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Introduction to Mycorrhiza

"Mycor" - "rhiza" is derived from the Greek words meaning "fungus" - "root" ^[1]. This symbiotic relationship occurs underground between a fungus and the 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^[2]. 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. 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.

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.^[3]

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^[4]

Endomycorrihza

Endomycorrizha are also known as arbuscular mycorrihizal (AM) fungi and are generally classified in the Zygomycota phylum^[5]. However, AM fungi lack the production of zygospores, which is a main and common characteristic of all fungi within Zygomycota. Therefore, according the AFTOL, AM fungi are apart of the Glomeromycota phylum^[5]. The Gloeromycota phylum contains 12 genra and 169 species^[5].

Habitat

Endomycorrihza are most abundant in areas where there is a massive decline in soil nutrients that is accessible to the vegetation. This is due to their purpose of invading hosts plants and aiding in nutrient retrieval^[5]. 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^{[6] [7]}

Reproduction and Growth

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^[5].

This diagram dipicts the life cycle of AM fungi. The starting point is the germinating spore, which then either grows infection structures known as appresoria or grows hyphae from root to explore soil. The appresoria move on the surface of host roots and forms hyphae between cells that penetrate cell walls ^[8]. This is one of 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. The picture below illustrates what AM fungi look inside of a host root cell.

Some of the other characteristics that define Glomeromycota are formation of arbuscules in plant roots and non-septate hypahe 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^[5]. These spores may be formed singly, in clusters or in fruiting bodies called sporocarps^[9]. Below are graphics depicting a sporocarp and spores of Glomus sinosum.

Biochemical Pathways

Applications

Genetics

Current Studies

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^[10]. Approximately 7750 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^[11]. Approximately 6,000 plant species have been identified that also take part in the symbiotic relationship^[10].

Ectomycorrhizal fungi associated with plant seedlings.^[12]

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^[11]. 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^{[10] [13]}.

Host-Fungi 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^[14]. 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 ^[14]. It is common to find mycorrhizas belonging to several different fungi on the root system of a single tree ^[5].

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^[5].

Communication

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.^[5]

The most important communication between plant and fungus in ectomycorrhizal association is nutrient exchange.

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. In return, the fungi absorb minerals and water from the soil through its mycelium, which it shares with the plant roots. The fungi also confers pathogen resistance and provides protection from water stress to the plant host^[5].

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^[15].

Unlike many 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 m^{2 [15]}.

Economical Importance

Comparison Table

Environmental concerns

Acid Rain

Acid rain is formed in the upper atmosphere as NO3 and SO2 are hydrolyzed ^[16] mixing with rain water and eventually falling back down to earth. Mycorrhizal associations can be affected either indirectly or through influence on host shoots. ^[17] Acid rain can have differeing effects on different species, one particular ECM fungi ascomycetes Cenococcum spp. has been reported to be more abundant in forests that have under gone acidification, most likely due to decreased competition from other species of fungi.^[18] The problem of acid rain is often compounded as an increase in PH leads to an increase in the rate that minerals dissolve such as toxic heavy metals. ^[19]

Head 2

Additional Information

Plant species that benefit from Endomycorrizha and Ectomycorrizha[1]

References

1. ↑ Frank, A. B. (1885). "Über die auf Würzelsymbiose beruhende Ehrnährung gewisser Bäum durch unterirdische Pilze". Berichte der Deutschen Botanischen Gesellschaft 3: 128–145.
2. ↑ Kirk, P. M.; Cannon, P. F.; David, J. C. & Stalpers, J. (2001). Ainsworth and Bisby’s Dictionary of the Fungi (9th ed.). Wallingford, UK: CAB International.
3. ↑ Syekhfani, Prof. Dr. Ir. MS. (2013). Succession Life on Earth. Soil-Function
4. ↑ Trappe, J. M. (1987). Phylogenetic and ecologic aspects of mycotrophy in the angiosperms from an evolutionary standpoint. Florida: CRC Press.
5. ↑ ^{5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9} Moore, D., Robinsion, G.D., & Trinci, A.P. (2011) 21st Century Guidebook to Fungi. Cambridge University Press, New York.
6. ↑ Gederman, H. A. Rev. Phytopath. 6, 397−418 (1968).
7. ↑ Wang, B.; Qiu, Y.L. (2006). "Phylogenetic distribution and evolution of mycorrhizas in land plants". Mycorrhiza 16 (5): 299–363. doi:10.1007/s00572-005-0033-6. PMID 16845554. Retrieved 2008-01-21.
8. ↑ Shchar-hil Lab doi:http://shachar-hill.plantbiology.msu.edu/?page_id=44
9. ↑ Redecker, Dirk. 2008. Glomeromycota. Arbuscular mycorrhizal fungi and their relative(s). Version 14 January 2008. http://tolweb.org/Glomeromycota/28715/2008.01.14 in The Tree of Life Web Project, http://tolweb.org/
10. ↑ ^{10.0 10.1 10.2} Tedersoo, L., May, T.W., & Smith, M.E. (2010). Ectomycorrhizal lifestyle in fungi: global diversity, distribution, and evolution of phylogenetic lineages. Mycorrhiza, 20, 217-263
11. ↑ ^{11.0 11.1} Rinaldi, A.C., Comandini, O., & Kuyper, T.W. (2008). Ectomycorrhizal fungi diversity: separating the wheat from the chaff. Fungal Diversity, 33, 1-45
12. ↑ Alberton, O., & Kuyper, T. (2009). Ectomycorrhizal fungi associated with seedlings respond differently to increased carbon and nitrogen availability: implications for ecosystem responses to global change. Global Change Biology, 15 (1), 166-175
13. ↑ Abuzinadah, R.A., & Read, D.J. (1986). The role of proteins in the nitrogen nutrition of Ectomycorrhizal plants. New Phytologist, 103, 481-493
14. ↑ ^{14.0 14.1} den Bakker, H.C., Zuccarello, G.C., Kuyper, T.H.W., & Noordeloos, M.E. (2004). Evolution and host specificity in the ectomycorrhizal genus Leccinum. New Phyologist, 163, 201-215
15. ↑ ^{15.0 15.1} Carriconde, F., Gryta, H., Jargeat, P., Mouhamadou, B., & Gardes, M. (2008). High sexual reproduction and limited contemporary dispersal in the ectomycorrhizal fungus Tricholoma scalpturatum: new insights from population genetics and spatial autocorrelation analysis. Molecular Ecology, 17, 4433-4445
16. ↑ Charlson, R.J., Rodhe, H., 1982. Factors controlling the acidity of natural rainwater. Nature 295, 683-685
17. ↑ Cairney,J. Meharg,A. Influences of anthropogenic pollution on mycorrhizal fungal.Environmental Pollution 106 (1999) 169-182
18. ↑ Danielson, R.M., Visser, S., 1989. Ef€fects of forest soil acidification on ectomycorrhizal and vesicular-arbuscular mycorrhizal development. New Phytologist 112, 41±47
19. ↑ Cairney,J. Meharg,A. Influences of anthropogenic pollution on mycorrhizal fungal.Environmental Pollution 106 (1999) 169-182