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Background  Beta-carotene is the main dietary precursor of vitamin A. Potato tubers contain low levels of carotenoids, composed mainly of the xanthophylls lutein (in the beta-epsilon branch) and violaxanthin (in the beta-beta branch). None of these carotenoids have provitamin A activity. We have previously shown that tuber-specific silencing of the first step in the epsilon-beta branch,  LCY-e , redirects metabolic flux towards beta-beta carotenoids, increases total carotenoids up to 2.5-fold and beta-carotene up to 14-fold.    Results  In this work, we silenced the non-heme beta-carotene hydroxylases  CHY1  and  CHY2  in the tuber. Real Time RT-PCR measurements confirmed the tuber-specific silencing of both genes  . CHY  silenced tubers showed more dramatic changes in carotenoid content than  LCY-e  silenced tubers, with beta-carotene increasing up to 38-fold and total carotenoids up to 4.5-fold. These changes were accompanied by a decrease in the immediate product of beta-carotene hydroxylation, zeaxanthin, but not of the downstream xanthophylls, viola- and neoxanthin. Changes in endogenous gene expression were extensive and partially overlapping with those of  LCY-e  silenced tubers:  CrtISO ,  LCY-b  and  ZEP  were induced in both cases, indicating that they may respond to the balance between individual carotenoid species.    Conclusion  Together with epsilon-cyclization of lycopene, beta-carotene hydroxylation is another regulatory step in potato tuber carotenogenesis. The data are consistent with a prevalent role of  CHY2 , which is highly expressed in tubers, in the control of this step. Combination of different engineering strategies holds good promise for the manipulation of tuber carotenoid content.

Silencing of beta-carotene hydroxylase increases total carotenoid and beta-carotene levels in potato tubers