Ethylene is one of the five major types of plant hormones and affects the aging and development of plants. The ethylene gas ripens plants and triggers programmed cell death. And because ethylene is a gas, it spreads from fruit to fruit (so once one fruit goes ripe the rest start to follow suit). During the ripening process, enzymes are produced to break down cell walls (which makes the fruit softer) and starches and acids are converted to sugars to make the fruit more sweet.
Fruits are not the only ones that produce ethylene- trees do as well. This aging process takes place in autumn and is marked by the characteristic leaf falls and the change of color. This is actually an adaptation- when trees lose their leaves, they do not lose as much water due to evaporation and, because it is difficult to get water from the frozen ground, this is very helpful. Environmental stimuli, such as shorter days and cooler temperature, change the levels of ethylene and auxin hormones in the leaf. Auxin hormones work to prevent an abscission layer (the separation from the base of the leaf stalk from the stem) while ethylene promotes the formation of an abscission layer. As the leaf ages more, the level of ethylene increase while the levels of auxin decrease until the abscission layer separates the leaf from the tree.