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Symbionts in Ericoid Mycorrhizas

Host Plants

As far as we know, only members of the order Ericales form symbioses of the ericoid type. Known exceptions in the order are the families Pyrolaceae, Monotropaceae and the genus Arctostaphylus in Ericaceae. However, study of members of the families Ericaceae provides the basis for our understanding of ericoid mycorrhizas.

Members of Ericaceae are found commonly in the heath vegetation of the northern hemisphere where genera such as Calluna are maintained. In contrast, Australian members of the Ericaceae such as Epacris and Woolsia form a small component of complex heath communities of the southern hemisphere. Various species are found together in these vegetation systems. They also form part of the understory components of various woodlands and open forests.

The Fungi

A few members of the Holobasidiomycetes and many Ascomycota (probably only Leotiales) have been observed as coils in the fine roots of field collected plants. On isolation, those in culture appear to be slow growing and dark in colour. Only a few have fruited in culture. Using molecular techniques, it has become clear that one root fragment may contain many fungi. Adjacent cells may be colonised by different strains or species of fungus.

One species, Hymenoscyphus ericae, has been studied in detail. Other confirmed symbionts include members of the anamorphic genus Oidiodendron.

The difficulty of determining identity of mycorrhizal endophytes is that some fungi in coils are difficult to isolate to culture, and once in culture confirmation of benefit requires specific plant and fungal cultural conditions. At this stage, conditions for germinating seed of many Australian species are unknown. Simply identifying fungi in coils does not mean those fungi are mycorrhizal symbionts.

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Conclusion

Most members of the order Ericales commonly form a typical mycorrhizal association. Other members of the order have associations that more closely resemble ectomycorrhiza. The ericoid mycorrhizal fungi mostly belong to a restricted group within the Leotiales. Again, however, a few Basidiomycota can associate with ericoid plants and form morphologically typical ericoid mycorrhizas.

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References

Smith SE & Read DJ (2008) Mycorrhizal Symbiosis. Academic press, London.

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