Mechanism of herbicide toxicity
The primary ingredient in the herbicide Roundup is glyphosate, though other chemicals including surfactants and adjuvants are also present. Glyphosate inhibits the enzyme 5-enolpyruvylshikimate 3-phophste (EPSP), which is the next to last enzyme in the Shikimate biosynthetic pathway (Funke). The Shikimate pathway is responsible for the production many aromatic molecules including the aromatic amino acids which are required for plant growth. While all plants contain the Shikimate pathway, this pathway is absent in animals (which is why humans must obtain aromatic amino acids from the diet) (Herrman). Inhibition of the Shikimate pathway leads to a lack of aromatic molecules and consequently death for the plant.
Mechanism of herbicide resistance
In an effort to engineer plants to be resistant to glyphosate, scientists starting looking for glyphosate resistant EPSP in the 1980’s. Several methods were employed including selection, directed evolution, site-directed mutagenesis, and microbial screens. It was difficult to obtain a suitable enzyme because typically the resistant enzymes had an undesirable decrease in catalytic activity. Eventually, naturally occurring glyphosate-tolerant microbes were identified including Agrobacterium sp. Strain CP4, Achromobacter sp. Strain LBAA, and Pseudomonas sp. Strain PG2982. The enzymes isolate from these maintained good catalyctic ativity while being resistant to glyphosate (Funke). Agrobacterium sp. Strain CP4 was obtained from a glyphosate rich waste area at a glyphosate production facility. There is substantial sequence variation between these resistant enzymes and those of plants or E. coli. Other versions resistant versions of EPSP have been identified in Streptococcus pneumonia and Staphylococcus aureus (Funke). To create glyphosate resistant plants, the gene for the resistant version of the EPSP enzyme was transformed into plant cells.
Usage of RoundupReady crops
Proir to RoundupReady crops, Roundup was used typically applied before planting to kill weeds. Because the crop species were sensitive to roundup, the herbicide could not be generally applied after the crop species started to grow. The use of RoundupReady plants allows for applications of Roundup during the growth of the crops has turned out to be a good way of controlling weed growth. In 2005, 87% of soybeans, 61% of cotton, 26% of corn planted in the US in 2005 were glyphosate resistant varieties (Funke).
Safety concerns / criticisms
While animals do not contain the Shikimate pathway, large amounts of Roundup can be toxic to humans. Some of this toxicity may come from the surfactants and other additives that are important to allow the glyphosate to be effective. A toxicity study found the toxicity of Roundup to be more severe than that of glyphosate alone, which had little effect (Peixoto). Because RoundupReady crops are sprayed with Roundup, there is potential for a higher amount of Roundup exposure for consumers of these crops. To determine if this is indeed a safety concern the amount and toxicity of Roundup on crops post harvest must be examined.
Funke, T., Han, H., Healy-Fried, M. L., Fischer, M., & Schonbrunn, E. (2006). Molecular basis for the herbicide resistance of roundup ready crops.Proceedings of the National Academy of Sciences of the United States of America, 103(35), 13010-13015.
Herrmann, K. M., & Weaver, L. M. (1999). The shikimate pathway. Annual Review of Plant Physiology and Plant Molecular Biology, 50, 473-503.
Peixoto, F. (2005). Comparative effects of the roundup and glyphosate on mitochondrial oxidative phosphorylation. Chemosphere, 61(8), 1115-1122.