Involved in the biosynthesis of germacrene C, one of the most abundant sesquiterpene in the leaf oil of tomato (PubMed:11090225). Produces mainly germacrene C, but also smaller amounts of germacrene A, B and D when used with farnesyl diphosphate (FPP) as substrate; able to use both (2E,6E)-farnesyl diphosphate ((EE)-FPP) and (2Z,6Z)-farnesyl diphosphate ((ZZ)-FPP) (PubMed:9482865, PubMed:21818683). No or low activity with geranylgeranyl diphosphate (GGPP) (PubMed:9482865). Can act with a low efficiency as a monoterpene synthase with geranyl diphosphate (GPP) as substrate, thus producing beta-myrcene, limonene and terpinolene (PubMed:21818683, PubMed:9482865). (2E,6E)-farnesyl diphosphate = diphosphate + germacrene C (2E)-geranyl diphosphate = diphosphate + terpinolene (2E)-geranyl diphosphate = diphosphate + limonene (2E)-geranyl diphosphate = beta-myrcene + diphosphate (2Z,6Z)-farnesyl diphosphate = diphosphate + germacrene C Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Mostly expressed in stem and trichomes, to a lower extent in roots, leaves and flowers and, at low levels, in fruits. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsa subfamily. It is unclear whether AAC39431 and AAC39432 represent different alleles or different products derived from two germacrene C synthase loci.