Tree strikes. Your ouch.

The leaves might look soft and invit­ing. Please don’t touch them.

The Times-Tribune - - HEALTH & SCIENCE - BY KATHER­INE J. WU

It’s like hav­ing a nail shoved into your flesh.”

Ed­ward Gild­ing

Bi­ol­o­gist at the Univer­sity of Queens­land and self-de­scribed sting con­nois­seur

The lore that shrouds Aus­tralia’s gi­ant sting­ing trees, of the genus Den­droc­nide, is per­haps as du­bi­ous as it is vast. Tales abound of night­mar­ish en­coun­ters with the hy­po­der­mic-nee­dle-like hairs of its leaves in­ject­ing a toxin that drives men to mad­ness and has prompted horses to hurl them­selves off cliffs.

Some of these sto­ries are cen­turies old and can­not be ver­i­fied. But as Ed­ward Gild­ing can attest, these leg­ends con­tain at least one lick of truth: the ab­so­lute agony of be­ing stabbed by the fine, downy hairs that adorn the leaves and stems of Den­droc­nide. The trees, which can grow taller than 100 feet, are found through­out the rain­forests of east­ern Aus­tralia, where they are known to tor­ment hik­ers.

“It’s like hav­ing a nail shoved into your flesh,” said Gild­ing, a bi­ol­o­gist at the Univer­sity of Queens­land and self-de­scribed sting con­nois­seur.

The sting from the trees’ hairs also has im­mense stay­ing power, dol­ing out an­guish in waves for hours or days. Some anec­dotes have re­ported in­ter­mit­tent pain last­ing months; a few es­pe­cially bad stings have even landed peo­ple in the hos­pi­tal.

For most vic­tims, such lin­ger­ing mis­ery may be in­cen­tive enough to shun the plants. But Gild­ing and a few like-mind­edly masochis­tic col­leagues have in­stead la­bored to de­ci­pher what gives Den­droc­nide its punch.

Dozens of ex­per­i­ments and count­less stings later, they have iden­ti­fied some of the in­gre­di­ents in­volved. As they re­port in the jour­nal Science Ad­vances, Aus­tralia’s sting­ing trees are packed with a toxin that, when in­jected, latches on to pain-de­tect­ing cells in the re­cip­i­ent and makes them go hay­wire, lock­ing the af­flicted area into the molec­u­lar equiv­a­lent of an in­fi­nite scream.

“So many things in­duce pain, and so lit­tle is known about why,” said Isaac Chiu, a neu­ro­bi­ol­o­gist at Har­vard Univer­sity who was not in­volved in the study. Chiu noted that the trees’

tox­ins tar­get a mol­e­cule, found on nerve cells, that is “fun­da­men­tal to mam­malian pain,” he said. “If this re­veals some­thing that blocks that, it would be re­ally ex­cit­ing.”

The painful po­tency of Den­droc­nide plants has be­dev­iled re­searchers for decades. The trees so of­ten harm peo­ple that many of their habi­tats are marked by cau­tion­ary sig­nage, warn­ing un­wary vis­i­tors to “be­ware the sting­ing tree.” Peo­ple who fre­quent these forests some­times carry res­pi­ra­tors, heavy-duty gloves and a fist­ful of an­ti­his­tamines.

But even sci­en­tists driven enough to in­ject them­selves with ex­tracts made from the trees’ tox­ins have not been able to work out the source of the sting, said Irina Vet­ter, a pain re­searcher at the Univer­sity of Queens­land and an au­thor on the new study.

Those ex­per­i­ments, which are eth­i­cally gray, can no longer be con­ducted, Vet­ter said. But she, Gild­ing and their col­leagues were still able to sep­a­rate out the chem­i­cal com­po­nents of the toxin from two Den­droc­nide species and cre­ate syn­thetic ver­sions of the com­pounds in the lab. One very small pro­tein found in both plants made mice lick and nip at the spots where it was in­jected. Dumped onto nerve cells, the mol­e­cule flipped the trig­ger­happy cells into an “on” po­si­tion, forc­ing them to send out a del­uge of sig­nals.

The re­searchers named the minute, pain-caus­ing mol­e­cules gympi­e­tides, in homage to gympie-gympie, the word for sting­ing tree in the lan­guage of the Gubbi

Gubbi peo­ple, a group of Indige­nous Aus­tralians.

Vet­ter was amazed to find that the gympi­e­tides bore a re­mark­able re­sem­blance to tox­ins made by ven­omous spi­ders and cone snails, which use the chem­i­cals to in­ca­pac­i­tate their hap­less prey.

“These are three widely di­ver­gent groups of or­gan­isms — spi­ders, cone snails and now these trees — pro­duc­ing a toxin that’s very sim­i­lar,” said Shab­nam Mo­ham­madi, a toxin re­searcher at the Univer­sity of Ne­braska-lin­coln, who was not in­volved in the study.

It’s a stun­ning ex­am­ple, she added, of dif­fer­ent branches of the tree of life con­verg­ing on the same so­lu­tion.

The re­searchers are not sure how the toxin ben­e­fits Den­droc­nide trees. Per­haps it serves as a sort of chem­i­cal ar­mor to ward off hun­gry her­bi­vores, Vet­ter said. But some an­i­mals, like bee­tles and pademel­ons — pe­tite rel­a­tives of kan­ga­roos — seem to gladly munch on Den­droc­nide fo­liage, sting­ing spines and all.

Chiu and Mo­ham­madi both said they sus­pect that gympi­e­tides are not the only fac­tors that make Den­droc­nide toxin so tough to take, es­pe­cially given the plants’ bizarre and per­sis­tent side ef­fects. Some of Vet­ter’s pre­vi­ous tus­sles with the trees have re­sulted in chest pain and shoot­ing dis­com­fort in her ex­trem­i­ties, among other symp­toms.

“I think they’ve just scratched the sur­face of what these plants con­tain,” Mo­ham­madi said.

Un­til more of those mys­tery in­gre­di­ents are iden­ti­fied, Gild­ing rec­om­mended steer­ing clear of sting­ing trees. “If you work with the plant, it’s pretty much im­pos­si­ble to not get stung,” he said.

That chal­lenge is made more dif­fi­cult by the plant’s invit­ing ap­pear­ance, Gild­ing noted. The same hairs that can de­liver a dose of un­be­liev­able pain make the leaves and stems look de­cep­tively soft and felty, “like it’s a furry, friendly green plant that you’d want to rub,” he said.

In case it’s not yet clear: Don’t.

THOMAS DUREK / THE UNIVER­SITY OF QUEENS­LAND

A mag­ni­fied view of the sting­ing tri­chomes of Den­droc­nide ex­celsa. Sci­en­tists have found a po­tent chem­i­cal that might give Aus­tralian gi­ant sting­ing trees their ex­traor­di­nar­ily painful punch.

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