The cell wall of plants is made of a network of cellulose, a polymer of the simple sugar glucose, and a complex polymer called lignin. Lignin is tough enough to support everything from buildings to the largest living thing on the planet, a giant sequoia. But, if we're to boost biofuel production, we need an efficient way of getting at the individual sugar molecules within the polymer. A team of biologists might have found a shortcut to better processing: they've made genetically modified switchgrass that's easier to digest.
Their target wasn't the cellulose itself, but the other polymer present in the cell wall, lignin, which seems to lock a lot of cellulose in place, preventing it from being digested. In a number of plants, mutations in the gene for a specific enzyme (caffeic acid 3-O-methyltransferase, for the aficionados) has been linked to reduced lignin production. So, they created a piece of DNA that encoded an interfering RNA that would, ideally, limit the enzyme's production, and hence the amount of lignin present. When the DNA was introduced into switchgrass, it worked, dropping the levels of enzyme below a quarter of that present in unmodified plants. This didn't seem to bother the switchgrass; with the exception of a reddish-brown tint, the plants looked normal.
But they weren't normal when it comes to conversion into ethanol—they were much better. In nearly every condition the authors tried, more of the biomass was converted into fuel when transgenic plants were used, up to nearly 40 percent more under some conditions. The genetic engineering also made the conversion process simpler. Pretreatment with hot acid is normally used to make the cellulose more accessible to enzymes that will digest it. It's possible to run this acid treatments at lower temperatures or in milder conditions, cutting down the costs a bit.
Things were also good when the digestion of cellulose into its component sugars was considered. This is done using enzymes that have to be purified from their sources (usually bacteria), and they represent the biggest cost in the biofuel production process. With the genetically modified switchgrass, the enzyme volumes could be reduced by two-thirds or more, meaning a substantial cost savings.
Switchgrass grows readily under a variety of conditions, and with minimal human input, so it has become a favorite of ethanol advocates. This genetically modified form may make it even more appealing, provided there are no long-term consequences to the reduced lignin levels.
PNAS, 2011. DOI: 10.1073/pnas.1100310108 (About DOIs).
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Chloë Sevigny Janet Jackson Georgianna Robertson Reese Witherspoon Jordana Brewster
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