American chestnut: An enduring classic

By Joan Conrow

April 29, 2016

Long gone are the days when the American chestnut was common in eastern deciduous forests, providing food, habitat, quality lumber, and welcome shade. But two State University of New York (SUNY) researchers hope to turn back the hands of time with the modern tools of genetic engineering.

Over the past century, some three to five billion trees have succumbed to the ravages of chestnut blight, an exotic pathogen inadvertently introduced from Asia. The fungus functions by colonizing a wound in the bark and producing oxalic acid, which creates a canker that eventually proves lethal by girdling the trunk.

Now researchers Charles Maynard and William Powell have found a way to reverse that deadly cycle. Working with a team of 100 university scientists and students at the SUNY College of Environmental Science and Forestry, they’ve identified a gene from bread wheat that detoxifies the oxalic acid, providing an effective defense against chestnut blight.

The wheat gene produces an oxalate oxidase enzyme that is found in all grain crops and many other familiar foods, Powell explains. This enzyme does not kill the fungus. It takes away the pathogen’s weapon so the fungus is actually changing its lifestyle on the bark of these trees. Instead of forming a canker, it now can just survive on the bark and be a saprophyte.

By adding the wheat gene, and a marker gene, to the 40,000 genes already found in an American chestnut, they’ve been able to achieve resistance without significantly altering the characteristics of the tree. Genetic engineering is one of the safest ways to go forward because it makes the least changes to the genome, Powell says.

For the past 26 years they’ve conducted meticulous research, looking at 28 other candidate genes, some of which conferred partial resistance. Maynard and Powell are now about to seek federal approval so that genetically-engineered, blight-resistant trees can be returned to the wild.

Though it’s a new concept for federal regulators accustomed to keeping GE crops contained, widespread dispersal is crucial to the overall goal of the project: returning the iconic chestnut tree to its rightful place in American forest.

“That’s our game plan, to see it come back as a keystone species,” Maynard says. “We’re not going to be happy if we just get it to be a yard tree or a domestic species. We would like to see it back in the forest, slugging it out with all the other trees.”

Powell is optimistic that they’ll be able to show reviewers at the US Department of Agriculture, Food and Drug Administration and Environmental Protection Agency that the GE chestnut is essentially equivalent to its wild counterpart, and thus no threat to humans, wildlife or natural ecosystems.

“It’s an expensive and time-consuming process that is better suited to well-staffed corporations than busy university researchers,” Powell says. “I would really love it they could simplify the process. Trying to puzzle out these forms is like learning a foreign language.”

Meanwhile, they’re continuing to raise blight-resistant GE chestnuts in the 10-acre SUNY test field. They want to have 10,000 trees ready for distribution by the time deregulation is achieved, a process that could take three-to-five years.

They’re also piquing public interest, and crowd-sourcing funds for their work, by distributing chestnuts from a non-resistant mother tree that folks can grow at home. When a genetically engineered father tree is later planted nearby, the resulting nuts will carry the blight-resistant trait. “This outcrossing process will allow us to rescue much of the surviving genetic diversity and build in local adaptability for the restoration program,” Powell says.

In this way, blight-resistance can be spread through both domestic trees and those remaining in the wild. Though blight ultimately kills trees, chestnuts have the ability to sprout from their root collar, Powell says. Millions of stump sprouts are still growing in the forest, comprising a natural reservoir of genetic diversity that will strengthen restoration efforts.

Pioneers also brought chestnuts along on their westward migration, and some of the largest trees are now found in Washington and Oregon, where they’ve been protected from blight. These trees can be outcrossed with resistant trees, further broadening the gene pool.

“This is a project for our grandchildren,” Powell says. “It will take 100 years before we’re able to get a token of what we had at one time. But it’s a start.”