Involved in the biosynthesis of jasmonic acid, a growth regulator that is implicated also as a signaling molecule in plant defense. Acts on a number of unsaturated fatty-acid hydroperoxides, forming the corresponding allene oxides. (13S)-hydroperoxy-(9Z,11E,15Z)-octadecatrienoate = (9Z,13S,15Z)-12,13-epoxyoctadeca-9,11,15-trienoate + H2O Lipid metabolism; oxylipin biosynthesis. Belongs to the cytochrome P450 family.