January 29, 2023

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

Zoom in / Oyster mushrooms (Pleurotus ostratus) grows quietly under a tree in a forest. But beware of nematodes! These oyster mushrooms want to eat you—and they’ve developed a new way to paralyze and kill you.

Arterra/Getty Images

Oyster mushrooms (Pleurotus ostratus) is a staple of many cuisines, with its mild flavors and faint aniseed aroma. These cream-colored mushrooms are one of several types of carnivorous fungi that prey on Nematodes (roundworms) esp. Mushrooms have developed a new mechanism to paralyze and kill their nematode prey: a toxin called toxocysts in lollipop-like structures, when emitted, causes widespread cell death in roundworms within minutes. Scientists have now identified the specific volatile organic compound responsible for this effect New paper Published in the journal Science Advances.

Carnivorous fungi such as oyster mushrooms eat nematodes because these tiny organisms are abundant in the soil and provide an easy protein source. Different species have developed different methods of hunting and eating their prey. For example, Oomycetes Fungal-like organisms that send out “predator cells” to search for nematodes. Once found, they form cysts near the mouth or anus of roundworms and then inject into the worms and attack the internal organs. Another group of oomycetes uses cells that act like prey-seeking harpoons, injecting fungal spores into the worm to seal its fate.

Other fungi produce spores with annoying shapes like sticks or stilettos. Nematodes ingest the spores, which become trapped in the esophagus and germinate by piercing the worm’s gut. There are sticky branch-like structures that act like superglue; Death collars, which are severed as nematodes swim, insert themselves into the worms; And a dozen or more fungal species use traps that shrink within seconds and squeeze the nematodes to death.

<em>Scanning electron microscopy (SEM) image of toxocysts in P .  ostreatus</em>Haifa.” src=”https://cdn.arstechnica.net/wp-content/uploads/2023/01/oyster1-640×428.jpg” width=”640″ height=”428″ srcset=”https://cdn .arstechnica.net/wp-content/uploads/2023/01/oyster1.jpg 2x”/></a><figcaption class=
Zoom in / Scanning electron microscopy (SEM) image of toxocysts B. Austratus Haifa.

Yi-Yun Lee

The oyster mushroom avoids these physical traps in favor of a chemical mechanism. B. Austratus A so-called “wood rotter” that targets dead wood, but wood is relatively low in protein. Its long branching filaments (called hyphae) are part of the bushroom that grows into decaying wood. Those hyphae are home to toxocysts. When the nematodes encounter the toxocysts, they burst and the nematodes usually become paralyzed and die within minutes. Once the prey dies, the hyphae grow into nematode bodies, digest the contents, and absorb the broth for nutrients.

In 2020, a team of scientists from the Academia Sinica in Taiwan tested all 15 varieties. B. Austratus And Found it All 15 poison drops can be spawned while starving. They also tested 17 species of nematode and found that none survived exposure to the toxin. Co-author Ching-Han Lee and colleagues suggested that the culprit might be calcium stored in the animals’ muscles, which, in response to nerve signals, causes the muscles to contract. Muscles relax when nerve signals stimulate the replenishment of calcium storehouses.

To test the hypothesis, the team conducted experiments where the worms were exposed to calcium and then monitored the response to exposure to oyster mushroom toxocysts. They found that the pharynx and head muscles of venomous nematodes were filled with calcium and the calcium did not go away, leading to widespread nerve and muscle cell death. They suggested that the toxin stimulates the initial calcium response, but inhibits the mechanism by which the nematodes renew their calcium supply.

A mitochondrial calcium wave propagates throughout the hypodermis tissue after communication B. Austratus.Credit: Ching-Han Lee

But Lee and many others. Although they noted that the chemical structure of the oyster mushroom is different from the nematicides currently used to control nematode populations, they were unable to identify the specific toxins responsible for the effect. For the new study, Lee and co-authors used gas chromatography-mass spectrometry to do just that. The first version of the experiment tested a culture medium and vial sample containing glass beads. The second version tested a vial model B. Austratus It was cultured for two to three weeks. The third version was a combination of the first two, testing a vial sample in which both were cultured B. Austratus and glass beads.

The culprit: a volatile ketone called 3-octanone, one of several naturally occurring volatile organic compounds (VOCs) that fungi use for communication. 3-Octanone also acts as a potent nematicidal mechanism. Exposure of four species of nematodes to 3-octanone induced a massive (and fatal) influx of calcium ions into nerve and muscle cells. According to the authors, dosage is key. Low doses are a repellent to slugs and snails, but high doses are lethal. The same is true for nematodes. A concentration greater than 50 percent of 3-octanone is required to induce rapid paralysis and widespread cell death. The team also induced thousands of random genetic mutations in the fungus. Mutants that do not form toxocysts in their hyphae are nontoxic to the nematode. Caenorhabditis elegans.

As for why oyster mushrooms developed an unusual mechanism for killing nematodes, the authors suggest it’s because dying or decaying trees are particularly low in nitrogen. Toxocysts may even serve a defensive purpose. Certain species of nematodes pierce the fungal hyphae and absorb the cytoplasm, so the toxocysts, which release a poisonous gas in the hyphae, protect the fungus from such predators.

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