Mycorrhizal Fungi as Beneficial Organisms to Plants

By Mark Thomas

Fungi, although not plants themselves are often closely linked to the survival of plants through the formation of symbiotic relationships, these are called Mycorrhizal Fungi. Up to 90% of plant species rely on such symbiotic relationships with fungi for the increased uptake of essential nutrients, increased disease resistance and drought tolerance. It has been estimated that up to 15% of the biomass of all plant roots are actually fungi1. Some individual fungal colonies can be massive with sizes of up to 15 hectares and weighing 100 tons1.

Fungi do not contain chlorophyll; the photosynthesising pigment found in plants and so must rely on the nutrients obtained from the breakdown of organic material. Mycorrhizal Fungi, like many other types of fungi have filamentous hyphae that form a web- like network called mycelium which release enzymes into the organic matter substrate and re-absorb the released nutrients. The difference is that in the case of the mycorrhiza these nutrients are transferred to the plant via a beneficial infection directly to the roots of the plant either outside of the root (ectomycorrhizae) or through the cell wall of the outer roots (arbuscular mycorrhizae).

Mycorrhizal Fungi

Mycorrhizal Fungi                 Photo: Mark Thomas

The fungi in mycorrhizal associations act as extensions to greatly increase the surface area of the plants root- system and increase soil contact. Nutrients such as phosphorous, zinc, copper and several forms of nitrogenous compounds are transferred from the fungi to the beneficially infected plant in exchange for sugars produced by the plant via photosynthesis2. Many authorities suggest that the increased surface area also extends the ability of the plant to uptake more moisture in the soil, increasing drought tolerance4. Dominance of the rhizosphere or root- zone of the soil by beneficial fungi interacting with the plant can act as a powerful defence mechanism.

Harmful fungi can be kept from infecting the roots of the plant if a beneficial infection already exists3. More than one species of fungi can infect the roots of a plant with many fungi infecting and in so doing connecting large numbers of plants in one area1.

The difference in soil texture may also be important for mycorrhizal fungi as lighter or more open soils are easier for the hyphae to penetrate and extend the root zone. There would be a definite benefit for heavier more compacted soils if organic matter such as compost was incorporated into the soil to encourage the rapid expansion of the plant- fungi association. The beneficial infection is very important in phosphorous depleted soils such as those throughout much of Australia as small amounts of the nutrient can be quickly cycled back to plants from dead organic matter. Some plants don’t have such a relationship with fungi, for example some native families; Proteaceae, Chenopodiaceae, Cyperaceae, Juncaceae. These plants have alternative methods for dealing with the cycling of small amounts of vital nutrients such as the Banksias, which have proteoid roots that have a very large combined surface area. They can uptake nutrients directly for the plant in a similar way to the fungi through exploiting organic matter that is not yet decomposed.

1– Raven,P, Johnson,G., Losos, J.& Singer, S. 2005, Biology,7th Ed,Mc Graw- Hill,U.S.A
2– Fuhrer, B 2005, A Field Guide to Australian Fungi, Bloomings Books, Melbourne
3– Marschner,P.2004, Soil Ecology & Nutrient Cycling lecture notes, University of Adelaide
4– Buchanan, R.,1989 Bush Regeneration, Recovering Australian Landscapes, Tafe N.S.W.

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