Catalyzes the cyclization of trans,trans-farnesyl diphosphate (FPP) to the bicyclic sesquiterpene aristolochene. Produces germacrene A as an enzyme-bound intermediate that is not released by the enzyme, but is further cyclized to produce aristolochene. Aristolochene is the likely parent compound for a number of sesquiterpenoid toxins produced by filamentous fungi. (2E,6E)-farnesyl diphosphate = (+)-aristolochene + diphosphate Binds 3 Mg(2+) ions per monomer. Optimum pH is 8.0. Sesquiterpene biosynthesis; aristolochene biosynthesis; aristolochene from farnesyl diphosphate: step 1/1. Homodimer. Belongs to the terpene synthase family.