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Introduction to Mycorrhiza

In 1885,Albert Bernhard Frank coined the term "Mycor" - "rhiza" which is derived from the Greek words meaning "fungus" - "root" [1]. This symbiotic relationship plays an important role in soil life and chemistry occuring underground between a fungus and the rhizosphere (root system) of vascular plants. Mycorrhiza colonize in host plant root systems either intracellularly(endomycorrhiza) or extracellularly(ectomycorrhiza).

It is possible upon invasion that a weakly pathogenic relationship is established, and has been studied infrequently upon these rare occasions[3]. However, commonly upon invasion a mutualistic relationship is established in which hundreds of thousands of fungal hyphael branches are formed from the vegitative mycelium, and extend outwards into the soil. Plant roots that do not engage in a relationship with the mycorrhizial fungus may be incapable of taking up phosphate ions that are demineralized in soils with a basic pH. However, if a plant engages in a symbioitic relationship with the fungus the mycelium will access these phosphorous sources, making them available to the host plant. [4]Nutrients are often depleted in areas directly around plant roots, thus by Mycorrhiza extending the root zone over a large area nutrient uptake of water, nitrogen, and phosphorous is increased. In return, in order to provide root growth the host plant provides the mycorrhiza with the necessary carbohydrates such as glucose and sucrose [5]. These carbohydrates are translocated from their source (usually leaves) to root tissue, and on to the plant's fungal partners. The mechanism for increased absorption is not only physical, but also chemical as well. The cell membrane chemistry of fungi is unique compared to plants due to exhibiting organic acid excretion which aids in ion displacement.

The photo on the left depicts a vascular plant engaging in a symbiotic Mycorrhizal relationship therefore increasing the total root area, and allowing a maximum nutrient uptake. The photo on the right depicts a vascular plant that does not engage in a Mycorrhizal relationship therefore producing a smaller total root area, and in turn receiving a smaller nutrient uptake.
The photo on the left depicts a vascular plant engaging in a symbiotic Mycorrhizal relationship therefore increasing the total root area, and allowing a maximum nutrient uptake. The photo on the right depicts a vascular plant that does not engage in a Mycorrhizal relationship therefore producing a smaller total root area, and in turn receiving a smaller nutrient uptake.[6]


Although this scientific area of research is still ongoing, and only a small number of vascular plants has been examined 95% of them partake in this symbiotic relationship with Mycorrhiza with arbuscular mycorrhizas being the predominant ancestral form. [7]The sedimentary rock chert houses the earliest plant Aglaophyton major in fossil form dating back 400 million years ago, and shows evidence of a arbuscular mycorrhizal relationship. [8]



In this short video, Dr. Mike Amaranthus overviews the colonization of Mycorrhiza, and how this symbiotic relationship is especially beneficial to agricultural farming. He touches on the most interesting and beneficial aspect of how cost effective the spores are in relation to buying organic chemicals such as phosphorous and nitrogen to aid in growth of vascular plants.


Contents

[edit] Endomycorrihza

Endomycorrizha are also known as arbuscular mycorrihizal (AM) fungi and are generally classified in the Zygomycota phylum[9]. However, AM fungi lack the production of zygospores, which is a main and common characteristic of all fungi within Zygomycota. Therefore, according to AFTOL, AM fungi are apart of the Glomeromycota phylum[9]. The Gloeromycota phylum contains 12 genra and 169 species[9].Some of the other characteristics that define Glomeromycota are formation of arbuscules in plant roots and non-septate hypahe as previously mentioned. The AM fungi are characterized within the Glomeromycota because of their relatively large multi-nucleated spores that range from 40-800µm in diameter[9]. These spores may be formed singly, in clusters or in fruiting bodies called sporocarps[10].

[edit] Habitat

Endomycorrihzae grow naturally in the North American, however, can be transplanted and utilized in areas where there is a massive decline in soil nutrients. In these conditions, AM fungi are more than often placed in poor soil conditions to aid in the growth and development of vegetation, more so agricultural crops[9].Moreover, Endomycorrihza can be considered as ecologically important for most vascular plants and is found in 85% of plant families, most of them being crop species[14] [15]

[edit] Reproduction and Growth

[edit] Life Cycle

To date, there is no evidence that proves that AM fungi produce sexually. Molecular genetic markers show that there is little to no recombination from different lineages, therefore supporting the notion that AM fungi reproduce asexually[9].
Reproduction Cycle.
Reproduction Cycle.[16]
The starting point of the AM fingi life cycle is the germination of the spore, which then either grows infection structures known as appresoria within the host plants' root system or grows hyphae from root to explore soil in order to uptake nitrogen. The appresoria move on the surface of host roots and forms hyphae between cells that penetrate cell walls [17]. This is the main reasons why AM fungi are not detrimental to the host plant because the hyphae grow only within the external membrane. These hyphae form coils or tree-like structures called arbuscules.

[edit] Nutrient Exchange

Endomycorrizhae have the ability to move carbon and nitrogen and utilize these molecules as an energy source.Carbon is transferred from the host plant to the internal mycelium of fungus in the form of hexose. Once within the fungi, hexose is then rearranged into glycogen and trehalose for short term storage. Then these forms of carbon are reconfigured into triacylglycerol (TAG) to be exported to the external mycelium system. TAG is then catabolized to be used at cell wall components via the glyoxylate cycle or as energy via the TCA cycle by the formation of ATP and amino acids. In this case, the fungi is using the host to benefit its’ growth.

However, when it comes to nitrogen, it functions in the opposite direction of the carbon flow. Inorganic nitrogen in the forms of ammonia (NH4+)and nitrate (NO3-) are absorbed by the external mycelium. These forms of nitrogen are not usable, thus they are assimilated and converted into Arginine; it is hypothesized that polyphosphate aids in this conversion. Arginine is then imported to the internal mycelium and catabolized further into [http:en.wikipedia.org/wiki/ammonium ammonium] (NH3+). Ammonium is a usable form of nitrogen for the fungus as well as the host plant. Ammonium is able to undergo many other chemical reactions and transformations that result in the production of ATP and amino acids. These end products are then, mainly, used by the host plant. In this case, the host plant is using the fungi to benefit its’ growth.

[edit] Applications

Dr. Gail Wilson, an Associate Professor of Natural Resource Ecology and Management, describes how beneficial mycorhizzae are for plant life. Dr. Wilson also provides an application of the mycrohizzae in rejuvenation of vegetation around areas were coal mines were present. Moreover, that the success of the transplant of pine tree to Costa Rico were dependent on the soil from North America, which contain mycorhizzae; AM fungi are only found naturally in North America.



[edit] Current Research & Applications

  • To preserve, document and characterize mycorrhizal germiplasm
    • This fungi is able to form a relationship with various plant species, thereby enhancing mineral nutrient acquisition and provides the plant with a tolerance towards different stresses in host plants. There is wide diversity within the arbuscular mycorrhizal fungi which is the symbiotic fungi. This would be a mycorrhizal culture depository, in which various cultures from twelve different ecological zones are housed and maintained. [20]
  • To harvest the fungal inoculum production under in vitro conditions and to develop root organ cultures for various arbuscular mycorrhizal fungi.
    • This would be considered an advanced cultivation methodology for arbuscular mycorrhizal fungi. The purpose would be to develop the mass production of contamination free inoculum with less space, making it more effieicnt. Root inducing transfer DNA transformed roots of a host plant in order to develop the symbiosis on an in vitro medium. [21]
  • Use as a Bio-diesel
    • Mycorrhiza developed by TERI exhibits early fruition and flowering from the seventh month. This method also had a 20% to 30% higher yield in comparison to the non-mycorrhizal plantations. [22]
  • Wasteland creation by industry
    • This method not only benefits the plants but also benefits the environmentally vulnerable sites. Augmentation of the supply of phosphorus and elements such ar iron, boron, zinc, copper and the defense of plant roots from diseases and high soil temperatures and high salt concentrations sum up the benefits of the plant. Environmental benefits include the hyphae from the mycorrhiza bind soil particles and improve aggregation capabilities. This stabilizes the soil aggregates and avoids leaching of vital elements and heavy metals. [23]
  • To replace potting soils used in the greenhouse, since they are typically formulated from mixtures of materials including peat moss, perlite, and dvermiculite. However they lack mycorrhizal fungi.
  • This fungi can be applied to turfgrass and golf fields since a mycorrhizal relationship can help improve grasses resistance to the negative effects of the parasitic nematodes. When mycorrhizal is present, the root infections by pathogenic nematodes are less severe on mycorrhizal plants than on non mycorrhizal plants. Symptoms of nematode infect are reduced and the population of the nematode is also reduced. Also, when an environment is polluted with heavy metals and toxins, mycorrhizal fungi can protect its plant partner against the negative effects. These benefits of mycorrhizal fungi, improve conditions of turn for any golf field. [24]

[edit] Ectomycorrihza

Ectomycorrhizal (ECM) symbiosis represents one of the most prominent and ecologically crucial mutualistic associations in terrestrial habitats. These fungi evolved from humus and wood saprotrophic ancestors[25]. Approximately 7,750 ECM fungal species are grouped within the phyla Basidiomycota, Ascomycota, and Zygomycota. However, it has been estimated that there could potentially be between 20,000 and 25,000 ECM fungal species[26]. Approximately 6,000 plant species have been identified that also take part in the symbiotic relationship[25].

Ectomycorrhizal fungi associated with plant seedlings.
Ectomycorrhizal fungi associated with plant seedlings[27].

[edit] Habitat

Ectomycorrhizas are the most advanced symbiotic association, where the fungi completely surrounds the root systems of vascular plants. Ectomycorrhizal association occurs with thousands of mostly woody plant species worldwide such as birches, oaks, and pines, where they play an important role in seedling establishment and tree growth in habitats across the globe[26]. Ectomycorrhizal fungal species exist in most of the temperate, boreal, and Mediterranean forests of the Northern Hemisphere and parts of South America, seasonal savanna and rain forest habitats in Africa, India and Indo-Malay. They have also been found to exist in temperate rain forest and seasonal woodland communities of Australia[25] [28].

[edit] Host Plant-Fungal Interactions

[edit] Association

ECM fungi are capable of different levels of specialization with plant hosts. Some ECM fungi, such as Amanita muscari, are generalists, as they associate with a phylogenetically broad range of hosts, while others are specialized, such as those of the genera Rhizopogon, to a phylogenetically narrow range of hosts[29]. Associations with specialized fungi reduce the chances of indirectly helping competing plant species, while generalist ECM fungi can connect individuals of hosts from the same or different species and are able to translocate carbon between hosts [29]. It is common to find mycorrhizas belonging to several different fungi on the root system of a single tree [9].

ECM fungi begin development and association with plant hosts when their hyphae infect the secondary or tertiary roots systems. Once the fungi infect the host’s roots, the hyphae grows back up the root system between the epidermal and cortical cells, both mechanically and through the excretion of pectinases, thus forming the Hartig net. It should be noted that the hyphae never penetrate into the cells, but instead the intercellular Hartig net forms completely around each cell. In this association, the fungi form a sheath of tissue, ranging in thickness between 50-100 μm around the entire root system, which provides the main interface for exchange of substances between plant and the fungi[9].

[edit] Communication

Model of the inter- and intracellular communications that might exist between fungal hyphae and root cells in the ectomycorrhizal symbiosis. Changes in environmental conditions may produce signals sensed by cells of both partners in the symbiosis, and both probably transduce this information to their nuclei to provoke modifications in gene expression and consequently in phenotypes.
Model of the inter- and intracellular communications that might exist between fungal hyphae and root cells in the ectomycorrhizal symbiosis. Changes in environmental conditions may produce signals sensed by cells of both partners in the symbiosis, and both probably transduce this information to their nuclei to provoke modifications in gene expression and consequently in phenotypes[9].

The ECM symbiosis triggers key developmental programs in both symbionts, where the fungi must have the ability to recognize and become associated with its host, escape the host defense mechanisms, and establish bi-directional nutrient transfer[30]. Multiple signal and communication genes are involved in a series of complex and overlapping developmental processes in the symbionts, which include switching off of fungal growth mode, initiation of lateral roots, aggregation of hyphae, arrest of cell division in ensheathed roots, and radial elongation of epidermal cells[30]. Fungal and plant interactions are also influenced by environmental conditions, such as climate, soil, and nutrient availability, which can either enhance or repress the establishment of the symbiotic relationship[9]. The entire purpose of development in ECM symbiosis is to extend the function of the root system[30].

There have been a number of molecules that have been found that control the complex symbiotic interactions (communication) between the host plant and fungi, which are classified as follows[30]:

  • Rhizospheric signals (flavonoids, diterpenes, hormones and various nutrients) secreted by the plant host into the rhizosphere, a narrow region of soil directly around the root hairs, which act to modify hyphal morphology.
  • Adhesins and hydrolases secreted by the hyphae and used for attachment and invasion of host plant tissues.
  • Hormones and secondary signals used in the induction of organogenetic programmes in both fungal and root cells.
  • Molecules that facilitate fungal survival in response to plant defense systems.
  • Molecules that coordinate strategies for exchanging carbon and other metabolites between the symbionts.

[edit] Nutrient Exchange

Nutrient exchange between fungus and host depends on one partner releasing nutrient into the apoplastic interface and the uptake of that nutrient from the interfacial apoplast by the other partner. This diagram summarises current ideas about the transporters acting in ectomycorrhizal tissues that achieve this nutrient exchange. Key: fp, fungal plasma membrane, rp, root plasma membrane. The circles represent transporters, with the arrows indicating direction of transport. Blue circles represent transporters where at least one member of the transporter family has been characterised by functional complementation of a yeast deficient strain; grey circles are putative transporters for which candidate genes exist in the genome; white circles represent hypothetical transporters.
Nutrient exchange between fungus and host depends on one partner releasing nutrient into the apoplastic interface and the uptake of that nutrient from the interfacial apoplast by the other partner. This diagram summarises current ideas about the transporters acting in ectomycorrhizal tissues that achieve this nutrient exchange. Key: fp, fungal plasma membrane, rp, root plasma membrane. The circles represent transporters, with the arrows indicating direction of transport. Blue circles represent transporters where at least one member of the transporter family has been characterised by functional complementation of a yeast deficient strain; grey circles are putative transporters for which candidate genes exist in the genome; white circles represent hypothetical transporters[9].

The most important communication between plant and fungus in ectomycorrhizal association is nutrient exchange. Metabolite exchange is essential for the persistence of both plant and fungus. Nutrient exchange between the two partners depends on one partner releasing nutrients into the apoplastic interface, within the plant cell wall, followed by the other partner taking up and utilizing the specific nutrient from the interface[9].

The majority of ectomycorrhizal fungi rely on the plant host for carbon sources due to their uncompetitive nature and inability to utilize cellulose and lignin [9]. The plant transfers rich carbon sources in the form of photosynthetic carbohydrates to the fungi, which the fungi use in the development of extensive hyphal growth into the soil. The plant host delivers these carbon sources to the apoplast through hydrolysis of sucrose by invertase enzyme, which produces six-carbon monosaccharides, called hexoses, that can be taken up by the fungal cells and utilized for growth and other fungal activities[31].

Likewise, host plants rely on the fungi for the capture of some nutrients such as phosphate (P), nitrate (NO3-), ammonium (NH4+), peptides, amino acids and potassium ions (K+), which the host’s root systems cannot access in the soil[31]. Fungi are capable of harvesting inorganic phosphate (Pi) from the environment through the aid of ectoenzymes, like phosphatases, which the hyphae then deliver to the apoplast interface and are subsequently taken up by the host plant[31]. Nitrate, ammonium, peptides and potassium ions are also taken up by the fungi through specific systems, which are converted into amino acids and transferred to the host plant through the apoplast[31]. The fungi also confers pathogen resistance and provides protection from water stress to the plant host[9]. Thus, this association greatly enhances mineral uptake and protection for the host plant; while at the same time provides a carbon source used by the fungus for growth.

[edit] Reproduction

Unlike fungi that form arbuscular and ericoid mycorrhizas, ECM fungi are capable of sexual reproduction where they develop fruiting bodies either above ground (epigeous, mushroom-like), or below ground (hypogeous, truffle-like), and produce thousands to millions of meiotic spores[32].

Compared to other fungal groups, it is very rare for ECM fungi to produce asexual spores. Instead, ECM fungi only utilize asexual propagation for the vegetative spread of mycelium in the soil, or through mycelium dispersal by mycophageous organisms. Their vegetative thallus is a mycelium that has been found to span large areas. For example, in different species of Suillus, the mycelium have been found to span up to 300 m2 [32].

[edit] Ecology

Ectomycorrhizae and endomycorrhizae play a large role in the redistribution of resources in soil. The saprotrophic network they create allows their hosts to utilize saccharides, protiens, and more that would otherwise be unacquirable. These mycorrhizae have used these methods to support vegetation in environments as hostile as the northern tundra[33]. Among other things, the benefits mycorrhizae supply to their surrounding organisms include improving soil structure, increased water availability and storage, and decreased leaching. These factors allow them and their beneficiaries to thrive more consistantly[1]. The ecological effects of mycorrhizal symbiosis have effects on more than one trophic level. In the video below, mycorrhizal fungus is afluently detailed as being the "earth's natural internet" by linking all of the resources from all the kingdoms together.

[edit] Redistribution of Resources

Nitrogen as a vital part of a food chain. It enters from one part of a plant root system, and once dead, is extracted to another plant via mycorrhizae.
Nitrogen as a vital part of a food chain. It enters from one part of a plant root system, and once dead, is extracted to another plant via mycorrhizae.[34]

A main contributor to the success mycorrhizae is its ability to capture and relocate resources. Since many orders of fungi have a plethora of different extracellular digestive enzymes, they can consume lignin, cellulose, chitin and more. When these resources are assimilated and spread through out the hyphal network, they can become vectors of the host plant. This action essentially changes the local habitat, creating sequestration of resources, diverting them so as to be useful to the host plant. This is especially notable in highly taxing crops such as corn and tomatoes. These types of crops can face severe nitrogen deficiencies. In one report, up to 10.6% of the radioactively marked nitrogen was leached into a tomato plant, from a dead tomato plant via its mycorrhizae [35]. The most useful ion these mycorrhizae can transmit is phosphorous, however. Since P is not mobile in soil, many plants must rely on these mycorrhizae for increased acquisition it provides[2]. The simple paradigm this creates allows many mycorrhizae to be a conduit or medium for nutrient acquisition, and as a result can determine the fate of specific species, in specific regions.

[edit] Relation to Parasites

Mycorrhizae are of considerable interest due to their ability to enhance their host plant's ability to ward of infection. This is not true of all mycorrhizae, in fact, some of them increase parasitism, making it an awkward relationship for the host plant. In one study, the presence of mycorrhizae generally increased the predation by herbivores, while it decreased the predation by gall makers like coleopterans[36]. This is of particular interest because of the natural methods of pest reduction that could be exploited. Since industry is under pressure to produce cost and environmental efficient techniques, this could be an avenue for further exploration. In one study, the parasitism of a hymenoptera, Chromatomyia Syngenesiae, was drastically lowered on its target, Barely. Considering the economic importance of barely, it is easy to see the potential of these methods[3].


[edit] Economic Importance

[edit] Agriculture

The majority of higher plants exist in some sort of symbiotic relationship with mycorrhiza connections with either Endomycorrihza, Ectomycorrhizal fungi. The most widely used fungi in agricultural development are part of the endomycorrihza family, many agricultural companies sell inoculant for use in fields such as Glomus intraradices. This is especially important in agriculture as many farmers employ fungi inoculation of the root system a key part of development of their crops. These fungi improve nutrient uptake and help to optimize fertilizer, water uptake and can also restore degraded soil, cutting a farmers costs [37]. Endomycorrihza fungi can increase water absorption and can absorb all 15 essential micro and macro nutrients essential for plant growth and development especially in areas of low soil quality [38] . Mycorrhizae fungi can also provide disease protection against necrotrophic competing fungi, insects and increased resistance to salinity and heavy metals in the soil [39] . Many modern farming practices however do not encourage the use of endomycorrihza fungi, many modern practices are actually detrimental to soil fungal population. The use of biocides, monocultures, constant tilling and high amounts of fertilizer in the soil can have negative impacts on many endomycorrihza communities [40] . In an example of this a study was conducted in a monoculture soybean field with high fertilizer use, it was noted that high levels of phosphorous from constant fertilization was seen to inhibit endomycorrihza infection. Large summer fallow periods associated with monocultures in which the land is left deliberately bare were seen to decimate endomycorrihza, as they are stripped of their symbiotic partner [41] .

[edit] Organic Farming

The practice of organic farming generally excludes the use of harsh pesticides and fertilizer use, organic farmers abide by the guidelines of the International Federation of Organic Agricultural Movement [42] . Because of these guidelines many organic fields have lower phosphorous and nitrogen concentration in the soil, have lower crop yields and are more prone to pests [43]. In organic farms phosphorus is usually the limiting factor of crop growth, not enough is available in the soil through weathering and other natural processes. Many farmers must use a complex mix of rotting material and manure in order to increase the nutrient value of the soil, endomycorrihza inoculation of crops helps to better utilize these nutrients and increases the yield on an organic farm. [43] Low yield of an organic farm is one of the major downfalls to this method of farming when compared to traditional practices, therefore increasing crop yield is one of the biggest challenges facing organic farmers today. Certain species of endomycorrihza have been shown to increase growth rates of plants by stimulating the release of certain plant growth hormones such as cytokinins and gibberellins [43]

[edit] Truffles

Truffles have been a prized delicacy in Europe dating back to ancient Greek and Roman civilizations being recorded as early as 20 AD [44]. Truffle fungi form complex ectomycorrhizal relationships with host root systems of various trees such as oak, hazel and certain species of shrubs; the hyphae eventually aggregate forming an ascoma or fruiting body [44]. The most prized truffles come from the genus tuber, and are the “black truffle” Tuber melanosporum, and the rarer “white truffle” Tuber magnatum which can cost anywhere from 300–400 Euros per 100g [44].Large scale mycorrhizal production of certain species of black truffles such as Tuber melanosporum, have been produced artificially for hundreds of years. At present almost 80% of France’s black truffles are produced from artificial truffle grounds, and have been grown as far away as the United States, Israel and Tasmania [44]. Artificial truffle farming is similar to fruit farming; only the fruit appears underground rather than on the tree. Tuber melanosporum is used to inoculate seedling trees such as oak or hazelnut and are than planted in the ground like a normal orchard [44]. In contrast Tuber magnatum “white truffles” has yet to be cultivated; therefore white truffles must still be harvested traditionally in the hillsides of northern Italy and neighboring provinces.

[edit] Environmental impacts and concerns

[edit] Forest communication

A view of a forest Mychorhiazal community, trees far from sources of water are connected through mycorrhizal connections to trees in areas of abundant water .
A view of a forest Mychorhiazal community, trees far from sources of water are connected through mycorrhizal connections to trees in areas of abundant water .[47]

Mycorrhiza applications go far beyond just nutrient and water absorption and play a larger role in the forest community. Networks of Mycorrhiza can connect two trees of a different or similar species together forming vast networks throughout the forest floor [48]. Most forests are interconnected through these networks of mycorrhiza with the oldest trees forming the central hub of the network while the younger trees and seedlings become established in these pre made networks [48]. The underground networks actually help support saplings and younger weaker trees as carbon, nitrogen, water and other nutrients can be transferred through these networks from older more established trees [48] . This transfer of nutrients back and forth throughout the forest floor can help these systems cope with climate changes such as unusually dry conditions.

[edit] Pollution

[edit] Acid Rain

Acid rain is formed in the upper atmosphere as NO and SO2 are hydrolyzed [49] mixing with rain water and eventually falling back down to earth. Mycorrhizal associations can be affected either indirectly through influence on host shoots, or directly by changes in soil PH. [50] Usually mycorrhizal fungi are either absent or show less diversity in soils with a lowered PH.[50] Acid rain can have differing effects on different species, one particular ectomycorrhiza fungi ascomycetes Cenococcum, has been reported to be more abundant in forests that have under gone soil acidification, most likely due to decreased competition from other species of fungi.[50] The problem of acid rain is often compounded as a decrease in PH leads to an increase in the rate that minerals dissolve such as Al,Cd, Cu, Ni, Pb and Zn; these heavy metals have been shown to greatly reduce the rate of mycorrhizal infection. [50] High aluminum concentrations have been shown to have detrimental effects on ectomycorrhiza fungi; causing massive cytoplasmic damage, affected ectomycorrhiza fungi were often also absent a Hartig net.[50].

[edit] Organic chemical pollution

Polycyclic aromatic hydrocarbons (PAHs) such as Acenaphthene, or flouoranthene are released into the environment as a by-product of various industrial processes and accidents. The by-products of coal gasification and oil spills can cause these toxic mutagenic compounds to be released into the soil. Ectomycorrhizae fungi show some interesting characteristics over a broad range of species; when exposed to crude oil most ectomycorrhizae fungi were inhibited.[50] Some species however showed no response to the presence of crude oil, or even had stimulated growth and colonization due to a decrease in competition .[50] Most species of endomycorrihza are inhibited by crude oil however some species such as Glomus aggregatum and Glomus mosseae are able to colonize oil contaminated soil.[50]

[edit] Timeline

  • Researchers today are well versed in the function of arbuscular mycorrhizai, and have researched their consequences for nutrient cycling and plant productivity and studied their relevance in ecology and botany.
  • In 1842, Nageli had first described mycorrhiza, thereby causing this field of research to become reputable in the last 35 years. [53]
  • This was a rather large discovery due to the obligate symbiotic nature of arbuscular mycorrhizai. [54]
  • In 1885, the name “mycorrhiza” was given to the association between tree roots and ectomycorrhizal fungi, by Frank. [55]
  • In 1889, Schlicht had observed anatomical relationships between the host and the mycorrhizal tissues. [56]
  • In 1897, Janse labeled the spores to be “vesicules” . [57]
  • In 1905, Gallaud found other structures and named them “arbuscules”. These “arbuscules” were located in the inner cortex of the fungal cell.
  • In 1927, there were differences noted in the fungal cultures further developing into the classification of ectomycorrhizaand arbuscular mycorrhizas. [58]
  • In 1940, Howard had studied their controversial linkage to the effectors of composting in India. [59]
  • In 1972, Bertrand had named them “mal aimee des microbiologists”. [60]
  • In 1974, Cox and Sanders confirmed that the fungal cell is surrounded by a host membrane, using transmission electron microscopy.
  • Techniques used in the past for mycorrhizai included cutting root systems into small pieces and determining the proportion of the pieces that were mycorrhizal. Current techniques are based on the line intersect which was devised by Newman in 1966 and was finally applied to mycorrhizai cultures in 1975. [61]
  • In 1985, Trappe and Berch, as well as Rayner in 1926-1927 had cited earlier observations of the symbiosis. [62]
  • In 2001, molecular data was used to establish a relationship between arbuscular mycorrhizal fungi as well as the relationship between two other fungi. This was done by Schubler et al. [63]

[edit] Additional Information

Potential Harm of Endo/Ectomyocorrhizal to Host’s Cells

There are many theories whether or not Arbuscular Myocorrhizal fungi interactions can be harmful to the host plant. Up until now little was known about this issue but emerging evidence is leading scientists to believe that there are some negative effects that come with this symbiotic relationship. There has been evidence shown of alteration and decomposition of the host plant’s cell wall due to Arbuscular Myocorrhizal fungi symbiosis with the host plants roots. A majority of this evidence however is based on the endomyocorrhizal fungi because it infects inside the host’s roots[64]. During initial colonization, the fungi penetrate the cortical cells via hyphae and many cell wall degrading enzymes such as polygalacturonase [65]. The enzymes cleave different polysaccharidic components (loosen) of the cell wall and allow the hyphae to penetrate the surface of the root much more efficiently[66]. Once in the cell wall, the hyphae form tree like systems (arbuscules) that disrupts the host cell’s organization (Bonfante et al. 1987). These hyphae produce pectin degrading enzymes that denature the material of the middle lamella and as a result the host cells surrounding the hyphae seperate[67] (Keon et al. 1987; Benhamou et al. 1990). This can be a hazard to the host plant because a weakening of the cell wall gives a potential for other invading pathogens and bacteria to enter the plant and cause devastating effects.

[edit] Chemistry

Despite the drastic increase in technology and advancements in the field of science, very minimal knowledge has developed from the chemical frontier relating to AM. The metabolic pathways relating to the signaling of compounds between the symbiosis and the role of secondary metabolites in the establishment and maintenance of a functioning AM has yet to be fully elucidated. Although secondary metabolism has been linked to various interactions between plants and their biotic and abiotic environments, it has yet to be shown how exactly the link between secondary metabolites affect mycorrhizal symbiosis. (Harborne, 1993 - Harborne, J. B. 1993. Introduction to Ecological Biochemistry, 4th Edition Academic Press, London.). Flavonoids have shown to provide an increase rate of spore germination in AM-fungi. (Gianinazzi-Pearson et al., 1989; Tsai and Phillips, 1991 - Gianinazzi-Pearson, V., Branzanti, B., and Gianinazzi, S. 1989. In vitro enhancement of spore germination and early hyphal growth of a vesicular–arbuscular mycorrhizal fungus by host root exudates and plant flavonoids. Symbiosis 7:243–255.& Tsai, S. M. and Phillips, D. A. 1991. Flavonoids released naturally from alfalfa promote development of symbiotic Glomus spores in vitro. Appl. Environ. Microbiol. 57:1485–1488.).

A common characteristic of mycorrihizal is a yellow coloration which develops on the roots of many plants. This allows qualitative knowledge of the degree of mychorrhization from the naked eye (Daft and Nicholson, 1969; Fyson and Oaks, 1992 - Daft, M. and Nicholson, T.H.C. 1969. Effect of Endogone mycorrhiza on plant growth. III: Influence of inoculum concentration on growth and infection in tomato. New Phytol. 68:953–963. & Fyson, A. and Oaks, A. 1992. Rapid methods for quantifying AM fungal infections in maize roots. Plant Soil 147:317–319.). The chromophore component of the yellow pigment was isolated and identified as it’s dimethyl derivative E-4,9-dimethyldodeca- 2,4,6,8,10-pentanedioic acid, named mycorradicin (Klingner et al., 1995a - Klingner, A., Bothe, H., Wray, V., and Marner, F.-J. 1995a. Identification of a yellow pigment formed in maize roots upon mycorrhizal colonization. Phytochemistry 38:53–55.). It was later determined through chemical analysis that this chromophore was actually the core structure of a mixture of various oligio- or polyesters with glycosylated cyclohexenone derivatives. Their importance in soil life and soil chemistry can’t be under stated.

Figure #1: Mycorradicin (H=H for the core structure of the yellow pigment), Yellow pigment (H=glycosylated cyclohexenone derivatives).

In lieu of determining mycorradicin’s significance, importance, and origin, research has poured into this field. With mycorracidin’s structural similarity to crocetin (C20-polyene) and azafrin (C27-apocarotenoid), it has been speculated that it derives from a C40-carotenoid precursor from the breaking of two C13-units (covalent bond cleavage) (1989, Klingner et al., 1995 – Klingner, A., Hundeshagen, B., Kernebeck, H., and Bothe, H. 1995. Localization of the yellow pigment formed in roots of gramineous plants colonized by arbuscular fungi. Protoplasma 185:50–57). With all the research put into mycorradicin, and in general secondary metabolites, blumenin; a breakdown unit of a carotenoid was identified (a mycorrhiza-induced glycosylated cyclohexenone derivative). Blumenin is generated via the colonization of AM fungus (Glomus intraradices) in roots of many plants (examples include wheat and oat) (Maier et al., 1995 - Maier, W., Peipp, H., Schmidt, J., Wray, V., and Strack, D. 1995. Levels of a terpenoid glycoside (blumenin) and cell wall-bound phenolics in some cereal mycorrhizas. Plant Physiol. 109:465–470.). This metabolite is impertinent to furthering research because of the direct correlation blumenin shows with respect to the degree of root mycorrhization for a symbiosis.

Figure #2: Blumenin; Glycosylated cyclohexenone derivtives isolated from mycorrhizal roots of various plants. R is dependent on the plant – fungus interaction, depending on the host plant, different cyclohexenone derivatives will be synthesized, but used for similar metabolic purposes.

To evaluate the AM-specific synthesis of cyclohexenone derivatives, pathogens (Gaeumannomyces graminis and Drechslera sp.) and endophytes (Fusarium sp.) were applied, as well as abiotic stressors (heat, cold, high intensities of light, heavy metals, and drought). The results of these experiments lead to no findings of cyclohexenone derivatives to a particular symbiosis. These findings show that the derivatives are specific for the symbiosis and not random, nor dependent on environmental conditions or interactions with other organisms (Maier et al., 1997 - Maier, W., Hammer, K., Dammann, U., Schulz, B., and Strack, D. 1997. Accumulation of sesquiterpenoid cyclohexenone derivatives induced by an arbuscular mycorrhizal fungus in members of the Poaceae. Planta 202:36–42.). Mycorrhizal roots synthesize both C13 and C14 apoparotenoids (cyclohexenone derivatives and mycorradicin) which may be similar in formation to that of abscisic acid, involving a dioxygenase-catalyzed cleavage of a carotenoid precursor (Maier et al., 1998; Walter et al., 2000 - Maier, W., Schneider, B., and Strack, D. 1998. Biosynthesis of sesquiterpenoid cyclohexenone derivatives in mycorrhizal barley roots proceeds via the glyceraldehyde 3-phosphate/pyruvate pathway. Tetrahedron Lett. 39:521–524. & Walter, M. H., Fester, T., and Strack, D. 2000. Arbuscular mycorrhizal fungi induce the nonmevalonate methylerythritol phosphate pathway of isoprenoid biosynthesis correlated with accumulation of the “yellow pigment” and other apocarotenoids. Plant J. 21:571–578.). With the limited knowledge thus far obtained on the chemical interactions between the plant and fungus, [13C] glucose tracing experiments were undertaken to try and discover the connection between the formation of cyclohexenone derivatives to their thought to be precursors, carotenoids (by using NMR spectroscopy). Mevalonate-independent biosynthesis was discovered to be the pathway in which the cyclohexenone derivatives were forming (Maier et al., 1998 - Maier, W., Schneider, B., and Strack, D. 1998. Biosynthesis of sesquiterpenoid cyclohexenone derivatives in mycorrhizal barley roots proceeds via the glyceraldehyde 3-phosphate/pyruvate pathway. Tetrahedron Lett. 39:521–524.).

Figure #3: 13C-labelling of glucose fed to barely roots. Carotenoid biosynthesis is strongly stimulated in AM roots, at least partially at the transcriptional level.

Illustrated above, 13C-labelling pattern of blumenol C after feeding barely roots with [13C] glucose. The assumption that the AM-specific isoprenoids are apocarotenoids (derived from carotenoids by oxidative cleavage) was recently supported by a study showing that carotenoid biosynthesis is strongly stimulated in AM roots, at least partially at the transcriptional level (Fester et al., 2002 - Fester, T., Schmidt, D., Lohse, S., Walter, M. H., Giuliano, G., Bramley, P. M., Fraser, P. D., Hause, B., and Strack, D. 2002 Stimulation of carotenoid metabolism in arbuscular mycorrhizal roots. Planta 216:148–154.). Alkaline hydrolysis reactions of the yellow pigment (purified from mycorrhizal maize roots), gave mycorradicin, and an isomer pair of a cyclohexenone derivatives. This is a further realization that mycorradicin is an important unit used by the symbiosis which originates from various carotenoids. The ordeal of unravelling the mystery of how secondary metabolism, and specifically the carotenoid splitting into its subunits affects mycorrhiza is only early in its research. This problem has to be tackled not only on the chemical front, but also the genetic and metabolic fields to help generate more data to provide answers for many unanswered questions. Research has only recently begun to help elucidate and explain the complexity of interactions and signaling chemically between the two partners. Currently the role cyclohexenone derivatives in mycorrhizal symbiosis has yet to be established, although there are supporting trends for its involvement in the control of mycorrhization. Exogenous applications of blumenin has shown to drastically inhibit colonization and formation of arbuscules within early life stages of mycorrhza development in barley (Fester et al., 1999 - Fester, T., Maier, W., and Strack, D. 1999. Accumulation of secondary compounds in barley and wheat roots in response to inoculation with an arbuscular mycorrhizal fungus and co-inoculation with rhizosphere bacteria. Mycorrhiza 8:241–246.). The connection between cyclohexenone derivative formation with a variety of plants when inoculated with different AM fungi has successfully been established. Although the compounds which are formed are specific for the symbiosis giving qualitatively identical results, but not quantitative ones in relation to the compounds formed (Vierheilig et al., 2000 - Vierheilig, H., Maier, W., Wyss, U., Samson, J., Strack, D., and Pich’e, Y. 2000. Cyclohexenone derivative- and phosphate-levels in split-root systems and their role in the systemic suppression of mycorrhization in precolonized barley plants. J. Plant Physiol. 157:593–599.).

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