Identification of flavonoids and expression of flavonoid biosynthetic genes in two coloured tree peony flowers

Tree peony (Paeonia suffruticosa Andr.) has been named the "king of flowers" because of its elegant and gorgeous flower colour. Among these colours, the molecular mechanisms of white formation and how white turned to red in P. suffruticosa is little known. In this study, flower colour variables, flavonoid accumulation and expression of flavonoid biosynthetic genes of white ('Xueta') and red ('Caihui') P. suffruticosa were investigated. The results showed that the flower colours of both cultivars were gradually deepened with the development of flowers. Moreover, two anthoxanthin compositions apigenin 7-O-glucoside together with apigenin deoxyheso-hexoside were identified in 'Xueta' and 'Caihui', but one main anthocyanin composition peonidin 3,5-di-O-glucoside (Pn3G5G) was only found in 'Caihui'. Total contents of anthocyanins in 'Caihui' was increased during flower development, and the same trend was presented in anthoxanthins and flavonoids of these two cultivars, but the contents of these two category flavonoid in 'Caihui' were always higher than those in 'Xueta'. Furthermore, nine structural genes in flavonoid biosynthetic pathway were isolated including the full-length cDNAs of phenylalanine ammonialyase gene (PAL), chalcone synthase gene (CHS) and chalcone isomerase gene (CHI), together with the partial-length cDNAs of flavanone 3-hydroxylase gene (F3H), flavonoid 3'-hydroxylase gene (F3'H), dihydroflavonol 4-reductase gene (DFR), anthocyanidin synthase gene (ANS), UDP-glucose: flavonoid 3-O-glucosyltransferase gene (UF3GT) and UDP-glucose: flavonoid 5-O-glucosyltransferase gene (UF5GT), and PAL, UF3GT and UF5GT were reported in P. suffruticosa for the first time. Their expression patterns showed that transcription levels of downstream genes in 'Caihui' were basically higher than those in 'Xueta', especially PsDFR and PsANS, suggesting that these two genes may play a key role in the anthocyanin biosynthesis which resulted in the shift from white to red in flowers. These results would provide a better understanding of the underlying molecular mechanisms of flower pigmentation in P. suffruticosa.