Chapter 5: Golden Rice 2

ABSTRACT:  Golden Rice 1 was developed to help control vitamin A deficiency (VAD). To construct Golden Rice 2, the phytoene synthetase gene ( psy ) from maize and the carotene desaturase gene ( crtl ) from Erwinia uredovora were inserted into rice. Evaluation of phytoene synthase (the rate limiting step in carotenoid biosynthesis) from several plant sources led to the identification of the psy gene from maize as the most efficacious source, resulting in the greatest accumulation of total carotenoids and ‐carotene. Golden Rice 1 contains about 1.6 g of total carotenoids per gram of dry weight of grain. Golden Rice 2 contains as much as 37 g total carotenoids per gram of dry weight of grain, of which 31 g/g is ‐carotene. While the quantity of ‐carotene is high, its bioavailability is unknown. Golden Rice 2 was developed with the expectation that it could make a major contribution to the vitamin A requirement. It is conservatively estimated that a breastfed 1‐ to 2‐y‐old child could derive 60% of the of U.S. recommended dietary allowance (RDA) from the consumption of approximately 70 g of uncooked Golden Rice 2; an average serving size for a child this age in Thailand, for example, is 160 g. The higher level of ‐carotene raises the possibility that Golden Rice 2 may help reduce many of the deaths attributed to VAD (). Biofortified rice with high levels of ‐carotene is in an early stage of development. The published data provide a description of the DNA construct introduced into rice and report the concentrations of both total carotenoids as well as that of the 5 major carotenoids present in representative transgenic rice plants. The safety assessment of Golden Rice 2 follows generally accepted international guidelines. Beta‐carotene biosynthesis is a peripheral biosynthetic pathway that diverts very small amounts of biosynthetic precursors from plant metabolism and would, thus, not be expected to produce major changes in composition. Hence, the safety assessment can be targeted to the impact of the insertion on cellular carotenoids and other metabolically related compounds. Plant phenotype, seed weight, and germination were not affected by the presence of the genetic modification. Apart from safety concerns, there exists an urgent need for studies to evaluate the efficacy of this product as a bioavailable source of ‐carotene in animals and humans. Proof of efficacy would allow Golden Rice 2 to be widely distributed. Appropriate safety assessments are needed and should at least include (1) characterization of the inserted DNA,(2) determination of the composition according to OECD consensus documents,(3) analysis of the carotenoid metabolite pool, and (4) evaluation of the efficacy of this product as a source of ‐carotene (although it is not related to safety). The safety of carotene desaturase gene ( crtl ) and its products from E. uredovora requires characterization because it has no history of safe use in foods.