July 2011

Pouring energy into biofuels

Another drawback researchers will need to overcome before vegetation rules the biomass roost is that in most plants, energy-rich cellulose is bound up with significant amounts of lignin, the cell wall component that provides mechanical strength. Currently, biofuel producers separate cellulose from lignin with harsh, expensive chemicals and high temperatures. Several researchers, including Loque, are looking for ways to avoid these.

Loque explains that altering lignin content is tricky. Remove too little, and deriving cellulose remains difficult; remove too much, and the plant has no support to grow.

He and his colleagues currently are working on two strategies to surmount the lignin problem. In the first, the researchers are tinkering with where plants deposit lignin. Loque notes that the entire lignin pathway is known and highly conserved. By using promoters throughout the pathway that produce different expression of lignin genes relative to the native ones, the researchers have successfully reduced lignin in undesirable areas while keeping it in necessary places, such as the vessels plants use for nutrient transport.

“In the end, we got plants that look like wild-type, but contain much less lignin,” he says.

He and his team also are working on engineering plants that make weaker lignin through genetic modifications that insert ester or amide bonds into the native structure, which has only carbon-carbon or carbon-oxygen bonds. These weak links eventually could reduce the amount of chemicals and lessen the temperatures needed to pretreat cellulosic feedstocks.

Escaping from ethanol

Another drawback of fuel ethanol is that researchers have calculated that, in many cases, it’s actually an energy sink rather than a source; the amount of petroleum used to plow and fertilize a cornfield, then transport and process the corn before fermentation, often contains more energy than the resulting ethanol. It’s also tremendous waste of the carbon atoms plants work so hard to fix. Only two thirds of a feedstock’s carbons are used in ethanol production, explains Katie Dehesh, a professor of plant biology at the University of California, Davis. The other one-third ends up as food for the fermenting yeast and in the air as carbon dioxide.

  
Katie Dehesh, a professor of plant biology at the University of California, Davis, is coaxing oats to make more oil than starch.

A possible solution is coaxing plants to make more oil than starch. Indeed, many plants already produce significant quantities of oil; it’s what fills the frying vats for much-loved fast-food fries. However, using these food crops for fuel oil has the same competitive disadvantages as creating ethanol from corn. Additionally, Dehesh points out, oil is only a minor component of most plants’ seeds and is even less abundant in their vegetative parts.

She and her colleagues recently published new research that could offer a possible solution to this problem by redirecting carbon flux toward oils and away from carbohydrates. The researchers used oat as their model organism, since this grain is a rare example of a plant that produces significant amounts of oil in its endosperm at the cost of carbohydrates. Using two varieties of oats— one that produced much more oil than the other— Dehesh’s team compared gene activity between the plants during seed development. Surprisingly, the fatty acid pathway that they expected to see upregulated in the high oil producer was actually the same between the two plants. However, the researchers found a variety of differences in the cofactors involved in respiratory metabolism. These cofactors, says Dehesh, appear to be the answer for determining carbon flux.

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