|
The
Wisconsin
Mycological Society
Chemical
Secrets of the Matsutake Mushroom
Mushroom hunters know how
hard it is to find the elusive matsutake. They hide in the forest duff,
just peaking out with a small portion of their cap or only showing as a hump in
the ground cover. In spite of their secretive nature, they are actively
sought out by amateur and commercial pickers because of their exquisite taste
and high commercial value.
Because of its unique flavor,
the matsutake has been revered for hundreds of years in Japan and has become deeply
ingrained in the culture. In recent years, the harvest of the Japanese matsutake
[Tricholoma matsutake (Ito et Imai) Sing.] has declined and so the American
matsutake [Tricholoma magnivelare (Peck) Redhead] is imported to fill
the gap.
The chemicals that make up the exquisite taste of this mushroom
have been the focus of many scientific studies on the Japanese species.
In fact, the very first studies as to the compounds responsible for odors in mushrooms
were done on extracts of Japanese matsutake. In 1936 and 1938, the Japanese
scientist, S. Maruhashi isolated and identified two highly odoriferous compounds
from matsutake extracts. The substance that is most characteristic of the
distinctive odor of the matsutake is the ester, methyl cinnamate. Esters
are pleasant smelling compounds and are found in many edible fruits. In
this case the ester is related to the compounds that give cinnamon its spicy flavor,
hence the origin of the name "cinnamate."
The
other compound that Maruhashi identified as being important to the flavor of the
matsutake was an alcohol. This compound has been dubbed, "mushroom alcohol,"
because it is found in many other mushroom species. The proper chemical
name for this alcohol is 1-octen-3-ol, and it is responsible for the typical mushroom
odor.
A recent scientific
study explains why these two pleasant tasting compounds are found in the matsutake.
In the September issue of Biochemical Systematics and Ecology [volume
35, 634-6 (2007)], William Wood and Charles Lefevre report the production and
function of these substances in the American matsutake. The spicy ester,
methyl cinnamate, is a potent slug repellent. The matsutake uses this compound
defensively to protect the sporocarp from being eaten by slugs before it can release
its spores.
The second compound, the "mushroom alcohol," is even more
interesting. When Wood and Lefevre extracted mushrooms that were not cut
up or crushed, they found this "mushroom alcohol" was absent. If they crushed
the mushroom before their analysis, large amount of this chemical is formed.
This is a second and equally potent way the matsutake protects itself from slug
predation. Previous research by William Wood has shown that "mushroom alcohol"
is a potent banana slug repellant (Biochem. Syst. Ecol. 29, 531). When a slug
tries to eat a mushroom, the chewing causes this alcohol to be released, which
repels the slug. It is interesting that these two chemicals, which humans
find as flavorful, are in reality produced by the mushroom to protect them from
slug predation.
Besides looking into the chemicals produced by the fruiting
body or sporocarp, these researchers investigated the chemicals found in the mycelium
of the American matsutake. This mushroom is mycorrhizal and only grows in
association with the roots of trees. In this association, the trees exchange
sugars produced in their leaves for nutrients collected by the mycelium from the
soil surrounding the tree roots. Because of this special mutualistic or
symbiotic arrangement these mushrooms cannot be artificially grown and harvested.
As part of his Ph.D. studies Charles Lefevre was able
to culture American matsutake mycelium in the absence of the symbiotic tree roots.
These cultures were slow growing, taking a number of months to grow to a reasonable
size. When these cultures were analyzed by William Wood, the chemist
on this study, he found to his surprise that the slug repellent chemicals observed
in the sporocarp were absent in the mycelium. The secret chemical life of
the matsutake continued to unravel.
The major chemicals Wood found in
the mycelium were of a type rarely found in terrestrial plants or animals, they
contained organic chlorine compounds. These types of compounds are best
known as substances that humans have used as pesticides, such as the insecticide
DDT or the herbicide 2,4-D. Why are these compounds being made by the mycelium?
This is the question these researchers asked.
The mycelium is not under
threat of being eaten by slugs since it is growing underground with the tree roots.
However, at this stage of its matsutake's life cycle, there is competition with
other fungi for space on the tree roots. The chlorinated compounds found
in the mycelium, 3,5-dichloro-4-methoxybenzaldehyde and 3,5-dichloro-4-methoxybenzyl
alcohol are known to stop important aspects of fungal metabolism. They inhibit
an enzyme that produces cell walls in other fungal species. These compounds
also halt the production of fungal melanin, a pigment that protects fungal hyphae
by forming a physical barrier between the cell and its surroundings. Chemical
warfare between different fungi for space on plant roots is not frequently observed,
but must be an important aspect of fungal life.
To exclude the possibility
that these chlorinated compounds were only produced in the artificial medium in
which the mycelium was grown, these researchers analyzed soil containing matsutake
mycelium. They identified the most abundant of the chlorinated compounds
in the soil, so these compounds are not artefacts and are produced by free-living
mycelia.
Thus, the matsutake uses defensive chemicals throughout it life
cycle. When it is underground and associated with tree roots, it fights
off other fungi's mycelium with exotic chlorinated compounds. On fruiting,
it protects the spores in the sporocarp with the volatile and spicy ester, methyl
cinnamate. Furthermore, if slugs trying to eat this mushroom are not
repelled by this potent ester, it releases large quantities of distasteful mushroom
alcohol upon tissue disruption.
Professor
William F. Wood
Department of Chemistry
Humboldt State University
Arcata, CA 95521
U.S. A.
Phone (707) 826-3109
FAX (707) 826-3279
E-mail wfw2@humboldt.edu
|
|
