Metabolic Engineering of Ketocarotenoid Formation in Higher Plants

Although higher plants synthesize carotenoids, they do not possess the ability to form ketocarotenoids. In order to generate higher plants capable of synthesizing combinations of ketolated and hydroxylated carotenoids the genes responsible for the carotene 4,4' oxygenase and 3,3' hydroxylase have been transformed into tomato and tobacco. The gene products were produced as a polyprotein. Subsequent cleavage of the polyprotein, targeting of the two enzymes to the plastid and enzyme activities have been shown for both gene products. Metabolite profiling has shown the formation of ketolated carotenoids from beta-carotene and its hydroxylated intermediates in tobacco and tomato leaf. In the nectary tissues of tobacco flowers a quantitative increase (10-fold) as well as compositional changes were evident, including the presence of astaxanthin, canthaxanthin and 4-ketozeaxanthin. Interestingly, in this tissue the newly formed carotenoids resided predominantly as esters. These data are discussed in terms of metabolic engineering of carotenoids and their sequestration in higher plant tissues.