Lichens | Herbarium | USU
Mycorrhizal Fungi and Plant Roots: A Symbiotic Relationship Lichens from Antarctica survived 34 days in a laboratory setting designed to simulate the. A mutualistic relationship is the win-win type of symbiosis in which both Thanks to its mycorrhizal partner, the chlorophyll-deficient Bird's nest orchid (Neottia) Lichens: symbiotic organisms composed of fungi and algae. If both partners benefit, the relationship is called mutualism. Lichens and mycorrhiza are examples of mutualists. Lichens. A lichen consists of a fungus and a.
Swiss botanist Simon Schwendener proposed in that this combination of creatures represented a symbiotic relationship. It earned him years of scorn from prominent lichenologists. It was more like a creed — a projection of the human sense of individual identity in Western culture.
As ofthousands of species of lichens have been identified. Their nature as a sort of biological alloy makes them tremendously self-sufficient and able to inhabit extreme environments. Lichens from Antarctica survived 34 days in a laboratory setting designed to simulate the environment on Mars.
For that matter, lichens have been shot into orbit and placed outside a spacecraft in a container that was then opened, directly exposing those composite creatures to the flash-freezing temperatures and cosmic radiation of space for 15 days. Upon returning to Mother Earth, they simply resumed growing! You just have to imagine the plants as equivalent to the single cells of symbiotic algae — big algae poking into the air above ground while enwrapped in a mesh of fungal threads below.
I am You, and You Are Me Perhaps this is where we should shift our gaze from other species to the one calling itself Homo sapiens. Some are harmless hitchhikers, but most are symbionts that contribute to our well-being. Roughly 30, species — primarily bacteria but also archaea, protists, and fungi mostly in the form of yeasts — typically inhabit the human stomach and intestinal tract.
Still others congregate on our skin and in its pores, in the conjunctiva of our eyes, and in …. People are increasingly aware of these facts nowadays. Yet the human-microbe symbiosis goes way deeper. Every cell in every plant and animal, many protists, and all fungi contains organelles known as mitochondria.
Commonly described as the power sources of the cell, they build the molecule ATP adenosine triphosphatewhose complex bonds, when broken, release the energy needed to drive other cellular functions. These organelles also reproduce on their own by splitting, just as bacteria do. It probably began with the bigger cell engulfing a bacterium to eat it. That combination became the primordial line that ultimately led to the larger life forms we know today.
Plants have an additional type of organelle in their cells: That in turn fuels the construction of sugars from ordinary carbon dioxide and water, with oxygen given off as a byproduct. About 20 species in the tropical and temperate rain forests are Basidiomycetes, the "mushrooms". About 40 genera of algae and cyanobacteria are found in lichen partnerships. How do Lichens Grow? They also provide vitamins to the fungus. Cyanobacteria can make amino acids directly from the nitrogen gas in the atmosphere, something neither fungi nor algae can do.
The fungus, in turn, protects its partners from drying out and shades them from strong sunlight by enclosing the photosynthesizing partners within the body of the lichen. This life habit has allowed lichens to successfully colonize many different habitats. Lichens have a truly remarkable resistance to drought. A dry lichen can quickly absorb from 3 to 35 times its weight in water!
Lichens can also absorb moisture from dew or fog, even from the air itself if the humidity is very high and the temperature is low. They also dry out slowly, making it possible for the photosynthesizing partner s to make food for as long as possible.
This ability to quickly absorb and retain water from many sources makes it possible for lichens to live in harsh environments like deserts and polar regions, and on exposed surfaces like bare rocks, roofs and tree branches. The thallus, or lichen body, comes in four shapes: Most lichens grow slowly, probably because they live in environments where water is available for only short periods.
They tend to live for many years, and lichens hundred of years old can be used to date the rock surfaces on which they grow. Lichens spread mostly by small pieces of their body being blown around. 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.
Mutualistic Fungi: Lichens and Mycorrhizae
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.
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. 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.
Mycorrhizal Fungi and Plant Roots: A Symbiotic Relationship
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.