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Poplar Trees Engineered to Produce More Biofuels, Wood Pulp

Poplars

Poplars (Luciano/Flickr)

3 April 2014. Researchers at the Great Lakes Bioenergy Research Center, a consortium of University of Wisconsin in Madison and Michigan State University in East Lansing, created a genetically modified poplar tree variety with weakened lignin bonds, making it easier to process into commercial biofuels and wood pulp. The team from the labs of Wisconsin’s John Ralph and Michigan State’s Curtis Wilkerson, with University of British Columbia’s Shawn Mansfield, published its findings today in the journal Science (paid subscription required).

Lignin is a polymer material in trees and other plants that gives strength to the cell walls, but that same quality in lignin makes it difficult to break down, thus adding considerable effort and cost when processing poplar wood for commercial products, such as biofuels and pulp for making paper. Poplars are fast-growing trees used in packing boxes, industrial pallets, and musical instruments. Poplars also grow on marginal land, thus do not compete with food crops.

While popular wood can today be made into pulp for paper, the quantities of lignin in poplar require pretreatment with chemicals. Ralph’s lab at Wisconsin-Madison began investigating genetic modification of poplars in the mid-1990s, to make it a better feedstock for pulp and paper. Previous attempts to genetically engineer poplars reduced the amount of lignin, which resulted in smaller trees that could not stand up against wind and pests.

With Michigan State’s Wilkerson, the researchers sought a process for creating a variety of poplar with enough lignin to maintain the trees’ strength, but also with weaker esters, the organic bonds making lignin’s chemistry difficult to break down. The team sought out plants with lignin that digested more readily, and found a Chinese medicinal herb, dong quai (Angelica sinensis), where the genes in its root tissue expressed an enzyme that does not add ester bonds into the lignin.

Wilkerson’s lab isolated the key gene from dong quai needed to create a more processable form of lignin, and Mansfield’s lab created the modified form of poplar with the gene from dong quai added. The researchers then grew the modified poplars in a greenhouse. Their tests of the greenhouse plants showed the modified poplars exhibited no abnormalities compared to poplars grown in the wild, including similar total levels of lignin.

The team also submitted the lignin in the modified poplars to gas chromatography–mass spectrometry, a combination of analytical techniques that makes it possible to reveal and quantify the substance’s underlying components. Their analysis showed the lignin in the modified poplars contained the non-native enzymes, with the ability to produce the enzymes apparently transferred from dong quai. Further tests showed the lignin in the modified poplars to be more digestible with mild treatments, releasing more energy-generating glucose sugars than wild-type poplars.

“We can now move beyond tinkering with the known genes in the lignin pathway to using exotic genes to alter the lignin polymer in predesigned but plant-compatible ways,” says Ralph in a Great Lakes center statement. “This approach should pave the way to generating more valuable biomass that can be processed in a more energy efficient manner for biofuels and paper products.”

The research center says the technology is available for licensing through the Wisconsin Alumni Research Foundation.

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