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Section 1.

INTRODUCTION TO MYCORRHIZAS

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Written and Illustrated by Mark Brundrett
CSIRO Forestry and Forest Products
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Index

  1. What are Mycorrhizas?
  2. Types of Associations
  3. Host plants
  4. Where are Mycorrhizal Plants Found?
  5. Mycorrhizal Fungi
  6. Terminology
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A. What are Mycorrhizas?

3-way diagram (7KB) Mycorrhizas are highly evolved, mutualistic associations between soil fungi and plant roots. The partners in this association are members of the fungus kingdom (Basidiomycetes, Ascomycetes and Zygomycetes) and most vascular plants (Harley & Smith 1983, Kendrick 1992, Brundrett 1991). In the mycorrhizal literature, the term symbiosis is often used to describe these highly interdependent mutualistic relationships where the host plant receives mineral nutrients while the fungus obtains photosynthetically derived carbon compounds (Harley & Smith 1983). Mycorrhizal associations involve 3-way interactions between host plants, mutualistic fungi and soil factors. Roles of mycorrhizal associations are explained further in Section 5.
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B. Types of Associations

At least seven different types of mycorrhizal associations have been recognised, involving different groups of fungi and host plants and distinct morphology patterns. Only ECM and VAM associations are considered in detail in this site.

  1. Vesicular-arbuscular mycorrhizas (= arbuscular mycorrhizas, VAM or AM) are associations where Zygomycete fungi in the Glomales produce arbuscules, hyphae and vesicles within roots. Spores are formed in soil or roots. These associations are defined by the presence of arbuscules. Fungi within roots spread by linear hyphae or coiled hyphae. VAM associations are fully described in Section 3.

MicroscopeHighly magnified views of cleared and stained roots of Asarum canadense (upper) and Allium porrum (lower)

Coils and arbuscules (11KB) Arbuscule (13KB)
  1. Ectomycorrhizas (ECM) are associations where Basidiomycetes and other fungi form short swollen lateral roots covered by mantle hyphae. These roots have Hartig net hyphae around cells in the epidermis or cortex. ECM associations are fully described in Section 4.

MicroscopeHighly magnified views of cleared and stained sections of Pinus (upper right) and Populus (lower right) ECM.
Microscope Lower magnification view of Betula roots with ECM (arrows) (below).

Ectomycorrhizal short roots (16KB)

Hartig net of pine (11KB) Hartig net of Poplar (15KB)
  1. Ectendo-, arbutoid- and monotropoid mycorrhizal associations are similar to ectomycorrhizas (see Table below). These associations have not been found in Australia.

MicroscopeHighly magnified view of a sectioned Monotropa root. Note hypha projecting into epidermal cell (arrow).

Monotropa mycorrhiza (13KB)
  1. Orchid mycorrhizas consist of coils of hyphae within roots or stems of plants in the family Orchidaceae. Young orchid seedlings and some adult plants which lack chlorophyll are entirely dependant on mycorrhizal fungi for their survival. Details of Orchid mycorrhizal associations are not provided here, but Australian Orchids found to have mycorrhizas are listed.

microscopeHighly magnified views of cleared and stained stem of Pterostylis vittata (upper) and root of Thelymitra crinita (lower)

Orchid mycorrhiza in rhizome (13KB) Hyphal coils in orchid root cells (12KB)
  1. Ericoid mycorrhizas have hyphal coils in outer cells of the narrow "hair roots" of plants in the plant order Ericales. These associations also occur in thicker roots of Australian members of the Epacridaceae. These associations are not described in detail, but Australian plants with these mycorrhizas are listed.

microscopeHighly magnified views of cleared and stained roots of Leucopogon verticillatus (Epacridaceae)

Leucopogon hair root (11KB) Leucopogon thicker root (16KB)
  1. The Australian lily Thysanotus has a unique mycorrhizal association where fungus hyphae only grow under epidermal cells.

MicroscopeHighly magnified views of a sectioned and stained root of Thysanotus (Anthericaceae)

Mycorrhiza of Thysanotus (11KB)

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Table summarising key differences between mycorrhizal association types (after Harley & Smith 1983).
Type VAM ECM Ectendo- Arbutoid Mono-
tropoid
Ericoid Orchid
Septate hyphae - (+) + - + - + + + +
Hyphae in cells + - + + + + +
Hyphal coils + - - - - - + +
Arbuscules + - - - - - -
Mantle - + (-) + (-) + + - -
Hartig net - + + + + - -
Vesicles + - - - - - - -
Plants Vascular
plants
Gymnosperms &
Angiosperms
Ericales Mono- tropaceae Ericales Orchid-
aceae
Chlorophyll + + + + - - + + -
Fungi Zygo- Glomales Most Basid-, but some Asco- and Zygo- Asco- (Basid-) Basid-
Notes: - = absent, + = present, (+)= sometimes present, (-)= sometimes absent, +- = present or absent, Basid- = Basidiomycetes, Asco- = Ascomycetes, Zygo = Zygomycetes.

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C. Host Plants

The relative importance of different mycorrhizal associations for plant species is graphically summarised below using data for Angiosperms from the mycorrhizal literature (Trappe 1987). Many Pteridophytes are also known to have VAM associations, while Gymnosperms may have ECM or VAM associations. Most information about the mycorrhizal status of plants is from the northern hemisphere in temperate regions, especially for crop plants and weeds (Trappe 1987).

Mycorrhizal Associations of Angiosperm Plants

Graph (5KB)

Proportion of angiosperm species:
  • 18% were not found to have mycorrhizas
  • 50% reported to have VAM
  • 12% reported to have VAM in some cases, but not in others
  • 20% had another type of association (ECM, orchid, ericoid, etc.)
Data compiled by Trappe (1987) from a dataset representing 3% of Angiosperm species.
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D. Where are Mycorrhizal Plants Found?

Field surveys have found that plants with mycorrhizal associations predominate in most natural ecosystems in Australia and throughout the world (Brundrett 1991). The Table below briefly summarises the importance of mycorrhizal associations throughout the world. More information about mycorrhizal associations in Australian plants in Section 6.

Association Occurrence
Vesicular Arbuscular Mycorrhizal (VAM) Plants
  • Plants with VAM are common in most habitats
  • It is easier to say where they are not found
Ectomycorrhizal (ECM) Plants
  • Trees with ECM are dominant in coniferous forests, especially in cold boreal or alpine regions
  • ECM trees and shrubs common in many broad-leaved forests in temperate or mediterranean regions
  • ECM trees also occur in some tropical or subtropical savanna or rain forests habitats
Nonmycorrhizal (NM) Plants
  • NM plants are most common in disturbed habitats, or sites with extreme environmental or soil conditions
  • NM plants appear to be more common in Australia than in other continents (See Section 5).
Notes: Data from Brundrett (1991).

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E. Mycorrhizal Fungi

Spores (12KB) Members of the fungus kingdom obtain nutrition from many sources, including decomposition of organic substrates, predation and parasitism, and involvement in mutualistic associations (Christensen 1989, Kendrick 1992). Many soil fungi are saprobes with the enzymatic ability to digest organic substrates of varying degrees of complexity, but some subsist on very low levels of organic or inorganic substrates (Wainwright 1988). Mycorrhizal fungi are a major component of the soil microflora in many ecosystems, but usually have limited saprophytic abilities (Harley & Smith 1983). Mycorrhizal fungi are considered to have many important roles in natural and managed ecosystems.

We know very little about the taxonomic or functional diversity of mycorrhizal fungi in Australia (Bougher 1995). Indeed, there is a paucity of knowledge about all Australian fungi, with only about 10% of these organisms having been named, while an estimated 80% of them remain undiscovered (Pascoe 1991). Laboratory and glasshouse experiments have shown that different fungal taxa vary in their capacity to utilise resources, withstand adverse conditions, etc. For this reason, mycorrhizal fungus diversity is thought to contribute to the resilience of ecosystems, but there is very little comparative data that can be used to test this hypothesis. More information about fungi forming ECM and VAM associations is provided in the relevant sections.

Fungi (11KB)
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F. Terminology

Vascular plants
"Higher plants" with conducting elements for water and nutrients, differentiated leaves and roots, with a dominant sporophyte.
Roots
Plant organs responsible for nutrient uptake, mechanical support, storage, etc. that are usually subterranean.
Fungi
Members of the fungus kingdom are eukaryotic, heterotrophic organisms with a tubular body that reproduce by spores.
Mineral nutrients
The basic form of substances required for life (N, P, K, etc. - excluding gases).
Photosynthesis
Solar energy capture by plants and conversion into organic carbon.

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This page was last revised April 10 2000


Start
Here

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this page

Roots

VAM

ECM

Roles

Australian
Plants

Eucalypt
Associates

Methods

Refer-
ences