Involved in the biosynthesis of leucosceptrane sesterterpenoids natural products, which are playing defensive roles toward herbivorus insects (e.g. Spodoptera exigua) (PubMed:26941091). Catalyzes the condensation of isopentenyl pyrophosphate (IDP) with the allylic pyrophosphates to yield geranylfarnesyl diphosphate (GFDP), the C(25) prenyl diphosphate precursor to all sesterterpenoids (PubMed:26941091). Geranylgeranyl diphosphate (GGPP) is the preferred substrate, however dimethylallyl diphosphate (DMADP), farnesyl diphosphate (FDP) and geranyl diphosphate (GDP) can also be used as allylic substrate (PubMed:26941091). (2E,6E,10E)-geranylgeranyl diphosphate + isopentenyl diphosphate = (2E,6E,10E,14E)-geranylfarnesyl diphosphate + diphosphate (2E,6E)-farnesyl diphosphate + 2 isopentenyl diphosphate = (2E,6E,10E,14E)-geranylfarnesyl diphosphate + 2 diphosphate (2E)-geranyl diphosphate + 3 isopentenyl diphosphate = (2E,6E,10E,14E)-geranylfarnesyl diphosphate + 3 diphosphate dimethylallyl diphosphate + 4 isopentenyl diphosphate = (2E,6E,10E,14E)-geranylfarnesyl diphosphate + 4 diphosphate Binds 2 Mg(2+) ions per subunit. kcat is 16.9x10(-6) sec(-1) with dimethylallyl diphosphate as substrate (in the presence of isopentenyl diphosphate) (PubMed:26941091). kcat is 8.2x10(-6) sec(-1) with isopentenyl diphosphate as substrate (in the presence of geranylgeranyl diphosphate) (PubMed:26941091). kcat is 6.16x10(-6) sec(-1) with geranyl diphosphate as substrate (in the presence of isopentenyl diphosphate) (PubMed:26941091). kcat is 7.19x10(-6) sec(-1) with farnesyl diphosphate as substrate (in the presence of isopentenyl diphosphate) (PubMed:26941091). kcat is 5.17x10(-6) sec(-1) with geranylgeranyl diphosphate as substrate (in the presence of isopentenyl diphosphate) (PubMed:26941091). Secondary metabolite biosynthesis; terpenoid biosynthesis. Isoprenoid biosynthesis. Monomer. Strongly expressed in glandular trichomes, and, at low levels, in leaves, stems and flowers. Induced by jasmonic acid, thus triggering sesterterpenoids accumulation and reducing feeding and growth of the herbivorus insect Spodoptera exigua (PubMed:26941091). Accumulates transiently after salicylic acid treatment (PubMed:26941091). Belongs to the FPP/GGPP synthase family.