Polyphenols - Fruit Ripening: Physiology, Signalling and Genomics - page 169

blueberries (Zifkin et al. , 2012). PA levels
usually decrease thereafter when expressed
per FW, but remain almost constant when
expressed per fruit. Ellagitannins exhibit
the same accumulation profi le as PAs, and
constantly decreased in strawberry when
expressed per fresh weight (Williner et al. ,
2003). Accumulation of stilbenes in
healthy grape berries is restricted to
ripening stages (Gatto et al. , 2008). In
grape, fl avonol synthesis occurs at two
distinct periods during berry development
(Downey et al. , 2003). The fi rst synthesis
phase occurs early in the infl orescence and
the second after veraison. Accumulation of
anthocyanin begins during ripening in
most fruits, and then stabilizes or decreases
slightly towards the harvesting stage (Boss
et al. , 1996; Lister et al. , 1996). While
phenolics are markers of development
stages, the molecular circuits connecting
their accumulation to the ripening process
are poorly understood.

2008). At maturity, ellagitannins are found
mostly (about 80%) in achenes (Williner et
al. , 2003). Stilbenes were also found in
much higher concentration in achenes than
in the receptacle of strawberries (Wang et
al. , 2007). Localization of stilbenes in grape
berry skin is in accordance with their role
as a barrier against pathogens (Fornara et
al. , 2008). Ellagitannins were detected in
equivalent concentrations in the peel and
mesocarp of pomegranate (Fischer et al. ,
2011) and only in trace amounts in arils
(Gil et al. , 2000). Anthocyanins accumulate
in the skins and, in some species or
varieties, in the pulp of red fruits, in
accordance with their roles as pigments to
attract animals for seed dispersion and
protectants against UV irradiation (Winkel-
Shirley, 2001). Similarly, fl avonols are
mainly detected in fruit skins, which is
consistent with their roles as UV fi lters
(Winkel-Shirley, 2002).

9.5 Biosynthesis Pathways

9.4.3 Distribution and role in
fruit tissues 9.5.1 General and particular pathways

The phenolic compounds, like most
secondary metabolites, present a very
uneven distribution within the fruit, and
for the same species, depending on variety.
In young unripe fruit skin, while seeds are
still immature, it is generally assumed that
PAs function as feeding deterrents thanks
to their astringency and bitterness
(Wrangham and Waterman, 1983). Skin
PAs may also protect the fruits against
pathogens, fungi and viruses due to their
protein-binding properties (Treutter, 2006).
In seeds, PAs are often present in the seed
coat, in agreement with their protective
role (Debeaujon et al. , 2000). To date, there
are few reports of the localization of PAs in
fruit: in grape (Cadot et al. , 2011), apple
(Lees et al. , 1995) and blueberry (Zifkin et
al. , 2012), higher concentrations of PAs
were detected in the skin and the seed
coat, but parenchyma is the major
contributor of PAs in apple (Guyot et al. ,
1998), and PAs were also found in grape
berry pulp (Mané et al. , 2007; Verries et al. ,

Most of the phenolics described in this
chapter, except hydrolysable tannins, derive
from the general phenylpropanoid with
phenylalanine as substrate (Fig. 9.3),
already described in numerous reviews. The
biosynthetic pathway of fl avonoids was
characterized fi rst in parsley (Kreuzaler et
al. , 1983) and then in maize, petunia
(Holton and Cornish, 1995) and Arabidopsis
(Shirley et al. , 1995). More recently, this
pathway has also been described in fruit
crops such as apple (Takos et al. , 2006b;
Espley et al. , 2007), bilberry (Jaakola et al. ,
2002) and grapevine (Boss et al. , 1996).
These compounds derive from a common
precursor, the fl avanone naringenin (Fig.
9.1f). This intermediate is subsequently
hydroxylated by fl avanone-3 E -hydroxylase
to dihydrofl avonol (Fig. 9.3). This part of
the pathway is common to the biosynthesis
of PAs, anthocyanins and fl avonols.
Hydroxylation catalysed by fl avonoid 3 c
hydroxylase and fl avonoid 3 c 5 c hydroxylase
gives rise to the different B-ring