Carnivorous oyster mushrooms can kill roundworms with “nerve gas in a lollipop”

Oyster mushroom (Pleurotus ostreatus) is a staple in many types of cuisine, prized for its mild flavor and faintly aniseed aroma. These cream-colored mushrooms are also one of several carnivorous fungi that specifically prey on nematodes (roundworms). Mushrooms have evolved a new mechanism for paralyzing and killing nematode prey.The toxin, contained in a lollipop-like structure called a toxocyst, is released and, when released, injects a wide range of cells into the roundworm within minutes. Causes death. According to a new paper published in the journal Science Advances, scientists have identified a specific volatile organic compound responsible for this effect.

Carnivorous fungi, such as oyster mushrooms, feed on nematodes because these small creatures are abundant in the soil and provide a convenient source of protein. We have evolved the mechanics. For example, oomycetes are fungal-like organisms that send out “hunter cells” to hunt for nematodes. When they find them, they form cysts near the roundworm’s mouth or anus and inject themselves into the roundworm to attack its internal organs. Another group of oomycetes uses cells that act like harpoons to search for prey, injecting fungal spores into worms to seal their fate.

Other fungi produce spores with irritating shapes like stickles or stilettos. It has sticky, branch-like structures that act like superglue. A death collar that is removed when the worm swims and injects itself into the worm. And a dozen fungal species use traps that contract in less than a second to squeeze nematodes to death.

Expanding / Scanning electron microscopy (SEM) images of toxocysts P. ostreatus Mycelium.

Lee Yoon

Oyster mushrooms avoid these physical traps in favor of chemical mechanisms. P. ostreatus It is called a “wood rotter” that targets dead trees, but wood is relatively low in protein. Its long branched filaments (called hyphae) are part of the “mushroom” that grows on rotting wood. These hyphae are the home of toxocysts. When a worm encounters a toxocyst, it bursts, and the worm is usually paralyzed and dies within minutes. When the prey dies, the hypha grows into the nematode’s body, dissolving the contents and absorbing the slurry of nutrients.

In 2020, a team of scientists from Taiwan’s Academia Sinica tested all 15 species. P. ostreatus We found that all 15 species can produce poisonous drops when starved. They also tested 17 species of nematodes and found that none survived exposure to the toxin. It is calcium and may cause muscle contraction when released in response to nerve signals. When nerve signals cause refilling of calcium stores, muscles relax.

To test the hypothesis, the team conducted a calcium-visible experiment in the worms to track their response to exposure to oyster mushroom toxocysts. They found that the toxin did not disappear, leading to widespread neuronal and muscle cell death. bottom.

but lee othersThey noted that the chemical mechanism in oyster mushrooms differs from the nematicides currently used to control nematode populations, but identifying the specific toxin responsible for the effect is difficult. We couldn’t. In a new study, Lee and co-authors used gas chromatography-mass spectrometry to do just that. In the first version of the experiment, we tested vial samples containing only medium and glass beads. For the second version, we tested vial samples containing: P. ostreatus Cultured for 2-3 weeks. The third version was a combination of her two firsts, testing vial samples containing both cultures. P. ostreatus and a glass ball.

The culprit is a volatile ketone called 3-octanone, one of several naturally occurring volatile organic compounds (VOCs) that fungi use to communicate. 3-octanone also appears to function as a potent nematode killing mechanism. Exposure of four species of worms to her 3-octanone caused an apparently large (and lethal) influx of calcium ions into nerve and muscle cells. Dosage is important, according to the authors. Low doses are repellent to slugs and snails, while high doses are lethal. High concentrations of His 3-octanone, greater than 50%, are required to cause rapid paralysis and widespread cell death. The team also induced thousands of random genetic mutations in the fungus.Mutants that did not develop toxocysts on their hyphae were no longer virulent to nematodes Caenorhabditis elegans.

As to why oyster mushrooms have evolved an unusual mechanism to kill nematodes, the authors suggest that dying or rotting trees are particularly deficient in nitrogen, and this mechanism is likely due to the fact that the mushroom compensates for that deficiency. Toxocyst may also serve defensive purposes. Certain species of nematodes can pierce the fungal hyphae and suck out the cytoplasm, so having toxocysts in the hyphae, which release poisonous gases, can protect the fungus from such predators.

DOI: Science Advances, 2023. 10.1126/sciadv.ade4809 (About DOI).