Mycorrhizae: fungi in partnership with plants

by Cassandra McLean

When a plant looks sick we tend to focus on problems such as lack of water and/or nutrients, too much fertiliser, or a pathogen. But what else is going on underground?

Mycorrhiza is the name given to the relationship formed between the roots of a plant and fungi. The word mycorrhiza literally means 'fungus-root' and the relationship is usually beneficial to both parties. Mycorrhiza have an important role to play in the soil and in a typical Australian environment most plants will have mycorrhiza of one kind or another.

In simple terms mycorrhizas can be grouped into two main types. Endomycorrhizas have most of the fungus inside the root (Figure 1. left) and ectomycorrhizas have most of the fungus on the surface of the root (Figure 2. right ). Ectomycorrhizas occur on woody plants such as eucalypts, pines, birches, willows and oaks. The fruiting structures of the fungi involved can be seen in winter under these trees as mushrooms, puff balls and earth stars. Plants with endomycorrhizas include many horticulturally important species.

A plant may be colonised by a number of different mycorrhizas simultaneously and these may change during its lifetime. The type of mycorrhizal relationship present depends on the species of plant, climate and nutrient composition of the soil in which the plant is growing.

Health and other benefits to Plants

Nutrients

Fungi are organisms consists of very fine filaments called hyphae that can spread for metres through the soil taking up nutrients. Plants in a mycorrhizal relationship have increased uptake of nitrogen, phosphorus, micronutrients and water due to the large surface area of the fungal hyphae. The result of this increased uptake is larger and healthier plants (Figure 3. left). Unlike plants, fungi do not contain chlorophyll and therefore need a source of energy. In return for providing nutrients to the plant the fungi receive sugars.

Soil chemistry and structure

Mycorrhiza can improve plant growth and reduce the need for large amounts of fertiliser but have other benefits as well. Mycorrhizal fungi can influence soil chemistry and structure. They produce an array of chemicals such as enzymes and organic acids that are secreted into the soil. These chemicals break down otherwise unavailable minerals that are then absorbed by the fungus and transferred to the plant. The structure of the soil is improved by the presence of mycorrhizal hyphae as they spread through the soil binding particles and reducing soil erosion. The presence of mycorrhiza and the production of the chemical residues help to form stable soil aggregates that bind organic matter and associated nutrients, improving soil structure and stability

Deterring pathogens

Both ectomycorrhizal and endomycorrhizal plants have shown some resistance to attack from nematodes and pathogenic fungi. The mechanism of this protection is not well understood but is thought to be due to one or more of the following. Ectomycorrhizas provide physical protection by covering the root, making penetration by nematodes and pathogenic fungi more difficult. Endomycorrhizas change the physiology of the root and may make them less attractive to the pathogen. Most mycorrhizal fungi produce inhibitory substances that may affect the viability of pathogens. The improved health of a mycorrhizal plant may also help to ward off attack.

Plant Production

In forestry, agriculture and horticulture, plants can be inoculated with mycorrhizal fungi to improve plant establishment. Both container grown and bare rooted plants can be inoculated and at first soil taken from under established plants was used as a source of fungi. Soil, however, is not a satisfactory source of mycorrhizal fungi as soil borne pathogens can also be introduced. 'Nurse seedlings' can be produced by growing plants under established trees but this a slow process and success is variable. The usual method of introducing mycorrhizal fungi is to use a commercially prepared product where the fungus has been grown without soil.

Mycorrhizas are important in establishment and growth of plants and can improve soil chemistry and structure. This has implications for disturbed sites where soil structure, chemistry and microflora have deteriorated. Failure of revegetation plantings at these sites may be due to the absence of the correct mycorrhizal fungi. The increasing use of mycorrhiza in production horticulture means that in the future your purchases from the local nursery may contain little fungal helpers.

About the writer: 
Cassandra McLean is a lecturer in Plant Science at Burnley. Her research interests include using mycorrhizas to improve plant health and survival in nursery production and revegetation, with particular focus on mycorrhizas in the Epacridaceae.   

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