Many gardeners would be horrified to hear that most of the plants in their garden have a fungal infection in their roots. Fungi have a bad reputation for causing plant diseases; but not all fungal infections of plants are pathogenic (disease-causing). The infection to which I am referring is a beneficial one, of the type called symbiosis (‘together-living'). In symbiosis, two organisms live in close association to the benefit of both. The symbiotic association between plant roots with fungi (of many species) is termed mycorrhiza, meaning ‘fungus-root'.
Mycorrhiza is a very intimate relationship. In some types (ectomycorrhiza), the fungal threads, the hyphae, make sheaths around the young root tips and penetrate into the root, growing between the plant cells. In the most common form (endomycorrhiza), however, the fungal hyphae grow right into the cells and there branch so profusely, that the mass of fungus in the cell is called an arbuscule, meaning ‘little tree'. One might expect such an invasion to kill the plant cell; but the root cells survive. The entry of the mycorrhizal fungus is a more subtle process than just digesting a hole, as a pathogen might do.
Dactylorhiza maculata, heath-spotted orchid, near Lake Iseo, Italy. Example of a plant dependent on mycorrhiza. Photo by Helgi Öpik.
A plant cell is surrounded by two barriers: a cell wall, and a plasma membrane. The cell wall is a tough, fibrous structure; cotton and linen are made of cell walls and so is wood. The plasma membrane, however, is a delicate structure too thin to be seen even under an ordinary microscope, only visible under an electron microscope. The wall is not an absolutely vital part of the cell, in the sense that in a laboratory, plant cells can be stripped of walls and kept alive, surrounded only by the plasma membrane - but only under very special conditions, sterile and in a very carefully selected medium of protective chemicals. The mycorrhizal fungus produces substances that loosen the wall at the place of contact and the fungal hypha pushes the minuscule wall fibres apart to gain entry through the wall; such a hole in the wall is not fatal. The plasma membrane is a vital, ‘living', structure and damage to it is usually fatal. Every living cell on earth has a plasma membrane. But the fungus does not pierce it: as the hypha pushes in, the membrane stretches and grows. Imagine the fungus growing in like a finger into a rubber glove of membrane, which grows and branches to accommodate it. The fungus is physically inside the cell, but physiologically it is outside, the plasma membrane still separates it from the living contents of the cell. Hence the cell can live on.
What is there in this relationship for the two partners? The fungus receives organic nutrients from the plant, and the plant receives water and mineral salts from the fungus. The fungal hyphae within the plant are connected to a profuse network of hyphae free in the soil, fine threads with an enormous total surface area for the absorption of minerals and water; the fungus thus in effect acts as an extension of the host plant's root system. We become aware of the mycorrhiza when the fungus outside the plant enters the reproductive phase and produces mushrooms or toadstools, which may form a regular ring round the tree.
Mushrooms, the fruiting (spore-bearing) bodies of a mycorrhizal fungus, associated with an oak tree, Newmiln (near Perth), Scotland . Photo by Helgi Öpik.
Many, if not all, of the conspicuous mushrooms under trees are mycorrhizal. Some are specific to particular tree species, others can live with numerous trees. Perhaps the most famous one is the red-and-white fly agaric, Amanita muscaria, which forms mycorrhiza with birch and some other tree species.
Fly agaric. Amanita muscaria. Photo by Michael Wood.
Orchis pallens, pale-flowered orchid, near Lake Iseo, Italy. Another plant dependent on mycorrhiza. Photo by Helgi Öpik.
The existence of mycorrhiza has been known for many years, but it used to be thought of as something rare. In recent decades, it has been found that most field-grown (i.e. as opposed to laboratory-reared) plants have mycorrhiza. Most plant species can grow without it, but the mycorrhiza is highly beneficial, especially in nutrient-poor soils. For soil-growing orchids, the mycorrhiza is obligatory, the orchid seeds cannot even germinate unless infected by the appropriate fungus. The reason that mycorrhiza was not recognized as widely important for a long period is, that laboratory workers generally supply their experimental plants abundantly with nutrient solution,so that
they grow very nicely even without the fungus. The mycorrhiza is not missed. But in the real world where life is tough and plants compete for a limited nutrient supply mycorrhiza comes into its own. Phosphate supply to the plant is particularly increased by mycorrhiza. Even inlaboratory culture, mycorrhiza may improve growth. Biodiversity of plant communities is enhanced. (Van der Heijden et al., 1998 (1)). Also some aspects of plant stress resistance are improved by the fungal symbiosis. In Florida, orange growers decided to kill off fungal pathogens by sterilizing the soil. The result on crop yield was disastrous - they had killed off the mycorrhiza! (Watling, 2001 (2)).
The symbiosis is thought to be ancient: fossils of the first land plants, which appeared some 400 million years ago and are long extinct, show cells infected by fungi which look very like the living mycorrhizal structures of to-day.
Whereas mycorrhiza can offer some protection against some environmental stresses, it is subject to stresses itself. Acid rain hits plants severely through mycorrhiza: the rain acidifies the soil and even if the fungus survives, its beneficial effect on the plant is reduced or lost altogether and nutrient deficiency is added to any direct effect of the acid rain on the plants.
We tend to think of plants as very independent, with very simple nutritional requirements, needing nothing except light, air, water, and certain mineral elements for existence. This is perfectly true. But under natural conditions, if ‘no man is an island', neither is a plant, and plants benefit a great deal nutritionally from close symbiotic relationships with microorganisms, i.e. fungi and also bacteria. Another symbiosis of plants, mainly of the pea and bean family, Fabaceae, (formerly called Leguminosae) with nitrogen-fixing bacteria, is very important in providing nitrogen. These bacteria live within nodules on plant roots and like mycorrhizal hyphae, the bacteria are right inside the root cells, but separated from the cell contents by the cells' plasma membrane.
Root nodules formed by bacteria in an Inga seedling. Microscopic examination would be expected to show also mycorrhiza in the roots. Photo by Gaston Bityo, (VSD), Cameroon 2009.
Finally, fungi are NOT plants. They go in a separate Kingdom Fungi, and in evolutionary terms are probably more closely related to the Animal than the Plant Kingdom.
1. Van der Heijden, M. G. A., Klironomos, J. N., Ursic, M., Moutogolis, P., Streitwolf-Engel, R., Boller, T., Wiemken, A. & Sanders, I. R. (1998). Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity. Nature, 396, 69 - 72.
2. Watling, R. (2001). Mycorrhiza - what every plant should have. Plantlife, Autumn 2001, 10 - 11.
Note: This article is reproduced with minor modification from an article published by the author in the Newsletter of the Friends of the City of Swansea Botanical Complex, no.17 (2005).
And thank you to Michael Wood for his kind permission to use his lovely image of the fly agaric. See the following 3 references:
Editor's Note: About Inga
The success of the Inga as an alley cropping tree depends on its symbiotic relationship with both mycorrhiza and the root nodule bacteria. Loss of phosphorus from the slashed and burned soil was found to be the major reason for the rapid loss of soil fertility. The mycorrhiza take up unused phosphorus, pass it into the tree, and when the tree is pruned the decomposing leaves that are left in the alleys where the crops are planted release the phosphorus to the crops. The surplus is again taken up, and so the cycle keeps being repeated.
Collecting ripe fruit in a well-grown Inga tree. Photo by Gaston Bityo 2009.
If mushrooms grow where there are mychorriza in the soil beneath under trees I wondered if we could ascertain whether the correct mychorriza were present in a new Inga plantation by looking for mushrooms. Apparently not. Inga has ENDOmycorrhiza (Pennington and Fernandes 1998 (3)), , and endomycorrhizal fungi are not formers of mushrooms. Ectomychprriza are the formers of mushrooms.
Young Inga edulis tree, at FunaVid, Honduras. Photo by Tiiu Miller.
3. Pennington, T.D. and Fernandes E.C.M. The Genus Inga: Utilization The Royal Botanic Gardens, Kew 1998