'Le Rouge et le Noir': A decline in flavone formation correlates with the rare color of black dahlia (Dahlia variabilis hort.) flowers

Black cultivars accumulate high amounts of anthocyanins, but show drastically reduced flavone contents. High activities were observed for all enzymes from the anthocyanin pathway whereas FNS II activity could not be detected or only to a low extent in 13 of 14 cultivars. cDNA clones and genomic clones of FNS II were isolated. Independently from the colour type, heterologous expression of the cDNA clones resulted in functionally active enzymes. FNS II possesses one intron of varying length. Quantitative Real-time PCR showed that FNS II expression in black cultivars is low compared to other cultivars. No differences between black and red cultivars were observed in the expression of transcription factors IVS and possible regulatory genes WDR1, WDR2, MYB1, MYB2, 3RMYB and DEL or the structural genes of the flavonoid pathway. Despite the suppression of FHT expression, flavanone 3-hydroxylase (FHT, synonym F3H) enzyme activity was clearly present in the yellow and white cultivars.An increased accumulation of anthocyanins establishes the black flowering phenotypes. In the majority of black cultivars this is due to decreased flavone accumulation and thus a lack of competition for flavanones as the common precursors of flavone formation and the anthocyanin pathway. The low FNS II activity is reflected by decreased FNS II expression.Dahlia (Dahlia variabilis hort.), a member of the Asteracea family, is a popular garden flower. More than 20,000 cultivars are available world-wide [1]. Garden dahlias are hybrids, which are classified according to shape and size of the plant and its inflorescences. A broad spectrum of flower colours exists ranging from white and yellow to almost every imaginable hue of red, orange and magenta (red tones), but blue flower colour does not occur [1]. In contrast to the multitude of available red-shaded cultivars, only a handful of so-called black cultivars exist (Figure 1). Such cultivars frequently have expressive names e.g. Chat Noir (French term