product on relatively little land. Some applications for therapeutic proteins such as serum
factors, hormones, or antibodies have traditionally relied on human or animal sources.
By using plants, the risk of transferring unknown infectious agents from the donor
source can be greatly reduced because plants typically do not carry animal pathogens.
The idea of producingtherapeutic proteinsin crop plants is not accepted by everyone.
Opponents worry that food products could be contaminated with tissue of plants intended
for drug production. Companies that rely on commodities for products to which certain con-
sumers may be sensitive have also opposed transgenic crops expressing pharmaceuticals.
A prominent example was when a large beverage company opposed a pharmaceutical
company who wished to grow transgenic rice near rice fields that would be used in their
beverage product. Another potential hurdle is the differences in glycosylation of proteins
that occur in plants and animals. The sugar moieties added to proteins can vastly affect
their function and immunogenicity, and some patterns of plant glycosylation can cause
unwanted allergic reactions in humans. To be used in humans, these proteins would
need to be produced so that they do not elicit an immune response in the patient.
8.4.4 Biofuels
With demands for energy increasing worldwide and supplies of fossil fuels being depleted,
finding alternative and renewable energy sources has become an important goal for plant
scientists. Both ethanol (ethyl alcohol) and biodiesel produced using plant materials can
be adapted relatively easily to existing fuel storage, movement, and uses with existing infra-
structure and machinery. Applications using transgenic plants have the potential to increase
the efficiency of biofuel production on several fronts.
Ethanol offers several attractive features as an energy source; it is biodegradable and
renewable, and burns cleaner than do most fossil fuels. Ethanol is produced by yeast-
driven fermentation of carbohydrates (sugars). In the United States corn is currently the
dominant source for fermentable sugars. In this case, the complex carbohydrates of
starch in corn grains are first converted to simple sugars, which the yeast can then use to
produce ethanol. One suggested approach to improve ethanol production is to transgenically
engineer plants to produce higher levels of the enzymes responsible for the initial steps of
starch breakdown (Himmel et al. 2007). The genes encoding enzymes such as amylase,
which degrades starch into simpler sugars, could possibly be expressed at high levels in
corn grains or in other plants, resulting in higher percentages of readily fermentable
sugars. The considerable inputs necessary for growing corn, in terms of nitrogen fertilizer,
fuel, and pesticides, mean that it is likely not going to be an efficient long-term solution as a
source for ethanol production. In Brazil, sugarcane is the plant source of choice for making
ethanol, as the high levels of simple sugars make it superior for fermentation. In addition,
sugarcane is a perennial crop that can be more easily grown with fewer inputs. The success
of the Brazilian adoption of ethanol as a fuel source is widely touted as an example of how
existing infrastructure and practices can be adapted for conversion to reliance on biofuels.
The use of plant material high incelluloseas a source for ethanol production is also
being widely studied. The conversion of high-cellulose materials into fermentable sugars
is an inefficient process, and so it is not currently viable as a method for biofuel production.
However, plant materials such as corn stover (stalks and leaves), wood chips, or biomass
crops such as perennial grasses contain energy that could potentially be converted to
ethanol. Biomass crops, such as switchgrass or fast-growing trees such as willow or
poplar, have advantages in that large amounts of biomass can be harvested multiple
8.4. TRAITS FOR IMPROVED PRODUCTS AND FOOD QUALITY 209