Carotenoids are ubiquitous isoprenoid pigments that are synthesized by archaea, bacteria, fungi, algae and higher plants: in these organisms, they have fundamental ecological, biochemical and physiological functions. Carotenoids also play a crucial role in human health by improving normal vision and by preventing degenerative diseases and cancer. Mammals do not produce carotenoids and therefore need to assume them through the diet. Most major crops (including rice, wheat and potato) contain little α- and β-carotene, which are the main carotenoids with provitamin A activity. For this reason, during the last 10 years, increasing carotenoid (especially β-carotene) levels in major crops through genetic transformation has been one of the main goals of plant metabolic engineering. This review will focus on past and recent advances in this field, with particular attention to the modification of carotenoid biosynthesis in rice, tomato and potato. 'Golden Rice' is probably the most widely known result of metabolic engineering; in tomato, expression of carotenoid genes, both of bacterial and plant origin, resulted in an increase of β-carotene and/or total xanthophylls in transgenic fruits. In the past 3 years several metabolic engineering efforts have also focused on potato, resulting in the obtainment of 'golden tubers', with the highest β-carotene content reported for the four major crops. © CABI Publishing 2007.

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