Signs of Summer 7: The American Chestnut: Trans-genetics and Hybrids

Forest History Society

The America chestnut (Castanea dentate), as I have written before, was once one of the most abundant trees in the forests of the eastern United States.  They were not the tallest tree in these forests, but they did have huge, ten or twelve feet diameter, trunks and thick, extending 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 also produced large numbers of extremely nutritious nuts that were eaten by squirrels, birds, deer, and bears and also humans. The American chestnut produced 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.

Photo by D. Sillman

The species, though, persisted even in the face of this awful disease.  The fungus is 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’s 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!

Photo by B. Marlin, Wikimedia Commons

In February 2016 I wrote about two scientists (Bill Powell and Chuck Maynard) at Deborah’s and my alma mater (State University of New York, College of Environmental Science and Forestry (“ESF” for short)) who have developed a transgenic American chestnut tree that is resistant to the chestnut blight fungus. Powel and Maynard 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 transgenic 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 generated as a consequence of the 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!

Photo by D. Sillman

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 transgenic 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.

Researchers at Penn State (see Penn State News, N0vember 10, 2016) have also been working on resurrecting the American chestnut tree using traditional breeding techniques and also gene transferring biotechnologies.

At the Chestnut Orchard at the Arboretum at Penn State a labor-intensive, controlled pollination program is attempting to interbreed blight resistant Chinese chestnut trees with blight susceptible American chestnut trees. The hoped for outcome is an American chestnut hybrid with a natural, blight resistance. The time involved, though, in the painstakingly controlled pollination, chestnut germination and seedling to sapling growth of the hybrid trees (followed by testing for relative susceptibility to the blight fungus) will run to many decades. Also, in a given cross-year, only 1% of the subsequent trees are at all resistant to the blight fungus. Small numbers for such a herculean effort!

Photo by J. Grandmont, Wikimedia Commons

To augment these traditional breeding efforts, molecular biologists are painstakingly going through the genomes of the American chestnut and the Chinese chestnut to try to determine the precise genes involved in the Chinese chestnut’s blight resistance. If these genes can be found, it might be possible to insert the resistance genes into the American chestnut’s genome and directly generate resistance to the fungus. Other teams of molecular biologists are also looking at the blight fungus itself in the hopes that something in its genetic makeup may allow a genetic mechanism of biocontrol that could weaken or maybe even kill the fungal pathogen.

Maintenance of genetic diversity of the American chestnut is vital regardless of how the blight resistant American chestnut is generated. These trees must not be “horticultural clones” or they will not be able to ecologically and evolutionarily thrive in the complex ecosystems of the northeast United States. Deborah and I are participating in the ESF program by germinating and growing a set of genetically diverse, wild American chestnut trees that we were sent back in 2014. These trees, growing in our garden and out in our orchard, along with the thousands of similar trees that are being grown throughout the northeast, will serve as a broad genetic base for either the transgenic or traditionally bred American chestnut that develops blight resistance.

 

 

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One Response to Signs of Summer 7: The American Chestnut: Trans-genetics and Hybrids

  1. Ruth says:

    Fascinating! Thanks, Bill.

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