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Studies: Transgenic American Chestnut Trees Show No Ill Effects on Seeds, Fungi or Larval Frogs

Two new studies on the environmental impact of transgenic American chestnut trees provide evidence that the trees have no harmful effects on germinating seeds, beneficial fungi, or larval frogs that are dependable indicators of environmental quality.

The findings were published by researchers at the SUNY College of Environmental Science and Forestry (ESF), where scientists have been working for 29 years to restore the valuable species after it was nearly wiped out by a pathogenic blight in the 20thcentury. Now that they have developed a process for growing blight-tolerant trees, ESF scientists have turned their attention to assessing how these trees could affect the environment.

The two recent studies found that leaf litter from transgenic trees has no harmful effects on germinating seeds, mycorrhizal fungi that benefit the tree and the ecosystem, or larval frogs that live in the forests that were once home to some 4 billion American chestnut trees.

"Since we were making an extremely small change to the tree as compared to other, more traditional breeding methods, we didn't expect to see any differences between the wild, blight-susceptible trees and the blight-tolerant American chestnut trees other than now being able to coexist with the invasive pathogen. These and other experiments support these conclusions," said Professor William Powell, a co-author on both studies and director of ESF's American Chestnut Research and Restoration Project.

The ESF technique neutralizes the pathogen's main weapon by using a common detoxifying enzyme found in many plants. When this single gene is added to the chestnut tree's approximately 38,000-gene genome, the tree can withstand an attack by the blight.

ESF's body of research on the subject includes participation by scientists beyond foresters and tree geneticists. The two recent studies included SUNY Distinguished Professor James Gibbs, director of ESF's Roosevelt Wild Life Station, and Professor Thomas R. Horton, a mycologist. Powell said the work of other biologists is key to determining how - if at all - a transgenic American chestnut might affect an ecosystem in ways a non-transgenic tree would not. So far, researchers have studied leaf herbivory by insects, bumblebees and pollen, and leaf litter composition, and found no evidence that transgenic American chestnuts present greater ecological risks than traditional breeding.

"Therefore, blight-tolerant trees should provide the same ecological benefits as those provided by the species before the blight," Powell said.

Powell and his colleagues are seeking regulatory approval from the federal government to distribute the trees publicly. That will involve review by the U.S. Environmental Protection Agency, the U.S. Department of Agriculture, and the Food and Drug Administration. Although regulatory approval has been sought, and obtained, for many agricultural crops, this is the first time such approval will be considered for a threatened plant that is intended to be reintroduced into its natural environment. The process could take two or more years.

"If federal regulatory approval is obtained to release transgenic American chestnut, ESF will collaborate with The American Chestnut Foundation to breed the transgenic trees with wild American chestnut trees. The aim of this breeding is to create a genetically diverse, and blight-tolerant population that will adapted to the diverse and changing climate of eastern North American forests," said Dr. Jared Westbrook, director of science with The American Chestnut Foundation.

The first of the recent studies, published in July in the journal Frontiers in Plant Science, reported that leaf litter from transgenic American chestnuts had no harmful effects on the germination of seeds from a variety of trees, shrubs and grasses. In addition, the blight-tolerant American chestnut did not inhibit colonization by mycorrhizal fungi, which grow among tree roots in a mutually beneficial relationship with the host tree. The fungi enhance the trees' ability to gather water and nutrients and receive energy from the trees. Mycorrhizal fungi are observed in fossils of the first plants to colonize land and are associated with over 80 percent of the world's plants. Plants cannot survive to a reproductive age in nature without these fungi. The research showed that while chestnut with the wheat gene inhibits chestnut blight on its stems, it does not inhibit the growth of the important mycorrhizal fungi on its roots.

"Along with other environmental impact comparisons, these conclusions provide further evidence that transgenic American chestnuts are not functionally different with regard to ecosystem interactions than non-transgenic American chestnuts," the paper states.

In the second study, published in August in the journal Restoration Ecology, the ESF research team evaluated the effect of transgenic American chestnut leaf litter on the growth and survival of larval wood frogs. The frogs, which are less than three inches long, have a range that includes forested habitat across the eastern United States - including much of the same region that was once home to American chestnut trees. In their larval stage, the frogs forage almost entirely on periphyton, a mixture of freshwater organisms that attach themselves to submerged plants and other objects that accumulate in temporary vernal pools in forests.

"Developing wood frogs are a product of the leaves they forage upon in the vernal pools where they live; these abundant amphibians are good indicators of environmental quality. We saw neutral to positive effects of blight-tolerant American chestnut leaves on developing wood frogs, suggesting chestnut restoration may actually benefit the species," Gibbs said.

The scientists reared wood frog larvae on leaf litter from American chestnuts that had been genetically engineered for blight tolerance. They also raised frog larvae on litter from non-transgenic American chestnut, Chinese chestnut, an American-Chinese chestnut hybrid and two non-transgenic "control" tree species, sugar maple and American beech.

The result: No observed differences in growth or survival of wood frog larvae reared on transgenic versus non-transgenic American chestnut leaves.

"Without supplementary food, wood frog larvae provided leaves from American chestnut (both types) developed faster and grew larger than those exposed to other leaf litter treatments," the scientists wrote.

The results suggest that American chestnut might have formerly been an important source of food for forest‐dwelling amphibians and that transgenic American chestnut litter generated as part of chestnut restoration efforts is unlikely to present unusual risks to developing amphibian larvae in the forest.

Two additional studies are expected to be submitted for publication in the coming months: one examining interactions between leaf-feeding insects and forest biocontrol treatments as applied to American chestnut, and another looking at pollen use by native bumblebees. As with the previous studies, both these experiments showed no measurable differences on the insects feeding on transgenic and non-transgenic American chestnuts as they may someday grow in the wild.

In addition to Powell, Gibbs and Horton, these ESF researchers contributed to the two publications: master's student Harrison B. Goldspiel; Ph.D. student Andrew E. Newhouse; Visiting Instructor Allison D. Oakes; master's student Hannah C. Pilkey; and Hannah E. Roden, who worked on the project for her undergraduate honor's thesis and earned her bachelor's degree in 2018. Additional work investigating the impact of blight resistant transgenic chestnut on mycorrhizal fungi was conducted by master's students Katie D'Amico and Sam Tourtellot in previous studies.