Practice question

Original Golden Rice event employed phytoene synthase gene cloned from Narcissus pseudonarcissus, common daffodil, a monocot bulbous geophyte storing high carotenoid levels in chromoplasts of perianth flowers bright yellow. Daffodil psy cDNA was available, well characterized, and known to encode plastid-localized enzyme with high activity converting GGPP to phytoene. Its coding sequence fused to endosperm-specific glutelin promoter Gt1 and pea rbcS transit peptide sequence MSMA to direct import into plastids via Toc-Tic translocon machinery. Expression in rice endosperm complemented missing endogenous activity, initiating flux toward carotenoids. Later analysis of Golden Rice 1 showed psy transcript level limiting, with phytoene accumulation modest. Syngenta improved design replacing daffodil psy with Zea mays maize psy1 under same promoter, achieving 23-fold higher total carotenoids and preferential beta-carotene accumulation because maize enzyme better codon optimized for monocot translation, higher catalytic efficiency and association with metabolon channeling substrate. Daffodil origin highlights cross-species utility of orthologous enzymes from ornamental plants for cereal biofortification using conserved plastid pathways.