| Summary: | <p>Abstract</p> <p>Background</p> <p>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, <it>LCY-e</it>, redirects metabolic flux towards beta-beta carotenoids, increases total carotenoids up to 2.5-fold and beta-carotene up to 14-fold.</p> <p>Results</p> <p>In this work, we silenced the non-heme beta-carotene hydroxylases <it>CHY1 </it>and <it>CHY2 </it>in the tuber. Real Time RT-PCR measurements confirmed the tuber-specific silencing of both genes <it>. CHY </it>silenced tubers showed more dramatic changes in carotenoid content than <it>LCY-e </it>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 <it>LCY-e </it>silenced tubers: <it>CrtISO</it>, <it>LCY-b </it>and <it>ZEP </it>were induced in both cases, indicating that they may respond to the balance between individual carotenoid species.</p> <p>Conclusion</p> <p>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 <it>CHY2</it>, 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.</p>
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