A team of scientists from the University of Queensland and King’s College London has found that the venom of Australian Dendrocnide trees contains previously unidentified neurotoxic peptides and that the 3D structure of these pain-inducing peptides is reminiscent of spider and cone snail venoms targeting the same pain receptors, thus representing a remarkable case of inter-kingdom convergent evolution of animal and plant venoms.
Australia notoriously harbors some of the world’s most venomous animals, but although less well known, its venomous flora is equally remarkable.
The giant stinging tree (Dendrocnide excelsa) reigns superlative in size, with some specimens growing to 35 m (115 feet) tall along the slopes and gullies of eastern Australian rainforests. However, these members of the family Urticaceae are far more than oversized nettles.
Of the six species in the genus Dendrocnide native to the subtropical and tropical forests of Eastern Australia, the giant stinging tree and the mulberry-like stinging tree (Dendrocnide moroides) are particularly notorious for producing painful stings, which can cause symptoms that last for days or weeks in extreme cases.
Like other stinging plants such as nettles, the giant stinging tree is covered in needle-like appendages called trichomes that are around five millimeters in length — the trichomes look like fine hairs, but actually act like hypodermic needles that inject toxins when they make contact with skin.
Small molecules in the trichomes such as histamine, acetylcholine and formic acid have been previously tested, but injecting these did not cause the severe and long-lasting pain of the stinging tree, suggesting that there was an unidentified neurotoxin to be found.
The scientists found a completely new class of neurotoxin miniproteins that they termed ‘gympietides,’ after the Indigenous name for the plant.
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.
The long-lasting pain from the stinging tree may be explained by the gympietides permanently changing the sodium channels in the sensory neurons, not due to the fine hairs getting stuck in the skin.
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. They can also potentially use the gympietides as scaffolds for new therapeutics for pain relief. They can also potentially use the gympietides as scaffolds for new therapeutics for pain relief.
They can also potentially use the gympietides as scaffolds for new therapeutics for pain relief.
References: Edward K. Gilding et al. 2020. Neurotoxic peptides from the venom of the giant Australian stinging tree. Science Advances 6 (38): eabb8828; doi: 10.1126/sciadv.abb8828 link: https://advances.sciencemag.org/content/6/38/eabb8828