What are Mycorrhizae?

The word mycorrhiza was first used by German researcher Albert Bernard Frank (1885). It originates from the Greek word ‘mycos’ meaning ‘fungus’ and ‘rhiza’ meaning ‘root’. It forms a symbiotic association in a similar fashion to the root-nodule bacteria in legumes. The fungus takes carbohydrates from the plant and in turn supplies the plant with nutrients, hormones, ect.
Types of mycorrhizal associations

1. Ectomycorrhizae
2. Endomycorrhizae
3. Ericoid mycorrhizae
4. Orchidaceous mycorrhizas

Ectomycorrhizae
Ø Fungus forms a sheath around the root, with hyphae emanating through the soil, greatly increasing the surface area
Ø Fungus penetrate between the cells of the cortex to facilitate nutrient exchange
Ø Fungus is almost a Basidiomycota although a few are Ascomycota species.

Endomycorrhizae
Ø Also known as Vesicular arbuscular-mycorrhizae
Ø Fungus does not forma a sheath around the root fungus, penetrate into the cortical cell but does not penetrate the cell membrane
Ø Fungus is the member of Glomeromycota
Ø More common than Ectomycorrhizae

Ericoid mycorrhizae
Ø The fungus grows loosely over the lateral "hair" roots of the host plant and the septate hyphae penetrate the single layer of cortical cells, often at several points, and fill them with intracellular hyphal coils. The apex of the "hair" root is not colonised and the stele is never penetrated.

Orchidaceous mycorrhizae
Ø The fungus grow into the plant cell, invaginating the cell membrane and forming hyphal coils within the cell. These coils are active for only a few days, after which they lose turgor and degenerate and the nutrient contents are absorbed by devloping orchid.
Ø The mycorrhizal fungi in orchids are Basidiomycota, and in particular species of Rhizoctonia.
Ø In mature orchids, mycorrhizae also have roles in nutrient uptake and translocation.
Ø The ericoid type is the most important and is found in such genera as Calluna, Erica, Rhododendron and Vaccinium.


Effects of Mycorrhizae on plants

ð Increase in nutrient uptake
ð Selective uptake of certain elements
ð Increase in drought resistance
ð Increase in survival after out planting
ð Increase in growth rate
ð Protection from root pathogens

Benefits
Ø Increased plant nutrient supply by extending the volume of soil accessible to plants.
Ø Increased plant nutrient supply by acquiring nutrient forms that would not normally be available to plants.
Ø Some ECM and ericoid fungi have the capacity to breakdown phenolic compounds in soils which can interfere with nutrient uptake.
Ø Root colonisation by ECM and VAM fungi can provide protection from parasitic fungi and nematodes.
Ø Non-nutritional benefits to plants due to changes in water relations, phytohormone levels, carbon assimilation, etc. have been reported, but are difficult to interpret.
Ø Mycorrhizal benefits can include greater yield, nutrient accumulation, and/or reproductive success. Mycorrhizas can cause growth form changes to root architecture, vascular tissue, etc.
Ø Suppression of competing non-host plants, by mycorrhizal fungi has been observed.
Ø Significant amounts of carbon transfer through ECM fungus mycelia connecting different plant species have been measured. This could reduce competition between plants and contribute to the stability and diversity of ecosystems.
Ø Networks of hyphae supported by dominant trees may help seedlings become established or contribute to the growth of shaded understorey plants.
Ø Nutrient transfer from dead to living plants can occur.