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Dec. 22, 2014

Picture of the Week: Transgenic American Chestnuts

by Chau Tu

Click to enlarge images
If you’re roasting chestnuts on an open fire this holiday season, they’re likely not from an American chestnut tree, but rather a related European or Chinese species. That’s because for nearly 100 years, the American chestnut has been “spiraling towards extinction,” according to Charles Maynard, a professor at SUNY College of Environmental Science and Forestry (ESF) and a co-director of its American Chestnut Research and Restoration Project.
 
The native range of American chestnut trees used to cover some 200 million acres, spanning from southern Canada to northern Florida and as far west as the Mississippi River. But at the turn of the 20th century, a severe blight ravaged the population. The culprit was an exotic fungus called Cryphonectria parasitica, which hitchhiked on imported Japanese chestnut trees that were resistant to the parasite.
 
The fungus spread across the American chestnut’s entire natural range in fewer than 50 years, infecting “close to 100 percent” of the population, says Maynard.
 
The fungus infects trees by entering through wounds in the bark, explains William A. Powell, another co-director of the restoration project. “It will colonize that wound,” he says, where it starts producing acids, including oxalic acid, or oxalate, which kills the tree’s tissues. After a couple weeks, the fungus also starts forming mycelial fans, “which are like mechanical wedges that help it pry into the tree tissues,” says Powell.
 
The dead tree tissue and fungal mycelium together form a canker that spreads around the tree “until it finally girdles it,” says Powell. “It eventually cuts off all circulation up and down, killing everything above it.”
 
If the canker forms at the base of the tree, the tree dies down to the stump, but the roots can survive. Millions of stumps still cling to life, according to Powell. The tree can re-sprout, but “eventually that will get infected and get killed back down,” he says. “It's in a Sisyphus-like cycle.”
 
Those regrown shoots almost never get big enough to reproduce, which is where the American Chestnut Research and Restoration Project comes in. For the past 25 years, researchers at SUNY-ESF have been developing a blight-resistant chestnut tree.
 
The first step was to find a blight-resistance gene that could be incorporated into the American chestnut tree genome. The game changer came when Powell’s lab discovered a gene from wheat that makes an enzyme that detoxifies oxalic acid, breaking it down into hydrogen peroxide and carbon dioxide—both chemicals that chestnut trees can metabolize. The idea is, “if you make enough of this enzyme, it will completely detoxify the oxalate,” says Powell. “Therefore, no cankers are formed.”
 
{"input":{"width":"480","photo":"chestnutembryo","row":"5043","table":"DOCUMENT"}}
Maynard’s lab, meanwhile, has focused on incorporating this gene into American chestnut embryos derived from the plant’s somatic, or body, cells (pictured above). Eventually, the embryos are regenerated into whole plants.
 
Earlier this year, the American Chestnut Restoration Project team announced that it had been successful in developing a blight-resistant chestnut. So far, 18 events, or lines of trees, have demonstrated good blight resistance, says Powell, and the trees are growing in their test plots. The new trees are nearly identical to the native species. “It’s 99.996 percent American chestnut,” says Maynard.
 
{"input":{"width":"300","photo":"powell","row":"5043","table":"DOCUMENT"}}
After the researchers whittle down the trials to find the most successful plants, they will apply for approval from three regulatory agencies, including the FDA, the USDA, and the EPA. Powell and Maynard estimate the process will take about five years, after which they’ll distribute the trees to the public as soon as possible. 
 
The goal is to integrate and cross-pollinate the transgenic trees with the few native ones still surviving in the wild in order to rescue the genetic diversity of the species. “We don't want to put out a clone of American chestnut,” says Powell. The researchers also hope to plant some of the new trees in restoration plots such as mine reclamation lands and old fields.
 
“Once we get [federal] approval, we want to pass these out to whoever wants them,” says Powell. “We want the general public helping us plant these trees.”
About Chau Tu

Chau is SciFri's web producer. She spends a lot of her time drinking coffee, seeking out street art, listening to music, and defending Los Angeles. Follow her @chaubtu

The views expressed are those of the author and are not necessarily those of Science Friday.
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