Australia Has Venomous Snakes, Spiders, and… Trees

Australia Has Venomous Snakes, Spiders, and… Trees
Hair-like structures on a stem of Dendrocnide excelsa. (Image: Institute for Molecular Bioscience/University of Queensland)

Notorious stinging trees from Australia cause agonising pain that can linger for weeks and even months. New research suggests this nettle relative is actually venomous, producing a toxin not unlike the venom of spiders.

From snakes and spiders to jellyfish and cone snails, Australia has no shortage of venomous animals. As new research published in Science Advances shows, Australia even harbours venomous plants belonging to the Dendrocnide genus, namely Dendrocnide excelsa and Dendrocnide moroide, both of which are known as “gympie-gympie” in the local indigenous Gubbi Gubbi language.

Gympie-gympie plants look harmless, but their stems and leaves pack a nasty punch.  (Image: The University of Queensland) Gympie-gympie plants look harmless, but their stems and leaves pack a nasty punch. (Image: The University of Queensland)

A chemical analysis conducted by researchers from the Institute for Molecular Bioscience at the University of Queensland in Brisbane has resulted in the discovery of an entirely new family of toxins, dubbed “gympietides,” which are produced by the Dendrocnide plants. This toxin is surprisingly similar to venom found in spiders and cone snails, according to the researchers.

These trees grow in eastern Australia, particularly along the slopes and gullies of rainforests. Dendrocnide trees technically belong to the nettle family of plants, which are known to produce annoying stings, but “they are far more than oversized nettles,” wrote the authors of the study. The stems and oval-shaped leaves from these trees are covered in needle-like hairs, and anyone unfortunate enough to rub against them is in for a nasty surprise.

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

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

In the state of Queensland, it is not uncommon to find warning signs along forest tracks, alerting unwary visitors to the presence of Dendrocnide species and the potency of their sting. This signage is justified given that D. moroides has been implicated in hospitalisation of two individuals requiring intensive care for 36 hours who suffered from acute pain that reportedly did not respond to morphine and ongoing symptoms lasting months. This long-lasting pain is also typical of other Dendrocnide species stings, with episodic pain typically subsiding over several weeks, although [painful tingling and prickling sensations] may persist longer.

Scientists have struggled to explain these exaggerated health effects, as the extensive, long-term stinging doesn’t seem to be caused by the fine hairs getting lodged into a person’s skin. What’s more, neurotransmitters and inflammatory mediators such as histamine, acetylcholine, and formic acid do not cause the observed pain effects, even though they’re found in trichomes. For the new study, Vetter and her colleagues sought to find a potentially overlooked neurotoxin in the two Dendrocnide trees, leading to the discovery of the gympietides molecule.

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

“Although they come from a plant, the gympietides are similar to spider and cone snail toxins in the way they fold into their 3D molecular structures and target the same pain receptors — this arguably makes the gympie-gympie tree a truly ‘venomous’ plant,” said Vetter in the University of Queensland release.

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 — the venom — has appeared in a plant and an animal. That’s unusual, as convergent evolution is often driven by similar environmental pressures and lifestyles.

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As the new research shows, this toxin makes permanent alterations to the sodium channels in sensory neurons. Sodium channels are a membrane protein that play a critical role in the formation of pain, which they do through the excitation of neurons. In tests, gympietides was shown to activate the sensory neurons of mice and then prevent them from shutting back down. So this venom — in addition to generating the pain signals — interrupts the mechanism responsible for stopping those signals. That is, in a word, nasty, and it explains why pain sometimes lasts so long after the encounter with the tree.

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

Which, thank goodness. I’ve been stung by “normal” nettles, and that was thoroughly unpleasant. It’s hard for me to imagine those sensations lasting longer than a few minutes, let alone days or weeks. An effective treatment for these venomous trees would be a most welcome development.