Mechanism of Action or How Do Plants that are Sensitive Die?

The following description explains the animation ‘Overall Picture of Auxinic Herbicide Action’.

This animation helps illustrate what processes change in a plant cell in response to auxin and how these changes may be involved in plant injury. Once the auxinic herbicide gets into the plant and reaches a living cell, auxinic herbicides are thought to bind to a receptor. This is the same receptor which recognizes the natural hormone, IAA, and this binding acts as a signal for the cell to turn on several genes. The signal that the auxin is present causes a cascade of secondary responses or messages which tell certain genes to turn on. The animation shows several genes being turned on by auxins; some have a known function and others do not. A classic response to auxins is cell elongation. Auxins actually induce a gene which helps loosen the cell-wall region and this loosening causes differential lengthening on one side of a stem compared to another, resulting in characteristic twisting, known as epinasty. Another example of a gene which auxins can turn on is for the enzyme (ACC synthase) that helps synthesize ethylene, another plant hormone. Ethylene can be involved in plant injury because ethylene causes epinasty or bending and twisting as well. Ethylene can also turn on the production of abscisic acid in sensitive plants; abscisic acid is yet another plant hormone and this hormone will close stomata so the plant can not access carbon dioxide for photosynthesis. Another byproduct of the ethylene synthesis pathway is cyanide which injures sensitive grasses. However, some plant species are insensitive to ethylene; therefore, other genes induced by auxinic herbicides, for which no function is yet known, are probably responsible for plant injury. Click on the animation above, ‘Overall Picture of Auxinic Herbicide Action,’ to see potential mechanisms of auxinic herbicide action.