Arbuscular vs Ectomycorrhizal fungi: limiting or stimulating tree mutualistic relation with the host plant, receiving carbohydrates from the plant. An arbuscular mycorrhiza is a type of mycorrhiza in which the fungus (AM fungi, or AMF) It is believed that the development of the arbuscular mycorrhizal symbiosis played a crucial role in the initial colonisation This symbiosis is a highly evolved mutualistic relationship found between fungi and plants, the most prevalent. We studied the relationships of plant and AMF (arbuscular mycorrhizal fungi) species richness and community composition to each other and.
There is some fossil evidence that suggests that the parasitic fungi did not kill the host cells immediately upon invasion, although a response to the invasion was observed in the host cells.
This response may have evolved into the chemical signaling processes required for symbiosis. Molecular evidence[ edit ] Increased interest in mycorrhizal symbiosis and the development of sophisticated molecular techniques has led to the rapid development of genetic evidence.
Arbuscular mycorrhiza - Wikipedia
This implies that mycorrhizal genes must have been present in the common ancestor of land plants, and that they must have been vertically inherited since plants colonized land. At the same time, Geosiphon-Nostoc symbiosis was reported previously.
The characterized AMF frq gene by same research  is the first frq gene identified outgroup of Dikarya, which suggest the frq gene evolution in fungal kingdom is much older than previously investigated. Presymbiosis[ edit ] The development of AM fungi prior to root colonization, known as presymbiosis, consists of three stages: Spore germination Spores of the AM fungi are thick-walled multi-nucleate resting structures.
However, the rate of germination can be increased by host root exudates. A concentration of 10 mM phosphorus inhibited both hyphal growth and branching. This phosphorus concentration occurs in natural soil conditions and could thus contribute to reduced mycorrhizal colonization. Spores of Gigaspora margarita were grown in host plant exudates. Hyphae of fungi grown in the exudates from roots starved of phosphorus grew more and produced tertiary branches compared to those grown in exudates from plants given adequate phosphorus.
When the growth-promoting root exudates were added in low concentration, the AM fungi produced scattered long branches. As the concentration of exudates was increased, the fungi produced more tightly clustered branches. At the highest-concentration arbuscules, the AMF structures of phosphorus exchange were formed. Spores of Glomus mosseae were separated from the roots of a host plant, nonhost plants, and dead host plant by a membrane permeable only to hyphae.
In it was shown how the AM undergoes physiological changes in the presence of exudates from potential host plant roots, to colonize it. Host plant root exudates trigger and turn on AM fungal genes required for the respiration of spore carbon compounds. In experiments, transcription rate of 10 genes increased half-hour after exposure and at an even greater rate after 1 hour. Genes isolated from that time are involved in mitochondrial activity and enzyme production.
Arbuscular mycorrhizal fungi in terms of symbiosis-parasitism continuum.
It may be part of a fungal regulatory mechanism that conserves spore energy for efficient growth and the hyphal branching upon receiving signals from a potential host plant. From this structure hyphae can penetrate into the host's parenchyma cortex. However, the hyphae did not further penetrate the cells and grow in toward the root cortex, which indicates that signaling between symbionts is required for further growth once appressoria are formed.
Arbuscules are the sites of exchange for phosphorus, carbon, water, and other nutrients. Paris type is characterized by the growth of hyphae from one cell to the next; and Arum type is characterized by the growth of hyphae in the space between plant cells. There is a decondensation of the plant's chromatinwhich indicates increased transcription of the plant's DNA in arbuscule-containing cells.
The vacuoles shrink and other cellular organelles proliferate. The plant cell cytoskeleton is reorganized around the arbuscules. There are two other types of hyphae that originate from the colonized host plant root.
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Once colonization has occurred, short-lived runner hyphae grow from the plant root into the soil. These are the hyphae that take up phosphorus and micronutrients, which are conferred to the plant. AM fungal hyphae have a high surface-to-volume ratio, making their absorptive ability greater than that of plant roots. The four types of hyphae are morphologically distinct.
They have limited saprobic ability and depend on the plant for their carbon nutrition.
Carbon transfer from plant to fungi may occur through the arbuscules or intraradical hyphae. Inside the mycelium, hexose is converted to trehalose and glycogen.
Trehalose and glycogen are carbon storage forms that can be rapidly synthesized and degraded and may buffer the intracellular sugar concentrations. Lipid biosynthesis also occurs in the intraradical mycelium. Lipids are then stored or exported to extraradical hyphae where they may be stored or metabolized. The breakdown of lipids into hexoses, known as gluconeogenesisoccurs in the extraradical mycelium.
Increasing the plant's carbon supply to the AM fungi increases uptake and transfer of phosphorus from fungi to plant  Likewise, phosphorus uptake and transfer is lowered when the photosynthate supplied to the fungi is decreased. Species of AMF differ in their abilities to supply the plant with phosphorus. This may be due to increased surface area in contact with soil, increased movement of nutrients into mycorrhizae, a modified root environment, and increased storage. Phosphorus travels to the root or via diffusion and hyphae reduce the distance required for diffusion, thus increasing uptake.
The rate of phosphorus flowing into mycorrhizae can be up to six times that of the root hairs. While significant advances have been made in elucidating the mechanisms of this complex interaction, much investigation remains to be done. Mycorrhizal activity increases the phosphorus concentration available in the rhizosphere.Mycorrhizal Fungi Animation
Decreased soil pH increases the solubility of phosphorus precipitates. 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. 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.