Flavonoids are major constituents of flower colour. Plants accumulate specific flavonoids and thus every species often exhibits a limited flower colour range. Three cytochromes P450 play critical roles in the flavonoid biosynthetic pathway. Flavonoid 3¢-hydroxylase (F3¢H, CYP75B) and flavonoid 3¢,5¢-hydroxylase (F3¢5¢H, CYP75A) catalyze the hydroxylation of the B-ring of flavonoids and are necessary to biosynthesize cyanidin-(red to magenta) and delphinidin-(violet to blue) based anthocyanins, respectively. Pelargonidin-based anthocyanins (orange to red) are synthesized in their absence. Some species such as roses, carnations and chrysanthemums do not have violet/blue flower colour due to deficiency of F3¢5¢H. Successful expression of heterologous F3¢5¢H genes in roses and carnations results in delphinidin production, causing a novel blue/violet flower colour. Down-regulation of F3¢H and F3¢5¢H genes has yielded orange petunia and pink torenia colour that accumulate pelargonidin-based anthocyanins. Flavone synthase II (CYP93B) catalyzes the synthesis of flavones that contribute to the bluing of flower colour, and modulation of FNSII gene expression in petunia and tobacco changes their flower colour. Extensive engineering of the anthocyanin pathway is therefore now possible, and can be expected to enhance the range of flower colours. Keywords Anthocyanin AE Cytochrome P450 AE Flavonoid AE Flavone AE Flower colour AE Cyanidin AE Delphinidin AE Dihydroflavonol 4-reductase AE Flavonoid 3¢-hydroxylase AE Flavonoid 3¢,5¢-hydroxylase AE Flavone synthase II AE Flavonol hydroxylase AE Monooxygenase AE Pelargonidin Abbreviations DFR dihydroflavonol 4-reductase DHK dihydrokaempferol DHQ dihydroquercetin DHM dihydromyricetin F3H flavanone 3-hydroxylase F3¢H flavonoid 3¢-hydroxylase F3¢5¢H flavonoid 3¢,5¢-hydroxylase FLS flavonol synthase FNS flavone synthase
Flavonoids and anthocyaninsFlower colour is due to flavonoids, carotenoids, betalains and some other pigments (