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ericoid mycorrhiza
The ericoid mycorrhiza is a mutualistic symbiosis formed between members of the plant family Ericaceae and several lineages of fungi. The symbiosis represents an important adaptation to acidic and nutrient poor soils that species in the Ericaceae typically inhabit,〔Cairney, J. W. G. and A. A. Meharg. 2003. Ericoid mycorrhiza: a partnership that exploits harsh edaphic conditions. European Journal of Soil Science 54: 735–740. doi:10.1046/j.1351-0754.2003.0555.x.〕 including boreal forests, bogs, and heathlands. Molecular clock estimates suggest that the symbiosis originated approximately 140 million years ago.〔Cullings, K. W. 1996. Single phylogenetic origin of ericoid mycorrhizae within the Ericaceae. Canadian Journal of Botany 74: 1896-1909.〕
==Structure and function==
Ericoid mycorrhizas are characterized by fungal coils that form in the epidermal cells of the fine hair roots of ericaceous species.〔Smith, S. E. and D. J. Read. 2008. Mycorrhizal Symbiosis, Third Edition. Academic Press.〕 Ericoid mycorrhizal fungi establish loose hyphal networks around the outside of hair roots, from which they penetrate the walls of cortical cells to form intracellular coils that can densely pack individual plant cells.〔 However, the fungi do not penetrate plasma membranes of plant cells. Evidence suggests that coils only function for a period of a few weeks before the plant cell and fungal hyphae begin to degrade.〔
The coil is the site where fungi exchange nutrients obtained from the soil for carbohydrates fixed through photosynthesis by the plant. Ericoid mycorrhizal fungi have been shown to have enzymatic capabilities to breakdown complex organic molecules.〔Cairney, J. W. G., and R. M. Burke.1998. Extracellular enzyme activities of the ericoid mycorrhizal endophyte Hymenoscyphus ericae (Read) Korf & Kernan: their likely roles in decomposition of dead plant tissue in soil. Plant and Soil 205: 181-192.〕〔Read, D. J., J. R. Leake, and J. Perez-Moreno. 2004. Mycorrhizal fungi as drivers of ecosystem processes in heathland and boreal forest biomes. Canadian Journal of Botany 82: 1243-1263.〕 This may allow some ericoid mycorrhizal fungi to act as saprotrophs. However, the primary function of these enzymatic capabilities is likely to access organic forms of nutrients, such as nitrogen, whose mineralized forms are in very limiting quantities in habitats typically occupied by ericaceous plants.〔
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