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).