Transgenic rice (Oryza sativa) endosperm expressing daffodil (Narcissus pseudonarcissus) phytoene synthase accumulates phytoene, a key intermediate of provitamin A biosynthesis

Rice ( Oryza sativa L.), the major food staple for more than two billion people, contains neither β-carotene (provitamin A) nor C₄₀ carotenoid precursors thereof in its endosperm. To improve the nutritional value of rice, genetic engineering was chosen as a means to introduce the ability to make β-carotene into rice endosperm tissue. Investigation of the biochemical properties of immature rice endosperm using ¹⁴C]-labelled substrates revealed the presence of geranyl geranyl diphosphate, the C₂₀ general isoprenoid precursor necessary for C₄₀ carotenoid biosynthesis. Phytoene synthase, which condenses two molecules of geranyl geranyl diphosphate, is the first of four specific enzymes necessary for β-carotene biosynthesis in plants. Therefore, the Japonica rice model variety Taipei 309 was transformed by microprojectile bombardment with a cDNA coding for phytoene synthase from daffodil ( Narcissus pseudonarcissus ) under the control of either a constitutive or an endosperm-specific promoter. In transgenic rice plants, the daffodil enzyme is active, as measured by the in vivo accumulation of phytoene in rice endosperm. Thus, it is demonstrated for the first time that it is in principle possible to engineer a critical step in provitamin A biosynthesis in a non-photosynthetic, carotenoid-lacking plant tissue. These results have important implications for longterm prospects of overcoming worldwide vitamin A deficiency.