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Sunday 11th March, 2007

Mycorrhiza - a brief introduction

By Ash

If you don’t have a clue what a mycorrhiza is, or if you have a vague idea but nothing substantial, then read on: I will do my best to edify you.

Simply put, mycorrhizas are intimate mutualisms between fungi and plant root tissue. Almost all higher plants are mycorrhizal, and those that are will be either obligative (the mutualistic species cannot survive if separated) or facultative (the mutualistic species can survive if separated, but mutualism is preferable). Trees are generally mycorrhizally facultative, whereas orchids are generally mycorrhizally obligative. ‘Mutualism’ is ecology jargon, but its meaning is fairly obvious – two organisms live in intimate association with mutual benefit.

With mycorrhizas, the host plant benefits by receiving nutrients from the fungal network that it is not capable of extracting from the soil itself. The mycorrhizal fungi benefits by receiving carbon from the plant in the form of photosynthate sugars. This mutualism is millions of years old, with the fossil record suggesting that the earliest land plants were heavily infected by mycorrhizal fungi.

There are a number of different types of mycorrhiza. When it comes to trees, two kinds are really important: arbuscular mycorrhizas and ectomycorrhizas. Arbuscular mycorrhizas are found in about two thirds of all plant species, particularly tropical trees and non-woody species. The fungi involved are not host specific, and there are only a few hundred known species. Ectomycorrhizal fungi generally are host specific, and there are about 6000 known species world-wide. They dominate in boreal and temperate forests.

What I find most interesting is that large (typically arbuscular) mycorrhizal fungal networks in forests often simultaneously colonise hosts of differing age and species. Even more interesting is the fact that nutrients can be exchanged between different tree individuals via the mycorrhizal fungi! A study* reported that "91% of paper birch and 56% of Douglas fir intermingled mycorrhizal roots examined were colonized by the same mycorrhizal fungi. In these tree species, a 4–7% net transfer of isotopically labelled carbon from birch to Douglas fir has been demonstrated". Amazing! Recently an ericoid mycorrhizal fungus (mutualistic with ericaceous plants such as heather) was shown to associate with Quercus (oak) roots. This suggests the possibility of some trees sharing nutrients with ericaceous plants!

* Durrell, D. M., Jones, M. D., Molina, R., Myrold, D. D., Perry, D. A. and Simard, S. W. (1997). Net transfer of carbon between ectomycorrhizal tree species in the field. Nature, 388, 579-582.

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