Australia has poisonous snakes, spiders and … trees

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Hair-like structures on a stem of Dendrocnidus excelsa.

Hair-like structures on a stem of Dendrocnidus excelsa.
Image: Institute for Molecular Bioscience / University of Queensland

Australia’s notorious biting trees cause excruciating pain that can linger for weeks or even months. New research suggests that this nettle relative is actually poisonous, producing a toxin similar to spider venom.

From snakes and spiders to jellyfish and conical snails, Australia has no shortage of poisonous animals. Like new research published in Science Advances shows, Australia is even home to poisonous plants belonging to the genus Dendrocnid, namely Dendrocnide excelsa and Dendrocnide moroide, both known as “gympie-gympie” in the local indigenous language Gubbi Gubbi.

Gympie-gympie plants look harmless, but their stems and leaves have a nasty blow.

Gympie-gympie plants look harmless, but their stems and leaves have a nasty blow.
Image: The University of Queensland

A chemical analysis conducted by researchers at the Institute for Molecular Bioscience at the University of Queensland at Brisbane has resulted in the discovery of a whole new family of toxins, called “gympietids”, produced by dendrocnid plants. This toxin is surprisingly similar to the venom found in spiders and conical snails, researchers say.

These trees grow in eastern Australia, particularly along the slopes and ravines of tropical forests. Dendrocnidia technically belong to the nettle plant family, which are known to produce annoying stings, but “they’re much more than oversized nettles,” the study authors wrote. The oval-shaped stems and leaves of these trees are covered in needle-like hairs, and anyone unlucky enough to rub against them will be in for a nasty surprise.

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Dendrocnid plants are “notorious for producing [an] excruciatingly painful bites, which, unlike those of their European and North American parents, can cause symptoms that last for days or weeks, ”said Irina Vetter, co-author of the study, in a Press release. Similar to other nettles, the prickly tree “is covered with needle-like appendages called trichomes that are about five millimeters in length,” she said. They look like fine hairs but “actually act like hypodermic needles that inject toxins when they come into contact with the skin,” said Vetter, an associate professor at the University of Queensland.

Indeed, these plants are no joke, as the researchers explain in their article:

In the state of Queensland, it is not uncommon to find warning signs along forest tracks, alerting unwary visitors to the presence of Dendrocnid species and the power of their sting. This signage is justified given that D. moroides was involved in the hospitalization of two people requiring intensive care for 36 hours who suffered from acute pain that allegedly did not respond to morphine and symptoms that persisted for months. This long-lasting pain is also typical of stings from other Dendrocnidia species, with episodic pain that usually subsides over several weeks, although [painful tingling and prickling sensations] may persist longer.

Scientists have struggled to explain these exaggerated health effects, as extensive, long-term tingling doesn’t appear to be caused by the fine hairs lodged in a person’s skin. Additionally, neurotransmitters and inflammatory mediators such as histamine, acetylcholine, and formic acid do not cause the pain effects seen, even though they are found in the trichomes. For the new study, Vetter and his colleagues sought to find a potentially neglected neurotoxin in the two dendrocnid trees, leading to the discovery of the gympietides molecule.

The needle-like hairs inject toxins when touched.

The needle-like hairs inject toxins when touched.
Image: The University of Queensland

“Although they originate from a plant, gympietids are similar to the toxins of spiders and cone snails in the way they fold into their 3D molecular structures and target the same pain receptors – this is arguably of ‘gympie-gympie tree a truly’ poisonous’ plant, ”Vetter said in the University of Queensland statement.

Interestingly, this could be an example of convergent evolution, in which similar traits appear in unrelated species. What makes this a particularly unique case, however, is that this same trait – venom – appeared in a plant and an animal. This is unusual, as convergent evolution is often driven by environmental pressures and similar lifestyles.

As the new research shows, this toxin permanently alters the sodium channels of sensory neurons. Sodium channels are a membrane protein that play an essential role in the formation of pain, which they do by arousing neurons. In testing, gympietids were shown to activate sensory neurons in mice and then prevent them from shutting down. So this venom – in addition to generating the pain signals – interrupts the mechanism responsible for shutting down those signals. In other words, in a nutshell, mean, and this explains why the pain sometimes lasts so long after meeting with the tree.

The good news is that by “understanding how this toxin works, we hope to provide a better treatment for those who have been bitten by the plant, to relieve or eliminate the pain,” said Vetter.

Who, thank goodness. I was stung by “normal” nettles, and it was really nasty. It’s hard for me to imagine these sensations lasting more than a few minutes, let alone days or weeks. An effective treatment for these poisonous trees would be a welcome development.

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