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Appendix D Herbicide Classifications and Mechanisms of Action Table D-1 contains the herbicide classifications and mechanisms of actions for herbicides discussed in the proceedings. It is based on Appendix A of the National Research Council report The Impact of Genetically Engineered Crops on Farm Sustainability in the United States (2010). Readers can find more information on herbicides, herbicide classifications, and mechanisms of action in that appendix and on the websites of the Weed Science Society of America ( and the Herbicide Resistance Action Committee ( TABLE D-1 Selected Herbicide Classifications and Mechanisms of Action Herbicide Classification Abbreviation or Herbicide Mechanism of Action Example Acetyl COA carboxylase ACCase inhibitors (e.g., Inhibitor of acetyl coenzyme-A inhibitors fluazifop) carboxylase, a pivotal enzyme in plant fatty acid biosynthesis. Acetolactate synthase ALS inhibitors (e.g., Inhibition of the acetolactate inhibitors sulfonylurea and chlorimon) synthase enzyme, resulting in cessation of the biosynthesis of essential branched chain amino acids (leucine, valine, and isoleucine). Enolpyruvyl shikimate-3- Glyphosate Inhibits the EPSP synthase, which phosphate (EPSP) synthase leads to depletion of essential inhibitor aromatic amino acids (tryptophan, tyrosine, and phenylalanine). Hydroxyphenylpyruvate HPPD inhibitors (e.g., Inhibition of dioxygenase inhibitors mesotrione) hydroxyphenylpyruvate dioxygenase, an enzyme involved in the synthesis of plastoquinone and tocopherol (vitamin E). Photosynthesis II inhibitors Symmetric and asymmetric Inhibit photosynthesis by binding triazines (e.g., atrazine) with a specific protein in the photosystem II complex. 53

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54 NATIONAL SUMMIT ON STRATEGIES TO MANAGE HERBICIDE-RESISTANT WEEDS Protoporphyrinogen PPO inhibitors (e.g., lactofen) Inhibits protoporphyrinogen oxidase inhibitors oxidase, which is in the chlorophyll synthesis pathway. The PPO inhibition starts a reaction in the cell that ultimately causes the destruction of cell membranes. The leaking cell membranes rapidly dry and disintegrate. Synthetic auxins Plant growth regulators (e.g., Mimics a plant growth regulator. dicamba and 2,4- The specific mode of action is not dichlorophenoxyacetic acid) well defined; however, these herbicides mimic endogenous auxin (IAA) and appear to negatively affect cell wall plasticity and nucleic acid metabolism.