Consider the oyster mushroom. It is a mild thing, a
pert, often creamy-colored whorl of fungus that goes well with thyme sautéed in
butter. But among scientists who study mushrooms, it has earned a reputation
for activities more sinister than you would expect from a fungus found in fine
dining. The oyster mushroom is a carnivore.
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The mushroom’s usual diet of damp logs is low in nitrogen.
To get that essential element, it feeds on microscopic nematodes, a type of
worm. When a worm makes the mistake of passing over the fungus, the oyster
mushroom paralyzes and kills it, devouring the animal’s nitrogen-rich flesh
with root-like tendrils called hyphae.
“They really paralyze the worms within a minute,” Hsueh said. “It’s very dramatic.”
Outside the animal kingdom, Venus flytraps, pitcher plants,
and others are well known for their macabre meal-making. But fungi, too, have
an appetite for flesh, and it is not just the oyster mushroom, although it is
the only carnivorous fungus you will generally find in your grocery store. Some
fungi craft sticky nets laced with tempting scents to snare their prey. Others
create deadly collars that constrict as the worm struggles, immobilizing the
prey as the fungus’ hyphae penetrate its body. Some even release tiny
sickle-shape spores that, when swallowed by a nematode, wreak havoc from
within.
The scenarios all end with the worm’s body invaded by the
threads of its hungry captor.
A paralyzing toxinThe oyster mushroom’s weapon of choice seems to be a toxin:
Worms that touch the fungus are paralyzed, and their cells fall apart as they
succumb to the hyphae. In a paper published earlier this year in the journal
Science Advances, researchers report that they have identified the substance,
which is contained in globes that they compare to lollipops. To the scientists’
surprise, it is a fairly common molecule, rather than an exotic, highly evolved
substance. But to the hapless worms, it is deadly.
Before they knew the toxin’s identity, the researchers were
familiar with its effects, said Yen-Ping Hsueh, a researcher at the Institute
of Molecular Biology of the Academia Sinica in Taiwan, and an author of the
paper. In 2020, the team described how toxins get into worms’ bodies through
the sensitive tips of the creature’s small sensing organs.
3-octanone is… a common ingredient in fragrances and flavors. But applying the substance to worms made it clear 3-octanone had all the gruesome effects of a brush with an oyster mushroom.
“They really paralyze the worms within a minute,” Hsueh
said. “It’s very dramatic.”
Once the toxin reaches the worm’s neurons and muscle cells,
it destabilizes the normal flow of ions across the cells’ membranes, causing
catastrophic failure.
For their latest paper, Hsueh and her colleagues used
ultraviolet rays and a chemical that causes mutations on oyster mushrooms and
looked for individuals whose touch did not kill worms. They found that all of
these fungal mutants lacked small globes called toxocysts that hang like fruit
from the hyphae. These, they reasoned, must be where the substance was kept.
But attempts to pinpoint the toxin by harvesting the
toxocysts from non-mutant mushrooms failed, and the researchers only understood
why when they found that physically disturbing the globes made them harmless to
worms. The substance must be volatile — floating away in the air as soon as it
is released.
If oyster mushrooms could be made to arm themselves, even in the richness of a fertilized field, we might someday see their talents for destruction deployed on our behalf
Using a machine to analyze the air above disturbed toxocysts
revealed a single molecule: 3-octanone. That was surprising, Hsueh said.
3-octanone is a relatively commonplace substance made by plants and fungi. It
is also a common ingredient in fragrances and flavors. But applying the
substance to worms made it clear 3-octanone had all the gruesome effects of a
brush with an oyster mushroom. They had found their culprit.
A new pest control?Nematode worms have been known to destroy the roots of
crops, and antiworm substances from nature have inspired drugs like ivermectin,
the antiparasite medication that made headlines during the height of the
pandemic. Because 3-octanone is volatile, it is unlikely it could be used as a
pesticide against worms — it would just drift away. Furthermore, the oyster
mushroom only goes to the trouble to make toxocysts when it is in a
nitrogen-poor environment. So oyster mushrooms likely could not serve as a form
of natural pesticide alongside crops slathered with nitrogen-rich fertilizers.
But perhaps, Hsueh said, understanding how the oyster
mushroom came to use this substance as a toxin and what triggers the creation
of toxocysts could open the door to a new kind of pest control. If oyster
mushrooms could be made to arm themselves, even in the richness of a fertilized
field, we might someday see their talents for destruction deployed on our
behalf — and not just in the service of making themselves plumper and tastier
for our plates.
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