Catalyzes the sequential condensation of isopentenyl pyrophosphate with the allylic pyrophosphates, dimethylallyl pyrophosphate, and then with the resultant geranylpyrophosphate to the ultimate product farnesyl pyrophosphate (By similarity). Component of the triterpene saponins (e.g. ginsenosides or panaxosides) and phytosterols biosynthetic pathways (PubMed:24933610, PubMed:27746309, PubMed:29378087). Promotes the accumulation of ginsenosides (PubMed:24933610). dimethylallyl diphosphate + isopentenyl diphosphate = (2E)-geranyl diphosphate + diphosphate (2E)-geranyl diphosphate + isopentenyl diphosphate = (2E,6E)-farnesyl diphosphate + diphosphate Binds 2 Mg(2+) ions per subunit. Stimulated by methyl jasmonate (MeJA). Isoprenoid biosynthesis; farnesyl diphosphate biosynthesis; farnesyl diphosphate from geranyl diphosphate and isopentenyl diphosphate: step 1/1. Isoprenoid biosynthesis; geranyl diphosphate biosynthesis; geranyl diphosphate from dimethylallyl diphosphate and isopentenyl diphosphate: step 1/1. Homodimer. Mostly expressed in roots and seeds, and to a lower extent, in leaves and stems. Rapid decrease in roots and leaves from the leaf opened to the green fruit stage. Induced by methyl jasmonate (MeJA) in hairy roots (Ref.1, Ref.2). Induced by A.niger mycelium-derived elicitor, thus improving ginsenosides production in adventitious roots culture (PubMed:27746309). Triggered by salicylic acid and yeast extract (Ref.4). Influenced in roots by relative humidity and rain, and in leaves by rain (PubMed:30577538). Belongs to the FPP/GGPP synthase family.