Abstract
Many transcriptional factors harboring the R2R3-MYB domain, basic helix-loop-helix domain, or WD40 repeats have been identified in various plant species as regulators of flavonoid biosynthesis in flowers, seeds, and fruits. However, the regulatory elements of flavonoid biosynthesis in underground organs have not yet been elucidated. We isolated the novel MYB genes IbMYB1 and IbMYB2s from purple-fleshed sweet potato (Ipomoea batatas L. Lam. cv Ayamurasaki). IbMYB1 was predominantly expressed in the purple flesh of tuberous roots but was not detected (or only scarcely) in other anthocyanin-containing tissues such as nontuberous roots, stems, leaves, or flowers. IbMYB1 was also expressed in the tuberous roots of other purple-fleshed cultivars but not in those of orange-, yellow-, or white-fleshed cultivars. Although the orange- or yellow-fleshed cultivars contained anthocyanins in the skins of their tuberous roots, we could not detect IbMYB1 transcripts in these tissues. These results suggest that IbMYB1 controls anthocyanin biosynthesis specifically in the flesh of tuberous roots. The results of transient and stable transformation experiments indicated that expression of IbMYB1 alone was sufficient for induction of all structural anthocyanin genes and anthocyanin accumulation in the flesh of tuberous roots, as well as in heterologous tissues or heterologous plant species.
Highlights
Many transcriptional factors harboring the R2R3-MYB domain, basic helix-loop-helix domain, or WD40 repeats have been identified in various plant species as regulators of flavonoid biosynthesis in flowers, seeds, and fruits
IbMYB1, a Regulator of Sweet Potato Anthocyanin Biosynthesis studies have been done in maize, Arabidopsis (Arabidopsis thaliana), and petunia (Petunia hybrida) by numerous mutation analyses, and many regulatory genes have been identified that control the transcription of flavonoid structural genes
MYB domain C1 protein regulating the anthocyanin pathway in maize needs a basic helix-loop-helix (bHLH) partner (B/R) to activate the flavonoid structural gene dihydroflavonol reductase (DFR) promoter, whereas MYB domain P protein regulating the phlobaphene pathway can activate the promoter without a bHLH partner (Sainz et al, 1997)
Summary
We constructed cDNA libraries from the tuberous roots of the purple-fleshed sweet potato cultivar Ayamurasaki and the yellow-fleshed sweet potato cultivar Kokei-14, and we determined the sequences of 3,783 randomly isolated cDNA clones from Ayamurasaki and 2,804 from Kokei-14. The cDNA clones were clustered, and 25 sequence groups were selected according to their frequency of appearance in the libraries and/or specificity to the Ayamurasaki library. The 25 selected groups were subjected to reverse transcription (RT)PCR analysis, and the results indicated that five groups, IT1, IT4, IT4785, IT666, and IT4097, were expressed in a tissue-specific manner (Fig. 1). Genomic PCR and RT-PCR performed on five clones isolated from a cDNA library derived from sweet potato tuberous roots revealed tissue- and/or cultivar-specific expression. Probes and only a small number of signals in the case of the EcoRV and HindIII digests, one would expect a small number of loci to encode several similar-type MYB genes in the two cultivars of sweet potato. The band patterns were similar, the presence of small variations indicated that a few of the MYB-like genes had structures that differed between Ayamurasaki and Kokei-14
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