts are dehydrated to a degree that halts most biochemical activity. In this cryptobiotic state, lichens can survive wider extremes of temperature, radiation, and drought in the harsh environments they often inhabit.
Lichens do not have roots and do not need to tap continuous reservoirs of water like most higher plants. Thus, they can grow in locations impossible for most plants, such as bare rock, sterile soil or sand, and various artificial structures such as walls, roofs, and monuments. Many lichens also grow as epiphytes epi—on the surface, phyte—plant on other plants, particularly on the trunks and branches of trees.
When growing on other plants, lichens are not parasites ; they do not consume any part of the plant nor poison it. Some ground-dwelling lichens, such as members of genus Cladina reindeer lichenshowever, produce chemicals which leach into the soil and inhibit the germination of plant seeds and growth of young plants. Stability that is, longevity of their substratum is a major factor of lichen habitats. Most lichens grow on stable rock surfaces or the bark of old trees, but many others grow on soil and sand.
In these latter cases, lichens are often an important part of soil stabilization; indeed, in some desert ecosystems, vascular higher plant seeds cannot become established except in places where lichen crusts stabilize the sand and help retain water.
Pine forest with lichen ground-cover When growing on mineral surfaces, some lichens slowly decompose their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of weathering by which rocks are gradually turned into soil. While this contribution to weathering is usually benign, it can cause problems for artificial stone structures.
For example, there is an ongoing lichen growth problem on Mount Rushmore National Memorial that requires the employment of mountain-climbing conservators to clean the monument. Lichens may be eaten by some animals, such as reindeerliving in arctic regions.