'Designer' plants could seed cheaper biofuels
Bioethanol has long been touted as a sustainable alternative to petrol but so far it has a poor report card. For one thing it is usually made from crops such as corn or sugarcane, which compromises the global food supply. And bioethanol barely delivers enough energy to justify the cost of growing and processing the plants.
Hopes for a more sustainable solution have focused on making ethanol from the cellulose in plant waste like corn stalks or wood chips. Cellulose can be broken down into sugars and fermented to produce ethanol, a process that is costly enough. But the biggest expense lies in getting to the cellulose in the first place – it is cemented together by a stringy molecule called lignin. Removing lignin requires treatment with caustic chemicals at temperatures of around 170°C. For the moment, that’s a major obstacle on the road to commercially viable biofuels.
But researchers in the United States and Canada might have cleared the roadblock by genetically engineering plants to produce a form of lignin that breaks down more easily than the natural version.
The new approach was demonstrated in poplar trees, a fast-growing wood crop that has potential as a biofuel feedstock. Described last week in Science, experts say the method could slash the costs of producing biofuels from wood, grasses or inedible parts of food crops like corn, and push the biofuel industry closer to realising its promise as a sustainable alternative to fossil fuels.
“I’m really impressed,” says Bruce Dale, a chemical engineer who works on biofuels at Michigan State University. “Although it’s not been proven quite yet, I fully expect this modified lignin to be easier to process and that’s a really big deal.”
The idea started with paper. A group led by John Ralph, a plant biochemist at the University of Wisconsin-Madison, was trying to figure out how to reduce the energy used to chop up lignin in the pulping process used to make paper. Lignin is a polymer comprised of chains of building blocks called monomers. If he could weaken the bonds between the monomers, Ralph thought, the lignin could be degraded with less energy.
The potential crossover to biofuel processing was obvious. So Ralph got together with scientists at the Madison-based Great Lakes Bioenergy Research Center, of which he is a member, to see if he could soften lignin from within.
Modified poplars released up to twice as many sugars as natural
poplars under the same conditions.
Researchers have been messing with lignin for years. They have engineered plants that produce less of it and altered the proportions of different monomers. But it turns out that meddling with lignin synthesis can stunt plants’ growth and make them floppy. It can also render them less resistant to pests and pathogens. “Those strategies have severe drawbacks,” says Dale.
Ralph and his colleagues took a different approach: redesign lignin with new building blocks that are easy to break apart but do not affect plant health. “We were trying to design a weak link into the backbone of the polymer,” says Ralph.
To do so, Ralph and Curtis Wilkerson of Michigan State University first isolated a gene from a plant called dong quai (Angelica sinensis) that produces an exotic monomer that can link into the lignin chain. When these new monomers are incorporated, they form easily broken linkages called esters.
Next the researchers got together with Shawn Mansfield of the University of British Columbia in Vancouver to introduce the gene into poplars. By equipping the gene with a specific DNA switch, they made sure it turned on just as the lignin was cemented during the development of the plant’s cell wall. The introduced component “was fully incorporated just as well as a normal monomer would have been”, Ralph says.
The altered poplars grew normally in the greenhouse and appeared healthy. When pulverised and subjected to a dilute alkaline treatment at 90°C for three hours, the modified poplars released up to twice as many sugars as natural poplars under the same conditions, suggesting that “designer” lignin falls apart more easily.
Ralph points out that the test released only a small percentage of the total sugars available in the plants – enough to make a comparison. To prove that “designer” lignin can transform the economics of the biofuels industry, the researchers must now demonstrate that transgenic plants bring significant efficiency gains when exposed to the various processes used in biorefineries where 100% of the sugars are released and converted into fuel. Ralph says they plan to tackle that this year.
“Say it takes two hours at 170°C with wild-type plants, the intent is that it will take half an hour with transgenic plants, or we can reduce the temperature,” says Ralph. “Either way, it has the potential to vastly reduce energy requirements.”
The team also has to show that modified plants grow normally in the field and that the technique can be applied to different plants. If everything works out and regulatory hurdles can be cleared, Dale says that plants altered to have weaker lignin could go a long way to making cellulosic biofuels economically viable – though he cautioned that it is competing with a petroleum-refining industry that has had a century to reduce its costs.
Ralph agrees. “This is such a compelling advance that it might almost be irresponsible, in future, to pulp or convert any biomass that doesn’t have weaker lignin,” he says. “It should dramatically improve biofuel processing.”
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