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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Why Is Golden Rice Golden (Yellow) Instead of Red?^1,[w]

Center for Applied Biosciences, University of Freiburg, D–79104 Freiburg, Germany (P.S., S.A.-B., P.B.);and Syngenta, Jealott's Hill International Research Centre, Bracknell, Berkshire RG42 6EY, United Kingdom (R.D.)

The endosperm of Golden Rice (Oryza sativa) is yellow due to the accumulation of -carotene (provitamin A) and xanthophylls. The product of the two carotenoid biosynthesis transgenes used in Golden Rice, phytoene synthase (PSY) and the bacterial carotene desaturase (CRTI), is lycopene, which has a red color. The absence of lycopene in Golden Rice shows that the pathway proceeds beyond the transgenic end point and thus that the endogenous pathway must also be acting. By using TaqMan real-time PCR, we show in wild-type rice endosperm the mRNA expression of the relevant carotenoid biosynthetic enzymes encoding phytoene desaturase, -carotene desaturase, carotene cis-trans-isomerase, -lycopene cyclase, and -carotene hydroxylase; only PSY mRNA was virtually absent. We show that the transgenic phenotype is not due to up-regulation of expression of the endogenous rice pathway in response to the transgenes, as was suggested to be the case in tomato (Lycopersicon esculentum) fruit, where CRTI expression resulted in a similar carotenoid phenomenon. This means that -carotene and xanthophyll formation in Golden Rice relies on the activity of constitutively expressed intrinsic rice genes (carotene cis-trans-isomerase, /-lycopene cyclase, -carotene hydroxylase). PSY needs to be supplemented and the need for the CrtI transgene in Golden Rice is presumably due to insufficient activity of the phytoene desaturase and/or -carotene desaturase enzyme in endosperm. The effect of CRTI expression was also investigated in leaves of transgenic rice and Arabidopsis (Arabidopsis thaliana). Here, again, the mRNA levels of intrinsic carotenogenic enzymes remained unaffected; nevertheless, the carotenoid pattern changed, showing a decrease in lutein, while the -carotene-derived xanthophylls increased. This shift correlated with CRTI-expression and is most likely governed at the enzyme level by lycopene-cis-trans-isomerism. Possible implications are discussed.

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Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.057927.

^* Corresponding author; e-mail peter.beyer{at}biologie.uni-freiburg.de; fax 49–761–203–2675.

Received December 9, 2004; returned for revision February 15, 2005; accepted February 15, 2005.

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