Catalyzes the conversion of geranylgeranyl diphosphate to phytoene. Mediates the first committed step in carotenoid biosynthesis. May play a role in regulating carotenoid flux in response to abiotic stress in roots. May control flux to carotenoid precursors that are required for abiotic stress-induced abscisic acid (ABA) formation in roots. 2 (2E,6E,10E)-geranylgeranyl diphosphate = 15-cis-phytoene + 2 diphosphate Expressed in roots and endosperm. Induced by drought stress, salt stress and abscisic acid (ABA) in roots. Belongs to the phytoene/squalene synthase family.