Three methods for bringing American chestnut trees back to North American forests can now converge, say researchers.
While traditional breeding has been taking place, so have parallel lines of research into genetic modification and also bio-control of the fungus that causes the blight.
“Teams of researchers are now at a crossroads where all three methodologies may be combined to provide a more robust product,” says Sara Fitzsimmons, a research technologist who is also director of restoration for the American Chestnut Foundation, the group leading the chestnut-restoration effort. “By merging successes in genetic modification, hypovirulence, and traditional breeding, restoration of a disease-resistant American chestnut tree is closer.”
The chestnut blight—which wiped out the American chestnut species across its 180-million-acre range in the first half of the 20th century—is caused by a fungus inadvertently introduced from Asia. Some view the loss of the chestnuts, which produced untold tons of food for wildlife and food and lumber for humans, as one of the worst US ecological disasters.
“We didn’t account in our time estimates for how long it would take after we got nuts with blight resistance to plant out orchards and select progeny with the strongest resistance and eliminate material susceptible to the blight,” says Fitzsimmons.
“When we plant these trees with nuts generated by our latest generation of backcrossed trees, only 1 percent have the resistance that we are looking for. So you can imagine, if we’re planting 27,000 trees, only about 270 have the combination of blight resistance and American chestnut characteristics we need.”
The chestnut orchard in Penn State’s arboretum and the Chestnut Foundation’s Meadowview Research Farms in Virginia contain the latest generation of traditionally bred plant material with the most chestnut blight resistance and American character.
“Because we still haven’t finished planting out the orchard and we still haven’t finished selecting and culling highly blight-susceptible trees we planted a few years ago, the blight-susceptible trees are pollinating trees that are selected and resistant,” she explains. “So, when we collect nuts in this orchard, they have a wide variety of resistance and very few have full resistance, because there is so much pollen at this location.”
Fitzsimmons and other researchers bagged flowers on selected trees to keep unwanted pollen away and introduced pollen from trees known to have a high level of blight resistance. Blight resistance is measured after the fungus that causes the disease is applied to wounds made in the young chestnuts’ trunks or branches.
“When we were collecting open-pollination nuts, we were hoping that they wouldn’t have this much susceptibility in the progeny, but because there is so much susceptibility in the pollen cloud, we were not able to get rid of it,” Fitzsimmons explains. “So this year, we took the best of the best, and we performed controlled pollination. Controlled pollination, however, yields about 50 percent or fewer nuts than open pollination does.
“These control-pollinated nuts will be a true test of levels of resistance possible in this population.”
Adding resistance genes
John Carlson, professor of molecular genetics at Penn State, has led a project to sequence and characterize the entire genome of one of the blight-resistant donor Chinese chestnut trees in the chestnut foundation’s breeding program, with an eye toward identifying all of the resistance genes.
Carlson, director of Penn State’s Schatz Center for Tree Molecular Genetics, is now collaborating with tree geneticists and other researchers to unravel the mystery of blight resistance. The group also is testing a genome-sequence-based system to accelerate the selection of blight-resistant plants that now are genetically American, using nuts harvested this fall from the trees that underwent controlled pollination in the Penn State orchard.
Developing blight resistance in American chestnut is complex and challenging, concedes Carlson, who also has applied molecular-genetics techniques to modify poplar trees for bioprocessing and biofuels. “Our aspirations are to move the resistance genes from Chinese chestnuts into American chestnuts and find out which combinations of genes would give the best resistance,” he says.
“That has proven to be extremely complicated because more than a few genes are involved, and we haven’t yet pinned down which ones are the most important. Genetic engineering groups have been testing about a dozen blight-resistance genes that have been identified. We have to test them in combination because we know that blight resistance is not a single-gene trait, so we have to test multiple combinations of genes, which is very difficult to do and takes time.”
If biotechnology researchers do develop a genetically modified, blight-free American chestnut, current federal regulations would limit distribution of the plants, Fitzsimmons notes. An estimated five years will be required to have the product deregulated by governmental agencies before it is available for widespread distribution and planting.
“We expect to have research plantings of GMO backcross trees within the next two years,” she says. “While GMO cannot be allowed to open pollinate under current regulations, GMO American chestnuts appear to offer an excellent chance of creating a blight-resistant American chestnut.”
Weaken the fungus
The Chestnut Foundation also is planning to soon deploy a biocontrol, developed by pathologists from West Virginia University and the University of Maryland, to weaken the fungus that causes chestnut blight. The biocontrol involves infecting the fungus that causes chestnut blight with a virus that makes the fungus sick and reduces its virulence.
“We are now focusing on the three Bs in concert to restore the American chestnut—breeding, biocontrols, and biotechnology,” Fitzsimmons says. “This gives us a bigger suite of tools to fight off this fungus and blight.”
But even if all of these initiatives to restore the American chestnut come off without a hitch, it may take a century or more to see chestnuts again across their former range, from Maine to Florida, she concedes. Based on research she is conducting in Maine and Vermont on naturally regenerating sites, it looks like it takes at least 20 years for a plot of chestnuts just to become established beneath a forest canopy.
“Tree breeding, especially hardwoods, takes extraordinary patience because results often aren’t seen over a lifetime—we knew that,” she says. “To see a naturally regenerating American chestnut population regaining its reproductive niche in the ecological landscape will take a long time—50 years at least after we plant nuts from truly blight-resistant trees.”
No matter how long it takes, the chestnut reintroduction effort is monumental, because it is likely the most complex and long-term attempt to rescue a plant species ever pursued, says Kim Steiner, professor of forest biology, Penn State arboretum director, and senior science adviser to the American Chestnut Foundation.
Breeding trees to develop blight resistance is difficult enough, but in this particular case the rescue requires transferring genes from one species to another while still maintaining the genetic diversity of the original species.
“A ‘horticultural’ solution—where a successful product is commonly a single, clonally propagated genotype—is not sufficient because we are attempting to restore a species to the wild where it must survive, reproduce, and eventually evolve on its own,” Steiner says.
Funding sources for the work include the Forest Health Initiative and the National Science Foundation.
Source: Penn State