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Symbionts in Ectomycorrhizas


The central concept to understand is that ectomycorrhizal associations appear to differ with environment. In the northern hemisphere, ectomycorrhizas appear to be primarily associated with forest trees and the diversity of plant species in these forests is limited. In the tropics, only a few forest trees form ectomycorrhizas and these appear in monospecific stands. In these cases, diversity is defined by the fungal symbionts.

In the southern hemisphere, ectomycorrhizas are found on a broad range of trees, shrubs and herbs. The diversity of plants and fungi can be very high in any one habitat: neither fungi nor plants define the habitat.

The Fungi

Many thousands of different fungi are known to form ectomycorrhiza (EM). Most of the fungi come from the Holobasidiomycota and Ascomycota and a few from the aseptate fungi such as Endogone and Densospora.

The level of specificity varies. Some fungal species are specific to a single host species, and some fungi can form EM with a wide range of plants.

Most of the fungi have an obligate ecological association with their hosts. Though we can culture the fungi on agar, the fungi do not survive long in soil in the absence of their host. The obligate association indicates that the fungi gain specific cues or nutrients from the plant partner.

Most of the fungi have the capacity to form fruiting bodies, which either release spores to the wind LINK and thus may be distributed widely, or are attractive to mycophagous animals LINK that distribute the spores.

Bettongia sp.
Bettong 1
Bettongia sp .


The Plants

Patch of sheath, sufficient to increase growth of colonised seedling.

The range of plants which may form EM needs to be qualified with the realisation that the ecological importance of EM to these plants is unknown in most cases. Most of the trees of temperate northern hemisphere forests form EM, and for these trees, the fungi are obligately associated with the survival of the host. The trees are found in the families including Betulaceae, Cuppressaceae, Fagaceae, Juglandaceae, Pinaceae, Platanaceae, Oleaceae, Salicaceae, Taxaceae, and Ulmaceae, and from the southern hemisphere, Casuarinaceae, Mimosaceae, and Myrtaceae.

A significant proportion of the trees also form AM in the field, and a few may form ectendomycorrhiza as seedlings. The remainder have not been examined in detail.

Many herbs, shrubs and trees have been noticed with EM in surveys of Australian vegetation. In some cases the morphology of the EM differs from what has been reported in trees, and this variation appears to be related to the fungal associate as well as the environmental conditions. Where tested, the fungi that form EM with an atypical morphology have a typical effect on relative rates of plant growth.

Australian families reported to contain members that form EM include Asteraceae, Campanulaceae, Casuarinaceae, Euphorbiaceae, Fabaceae, Goodeniaceae, Mimosaceae, Myrtaceae, Nyctaginaceae, Polygonaceae, Rhamnaceae, Rubiaceae, Sterculiaceae, Stylidiaceae, and Thymelaeaceae. Many of the plants are herbs, some are annuals. The functions of the association in annual herbs may well differ from what we understand from studying trees.



M.F. Allen (Ed)(1992) Mycorrhizal Functioning. Chapman Hall, New York.


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