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 (PubMed:15247400, PubMed:18326788). Highly expressed in developing leaves. Expressed at low levels in roots (PubMed:18326788). Induced by salt stress and abscisic acid (ABA) in roots. Belongs to the phytoene/squalene synthase family.