Symbiosis: Mycorrhizae and Lichens
Plants cannot extract the necessary nitrogen from soil, so they form symbiotic relationships Describe the symbiotic relationship of mycorrhizae and plant roots. Here, we show that the presence of arbuscular mycorrhizal fungi (AMF), a dominant group of soil fungi, which form symbiotic associations with. A mycorrhiza is defined as a symbiotic relationship between the roots of plants in the soil, there is also a lot going on with respect to interaction of plant roots.
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. Soils disturbed by urbanization Urban landscaping in recent years, is challenged by climate change and urban heat islands that are generated with the use of concrete surfaces in city environments.
This means that the plants selected for these locations must be reviewed periodically to ensure they can survive in these landscaped areas with extreme temperatures, drought, human activity, etc. The soil must also be considered when it comes to plant survival, as it plays a crucial part in the retention of water and health of plants, especially for trees and shrubs, whose life cycle is longer than annuals.
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.
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. 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.
All about Mycorrhizae, its benefits, application and research and development
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.
31.3B: Mycorrhizae: The Symbiotic Relationship between Fungi and Roots
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.
In order to understand both sides of the issue, lets look at the morphology and anatomy of lichens. The Lichen Thallus In the traditional sense of lichens, their thallus can be artificially divided into four forms: Foliose Lichens Lichen thallus which is generally "leaf-like", in appearance and attached to the substrate at various points by root-like structures called rhizines.
Because of their loose attachment, they can easily be removed. These are the lichens which can generally be mistaken for bryophytes, specifically liverworts. It is possible, or even probable, that herbaria still contain lichens that have been mistakenly identified as liverworts. If we look at these a foliose lichen in longitudinal section, from top to bottom, we would be able to distinguished the following layers: Often composed of tightly interwoven mycelium, which gives it a cellular appearance.
Mycorrhiza - Wikipedia
This cellular appearance is referred to as pseudoparenchymatous. Composed of interwoven hyphae with the host algal cells.
This is the ideal location for the algal cells. Beneath the upper cortex so that it receives the optimal amount of solar radiation, for photosynthesis, but not direct solar radiation which would be harmful.
Composed of loosely interwoven mycelium.
Layer is entirely fungal. Usually same composition as the upper cortex and attached to the substrate by root-like structures called rhizines.
The rhizines are entirely fungal, in origin, and serve to anchor it to the substrate. Thus, the foliose lichens also have what is referred to as a dorsiventral thallus, i. Sectional views, illustrating how the three thallus types of lichens differ.
The entire lower surface is attached to the substrate. These lichens are so thin that they often appear to be part of the substrate on which they are growing. The following link shows an image of several lichen thalli. Crustose species that are brightly colored often give the substrate a "spray-painted" appearance. The thallus has the upper cortex, algal and medullary layers in common with the foliose lichens, but does not have a lower cortex.
The medullary layer attached directly to the substrate and the margins are attached by the upper cortex. This type of lichen is tightly flattened to its substrate and the entire lower surface medulla is attached, making it impossible to remove the thallus from its substrate. Fruticose Lichens The thallus is often composed of pendulous "hair-like or less commonly upright branches finger-like. The thallus is attached at a single point by a holdfast.
In cross section, the thallus can usually be seen to be radially symmetrical, i. The layers that can be recognized are the cortex, algal layer, medullary layer, and in some species the center has a "cord" which is composed of tightly interwoven mycelium.
Other species have a hollow center that lack this central cord. Fructicose lichen thallus is attached to its substrate at a single point, but finding that point is not that easy! Biology of Lichens In looking at the anatomy of the lichen, it is obvious that there is interaction between the phycobiont and mycobiont, but what kind of interaction is occurring.
One school of thou0ght is that the alga produces the food material and that the fungus protects alga from desiccation, high light intensities, mechanical injuries and provides it with water and minerals. This is the reasoning that many introductory text books have adopted and they define a lichen as a mutualistic symbiosis.
However, in studies that have been done that examines the alga-fungus interface, it can be clearly seen that haustoria, specialized feeding structures present in parasitic fungi, penetrate the alga cells. Thus, many lichenologist have defined this relationship as a controlled form of parasitism. There is more evidence and I would like to go over some of these. Illustration of haustoria penetrating algal cells give evidence that the lichen symbiosis is really a controlled form of parasitism.
But if one of the young conifers can get an infusion of that element through hyphae linked to an alder or birch tree, whose roots host symbiotic nitrogen-fixing bacteria, that particular sapling may be good to go. Make that good to grow. If hyphae from the impoverished plant only reach the soil near the second plant, this can be enough. Some farmers might have guessed that the roots of one plant borrowed good stuff from the soil around another, but nobody was aware of the bacteria in nodes on the legume roots making the nitrogen available or aware of the mycorrhizal hyphae gathering it.
They just knew the maize grew better. They offer packets and jars of inoculants to treat roots or seeds prior to planting and larger quantities for broadcasting onto croplands, especially those whose mycelial structures have been disrupted by chemical treatments, over-tilling or compaction from trampling.
To learn more gardening with mycorrhizal fungi in mind, read Mycorrhizal Fungi: It will be a microbe, single-celled algae or else cyanobacteria, which can convert sunlight to energy as well. Some fungi partner with both types at once. As in a mycorrhiza, the fungus takes a share of the sugars produced by its solar-powered collaborator. Cyanobacteria also fix nitrogen, making that available to any resident algae as well as to the fungus. The fungus meanwhile shelters the partner cells nested among its filaments and keeps them moist by absorbing water from rain, mists, and dew.
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.