Use of the cauliflower Or gene for improving crop nutritional quality

Carotenoids are a group of pigments that are essential to human diets. An increasing interest in carotenoids as a nutritional source of vitamin A and health-promoting compounds has prompted the recent progress in metabolic engineering of carotenogenesis in food crops. Current strategies have been mainly focused on manipulating genes encoding carotenogeic enzymes. In many cases, it is difficult to reach the desired levels of carotenoid enhancement. In this chapter, we briefly summarize the recent progress on our understanding of carotenoid biosynthesis. We describe the isolation of a novel gene, the Or gene, from a high-beta-carotene orange cauliflower mutant. The Or gene encodes a plastid-targeted protein containing a cysteine-rich zinc finger domain and appears to be plant-specific. The insertion of a copia-like LTR retrotransponson in the Or gene confers high levels of carotenoid accumulation in the normally low-pigmented tissues. Rather than directly regulating carotenoid biosynthesis, the Or gene controls carotenoid accumulation by inducing the formation of chromoplasts, which provide a metabolic sink to sequester and deposit carotenoids. Examination of the Or transgenic potato tubers confirms that the Or-induced carotenoid accumulation is associated with the formation of a metabolic sink. Thus, the Or gene offers a new molecular tool to complement current approaches for nutritional enhancement in agriculturally important crops.