Mode of action of auxin herbicides: a new ending to a long, drawn out story

Abstract

In the more than fifty years since their initial discovery, synthetic compounds possessing phytohormonal auxin activity have been among the most successful herbicides and bioregulators used in agriculture 1. Cobb A. Cobb A. Auxin-type herbicides. In Herbicides and Plant Physiology. Chapman & Hall, 1992: 82-106 Google Scholar , 2. Devine M. Physiology of Herbicide Action. Prentice Hall, 1993 Google Scholar , 3. Sterling T.M. Hall J.C. Mechanism of action of natural auxins and the auxinic herbicides. in: Roe R.M. Herbicide Activity: Toxicology, Biochemistry and Molecular Biology. IOS Press, Amsterdam1997: 111-141 Google Scholar , 4. Grossmann K. Quinclorac belongs to a new class of highly selective auxin herbicides. Weed Sci. 1998; 46: 707-716 Google Scholar . With their ability to control dicot weeds in cereal crops, these so-called synthetic auxins were among the first selective organic herbicides to be developed for agricultural use. They have revolutionized modern crop production. Various chemical classes of auxin bioregulators and herbicides with different weed spectra and types of selectivity have been synthesized and commercially introduced over the years. They include indole acids [derivatives of indole-3-acetic acid (IAA)], naphthalene acids, phenoxycarboxylic acids, benzoic acids, pyridine carboxylic acids and a relatively new category, the quinolinecarboxylic acid-type, exemplified by quinmerac and quinclorac (Fig. 1). Unlike other auxin herbicides, quinclorac has herbicidal activity on grass weeds 4. Grossmann K. Quinclorac belongs to a new class of highly selective auxin herbicides. Weed Sci. 1998; 46: 707-716 Google Scholar . In spite of extensive research following F.A. Gilbert's statement that auxin herbicides cause susceptible plants ‘to grow themselves to death’ 5. Gilbert F.A. The status of plant-growth substances and herbicides. Chem. Rev. 1946; 39: 199-218 Crossref PubMed Scopus (9) Google Scholar , until recently auxin herbicides had resisted all efforts to elucidate the details of their molecular interactions and the biochemical basis of their phytotoxicity.