Did you know … ? – Mycorrhizae

SESL Australia

Arbuscules (tiny bushy structures) inside maize root cellsThe word mycorrhiza comes from the Greek mukes, fungus, and rhiza, root. A mycorrhiza (plural mycorrhizae, which confusingly is Latin, not Greek; or just mycorrhizas) is a “fungal plant root”. It is a fungus that invades a plant root to form a mutually beneficial relationship – a symbiosis.

Most plants have mycorrhizal roots – around 92% of plant families and 80% of plant species.

Fungi from many different groups have developed an association with plant roots, so the origin of the symbiosis appears to have been a common discovery by various fungi and plants because of the benefits it brings both members of the partnership. Indeed, the oldest known fossils showing mycorrhizae are about 400 million years old.

The fungal hyphae (the very fine threads that form the bulk of most fungi) are much finer than plant roots: they can therefore explore much tighter spaces in the soil, and increase the effective root surface area by up to 100 times, maximising the extraction of both nutrients and water. In particular, mycorrhizae are adept at extracting tightly bound phosphorus from soil, using enzymic extraction methods not available to plants.

Mycorrhizae fall into two different types – the endomycorrhizae (“inner mycorrhizae”), which invade plant cell walls (the outer layer), though not the cell membrane (the inner layer); and the ectomycorrhizae (“outer mycorrhizae”), which sheathe the roots and invade the spaces between the cells. Most mycorrhizae are endomycorrhizae.

Mutual benefits

The benefits of the relationship go both ways. The plant benefits by receiving nutrients and water gathered by the fungi, and the fungus benefits by receiving carbon compounds created by the plants. The soil benefits too: the various polysaccharides exuded by the fungi help bind soil aggregates together, improving soil structure, resilience and moisture-holding capacity. In addition, mycorrhizae can protect plant roots against pathogenic fungi.

Many crop plants have mycorrhizal roots, notably the cereals. Experiments show that mycorrhizae increase P uptake by plant roots several-fold, and consequently plant yield.

Mycorrhizal fungi are natural inhabitants of undisturbed soils, but repeated ploughing, tilling, fertilising and use of fungicides can effectively sterilise soils. Crop yields then become entirely dependent on fertilisers. The reintroduction of mycorrhizae can significantly increase yields.

Mycorrhizal inoculum is available commercially.

Further information

Brundrett MC. 2008. Mycorrhizal Associations: The Web Resource. Site visited Nov 2008.