Soybean oil is also used in a variety of food and industrial applications. By decreasing
the levels of the enzyme calledD 12 -desaturase in transgenic soybeans, the amount of oleic
acid can be increased. To decrease levels of enzyme expression, the normal soybeanfad2
gene encoding D 12 -desaturase was repressed using a technique calledgene silencing,
whereby a second copy of the gene is introduced into the plant. By overexpressing a
second copy of the target gene, a response in the plant is triggered to shut down expression
of both the endogenous gene and the transgene. In this case, silencing thefad2gene results
in higher levels ofoleic acidand corresponding lower levels of two other 18-carbon fatty
acids and linoleic and linolenic acids. The only differences in the structures of these three
fatty acids are the number of double bonds in the chain. As a result, high–oleic acid soy-
beans have low levels of saturated fats and transfats. This can alleviate the need for the
hydrogenation process that is often used to make soybean oil suitable for foods like mar-
garine, resulting in a healthier product. It also keeps the oil in a liquid form and makes it
more heat-stable for cooking applications.
8.4.3 Pharmaceutical Products
Plant-manufactured pharmaceuticals(PMPs) are one of the most widely discussed appli-
cations of transgenic plants. The tremendous variety and potency of chemicals produced in
plants has been long recognized, as many have powerful effects on human health and physi-
ology (salicylic acid, cocaine, morphine, taxol, etc.). In addition to being able to produce
complex metabolites, plants can also produce high levels of specific proteins when a
novel transgene is introduced.
Production of human and animaloral vaccinesin plants has been proposed as an attrac-
tive approach, especially in areas of the world where infrastructure and costs might limit
storage, transfer, and administration of traditional vaccines. By including an immunogenic
protein in a food, vaccination could be effected using a product that is easily grown and
stored and that could be administered via consumption of the food source. For example,
production of the surface antigen of the hepatitis B virus in transgenic potato has been
demonstrated in clinical trials to lead to an immune response in humans consuming the
potatoes. Production of proteins in transgenic bananas is also often cited as a potential
source for these oral vaccines. There are several potential problems with this approach,
such as the timing of administering the vaccine, dosage, and the ability of the protein to
induce immunity on oral administration. Nonetheless, this strategy might have application
in some specific instances for humans or in vaccination of farm animals.
Antibodiesare large, complex proteins with the powerful ability to recognize and bind to
specific molecular targets. Plants do not normally produce antibodies, but it has been
repeatedly demonstrated that they can form functional antibodies when the encoding
genes are expressed transgenically. One of the more promising approaches is the production
of a specific monoclonal antibody that recognizes a cell surface protein ofStreptococcus
mutans, a bacterium that is one of the major causes of tooth decay. By binding to its
surface, the antibody interferes with the bacteria’s binding to tooth enamel. The planned
applications for this product, produced in tobacco and calledCaroRX, would be primarily
in toothpastes and mouthwashes.
To date, the vast majority of transgenic biopharmaceuticals are produced usingE. coli,
yeast, or mammalian cell cultures. The strategy of producing pharmaceutical proteins in
plants could have several advantages (Giddings et al. 2000). Transgenic plants offer the
economies of scale to grow and harvest large amounts of biomass expressing the target
208 GENES AND TRAITS OF INTEREST FOR TRANSGENIC PLANTS