Polyphenols - Fruit Ripening: Physiology, Signalling and Genomics - page 171

the formation of catechin from leuco-
anthocyanidin (Tanner et al. , 2003). In
fruits, these compounds were isolated in
grapevine (Bogs et al. , 2005), apple (Takos
et al. , 2006b), pear (Fischer et al. , 2007),
strawberry (Almeida et al. , 2007), kaki
(Ikegami et al. , 2007; Akagi et al. , 2009a)
and blueberry (Zifkin et al. , 2012). ANR is
suspected to produce catechin in addition
to epicatechin (Akagi et al. , 2009a;
Gargouri et al. , 2009; Han et al. , 2012).
Mechanisms of PA polymerization are still
unknown in fruits and in model plants
(Terrier et al. , 2009a).
Biosynthesis of hydrolysable tannins is
much less understood. The origin of gallic
acid (GA) was unclear until recently.
Labelling experiments revealed that it
derives from an intermediate product of
the shikimate pathway, probably de-
hydroshikimate (Werner et al. , 1997, 2004).
Dehydroshikimate was successfully re-
duced to GA by an enzymatic extract of
Betula leaves (Ossipov et al. , 2003), but the
enzyme was not identifi ed. Recently, Muir
et al. (2011) demonstrated that shikimate
dehydrogenase, an enzyme from the
shikimate pathway essential for aromatic
amino acid synthesis, is also able to
catalyse GA production.
The following step, esterification of GA
and glucose to yield E -glucogallin, was
observed with oak enzymatic extract
(Gross, 1983). Gallotannins from di- to
pentagalloylglucose are formed by succes-
sive position-specifi c steps, using
E -glucogallin as both acyl donor and acyl
acceptor. The enzymatic transfer of the
galloyl moiety to one of the galloyl
hydroxyls (transacylation forming a meta-
depside group), leading to more complex
gallotannin, was demonstrated with extract
from oak and sumach leaves (Gross et al. ,
1990; Gross and Denzel, 1991). Several
enzymes with different substrate spe-
cifi cities have been isolated and may
cooperate in synthesizing the various gal-
lotannins found in these plants (Niemetz
and Gross, 2005). Further oxidation steps
catalysed by laccases are then involved in
the formation of the hexahydroxy-
diphenoyl groups of ellagitannins (Niemetz

and Gross, 2003a,b). However, the proteins
have not been molecularly identifi ed.
E -Glucogallin is probably common to PA
galloylation and gallotannin biosynthesis.
Recently, Terrier et al. (2009b) identifi ed
glucosyltransferases induced in parallel
with PA synthesis in grapevine hairy roots.
These glucosyltransferases are able to form
E -glucogallin, and it was hypothesized that
this compound is an intermediate for PA
galloylation (Khater et al. , 2012). Serine
carboxy peptidase-like proteins (SCPLs)
have been identifi ed during transcriptomic
screening comparing grape or persimmon
samples differing in their PA content
(Ikegami et al. , 2007; Akagi et al. , 2009a;
Terrier et al. , 2009b). As both fruits contain
galloylated PA and SCPLs are able to
catalyse transacylation with glucose esters
as acyl donors (Strack and Mock, 1993;
Steffens, 2000; Milkowski and Strack,
2004), these genes represent good can-
didates for the enzymes involved in the
second step of galloylation.
Stilbene synthase is the fi rst enzyme
specifi c for stilbene biosynthesis. The
genes encoding those enzymes were
isolated fi rst from groundnuts and then
from grapes (Schröder et al. , 1988;
Melchior and Kindl, 1991). They belong to
the superfamily polyketide synthases, like
chalcone synthases. Hall and DeLuca
(2007) suggested that a bifunctionnal
glucosyltransferase could catalyse the
glucosylation of resveratrol to form piceid,
despite the optimum pH for this reaction
being quite alkaline (pH 9) and its rate
much lower than that of its other activity
(i.e. forming glucose esters with phenolic
acids). A resveratrol O -methyltransferase
gene isolated from V. vinifera was char-
acterized, and the corresponding enzyme
was found to be able to catalyse resveratrol
methylation to yield pterostilbene both in
vitro and in planta (Schmidlin et al. , 2008).
Several hypotheses have been proposed to
explain the oxidative polymerization of
stilbenes: by laccase-like stilbene oxidases
from the pathogens (Breuil et al. , 1999) or
by host peroxidases localized in the
vacuole, cell wall or apoplast (Ros Barcelo
et al. , 2003).