From Research to Restoration

The American Chestnut Foundation (TACF) is committed to supporting all approaches toward the ultimate restoration of the American chestnut tree since its inception in 1983.

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American chestnut burs typically have three seeds.

Our work includes three major research tracks – all of which integrate efforts to benefit American chestnut restoration. These tracks include: Breeding, Biotechnology, and Bio-control. The 3BUR (Breeding, Bio-control, and Biotechnology United for Restoration) ad-hoc committee was established in 2016 to propose ways that the three major research tracks might integrate their efforts to benefit American chestnut restoration.

We also recently drafted our 2015-2025 Breeding and Selection Plan. This document provides details on genomic methods to speed up selection in the backcross breeding program as well as detailed plans to incorporate transgenic blight resistance.

There are multiple efforts currently underway within each of these tracks such as traditional breeding methods, modern genetic transformation techniques, simple conservation strategies, and biocontrol methods that would reduce the virulence of the chestnut blight fungus.

Our backcross breeding program is based on methodology proposed by Dr. Charles Burnham. This breeding program uses Chinese chestnut trees, naturally resistant to the blight, and crosses them with American chestnuts. These trees are then backcrossed to the American species. Each generation is inoculated with the blight fungus and only those trees with the highest resistance are used to breed further generations. This process continues over seven generations to produce an American chestnut tree that retains no Chinese characteristics, other than blight resistance.

TACF has implemented backcross breeding at its research farm in Meadowview, Virginia and at orchards planted by sixteen different TACF state chapters. This program originally focused on selection for blight (Cryphonectria parasitica) resistance. More recently, it has been expanded to select for root rot (Phytophthora cinnamoni) resistance and to incorporate genome mapping and marker assisted selection. These additional selection criteria and methods will require transitioning the backcross program into a recurrent mass selection program, currently underway.

The biotechnology program has developed under the auspices of the State University of New York, College of Environmental Science and Forestry (SUNY-ESF) and the New York chapter of TACF. In this program, individual genes are being tested in American chestnut for their ability to enhance pathogen resistance using the tools of genetic engineering and molecular biology. Through this search a gene that produces an oxalate detoxifying enzyme has been found to enhance blight resistance significantly.

The breeding and biotechnology programs associated with TACF and SUNY-ESF should be the basis for successful reintroduction of the American chestnut tree. It’s also important to consider the genetics and evolution of C. parasitica, the fungus that causes chestnut blight. This pathogen population is comprised of genetically different strains that, during the early part of the 20th century, rapidly colonized the entire native range of the tree. It happens that some strains of C. parasitica have been weakened by viruses they carry in their cytoplasm. The viruses, termed hypoviruses, do not kill the virulent strains but significantly reduce their ability to cause lethal infections. When this condition exists the natural defenses of the chestnut may enable the tree to halt canker growth. This phenomenon has been termed hypovirulence and has resulted in some degree of biological control of chestnut blight in several regions of the world.  Other bio-control options may prove to be useful in the larger endeavor as well. To the extent that breeding, biotechnology, and hypovirulence individually offer incomplete control of chestnut blight, TACF is committed to studying ways to combine them for a more synergistic result.

As the biotechnology and backcross breeding are reaching a point of maturation, TACF is integrating these programs to plan for the first stages of reintroduction of the American chestnut to American forests. This type of merger will allow stacking of blight resistance genes, combining Phytophthora-resistance with transgenic blight resistance, and increasing the proportion of American chestnut genes in the resulting progeny.