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J Sci Food Agric 79 :421–424 (1999)
Journal of the Science of Food and Agriculture
Review
Biosynthesis of cell-wall ferulate and diferulates¹
Chris topher T Brett,1* Gundolf Wende,1,2 Andrew C Smith2 and
Keith W Waldron2
1 Plant Molecular Science Group , Ins titute of Biomedical and Life Sciences , Bower Building , Glas gow Univers ity , Glas gow G12 8QQ , UK
2 Ins titute of Food Res earch , Norwich Res earch Park , Colney Lane , Norwich NR4 7UA , UK
Abstract : Cell-wall hydroxycinnamates are derived from feruloyl-CoA, coumaroyl-CoA and other
intermediates of the phenylpropanoid pathway. In normal (unelicited) tissues, hydroxycinnamates are
ester-linked to pectins in some dicots and to arabinoxylans in grasses. The ester-linked hydroxycinnamates are thought to be formed by feruloylation of nascent polysaccharides in the Golgi apparatus, feruloyl-CoA perhaps being the substrate. It is not known whether dimerisation of
polysaccharide-linked ferulate occurs in the Golgi apparatus as well as in the wall. In elicited dicot
cells the immediate precursors of wall ferulate and coumarate include low molecular weight hydroxycinnamate derivatives (amides and esters) which are secreted into the cell wall and then oxidatively
linked to polymers. The cell-wall hydroxycinnamates may, in some circumstances, be subject to turnover, though this may be a property of suspension cultured cells that is not necessarily shared with
normal plant tissues.
( 1999 Society of Chemical Industry
Keywords : cell-wall ; ferulate ; diferulate ; beet ; biosynthesis
INTRODUCTION
Ferulic acid is the major hydroxycinnamic acid in
growing cell walls. In grasses it is esteriüed to
arabinoxylans1 and in beet to arabinans and to galactose residues in pectin.2 It has been known for more
than 20 years that ferulate can dimerise, forming a
5–5 linkage. Recently several other coupled dehydrodiferulates have been discovered,3 in quantities that
often exceed that of the 5–5 dimer.
Feruloylated polysaccharides are thought to
modify the mechanical properties (eg extensibility) of
cell walls through peroxidase-mediated oxidative
coupling of ferulates to form diferulate bridges
between wall polysaccharides.4,5 In Avena coleoptiles, increasing ferulate and diferulate levels in cell
walls are correlated with decreased wall extensibility
and decreased growth rate during maturation,6 and
similar correlations between diferulate levels and
growth rate were observed in pine hypocotyls.7 It
has been proposed that diferulate cross-linking is
responsible for light-induced reduction of coleoptile
growth in Avena8 and Oryza.9 In a recent investigation, however, rice seedlings which were treated with
2-aminoindan-2-phosphonic acid (AIP) showed a
90% reduction in cell-wall ferulate content without
any signiücant eþect on the growth of rice coleoptiles in dark or light, indicating that light-mediated
growth cessation in rice coleoptiles may not be
directly controlled by ferulate substitution or diferulate cross-linking (Wende G, Hatüeld RD, Grabber
J H and Ralph J , unpublished results). Cell-wall
diferulate cross-linking is also thought to be important in cell–cell adhesion,10,11 and therefore has great
signiücance in inýuencing the texture of fruit and
vegetables.12 Ferulic acid and its dimers may also act
as nucleation sites for ligniücation,13 and diferulates
and ferulate-lignin cross-links limit the degradability
of cell walls in grasses.14
The biosynthesis of cell-wall ferulates and other
hydroxycinnamates is therefore important in plant
growth and development. This review summarises
the present state of knowledge and highlights areas
where more research is needed.
¹ Bas ed on a paper pres ented at Ferulate ’98, IFR, Norwich, 8–11
July 1998.
* Corres pondence to : Chris topher T. Brett, Plant Molecular
Science Group, Ins titute of Biomedical and Life Sciences , Bower
Building, Glas gow Univers ity, Glas gow G12 8QQ, UK
ORIGIN OF CELL-WALL HYDROXYCINNAMATES
Cell-wall hydroxycinnamates are derived from the
phenylpropanoid pathway, which originates from
phenylalanine and tyrosine. This pathway gives rise
to a wide range of aromatic secondary metabolites,
(Received 9 July 1998 ; revis ed vers ion received 2 September
1998 ; accepted 15 October 1998 )
( 1999 Society of Chemical Industry. J Sci Food Agric 0022-5142/99/$17.50
421
CT Brett et al
synthesis of many of which is induced or stimulated
by wounding, pathogen attack or other stresses. A
major product is lignin, and both ferulic acid and
coumaric acid are intermediates in lignin synthesis,
as are feruloyl-CoA and p-coumaroyl-CoA.15 These
CoA derivatives are therefore obvious candidates for
the role of precursors for the addition of hydroxycinnamates to cell-wall polysaccharides, though other
compounds such as hydroxycinnamoyl-sugar esters
could also be precursors.
FERULOYLATION OF CELL-WALL
POLYSACCHARIDES
A feruloyltransferase has been reported which transfers ferulic acid from feruloyl-CoA to a polysaccharide, probably pectin, in microsomes from
parsley suspension cultures.16 The feruloylpolysaccharide was characterised by treatment with
Driselase (a mixture of fungal cell-wall degrading
enzymes), which led to formation of compounds with
the chromatographic mobility similar to feruloyloligosaccharides. The feruloylated polysaccharide
was, however, only a minor product of the reaction
system, the majority of the feruloylated product
being feruloylated protein.17 Treatment of the cell
walls of parsley suspension cultures with Driselase
did not give rise to detectable amounts of feruloylated oligosaccharides, either from control or elicited
cells, even though the cell walls of elicited cells contained signiücant amounts of ferulate.18 It was therefore concluded that the enzyme responsible for the
small amount of ferulate transfered onto polysaccharide in unelicited cells was probably not
responsible for the increase in wall-bound ferulic
acid found in the elicited cells. The low amount of
polysaccharide feruloylation observed in unelicited
cells, however, may still represent the normal route
for incorporation of ferulic acid esters into cell walls
under non-elicited conditions ; ferulic acid levels are
low in these cells, so a relatively low enzyme activity
would be expected. These observations suggest that
the mechanism of incoporation of ferulic acid into
cell walls is diþerent in elicited and unelicited cells.
A number of similar experiments in other systems
have not revealed any further evidence for feruloylCoA : polysaccharide feruloyltransferases. Incubation of membranes from cell-suspension cultures
of maize with feruloyl-CoA resulted in oxidative polymerisation of the feruloyl groups rather than esteriücation of polysaccharides.19 It was suggested that
one possible reason for this might be that the mechanism for feruloylation of arabinoxylans (the main
feruloylated polymer in walls of maize and other
Gramineae) might be diþerent from that of pectic
arabinogalactans in dicots. It is likely that diþerent
enzymes are involved in monocots and dicots, since
not only are the polysaccharide acceptors diþerent,
but also the linkage position on arabinose is diþerent
(2–O–feruloylarabinose
in
dicots,
5–O–
422
feruloylarabinose in grasses).20 Microsomal preparations from French beans catalysed only
peroxidative addition of ferulic acid from feruloylCoA onto polysaccharides ; in this tissue, feruloyllipids were formed, a process that was stimulated by
elicitation and was probably involved in suberin production.21
CELLULAR LOCATION OF POLYSACCHARIDE
FERULOYLATION
The feruloyltransferase involved in polysaccharide
feruloylation in parsley microsomes (see above16)
was thought to be located in the endomembrane
system, perhaps in the Golgi apparatus. Pulse-chase
studies in Festuca22 and spinach23 indicated that
feruloylation of primary wall polysaccharides occurs
intracellularly. Since wall matrix polysaccharides are
synthesised chieýy in the Golgi apparatus,24 it is
likely that feruloylation occurs shortly after polysaccharide formation, prior to export to the cell wall.
Yamamoto and Towers25 observed that wallbound ferulate in barley seedlings continued to accumulate even after wall-bound arabinose had ceased to
increase, and suggested that feruloylation might
occur in the cell wall. This could, however, be due to
enzyme-mediated loss of arabinose from the wall
occuring at the same time as continued deposition of
feruloylated arabinose in arabinoxylans,22 ie to arabinose turnover, and hence does not necessarily imply
an extracellular site of feruloylation.
MECHANISM OF FERULIC ACID INCORPORATION
INTO CELL WALLS IN ELICITED TISSUE
As noted above, there is evidence that, in some
tissues, the increase in wall ferulic acid after elicitor
treatment occurs by a mechanism diþerent from that
operating in unelicited tissue. Ferulic acid amides
(feruloyltyramine and feruloyloctopamine) were
found to be ether-linked through their ferulic acid
moiety to cell walls in natural and wound periderms
of potato tubers, and that these compounds increased
in wounded periderms.26 In parsley suspension cultures, walls of elicited cells contained low-molecular
weight hydroxycinnamoyl esters, probably linked to
glucose and arabinose, which could become peroxidatively linked to wall polymers.18 The artiücial secondary metabolite, 4-hydroxybenzoate, formed in
transgenic tobacco, has also been reported to become
linked to wall polymers, via its 1-b-glucosyl ester.27
These reports all indicate that hydroxycinnamates
and related compounds can become incorporated
into wall polymers via low molecular weight derivatives, which are presumably secreted into the wall
and there undergo peroxidative coupling to existing
wall material. This pathway seems to be especially
important in elicited and wounded tissues.
It should be noted that elicitation often results in
an increase in wall-bound ester-linked ferulate as
J Sci Food Agric 79 :421–424 (1999)
Biosynthesis of cell-wall ferulate and diferulates
well as ether-linked ferulate.28h30 This increase in
ester-linked ferulate presumably occurs by stimulation of the ‘normal’ feruloylation pathway involving feruloylation of nascent polysaccharides in the
Golgi apparatus.
CELLULAR SITE OF DIFERULATE FORMATION
As noted by Myton and Fry,19 maize microsomes are
capable of oxidative coupling of feruloyl groups. Peroxidases are present in plant microsomes,31 since
they are synthesised in the endoplasmic reticulum
and exported to the wall. Peroxidases might also be
taken up from the wall by endocytosis, a process that
has been observed for exogenously-added but not
endogenous peroxidases.32 It is possible, therefore,
that in the constitutive (unelicited) pathway of wall
hydroxycinnamate synthesis, the hydroxycinnamates
might not only be added to polysaccharides in the
endomembrane system, but also that cross-linking
might begin there, before export to the cell wall,
where cross-linking would continue. Wende et al33
have reported the presence of alkali-extractable ferulate dimers in microsomes obtained from suspension
cultures. Radiolabelling studies, however, indicated
that these might be attributed to wall polymers
which had been released or degraded from the wall
and taken up by endocytosis (see following).
DOES TURNOVER OF CELL-WALL
HYDROXYCINNAMATES OCCUR ?
Many cell-wall matrix polysaccharides are subject to
turnover, ie continued synthesis accompanied by
degradation and removal from the wall.34h36 It is
therefore a possiblity that cell-wall hydroxycinnamates may be subject to turnover, though
diferulate cross-linking might interfere with wall
polymer degradation and hence decrease the degree
of turnover.37 Wende et al33 reported preliminary
investigations in which 14C-cinnamate was fed to
suspension cultured beet cells and 14C-diferulates
were detected in microsomal membranes. The labelling kinetics of 14C-diferulates compared to 14Cferulic acid in the microsomes, however, indicated
that the diferulates were likely to be derived not from
nascent cell-wall polymers which had undergone
dimerisation prior to export to the wall, but from
cell-wall polymers that had been taken up from the
wall into microsomes, perhaps after partial degradation and endocytosis. This is clearly a topic which
requires further investigation.
CONCLUSION
While the cell-wall hydroxycinnamates are likely to
be derived from feruloyl-CoA and other intermediates of the phenylpropanoid pathway, the details of
the biosynthesis are not clear. In normal (unelicited)
tissues, feruloyl-CoA is thought to feruloylate
nascent polymers in the Golgi apparatus, though the
J Sci Food Agric 79 :421–424 (1999)
role of feruloyl-CoA as a direct precursor of cell-wall
ferulates has still only been reported in a single dicot
tissue and not at all in grasses. The deposition of
ester-linked wall hydroxycinnamates is oten
increased in elicited tissues. In addition, in elicited
cells a diþerent pathway is activated, in which the
immediate precursors of wall ferulate and coumarate
include low-molecular hydroxycinnamate derivatives
which are secreted into the cell wall and then linked
to polymers, giving rise to ether-linked rather than
ester-linked hydroxycinnamates. The cell-wall
hydroxycinnamates may in some circumstances be
subject to turnover, though this may be a property of
suspension cultured cells that is not necessarily
shared with normal plant tissues.
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