Chapter 16: MOA Part 2: Amino Acid Synthesis Inhibitors & Nitrogen Metabolism Inhibitors
16.7 Herbicides that Inhibit EPSP Synthase
The following animation visually depicts the main part of the shikimate pathway we are most interested in. It also shows how the herbicide, glyphosate, interferes with the pathway. You can either click through the animation on your own and/or watch the video clip of Dr. Namuth-Covert explaining the animation.
Please note the animation requires Flash player. If you are utilizing a mobile device, we recommend viewing the video.
Access the EPSP Synthase Animation Here: http://passel.unl.edu/pages/animation.php?a=EnzymeAction.swf
Here is a time lapse video of susceptible plant response to these herbicides:
For the full list of Ohio State University’s Weed Science herbicide time lapse videos, go to: https://www.youtube.com/playlist?list=PLrQLElJHkjuh65RjQZ8nWDBh0Da_MekB0
Temporary Yellow Flash in Soybeans
When you are spraying Roundup Ready soybeans with glyphosate, you might see what is called a “yellow flash”. This is just a temporary yellowing of the soybean leaves sometimes seen a week after application when the weather is warm and humid.
To read more, visit this article provided by North Carolina Cooperative Extension: “Yellow Flash” in Soybeans
Review and Reflection
Interesting Glyphosate Facts
Glyphosate, one of the most successful herbicides ever discovered, inhibits the production of aromatic amino acids: phenylalanine, tyrosine, and tryptophan. Glyphosate is a potent inhibitor of 5-enolpyruvylshikimate-3-phosphate synthase (often referred to as EPSP synthase or EPSPS).
The chemical structures of PEP (from the shikimate pathway) and glyphosate are very similar. As a result, glyphosate acts as a competitive inhibitor of PEP and binds more tightly to the EPSP synthase-S3P complex than does the normal substrate PEP. However, like PEP, glyphosate has no affinity for the enzyme alone. A major difference between glyphosate and PEP is that the dissociation rate for glyphosate is 2,300 times slower than PEP. Therefore, once glyphosate binds the enzyme-substrate complex (EPSP synthase-S3P) the enzyme is essentially inactivated.
Glyphosate can also be considered an uncompetitive inhibitor of EPSP synthase with respect to S-3-P. There are other factors that contribute to glyphosate’s herbicidal activity. The shikimate pathway is normally controlled by a process called feedback inhibition. In the shikimate pathway, arogenate (a product of the pathway) is a potent inhibitor of the first enzyme in the shikimate pathway, 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (DAHP synthase). Inhibition of EPSP synthase by glyphosate results in the decreased levels of arogenate causing the deregulation of the shikimate pathway due to increased DAHP synthase activity.
Deregulating the shikimate pathway results in the accumulation of very high levels of shikimate and S3P and in some plant species this accumulation can account for up to 16% of plant dry weight in sink tissues. Important building blocks for other metabolic pathways are reduced by uncontrolled carbon flow through the shikimate pathway and reduced levels of aromatic amino acids causes significant reductions in protein synthesis.
Organophosphorus (Group 9)
** Glyphosate Technical Fact Sheet. National Pesticide Information Center. http://npic.orst.edu/factsheets/archive/glyphotech.html
Review and Reflection