The American chestnut: A new frontier in gene editing
After being driven to near extinction, the historic American chestnut tree may begin repopulating Eastern U.S. forests later this year. Scientists have inserted a gene into the tree’s DNA to protect it from disease, and the U.S. Department of Agriculture is expected to announce soon whether the transgenic tree is safe enough to be released into nature.
The blight-resistant American chestnut — known as Darling58 — was developed by researchers at the State University of New York. Graduate student Erik Carlson contributed to the work.
“This research lays the foundation for the technology to be used in other forest trees facing extinction,” Carlson said.
Regulators are awaiting the results of a plant pest risk assessment. If they determine the tree is unlikely to pose an increased plant pest risk, then Darling58 will be deemed safe to grow in the U.S.
In the meantime, Carlson and others are working to educate a sometimes skeptical public on the safety and value of genetic engineering.
When people realize that genetic engineering can enable noble endeavors such as saving an entire species of trees, Carlson said, “They say, ‘Oh, wait a minute, maybe we've been looking at this the wrong way.’”
The American chestnut tree faces extinction
The American chestnut, Castanea dentata, was once one of the largest and most prominent trees in American eastern forests. Some of these trees would reach 100 feet tall and 10 feet in diameter, earning them the nickname the “redwoods of the east.”
These giants used to thrive in the eastern U.S. forests alongside the loblolly pines, tulip poplars, and white oaks, until a fungal blight pathogen from Asia, Cryphonectria parasitica, wiped out an estimated four billion trees. The fungal infection was impossible to control and ultimately left the American chestnut functionally extinct since the middle of the 20th century.
The science behind Darling58
The rapid decimation of the American chestnut kicked off a race to protect and restore it to its former glory.
The first attempt involved breeding it with Asian chestnut tree species that have a natural blight resistance, but that effort took nearly three decades to consistently maintain resistance. Also, those hybrids faced additional downsides, such as the low transmission of blight resistance to offspring and reduced forest stature. Plus, as hybrids, some may view them as no longer being true American chestnuts.
Researchers at the SUNY College of Environmental Science and Forestry took a different approach, using genetic engineering to produce a blight-resistant American chestnut.
The group inserted a gene from wheat into the tree’s genome. This gene encodes an enzyme known as oxalate oxidase that detoxifies oxalate, the harmful compound produced by C. parasitica and many other pathogenic fungi.
Unlike making hybrid crosses that jumble the genome with little control over the traits produced, genetic engineering provides a more precise introduction of the OxO gene, which allows for the conservation of the entire American chestnut genome.
Genetic engineering also cut the timeline for blight-resistant tree production and accompanying studies on its growth and effects from 30 years to 10 years.
Norman G. Lewis, a distinguished professor at Washington State University’s Institute of Biological Chemistry, said, “As the world changes, we, as scientists and humans, will need to change with it along with our approach to solving the problems. Our solutions will not be the same as in years prior because the shoes we fill are different than those of the past.”
Doubts about Darling58
Trees often symbolize our human connection to nature, evoking strong emotions from the public about genetically modifying them. During an open comment period, members of the public expressed concern about the environmental impact of Darling58 trees.
Carlson emphasizes that the trees have undergone extensive environmental impact studies.
“Leaves were fed to multiple insect species and wood frog tadpoles. Colonization of mycorrhizal fungi was examined. Effects of transgenic leaf litter on seed germination of multiple plant species and many other similar examples were included in the petition submitted to the USDA, and no unexpected negative effects were observed in any of these studies,” Carlson said.
Researchers also explored other species that are sensitive to ecosystem changes, such as bumblebees.
Experts point out that the OxO gene in Darling58 is also in myriad other species within the American chestnut’s ecosystem.
“We can’t possibly know all the risks of introducing Darling58, but we have ruled out the big ones,” said Carlson.
Edward Eisenstein, an associate professor at the University of Maryland, said that scientists and the public need to “meet in the middle.” Eisenstein said science should be publicly accessible and carefully evaluated for safety, as SUNY group and the American Chestnut Foundation have done, to encourage uptake of the “new normal” by the public.
Stuck in regulatory limbo
While Darling58 has been awaiting its regulatory fate, a transgenic poplar tree from Living Carbon became the first genetically modified tree planted in a U.S. forest.
Unlike the modified chestnut, the engineered poplars avoided the USDA plant pest risk assessment on the technicality that they were made using genes only from other plants and were inserted biolistically — using pressurized air and DNA-coated particles to insert new pieces of DNA — rather than genetically modified via a bacterium.
Though both the poplar and Darling58 possess foreign DNA, regulators subjected them to different levels of scrutiny based on the techniques used to generate them. These loopholes have since closed, thanks to updated regulatory policy. Now, all genetically engineered plants will be subject to the more stringent review undergone by Darling58.
Carlson said that, more than anything, he hopes the public will realize that Darling58 will allow scientists to have “a real opportunity to rescue genetic diversity” and ensure the longevity of the American chestnut tree as the world continues to change around it.
At the University of Georgia, I am using the method that was used to produce Darling58 in a faster-growing species, Populus, also known as the cottonwood.
The goal of my research in Chung-Jui Tsai’s lab is to improve biomass accumulation for the use of biofuel production. I use transgenic trees to study how the distribution of the macronutrient sulfate affects biomass accumulation during climate change–induced stressors, such as drought.
It is exciting to see these technological advancements occur during my training, and I hope reintroducing the gentle giants (American chestnuts) to the North American Eastern forests fortifies the continued advancement of genetic engineering in improving ecosystems for years to come.
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