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. Monomer (By similarity). Interacts with OR (PubMed:26224804, PubMed:25675505). Interacts with ORLIKE (PubMed:25675505). A number of isoforms are produced. According to EST sequences. Belongs to the phytoene/squalene synthase family.