A mycorrhiza (Greek for fungus roots; typically seen in the plural forms mycorrhizae or mycorrhizas) is a symbiotic (occasionally weakly pathogenic) association between a fungus and the roots of a plant (Kirk et al. 2001). In a mycorrhizal association the fungus may colonize the roots of a host plant either intracellularly or extracellularly.
This mutualistic association provides the fungus with relatively constant and direct access to mono- or dimeric carbohydrates, such as glucose and sucrose produced by the plant in photosynthesis (Harrison 2005). 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 (Selosse et al. 2006). Plant roots alone may be incapable of taking up phosphate ions that are immobilized, for example, in soils with an basic pH. The mycelium of the mycorrhizal fungus can however access these phosphorus sources, and make them available to the plants they colonize (Li et al. 2006). 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 pathogens, 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 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) (Trappe 1987).
Plants grown in sterile soils and growth media often perform poorly without the addition of spores 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 (Jeffries et al. 2003).References
- Harrison MJ (2005). Signaling in the arbuscular mycorrhizal symbiosis. Annu Rev Microbiol. 59: 19-42.
- Jeffries, P; Gianinazzi, S; Perotto, S; Turnau, K; Barea, J-M (2003). "The contribution of arbuscular mycorrhizal fungi in sustainable maintenance of plant health and soil fertility". Biol. Fertility Soils 37: 1-16.
- 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.
- Li H, Smith SE, Holloway RE, Zhu Y, Smith FA. (2006). Arbuscular mycorrhizal fungi contribute to phosphorus uptake by wheat grown in a phosphorus-fixing soil even in the absence of positive growth responses. New Phytol. 172: 536-543.
- Selosse MA, Richard F, He X, Simard SW (2006). Mycorrhizal networks: des liaisons dangereuses?. Trends Ecol Evol. 21: 621-628.
- 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
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