Ethylene is derived from the cyclic amino acid, ACC which is itself a derivative of the more common amino acid methionine. This is true for ABA and gibberellins but not ethylene. This is the major use of the gas but it can also be used in the form of ethrel, an ethylene generating liquid sprayed on plants. In general ethylene damages or causes abscission of flowers and flower buds. Its use to promote flowering in pineapple is exceptional.
ABA is an is a terpenoid and contains three isoprene units.
This is true for cytokinins but not gibberellins. Like abscisic acid, gibberellins are derived from the terpenoid biosynthetic pathway. Hormones do not get directly involved in this kind of process. GA is involved in germination itself. Amyloplasts are colorless and light perception would have to involve a pigment.