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
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.