Cheaper biofuels from transgenic trees

Tough, fibrous lignin is a natural scaffold in plants, giving them support as they grow up from the ground. It also surrounds the sugars in plant cell walls, making it hard for scientists to extract those sugars and turn them into biofuels.

By engineering a plant enzyme involved in lignin production, researchers have now transformed the lignin in aspen trees so that it more easily gives up the sugars trapped inside. Aspen are promising for biofuels production because they are fast-growing, hardy, and widespread in North America.

The team from Brookhaven National Laboratory, who reported their findings in Nature Communications, were able to get almost 50 percent more ethanol yield from the transgenic aspen trees. The advance could help bring down the cost of producing cellulosic biofuels, which are made from non-food biomass like grasses and woods as opposed to starch-based feedstock like corn.

Cellulosic fuels today comprise a minuscule percentage of total U.S. biofuel production. That’s mainly because of the costly pretreatment step—which requires high temperatures and expensive chemicals—needed to break down lignin and release sugars that are fermented to make biofuels. Pretreatment can account for one-third of the cost of making cellulosic fuel.

The Brookhaven team previously engineered plants to produce less lignin. But that compromised the structural integrity of plants, stunting their growth and making them weaker, which results in less biomass to convert to fuel. Other teams have also been trying various genetic tricks to alter lignin’s building blocks.

In the new work, the team modified an enzyme so that it hinders the formation of one of the two main lignin components typically found in aspen trees. Then they transplanted the gene that expresses this modified enzyme into a strain of fast-growing aspens.

Compared to unaltered trees, the engineered trees had only slightly less lignin, and it was more condensed. The researchers also found that altering the lignin increased the production of cellulose, which is the main fermentable sugar in plants. Wood containing this lignin released about 62 percent more sugar than the normal wood when treated with digestive chemicals, increasing ethanol yield by 50 percent. Most importantly, the changes did not affect the growth of the modified aspens.

The trees in the study were grown in a greenhouse for six months. The researchers plan to do an extended field trial to test the trees’ viability as a biofuel source. —Prachi Patel | 30 June 2016

Source: Cai, Y., et al. Enhancing digestibility and ethanol yield of Populus wood via expression of an engineered monolignol 4-O-methyltransferase. Nature Communications 2016. DOI: 10.1038/NCOMMS11989