Anthony King reports on progress towards controlling a new influx of recent tree pathogens, and asks: can biotechnology rescue forests from alien invaders?
The emerald ash borer (Agrilus planipennis) is a metallic green beetle from Asia that is wiping out trees across the eastern US. First detected in Michigan in 2002, the pest is spreading rapidly and has killed billions of ash trees, with seven out of nine ash trees in North America threatened by this newcomer.
This is only the latest in a litany of exotics to ravage American forests. Sixty-two high-impact insect species and a dozen pathogens have arrived since the 1600s. Only two were detected before 1860. Increased global trade and travel, along with climate change and warmer winters, are all fuelling the problem. And the devastation has pushed scientists and foresters to look towards biotechnology for a remedy.
‘Almost every day there appears to be a new forest pest and some of these are quite devastating,’ says tree geneticist Jeanne Romero-Severson at the University of Notre Dame, Indiana, US. ‘Biotech approaches such as transgenic technology and CrispR gene editing could be valuable tools in saving specific species.’
This is closest to reality for the most high-profile US tree victim, the American chestnut (Castanea dentata). One of the most common trees in the hardwood forests along the Appalachian Mountains, it was famed for providing a bountiful crop of chestnuts every year. Then, in the early 1900s, chestnut blight – caused by Cryphonectria parasitica – arrived from Asia and steadily decimated the trees, wiping out an estimated 4bn chestnuts in 50 years.
The blight forms a canker and eventually chokes the chestnut’s circulation, but the roots live on. ‘Fortunately, the American chestnut can survive at the roots and re-sprout them there. The fungus cannot compete with soil microbes and so the root is protected,’ says Bill Powell, plant scientist at the State University of New York (SUNY), US. ‘Unfortunately, the blight will eventually kill the sprout.’ While millions of living chestnut roots survive across large areas of the US, nevertheless the fungus lives throughout the forests, so continues to kill all chestnut sprouts as they grow.
The American Chestnut Foundation has worked for decades to try to save the tree. Its main strategy to hybridise the American chestnut with blight-resistant Chinese chestnut was unsuccessful. Recently, a wheat gene was genetically engineered into an American chestnut and seems to confer tolerance of the fungal foe. The gene produces oxalate oxidase, an enzyme present in plants like strawberry and broccoli, which breaks down oxalic acid.
The blight-causing fungus oozes oxalic acid at the edge of the canker. The acid lowers the pH ahead of the fungus’ growing tips and weakens plant cell walls, paving the way for fungal enzymes to eat through the tissue. ‘By adding this enzyme, we take away a weapon from the fungus. The fungus still colonises wounds and will feed on dead tissue, but it can no longer kill fresh tissue,’ says Powell.
No wild transgenic trees have been planted in American forests, but this could soon change. The American chestnut is in field trials, as are hybrid poplars in plantations in the northwest US, transformed to contain the Bt insecticide widely used in GM cotton. Scientists at SUNY have applied to regulators to have the GM chestnuts released into the forests. Three federal agencies will consider this move. It is likely that the US Department of Agriculture will see no threat to farmers, and that the Food and Drug Administration (FDA) will rule the chestnuts are safe to eat. The position that the Environmental Protection Agency (EPA) will take is far from certain, however.
Early in 2019, the National Academy of Sciences released a report: Forest Health and Biotechnology: Possibilities and Considerations. The two motivations behind the study, according to study author Jason Delborne, environmental policy expert at NC State University, US, were a recognition of the growing threat to forests and the rising interest in addressing the problem by biotech. Genetically modifying wild forests and even the iconic chestnut raises a number of thorny issues, not least of them concerning public attitudes.
Any decision to release GM chestnut in the US falls under the Coordinated Framework for Regulation of Biotechnology, passed in 1986 and updated in 1992. Regulators have used the framework to deal with GM row crops, but never a long-lived wild organism. ‘My personal opinion is that it would be highly unlikely that this transgenic [chestnut] would be a problem given that the enzyme occurs in nature in crops that we eat,’ says Romero-Severson. ‘But the regulatory process has been designed for row crops, not for this circumstance.’
Moreover, releasing GM trees is bound to be controversial. Environmentalists see the chestnut as a slippery slope towards wild forests filled with GM trees. The ash species being chewed by a newly arrived nemesis from Asia could be genetically altered to contain a protective chemical – an insecticide. ‘The chestnut is the flagship,’ says Powell. ‘If it works out well then we can start working with other trees being threatened. Everyone where I live is losing their ash trees right now, because of the emerald ash borer.’ Walnut is under threat from thousand cankers disease, white pine is being further restricted by a blister rust, and hemlock is under pressure from sap-sucking woolly adelgids.
Powell’s view is that a re-introduced American chestnut tree will restore forests, while introducing just two foreign genes in a tree genome that contains 38,000 genes. This is far closer to the natural tree than a tree crossed with a Chinese chestnut, which would have half its genes from Asia. ‘Wild is the closest to what was originally there,’ says Powell. ‘Would you rather have a tree with Bt [insecticide] in it and save ash trees, or no ash trees? For me, I would like to keep the diversity of the forest as high as possible.’
No silver bullet
Still, the National Academy of Sciences report makes it clear that a biotech solution to forestry health is far from an easy option. A first step is to identify genes for modification, introduction or silencing. If a gene is not at hand, a gene discovery effort will be necessary. This is daunting because of trees’ size and long generation time, as well as – for conifers – their colossally big genomes. ‘Trees are a pretty challenging experimental organism,’ says Steve DiFazio, a plant geneticist at West Virginia University, US, and an author of the NAS study who spoke about biotech and forest health at the AAAS annual meeting in Washington DC, in February 2019. ‘There is also a general lack of information about the molecular biology of trees. People are working on it, but progress is slow,’ says DiFazio.
Agricultural crops have not prepared scientists for the difficulties that wild trees might pose. Modified row crops require a gene to persist in a single individual for a growing season, but expressing a gene in a tree that might live 200 years is uncharted territory. ‘The stability of a transgene in an organism that lives for several hundred years is untested,’ warns Romero-Severson. Row crops like corn and cotton are ‘like pets’, she adds. ‘They only succeed because we tend to them. In tree restoration, the goal is to be able to put trees out in a forest and have them strong and adapted enough to take care of themselves. That is a very different end goal.’
There is also no ready-made solution for lots of the threats facing forest trees. The American chestnut is somewhat of an exceptional case. The NAS study used a number of case studies to illustrate that transgenic technology ‘is not a silver bullet,’ says Romero-Severson. ‘It is not quicker or cheaper than traditional approaches. For example, the transgenic chestnut was only possible because there was 13 years of R&D into it.’ For many tree species, there is simply not a reliable way to stably move foreign genes into their genome. She says there is presently no viable biotech option to stop the emerald ash borer, for example. Also, cell culture techniques for many tree species are not well developed.
Romero-Severson complains that a big barrier to saving American trees is a lack of long-term funding for forestry research. ‘The US Forest Service has a greatly reduced workforce in terms of forest geneticists and there are far fewer operational breeding programmes now,’ she explains. A breeding programme for trees takes time to even start up – five to ten years. There are no flash results early on for administrators. In contrast, it is much easier to fund a single-lab to develop a transgenic technique. ‘We have perverse incentives for funding short-term programmes,’ says Romero-Severson, whereas only the Federal government can undertake the sort of long-term commitment needed for maintaining forest health.
No wild transgenic trees have been planted in American forests, but this could soon change. The American chestnut is in field trials, as are hybrid poplars in plantations in the northwest US, transformed to contain the Bt insecticide used in cotton. Scientists at SUNY have applied to regulators to have the GM chestnuts released into the forests.
A GM American chestnut tree could get the greenlight from regulators as soon as 2020. It promises to restore forests, while introducing just two foreign genes in a tree genome that contains 38,000 genes. This is far closer to the natural tree than a tree crossed with a Chinese chestnut, which would have half its genes from Asia.
Agricultural crops have not prepared scientists for the difficulties that wild trees might pose. Modified row crops require a gene to persist in a single individual for a growing season, but expressing a gene in a tree that might live 200 years is uncharted territory.
This view is shared by others. ‘These biotech solutions look sexier to funders, and policymakers, and that is where the resources go. But in many ways, it is a dead end if you don’t have a foundational breeding programme to feed into,’ warns DiFazio. Knowledge of tree genetics also needs to improve. A technology like Crispr for gene editing is fast and powerful, but mostly it is used in lab organisms where much is known about their genetics. Without deep knowledge of a tree’s genome, Crispr will be far less useful.
If the American chestnut gets a regulatory greenlight, which could happen in 2020, the tree could be planted very quickly by private landowners, and then more widely by the American Chestnut Foundation, which has thousands of volunteers. But there are legitimate questions about the wisdom of this action. ‘What are we restoring and are there potentially negative ecological effects,’ asks DiFazio. ‘It raises also philosophical issues about what wildness is.’ The National Academy of Sciences discusses the ethical and philosophical issues overshadowing biotech in wild forests and the importance of public acceptance. ‘This raises new and different issues than for an agricultural crop,’ says Delborne. ‘It is about how we value forests and understand forests as a natural or wild environment. The [study] committee pointed out paradoxically that a biotech tree could be seen to reduce or enhance the wildness of a forest.’
After spending decades trying to revive the chestnut, Powell is a strong believer in leveraging biotech to boost forest health. But he acknowledges that it won’t just be up to scientists. ‘The biggest thing is to the get the public onboard; a lot of people are afraid of genetic engineering. Hopefully, over time they will see the benefits and change their mind,’ says Powell. Surveys suggest that knowledge about genetic engineering technology, as well as about threats to forest health, is fairly low amongst the general public. Given these deficits, ‘public opinion might be vulnerable to changes,’ notes Delborne.
Ultimately, the fate of the GM American chestnut could be decided by the EPA. ‘We are still trying to find out if [the EPA] regulates us,’ says Powell. The modified chestnut could conceivably be judged to fall under pesticide legislation, but Powell says the tree is not killing the fungus. If the tree were considered a pesticide, it would need to be re-registered every year, which is a peculiar requirement for an organism that could live for hundreds of years. ‘It is unclear what the EPA will do if it rules the chestnut falls under the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA),’ says Delborne. There are still regulatory unknowns.
Meanwhile, with the problem of invasive species only set to get worse, he warns: ‘We as a society are going to have to make some difficult decisions in the next 10 to 15 years in the ways we might use biotechnology in unmanaged forests.’
Europe says no