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Herbicide Resistance
Herbicide resistant plants were created in the
1980s (Jung, 2000). In 1999, there
were approximately 28 million hectares of herbicide resistance crops, with
herbicide-tolerant soybean the one most planted. Herbicide resistance allows for
widespread application of more effective herbicides to the crop field,
killing the weeds but not damaging the crops (Qaim et al., 2000; Pretty, 2001; Klotz-Ingram et al., 1999). Glyphosate and
glufosinate are the two major herbicides (Jung, 2000).
Glyphosate: Glyphosate is the ingredient used in Roundup. To kill weeds, glyphosate inactivates the
enzyme 5-enolpyruvate-shikimate-3-phosphatesynthase (Jung, 2000). This enzyme is necessary for plastids to
synthesize aromatic amino acids: phenylalanine, tyrosine, and tryptophan
(Jung, 2000; Universita’ Degli Studi Di Brescia 2003). A tolerance gene from bacteria has been
transformed to higher plants, and expression is controlled by a
constitutive promoter. The
polypeptide is directed into the plastid by a targeting sequence. Glyphosate tolerance has been proven to
be the most useful in soybean, rapeseed, and maize (Jung, 2000).
Permission
Pending from Monsanto Company. Image reproduced from Roundup.com.
Glufosinate: The enzyme glutaminesynthetase is inactivated by
glufosinate (phosphinotricin). The
inactivation of this enzyme causes the buildup of NH4+. This accumulation is toxic to the plant
cell. A resistance gene has been
cloned from Streptomyces and transformed into different crops. The gene, which encodes
N-acetyltransferase, metabolizes phosphinotricin to acetylphosphinotricin,
inactivating the herbicide.
Currently, glufosinate resistant rapeseed and maize are grown in the
field (Jung, 2000).
Pest Resistance
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Questions
or comments can be directed to nihesson@davidson.edu
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Page maintained by
Matt Talbert, Nicole Hesson, and Sarah Parker
Last modified April
2004
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