Getting to the Root of the Matter
Close up of fly agaric – Amanita muscaria, a common mycorrhizal fungus
As an undergraduate, over 40 years ago, I learnt about the way in which plants absorb water and nutrients through their roots. Descriptions of the chemical and physical processes involved were accompanied by pictures of root hairs and estimates of their phenomenal surface area. No mention was made of fungi. In separate lectures I learnt that some species of mushroom and toadstool formed intimate relationships with the roots of trees.
Some of the earliest work on the link between trees and fungi was initiated in the 1880s. It followed a request by the Prussian Minister for Agriculture, Lands and Forests for research into the commercial growing of truffles. A.B. Frank examined tree rootlets and observed their associated sheath of fungal threads. He called the association a mycorrhiza [from the Greek mykes, (Latin mycos) fungus, and rhiza, root].
A mycorrhiza is now defined as a symbiotic relationship (in that usually both plant and fungus benefit) in which the mycelium of a fungus supplies soil-derived nutrients to a plant root. In return the fungus is supplied with sugars manufactured by the plant. About 40% of the world’s woodland mushrooms and toadstools are believed to be involved in mycorrhizal relationships with trees. However, trees are not the only plants involved and recent research has estimated that over 90% of plant species in the world develop fungus roots through which nutrients and water are absorbed.
Given the Greek/Latin basis of the word mycorrhiza, the plural was originally written as mycorrhizae; a term I remember using in my student days. In the modern world it appears that mycorrhizae have been replaced by mycorrhizas, a reversal of the Victorian pedantry that fought against funguses in favour of fungi as the plural of fungus. More confusing still is the use of mycorrhiza to imply the plural; a cause of sleep loss only alleviated by the counting of sheeps.
Only 3% of the world’s plant species produce sheathing mycorrhizas of the type first studied in the 19th century and now termed ectomycorrhizas (Greek ektos, outside). Among these plants are many common trees (and some woody shrubs) of temperate regions, while the fungal partners include well-known mushrooms and toadstools. Ectomycorrhizas are commonly associated with trees such as oak, beech, birch, willow, alder and hazel, together with conifers including pine, larch and spruce. Tree species rarely, or never, forming ectomycorrhizas include sycamore, ash, elm, rowan and hawthorn; a major reason why the ground under these trees is less productive for mushroom hunters.
Fungi that form ectomycorrhizas with only one, or a small number of tree species include edible species of truffle, chanterelle, bolete, milkcap and hedgehog mushroom along with toadstools such as death cap (Amanita phalloides), fly agaric (Amanita muscaria) and brown rollrim (Paxillus involutus). A tree’s roots may cover an area that is much greater than its canopy and this can result in the production of fruitbodies of ectomycorrhizal fungi as far as 100m from their host’s trunk; the connection is not always obvious. Confusingly, the hyphae of some ectomycorrhizal fungi obtain their nutrients from fallen leaf litter rather than via plant roots. Woodland toadstools occurring on areas of chalk grassland where there are no trees have been shown to form mycorrhizas with rock-roses, which are small woody shrubs. While leading a group of botanists to see the marsh helleborine orchid in the wet dune slacks near Harlech I was fascinated to observe several large fruitbodies of brown rollrim (Paxillus involutus) towering above the creeping willow with which they had formed a mycorrhizal association; an Alice in Wonderland wood where the mushrooms were bigger than the trees.
The importance of ectomycorrhizas lies in their influence on the growth of the trees that are involved. It is still a commonly held fallacy that a tree ‘infected’ with toadstools, appearing from the ground around it, will be less healthy than an uninfected one. That there is a cost to the tree is shown by research indicating that as much as 25% of the host’s photosynthetic products, mostly in the form of glucose and fructose, pass to the fungus, but what the tree gains will often outweigh this drain on its carbohydrates. Plant roots only absorb a small percentage of soil-based nitrogen and phosphorus, two nutrients essential for plant growth. Fungus roots which develop long-lived side branches, increase this absorption by up to 50-fold, especially in poor soil areas. The implications for forestry are enormous. For many years tree seedlings have been inoculated with ectomycorrhizal fungi before being transferred to new plantations on land that has not previously grown trees. The growth rates of such trees have been shown to be up to twice those of non-inoculated trees that have not formed beneficial fungus roots. Ironically, the addition of inorganic fertilisers in poor soil regions used for forestry may have a detrimental effect on the mycorrhizas with the result that less nutrient reaches the trees.
Brown rollrim –Paxillus involutus
{Martin Garwood/NHPA}
Before Frank’s pioneering work, similar, short stubby rootlets, encased in a matted sheath, had been observed by American scientists trying to determine how ‘parasitic’ Indian pipe plants (which lack the green pigment chlorophyll and thus the ability to manufacture sugars) obtained their nutrients. In Britain the bird’s nest orchid, a woodland plant that also lacks chlorophyll, has been shown to form a parasitic relationship with a nearby existing tree/fungus mycorrhizal association. Bird’s nest describes the appearance of the orchid/fungus/tree roots, a feature that remains hidden underground.
Trees rarely or never forming ectomycorrhizas include holly, yew and field maple, along with introductions such as horse chestnut and London plane. Such trees and a very wide range of shrubs and non-woody plants (herbs) form endomycorrhizas in which the fungal partner invades the root cells of the plant. The fungi that are involved belong to a group known as the Glomales. These differ from mushrooms and toadstools in that their hyphae lack cross-walls and very few species produce large fruitbodies. Partly for this reason, VA (vesicular-arbuscular) mycorrhizas, as they are now known, were largely ignored until the 1970s.
It is now becoming apparent that VA mycorrhizas aid growth in trees that do not benefit from ectomycorrhizas. Discussing the implications of this, a leading researcher in the field observed ‘If a tree is not mycorrhizal, it is a dead tree’. VA mycorrhizas are also found in most herbaceous plants, including grasses and cereals. Here too the result is increased absorption of nitrogen and phosphorus by the plants. The transportation of phosphorus by fungal mycelia has been observed at rates as high as 0.5m per hour; a very efficient distribution network. This may necessitate a review of agricultural practices. The cultivation of soil destroys the mycelia that forms part of the beneficial mycorrhizas, not to mention the negative effects on the fungal partners of inorganic fertiliser additions.
VA mycorrhizas are also found in mosses and ferns. Fossil relatives of the ferns, complete with fungus roots, have been found in rock deposits from 400 million years ago. It is probable that mycorrhizas helped plants to colonise the land and thus played an important role in the evolution of higher plants.
In addition to increased nutrient absorption, plants benefit from mycorrhizas through an increased uptake of water. The ability of many plants to cope with climate change may be dependent on the presence of healthy mycorrhizas. Successful plant colonisation of soils of unusually high or low acidity, a common feature of land reclaimed after mining or industrial activity, is also dependent on the beneficial results of mycorrhizas. Additionally, mycorrhizas appear to give plants some protection from pathogenic fungi and parasitic nematodes.
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