By genetically modifying aspen trees, Dr. Vincent L. Chiang, professor of forest biotechnology, and his colleagues have reduced the trees' lignin content by 45 to 50 percent – and accomplished the first successful dual-gene alteration in forestry science. Their results are described in the current issue of the Proceedings of the National Academy of Sciences (PNAS). According to Chiang, the NC State research shows not only a decrease in lignin but also an increase in cellulose in the transgenic aspens. And their work demonstrates another benefit: the trees grow faster.
That is very good news for the wood, paper and pulp industries, which do multibillion-dollar business worldwide. Fast-growing, low-lignin trees offer both economic and environmental advantages, because separating lignin from cellulose – using harsh alkaline chemicals and high heat – is costly and environmentally unfriendly. Harvesting such trees, using them as "crops" with desirable traits, would also reduce pressure on existing forests.
Chiang and his team chose aspens because, he says, "they're the lab rats of forestry research." The scientists scratch the leaves and expose the wound to bacteria carrying the beneficial genes. Treated leaf-disks, with their enhanced genomic structure, are then cloned, producing trees with predictable qualities.
As with any research involving genetic engineering, Chiang's modified aspens have faced questions of real-world properties, resistance to insects and diseases, and the possibility of unforeseen ecological impacts. "There is a need for more data concerning the environmental effects and field performance of transgenic trees," said Chiang, "but four-year field trials of such trees in France and the United Kingdom show that lignin-modified transgenic trees do not have detrimental or unusual ecological impacts in the areas tested."
In previous work, Chiang and his team had successfully reduced lignin in aspens by inhibiting the influence of a gene called 4CL. The current research modifies the expression of both 4CL and a second gene, CAld5H, in the trees. This dual-gene engineering alters the lignin structure, and produces the favorable characteristics of lower and more degradable lignin, higher cellulose and accelerated maturation of the aspens' xylem cells.
The research is described in the paper "Combinatorial modification of multiple lignin traits in trees through multigene co-transformation," published online by PNAS on March 31.
Chiang is co-director of the Department of Forestry's Forest Biotechnology Group in the College of Natural Resources at NC State. Headed by Chiang and Dr. Ron Sederoff, Edwin F. Conger and Distinguished University Professor of Forestry and a member of the National Academy of Sciences, the group is one of the world's leading research organizations studying the molecular genetics of forest trees. The Forest Biotechnology Group is a key part of NC State's research strength in genomics, an important new area of scientific research focused on identifying and mapping all the genes of living organisms. Its work is leading to a better understanding of the genetic basis of biological diversity, improved disease resistance in important tree species, and increased commercial forest productivity.
According to Dr. Bailian Li, associate professor of forestry at NC State, Dr. Chiang's results in this aspen model species are "very significant" and will have dramatic impacts on the future genetic improvement of forest trees for pulp and paper production. "The improved tree growth and high cellulose content will increase pulp-yield production, while the reduced lignin content will reduce the pulping cost and energy consumption in the pulping process," he said. "The ability to produce high-yield plantations with these desirable characteristics will enable us to produce wood more efficiently on less land, allowing natural forests to be managed less intensively – for habitat conservation, aesthetics and recreational uses."
Citing the Forestry Department's Industry-Cooperative Tree Improvement Program – working to improve plantation productivity, adaptation and disease-resistance in North Carolina's loblolly pines – Li said, "Results from Dr. Chiang's research are very encouraging to our research. Although his research is on aspen, the valuable information on genetic regulation of wood formation should be useful for our efforts in producing pine plantations with lower lignin, higher cellulose, and faster growth rates."
Note to editors: An abstract of the Proceedings of the National Academy of Sciences paper follows.
"Combinatorial modification of multiple lignin traits in trees through multigene co-transformation" Authors: Laigeng Li (NC State); Yihua Zhou (Chinese Academy of Sciences, Beijing); Xiaofei Cheng (the Noble Foundation); Jiayan Sun (NC State); Jane M. Marita, John Ralph (University of Wisconsin); and Vincent L. Chiang (NC State). Date: Published in the March 31 early online edition of Proceedings of the National Academy of Sciences
Abstract: Lignin quantity and reactivity (which is associated with its syringyl:guaiacyl (S/G) constituent ratio) are two major barriers to woodpulp production. To verify our contention that these traits are regulated by distinct monolignol biosynthesis genes, encoding 4-coumarate:coenzyme A ligase (4CL) and coniferaldehyde 5-hydroxylase (CAld5H), we used Agrobacterium to co-transfer antisense 4CL and sense CAld5H genes into aspen (Populus tremuloides). Trees expressing each one and both of the transgenes were produced with high efficiency. Lignin reduction by as much as 40% with 14% cellulose augmentation was achieved in antisense 4CL plants; S/G increases as much as 3-fold were observed without lignin quantity change in sense CAld5H plants. Consistent with our contention, these effects were independent but additive, with plants expressing both transgenes having up to 52% less lignin, 64% higher S/G ratio and 30% more cellulose. S/G increase also accelerated cell maturation in stem secondary xylem, pointing to a role for syringyl lignin moieties in coordinating xylem secondary wall biosynthesis. The results suggest that this multigene co-transfer system should be broadly useful for plant genetic engineering and functional genomics.
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Proceedings of the National Academy of Sciences