While researchers originally believed abscisic acid to be the hormone that stimulates abscission (for which the hormone was named), it was later proven that it does not play a primary role. In fact, auxin, a plant hormone, and ethylene have been implicated as prominent regulators of abscission signaling. The two compounds work in a synergistic fashion: As the auxin levels decrease, the flux of auxin to the abscission zone is reduced. Exhaustion of auxin makes the abscission zone sensitive to ethylene. When the plant is then exposed to ethylene, gene expression of cell wall-degrading enzymes such as cellulase and polygalacturonase are activated. However, this is not to say that ethylene directly activates WDE gene expression, because the elements responsible for detecting ethylene have not been found in the gene's promoter region.