The legendary children’s book, ‘The Giving Tree,’ could have easily had an American Chestnut (Castanea dentata) as a central figure. The tree provided so much to both natural and human communities that it was once romantically dubbed ‘the perfect tree.’
However, unlike ‘The Giving Tree,’ humans didn’t directly cause the species’ demise (although they did chop down quite many of them). Chesnut blight, a fungal disease imported from Asia, wiped out almost the entire four billion-strong population from its natural range. Stubborn stumps still survive and produce sprouts, but the disease kills them before they mature into real trees.
With good reason, the fall of the American chestnut is considered one of the biggest North American ecological disasters and “the greatest tragedy in American forest history.”
Scientists and conservationists are working to rescue the American Chestnut from going completely extinct. Advances in genetic engineering have opened the door to the development of a blight-resistant variant through transgenic methods. Yet, the journey towards this solution has proven to be challenging.
‘The Perfect Tree’
The widespread range of ecosystem roles made the American chestnut one of the keystone plant species of the Northeast and its legendary old-growth woodlands. The American chestnut used to be found in over 200 million acres of eastern forests. Its range spanned from Maine to Alabama and from the Piedmont west to Ohio Valley. The largest chestnut specimens measured up to 130 feet in height and were more than 10 feet wide, earning the nickname ‘the Redwoods of the East.’
The large and elaborate canopy created homes for many tree-dwelling animals, plus a dense shade. The bountiful flowers attracted many pollinators and other insects, some associated exclusively with the American chestnut. By autumn, the former blooms would turn into nutritious nuts, feeding wildlife – squirrels, raccoons, Black bears, White-tailed deer, and even the Passenger pigeon before its extinction. Humans, boars, and livestock also feasted on the nuts that were a generous source of vitamin C, manganese, dietary fiber, and healthy fats. Also, the fallen autumn leaves enriched the forest soil and provided additional microhabitats.
For humans, the tree was also invaluable. Its wide range of commercial uses engraved it in history as one of North America’s most critical natural resources until the 20th century. Besides the nuts, the American chestnut multi-purpose hardwood was also immensely appreciated across the nation(s). The famous 20th-century plant pathologist George Hepting noted how the American chestnut supported the people of North America “from cradle to grave,” writing: “Not only was baby’s crib likely made of chestnut, but chances were, so was the old man’s coffin.” The natives used the bark to make the famous wigwams; European settlers used it to build their houses. Because the chestnut coppiced abundantly from stumps, it was an abundant source of wood and firewood.
You could also argue that the Chesnut was at the forefront of American modernization – its wood was a favorite choice for railroad ties and telephone poles because of its sturdiness and rot resistance.
The part about rot resistance is especially ironic in the context of the American chestnut’s 20th-century downfall – because it was an imported fungal pathogen that destroyed the population.
Chesnut Blight, The Disease That Killed The American Chestnut
The ever-increasing trade and import accidentally brought the Asian pathogenic fungus, chestnut blight (Cryphonectria parasitica), to North America and Europe. Although the first reported case was in 1904, it has probably been around since the end of the 19th century.
While the source of the disease – the Japanese Chestnut, is fairly resistant to both chestnut blight and ink disease (another problematic pathogen, Phytophthora cinnamomi), the American Chestnut, unfortunately, proved highly susceptible.
The chestnut blight is not a true ‘blight’ – the shoots do die off, but the fungus doesn’t attack them directly. It’s a canker disease – causing large, diffuse cankers in the trunk. The abnormal growth gradually cuts away the nutrient flow from the root system. The aboveground part of the tree then dies a slow death. The root system often survives and continues to give off shoots, but the fungus also kills these before they reach maturity.
Although the American chestnut is listed as ‘Critically Endangered,’ its population largely came down to old stumps that are still alive and continue to give off shoots. However, the inability of trees to reach maturity and continue reproducing means that the species is ‘functionally extinct.’
Besides the American Chestnut, Endothia parasitica induces high mortality in other American chestnuts like the American chinquapin (Castanea pumila) and the Ozark chinquapin (Castanea ozarkensis).
Efforts To Save the American Chestnut
Luckily, the American Chestnut has that ‘Original North American wilderness’ allure that raises interest in its conservation, even beyond the scientific community.
The main challenge in chestnut conservation is the creation of blight-resistant trees that would be reintroduced across the natural range. The main hope is that these trees will be able to regain their former glory and give way to a new generation of disease-free American Chestnuts.
The leading conservation organization is The American Chestnut Foundation. It was founded in 1983 “by a group of prominent plant scientists and lay persons who recognized the severe impact the demise of the American chestnut tree imposed upon the local economy of rural communities and upon the ecology of forests within the tree’s native range.”
Over the years, TACF has invested a great deal in various attempts at breeding blight-resistant cultivars using techniques varying from classic hybridization to mutational breeding to gene technologies. Here is a brief timeline.
- In the 1900s, USDA appointed Frank Meyer, a famous plant explorer, to find an adequate replacement for the American Chestnut. The Chinese Chestnut seemed to be the most promising and was thus imported in substantial quantities.
- By the 1940s, it was clear that imported Chinese chestnuts couldn’t fill the ecological role of the native chestnut. Thus, USDA and others began to crossbreed all the available material together to create a resistant hybrid. The move was significant for future programs, but it didn’t result in something that would promise species restoration at the time.
- The 1950s were the era of mutational breeding – exposing seeds to radiation to create novel mutations. The scientists Ralph Singleton and Albert Dietz hoped there would be some specimens with induced blight-resistance mutation.
- In the 1970s, hypovirulence was discovered – meaning that a certain virus can attack the fungus within the tree, reducing the aggressiveness of chestnut blight.
- The 1980s brought traditional backcross breeding back on the table. The results of these experiments provided crucial materials for further (and ongoing) restoration efforts.
- In the 1990s, a method for genetic modification of American Chestnut was created by Drs. Bill Powell and Chuck Maynard, giving rise to transgenic trees.
- The early 2000s brought the Darling trees – a transgenic chestnut line with inserted oxalate oxidase (OxO) wheat gene. For now, the results are mixed and somewhat controversial.
The Darling Line – Real Breakthrough or Wrong Turn?
As said, the Darling line was produced by inserting an oxalate oxidase (OxO) gene from wheat. Chestnut blight fungus produces oxalic acid, so in theory, the OxO gene helps the tree to get rid of the acid and thus stop the canker spread.
The gene was inserted randomly into the chestnut’s genome in various places. Depending on its placement, scientists created many Darling lines, all numbered. The Darling 58, developed by the State University of New York College of Environmental Science and Forestry (SUNY ESF), looked especially promising and resulted in further lab and field testing. In the field experiments, the young modified trees are inoculated with the blight fungus to see how much resistance they show.
Not long after the field tests began, scientists noticed that the Darling 58 trees were performing poorly. They were short and developed slower than the controls; their leaves were often curled and browned; the degree of resistance to the disease also varied significantly. It was all in contrast with the promising first tests.
Then, the mystery unfolded. In 2023, geneticists from the University of New England and the University of Maine discovered that the trees they were working with were not Darling 58 but another line called Darling 54.
The news about the mixup created a rift in the chestnut restoration community. In the phone interview with The Washington Post, the SUNY ESF chestnut restoration program director, Andrew Newhouse, said they are unsure how the mixup occurred. “It must have been a label swap between these two trees that we were working with at the same time in or around 2016,” he stated.
Because of the mixup and long-term poor performance of the Darling Line, the American Chestnut Foundation decided to pull their support for the project. The American Chestnut Foundation’s chief conservation officer, Sara Fern Fitzsimmons, insisted it was a science-based decision. “Virtually every month we turn around, we’re getting even more red flags,” she said, “there’s no way we would have pulled this if we weren’t really concerned.”
Despite the lack of external support from the biggest American Chestnut conservation foundation, SUNY ESF is preparing to continue the project and seek federal approval for seed distribution. According to current laws, genetically modified trees cannot be freely planted unless a special permit is obtained.
Newhouse says that the concerns about Darling trees field growth are ‘overblown’ and ‘can be overcome.’ He also adds that the Darling trees have so far turned out to be risk-free for other organisms and that it is important to keep studying and improving them. At this point, they are awaiting approval from the Agriculture Department and Environmental Protection Agency.
Is It Too Late for the American Chestnut?
What is the future of American Chestnut? Do the failures of the Darling 58 transgenic line and the frayed trust in genetic modification programs effectively put an end to restoration efforts? Not really. However disappointing the Darling 58 failure may sound, the continuation of the SUNY ESF program will almost surely result in new discoveries regarding blight resistance.
TACF continues to develop and support many other initiatives. Besides the OxO-based genetic engineering, there are other options on the table. For example, fungus biocontrol via hypovirulence – the phenomenon that made European chestnut populations more resistant to chestnut blight.
Last but not least, the classic breeding program shouldn’t be underestimated. As the Foundation writes, ‘During the past 36 years, offspring from blight resistant hybrids have been bred with American chestnuts from across the species’ range. Four generations later, our traditional breeding program has produced a genetically diverse population of American chestnut hybrids with improved blight tolerance from Chinese chestnuts (Castanea mollissima).’
There are around 500 TACF-led American Chestnut orchards.
As Thomas Klak, one of the scientists who discovered the Darling 58/54 mix-up, puts it, “Science never is smooth; two steps forward, one step back is normal in science, but you just keep pressing on.”
Although we’ll probably have to wait sometime to see another groundbreaking development, the ‘Redwood of the East’ certainly deserves that we press on.
Resources
The Studies
Burnham, C.R. (1988) ‘The Restoration of the American Chestnut: Mendelian genetics may solve a problem that has resisted other approaches.’ American Scientist Vol. 76, No. 5 (September-October 1988), pp. 478-487 (10 pages)
Faison, E. K., and Foster, D.R. (2014) ‘Did American Chestnut Really Dominate the Eastern Forest?’ Arnoldia – Volume 72, Issue 2, Oct 15, 2014.
Little, E. L., Jr. (1977) Atlas of United States trees. Volume 4. Minor eastern hardwoods. Misc. Pub. No. 1342. Washington, DC: U.S. Department of Agriculture, Forest Service. 17 p.
Newhouse, A.E. et al. (2021) ‘Bumble bee (Bombus impatiens) survival, pollen usage, and reproduction are not affected by oxalate oxidase at realistic concentrations in American chestnut (Castanea dentata) pollen.’ Transgenic Studies, Volume 30, pages 751–764, (2021)
Powell, W.A., Newhouse, A.E., Coffey, V. (2019) ‘Developing Blight-Tolerant American Chestnut Trees.’ Cold Spring Harb Perspect Biol. 2019 Jul 1;11(7):a034587. doi: 10.1101/cshperspect.a034587
Articles
‘Background on American chestnut and chestnut blight,’ SUNY College of Environmental Science and Forestry
‘Chestnut Blight,’ Forest Pathology
‘Control of Chestnut Blight,’ Penn State Department of Ecosystem Science and Management
Grandoni, Dino. ‘Genetic engineering was meant to save chestnut trees. Then there was a mistake.’ The Washington Post, 24 December 2023.
Grandoni, Dino. ‘Gene editing could revive a nearly lost tree. Not everyone is on board.’ The Washington Post, 22 August 2022.
Wu, Katherine J. ‘America Lost Its One Perfect Tree.’ The Atlantic, 24 December 2023
The American Chestnut Foundation Articles
‘History of the American Chestnut Tree,’ TACF
American Chestnut Conservation and Restoration, TACF
‘Darling 58,’ TACF
‘Darling 58 Performance’ TACF
‘3BUR – Using Science to Save the American Chestnut Tree’ TACF
Related
Fonte : National Geographic
