There has been an allusion to Morchella being mycorrhizal. The question is its relationship to plant roots.

The term mycorrhizal is becoming almost meaningless in the broadness of its use. All soil microbes more or less have a relationship to plant roots. The rhizosphere is where everything happens in the soil. Soil bacteria, including Pseudomonas fluorescens, cluster around plant roots, because nutrients accumulate over time in that area. Roots promote that result, since they benefit from the accumulation of nitrogen.

It has been found that Morchella mycelium aggregates around plant roots and breaks down the outer layers of tissue (Buscot). This result is not the same as that of true mycorrhizal fungi. The observed result with Morchella was clumping around large roots rather than an association with fine roots.

If one therefore chooses to refer to Morchella as mycorrhizal, which is arbitrary, doing so tells nothing about the biology involved. It certainly does not lead to additional conclusions in regard to nutrition or plant specificity.

Certainly plant roots are not required for the growth or nutrition of Morchella, since it grows readily in various composts. The study showing Morchella mycelium aggregating around roots also indicated that the mycelium initially grows in the soil without such aggregations.

It is possible that Morchella derives some nutrients directly from plant roots. But if it does, some form of root damage would apparently have to occur before root nutrients would be significantly available to the mycelium. Evidence for the role of root damage is in the tendency for morels to be found in large numbers where there have been forest fires or trees dying from such factors as dutch elm disease. However, these nutrients are probably indirect, as bacteria probably precede Morchella in utilizing them.

Buscot, F. 1989. Can. J. Bot. 67:589-593./ 1992. J. Plant Physiol. 141:12-17.

 

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2. Photos of Pigmented Mycelium

The figures in the research section show pigmented mycelium. Pigment cannot have a real function for mycelium which is located under the ground where light is ineffective. The pigment should not exist, because organisms do not waste resources producing complexities which they cannot use.

However, the pigmented mycelium is not normal mycelium; it is high density mycelium with a complex structure of cells below it. Normal Morchella mycelium is not pigmented, though die-off does result in a brown pigment. The die-off pigment appears to stem from a differentiation process including residual autolysis during die-off.

There is also a differentiation process which creates incompetent cells which are darkly pigmented. Incompetent cells appear to have the purpose of preventing resources from being wasted where they cannot be effectively used, as light sensitivity does with Agaricus. But why would incompetent cells be dark pigmented? Probably because that particular process is closely related to pigment formation within the mushroom tissue resulting in a carry-over of functions. Morchella is highly prone to mixing functions, because its evolution has not had time to sort out the details.

On a petri plate, the ascocarp pigment only forms within the high density mycelium of the anomaly, as shown by the sharp boundary around the black pigment in figures 5 and 6. So the anomaly is a complex structure which promotes formation of the pigment of the above ground structure.

The two species studied produced different pigments. The common morel produced gray and orange pigments, while the black morel produced brown and black pigments. It is also interesting that the orange pigment formed below the gray pigment on the petri plates. The orange pigment is normally located in the indents on the surface of common morels (varying of course from area to area, while the orange is more extreme than usual in the Michigan morel, and it varies with the spore strain), while the gray pigment is normally on the ridges. So the structure on the plates had a relationship to the structure of the mushrooms.

The anomaly was not doing something that mycelium would normally be expected to do. Not only was it a complex structure, it promoted properties of the above ground structure including pigment as well as the rubbery-brittle tissue of the mushroom. Of course, the anomaly was nothing resembling a normal and functioning structure.

Therefore, all else follows. Organisms cannot normally revert to such nonfunctional forms. First, they cannot waste resources on nonfunctionality; and second, they have their characteristics locked in place by millions of years of evolutionary refinement. The anomaly demonstrates that the morel does not have millions of years of evolutionary refinement in its macromorphology.

Organisms usually evolve from highly similar organisms, which prevents extreme variations from occurring. For Morchella to revert to a reduced structure means that it evolved from an ancestor that did not have structural complexity, which of course means a yeast. If it would have evolved from a mold, it would have conidia instead of ascospores in addition to all of the other physiological characteristics of higher, filamentous fungi.

Five indications of Morchella evolving from a yeast are these:

1. The reversion anomaly lacks normal structure.
2. Mycelium lacks extracellular enzymes.
3. Residual autolysis is disadvantageous for mycelium.
4. Intolerance of dehydration of mycelium is yeast-like.
5. Ascospores are disadvantageous.

 

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3. Yeast Endotrophism

Endotrophism was a baffling concept, when, in graduate school, I found the yeast Nadsonia fulvescens to be forming spores in distilled water. The general assumption was that yeasts required a nutrient medium to form spores. After deriving the term endotrophism from its component elements in pidgin (or is it Latin?), I looked in the index of a bacteriology book for endotrophism and was shocked to find it. The authors said some bacteria were known to be endotrophic by the criterion that they formed spores in distilled water. Nothing more was said, and no specific references were given.

In retrospect, all filamentous yeasts are probably endotrophic, but Nadsonia is not filamentous. The filamentous form provides a reserve of cellular nutrients which can be used to create spores, and doing so removes environmental limitations on the process. With unitary yeasts, the environment is depended upon for nutrients during sporulation, because there are not adequate reserves within the cells. So it is surprising that Nadsonia created an alternative. I'll describe in some detail the unusual alternative that Nadsonia created.

The morphology is highly informative. There is a migration of cell material from the mother cell and a bud into a bud-like chamber which becomes the ascus. The resulting spore is larger than the bud on the opposite side. This migration of cell material (other than spores or nuclei) is unique among unitary yeasts, and it is a telltale sign of endotrophism, which indicates that other unitary yeasts are not endotrophic.

The function of the cellular migration is to allow the volume to shrink to about one third due to metabolic breakdown of cell material for energy and metabolites. A new chamber is needed to accommodate the reduced volume without diluting the cell material, because spatial arrangements are critical inside of a cell. There must be a large amount of starting material, which is why an additional bud forms—it increase the cell mass. The extra bud may also have a role in differentiation by allowing nuclei to recombine and create a short-lived diploid state.

So the morphological evidence for endotrophism in a unitary yeast is a migration of cell material and reduction in cell mass. Since other unitary yeasts do not show this morphology, they must not be endotrophic. And since they are not endotrophic, they must not be adapted to tree exudate.

A large variety of unitary yeasts have been found in tree exudate, but they apparently have not adapted to it. They don't need to adapt to the nutrient medium of tree sap, because it is ideal for yeasts. What they need to adapt to is survival after being stranded on tree bark. Endotrophism allows Nadsonia to form spores when the cells are isolated on the bark without nutrients. In fact, the presence of nutrients prevents spores from forming due to an acetate repression mechanism. This restriction assures that a maximum amount of vegetative growth occurs before spores are formed.

The primary significance of endotrophic sporulation is that it promotes survival by preventing starvation and dehydration from killing the cells, after they are washed over the surface by rain or left stranded by a reduction in tree exudate.

 

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4. Induction Physiology

The growing procedure for common mushrooms involves two major steps. First mycelium is grown on a compost, and then a soil-like medium, called a casing layer, is placed over it. The mushrooms come up through the casing.

This procedure has existed longer than the science of the subject. So for at least a century, scientists have been trying to explain why a casing layer is needed over the mycelium for mushrooms to form.

The first concepts were that there must be bacteria in the casing telling the mycelium to form mushrooms. Then chemical signals such as cyclic amp were preferred for an explanation. Eventually, someone found acetylene in the casing, and it became the preferred theory.

In studying yeast sporulation in graduate school, I found an induction physiology for Nadsonia yeast which was very similar to that of mushrooms. There were two phenomena involved. The first was a trigger mechanism causing sporulation to occur. It was found to be linked to the energy level in studies of bakers yeast by A.F. Croes. The second was endotrophism, which was peculiar to Nadsonia.

I realized that mushrooms had to have the same induction physiology involving an energy linked trigger mechanism and endotrophism. The endotrophism was obvious from the fact that the mycelial mass had to be built up first, and then it disappeared while the mushrooms formed. The link to the energy level was indicated by necessity and the casing.

The function of the energy link is to make sure nutrients and cell machinery are adequate before the irreversible process of differentiation begins. Mushrooms have the same need as yeasts in that regard.

The casing layer should produce a high energy peak. It first inhibits respiration by restricting oxygen. At this point, energy yielding molecules (NADH) accumulate, because they need respiration for producing usable energy as ATP. These molecules then become available in large quantities to the surface mycelium which can utilize them through respiration. The result is a high energy peak in the mycelium located on the surface of the casing, which is where the mushrooms form.

So I began studying mushrooms to find verifying evidence for this mechanism. The Morchella anomaly provided a perfect tool for acquiring such evidence, because it created a differentiation process on the surface of agar, where it is easy to study.

An important point of evidence for the energy link is that nitrogen depletion is stimulatory to induction. Nitrogen depletion blocks synthetic processes, because most of the products must have nitrogen incorporated into their structures. But the shortage of nitrogen does not inhibit energy yielding metabolism. With energy being produced and little being used for synthesis, a peak occurs in the available energy supply as ATP.

The extent to which nitrogen depletion is involved in induction under natural conditions is difficult to evaluate, since there are other factors that can contribute to the energy peak including oxygen availability. The primary inducing factor for soil type mushrooms including Morchella appears to be a restriction of oxygen availability. The shortage of oxygen starts with a built up mycelial mass which creates a high oxygen demand. Rain usually, but not always, contributes to the induction process by inhibiting the diffusion of oxygen. The mycelial mass then transports energy yielding molecules to the mycelium near the surface, which is not so devoid of oxygen. This combination produces the energy peak for induction at the surface, but only if the mycelial mass is built up well enough to supply sufficient energy yielding molecules.

 

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5. Arthrospores

Morel mycelium breaks down into arthrospores under suitable conditions, which involve high aeration, high humidity and rich nutrients. The purpose is to produce water-born dissemination, when mycelium reaches the soil surface during rains.

I first observed this phenomenon on filter pads, which were placed on agar nutrients to start mycelial growth, and then transferred to sterile chambers, where I dropped on sterile liquid nutrients for a couple of days. The mycelium broke down into massive quantities are arthrospores.

The arthrospores are also observed within the anomaly (high density myclium). They are located in the upper-most layer of cells between the polymorphic cells and extended mycelium.

The arthrospores have a blocky shape, as if the mycelium were cut into pieces by a chef.