Mycorrhizas (5)
Fall Mushrooms, Ghostly Fungus-Robbers, and a Definition Revisited

© Steve Trudell
Ecosystem Science Division, College of Forest Resources
Box 352100, University of Washington
Seattle, Washington 98195-2100

Inquiring mycophiles want to know. Why do mushrooms sprout when they do? Is it all a matter of rainfall, or is there more to it than that? Miles of fungal hyphae thread through our soils, conveying food, vitamins, and minerals here and there. Might plants usurp this network of tiny pipelines for gifting or barter among themselves?

If a ghost-white plant steals nourishment from a fungus, can their relationship really be mycorrhizal? Who holds the keys to the vault of mycorrhizal knowledge? Answers to all these and more, this time, in Mycorrhizas Part 5.

Fruiting of Ectomycorrhizal Mushrooms

In the last issue, fellow Washingtonian Fred Rhoades (who, by the way, is an excellent teacher and takes fine 3-D photographs of mushrooms and lichens) passed along Eric Swisher's question concerning the relationship between the activities of trees and the fruiting of ectomycorrhizal mushrooms. The question was, "Does the 'shutting-down' of photosynthesis by a mycorrhizal host tree in the fall stimulate the production of fruit bodies by its (ectomycorrhizal) fungal associates?"

Fred recounted how his search for information on this question turned up very little of substance, and suggested that the topic might be a good one for a Master's thesis or PhD dissertation. The fact that there is little relevant information out there reflects two things. First, most mycology textbooks concern themselves primarily with describing what fungi ARE, and how we classify them, but contain much less about what fungi DO and how they do it. So it's not surprising that the most relevant stuff Fred found came from a book entitled Fungal Ecology.

The second reason is that, surprisingly, there just hasn't been much research done on what causes mushroom-fungi to fruit. With the growing culinary popularity of mushrooms produced by ectomycorrhizal fungi such as boletes, chanterelles, and matsutake, perhaps there will be interest in funding the necessary research. For until there's funding, there will be little research!

Nevertheless, there is a bit of information available. While it doesn't provide an iron-clad answer, it demonstrates, at least to me, that Eric's question was right on the money. The coming of fall affects the trees and this, in turn, affects mushroom fruiting. The information comes from two unrelated studies, one in a Swedish forest and the other in a Canadian laboratory.

The Swedish study, conducted by Peter Högberg and several co-workers, was primarily concerned with finding out to what extent respiration by soil organisms is fueled by current photosynthates. Before continuing with the story, let's take a brief physiology detour.

Photosynthesis ("light-making") is the process by which green plants make their food. Using energy obtained from sunlight, the plants produce sugar from carbon dioxide taken from the atmosphere and water obtained from the soil. This sugar and other carbohydrates (together called photosynthate) provide the raw material for manufacturing the plant's body and all the chemicals needed to make it function.

Photosynthate also provides energy, which is released via the process of respiration. In its simplest form, respiration is the opposite of photosynthesis. Complex carbon-containing materials are "burned," creating carbon dioxide and water, and releasing energy. In fact, burning wood in your fireplace or gasoline in your car is essentially the same process. Respiration is not just for plants; all organisms must do it. Thus, the carbon dioxide and water vapor that are in our exhaled breath are the products of respiration in our bodies.

Now back to the Swedish forest. Textbooks and professors' lectures taught me that "soil respiration" (actually the combined respiration of all the organisms living in the soil) reflects the activities of the organisms hard at work decomposing leaves, wood, animal remains, and other soil organic matter. Ultimately, all organic matter is derived from photosynthate, either by being formed by a green plant, or by coming from an organism whose food source can be traced down a food chain to green plant material. Thus, the organic material being decomposed can be considered old photosynthate. The activities of mycorrhizal fungi, which are powered by current photosynthate from their plant partners, weren't (and generally still aren't) included in these textbook and lecture discussions. Högberg and friends set out to see how important a contribution the current photosynthate, piped into the soil via mycorrhizal fungi, might be making to overall soil respiration. How did they do it?

You could tell how much respiration your body was doing by measuring the amount of carbon dioxide you were exhaling over some specific time period, say a minute or an hour. Little respiration yields little carbon dioxide; more respiration yields more carbon dioxide. Similarly, as the soil organisms respire, they give off carbon dioxide, which is released from the soil surface into the atmosphere. So, by trapping the carbon dioxide given off from a measured area of soil over a measured period of time one can calculate the rate of soil respiration. And that is what the Högberg team did.

Now, in order to tell how much respiration might be due to the current photosynthate compared to the old photosynthate, the researchers had to devise a means of creating two different soil areas -- one with decomposition and "mycorrhizal" respiration, and the other with just decomposition. The difference between the two would then indicate how large the mycorrhizal contribution was. Thus, large plots were established. One-half of them were designated control plots and the other half were the experimental plots. Nothing was done to the control plots. But on the experimental plots, all of the trees (Scots pines, Pinus sylvestris) were girdled.

To girdle a tree (an inhumane act in my estimation, but the trees in Högberg's study were already planned for logging), you remove a strip of bark from the entire circumference of the tree's trunk. This removes the phloem tissue, which is the piping system that conveys photosynthate from the leaves where it is manufactured down to the roots and their mycorrhizal fungi. Deprived of food in this manner, the roots stop taking up water and nutrients and the tree dies. Thus, by girdling all the trees in the experimental plots, inputs of current photosynthate were cut off over large areas of soil.

Having done this, the researchers then measured the amount of carbon dioxide given off from several locations in each plot and calculated the amount of soil respiration taking place. These measurements showed that respiration in the experimental plots was less than half that in the control plots. Thus, current photosynthate was driving more than half of soil respiration and so the conventional textbook story explains only part of the picture. "Yeah, yeah", you say, "but what does that have to do with Eric and Fred's mushroom question"? Read on.

The researchers also collected, dried, and weighed all the mushrooms that fruited on the plots the following fall. Mushrooms of saprotrophic (decomposer) fungi were present in roughly equal amounts in the control and experimental plots. However, the difference in fruiting of ectomycorrhizal fungi was striking -- an average of 250 mushrooms, belonging to an average of 11 species occurred on the control plots, whereas an average of 4 mushrooms belonging to an average of 1 species occurred on the experimental plots. The dry weight of the mycorrhizal mushrooms harvested from the control plots was nearly 200 times more than that of the mushrooms from the experimental plots.

To me, this is pretty convincing evidence that fruiting of ectomycorrhizal fungi is strongly tied to current delivery of sugars from their tree partners. But what does the onset of the fall season have to do with it, besides bringing rainfall?

To answer that, we turn to the Canadian study, conducted by Andre Fortin and two of his graduate students -- Mohamed Lamhamedi and Christian Godbout. Their experiment involved growing, under controlled conditions in a laboratory chamber, eastern white pine (Pinus strobus) seedlings together with Laccaria bicolor, an ectomycorrhizal fungus. After the ectomycorrhizas were well established, the researchers manipulated the environmental conditions in the growth chambers to make the trees think they were in eternal autumn.

In temperate-latitude parts of the world, trees sense the onset of winter by the decreasing length of daylight and decreasing temperatures, particularly episodes of sub-freezing weather. As summer fades, the trees cease devoting photosynthate for growth and utilize it for formation of next year's leaves (contained pre-formed in the buds). After the buds have been formed ("set"), above-ground activity largely stops and the tree sends its photosynthate below-ground to the root system until the onset of freezing temperatures calls a halt to the year's activity. (Actually most conifers and other evergreens do continue to carry out photosynthesis, at least on an intermittent basis, throughout the winter whenever conditions allow it. However, the rate of the process during winter is much lower than it is during spring, summer, and fall.)

So, to create eternal fall, the Fortin group decreased the length of daylight the trees received, and kept the temperature low, but above freezing. Above-ground growth stopped, the buds set, and Laccaria mushrooms began to appear in the pots (Figure 1). If the mushroom beneath or alongside a seedling was picked, another often would be produced. This process could be repeated, such that in some cases, the weight of the mushrooms produced by a seedling actually exceeded the weight of the seedling itself!

Figure 1: A mushroom of Laccaria bicolor fruiting with a white pine seedling. The size of the mushroom indicates that abundant photosynthate must be transported from the needles of the seedling to its root/mycorrhiza system.
Photo by Christian Godbout and Andre Fortin; used with permission.

Between them, these two studies provide a good general answer to Eric's question. Ectomycorrhizal fungi depend on photosynthate from their tree partners to form mushrooms, and it's not until fall that the trees divert a large enough amount of photosynthate below-ground to allow the fungi to form abundant mushrooms.

To those of you who had your heart set on making this your graduate school project, don't worry. There's still plenty of work left to be done, as not all trees work like white pine seedlings, and not all fungi behave like Laccaria bicolor. I know Andre Fortin would love to see someone continue with this sort of research and do it in a more rigorous fashion than they were able to. For, as too often happens, when Lamhamedi and Godbout finished their degrees they were forced to abandon mycorrhiza research in order to make a living.

Mycorrhizal Networks

Last time I promised we'd visit the topic of mycorrhizal networks. Having devoted considerable space to the mushroom-fruiting question, I won't be able to go into as much detail on networks as I had intended originally. So, for now, I'll introduce the general concept briefly, indicate some reasons why these networks might be important, and then describe the best documented example in some detail. For those who want more information, there's an up-to-date review article written by Suzanne Simard, Melanie Jones, and Dan Durall in the book, Mycorrhizal Ecology, which is listed at the end of this article.

In the third installment of this series I introduced the notion that the hyphae of ectomycorrhizal fungi could form links between different plant individuals. For example, Figure 2 shows an experimental set-up in which the central Scots pine (Pinus sylvestris) seedling was planted in soil containing an ectomycorrhizal fungus, Suillus bovinus. After letting the initial seedling and fungus become well established, two more seedlings (this time lodgepole pines, Pinus contorta) with no fungi on their roots were added to the system. After a short time, a well developed root system had formed. However, note that in the figure the actual roots of the pines are very limited in extent, comprising only the thickest main branches seen in the figure. Most of the "root" system actually consists of fungal hyphae.

Figure 2: Mycorrhizal network formed between Suillus bovinus and two species of pine, Pinus sylvestris and Pinus contorta, in a laboratory microcosm. The original photograph (with no blue sky and clouds) is from David Read's laboratory and here appears on the cover of Mycorrhizal Symbiosis, the "bible" of mycorrhiza study. The book is listed at the end of this article and is highly recommended reading.

When you consider the characteristics of fungi that make them so well suited for life under ground (Mycorrhizas 3) -- hyphal body type, coverage of large areas with relatively little investment of tissue and energy, and ability to span poor-quality territory and move materials from resource-rich areas to not-so-rich areas or areas of rapid growth -- and the fact that each mycelium is connected to one or more plants, it is easy to imagine that they could serve as a pipeline system connecting individual plants into an integrated network.

Figure 2 illustrates two important things. First, the two more recently added seedlings became mycorrhizal very quickly. Second, many continuous pathways can be traced from the main root of one seedling to the main root of another seedling. Thus, the three seedlings and the fungus mycelium form a single physically interconnected system within which, potentially at least, materials such as photosynthates and nutrients could be moved from place to place. These networks have been termed common mycelial (or mycorrhizal) networks, or CMNs.

CMNs can be formed not only by ectomycorrhizal fungi, but arbuscular mycorrhizal fungi as well. Some researchers have also speculated that ericoid mycorrhizal fungi could form networks, but that seems less likely to me, given the small amount of external mycelium that these fungi produce.

If indeed movement of material can take place from one plant to a fungus and then to a second plant, this could have important ecological consequences. For instance, imagine that the central seedling in our experimental system (often called a microcosm or mesocosm) is a mature tree in a forest, tall enough for the leaves on its upper branches to be exposed to full sunlight. Assuming the tree has access to sufficient water and nutrients, the high light levels will allow it to manufacture large amounts of sugar through photosynthesis.

Further imagine that the smaller flanking seedlings are growing on the forest floor in the dark shade of the mature canopy trees. The low level of light there makes it difficult to produce much food by photosynthesis. Not only would this make growth of the young tree a difficult task, but it would also limit its ability to "feed" the mycorrhizal fungi it needs to obtain nutrients and water from the soil -- a double whammy. If the seedling could tap into a CMN formed among the mature trees, it is conceivable that photosynthate from those trees could be accessed by the seedling, thus providing a sort of nutritional subsidy. This would be analogous to a son or daughter away at college receiving a care package of groceries from mom and dad.

Another, perhaps less radical, possibility is that photosynthate from the mature trees maintains the mycorrhizal network, such that the seedlings benefit from it without having to contribute much of their own photosynthate. This would be like mom and dad paying the student's rent directly to the landlord. In general, the existence of CMNs provides a possible mechanism for the redistribution of resources from more dominant to less dominant individuals, or from dying plants or mycelium to young vital individuals. One expected outcome would be a greater diversity of plants than would occur in the absence of the CMN.

Thus, CMNs provide a means by which one plant can benefit nutritionally from another plant of the same species or even of a different species. Botanists and ecologists nearly always view the world in terms of competition among organisms. Thus, "nature is red in tooth and claw," "survival of the fittest," and so on. In this view, the above-ground portions of plants compete with each other for light, while the below-ground portions fight over water and nutrients. The possibility that plants might actually "help" one another doesn't at all fit into this widespread view of nature.

So it is not surprising that plant biologists have not leapt to embrace the notion of CMNs. Indeed, the requirements for proving their existence are difficult to accomplish. First it must be shown that the hyphal connections occur. Then it must be shown that material actually moves from plant to plant through the hyphal network. Finally it must be demonstrated that material moves between plants in sufficient quantity to be physiologically and ecologically important.

In thinking about how one could go about making the necessary demonstrations in an actual forest or other ecosystem, reflect for a moment on how extraordinarily complex these networks could be. Each tree in a forest can form mycorrhizas simultaneously with tens or even hundreds of different fungi, and each fungal mycelium can be associated with many different trees. And it is likely that the individual associations are coming and going constantly.

Although there is good evidence of the utility of CMNs from small-scale laboratory and greenhouse studies with both ectomycorrhizas and arbuscular mycorrhizas, there is very little evidence from whole ecosystems in the field. The best evidence we have so far that meets all three of the above requirements comes from so-called achlorophyllous plants. These plants lack chlorophyll, the green pigment that allows other plants to capture light energy for photosynthesis, and thus cannot make their own food. They are heterotrophs, not autotrophs like other plants. Some are directly parasitic on the roots of green plants, but many are not.

There are many such plants in the world. They include the "monotropes" such as Indianpipe (Monotropa uniflora), pinesap (Monotropa hypopitys), and snowplant (Sarcodes sanguinea) of the heath family (Figure 3) and orchids such as coral-root (Corallorhiza spp). Most often these plants are either white or pink.

Figure 3: An achlorophyllous plant, Indianpipe, Monotropa uniflora.
Photo by Steve Trudell.

It has long been recognized that these plants must obtain their food from a source other than photosynthesis. In the early-1800s, microscopic examination of monotrope roots indicated that they were not connected to tree roots, but were closely associated with abundant fine threadlike structures. However, the nature of this mycorrhizal association and even the fact that the threads were fungus hyphae was initially not appreciated.

Eventually the fact that fungi were always closely associated with these roots became well established, although widely ignored. Botanists assumed that these plants obtained their food by decomposing leaf litter, and references to these "saprophytic" plants continue to appear in many college biology and botany textbooks. However, there is no evidence that these plants have any decomposer ability and it takes only one look at their short stubby roots to see that they are not designed for efficient decomposition and nutrient capture (Figure 4).

Figure 4: Root mass of an Indianpipe. The plant's short thick roots clearly are not well suited for nutrient capture. Ectomycorrhizal root tips and fungus rhizomorphs also can be seen.
Photo by Steve Trudell.

Although bits of the puzzle were worked out in the late 1800s and early 1900s, unequivocal evidence of the source of nutrition for this group of achlorophyllous plants was provided first by a Swede, Erik Björkman, in the late 1950s. Björkman conducted three types of experiments.

First he located vigorous clumps of pinesap plants, designated some of them as controls, and drove stainless steel cylinders deep into the soil surrounding the experimental plants. These cylinders were intended to sever the inferred connections between the pinesaps and surrounding trees. The following year, new vigorous shoots appeared in the control locations, but those inside the steel cylinders were stunted and sickly looking. This result supports the idea that the cylinders prevented the experimental plants from receiving food.

In the second type of experiment, Björkman injected radioactive isotopes (14-carbon and 32-phosphorus) into spruce and pine trees, waited a short time, and then measured the radioactivity of pinesap plants growing near each tree compared to control plants growing much farther away. The amount of radioactive carbon or phosphorus in the close-by plants indicated that they were receiving both photosynthate and nutrients from the injected tree. These substances could not have moved through the air, nor could they have traveled through the soil rapidly enough to have shown up in the pinesaps so quickly. Because the pinesap roots and tree roots are not connected directly, the only plausible explanation was that the carbon and phosphorus had been transported via mycorrhizal fungus hyphae connecting the two types of plant.

In his third set of experiments, Björkman isolated a fungus from pinesap plants and showed that it could form ectomycorrhizas with pines.

Later studies repeatedly have confirmed Björkman's finding that achlorophyllous plants receive their food from the ectomycorrhizal fungi with which they form mycorrhizas. With respect to the carbon-rich photosynthates, the ultimate source is the trees. Soil-derived nutrients such as nitrogen and phosphorus for the most part come directly from the fungus. In both cases, the plant is receiving its nutrients from the fungus, hence the term mycoheterotroph increasingly is being used to refer to them.

Thus, mycoheterotrophs are plants that obtain their food from fungi rather than through photosynthesis. As an aside, it now appears that some chlorophyll-containing plants also can be mycoheterotrophic, at least to a degree. Their chlorophyll levels are not sufficiently high to allow them to completely feed themselves.

Pertinent to our discussion of CMNs is the fact that some of what the mycoheterotroph receives came originally from another plant and was transported to it via fungal pipelines. So, in this case at least, CMNs can be shown to exist in nature and to be ecologically relevant. However, whether or not CMNs have broader importance is a more difficult question to answer.

So what can we conclude? CMNs do exist. Under some circumstances material can move from one plant to another by means of them. Under some circumstances, this movement is physiologically meaningful. In some experimental systems (which I did not have room to discuss), movement of material between plants correlates with changes in the species composition of the plant community and/or relative growth of different plant species. However, whether CMNs will prove to be as important as some think they are remains to be seen. There is wonderful thesis and dissertation fodder here.

Some Thoughts on the Definition of Mycorrhiza

You might have noticed that I termed the mycoheterotroph-fungus associations as mycorrhizas. In fact, they are called monotropoid mycorrhizas, and are usually included as one of the seven or so main types of mycorrhiza. However, those of you with long memories and a penchant for detail might be puzzled by this. You will recall that in the first installment of this series (Mycorrhizas 1) I went into considerable detail explaining a typical definition of mycorrhiza. The key elements of that definition are:

Now let's consider the monotropoid mycorrhiza in light of those elements. The first of them fits. The mycoheterotroph's roots are indeed associated closely with hyphae of a fungus. The second also fits. The mycoheterotroph receives nutrients from the fungus. However, what about the third element? The mycoheterotroph receives photosynthates from the fungus, which is contrary to our definition. And on the fourth element, we can't yet be sure.

As far as we know, the mycorrhizal fungus, or the trees that produce the photosynthate, receive no benefit from the mycoheterotroph. However, it is possible that such benefit does exist. In another of his experiments, Björkman showed that extracts from pinesap plants stimulated the growth of ectomycorrhizal fungi growing in culture. Although the specific compound(s) responsible for the effect were not identified, Björkman suggested it might be a vitamin.

Nonetheless, as it stands now, the association between mycoheterotrophic plants and ectomycorrhizal fungi fails to meet key elements of the widely accepted definition of mycorrhiza. The mycoheterotrophs appear to be parasites rather than mycorrhizal mutualists.

This example hints at a more fundamental issue -- how to make a clearcut distinction between mutualistic and parasitic interactions. For instance, in the last installment I mentioned the above-ground versus below-ground study conducted by Monique Gardes and Tom Bruns. One of their findings was that Suillus pungens produced large numbers of bulky mushrooms, but occupied only a small proportion of the root tips in a Bishop pine (Pinus muricata) woodland. In contrast, Russula amoenolens occupied a substantial portion of the root tips, but produced only one mushroom over the 3-year study period.

The Suillus obviously is receiving abundant photosynthate from the trees, as it is manufacturing lots of large mushrooms as well as lots of external mycelium throughout the soil. Is it returning equal benefit to the pines, in terms of nitrogen uptake or other function? Or is it more nearly a parasite, at least during its fruiting period? (Note that the first part of this installment might be relevant to such ponderings.)

Is the Russula overall a "better" fungus for the pines because it requires less photosynthate (in addition to apparently lower carbon requirements for fruiting, russulas produce much less external mycelium than do suilluses)? It is likely that as we learn more about what individual fungi do differently from one-another, how relationships change with the season or over time, and how they differ from place to place, we will have to abandon the apparently straightforward definition of mycorrhiza for a much fuzzier one that acknowledges that the distinction between mutualist and parasite is not always clear.

But this sort of thing is what makes biology in general and mycology in particular so fascinating. If mycorrhizal fungi did just one thing (deliver all manner of nutrients in return for food), they could be easily defined. But then there would probably not be thousands of different mycorrhizal fungi in the world, and the occurrence of tens, or even hundreds, of different species on the roots of a single tree. Understanding this incredible diversity probably will require that we remain flexible in our definition-making.

Where to Learn More about Mycorrhizas

In light of the changes taking place with Mushroom the Journal, it's not clear what the future of this series might be. Whether or not it continues, this seems like a good time to list some resources for those of you who might be interested in pursuing additional mycorrhiza knowledge on your own. Obviously it will help if you happen to live near a major university library. But, increasingly, information is appearing on the Internet. So here are some good, relatively available, sources of information that you might find interesting. Of course, you can always get in touch with me with questions and I'll do my best to help you out.





Copyright Notice

This material appeared in slightly modified form in Mushroom: the Journal of Wild Mushrooming, Issue 77, Fall 2002. It can be used freely for not-for-profit personal and educational purposes provided that its source is clearly credited.