Wood is great for building and heating homes, but it’s the bane of biofuels. When converting plants to fuels, engineers must remove a key component of wood, known as lignin, to get to the sugary cellulose that’s fermented into alcohols and other energy-rich compounds. That’s costly because it normally requires high temperatures and caustic chemicals. Now, researchers in the United States and Canada have modified the lignin in poplar trees to self-destruct under mild processing conditions—a trick that could slash the cost of turning plant biomass into biofuels.
“This work has the potential to fundamentally change the economics of lignin degradation,” says Ronald Sederoff, a plant geneticist at North Carolina State University in Raleigh. If researchers can add the same self-destructing lignin to agricultural plants such as corn and energy crops such as switchgrass—an effort already under way—that could open the spigots for cellulosic ethanol, made from plant waste rather than food. The U.S. Department of Energy has backed a number of cellulosic ethanol producers, and in 2007 it forecast that by this year they would be making more than 6 billion liters of cellulosic ethanol. Yet this year’s actual production is expected to be just 1% of that volume.
The problem is the lignin. More than two-thirds of plant matter consists of cellulose and hemicellulose fibers, both made up of long chains of glucose and other sugar molecules. Much of the rest is the lignin that wedges between the other fibers and glues them together, providing rigidity and preventing pathogens from lunching on the sugary materials. To remove that glue, engineers typically heat biomass to 170°C for several hours in the presence of sodium hydroxide or other alkaline compounds that break lignin apart. This “pretreatment” accounts for between one-quarter and one-third of the cost of making cellulosic ethanol, says Bruce Dale, a chemical engineer and biomass pretreatment expert at Michigan State University in East Lansing.
Plant biologists have tried for decades to work around their troubles with lignin. One early approach decreased the expression of plants’ lignin-producing genes. But that backfired, as the plants either wound up with stunted growth or keeled over when hit with a gust of wind. “Plants really need lignin,” says John Ralph, a plant biochemist at the University of Wisconsin, Madison.
Among the strategies for dealing with lignin, numerous teams have tried altering the chemicals that make up lignin. Although the structure of lignin varies from species to species, most plants assemble it from three main building blocks called coniferyl alcohol (CA), sinapyl alcohol (SA), and p-coumaryl alcohol, producing chains abbreviated G lignin, S lignin, and H lignin, respectively. Several teams have manipulated plant genes to change the proportions of building blocks, hoping to create a lignin that degrades more easily. Researchers led by Clint Chapple of Purdue University reported in Nature last month, for example, that plants engineered to produce only H lignin wound up dwarfed, but knocking out certain regulatory genes enabled them to grow to a near-normal size. H lignins give up their sugars with less pretreatment, but the chemical bonds between the remaining lignin molecules are still hard to break.
Ralph and his colleagues opted for another path. Instead of altering the proportions of lignin building blocks, they added a new one—ferulic acid (FA)—that pairs up with CA and SA building blocks. These pairs then form bonds with their neighbors that are easier for chemists to break. (A few plants naturally use these FA-contianing pairs in making lignins that serve as plant defense compounds, Ralph says.) They hoped that by introducing paired building blocks throughout the lignin, they could later “unzip” the lignin’s structure during pretreatment.
The feat took several years to pull off. Ralph’s team had to isolate the genes for the synthesis of FA-containing building blocks, insert them into plants, show that the plants could make the compounds, send them to the cell walls, and incorporate them into lignins. But online today in Science, Ralph and his colleagues report that they’ve now produced “zip-lignins” in young poplar trees. The plants appear healthy and show every sign of normal growth in the greenhouse. But when ground up and subjected to a mild base at 100°C, the lignins readily fall apart, releasing twice as many sugars as their wild-type kin do under the same conditions. “It’s the most promising method of changing lignin that I’ve seen so far,” Sederoff says.
Ralph says his team is already working to insert zip-lignins into corn plants. If the effort succeeds, it could save cellulosic biofuel companies serious cash and may even propel them to profitability, Dale says. It could also spawn a new generation of biorefineries that convert plant cellulose into plastics and other industrial materials. But Dale and others caution that it could take a decade or more. Any newly engineered plants and trees must still be field-tested to show that they grow normally and aren’t more susceptible to pests, among other things. Then researchers must also show that they pass economic muster in pilot-scale and demonstration biorefineries. But if the strategy works, biofuel-makers may finally find a way out of the glue that has trapped them for decades.