Summary - Auxin and Auxinic Herbicides - Part 2 - Advanced
The auxinic herbicides have been around since World War II and were the first selective herbicides developed. Although they continue to be a very important class of herbicides, their precise mode of action is still unknown. Auxinic herbicides have structural similarities to the natural plant hormone, auxin, and also cause similar physiological effects on plants including increased cell elongation, epinasty, hypertrophy, root initiation and ethylene biosynthesis. Specific proteins in the plasma membrane that recognize auxin and auxinic herbicides include the auxin influx and efflux carriers which help regulate cellular concentrations of this plant hormone. Other receptors on the membrane are thought to bind auxin or auxinic herbicides and this binding initiates a biochemical cascade resulting in many cellular changes including a rapid influx of calcium into the cytoplasm, activation of plasma membrane ATPases, and changes in gene expression. The precise nature of putative auxin receptors and the nature of the signal that initiates the observed cascade of biochemical events remains unknown. It is also unknown why sensitive plants die after exposure to auxinic herbicides, but symptoms include tissue proliferation, vascular system disruption and bending and twisting of tissue. Ethylene is induced in most species in response to auxin or auxinic herbicides and does play a role in plant death in some cases. For example, grass species susceptible to auxinic herbicides are sensitive to the hydrogen cyanide released during synthesis of induced ethylene. Several species are sensitive to ethylene directly and the auxin-induced ethylene is involved in plant death possibly through the induction of abscisic acid which causes stomatal closure resulting in a plant that can no longer access carbon dioxide for photosynthesis; however, other species are insensitive to ethylene with the epinasty caused by the auxinic herbicide being a result of some other mechanism. Identification of plants that are resistant to auxins and/or auxinic herbicides may help us elucidate the exact mechanism of action of auxinic herbicides.