The biosynthesis of flavonoid pigments
Anthocyanins are key pigments in fruit, generating red, blue and purple hues. These compounds usually accumulate in the epidermal or sub-epidermal tissues of different plant organs, offering protection against variable environmental stress factors, especially excess of light. The biosynthesis of anthocyanins occurs in the cytosol via the phenylpropanoid and flavonoid pathways. The precursors for flavonoids, including the pigmented anthocyanins, are malonyl-CoA andp -coumaroyl-CoA. From these precursors the first committed enzyme, chalcone synthase (CHS), forms chalcone, establishing the C15 backbone (Figure 3). Chalcone is isomerised by chalcone isomerase (CHI) to produce chalcone naringenin, which is hydroxylated by flavanone 3β-hydroxylase (F3H), converting naringenin to dihydroflavonol. Further hydroxylation is carried out by flavonoid 3′-hydroxylase (F3’H) to form cyanidin, or flavonoid 3′5′ -hydroxylase (F3’5’H) to form delphinidin. Dihydroflavonol is reduced by dihydroflavonol 4-reductase (DFR ) to produce leucoanthocyanidin, which is converted into the pigmented compound anthocyanidin by anthocyanidin synthase (ANS), also sometimes referred to as leucoanthocyanidin dioxygenase (LDOX). Finally, glycosylation is carried out by uridine diphosphate (UDP)-glycosyl:flavonoid 3-0 -glycosyltransferase (UFGT). Further addition of sugars and acyl side groups may follow. After biosynthesis, anthocyanins are transported to vacuoles or, rarely, cell walls. The differences in anthocyanin colours result from various factors, including the number of hydroxyl groups on the B-ring, the sugars and acyl side groups, the vacuolar pH or the accumulation of specific metal ions. To date, more than 650 different anthocyanins have been identified, the most common ones being glucosides of six aglycons: cyanidin, delphinidin, pelargonidin, petunidin, peonidin and malvidin (Jaakola 2013; Zhang, Butelli & Martin 2014).