and are now the most popular transgenic plant in the world. Glyphosate has several features
that make it an attractive herbicide for growers. The compound is readily taken up and trans-
ported throughout the treated plant, traits that make it especially effective as an herbicide.
Because glyphosate is rapidly degraded by soil microorganisms, it does not persist long in
the environment after application. This is a benefit from both an environmental standpoint
and a crop management perspective, because farmers can plant any crop in a sprayed field
relatively soon after herbicide application. Because it is so effective at selectively killing
weeds and not the herbicide-resistant crop plant, more farmers using glyphosate have
adopted “no-till(age)” or “low-till(age)” methods, resulting in less soil erosion and lower
fuel costs because they take fewer trips through a field. Furthermore, because animals do
not make aromatic amino acids, they do not possess the shikimate pathway that is the
target of glyphosate and so the herbicide has low toxicity in animals. In 1996, the first
year they were commercially available, RoundUp Ready soybeans made up about 2% of
the total soybeans grown in the United States. By 2000, that amount had risen to 54%,
and in 2005 it was up to 87% (US National Agriculture Statistics Service; http://www.
nass.usda.gov/). Now, glyphosate resistance has been engineered into a large number of
crops that are grown globally, including Latin America and Asia. Predictably, adoption of
glyphosate-resistant crops has resulted in a vast increase in the amount of this herbicide
applied worldwide; however, there has been a decrease in the use of other herbicides,
especially on soybean. This increase has also been encouraged by glyphosate coming off
patent in 2001. Now glyphosate is sold as a generic by many companies as well in
RoundUp formulations by Monsanto. The large amounts of glyphosate that are now
applied to crops have led to concerns that glyphosate-resistant weed biotypes will be selected
for and propagate in agricultural fields. Furthermore, farmers are required to pay a significant
technology fee to Monsanto for the right to grow RoundUp Ready crops.
Glyphosate resistance is conferred through the expression of an active target enzyme,
EPSPS, which is not affected by the herbicide. An alternative strategy to engineer herbicide
resistance is to express a protein that will inactivate the herbicide if it is sprayed onto plants.
This is the approach used in resistance against the herbicide glufosinate, the active ingredi-
ent in the product LibertyTM, generating a trait in crop plants often called “LibertyLink.”
Glufosinatekills plants by inhibiting the plant enzyme glutamine synthetase (GS), which
is responsible for synthesis of the amino acid glutamine. As part of the chemical reaction
that produces glutamine, GS utilizes excess plant nitrogen in the form of ammonium that
is incorporated into the amino acid. When GS is inhibited in glufosinate-treated plants,
ammonium concentrations inside the plant rise to toxic levels (Fig. 8.2).
The glufosinate compound is naturally produced in someStreptomycesbacteria. In
addition to having phytotoxic activity, glufosinate also servers as an antibiotic because it
is toxic to some other bacteria. Bacterial strains that are resistant to glufosinate produce
an enzyme, encoded by thebargene, calledphosphoinothricine acetyltransferase(PAT)
(Thompson et al. 1987). Thebargene was isolated from a strain ofStreptomyces hygrosco-
picus, which degrades glufosinate, and has been transferred into several crop plants. The
LibertyLink trait is currently widely used in transgenic corn, canola, and cotton varieties.
Similar to the strategy in making LibertyLink crops, resistance to the herbicide bromox-
ynil (BuctrilTM) was engineered by expressing the protein of a bacterial gene that will inac-
tivate the herbicide. Bromoxynil kills plants by inhibiting function of photosystem II, a
crucial component of photosynthesis. Buctril-resistant cotton is already widely grown in
the United States, and other crops resistant to this herbicide, such as tobacco and potato,
are nearing final stages of commercialization.
8.3. TRAITS FOR IMPROVED CROP PRODUCTION 199