Practice question

Golden Rice is paradigm of metabolic engineering for nutritional biofortification to combat vitamin A deficiency blindness and mortality affecting 250 million children in Asia and Africa dependent on polished rice lacking provitamin A. Endosperm of Oryza sativa does not accumulate carotenoids because pathway blocked downstream of geranylgeranyl diphosphate due to absent psy expression and limited desaturase activity. Ingo Potrykus and Peter Beyer introduced minimal carotenogenic pathway: phytoene synthase condensing two GGPP to 15-cis-phytoene, phytoene desaturase performing four desaturations to all-trans lycopene, and lycopene beta-cyclase forming beta-carotene yellow pigment localized in amyloplasts as crystals requiring plastid transit peptide. Second-generation Golden Rice 2 using maize phytoene synthase under glutelin promoter accumulates up to 37 microgram per gram dry weight beta-carotene, turning endosperm golden. Conversion by intestinal BCMO1 cleavage yields retinol delivering estimated 30-50 percent RDA from 130 g rice per day, demonstrating how plant engineering can deliver essential micronutrients lacking in staple without agronomic penalty or yield loss.