Through Genetics, Tapping a Tree’s Potential as a Source of Energy
It might be true that “only God can make a tree,” as the poet Joyce Kilmer wrote. But genetic engineers can fundamentally redesign them.
Aiming to turn trees into new energy sources, scientists are using a controversial genetic engineering process to change the composition of the wood. A major goal is to reduce the amount of lignin, a chemical compound that interferes with efforts to turn the tree’s cellulose into biofuels like ethanol.
Vincent L. Chiang, co-director of the forest biotechnology group at North Carolina State University, has developed transgenic trees with as little as half the lignin of their natural counterparts. “I think the transgenic tree with low lignin will contribute significantly to energy needs,” he said.
Environmentalists say such work can be risky, because lignin provides trees with structural stiffness and resistance to pests. Even some scientists working on altering wood composition acknowledge that reducing lignin too much could lead to wobbly, vulnerable trees.
“Nature would have selected for lower-lignin trees if they could survive,” said Shawn Mansfield, associate professor of wood science at the University of British Columbia.
People working in the field also acknowledge that they will face resistance from others who see trees as majestic symbols of pristine nature that should not be genetically altered like corn and soybeans.
“The general public is not going to look at trees at this point as a row crop,” said Susan McCord, executive director of the Institute of Forest Biotechnology in Raleigh, N.C. “The same is true of foresters. The people who go into that work, they love trees. They view them very differently than a row of corn.”
Ethanol is mainly made from the starch in corn kernels. To increase the supply to make a dent in the nation’s energy picture, scientists are looking at using cellulose, a component of the cell wall in plants.
Proponents of using trees for this say they are good sources of cellulose and are also good at absorbing carbon dioxide, helping to fight global warming. Also, trees can be cut as needed rather than having to be harvested at a given time each year like a crop.
But the cellulose is covered by lignin, another component of the cell wall, making it difficult for enzymes to reach the cellulose and break it down into simple sugars that can be converted to ethanol. Pulp and paper companies break down lignin using acids and steam. Ethanol producers would have to do the same.
Trees that have less lignin might reduce or eliminate these steps. That could save at least 10 cents a gallon in ethanol costs, said Michael Ladisch, director of the Laboratory of Renewable Resources Engineering at Purdue.
Scientists understand the steps in creating lignin and can make lower-lignin trees by blocking one of them. One way is to put in a reverse copy of a gene that codes for an enzyme in lignin formation. The reverse copy silences that gene and reduces production of that enzyme.
Dr. Chiang said a 50 percent reduction in lignin appeared to be the maximum achievable, adding, “The tree doesn’t allow you to go further.”
The new focus on biofuels has brought a renewed interest in tree biotechnology, and new money for it, from the Energy Department. The field has been languishing because of technical challenges, costs, environmental concerns and financial problems in the forest products industry.
The revival has dismayed critics like Anne Petermann, a leader of the Stop Genetically Engineered Trees Campaign. She said energy concerns were being used “as a really great opportunity to sell this controversial technology to the public.”
Just one company in the United States is known to be pursuing genetic engineering of forest trees vigorously. The company, ArborGen, is small but has some big backers, being jointly owned by three forest products companies: International Paper, MeadWestvaco and Rubicon, based in New Zealand
ArborGen, based in Summerville, S.C., is developing a low-lignin eucalyptus that it hopes to sell in South America, where the fast-growing trees are already used for pulp and paper. For the United States, the company is developing a eucalyptus genetically engineered to survive cold snaps, allowing the trees to be grown more widely.
“In the next 5 to 10 years, you’ll be seeing transgenic trees on the market,” said Maud Hinchee, the chief technology officer at ArborGen.
Two genetically engineered trees are approved by the Agriculture Department, both for crops: papaya trees resistant to the ringspot virus, and plum trees resistant to plum pox virus.
The only known approval of a genetically engineered forest tree has come in China, where insect-resistant poplars have been widely planted.
Genetically modifying forest trees raises questions beyond those of crops. Trees can establish themselves in the wild, while corn would have trouble surviving without a farmer’s tender care.
A biologist, Claire Williams, said the wind could carry pollen from some trees like pines hundreds of miles, making it difficult to prevent a trait like reduced lignin from spreading to wild trees.
Dr. Williams, who works for the State Department but was interviewed while she was working at Duke, said the long life spans of trees made it “almost impossible to evaluate the long-term consequences of transgenic trees.”
Loblolly pine, the main tree the forest industry grows in the Southeast, takes 25 years to go from seed to harvest.
Critics also say transgenic trees would usually be grown on plantations, which, they say, lack the beauty and wildlife of natural forests.
Supporters of transgenic tree research say that because of the long time it takes to grow trees, conventional breeding is difficult.
“The only way to domesticate trees is through genetic engineering,” said Richard Meilan, associate professor of molecular tree physiology at Purdue. He said plantations of fast-growing trees for energy production would reduce the need to cut trees in natural forests. “Let’s domesticate those trees and grow them as commodities and not sacrifice our wild forests,” Dr. Meilan said.
The low-lignin trees, some experts say, have not been tested enough under real field conditions. “To mess with physiology like this, you really need to get out of the laboratory,” said Steven H. Strauss, a professor of forest science at Oregon State University who has conducted field tests of transgenic trees.
The one big field trial of low-lignin trees, conducted over four years in Britain and France, found that they appeared to grow normally and were not more vulnerable to insects, according to a paper published by the investigators in Nature Biotechnology in 2002.
And Jeffrey F. Pedersen, a research geneticist for the Agriculture Department in Lincoln, Neb., found that sorghum with reduced lignin was actually more resistant to a particular fungus than similar varieties with normal levels. He said arresting lignin production could lead to a buildup in the plant of chemical lignin precursors that also have pathogen-fighting properties.
Dr. Chiang of North Carolina State said his trees appeared normal, at least in the greenhouse. He has found that trees that produce less lignin might produce more cellulose, making them even more useful in producing ethanol, pulp or paper without reducing tree strength.
Some field tests are under way outside the United States, Dr. Chiang said, by corporate sponsors of his research who do not want to be identified.
Dr. Hinchee said ArborGen was aiming to reduce lignin 10 percent to 20 percent, to be on the safe side. “It’s not to our advantage to have a tree that’s weak in some other way,” she said.
Rather than reduce lignin, Purdue researchers, working under a $1.4 million three-year grant from the Energy Department, are trying to alter it.
Lignin can be made of two types of alcohols, said Clint Chapple, a biochemist who is working on the project with Professors Meilan and Ladisch. Pulp and paper companies know that one type is easier to remove. By boosting or inactivating various genes, the scientists plan to create trees with different mixes of the two alcohols and test how easy it is to make ethanol.
Dr. Meilan said that after determining an optimal composition, the team hoped to find such trees in the wild that could be reproduced, eliminating the need for genetic engineering.
But it is not certain that can be done. “I believe in the end,” he said, “we will have to rely on genetically engineered trees for our energy plantations.”
