| Summary: | R2R3-MYB transcription factors play important roles in the regulation of plant flavonoid metabolites. In the current study, <i>NtMYB3</i>, a novel R2R3-MYB transcriptional factor isolated from Chinese narcissus (<i>Narcissus tazetta</i> L. var. <i>chinensis</i>), was functionally characterized. Phylogenetic analysis indicated that NtMYB3 belongs to the AtMYB4-like clade, which includes repressor MYBs involved in the regulation of flavonoid biosynthesis. Transient assays showed that <i>NtMYB3</i> significantly reduced red pigmentation induced by the potato anthocyanin activator <i>StMYB-AN1</i> in agro-infiltrated leaves of tobacco. Over-expression of <i>NtMYB3</i> decreased the red color of transgenic tobacco flowers, with qRT-PCR analysis showing that <i>NtMYB3</i> repressed the expression levels of genes involved in anthocyanin and flavonol biosynthesis. However, the proanthocyanin content in flowers of transgenic tobacco increased as compared to wild type. <i>NtMYB3</i> showed expression in all examined narcissus tissues; the expression level in basal plates of the bulb was highest. A 968 bp promoter fragment of narcissus <i>FLS</i> (<i>NtFLS</i>) was cloned, and transient expression and dual luciferase assays showed <i>NtMYB3</i> repressed the promoter activity. These results reveal that <i>NtMYB3</i> is involved in the regulation of flavonoid biosynthesis in narcissus by repressing the biosynthesis of flavonols, and this leads to proanthocyanin accumulation in the basal plate of narcissus.
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