Involved in the biosynthesis of germacrene-derived sesquiterpene lactones (PubMed:20351109). Catalyzes three consecutive oxidations of germacrene A to produce germacrene A acid (PubMed:20351109). Could also catalyze the three-step oxidation of non-natural substrate amorphadiene to artemisinic acid (PubMed:20351109). (+)-(R)-germacrene A + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = germacra-1(10),4,11(13)-trien-12-oate + 4 H(+) + 4 H2O + 3 oxidized [NADPH--hemoprotein reductase] Secondary metabolite biosynthesis; terpenoid biosynthesis. Expressed in leaf primordia. In developing leaves, expressed at very low levels in young leaf primordia, but strongly induced after the fourth and fifth days following leaf primordia initiation. Belongs to the cytochrome P450 family.