John Donne’s famous words ‘No man is an island entire on itself’ let themselves easily be translated into ‘No tree is an island’. Every tree provides food and shelter to a host of other organisms, and is also dependent on many other species itself. Fungi mediate the supply of water and nutrients, insects and other animals. They also aid in pollination and seed dispersal.
One particular group of organisms, invisible to the eye of the tree hugger, makes its home inside the tree. These are the endophytic fungi – fungi that grow within plants at least during part of their stay, without obviously causing disease.
These fungi have even a more hidden lifestyle than the mushroom species we know from our walks in the woods and are less familiar than those mycorrhizal associations underfoot. And yet, they are numerous and ubiquitous. Almost every plant on earth harbours one or more species of fungi among or within its cells. Best known are the fungi in grasses and in trees.
Leafing through old issues of Science, I came across a story written a century ago, describing the experiences of the author and his company in the mountains of New Mexico: “We had made camp one evening in a beautiful park, bordered with spruces and firs, and covered with tall grass that, with its green base leaves and ripe heads loaded with heavy rye-like grain, offered a tempting feast for our hungry animals. The moment saddles and harnesses were off, the horses were eagerly feeding. A few minutes later, a passing ranch-man stopped his team and called over to us, ‘Look out there, your horses are getting sleepy grass,’ and added, ‘If they get a good feed out of that grass, you will not get out of here for a week’.” The following morning the author is met with the sight of his horse Old Joe: “The horse was standing on a side hill, asleep, his feet braced far apart, head high in air, both ears and under lip dropped, a most ridiculous picture of profound slumber.” (Bailey, 1903).
Now we know that sleepy grass, Achnatherum robustum, hosts a Neotyphodium species, an endophytic fungus, that produces lysergic acid amide, the sedative to which Old Joe fell victim. Horses and cattle who have eaten this grass will avoid it for the rests of their lives, so it offers the grass perfect protection against herbivory. This fungus does not produce fruitbodies, but grows into the seeds of the plants and is spread to the next generation of grass when the seeds germinate.
This is a very beneficial partnership for the plants. However, in the long run, the fungi, which only seem to reproduce asexually, are not so well off.
On the other end of the spectrum of grass-fungus relationships, are the Epichloe species. These do have sex and form their fruitbodies around the stems of the grass, choking the grass stems and preventing them from forming flower spikes. The grass reacts by forming lots of shoots, technically known as tact, and this has long-ranging effects. To take just two examples to illustrate this: perennial ryegrass, Lolium perenne, introduced into New Zealand from Europe and is now the dominant, naturalized grass in New Zealand. With it came Neotyphodium lolii, which makes the grass less palatable for cattle, but also protects the grass from attacks by the equally introduced Argentinean stem weevil.
The other example also relates to an introduced grass. The originally European species, Festuca arundinacea, tall fescue, now covers millions of acres in the eastern US. The endophyte Neotyphodium makes it grow even better, but causes toxicoses in the cattle that graze in it. However, it also enhances the growth of the grass, and makes it more drought and heat resistant. As a result, it can easily out compete local plants and grasses, significantly decreasing biodiversity in grasslands. Nor is that the end of fungal influence. Underground, the endomycorrhizal fungi - the microscopical fungi in the roots of many plants-also play a crucial role.
The fact that endophytic fungi often produce secondary compounds, with an effect on other organisms, makes them an ideal target for the pharmaceutical industry in its search for new medicines. Especially since the compounds are so unlike existing drugs against which many microorganisms have developed resistance.
The anti-cancer drug taxol is present in all the earth’s yew species, but the natural source is very expensive, and organic synthesis is not economically feasible. Recently a taxol-producing endophyte of Taxus brevifolia was discovered, Taxomyces andreanae. Already, other fungi, some growing within yew, but an even larger number also in other trees, have been shown to produce taxol. Taxol may also play a role in defense against plant pathogens, like Phytophthora, as it seems that these organisms do not easily attack yews.
Plants growing in extraordinary habitats are more likely to have unusual defense mechanisms against the perils their niche brings with it. So the search for those novel medicines focuses on rain forests, and especially plants growing in very wet places. Waterfalls, with their falling water and stones, are favourites.
Traditional medicinal practice may also point to the antibacterial or antifungal activities of endophytes. Aboriginal people in Australia use the snakevine (Kennedia nigricans) to heal wounds and infections. And yes, this plant hosts a Streptomyces species which is active against a host of fungi and bacteria.
All those endophytes do seem to have a beneficial function in the host plant; their presence alone makes it impossible for another (read more harmful) fungus to be there.
It is not only exotic species from faraway wild areas that have endophytes. Our local redwoods have many fungi living within their leaves. And so we are back where we started. Fungi are on trees, in trees, around trees, profiting or protecting trees, but all contributing to the tremendously interesting and diverse world around us.
- Bacon, C.W. & J.F. White Jr (eds), 2001. Microbial endophytes. Marcel Dekker, Inc., New York, Basel.
- White, J.F. Jr., C.W. Bacon, N.L. Hywel-Jones & J.W. Spatafora (eds), 2003. Clavicipitalean fungi. Evolutionary biology, chemistry, biocontrol, and cultural impact. Marcel Dekker, Inc., New York, Basel.
- Arnold, A.E., Z. Maynard & G.S. Gilbert, 2001. Fungal endophytes in dicotyledonous neotropical trees: patterns of abundance and diversity. Mycological Research 105: 1502-1507. (PDF)
- Strobel, G.A., 2003. Endophytes as sources of bioactive products. Microbes and Infection 5: 535-544.