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 billio n people, contains neither beta-carotene (provitamin A) nor C-40 carot enoid 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 beta-carotene into rice endosperm tissue. Investig ation of the biochemical properties of immature rice endosperm using [ C-14]-labelled substrates revealed the presence of geranyl geranyl dip hosphate, the C-20 general isoprenoid precursor necessary for C-40 car otenoid biosynthesis. Phytoene synthase, which condenses two molecules of geranyl geranyl diphosphate, is the first of four specific enzymes necessary for beta-carotene biosynthesis in plants. Therefore, the Ja ponica rice model variety Taipei 309 was transformed by microprojectil e 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 daff odil enzyme is active, as measured by the in vivo accumulation of phyt oene in rice endosperm. Thus, it is demonstrated for the first time th at it is in principle possible to engineer a critical step in provitam in A biosynthesis in a non-photosynthetic, carotenoid-lacking plant ti ssue. These results have important implications for longterm prospects of overcoming worldwide vitamin A deficiency.