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| The order Ericales is a worldwide group of closely related families, perhaps best known in the UK by the Ericaceae family - heathland plants such as Calluna (heather), Vaccinium and Erica. | |||||
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| The order Ericales is a worldwide group of closely related families, perhaps best known in the UK by the Ericaceae family - heathland plants such as Calluna (heather), Vaccinium and Erica. | |||||
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In the northern hemisphere, large areas of land can be dominated by the Ericaceae, whereas in the southern hemisphere, the Epacridaceae are most important. Heathland habitats are typically found at high altitudes and colder latitutudes, and have nutrient poor, acidic soils. It is likely that the success of the Ericales in this type of habitat is due to the endomycorrhizal associations they from - termed ericoid mycorrhizas. |
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| Some members of the order Ericales form arbutoid and monotropoid associations, which are dealt with in their own section. | ||
| The fungi involved in ericoid associations are Ascomycetes and/or Deuteromycetes. The mycorrhizal roots formed by ericoid fungi show considerable uniformity through the Ericales (Smith & Read, 1997). | ||
 Ericoid mycorrhizal root of Epacris impressa. Reprinted from McLennan (1935) © New Phytologist with permission. |
The primary roots for Calluna vulgaris, for example, grow vertically in the soil, giving rise to secondary roots, which terminate in very fine hair roots. It is the hair roots that are primarily colonised by ericoid fungi. The hair roots consist of an inner stele, surrounded by a cortex of layers - the outer exodermis and the inner endodermis. A single layer of epidermal cells forms the outer surface of the hair root (Smith & Read, 1997). |
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The ericoid fungal hyphae form a loose network over the hair root surface, as shown left. The hyphae can also penetrate the epidermal cells, often at several points in each cell. Coiled hyphae fill the cell (as shown in the diagram above, and the picture below). Up to 80% of root volume can be fungal tissue (Carlile & Watkinson, 1994). It is through these coils that nutrient exchange is thought to occur. |
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| Electron Micrograph of Rhododendron root colonised by loose wefts of fungi. Reprinted from Smith & Read. Mycorrhizal Symbiosis. Fig. 12.3 © (1997) by permission of the publisher Academic Press London. | |
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Colonisation by ericoids is restricted to expanded epidermal cells (i.e. mature cells). Therefore the apical region of the hair root, behind the growing meristem, remains uncolonised until the cells differentiate and mature (Jackson & Mason, 1984). |
 Coiled hyphae inside epidermal cells. From Kendick (1999). © Mycologue Publications with permission. |
The fungi involved in ericoid associations, the Ascomycetes and the Deuteromycetes are facultative i.e. they can be free-living in soil, and can also be cultured. When grown on nutrient agar the fungi, termed isolates, produce dark-coloured, slow growing, sterile mycelia (Smith & Read, 1997). |
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 Zigzag segmentation shown by hyphae. Reprinted from Smith & Read. Mycorrhizal Symbiosis. Fig. 12.10 © (1997) by permission of the publisher Academic Press London. |
The absence of spores, the reproductive structures, has previously complicated the identification and classification of ericoid fungi. The segmentation of hyphae into zigzags by some species such as Hymenoscyphus ericae can help classify the fungi somewhat, but it is the increasing use of techniques examining DNA and RNA profiles that has advanced this area of research. It has been shown by Hutton et al (1994) that there may be considerable genetic differences between isolates that are superficially similar in appearance. |
Very little is known about the evolution of ericoid mycorrhizas. The association is geologically fairly recent, with fossil records of Ericaceae known from the early Tertiary (around 55 mya), and Epacridaceae from the Cretaceous (Lewis, 1987). (To see a discussion of the evolution of mycorrhizas, click here) |
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