As the amount of genomic detail for crop plants continues to rapidly expand and be
understood, it will provide more candidate genes as tools for biotechnological applications.
Uses for this knowledge could come in the form of transgenes to be transferred between
species, or as tools for plant breeders who utilize DNA marker-assisted selection in
crop improvement. The amount of information contained within a single plant species’
genome is immense, and the potential that it holds for genetic improvement is therefore
also large. Understanding and applying that potential is the challenge for scientists trying
to identify genes that can contribute to traits of value to growers and consumers.
8.3 Traits for Improved Crop Production
The growth of healthy plants that yield quality products requires farmers to deal with ever-
changing environmental conditions and pests. Transgenic approaches to helping farmers
with these challenges are being broadly used today, while additional products are in the
developmental pipeline. Plants with improved tolerance to high temperatures, saline
conditions, and drought are likely to find their way into production in the future. The
most common uses of transgenic plants in agriculture today are engineered resistance to her-
bicides, insects, and pathogens. In doing so, transgenic plants are addressing some of the
oldest problems in crop production.
8.3.1 Herbicide Resistance
The first transgenic application to be widely adopted in agriculture wasresistance to herbi-
cides. Weeds are generally regarded to be the most serious problem for farmers and result in
reduced yields because they compete with crop plants for water, light, and nutrients.
Chemical herbicides are widely used by many farmers because they are cost-effective
and efficient at killing weeds. Most effective herbicides for agricultural production must
be somewhat selective, meaning that they should kill the target weeds but not the crop
plant. Using single-gene traits in transgenic plants can provide a very specific way to
protect the crop plant from the effects of a given herbicide.
Herbicides generally work by targeting metabolic steps that are vital for plant survival.
For example,glyphosatekills plants by inhibiting the production of certain amino acids that
the plant requires for survival. Glyphosate is the active ingredient in the herbicide
RoundUpTM. Thus, crops such as soybean and corn that have been engineered to be resist-
ant to glyphosate were given the name “RoundUp Ready.”
Glyphosate works by binding to and inhibiting the enzyme 5-enolpyruvylshikimate-
3-phosphate synthase (EPSPS), which is active in the shikimate pathway leading to the
synthesis of chorismate-derived metabolites, including the aromatic amino acids (tyrosine,
phenylalanine, and tryptophan) (Fig. 8.1).
To make plants resistant to glyphosate, a form of the EPSPS enzyme that is functional in
plants but is not affected by the herbicide was used. In addition to being present in plants,
the EPSPS protein can also be found in bacteria. So scientists at Monsanto, the inventors of
RoundUp, looked for and identified a form of EPSPS from a soil bacterium that was not
sensitive to treatment with glyphosate. The initial steps in this process were relatively
straightforward, as they simply plated soil bacteria on media containing glyphosate to ident-
ify strains that were resistant to the chemical. The EPSPS gene from the bacterium was then
isolated and transferred into plants where its expression was regulated by putting it
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