With a little genetic tinkering, scientists have managed to create trees that contain a more easily processable version of the biopolymer lignin. The advance could make it easier and cheaper to pulp wood into paper.
Trees need lignin to grow. The phenolic polymer provides support and strength to trees’ cell walls. But lignin’s strength-giving properties become a liability when turning wood into high-quality paper. So papermakers have to remove the biopolymer during the pulping process. Lignin’s most labile bonds are β-aryl ethers, and breaking these bonds requires elevated temperatures and caustic conditions.
Scientists have tried to tweak the lignin in trees via genetic engineering to make the pulping process more environmentally friendly. They’ve made trees with less lignin or with lignin that has fewer chemical cross-links. Now, researchers led by John Ralph, a biochemistry professor at the University of Wisconsin, Madison, have taken an entirely different approach by adding a new chemical entity—an ester linkage—into lignin’s polymer backbone (Science 2014, DOI: 10.1126/science.1250161).
“We have all these genes for making the lignin monomer, and if you prod them, you can end up changing the structure of lignin,” Ralph explains. “We took a step back and said, ‘What would we want lignin to be like? We want it to chemically fall apart easily.’ ”
The polymerization process that transforms monolignols into lignin in plants takes place in the cell wall. It’s not governed by any enzyme but instead occurs via a purely chemical mechanism. Ralph and his colleagues reasoned that if they could get trees to produce a monomer that was chemically compatible with this polymerization process and also contained an ester moiety, trees could make lignin that is, as Ralph puts it, “designed for destruction.”
The Chinese medicinal herb Angelica sinensis produces just such monomers—monolignols that are connected to ferulate groups via ester bonds. So Ralph’s team engineered a monolignol gene from the herb into fast-growing poplars and found that the new monolignol ferulate conjugates were incorporated into the trees’ lignin. The trees, Ralph says, don’t appear to be any different from regular poplars.
“This is exciting because it shows that lignin is really malleable,” comments Björn Sundberg, vice president of forest biotechnology with papermaker Stora Enso. “You can modify the lignin without changing the tree or the amount of lignin.” Sundberg adds, however, that further tests and optimization are required to determine if the transgenic trees are commercially viable.
“I particularly like the translational aspect that Ralph, as a chemist, has brought to the plant biology world,” adds Harry Brumer, an expert in biomass enzymology and chemistry at the University of British Columbia. “The work raises the intriguing possibility of engineering other reactive groups into lignin in vivo, thereby opening new avenues for modification.”