If you are an outdoor person and you read this blog in Australia, it is not too unlikely that you have experienced an unpleasant encounter with the Stinging bush yourself. The Stinging bush, Dendrocnide moroides (Wedd.) Chew, also called Gympie-Gympie or Gympie stinger, is a remarkable example of a venomous plant. Although very few fatal incidents have been described, and even hospitalisation is rare, everyone who has come in contact with the plant will have suffered very intense, long-lasting pain, which is described as “ten times worse than any other pain”. And while the acute pain might subside in the days or weeks after the encounter, often burning sensations remain which might last for months on end. No wonder why some people call Dendrocnide moroides “the most painful plant in the world”.
The Stinging bush is a member of the nettle family (Urticaceae). It is one of six species in the genus Dendrocnide (which literally translated means “Stinging tree”). It is native to the subtropical and tropical forests of eastern Australia in Queensland and the northern parts of New South Wales. The species is also native to some of the Indonesian Lesser Sunda islands and to Vanuatu.
The Stinging bush is a perennial understorey shrub, usually between 1-4 meters tall. The broadly ovate leaves are about 15-25 cm x 10-20 cm large, the leaf blades sitting on long petioles (they are “peltate”, which simply means the petiole, the stalk of the leaf, is attached to the lower surface of the leaf and not at the margin). They have acuminate (= pointed) leaf tips and regularly toothed margins. The small male and female flowers are arranged in inflorescences, which are borne in the leaf axils and which are about 15 cm long. The perianth is rather small (just about 1 cm). Flowering occurs throughout the year but mostly in summer. The fruits, botanically speaking small achenes, each with a single seed inside, are surrounded by swollen, fleshy pedicels. The fruits inside their fleshy pedicels are grouped together in a round structure, which resembles a mulberry (hence the epithet moroides = mulberry-like). The fruits are eaten by birds regardless of the stinging hairs, which in that way disperse the seeds in their droppings to other parts of the forest.
In its native range, Dendrocnide moroides grows at altitudes up to 900 m asl. As a pioneer forest species, the seeds germinate in full sunlight after soil disturbance. The species grows in canopy gaps, in disturbed areas, man-made forest clearings, along watercourses or along roads or small paths.
Almost every part of the plant – the stems, petioles, and the leaves – is covered with needle-shaped, stinging hairs, so called trichomes (think of hairs of the common Stinging nettle Urtica dioica). In fact, already the cotyledons, the first leaves that appear after germination, are covered with trichomes. These trichomes act like hypodermic needles and inject what is called “pharmacological mediators” onto the skin. These “mediators” are not one but multiple substances, a true cocktail of neurotoxins, whose exact nature is not yet fully understood. It is clear that these neurotoxins are chemically very stable as even dead leaves from the forest floor can sting. These neurotoxins activate nerve cells that innervate the skin. The reaction of the nerve cells of the skin materialises itself as pain of increasing intensity (which has been described as “burning”) and a deep aching which can radiate out. Some patients describe the feeling as a sharp stabbing pain. Without going into the biochemical and pharmacological details, I should mention that recent scientific research showed that a variety of different chemicals in the trichome liquid is responsible. Among these compounds is a metabolite called “moroidin”; and very recently a completely new group of very stable peptides was identified, so called “gympietides”. These “gympietides” are active at the sodium (Na) channels of pain receptors of specific nerve cells of the skin that normally respond to injuries and damage. In other words, the open (= activated) sodium channels constantly send neural alarm signals to the brain, which are then translated into “danger” and are felt as strong pain. And because these “gympietides” are so stable, the sodium channels remain activated for so long, and the alarm signalling lasts for so long. To be clear – while the pain is very real, there is not any actual damage to the skin. The brain is tricked!
Most intriguingly, the structure of these “gympietides” is very similar to certain animal venoms as found in some spiders or cone snails (little sea creatures with a rather notorious reputation, also from Australia), and they target the same type of neural receptors. The authors of one research article I have read talk about a remarkable case of convergent evolution across plant and animal kingdoms. And interestingly, there are at least some native animals that do not seem to be deterred by the stinging hairs. One example is the herbivorous Red-legged pademelon, a small, forest-dwelling marsupial, related to kangaroos, which is reported to happily eat Stinging bush leaves with no apparent side effects.
Plants worldwide 