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 Carotenoid biosynthesis; phytoene biosynthesis; all-trans-phytoene from geranylgeranyl diphosphate: step 1/1. Monomer. Forms punctuate spots in plastid stroma. Expressed in embryos, endosperm and seedling leaves (PubMed:8722797). Expressed in leaves and endosperm (PubMed:8722797). Belongs to the phytoene/squalene synthase family.