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Designing Trees

The same, often controversial gene-tweaking techniques that brought us pest-resistant fruits and vegetables now have a new target: trees
aging



J. W. Stewart
chestnut
Image: Courtesy of S. MERKLE

UNTIL THE BLIGHT, majestic American Chestnut trees dominated many U.S. forests. This image was taken in 1910 in the Great Smokey Mountains of North Carolina.

To gain an appreciation for the promise--and perils--of genetically modified trees, consider the case of the American chestnut.

Few trees have had such a remarkable presence and impact on the American landscape and ecosystem as the chestnut. Mature trees grew to 100 feet tall and their trunks had diameters that spanned five feet. Each fall, their fruit--polished, nutritional, protein-packed chestnuts--rained down on forest denizens (including human gatherers) as manna from heaven. Then, in 1904, an accidentally imported fungus began to attack the trees. In less than 50 years, the trees that had once defined forests from Maine to Georgia were virtually wiped out. Today, stumps of once-mighty chestnut trees may sprout small clones. But even if these saplings avoid the relentless fungus, they can reach only half the height of the previous chestnuts. The young clones rarely attain reproductive maturity.

Breeding American chestnuts with their fungus-resistant Asian cousins could bestow protection--but only slowly, over many generations of the long-lived trees, and through several lifetimes of their human breeders.

Or, with genetic-modification techniques, the protective genes could be inserted into the trees in just a few years.

No solution, of course, is ever that easy: the downside is that transgenic trees, just as with any other genetically modified (GM) plant, could breed and swap lab-inserted genes with wild trees--with unpredictable, and perhaps ecosystem-threatening, results.

According to researchers and industrial groups, trees are the next big crop plant, and GM trees could provide a variety of advantages over natural forests and traditionally bred timber. GM trees could be designed so that they ease paper processing, for instance. They could grow faster, requiring less land to generate the same amount of lumber. They could be engineered to be pest-resistant--saving on chemical treatments--or allergen-free (see "Allergen-Free Trees"). Last, they might even be used to assist in bioremediation of heavy metals.

Researchers developed the first transgenic trees by the end of the 1980s. By 1998 a team at Michigan Tech University, led by Vincent Chiang, had designed a better tree for paper. Chiang and his colleagues determined how to insert the correct genes in aspens to halve the lignin content--which has to be isolated during papermaking by chemical treatments--and increase paper-strengthening cellulose by up to 15 percent. The trees would require fewer treatment chemicals, making them "greener" and less expensive to process than conventional trees. Michigan Tech patented the gene-insertion process; the trees are now in field trials.

Trees as Crops


For a list of trials of genetically modified trees currently being conducted in the U.S., click here.

Timber is big business, and another area that GM trees could help improve. The U.S. Department of Agriculture reported 2.9 billion cubic feet of timber cut in 1999 on national forest lands, valued at over U.S.$340 million. Banking on the potential profitability of GM trees, in 2000 several major timber and paper companies, including giants International Paper and the Westvaco Corporation, joined the New Zealand biotechnology company Genesis in a $60-million venture called ArborGen; it was the largest international commercial venture dedicated to transgenic trees for lumber at that time. A selling point for transgenic trees is that they would reduce land needs for tree plantations in the U.S. Southeast and reduce pressures on remaining forests in the West. "The environmental benefit in a shift to planted from wild is that you could get all the wood the world needs pretty much from 5, 10 or 20 percent of the land used now," says Steve Strauss, professor of forest genetics and biotechnology at Oregon State University.

"Greener" Trees?

Pest-resistant GM trees would also reduce the need for biocides, another environmental--and cost-saving--advantage for the timber industry. Genes for expressing the Bt toxin--originally derived from Bacillus thuringiensis, an insecticidal bacterium used worldwide for pest control--could be inserted in trees; Bt genes have already been engineered into corn, cotton, potatoes and soybeans.

GM trees could also be "greener" because of their potential to function as bioremediation tools. In addition to his work on the American chestnut, Scott Merkle, a forestry professor and biogeneticist at the University of Georgia at Athens, has inserted genes from the Escherichia coli bacterium cloned and modified by his colleague Richard Meagher into eastern cottonwood trees. Eventually, he says, the added genes will allow a tree to biosynthesize certain forms of mercury, transpiring it into the atmosphere in a less-toxic form, just as microbes might. "You could accumulate heavy mercury [in a tree] and haul it away," Merkle says, effectively removing both mercury and the transgenic organism before either could spread into the local ecosystem.

The issue Merkle raises--the potential spread of transgenic trees--is a major concern raised by critics of the technology. If the GM trees flower, they would spread their genes to related trees nearby, because tree pollen can travel for miles. "Trees are apt to hybridize with their wild relatives," explains Peter Raven, director of the Missouri Botanical Garden and current president of the American Association for the Advancement of Science. "As a practical thing, with plantations around natural forests, you can expect progeny that would have whatever features there are in the ones that are planted, and you can expect that regardless of whether they came about as a result of normal selection or of genetic modification."

Frankentrees?

propagules
Image: Courtesy of S. MERKLE

LAB-GROWN CULTURES of embryogenic American chestnut trees such as these can grow into thousands of propagules--asexual parts of the plant that can be used to grow new specimens.

For these reasons, environmental activist groups such as the World Wildlife Foundation and the Sierra Club have encouraged a moratorium on the planting of any genetically modified organisms, including trees. Because trees live long and have complex ecosystem interactions, says Jim Diamond, a senior fellow with the Sierra Club, "the precautionary principle should keep us from deploying this technology until we understand those risks much more completely than we do now."

Strauss acknowledges some of these concerns. He agrees that "absolutely complete containment is impossible." Farmed crops, including trees, are "very different products than what nature had in mind," he adds. But he says that, if engineered properly, GM trees would not be a threat--if anything, the lab-coddled trees would be less vigorous than natural ones. A tree with less lignin, for instance, is unlikely to survive insect assaults without pesticides and would not survive in the wild. That means that the so-called superweed syndrome--a concern raised about other GM crops--may be unlikely with trees. "[Transferred] genes in the wild will have very, very little effect," Strauss says, because natural selection will take over and transferred genes will blend into the background or disappear.

Raven of the Missouri Botanical Garden says we have to carefully monitor GM plants that are introduced into any ecosystem. We may want engineered American chestnuts to mingle with the natives, for instance. But other tree species containing genes, for example, for expressing the Bt toxin may have unplanned deleterious effects on a natural forest¿s butterfly and moth populations. Researchers may think they can predict the outcomes of the addition of a specific gene to a plant, Raven says, "but nature has a way of doing something completely different."

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