15.1.2 Anthocyanins
The flavonoid biosynthesis pathway has been well characterized in an endless
number of higher plants. The biosynthesis of flavonoids is initiated by the enzymatic
step catalyzed by chalcone synthase (CHS); in the majority of plants chalcones are
not the end-products, but the pathway proceeds with several enzymatic steps to other
classes of flavonoids such as flavanones, dihydroflavonols, and finally anthocyanins.
Other flavonoids classes, isoflavones, aurones, flavones, flavonoles, and proanthocyanidins (tannins), represent side branches of the flavonoid pathway and are
derived from intermediates in anthocyanin formation (Schijlen et al. 2004). When
CHS alone is present in a plant, 6
0 -hydroxychalcones are produced. The first deoxy
flavonoid, isoliquiritigenin is synthesized by the co-action of CHS and polyketide
reductase (Welle and Grisbach 1988) later called chalcone reductase (CHR)
(Forkmann and Martens 2001). The isoliquiritigenin is the precursor of daidzein
(5-deoxy-isoflavonid); this isoflavonoid is synthesized in two subsequent steps by
the action of chalcone isomerase (CHI) and isoflavone synthase (IFS). IFS is a key
enzyme responsible for the migration of the b-ring, resulting in the formation of
isoflavonoids daidzein and genistein (Yu et al. 2003). CHI is able to isomerize
6
0 -deoxychalcone and isoliquiritigenin, to the 5
0 -deoxyflavanone liquiritigenin
(Joung et al. 2003). The hydroxylation in C3 for the conversion of flavanones into
dihydroflavonols is carried out by flavanone-3-hydroxylase (F3H). Consecutively,
the enzyme dihydroflavonol-4-reductase (DFR) catalyzes the stereospecific
reduction of dihydroflavonols to leucoanthocyanidins (flavan-3,4-diols). This
enzyme converts a dihydroflavonol into leucoanthocyanidins via the reduction of
ketone in the central ring of the flavonoids (Petit et al. 2007). The leucoanthocyanidins formed are the immediate precursors for the synthesis of anthocyanins.
They are also precursors for the production of catechins and proanthocyanidins
(Schijlen et al. 2004).
15.2 Methods to Increase the Yields of Secondary
Metabolites in Plant Cell Culture
Plant cell culture (PCC) is one choice for producing secondary metabolites such as
pharmaceuticals, agrochemicals, flavors, colorants, fragrances, and food additives.
Under specific conditions and plant hormone balances, plant cell can become
totipotent and are able to produce all compounds found in the whole plant
(Ramachandra and Ravishankar 2002). PCC has some advantages over conventional agricultural production, like independence of geographical variables and the
environment, the possibility of define production, quality and yield, production of
new compounds not found in the original plant or with specific stereochemical
requirements, and manufacture them in easy and efficient way.
15 Plants as Bioreactors for Human Health Nutrients
425
The flavonoid biosynthesis pathway has been well characterized in an endless
number of higher plants. The biosynthesis of flavonoids is initiated by the enzymatic
step catalyzed by chalcone synthase (CHS); in the majority of plants chalcones are
not the end-products, but the pathway proceeds with several enzymatic steps to other
classes of flavonoids such as flavanones, dihydroflavonols, and finally anthocyanins.
Other flavonoids classes, isoflavones, aurones, flavones, flavonoles, and proanthocyanidins (tannins), represent side branches of the flavonoid pathway and are
derived from intermediates in anthocyanin formation (Schijlen et al. 2004). When
CHS alone is present in a plant, 6
0 -hydroxychalcones are produced. The first deoxy
flavonoid, isoliquiritigenin is synthesized by the co-action of CHS and polyketide
reductase (Welle and Grisbach 1988) later called chalcone reductase (CHR)
(Forkmann and Martens 2001). The isoliquiritigenin is the precursor of daidzein
(5-deoxy-isoflavonid); this isoflavonoid is synthesized in two subsequent steps by
the action of chalcone isomerase (CHI) and isoflavone synthase (IFS). IFS is a key
enzyme responsible for the migration of the b-ring, resulting in the formation of
isoflavonoids daidzein and genistein (Yu et al. 2003). CHI is able to isomerize
6
0 -deoxychalcone and isoliquiritigenin, to the 5
0 -deoxyflavanone liquiritigenin
(Joung et al. 2003). The hydroxylation in C3 for the conversion of flavanones into
dihydroflavonols is carried out by flavanone-3-hydroxylase (F3H). Consecutively,
the enzyme dihydroflavonol-4-reductase (DFR) catalyzes the stereospecific
reduction of dihydroflavonols to leucoanthocyanidins (flavan-3,4-diols). This
enzyme converts a dihydroflavonol into leucoanthocyanidins via the reduction of
ketone in the central ring of the flavonoids (Petit et al. 2007). The leucoanthocyanidins formed are the immediate precursors for the synthesis of anthocyanins.
They are also precursors for the production of catechins and proanthocyanidins
(Schijlen et al. 2004).
15.2 Methods to Increase the Yields of Secondary
Metabolites in Plant Cell Culture
Plant cell culture (PCC) is one choice for producing secondary metabolites such as
pharmaceuticals, agrochemicals, flavors, colorants, fragrances, and food additives.
Under specific conditions and plant hormone balances, plant cell can become
totipotent and are able to produce all compounds found in the whole plant
(Ramachandra and Ravishankar 2002). PCC has some advantages over conventional agricultural production, like independence of geographical variables and the
environment, the possibility of define production, quality and yield, production of
new compounds not found in the original plant or with specific stereochemical
requirements, and manufacture them in easy and efficient way.
15 Plants as Bioreactors for Human Health Nutrients
425