Another classic response to auxin or auxinic herbicides is the induction of ethylene production (Figure 12). The ’Ethylene synthesis’ animation follows the major steps in its synthesis.

Fig. 12: Pathway for ethylene biosynthesis

Animation 4: Ethylene Synthesis

Ethylene synthesis begins with methionine and ATP combining to produce S-adenosyl-methionine (SAM) as catalyzed by SAM synthetase, which is also known as AdoMet synthetase. SAM is then converted to 1-aminocyclopropane-1-carboxylic acid (ACC) as catalyzed by ACC synthase. ACC synthase is induced by several stresses including flooding, wounding, IAA and auxinic herbicides (Figure 13).

Fig. 13: Stresses induce ACC synthase activity

The final step for ethylene production is catalyzed by ACC oxidase which produces ethylene, hydrogen cyanide and carbon dioxide from ACC (Figure 12). Depending on the plant species, ethylene induced by auxin or auxinic herbicides may or may not play a role in subsequent plant death. The animation ’Overall Picture of Auxinic Herbicide Action’ illustrates how ethylene may be involved in plant death.

Animation 3: Overall Picture of Auxinic Herbicide Action