Principles of Plant Genetics and Breeding

milling, extraction of oil, starch production, or sugar
production. Breeders have developed special purpose
hybrids for these uses. For example, by inserting the
endosperm gene, sugary (su), and another mutant gene,
shrunken (sh2), in the same genotype, the resulting
hybrid has increased sugar content (called supersweet or
extrasweet corn). Similarly, waxy corn is developed for
use in the production of adhesives, gums, and puddings,
because of its high amylopectin content.
Edible oils that are high in polyunsaturated fatty acids
are considered more healthy because they have the
capacity to lower blood cholesterol. However, such oils
are unstable for high-temperature cooking because they
oxidize and break down readily at high temperatures.
The hydrogenation process that is used to artificially
stabilize oils converts the polyunsaturates back to
monounsaturates. Oil plants such as soybean and cotton
have genes that naturally convert oleic acid, a monounsaturated
fatty acid, into polyunsaturated fatty acid.
In cotton, about 25% of the seed oil comprises two fatty
acids – palmitate and stearate. Conventional cotton seed
oil contains primarily palmitate, which is thought to
raise blood cholesterol more than stearate. Scientists at
the Commonwealth Scientific and Industrial Research
Organization (CSIRO), Australia, turned off the genes
for polyunsaturates to produce cotton seed with high
levels of oleic acid (monounsaturates). This high oleic
oil is more stable for high-temperature cooking and
can be used in place of hydrogenated oils. Further, the
scientists modified the plants to produce more stearate
than palmitate, making the genetically modified (GM)
product healthier for human use.
Similarly, in soybean, other scientists used the technology
of gene silencing to develop high oleic cultivars.
Using biolistic techniques, a second copy of the fad2
gene (fatty acid desaturase gene) that encodes the enzyme
Bjarnason, M. 1990. CIMMYT’s Quality Maize Program:
Present status and future strategies. Paper presented at the
18th Congresso Nacional de Milho e Sorgo, Vitoria, Brazil,
29 July–3 August.
Helms, T.C., and J.H. Orf. 1998. Protein, oil, and yield in
soybean lines selected for increased protein. Crop Sci.
38:707–711.
Hulke, B.S., W.R. Fehr, and G.A. Welke. 2004. Agronomic
and seed characteristics of soybean with reduced phytate
and palmate. Crop Sci. 44:2027–2031.
BREEDING COMPOSITIONAL TRAITS AND ADDED VALUE 415
References and suggested reading
delta-12 desaturase, which is involved in fatty acid synthesis,
was introduced into the genome of soybean. This
event switched off the desaturase gene, causing the
accumulation of oleic acid in the seed only (normal fatty
acid biosynthesis occurs in other plant parts). Consequently,
only small amounts of the polyunsaturated
fatty acids, linoleic and linolenic, are produced in the seed.
The high oleic soybean GM cultivars such as G94-1
contain about 80% more oleic acid than conventional
seed, and higher levels than in olive oil and rapeseed oil.
Breeding plants for novel traits
An application of genetic engineering to breed novel
traits is the use of organisms as bioreactors to produce
pharmaceuticals. One of the earliest applications of this
technology was the commercial production of human
insulin in microbial systems. Similarly, certain pharmaceuticals
are commercially produced in mammalian
milk of sheep, goats, and rabbits. The application is
being applied to plants to produce selected chemical
compounds. Plant-made vaccines are currently under
development for protection against cholera, diarrhea
(Norwalk virus), and hepatitis B. The most common
plants that are being used in plant-made pharmaceuticals
are corn, tobacco, and rice. Other crops being
investigated include alfalfa, potato, safflower, soybean,
sugarcane, and tomato. To be usable, the plant should
be readily amenable to genetic engineering and capable
of producing high levels of protein. Further, there
should be an efficient method for extracting the protein
products from the plant tissues. Another example of a
plant-manufactured pharmaceutical is taxol, a secondary
product derived from the Pacific yew tree. This product
has been found to be effective against certain cancers.
Mertz E.T., L.S. Bates, and O.E. Nelson. 1964. Mutant gene
that changes protein composition and increases lysine content
of maize endosperm. Science 145:279–280.
National Research Council. 1988. Quality-protein maize.
Academy Press, Washington, DC.
Wang, T.L., C. Domoney, C.L. Hedley, R. Casey, and M.A.
Grusak. 2003. Can we improve the nutritional quality of
legume seeds? Plant Physiol. 131:886–891.