London (20.4.15) –
In recent years, American chestnuts, glorious trees growing all over the
eastern states of the US. Once one of the most important forest trees in
the area, they have been severely affected by a fungal blight caused by
infection with Cryphonectria parasitica. It is estimated that very
few mature trees remain although small shoots appear to be surviving (1,
Two approaches have been used to deal with the problem. In one, attempts have been made to introduce resistance to the American trees by crop-breeding with relatives from China.
“Although the Chinese genes for resistance are only incompletely dominant, they nonetheless usually express themselves clearly when present in seedlings purposely inoculated with a virulent form of the blight fungus. And that is how each backcross generation is tested - by inoculation with blight. Only those seedlings that show the greatest resistance are used for further backcrossing to an American parent.
But every backcross, although necessary to recover desirable American traits, also reintroduces the genes for blight susceptibility from the American parent. In order to remove those genes, the next steps at TACF are intercrosses. In the first intercross, the most blight-resistant 15/16ths American trees are crossed with other blight-resistant 15/16ths American trees. Again, only resistant seedlings are saved.”
Tree breeding is slow: about six years for each backcross generation and five years for intercross generations.
Meanwhile a genetic modification solution has been sought. A gene found in wheat (4) when transferred to the chestnut allows the trees to defend themselves. The gene codes for oxalate oxidase and enzyme thought to be suitable for helping this blight in chestnut because the causative fungus generate large amounts of oxalate at the margins of the canker; the enzyme catalyses the oxidation of oxalate to carbon dioxide and hydrogen peroxide. Furthermore, it is though that hydrogen peroxide might strengthen the lignin in the barrier produced by the chestnut in an effort to wall off the fungal infection.
In May 2013 the U.S. Department of Agriculture supervised the planting of three experimental patches of the GM-trees, a project organized by the Forest Health Imitative (FHI). If the trial works, the FHI will request permission to plant transgenic chestnut trees in the wild to re-establish the species in America’s woodlands. The team hopes to use the model for future projects to re-establish threatened species such as the elm tree, ash tree, and a fir tree known as the eastern hemlock (5, 6, 7).
It is not yet clear which approach will be most effective and practical. Both have their strengths and weaknesses but it is not yet clear how the resistance compares nor what the potential environmental impacts might be and the genetic identity of the trees that result (8).
1. William Powell (March 2014). The American chestnut's genetic rebirth. Scientific American, 310(3) (http://www.scientificamerican.com/article/the-american-chestnut-genetic-rebirth/)
2. Restoring the American chestnut tree. The American Chestnut Foundation (no date) (http://www.acf.org/pdfs/help/restoration_crop_sm.pdf)
3. The Backcross Method. The American Chestnut Foundation (no date) (http://www.acf.org/r_r.php)
4. The search for blight resistance-enhancing genes. The American Chestnut Research and Restoration Project (2015) (http://www.esf.edu/chestnut/genes.htm)
5. Genetic engineering helps American chestnut trees to rise again. Crop Biotech Update (15.5.13) (http://www.isaaa.org/kc/cropbiotechupdate/article/default.asp?ID=10959)
6. A GM species may soon be liberated deliberately. The Economist (4.5.13) (http://www.economist.com/news/science-and-technology/21577033-gm-species-may-soon-be-liberated-deliberately-wildwood)
7. Andy Coghlan (30.5.14). American chestnut set for genetically modified revival. New Scientist (http://www.newscientist.com/article/dn25644-american-chestnut-set-for-genetically-modified-revival.html#.VTPXtReKITN)
8. Karl Haro von Mogel (8.10.13). What do you want to know about restoring the American Chestnut? Biology Fortified (http://www.biofortified.org/2013/10/what-do-you-want-to-know-about-restoring-the-american-chestnut)