Mycorrhiza
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A '''mycorrhiza''' (Greek for ''fungus roots'' coined by Frank, 1885<ref name=Frank1885>AB Frank (1885) Über die auf Würzelsymbiose beruhende Ehrnährung gewisser Bäum durch unterirdische Pilze. Berichte der Deutschen Botanischen Gesselschaft 3, 128-145.</ref>; typically seen in the plural forms ''mycorrhizae'' or ''mycorrhizas'') is a [[symbiosis|symbiotic]] (occasionally weakly [[pathogen]]ic) association between a [[fungus]] and the roots of a [[plant]].<ref>Kirk, P.M., P.F. Cannon, J.C. David & J. Stalpers 2001. ''Ainsworth and Bisby’s Dictionary of the Fungi''. 9th ed. CAB International, Wallingford, UK.</ref> In a mycorrhizal association the fungus may colonize the roots of a host plant either intracellularly or extracellularly.
This [[mutualism|mutualistic]] association provides the fungus with relatively constant and direct access to mono- or dimeric [[carbohydrate]]s, such as [[glucose]] and [[sucrose]] produced by the plant in [[photosynthesis]].<ref name="Harrison MJ.">{{cite journal|author=Harrison MJ|year= 2005|title=Signaling in the arbuscular mycorrhizal symbiosis|journal=Annu Rev Microbiol.|volume=59|pages=19–42|pmid=16153162 | doi = 10.1146/annurev.micro.58.030603.123749}}</ref> The carbohydrates are translocated from their source location (usually leaves) to the root tissues and then to the fungal partners. In return, the plant gains the use of the [[mycelium]]'s very large surface area to absorb water and mineral nutrients from the soil, thus improving the mineral absorption capabilities of the plant roots.<ref name="Selosse">{{cite journal|author=Selosse MA, Richard F, He X, Simard SW|year= 2006|title=Mycorrhizal networks: des liaisons dangereuses?|journal=Trends Ecol Evol.|volume=21|pages=621–628|pmid=16843567 | doi = 10.1016/j.tree.2006.07.003}}</ref> Plant roots alone may be incapable of taking up [[phosphate]] [[ions]] that are immobilized, for example, in soils with a [[pH|basic pH]]. The [[mycelium]] of the mycorrhizal fungus can however access these phosphorus sources, and make them available to the plants they colonize.<ref name="Li">{{cite journal|author=Li H, Smith SE, Holloway RE, Zhu Y, Smith FA.|year= 2006|title=Arbuscular mycorrhizal fungi contribute to phosphorus uptake by wheat grown in a phosphorus-fixing soil even in the absence of positive growth responses.|journal=New Phytol.|volume=172|pages=536–543|pmid=17083683 | doi = 10.1111/j.1469-8137.2006.01846.x}}</ref> The mechanisms of increased absorption are both physical and chemical. Mycorrhizal mycelia are much smaller in diameter than the smallest root, and can explore a greater volume of soil, providing a larger surface area for absorption. Also, the cell membrane chemistry of fungi is different from that of plants. Mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils.
Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne [[pathogen]]s, and are also more resistant to the effects of drought. These effects are perhaps due to the improved water and mineral uptake in mycorrhizal plants.
Mycorrhizae form a [[mutualism|mutualistic]] relationship with the roots of most plant species (although only a small proportion of all species have been examined, 95% of all plant families are predominantly mycorrhizal).<ref name=Trappe1987>Trappe, J.M. (1987) ''Phylogenetic and ecologic aspects of mycotrophy in the angiosperms from an evolutionary standpoint''. Ecophysiology of VA Mycorrhizal Plants, G.R. Safir (EDS), CRC Press, Florida</ref>
Plants grown in sterile [[soil]]s and growth media often perform poorly without the addition of [[spore]]s or hyphae of mycorrhizal fungi to colonise the plant roots and aid in the uptake of soil mineral nutrients. The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes.<ref name="Jeffries">{{cite journal|author=Jeffries, P; Gianinazzi, S; Perotto, S; Turnau, K; Barea, J-M |year= 2003|title= The contribution of arbuscular mycorrhizal fungi in sustainable maintenance of plant health and soil fertility|journal=Biol. Fertility Soils|volume=37|pages=1–16|id=http://cat.inist.fr/?aModele=afficheN&cpsidt=14498927}}</ref>
==Occurrence of mycorrhizal associations==
At around 400 million years old, the [[Rhynie chert]] contains the earliest fossil assemblage yielding plants preserved in sufficient detail to detect mycorrhizae - and they are indeed observed in the stems of [[Aglaophyton|''Aglaophyton major'']].<ref name="Remy et al."/>
Mycorrhizae are present in 92% of plant families (80% of species)<ref name=Wang2006>{{cite journal
| author = Wang, B.
| coauthors = Qiu, Y.L.
| year = 2006
| title = Phylogenetic distribution and evolution of mycorrhizas in land plants
| journal = Mycorrhiza
| volume = 16
| issue = 5
| pages = 299–363
| url = http://www.springerlink.com/index/X7151P60502078U1.pdf
| accessdate = 2008-01-21
| doi = 10.1007/s00572-005-0033-6
}}</ref>, with arbuscular mycorrhizae being the ancestral and predominant form,<ref name=Wang2006/> and indeed the most prevalent symbiotic association found in plants at all.<ref name="Harrison MJ."/>
The structure of arbuscular mycorrhizae has been highly conserved since their first appearance in the fossil record,<ref name="Remy et al.">{{cite journal|author=Remy W, Taylor TN, Hass H, Kerp H |year=1994|title= 4 hundred million year old vesicular-arbuscular mycorrhizae|journal= Proc. National Academy of Sciences|volume=91|pages=11841–11843|pmid=11607500|doi= 10.1073/pnas.91.25.11841}}</ref> with both the development of ectomycorrhizae, and the loss of mycorrhizae, [[convergent evolution|evolving convergently]] on multiple occasions.<ref name=Wang2006/>
==Types of mycorrhiza==
[[Image:Wheat_field.jpg|thumb|150px|Arbuscular mycorrhizal [[wheat]]]]
[[Image:Grib skov.jpg|thumb|150px|Ectomycorrhizal [[beech]]]]
[[Image:Ericoid mycorrhizal fungus.jpg|thumb|200px|An ericoid mycorrhizal fungus isolated from ''[[Woollsia pungens]]''.<ref name="Midgley">Midgley, DJ, Chambers, SM & Cairney, JWG. 2002. Spatial distribution of fungal endophyte genotypes in a Woollsia pungens (Ericaceae) root system. ''Australian Journal of Botany'' '''50''', 559-565</ref>]]
Mycorrhizas are commonly divided into ''ectomycorrhizas'' and ''endomycorrhizas''. The two groups are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate individual [[cell (biology)| cells]] within the root, while the hyphae of endomycorrhizal fungi penetrate the cell wall and invaginate the [[cell membrane]].
===Endomycorrhiza===
Endomycorrhiza are variable and have been further classified as arbuscular, ericoid, arbutoid, monotropoid, and orchid mycorrhizae <ref name="Peterson et al. 2004">Peterson, R.L., H.B. Massicotte and L.H. Melville (2004) ''Mycorrhizas: anatomy and cell biology.'' National Research Council Research Press. [http://pubs.nrc-cnrc.gc.ca/cgi-bin/rp/rp2_book_e?mlist7_558]</ref>. [[Arbuscular mycorrhiza]]s, or AM (formerly known as vesicular-arbuscular mycorrhizas, or VAM), are mycorrhizas whose hyphae enter into the plant cells, producing structures that are either balloon-like (vesicles) or dichotomously-branching invaginations (arbuscules). The fungal [[hyphae]] do not in fact penetrate the [[protoplast]] (i.e. the interior of the cell), but invaginate the [[cell membrane]]. The structure of the arbuscules greatly increases the contact surface area between the hypha and the cell [[cytoplasm]] to facilitate the transfer of nutrients between them.
Arbuscular mycorrhizae are formed only by fungi in the [[Division (biology)|division]] [[Glomeromycota]]. Fossil evidence<ref name="Remy et al."/> and DNA sequence analysis<ref name=simon1993>L Simon, J Bousquet, RC Lévesque, M. Lalonde (1993) Origin and diversification of endomycorrhizal fungi and coincidence with vascular land plants. Nature, 363, 67-69</ref> suggest that this mutualism appeared 400-460 million years ago, when the first plants were colonizing land. Arbuscular mycorrhizas are found in 85% of all plant families, and occur in many crop species.<ref name=Wang2006/> The hyphae of arbuscular mycorrhizal fungi produce the glycoprotein [[glomalin]], which may be one of the major stores of carbon in the soil. Arbuscular mycorrhizal fungi have (possibly) been asexual for many millions of years and, unusually, individuals can contain many genetically different nuclei (a phenomenon called [[heterokaryosis]]).<ref name="Hijri">Hijri M & Sanders IR. 2005. Low gene copy number shows that arbuscular mycorrhizal fungi inherit genetically different nuclei ''Nature'' 433:160-163</ref>
Many plants in the order [[Ericales]] form [[ericoid mycorrhiza]]s, while some members of the Ericales form arbutoid and monotropoid mycorrhizas. All [[Orchidaceae|orchids]] are mycoheterotrophic at some stage during their lifecycle and form [[orchid mycorrhiza]] with a range of basidiomycete fungi.
===Ectomycorrhiza===
Ectomycorrhizas, or EcM, are typically formed between the roots of woody plants and fungi belonging to the [[Basidiomycota]], [[Ascomycota]], and [[Zygomycota]]. Ectomycorrhizas consist of a hyphal sheath, or mantle, covering the root tip and a [[hartig net]] of hyphae surrounding the plant cells within the root [[cortex]]. In some cases the hyphae may also penetrate the plant cells, in which case the mycorrhiza is called an ectendomycorrhiza. Outside the root, the fungal mycelium forms an extensive network within the soil and leaf litter.
Ectomycorrhizas are found in around 10% of plant families, including members of the [[Betulaceae|birch]], [[Dipterocarpaceae|dipterocarp]], [[Myrtaceae|eucalyptus]], [[Fagaceae|oak]], [[Pinaceae|pine]] and [[Rosaceae|rose]] families.<ref name=Wang2006/>
The ectomycorrhizal fungus ''[[Laccaria bicolor]]'' has been found to lure and kill [[springtail]]s to obtain nitrogen, some of which may then be transferred to the mycorrhizal host plant. In a study by Klironomos and Hart, [[Eastern White Pine]] inoculated with ''L. bicolor'' was able to derive up to 25% of its nitrogen from springtails.<ref>[http://www.24hourscholar.com/p/articles/mi_m1200/is_14_159/ai_104730213 Fungi kill insects and feed host plants] 24hourscholar.com</ref><ref>Klironomos, J. N. and Hart, M. M. 2001. Animal nitrogen swap for plant carbon. Nature, 410: 651-652.</ref>
==See also==
*[[Glomeromycota]]
*[[Basidiomycota]]
*[[Ascomycota]]
==References==
{{reflist}}
==External links==
*[http://www.gmo-safety.eu/en/grain/502.docu.html Mycorrhizas – a successful symbiosis] Biosafety research into genetically modified barley
*[http://wiki.terrorchid.org/mycorrhiza:mycorrhiza Orchid mycorrhiza extraction & culture] on the terrorchid wiki
[[Category:Soil biology]]
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