Catalyzes the reaction from prephytoene diphosphate to phytoene. 2 (2E,6E,10E)-geranylgeranyl diphosphate = 15-cis-phytoene + 2 diphosphate Carotenoid biosynthesis; phytoene biosynthesis; all-trans-phytoene from geranylgeranyl diphosphate: step 1/1. Belongs to the phytoene/squalene synthase family.