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How Roundup® Agricultural Herbicide Works
The active ingredient in Roundup® agricultural herbicides, glyphosate, is the most studied herbicide molecule in history.
From the first test of the compound in 1970, it was apparent that glyphosate stopped plant growth at the meristems – the growing tips of the plant both below and above the ground. That’s what made it a great herbicide for perennial weeds as well as annual weeds. In 1970, however, it wasn’t clear exactly how glyphosate accomplished its task.
Monsanto scientist Dr. Ernie Jaworski began the search for “how glyphosate works” by looking for compounds that could help plants survive an application of glyphosate.
In 1972 Dr. Jaworski demonstrated that glyphosate’s effect could be lessened by applying a mixture of several aromatic amino acids to plant tissue treated with glyphosate. From those experiments they concluded that glyphosate must interfere with the plant’s ability to make these aromatic amino acids, which are required for the production of proteins and other compounds plants need for growth.
The next step was to examine the biochemical pathway by which plants make the aromatic amino acids to find out how glyphosate stops it from functioning. Many researchers within and outside of Monsanto focused on the “shikimic acid pathway,” a series of molecular-level biochemical events leading to production of aromatic amino acids. This pathway is most active in the plant tissue in the growing tips of roots and plant parts above the soil – the meristematic tissue.
Further experiments showed that glyphosate inhibits a key enzyme in the shikimic acid pathway known as 5-enolpyruvoylshikimate-3-phosphate synthase (EPSPS). Enzymes are molecules that speed up chemical reactions.
The glyphosate molecule fits into a part of the EPSPS enzyme molecule and keeps it from binding with its intended substrate - a molecule in the shikimic acid pathway. Substrates, the reactants that change when bound to the enzyme, must fit like a lock and key, so only certain substrates fit inside certain enzymes.
When the EPSPS enzyme is prevented from binding with its substrate, the shikimic biosynthetic pathway can’t function properly and the aromatic amino acids are not produced. And, without these aromatic amino acids, the proteins required for plant growth cannot be made and the plant dies.
The EPSPS enzyme is present in all plants, bacteria and fungi, but NOT in mammals, which do not make their own aromatic amino acids. This negligible animal toxicity is one of the most significant properties of glyphosate.
When a droplet of a Roundup® agricultural herbicide lands on a leaf surface of a growing plant, the active ingredient, glyphosate, is absorbed through the leaf’s surface and into the interior cells of the leaf. Some of the glyphosate translocates (moves) within phloem tissue from the leaves to the growing tips of the roots and shoots of the plant – the apical meristems.
It is glyphosate’s ability to translocate to the growing root and shoot tissues - where it shuts down growth by the biochemical mechanism outlined above - that makes it useful for killing not only annual weeds, but perennial weeds as well.
A more complete look at glyphosate’s environmental profile may be found at http://www.monsanto.com/products/techandsafety/roundup_background.asp.
Note: With respect to the time period prior to Sept. 1, 2000, references to Monsanto or the company refer to the agricultural business of Pharmacia Corporation, which was known as Monsanto Company until March 1, 2000.
