Abstract
BackgroundBeta-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.ResultsIn 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.ConclusionTogether 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.
Highlights
Beta-carotene is the main dietary precursor of vitamin A
The tomato wf mutant, which maps to the CHY2 gene, is sufficient to severely impair flower β-xanthophyll biosynthesis, while leaf β-xanthophyll levels remain similar to those found in wild-type plants [6]
In order to verify the tissue-specificity of expression of the genes controlling carotenoid biosynthesis in potato, we conducted Real Time RT-PCR experiments on leaf and tuber RNA
Summary
Beta-carotene is the main dietary precursor of vitamin A. BMC Plant Biology 2007, 7:11 http://www.biomedcentral.com/1471-2229/7/11 companion paper, we reported the results of the tuberspecific silencing of the first dedicated step in lutein biosynthesis, LCY-e [3] This resulted in increases of β-carotene (up to 14-fold) and of total carotenoids (up to 2.5fold). No changes in carotenoid content, or in endogenous carotenoid gene expression, were observed in leaves, indicating that, in agreement with previous reports [1], gene silencing remains confined in tubers Encouraged by this result, we decided to silence a second important regulatory step in carotenogenesis, the hydroxylation of β-carotene. The tomato wf mutant, which maps to the CHY2 gene, is sufficient to severely impair flower β-xanthophyll biosynthesis, while leaf β-xanthophyll levels remain similar to those found in wild-type plants [6] In keeping with this result, the tomato CHY2 transcript is expressed preferentially in flowers, while the CHY1 transcript is expressed preferentially in leaves [6]. In order to elucidate the role of CHY genes in the hydroxylation of β-carotene in potato tubers, we took a tuber-specific gene silencing approach
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