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Plants that bite and sting

The stuff they inject has been found to be like the venom of spiders and sea snails


Some plants, like the Venus flytrap – move and act to get their prey. Some do not move, but they use venom, which is more common among animals, to keep enemies at bay.

Edward K Gilding, Sina Jami, Jennifer R Deuis, Mathilde R Israel, Peta J Harvey, Aaron G Poth, Fabian BH Rehm, Jennifer L Stow, Samuel D Robinson, Kuok Yap, Darren L Brown, Brett R Hamilton, David Andersson, David J Craik, Irina Vetter, Thomas Durek, from the University of Queensland at Brisbane, Australia and King’s College, London, describe in the journal, Science Advances, what gives the giant Australian stinging tree, Dendrocnide excelsa, its legendary sting, which is as bad, and often far worse, than those of poisonous insects.

They find that the active agents are akin to those found in the animal kingdom, which suggests an intriguing path of evolution, where plants diverged from animals, but developed related instruments of predator control.

“Australia notoriously harbours some of the world’s most venomous animals, but although less well known, its venomous flora is equally remarkable,” says the opening line of the paper. The giant Australian stinging tree, also known as the stinging brush, mulberry-leaved stinger, gympie gympie or moonlighter, which is found in the rainforests of North-east Australia and parts of Indonesia, the paper says, stands supreme in size — it is usually one to three metres high, but can grow as high as 35 metres.

The stinging tree belongs to the family Urticaceae, a family that includes the nettles or stinging nettles, well-known shrubs that have stinging hairs. Readers may be familiar with the term, urticaria, for the red, itchy welts that sometimes form on the skin. The Australian stingers, however, the paper says, “are far more than oversized nettles.” The Australian stinging tree species “are particularly 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. Like other stinging plants such as nettles, the giant stinging tree is covered in needle-like appendages called trichomes that are around five millimetres in length – the trichomes look like fine hairs, but actually act like hypodermic needles that inject toxins when they make contact with skin,” says co-author Irna Vetter, in a press release.

The state of Queensland, the paper says, has put up signs along forest tracks, to alert visitors of Dendrocnide species and the potency of their sting. The symptoms are usually severe and persistent pain, sometimes with neuromuscular symptoms and respiratory distress. Although most cases do not require hospitalisation, there are instances of patients in intensive care for 36 hours – with acute pain that did not respond to morphine and symptoms that continued for months!

Apart from analgesics, the severe pain is often treated with dilute hydrochloric acid, to inactivate the venom, and, maybe under the mistaken impression that it is the stinging hairs stuck in the skin that cause the pain, with wax strips, to pull them out. The needle shaped structures on the leaves and stem of D excelsa and D Moroides, the leading giant stingers, look like a benign felt cover, the paper says, but they are, in fact, hollow, silica impregnated needles that can inject chemical agents that affect pain-sensitive nerve terminals in ? the skin.

One proposed component of the Urtica sting has been the ‘small molecule”, a structure that is common in developing drugs that can form a “lock and key” fit with portions of cells, to block or promote their action. Examples are histamine (which sets of sneezing or allergic reactions), other agents that affect nerves, formic acid (the component in the ant sting and in nettles) and some other acids. Injecting those compounds, even in combinations, however, the paper says, could not replicate or change the severity of extended duration of the pain that Dendrocnide extract could cause. There was hence clearly some other substance, that affected nerves, which they had to find.

It is the tip of the stingers’ needles that breaks when it enters the skin and allows the contents of the needle to leak out. The team prepared extracts of the sting hairs and analysed the extract — using methods to separate the contents. One component, which showed pain-inducing behaviour, was further analysed and a group of small-sized proteins were isolated as the active agents. The proteins, unlike any encountered before, were named gympietides, from the tribal name, gympie-gympie, for the stinging trees.

The effect of gympietides was found to arise from the effect that they have on the sodium channels of nerve cells. Nerves function by passing electric signals, which are generated by changes in the material within the nerve cells. The material inside a nerve cell “at rest” has a slight excess of positively charged potassium ions and some negatively charged protein molecules. At the same time, the cell has a slight deficiency of positive, sodium ions. The relative concentration of the charged “ions” within the cell is thus some 70-80 mV negative compared to outside the cell. And, as the cell wall does not allow ions to go through, there is electrical ‘tension” across the two sides of the cell wall.

That changes when an electrical signal arrives at the cell from the environment or a neighbouring cell. This signal causes gaps, called gateways, in the cell wall to open, and positive sodium ions rush in, and the net change is reversed to about +40 mV. This change opens another set of gateways that allow positive potassium ions to rush to the potassium deficient exterior, which restores the 70 mV polarity of the cell. This inandout shiver passes along the body of the cell, until it affects the neighbouring cell, and so the signal is passed on.

What the Queensland team found was that gympietides activated sodium channels and started pulses of pain signals. The team developed a synthetic substance that resembled the gympietides and showed that it had the same effect on sodium channels and again, pain in experimental animal subjects for extended periods.

The work is a step forward in understanding how pain-causing agents work and a suggestion that agents that block the sodium channels could be a way to relieve pain. It also reveals a similarity of plant proteins with agents in animal venoms, whose ancestral connection is an exceedingly remote function to cause pain.

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