Their main ingredient, cellulose fibers, literally grows on trees. But separating the cellulose from other substances in the plant, such as the stiff, woody material called lignin, comes with a heavy environmental toll. Every year, paper mills generate millions of tons of chemical waste and more than 150 million tons of greenhouse gas emissions. Today in Science, researchers report they have found a way to reduce that burden. By using CRISPR gene-editing tools, they grew engineered poplar trees with far less lignin than usual. Milling these trees, they argue, could lower papermaking pollution while saving the industry billions of dollars.
This is impactful work,” says Vânia Zuin Zeidler, a chemist at Leuphana University of Lüneburg who wrote a companion piece in Science describing the result’s significance. “Rather than remediate an existing problem, they are trying to prevent pollution.
Papermakers start by cutting wood into small chips. Poplars, a fast-growing tree common in plantations, are a popular source. Water and chemicals are added to break apart the tough lignin structures and separate out a wet pulp of cellulose fibers, which are then pressed and dried into paper. In the past, researchers have tried to breed and engineer trees with lower lignin content, but the gains have been modest.
Part of the reason is that lignin is woven together from three precursor compounds in a complex process that’s governed by 11 gene families and hundreds of genetic and metabolic regulatory elements. Past research has focused on tweaks to individual genes and gene families to make lignin easier to break down rather than wholesale changes to the system that regulates overall lignin production.
But the ease of CRISPR gene editing led a team of researchers to try. Jack Wang and Rodolphe Barrangou, biotechnologists at North Carolina State University (NC State), and dozens of colleagues built a computer model, based on decades of forest biotechnology studies, to predict how simultaneously changing poplar genes related to lignin production might impact the trees’ wood composition, growth rate, and other factors.
After evaluating nearly 70,000 different gene-editing combinations, they determined that 99.5% of these changes would be detrimental, leading to effects such as drooping limbs and stems. But 347 combinations, each comprising a handful of individual gene changes, appeared to safely boost cellulose, reduce lignin, or both, thus improving the papermaking potential of the trees.
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The NC State team used CRIPSR to make the gene changes associated with 174 of the most promising combinations, then grew these engineered trees in a greenhouse. After 6 months, the most promising varieties had their lignin content reduced by 49.1% and their cellulose-to-lignin content increased by 228%. If a typical paper mill used these varieties, the team reports, it could increase its paper output by 40%, cut its greenhouse emissions by 20%, and boost its lifetime profits by about $1 billion. “That would create an impetus for industry to adopt the new technology,” Barrangou says.
Still, adopting the approach may not happen quickly. First off, Barrangou notes, the researchers need to conduct field trials to ensure the novel trees can grow to maturity and withstand real-world insults. Lignin not only helps trees stand up to windstorms, but also protects them from insects—less of a problem for greenhouse trees. And some of the edited lines, Zeidler notes, did not grow to have the same volume as regular poplars.
The modified trees also need to pass muster with regulators. However, Barrangou notes that the engineered poplar trees do not include any transgenes, or genes from other organisms. CRISPR was used only to either eliminate genes the trees already carry or reduce their expression. That is likely to ease the pathway to regulatory acceptance, at least in the United States, Barrangou says. Wang and Barrangou, who founded a spin-off company called TreeCo, aim to start field trials of CRISPR-edited poplars as soon as possible, and they working to introduce similar gene edits into eucalyptus and pine trees, also widely used for making paper.
