The Molecular Mode of Action of Auxin Herbicides

Auxin mimics or synthetic auxin herbicides (HRAC Group 4) are represented by over 20 compounds utilized in commercial herbicides that cause profound morphological effects on plant growth and development, eventually leading to plant death. This chapter describes breakthrough discoveries in indole acetic acid (IAA) perception and signal transduction in plants that have revolutionized current understanding of the molecular mode of action (MoA) of auxin herbicides and their impact on auxin herbicide discovery. Recent advances in herbicide resistance research and weed genome availability have facilitated a better understanding of how weeds can evolve target-site resistance to auxin herbicides by mutations in the Aux/IAA genes, though metabolic resistance remains the primary mechanism of resistance to these herbicides. Even though scientists have made progress in our understanding of the complex MoA of auxin herbicides, further intriguing questions remain, such as the role of Aux/IAA protein families on downstream auxin-responsive genes, species selectivity mechanisms, chemical/receptor specificity, and mechanisms of resistance at the target site level. Continued research in this fascinating MoA can enable further discoveries of efficacious and sustainable auxin herbicides.