A New Hope

The greatest success story in the history of forest pathology is just around the corner — if it doesn’t get buried in red tape.

If you’re familiar with the story of chestnut blight, and more importantly what American chestnut was and the bounties it provided before the disease all but eliminated it from American forests, you know the greatest tragedy in American forest history. Hypovirulence seemed like a cool biocontrol for the disease, but it hasn’t really worked in North America. The American Chestnut Foundation has been uncommonly perseverant in trying to breed a resistant tree that is mostly American chestnut, but it is a long, slow process and there have been setbacks.

Scientists at ESF, led by Bill Powell, have made stunning progress with a novel, biotechnology approach ​[1–4]​. The result is a blight-tolerant chestnut tree that is completely American chestnut, but suffers little to no significant damage from the disease (see figure). It’s about as close to a silver bullet as you’re ever going to find in plant pathology, short of chemical control. How the heck did they do that, you might ask.

Science is awesome

Thirty days after greenhouse inoculations to test resistance of transgenic American chestnut to chestnut blight. A. Wild-type American chestnuts die within 19-30 days. B. Transgenic Darling 54, American chestnut produced from the same line as ‘A’, but with the gene for oxalate oxidase added, is quite resistant. None wilted within 30 days and all survived with small healing cankers at the inoculation sites. Plants were still surviving through 10 months. C. Chinese chestnut, which has a fair degree of natural resistance. Half wilted between 24 and 30 days and only one eventually survived. Photo courtesy of Bill Powell, ESF.

They did it by capitalizing on the years of basic research on the weapons that pathogens use to attack plants and the defense mechanisms that plants have evolved to counter them (along with a lot of work to develop and apply that knowledge in this system).

Cryphonectria parasitica, which causes chestnut blight, is one of the pathogens that produces oxalic acid. Oxalic acid is toxic to and kills plant cells in advance of the mycelium to facilitate invasion of the host. Well, plants don’t just take this lying down. Those that have evolved with pathogens that use this weapon often develop the ability to produce oxalate oxidase, an enzyme that breaks down and detoxifies oxalic acid.

Unfortunately, American chestnut did not evolve with this pathogen, and has no oxalate oxidase gene. Wheat, banana, strawberry, peanuts, and other plants do have it. Powell and colleagues worked out a tissue culture system for chestnut and a system for transferring oxalate oxidase to American chestnut. It’s easy to select cultures that express the gene, then plantlets are developed from the cultures.

The next step will be to cross these resistant lines with the many locally adapted populations across the range of chestnut and outplant those that inherit the resistance gene, a task the American Chestnut Foundation is wonderfully prepared for. Wow, it’s gonna be great having chestnut dominating forests again!

Regulations? Not so awesome

To put it mildly, there are some regulatory hurdles to clear before the plan can be realized. Three agencies in three government departments are involved:

  • Department of Agriculture, Animal-Plant Health Inspection Service regulates transgenic plants and animals.
  • Department of Interior, Environmental Protection Agency regulates pesticides. Apparently that is defined quite broadly to include many non-pesticide methods of controlling diseases, insects, and pests.
  • Department of Health and Human Services, Food and Drug Administration regulates human and animal foods. Since people and animals eat chestnuts, they have to approve.

ESF submitted an enormous ‘Petition for Nonregulated Status’ to APHIS (read the Executive Summary or the full Petition), and documents must be submitted to other agencies as well. The whole process may take 3-5 years, and the outcome is not certain. Some people are against food made from GMOs (genetically modified organisms), although traditional breeding modifies genetics far more than adding a single gene.

In order to provide evidence for regulatory issues, the scientists have conducted many studies into the effect of the modified tree on various aspects of the environment, with entirely predictable results. I feel sorry for people having to do studies like that. Will the transgenic tree disrupt forest ecosystems? Yes, hopefully, it will undo the terrible disruption caused by the introduction of chestnut blight.

Check out the video, which briefly summarizes the whole story. Do you think this work should go forward? Why or why not?

  1. 1.
    Newhouse AE, Polin-McGuigan LD, Baier KA, Valletta KER, Rottmann WH, Tschaplinski TJ, Maynard CA, Powell WA. 2014. Transgenic American chestnuts show enhanced blight resistance and transmit the trait to T1 progeny. Plant Science 228:88–97 <https://www.esd.ornl.gov/mb/pdfs/Newhouse%20et%20al%202014%20Plant%20Science%20Final.pdf>.
  2. 2.
    Newhouse AE, Powell WA. 2021. Intentional introgression of a blight tolerance transgene to rescue the remnant population of American chestnut. Conservation Science and Practice 3(4) <10.1111/csp2.348>.
  3. 3.
    Powell WA, Newhouse AE, Coffey V. 2019. Developing blight-tolerant american chestnut trees. Cold Spring Harbor Perspectives in Biology 11(7):a034587 <10.1101/cshperspect.a034587>.
  4. 4.
    Steiner KC, Westbrook JW, Hebard FV, Georgi LL, Powell WA, Fitzsimmons SF. 2017. Rescue of American chestnut with extraspecific genes following its destruction by a naturalized pathogen. New Forests 48(2):317–336 <10.1007/s11056-016-9561-5>.

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