An international team of scientists has discovered a new plant enzyme that could eventually lead to a breakthrough in the production of cellulosic bio-based fuels made from crop wastes, as well as chemicals and plastics.
Caffeoyl shikimate esterase (CSE) is an enzyme whose genes can be switched off to control the formation of lignin. Lignin is a lattice-like structure of cells that makes plants rigid. It is also a tough substance that makes it difficult to extract sugar from agricultural wastes to make biofuels. As such, it has been a roadblock to commercial production for years.
The series of steps that plants use to make lignin has been studied for decades, said Wout Boerjan, a co-author of the paper who does research in plant biotechnology at Ghent University and Belgian life sciences institute VIB. But CSE had remained an elusive factor in it until now.
"It had been overlooked all these years," Boerjan explained.
Until recently, scientists assumed another gene was responsible for making lignin, said John Ralph, another author of the paper and a professor at the Energy Department's Great Lakes Bioenergy Research Center.
Most biofuels are based on sugars. Once they are extracted, the sugars are fermented into an alcohol -- like ethanol or butanol -- that can then be used as a fuel. By "downregulating," or switching off the function in the CSE gene that causes lignin formation, the researchers were able to engineer a small flowering plant to make 36 percent less lignin per gram of stem material.
This is enough to boost the production of the sugars up to four times, Boerjan said.
The plant -- Arabidopsis thaliana, a relative of the mustard family -- had been studied by researchers for years because it is often used as a model in examining the biological structures of plants. By blocking the production of polymers, the researchers found that molecular structure of its lignin was altered and became less complex, making for a more easily digestible feedstock to turn into fuels.
Cellulosic's slow path to commercial use
Two weeks ago, INEOS Bio announced it had begun commercial production of cellulosic ethanol from yard waste in its Vero Beach, Fla., plant. A handful of other cellulosic ethanol plants, which will make biofuels from corn stover, wheat straw and municipal waste, plan to begin production by next year (ClimateWire, Aug. 5).
Under the federal renewable fuel standard, the United States must ramp up biofuels production to 36 billion gallons by 2022. When the law was enacted, U.S. EPA expected 16 billion of the 36 billion gallons would be cellulosic biofuels. But to date, there is a minuscule amount of cellulosic fuels on the market.
EPA recently revised its cellulosic target for 2013 from a proposed 14 million gallons of ethanol-equivalent to a final requirement of 6 million gallons (Greenwire, Aug. 6).
Many companies have been stalled by high costs, and difficulties have prevented smaller firms from jumping from a small scale to commercial scale.
"The main bottleneck is the pretreatment," Boerjan explained, referring to the expensive process of cooking or otherwise softening the plant material so the sugars can be extracted and converted. "[The industry] wants to make their products in an inexpensive way."
"Pulping recalcitrance," or the difficulty in breaking down tough plant matter, has been a struggle for the industry for years, said a spokeswoman from Danish company Novozymes, which makes enzymes for the production of biofuels. As a result, companies have needed to add more chemicals to facilitate the work.
The next step is to ensure that these kind of genetic modifications don't negatively affect plants, said Mads Torry-Smith, director of research and development in the biomass applications division of Novozymes, who was not involved in the study.
"There remains much work ahead to prove that these modifications do not affect the plant's ability to maintain its structure," Torry-Smith added. "Nonetheless, we strongly encourage research in this area and consider it an important supporting technology for biofuel production."
Next experiment could be poplar trees
Ralph of the Great Lakes Bioenergy Research Center said it would be premature to think that this specific discovery could change the way biofuels are made, but it could pave the way to more insights on how to make better biofuel crops.
"At some point I think you will see plants that are much easier to pretreat," he said.
But downregulating the gene could also hinder the larger yields. Lignin is found in two types of cells: fiber cells that make up the structure of a plant and cells that make up the vessels that carry water throughout the plant. Engineering CSE to alter lignin formation could affect the flow of water within a plant and determine how well it grows.
"We hope to overcome the yield penalty by downregulating in fiber cells only," Boerjan said. Poplar trees, a promising feedstock for biofuels, could be the next subject in the downregulating experiment, he added.
Eighteen researchers from Belgium, the United Kingdom and the United States participated in the study. The paper appears in this week's edition of Science, which is published by the American Association for the Advancement of Science.
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