Last week I wrote about the emerald ash borer and its devastating assault on our eastern forests. In that essay I mentioned the America chestnut and the terrible blight that attacked it over one hundred years ago. American chestnuts were once one of the most abundant trees in the eastern United States. They were not the tallest tree in the forest, but they did have huge trunks that could be ten or twelve feet in diameter and thick, extending, shading branches that spread out over remarkably large areas (Photo of virgin American chestnut trees used with permission from the Forest History Society).
The American chestnut (Castanea dentata) also produced large numbers of extremely palatable nuts that were eaten not only by squirrels, birds, deer, and bears but also humans. The American chestnut made these nuts in abundance every year (unlike oak trees, say, that make their acorns over multi-year, boom and bust cycles). Many animals relied on the yearly production of chestnuts to sustain their populations.
In 1904, though, the American chestnuts lining the roads and walkways of the Bronx Zoo began to sicken. Their leaves withered and great lesions appeared in their bark. The trees then died one by one. They were the first recorded casualties of Chestnut Blight epidemic that swept through the eastern United States. There is evidence that the fungus responsible for this disease (Cryphonectria parasitica) had been present in the southern U.S. since the 1820’s, but the death of the chestnuts in New York set off alarms that reverberated through the country. By 1950, the American chestnut was for all intents and purposes “gone.” It was no longer a reliable source of nuts or timber. It was no longer a tree of size and majesty.
The species, though, persisted even in the face of this awful disease. The fungus can be transported either via insects or on the wind and infects a tree through cracks in its bark. The fungal mycelia then grow into the cambium layer of the tree (the part of the tree that includes the vascular system that transports sugars and nutrients). The tree responds to the infection by sealing off the infected cambium with a dense, callus tissue. But the fungus grows faster than the callus and eventually the tree loses its ability to transport nutrients and dies. The fungus, though, does not affect the tree’s roots, and new chestnut trees are then able to sprout from the still living roots and stumps. Depending upon the site density of the chestnut trees and the abundance of the fungal spores, these new sprouts may grow for ten to fifteen years before the fungal infection kills them. They can reach heights of fifteen to twenty feet and can even produce nuts for several years before they die back. This growth and die-back cycle has caused the American chestnut to become more of a shrub than a tree!
All this, though, might be changing dramatically. Two researchers (Bill Powell and Chuck Maynard) and their teams at one of Deborah’s and my alma maters, the State University of New York College of Environmental Science and Forestry (SUNY-ESF) (a college whose students are affectionately referred to as “Stumpies!”) have unveiled a viable and thriving, transgenetic American chestnut that is resistant to the fungus that causes chestnut blight. They are waiting for final federal review and approval so that they can begin planting these trees back into the forests of the eastern United States.
How they developed this hybrid tree is simple to explain but astoundingly complex in its actual accomplishment. Some forty years ago researchers at another institution noted that strains of pathogenic tree fungi that produce low levels of oxalic acid are far less virulent than the strains that produce high levels of this acid. So the researcher team at SUNY-ESF isolated a gene from wheat plants that codes for an enzyme (oxalate oxidase) that breaks down oxalic acid and inserted it into the genome of the American chestnut tree. It turns out that wheat and many other grass species use this oxalate oxidase as a generalized protection against their own fungal infections, and it further turns out that this enzyme is equally as effective as a fungal control agent in the transgenetic chestnut tree! The chestnut blight fungus makes large amounts of oxalic acid at the margins of those calluses (or “cankers”) that the chestnut trees make to try to seal the fungal infection off from the tree’s healthy tissues. The acid eats away the wall of the protective callus and allows the fungus to then run riot through the tree’s tissues. Breaking down the oxalic acid at this margin not only neutralizes the erosive tool of the fungus but also, via the impact of the hydrogen peroxide that is being generated as a consequence of the catalyzed oxalic acid oxidation, strengthens the lignins in the wood of the callus! The callus, then, very effectively seals off the blight fungus and the tree remains healthy!
The Chinese chestnut tree and other Asian varieties are resistant to the chestnut blight fungus through other genetic mechanisms, but it turns out that the transgenetic American chestnut with its oxalic oxidase enzyme and lignin enhanced calluses is even more resistant to the fungus than those Asian chestnut species to this disease!
The final phase of this SUNY-ESF project is to raise and plant ten thousand of these blight resistant, transgenetic American chestnut trees. This past fall, to raise funds for this effort, the Development Office of SUNY-ESF set up a crowd funding campaign called the “Ten Thousand Chestnut Challenge.” They sent emails to SUNY-ESF alumnae and to past donors to the college hoping to raise the fifty thousand dollars needed to cover the costs of this tree production program. In the four brief weeks of the campaign, they exceeded their goals and raised over one hundred and four thousand dollars!
A benefit of becoming a contributor to this chestnut tree crowd funding challenge was that you received a small bag of American chestnut tree seeds as a token of thanks. These seeds are not for the transgenetic, blight resistant American chestnut, but instead are from a genetically diverse array of wild American chestnut trees. The trees grown from these seeds will serve as “Mother” trees that will be crossed with the transgenetic chestnuts to greatly expand their genetic base! My zip-lock bag of chestnut seeds are in the salad crisper of my refrigerator getting their three months of “winter weather” so that they will germinate when planted in the spring. I will share pictures this summer!!
If you want to read more about this fantastic research and rehabilitation effort, check out www.esf.edu/chestnut . Happy winter, everyone, and happy chestnut forests!