Catalyzes the conversion of geranylgeranyl diphosphate to phytoene. Mediates the first committed step in carotenoid biosynthesis. 2 (2E,6E,10E)-geranylgeranyl diphosphate = 15-cis-phytoene + 2 diphosphate Expressed in leaves and endosperm. Belongs to the phytoene/squalene synthase family.