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American chestnut trees were all over the US at the end of the 19th century until the fungus wiped most of them out. Scientists have been trying to figure out ways to bring those endangered American chestnuts back to their former glory.

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Picture a forest full of gigantic trees soaring 30 meters into the sky with five-meter wide trunks. You probably envisioned something like the giant sequoias and redwoods that grow on the western coast of the United States.

But a little over a century ago, the east coast of America was also home to giant trees. Though somewhat smaller than their western counterparts, American chestnuts were huge, and they were all over the eastern US at the dawn of the 20th century. Then, within a few decades, they were almost extinct.

The culprit: a fungus that strangled the trees from within, brought by accident from Asia. Since their demise, scientists have been trying to figure out if there's a way to bring the American chestnut back. And thanks to technological advances, they may finally have a solution — if they can convince the government to let them plant genetically modified trees.

To understand what happened to the American chestnut, we have to go back in time to the end of the 19th century. Back then, American chestnut trees were known as the ‘Sequoias of the East' because they had huge trunks and were tall like the West Coast giants. And they were all over.

In 1900, around a quarter of the hardwood trees east of the Mississippi were American chestnuts — in some places, they made up as much as 40% of the forest. But by the 1940s, they were all but gone. The first signs of trouble were seen in the Bronx Zoo in 1904, when sores called cankers were discovered on a stand of dying trees.

Scientists soon realized the disease was widespread, and by 1912, botanists had managed to identify both the fungus responsible and its point of origin. The chestnut blight fungus gets under tree bark by hitching a ride on insects. The fungus then attacks and feeds off of the trees water-transmitting cambium tissues, essentially choking the tree.

The blight fungus probably arrived in New England in the 1870s, when Japanese chestnut trees became popular ornamental plants. The imports are resistant to the blight, so it's likely they carried it to America where the chestnut trees were totally susceptible. And by the 1940s, it's estimated that nearly 4 billion trees had died.

But they didn't go extinct entirely. A few scattered populations still exist, mostly trees that people planted outside of their original range. There are also smaller specimens along the east coast that were isolated enough from their kin to avoid infection.

And it turns out that, like the Dread Pirate Roberts, even the ”dead” trees are only mostly dead. While the blight destroyed their trunks, their root systems remained. And even decades later, these “living stumps” occasionally eke out a shoot of new growth.

But it's usually in vain because the blight is still around. Although doesn't do much damage to them, it's lurking in those oaks that took over after the chestnuts were wiped out. So before any chestnut shoots can reach reproductive maturity, they catch the blight.

But where there's growth, there's hope, so scientists have been trying to figure out a way to bring American chestnuts back to their former glory. Since the 1980s, forestry specialists and geneticists have tried all sorts of things make blight-resistant trees. They attempted a technique called backcrossing, for example, where surviving specimens and their offspring were carefully bred together to select for natural resistance genes.

But, while this method seems to work for European chestnuts, it hasn't worked as well with. American ones — probably because the European ones were more resistant to begin with. Researchers have also tried hybridizing American chestnuts with blight-resistant Chinese chestnuts, but so far, they haven't been able to get the resistance traits to reliably pass down from generation to generation.

But one method that does seem to work is genetically modifying the trees. It turns out that wheat rust, a fungal disease of wheat, has a similar mechanism of infection to chestnut blight. Both use a compound called oxalic acid to soften up important structural tissues, while also attacking their hosts' cambium by stimulating the growth of calcium oxalate crystals, blocking the flow of nutrients.

Resistant forms of wheat produce an enzyme called oxalate oxidase, which breaks down the acid, thereby blocking the dispersal of the disease and preventing the growth of those crystals. Scientists have introduced this wheat gene into American chestnuts. And in 2014, they revealed that they'd produced a 100% resistant tree that passed the trait onto its offspring — success!

But… the trees haven't been planted. Yet. The researchers have conducted some preliminary studies to show the trees don't cause any unexpected harm to the organisms that live in the environments they once inhabited.

And then, they requested permission from the US Department of Agriculture to release the transgenic trees into the wild. But they're still waiting for the green light. And that could take a while, if it's ever granted at all.

Aside from the general anxiety that accompanies the development of any GMO, some ecologists worry that a return of the American chestnut would disrupt a century-old ecosystem that's developed without it. On the other hand, if successfully put in action, this method could also work for restoring other wild tree populations beleaguered by fungal invasives, like elm trees. I guess only time will tell if the Sequoia of the East will once again stand tall.

Thanks for watching this episode of SciShow! If you liked learning about the efforts to revive the American chestnut, you might like our episode on how scientists could bring extinct animals back. And if you like what you see in general, click that subscribe button to catch every episode! [ ♪OUTRO ].