Arikah Map

Glyphosate

Glyphosate:Skeletal formula of glyphosate
Enlarge
Skeletal formula of glyphosate
Glyphosate:3D model of glyphosate
Enlarge
3D model of glyphosate

Glyphosate (N-(phosphonomethyl) glycine, C3H8NO5P) is a non-selective herbicide to kill weeds, especially perennials.

Glyphosate is the primary ingredient in Monsanto's popular herbicide, Roundup. Many crops have been genetically engineered to be resistant to it. The chemical is only absorbed by the leaves of plants and it is not absorbed by roots from the soil.


Contents

Chemistry

Glyphosate is an aminophosphonic analogue of the natural amino acid glycine and the name is a contraction of glycine, phospho-, and -ate.

Biochemistry

Glyphosate kills plants by inhibiting the enzyme 5-enolpyruvoyl-shikimate-3-phosphate synthase (EPSPS), which forms the aromatic amino acids: phenylalanine, tyrosine and tryptophan. EPSPS catalyzes the reaction of shikimate 3-phosphate (S3P) and phosphoenolpyruvate (PEP) to form ESP and phosphate. The aromatic amino acids are also used to make secondary metabolites such as folates, ubiquinones and naphthoquinones. The shikimate pathway is not present in animals. CAS: 1071-83-6

Environmental science

Glyphosate is one of the most toxic herbicides, with many species of wild plants being damaged or killed by applications of less than 10 micrograms per plant. Glyphosate can be more damaging to wild flora than many other herbicides, as aerial spraying with glyphosate can give average drifts of 1200 to 2500 feet and ground spraying with glyphosate may cause damage to sensitive plants up to 300 feet from the field sprayed. Glyphosate use is thought to affect hedgerow trees, causing die-back, and may reduce trees' winter hardiness and resistance to fungal disease

The direct toxicity of glyphosate to mammals and birds is low. However, its effect on flora can have a damaging effect on mammals and birds through habitat destruction. The US EPA concluded that many endangered species of plants, as well as the Houston toad, may be at risk from glyphosate use.

Fish and invertebrates are more sensitive to formulations of glyphosate. As with humans, the surfactants are responsible for much of the harm . Toxicity is increased with higher water temperatures, and pH. In Australia, guidelines state that most formulations of glyphosate should not be used in or near water because of their toxic effects on tadpoles and adult frogs. The newer, non-irritant formulations such as Roundup Biactive are not included in this advice.

Of nine herbicides tested for their toxicity to soil microorganisms, glyphosate was found to be the second most toxic to a range of bacteria, fungi, actinomycetes and yeasts. However, when glyphosate comes into contact with the soil it rapidly binds to soil particles and is inactivated. Unbound glyphosate is degraded by bacteria. Low activity because of binding to soil particles suggests that glyphosate's effects on soil flora will be limited. However, some recent work shows that glyphosate can be readily released from certain types of soil particles, and therefore may leach into water or be taken up by plants. Low glyphosate concentrations can be found in many creeks and rivers in U.S. and Europe.

Health concerns

There are concerns about the effects of glyphosate (and Roundup) on non-plant species. For more information, see the Roundup article.

Endocrine disruptor debate

In-vitro studies (Walsh, et al 2000) have shown glyphosate to have an effect on progesterone production in mammalian cells and can affect mortality of placental cells in-vitro (Richard, et al 2005). Whether these studies classify glyphosate as an endocrine disruptor is a matter of debate.

Some feel that in-vitro studies are insufficient, and are waiting to see if animal studies show a change in endocrine activity, since a change in a single cell line may not occur in an entire organism. Additionally, current in-vitro studies expose cell lines to concentrations orders of magnitude greater than would be found in real conditions, and through pathways that would not be experienced in real organism. Current toxicological studies of higher order mammals (EU 2002) have suggested no endocrine disruption even at high doses.

Others feel that in-vitro studies, particularly ones identifying not only an effect, but a chemical pathway, are sufficient evidence to classify glyphosate as an endocrine disruptor, on the basis that even small changes in endocrine activity can have lasting effects on an entire organism that may be difficult to detect through whole organism studies alone. Further research on the topic has been planned, and should shed more light on the debate.

Glyphosate resistance

Some microorganisms have a version of 5-enolpyruvoyl-shikimate-3-phosphate synthetase (EPSPS) that is resistant to glyphosate inhibition. The version used in genetically modified crops was isolated from Agrobacterium strain CP4 (CP4 EPSPS) that was resisitant to glyphosate. The CP4 EPSPS gene was cloned and inserted into soybeans. The CP4 EPSPS gene was engineered for plant expression by fusing the 5' end of the gene to a chloroplast transit peptide derived from the petunia EPSPS. This transit peptide was used because it had shown previously an ability to deliver bacterial EPSPS to the chloroplasts of other plants. The plasmid used to move the gene into soybeans was PV-GMGTO4. It contained three bacterial genes, two PC4 EPSPS genes, and a gene encoding beta-glucuronidase (GUS) from Escherichia coli as a marker. The DNA was injected into the soybeans using the particle acceleration method. Soybean cultivar A54O3 was used for the transformation. The expression of the GUS gene was used as the initial evidence of transformation. GUS expression was detected by a staining method in which the GUS enzyme converts a substrate into a blue precipitate. Those plants that showed GUS expression were then taken and sprayed with glyphosate and their tolerance was tested over many generations.

Genetically modified crops

In 1996, genetically modified soybeans were available commercially([1]). This greatly improved the ability to control weeds in soybean fields since glyphosate could be sprayed on fields without hurting the crop. As of 2004, glyphosate was used on 80% of U.S. soybean fields to eliminate weeds.

Tradenames

It was first sold by Monsanto under the tradename Roundup but is no longer under patent so is now marketed under various names (for example TOP UP48 in Thailand).

Other uses

Glyphosate is one of a number of herbicides used by the United States government to spray Colombian coca fields through Plan Colombia. Its health effects, effects on legal crops, and effectiveness in fighting the war on drugs have been widely disputed.

References

Categories


Herbicides | Organic compounds | Phosphorus compounds

Find

Find

Find