How Genetic Engineering Can Save the Iconic American Chestnut Tree

The original habitat of the American chestnut. Source: U.S. Department of Agriculture / Wikimedia Commons.

An estimated 3 billion to 6 billion American chestnut trees once covered forests spreading from southern Mississippi to central Maine. They're well documented in our nation's history and literature, from Henry David Thoreau's Walden to Bob Wells and Mel Torme's classic song Merry Christmas to You. A 1540 expedition led by Spanish explorer Hernando de Soto captured their awesome prevalence by simply saying, "Where there be mountains, there be chestnuts."

So when's the last time you saw an American chestnut tree? Well, chances are you haven't.

When Chinese chestnuts were imported in the early 1900s, a pathogenic fungus, Cryphonectria parasitica, was unknowingly imported with them. By 1940, most mature American chestnuts were reduced to rotting skeletons or erased from the landscape completely. In a little over a century, populations plummeted from as many as 6 billion to a handful of wild stands. Life may be getting on fine without them, but they played an integral role in the American economy, environment, and cultural history.

It's a pretty sad story, right? Yes, it is, but who said it was over?

A team of researchers at the State University of New York's College of Environmental Science and Forestry, led by Dr. William Powell and Dr. Charles Maynard (the men who taught me college genetics), is leading the charge against chestnut blight in an effort to revive the iconic American chestnut tree. They're using genetic engineering to introduce resistance genes into American chestnut seeds, and then crossing them with existing wild trees to inject some genetic diversity. If successful, they'll be the first GM forest trees released into the American wild. Moreover, the methods are similar to those used by Monsanto (NYSE: MON  ) , Syngenta (NYSE: SYT  ) , Dow Chemical (NYSE: DOW  ) , and DuPont (NYSE: DD  ) when producing enhanced agricultural crops. Can their efforts build a bridge to the hearts and minds of consumers opposed to genetic engineering?

Economic, environmental, and cultural dominance
It's amazing how important the American chestnut tree was to the environment and our nation's early economy. As a keystone species, or one that provides a disproportionately large service to the environment in relation to its population, American chestnuts produced a stable annual crop of edible nuts to countless species -- including the now extinct Carolina parakeet and passenger pigeon. (If scientists are ever going to successfully resurrect and reintroduce either species into the wild, they will first need to bring back the American chestnut.) While the tree's niche has been replaced by opportunistic oak varieties, acorns are not produced in stable amounts year after year. In addition, chestnuts were a more important food for humans -- with the nation producing an annual crop of 20 million pounds -- and were even used to brew gluten-free beers.

The nation's chestnut crop is now mostly imported. Source: Peatcher / Wikimedia Commons.

Their economic impact went well beyond food, however. Few sources of timber were more valuable than the wood harvested from American chestnuts, which was naturally rot-resistant and straight-grained. It's no coincidence that chemically treated wood first appeared nearly 70 years ago -- exactly the same time American chestnut populations reached virtually non-existent levels. Chemically treated wood is generally safe -- it must be approved by the U.S. Environmental Protection Agency or parallel state regulatory bodies -- but can add up to $2,500 to the cost of building a new home. That's nearly 1% of the median home price in the United States! We just don't realize the added cost because a suitable natural alternative, such as American chestnut timber, doesn't exist. Not anymore, anyway.  

Chestnut blight destroyed an estimated 3 billion to 5 billion trees throughout the country. While a handful of stands have been found to exist in isolated pockets in the original habitat -- sometimes only supporting one or two trees -- American chestnut roots are still alive under the soil. In fact, up to 60 million root systems may be alive in New York state alone. That doesn't mean much in terms of bringing the population back initially, but it explains why the species is only functionally extinct, not entirely extinct. Root systems could also provide a big boost to recovery efforts if researchers can find a way to inject resistance in the future. 

At 12 feet in diameter and 200 feet tall, the American chestnut was impossible to overlook. Image used with permission from the Forest History Society. Credit: Sidney Streator.

Of course, they'll need to master that whole resistance thing first, but the payoff could be huge. If we could create a national population of American chestnuts that were blight-resistant, we could begin to recapture some of the environmental and economic benefits. While multiple cross-breeding projects have attempted to answer the call, one project relying on genetic engineering holds the most promise.

Genetic engineering: Our best shot to save the American chestnut
Efforts to create a blight-resistant American chestnut tree began with a simple idea: Introduce genes from the naturally resistant Chinese chestnut. There are 21 resistance genes from C. mollussima and 6 resistance genes from C. seguinii -- both Chinese chestnut varieties -- to choose from. Seems simple enough, and we've been cross-breeding plants for at least 10,000 years. So it shouldn't be too difficult to introduce the desired genes into new seeds, right? Unfortunately, cross-breeding is relatively messy and also introduces a wide array of undesirable genes into the hybrid chestnuts.

While both trees are technically chestnuts and share large swaths of genes, problems arise when you consider the physical characteristics, or phenotype, of each. The Chinese chestnut was bred for thousands of years as a low-growing orchard tree. The American chestnut is a massive wild timber tree. Names can be misleading.

The Chinese chestnut looks drastically different from the mighty American chestnut. Source: Richard Webb / Wikimedia Commons.

Good news: Researchers can remove the undesired traits with more breeding. Bad news: It can require a ridiculous amount of time and doesn't exactly scream precision.

Enter genetic engineering. Powell and Maynard can introduce the most potent genes coding for resistance into the American chestnut while completely avoiding the undesired traits altogether. And since they aren't limited by breeding, the team isn't limited to genes found in chestnut varieties. As it turns out, the best genes for saving the American chestnut reside in wheat and code for a compound called oxalate oxidase, which naturally neutralizes the acid produced by the pathogenic fungus responsible for canker growth -- the physical sign of blight.

The drastic physical differences between the American chestnut and Chinese chestnut provide a powerful visual example as to why cross-breeding is not always the best way to introduce new traits into an organism, or, in this case, plants. It parallels the obstacles faced by biotech seed manufacturers in many ways. The phenotypes of corn and soy varieties may not differ as widely as in chestnuts, but there is no shortage of undesirable traits that can be introduced, whether physical or genetic. Why should Monsanto, Bayer, or Dow Agrosciences tack on several years to an already lengthy regulatory process to breed traits into plants and then screen out undesired traits with more breeding when they can introduce the desired traits more quickly and precisely? 

Before you answer that with your interpretation of the laws of nature, consider the following. Powell and Maynard naturally integrate the 2-4 genes using Mother Nature's genetic engineer: Agrobacterium tumefaciens. The bacterium moves genes across species (and kingdoms) in the environment, so the team simply hijacked it to save the American chestnut tree. In fact, many bacteria and viruses have provided this free transportation of genetic material every millisecond for the past several billion years. That is something that seems to be lost in most conversations involving genetic engineering or the products created by DuPont and Syngenta, as is the fact that all genes originated in nature. Why shouldn't we be allowed to put them together more efficiently to create enhanced products -- whether they come from the same kingdom or not? Many seemingly unrelated organisms share genes. For instance, you share 76% of your genes with zebrafish, 51% with fruit flies, and 26% with thale cress (a type of weed). Tell me, again, how swapping genes across kingdoms violates the laws of nature?

Powell explained the difference between cross-breeding and more precise genetic engineering with the following analogy in his TEDx Talk (embedded below). Think of the American chestnut genome as a 180-page book where each of the 45,000 genes is represented by an English word. In cross-breeding, the resulting trees are 1/16 Chinese and 15/16 American. In genetic engineering, only a handful of new genes are introduced. How many English words would be replaced by Chinese words for each method?

Method

Number of Chinese Words

Pages of Chinese Text

% of Total Book

Cross-breeding

2,812

11

6.25%

Genetic engineering

Up to 4

0.016

0.0089%

Source: Dr. William Powell / TEDx Talk.

Could you read a book that had 11 pages of Chinese text in it and still understand what was happening? Probably not. Luckily, scientists can still read the genes introduced from the Chinese chestnut, but it's nearly impossible to breed out the undesired genes to produce a tree with the exact characteristics to those created with precise genetic engineering.

How's the research going? Take a look at how the SUNY-ESF team's first genetically engineered tree, Darling 4 American chestnut, compared to American chestnut and Chinese chestnut in a resistance study. Here, resistance is quantified by canker growth over time. 

Source: Dr. William Powell, SUNY-ESF / TEDxTalk.

The engineered trees closely match the resistance of Chinese chestnuts, but will still display all of the characteristics of the American chestnut, such as size and wood type. If you think that's impressive, consider this: The team has taken what they have learned to create a next-generation tree, Darling 311 American chestnut, which produces more oxalate oxidase than Darling 4 and should be more resistant to blight than Chinese chestnut while retaining the desired phenotype.

Source: Author snapshot of TEDx Talk given by Dr. William Powell.

Who said genetic engineering couldn't be used to solve real-world problems?

Foolish bottom line
Genetic engineering doesn't have to be scary, nor is it unnatural as many opponents claim. I think it all just comes down to a simple misunderstanding. Mother Nature has enlisted viruses and bacteria to transfer genes across species and kingdoms since life first emerged billions of years ago. Why is it so appalling when humans corral the same natural processes to create a more robust organism to fill an environmental niche, such as an agricultural crop that can better resist pests or a mighty American chestnut that resists an invasive parasite?

The team at SUNY-ESF must still conduct field tests to prove their hypothesis for the Darling 311 and replicate their data from the original field tests of Darling 4 -- a process that will take several years -- but Powell and Maynard are optimistic that regulatory agencies (the USDA, the Food and Drug Administration, and the EPA) will one day provide clearance to begin planting transgenic American chestnut trees in the wild. The first chestnuts produced from the university's hybrid plot have already been sent to Oak Ridge National Laboratory to ensure they are exactly identical to wild-type chestnuts. There's a long way to go to restore the original population of 3 billion to 6 billion mature trees -- a journey that will take 100 years, according to Powell -- but it can be done. We can thank genetic engineering for getting us started.

To find out how you can help be a modern-day Johnny Appleseed when seeds are approved, visit the American Chestnut Foundation, and watch Dr. Powell's TEDx Talk below.

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