Symbiotic relationships

Scleroderma sp. (a mycorrhizal fungus)

Mycorrhizal fungi make the survival of most of earth’s land plants possible by partnering with them in the mutually-beneficial exchange of nutrients. ‘Mycorrhiza’ means fungus-root. Put simply, a fungus is intimately connected to a plant partner and absorb sugars from it. (10-30% of the food produced by the plant can be transferred to the fungal partner.)

Plant partners in a mycorrhiza receive inorganic nutrients such as nitrogen, potassium and phosphorus in forms that are easily absorbable by the roots, together with water from the soil.

In good conditions, hyphae are able to grow very quickly in response to rain, much sooner than can plants. The fungus can collect water and nutrients and quickly make them available to the plant. If the soil dries out for extended periods, mycelia may die off but the fungal material inside the plant root will be protected and able to regrow the mycelium once conditions become favourable.

Mycorrhiza fungi can produce plant hormones, and may confer resistance to environmental stress, tolerance to heavy metals and salt, and give protection against soil microbes and plant pathogens.

90% of plants benefit from this relationship and studies in different Australian habitats have found that at least two thirds of plant species form mycorrhizal relationships.

Mycorrhizal fungi appear to be less diverse in disturbed sites and plantations. There is great scope for investigating the importance of mycorrhizal fungi for re-vegetation and environmental remediation.

Here are the two main ways in which this partnership works.

Ectomycorrhizal fungi are the most prevalent form of mycorrhizal fungi, numbering more than 6,000 species. (Examples Amanita xanthocephala and Cortinarius archeri)

These fungi wrap a dense mantle of specialised hyphal filaments around young roots and grow into the spaces between plant cells but do not penetrate the cells. A complex exchange occurs with mineral nutrients passing from fungus to plant while metabolites are transferred from plant to fungus. Ectomycorrhizal fungi can also protect plants from disease and damage and the fruit bodies are food and shelter for animals.

The fruit-bodies of ectomycorrhizal fungi tend to be visible near to the base of the plant. Ectomycorrizal fungi include Hebeloma, Inocybe, Lactarius, Austropaxillus and Russula but not all ectomycorrizal fungi are ‘mushroom-like’. Most truffles and some cups, corals, and boletes and a few puffball-like fungi are also ectomycorrhizal.

Endomycorrhizal fungi penetrate or colonise the cells of plants. Australian orchids need endomycorrhizal partners for seedlings to survive beyond germination stage and some need these fungi throughout the orchid’s life.

Remarkably, there are several Australian orchids that have no chlorophyll and their mycorrhizal partners harvest carbohydrates from other plants to be shared with those orchids.