Transgenic tomato lines expressing either the Erwinia uredovora phytoene synthase (crtB) or phytoene desaturase (crtI) gene have been produced using the polygalacturonase (PG) or CaMV 35S promoter respectively. Over three generations, tomato fruits expressing the additional phytoene synthase showed increased total carotenoids approaching twofold. Ripe tomato fruits expressing crtI showed significant increases in β-carotene (threefold) but a reduction in the total carotenoid content (twofold). The contents of other biosynthetically related isoprenoids, including tocopherols (vitamin E), vitamin K, ubiquinones and plastoquinones, were not altered by manipulations to the carotenoid pathway. Detailed biochemical analysis indicates that both enzymes are targeted to the correct location and reside in active states. Increases in enzyme activity and the levels of carotenoid end-products do not correlate. The data suggest that enhancement of carotenoids can be achieved by metabolic engineering of the pathway, but intrinsic regulatory mechanisms such as feedback inhibition and metabolite channelling may limit the potential shown by the increased enzyme activities.
© 2001 Society of Chemical Industry