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Herbicides which inhibit the enzyme EPSP synthase (5-enolpyruvylshikimate-3-phosphate synthase) contain the active ingredient glyphosate and are nonselective, post emergence herbicides.
The glyphosate molecule binds to EPSP synthase, changes its shape, so it becomes nonfuctional. ESPS synthase is responsible for the production of the amino acids phenylalanine, tryptophan
and tyrosine which prevent plants from producing proteins necessary for survival. EPSP synthase occurs in bacteria, fungi and plants. It does not occur in animals.
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Crop plants tolerant of glyphosate have been produced. The first crop plants tolerant of glyphosate were produced by insertion of a gene from a bacterium which produces a EPSP synthase
that is a different shape than the "wild-type" EPSP synthase in plants. This change in shape does not allow glyphosate to bind and thus allows the bacterial EPSP synthase to produce phenylalanine, tryptophan
and tyrosine. More recently, genes from a bacterium which codes for production of a glyphosate degrading enzyme (glyphosate acetyltransferase) had been inserted into
plants. This confers resistance to glyphosate by degradation of glyphosate.
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Symptoms on plants from glyphosate vary depending on rate of application and plant species. Symptoms include leaf shape distortion, yellowing within or between leaf veins, stunting and dying/brown tissue.
Perennials may exibit a stunting of regrowth from buds below dead growing points. At high rates (1-2% solutions), yellowing of meristematic regions is seen followed by stem dieback.
Symptoms are slow to develop in cool weather. The incidence of resistance to this herbicide is increasing.
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Photographs by Dr. Michael Pfeiffer |
