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. Belongs to the cytochrome P450 family.