JXB Advance Access published online on February 17, 2006
Journal of Experimental Botany, doi:10.1093/jxb/erj086
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1 Albert-Ludwigs University of Freiburg, Center for Applied Biosciences, Schaenzlestr. 1, D-79104 Freiburg, Germany
* To whom correspondence should be addressed. To increase the
Received August 11, 2005
Accepted December 5, 2005
RESEARCH PAPER
Exploring the potential of the bacterial carotene desaturase CrtI to increase the
Salim Al-Babili 1 *,
Tran Thi Cuc Hoa 2,
and
Patrick Schaub 1
-carotene content in Golden Rice
2 Cuu Long Delta Rice Research Institute, Omon, Cantho, Vietnam
Salim Al-Babili, E-mail: salim.albabili{at}biologie.uni-freiburg.de
Abstract 
-carotene (provitamin A) content and thus the nutritional value of Golden Rice, the optimization of the enzymes employed, phytoene synthase (PSY) and the Erwinia uredovora carotene desaturase (CrtI), must be considered. CrtI was chosen for this study because this bacterial enzyme, unlike phytoene synthase, was expressed at barely detectable levels in the endosperm of the Golden Rice events investigated. The low protein amounts observed may be caused by either weak cauliflower mosaic virus 35S promoter activity in the endosperm or by inappropriate codon usage. The protein level of CrtI was increased to explore its potential for enhancing the flux of metabolites through the pathway. For this purpose, a synthetic CrtI gene with a codon usage matching that of rice storage proteins was generated. Rice plants were transformed to express the synthetic gene under the control of the endosperm-specific glutelin B1 promoter. In addition, transgenic plants expressing the original bacterial gene were generated, but the endosperm-specific glutelin B1 promoter was employed instead of the cauliflower mosaic virus 35S promoter. Independent of codon optimization, the use of the endosperm-specific promoter resulted in a large increase in bacterial desaturase production in the T1 rice grains. However, this did not lead to a significant increase in the carotenoid content, suggesting that the bacterial enzyme is sufficiently active in rice endosperm even at very low levels and is not rate-limiting. The endosperm-specific expression of CrtI did not affect the carotenoid pattern in the leaves, which was observed upon its constitutive expression. Therefore, tissue-specific expression of CrtI represents the better option.
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