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Plant Cell Wall Biosynthesis
Secondary article
Article Contents
Stephen C Fry, The University of Edinburgh, Edinburgh, UK
. Introduction
Most plant cell wall polymers are synthesized from ‘activated’ precursors by the action of
transferases. These enzymes are located in cell membranes (synthesizing polysaccharides
and glycoproteins) or in the cell wall itself (synthesizing cutin and suberin). Lignin, in
contrast, is synthesized in the wall by oxidative rather than transferase-catalysed reactions.
. Mechanisms of Polymer Assembly
. Synthesis of the Building Blocks of Cell Wall Polymers
. Polymer Assembly in Endoplasmic Reticulum and Golgi
Cisternae
. Polymer Assembly at the Plasma Membrane
. Polymer Assembly in the Cell Wall
Introduction
Plant cell walls are composed of numerous complicated
polymers, principally polysaccharides, glycoproteins,
polyesters and lignin. This sophisticated repertoire of
polymers is essential for the wall’s ability to govern growth,
morphogenesis and disease resistance. The plant invests
many enzymes, and thus genes, in the business of
manufacturing walls. Also, since walls often constitute
the majority of a plant’s dry mass, the plant must invest
much carbon and energy in wall production.
The building blocks (sugars, amino acids, etc.) for wall
biosynthesis are made, and often ‘activated’, in the
protoplast; the polymerization process then occurs either
in association with membranes or in the wall itself,
depending on the polymer under consideration.
Mechanisms of Polymer Assembly
Transferase-catalysed polymerization
Most wall polymers are assembled by transferase-catalysed, nucleophilic substitution (SN2) reactions (Table 1), in
which each new bond is synthesized by transfer of a group
from an ‘activated’ donor substrate (containing an
electrophilic carbon atom) to a nucleophilic group (e.g. –
OH or –NH2) in an acceptor substrate. The donor
substrate is described as ‘activated’ because its free energy
(DG8’) of hydrolysis is sufficient to drive net polymer
synthesis.
Glycosyl donors
The donor substrates for transfer of sugar groups are
nucleoside diphosphate sugars (NDP-sugars), the NDP
moiety being an excellent leaving group. The most
important NDP-sugars are listed in Table 2. All except
UDP-Api have the sugar residue in the pyranose (sixmembered) ring form.
The acceptor substrate is the nascent polymer. Linear
polysaccharides (e.g. cellulose, callose, homogalacturonan) and the backbones of branched polysaccharides (e.g.
xyloglucan, galactomannan) appear to be assembled by the
repeated addition of single sugar residues to the polymer’s
nonreducing terminus. This is described as ‘tailward
growth’ and results in the most recently added sugar
residue being at the nonreducing end (‘nucleophile end’) of
the polymer chain. (In bacterial a-dextrans, polysaccharides synthesized by a different mechanism involving
‘headward growth’, the newest glucose residue is at the
reducing end.)
Acyl and alkyl donors
In protein synthesis, the ‘activated’ donor substrates are
peptidyl-tRNAs (Table 1). An aminoacyl-tRNA first acts as
the acceptor substrate for the nascent polymer, thus
becoming a peptidyl-tRNA molecule. This then acts as a
donor substrate, with the next aminoacyl-tRNA as
acceptor. The newest amino acid residue is therefore at
the C-terminus (furthest from the ‘nucleophile end’) of the
polymer – an example of ‘headward growth’.
The ester bonds in cutin and suberin and in O-acetyl and
O-feruloyl-polysaccharides all appear to be synthesized
using coenzyme A (CoA) thioesters as donor substrate, by
transesterification reactions. Methyl-ester and methylether groups are added to polysaccharides using Sadenosylmethionine (SAM) as donor.
Oxidative polymerization
Lignin is synthesized by homolytic reactions, not nucleophilic substitutions. The building blocks (monolignols) are
not ‘activated’. Instead, the monolignols are oxidized to
form free radicals, which then polymerize (see below).
Synthesis of the Building Blocks of Cell
Wall Polymers
This section discusses the biosynthesis of ‘activated’ forms
of these building blocks.
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Plant Cell Wall Biosynthesis
Table 1 Substrates used in the biosynthesis of plant cell wall polymers
Bond to be made
Bond broken in
donor substrate
Ester
Peptide bond
(secondary amide)
in protein
Acceptor substrate
(nucleophile)
Example of donor substratea
Peptidyl-tRNAAla
70–90 additional
nucleotide
residues
H 2N
N
N
O
O
n additional
aminoacyl
residues
O
CH 2
N
O
CH 3
C
OH
O
C
O
UDP-Galactose
O
C
OH
O
OH
O
O
P
O
P
O
CH 2
O
O
O
OH
Thioester
(a) –OH group on
nascent
polysaccharide
O
chain or
(b) –OH group of
NH
Hyp, Ser or Thr in
polypeptide or
N
O (c) –CONH2 in
side-chain of Asn
CH 2 OH
HO
Fatty acyl ester
bond in cutin or
suberin
N
O
N
H
Glycosidic bond in Glycosylpolysaccharide or pyrophosphate
glycoprotein
P
OH
10,16-Dihydroxyhexadecanoyl–CoA
OH
CH 2 OH
O
α-NH2 group of the
‘next’
aminoacyl-tRNA,
according to the
genetically
encoded peptide
sequence
–OH group in
growing
polyester
molecule
C
S
CH 2
H 2C
N
H
pantothenate
CoA
phosphate
phosphate
ribose-3'-phosphate
adenine
O-Acetyl ester in
polysaccharide
Thioester
Acetyl–CoA
CH3
O
C
–OH group on
(nascent?)
polysaccharide
chain
S
CoA
continued
2
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Plant Cell Wall Biosynthesis
Table 1 – continued
Bond to be made
Methyl ester
or O-methyl
ether in
polysaccharide
Bond broken in
donor substrate
Example of donor substratea
Trialkylsulfonium
S-Adenosyl-methionine
Acceptor substrate
(nucleophile)
COOH
HC
H 2N
NH 2
N
CH 2
N
CH 2
S
H 3C
CH 2
OH
a
N
O
(a) –OH group on
(nascent?)
polysaccharide
chain
(b) –COOH group
on (nascent?)
pectin chain
N
OH
Pink, group transferred (the electrophilic carbon atom is shown in bold type); black, leaving group; ---, indicates that the bond is broken.
Table 2 Important NDP sugars
Abbreviation
Full name
UDP-Glc
UDP-Gal
UDP-GlcA
UDP-GalA
UDP-GlcNAc
UDP-GalNAc
UDP-Xyl
UDP-Ara
UDP-Rha
UDP-Api
GDP-Man
GDP-Fuc
GDP-Gal
UDP-a-d-glucose
UDP-a-d-galactose
UDP-a-d-glucuronic acid
UDP-a-d-galacturonic acid
UDP-(N-acetyl-a-d-glucosamine)
UDP-(N-acetyl-a-d-galactosamine)
UDP-a-d-xylose
UDP-b-l-arabinose
UDP-b-l-rhamnose
UDP-a-d-apiofuranose
GDP-a-d-mannose
GDP-b-l-fucose
GDP-b-l-galactose
UDP, uridine diphosphate; GDP, guanosine diphosphate.
Sugar nucleotides
NDP-sugars are primarily derived from a cytosolic pool of
five rapidly interconverting hexose monophosphates
(Figure 1; enzymes 1–4). Glc-1-P and Man-1-P are drawn
off from the pool to form UDP-Glc and GDP-Man,
respectively (5, 6). Also, some Glc-6-P may be drawn off to
form UDP-GlcA via the inositol pathway (7–11).
UDP-Glc and GDP-Man are precursors of all other
major NDP sugars (12–20), including UDP-GlcA: enzyme
12 potentially short-circuits the inositol pathway.
In addition to the primary pathways of NDP sugar
formation (1–20), there are ‘scavenger’ pathways by which
certain monosaccharides, released for example during
polysaccharide turnover, can be recycled by kinases (10,
21–27) and pyrophosphorylases (5, 6, 11, 28–31). To
biochemists, scavenger pathways are valuable because they
enable exogenous 3H- or 14C-monosaccharides to be used
to radiolabel selected sugar residues in newly synthesized
polysaccharides in vivo.
Cell wall phenolics
Phenolic building blocks (except tyrosine) are formed from
cinnamate, itself synthesized from l-phenylalanine by the
action of phenylalanine ammonia-lyase (reaction [I]).
phenylalanine!cinnamate 1 NH3
[I]
Cinnamate is rapidly converted to a family of hydroxycinnamates through reactions [II], where [h] represents
the action of hydroxylases, using O2; and [m] represents the
action of methyltransferases, using S-adenosylmethionine.
cinnamate }[h]! p-coumarate }[h]!
caffeate }[m]! ferulate }[h]!
5-hydroxyferulate }[m]! sinapate
[II]
Hydroxycinnamates do not usually occur as free acids but
as esters and amides. Particularly important, although
present at low concentrations, are CoA thioesters, formed
by (hydroxy)cinnamate:CoA ligase(s) (CCL), as shown in
reaction [III] (PPi represents inorganic pyrophosphate).
p-coumarate 1 CoA 1 ATP!
p-coumaroyl–CoA 1 AMP 1 PPi
[III]
The CoA thioesters may be converted to the corresponding
alcohols (monolignols, e.g. coniferyl alcohol; Figure 2) by
(hydroxy)cinnamoyl–CoA reductase (CCR) and (hydroxy)cinnamyl alcohol dehydrogenase (CAD).
Hydroxy fatty acids (HFAs)
HFAs are assembled on an acyl carrier protein (ACP),
using acetyl–CoA and malonyl–CoA as C2 donors, with
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
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Plant Cell Wall Biosynthesis
the reduced form of nicotinamide–adenine dinucleotide
phosphate (NADPH) as reductant. Major intermediates
are palmityl-ACP and oleyl-ACP, which are hydrolysed to
release palmitic and oleic acids (saturated C16 and 9,10unsaturated C18 chains, respectively). Palmitic acid is
hydroxylated (using O2 1 NADPH) to form the C16
HFAs. Oleic acid may be hydroxylated at C18 and/or
epoxidated at the C=C group; the epoxide thus formed
may be hydrolysed to yield a 9,10-diol. HFA–CoA
thioesters are then presumably formed by ligases (see
above).
Gal
ATP
21
23
ADP
ADP
GalA-1-P
Gal-1-P
UTP
Ara-1-P
UTP
28
UTP
29
30
PPi
NADP+ + H2O
19
ATP
22
ADP
UDP-Rha
Ara
GalA
ATP
PPi
PPi
UDP-D-Gal
UDP-GalA
UDP-Ara
* 14
* 15
* 16
CO2
Glc-1-P
5
12
UDP-Glc
UDP-GlcA
2NAD+ 2NADH
+2H+
UTP PPi
25
7
Glc-6-P
Ins-1-P
*
CO2
20
GlcA-1-P
Pi
ADP
26
Fru-6-P
UDP-Api
ADP
8
* 2
10
ATP
O2
9
Inositol
GlcA
* 3
ATP
Man
ADP
27
Man-6-P
GDP-L-Gal
Fuc
ATP
* 4
* 18
24
ADP
PPi GTP
Man-1-P
6
GTP PPi
4
Wall proteins are synthesized by ribosomes on the outer
surface of the rough endoplasmic reticulum (ER). At the
N-terminus of the nascent polypeptide is a hydrophobic
‘signal sequence’ that leads this end of the chain through
the membrane into the ER lumen. The signal sequence is
later removed enzymatically. The new protein is carried
through the endomembrane system: ER!cis-Golgi!
medial-Golgi!trans-Golgi!plasma membrane. During
this journey, the protein may be posttranslationally
modified. In extensin, for example, most of the proline
residues get oxidized to 4-hydroxyproline (Hyp) about 2.5
min after their incorporation into the polypeptide chain.
This was shown by pulse-labelling carrot roots with
[14C]proline (Sadava and Chrispeels, 1971). Later, most
of the Hyp residues are O-arabinosylated as in reaction [IV]
(where n 5 1–4) and some Ser residues are O-galactosylated.
[IV]
Ara n
Many wall enzymes are N-glycosylated: oligosaccharides
(rich in Man and GlcNAc; sometimes also Fuc, Gal, Xyl,
etc.) are attached to the –CONH2 groups of specific Asn
residues. In some enzymes, e.g. xyloglucan endotransglycosylase (XET), this N-glycosylation is essential for full
enzymic activity.
ADP
H2O
Fru
UDP-Xyl
PPi
UTP
ATP
13
11
* 1
ATP
Proteins and glycoproteins
… −(Hyp)− … + nUDP-Ara → … −(Hyp)− … + nUDP
?NADPH + H+
Glc
Polymer Assembly in Endoplasmic
Reticulum and Golgi Cisternae
GDP-Man
17a–c
GDP-Fuc
NADPH NADP+
+ H+
+ H2O
31
Fuc-1-P
Figure 1 Major pathways for the synthesis and interconversion of the
NDP sugars used in the biosynthesis of plant cell wall polysaccharides.
Some indication of the relative flux through the various pathways is given
by arrow thickness. In addition, solid arrows indicate primary pathways for
de novo synthesis of NDP sugars and inositol. Pecked arrows (————)
imply great variation between tissues. Dot-dashed arrows (–––) indicate
scavenger pathways involved in recycling monosaccharides, e.g. released
by polysaccharide turnover. The ‘box’ is the pool of hexose
monophosphates mentioned in the text. Reactions marked * are
isomerizations, with no other reactants. Numbered enzymes are: 1,
phosphoglucomutase; 2, glucose 6-phosphate isomerase; 3, mannose 6phosphate isomerase; 4, phosphomannomutase; 5, UDP-glucose
pyrophosphorylase; 6, GDP-mannose pyrophosphorylase; 7, myo-inositol
1-phosphate synthase; 8, myo-inositol 1-phosphatase; 9, myo-inositol
oxygenase; 10, glucuronokinase; 11, UDP-glucuronate
pyrophosphorylase; 12, UDP-glucose dehydrogenase; 13, UDPglucuronate decarboxylase; 14, UDP-glucose 4-epimerase; 15, UDPglucuronate 4-epimerase; 16, UDP-xylose 4-epimerase; 17, ‘GDP-fucose
synthase’ (three individual activities: (a) GDP-D-mannose 4,6-dehydratase,
(b) GDP-4-keto-6-deoxy-D-mannose 3,5-epimerase, and (c) GDP-4-keto-Lfucose 4-reductase); 18, GDP-mannose 3,5-epimerase; 19, ‘UDPrhamnose synthase’ (probably three activities, cf. 17); 20, UDP-apiose
synthase; 21, D-galactokinase; 22, galacturonokinase; 23, arabinokinase;
24, fucokinase; 25, hexokinase or glucokinase; 26, fructokinase; 27,
mannokinase; 28, UDP-D-galactose pyrophosphorylase; 29, UDPgalacturonate pyrophosphorylase; 30, UDP-arabinose pyrophosphorylase;
31, GDP-fucose pyrophosphorylase.
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Plant Cell Wall Biosynthesis
OH
CH 3O
HC
CH
CH 2OH
Coniferyl alcohol
[H]
O•
Free
radicals:
O
CH 3O
CH 3O
O
C
•
O
CH 3O
CH 3O
C•
HC
HC
CH
CH 2OH
HC
CH
CH 2OH
A
HC
CH
CH 2OH
B
CH 2OH
C
D
A+B
coupling
D+D
coupling
O
O
O
CH 3O
•
CH
CH 3O
CH 3O
O
CH 3O
Initially
formed
dimers
HC
HC
CH
CH 2OH
HC
HC
HC
CH 2OH
CH
CH
CH 2OH
CH 2OH
OH
OH
CH 3O
CH 3O
O
Stable
rearrangement
products
CH 3O
O
HC
HC
CH
CH 2OH
HC
HC
CH
H 2C
CH 2
CH
CH
O
CH 2OH
OCH 3
OH
Figure 2 Early steps in lignin synthesis. Coniferyl alcohol (one of three monolignols) is oxidized enzymically, losing one hydrogen atom to form a free
radical, which rapidly interconverts between four tautomers (A, B, C, D). These pair off nonenzymically to form dimers (two of the several possible dimers
are illustrated), some of which (e.g. the D 1 D dimer) undergo intramolecular substitution reactions to form more stable products.
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Plant Cell Wall Biosynthesis
Matrix polysaccharides
Pectins and hemicelluloses (except callose) are synthesized
in Golgi bodies. Polysaccharide assembly begins in ciscisternae, the finishing touches being applied in the medialand/or trans-cisternae. The basic reaction [V] is catalysed
by transferases called NDP sugar : polysaccharide transglycosylases or polysaccharide synthases.
NDP-sugar 1 (sugar)n!NDP 1 (sugar)n+1
[V]
Specific permeases carry NDP-sugars into the Golgi
lumen, where polysaccharide synthesis occurs.
All NDP-d-sugars are a-anomers, whereas the d-sugar
residues in polysaccharides may be a- and/or b-anomers.
Simplistically, transglycosylation (as an SN2 reaction)
should reverse the anomerism: thus UDP-a-Xyl could
directly add a b-Xyl residue to a xylan chain. Retention of
anomerism, on the other hand, e.g. when UDP-a-Xyl adds
an a-Xyl residue to xyloglucan, suggests two consecutive
transglycosylations (a!b!a). In such cases, the enzyme
itself may accept the sugar residue before adding it to the
nascent polysaccharide, reversal of anomerism occurring
at each step (reactions [VI] and [VII]).
UDP-a-Xyl 1 [enzyme]!b-Xyl–[enzyme] 1 UDP
[VI]
b-Xyl–[enzyme] 1 (sugar)n!a-Xyl–(sugar)n
1 [enzyme]
The activity of the transglycosylases (rather than supply of
NDP-sugars) is the main variable regulating polysaccharide biosynthesis. For example, pectin and xylan synthases
decrease and increase, respectively, during the transition to
secondary wall production during xylem differentiation
(Bolwell and Northcote, 1981).
Nonsugar groups (methyl, acetyl and feruloyl esters;
methyl ethers) may also be added to polysaccharide chains
in Golgi cisternae, as shown in reactions [VIII] and [IX]
(Me, methyl ester or ether; Ac, acetyl ester). The feruloyl
donor may be feruloyl–CoA.
S-adenosyl-methionine 1 (sugar)n!(sugar)n –Me
1 S-adenosyl-homocysteine
Ac-CoA 1 (sugar)n!(sugar)n –Ac 1 CoA
[VIII]
[IX]
The completed polysaccharides in the trans-cisternae are
finally carried in vesicles to the plasma membrane, and
thence into the apoplast, to be integrated into the wall.
[VII]
During xyloglucan synthesis, the a-Xyl side-chains are
added only to those b-Glc residues that have themselves
very recently been incorporated into the nascent polymer’s
backbone; equally, further elongation of the backbone
appears to depend on successful a-xylosylation at or near
the nonreducing end. The enzymes cannot complete a
glucan backbone and then add a-Xyl side-chains. In
contrast, a-Fuc side-chains can be added to a preformed
xyloglucan core. Similar conclusions have been reached
through studies of galactomannan biosynthesis (Reid et al.,
1995).
Specific polysaccharides synthesized in the Golgi bodies
include homogalacturonan, xyloglucan, (glucurono)(arabino)xylans and (galacto)(gluco)mannans, as demonstrated by two main approaches:
1. Isolated Golgi bodies can synthesize these polysaccharides from the appropriate NDP sugars (Hobbs
et al., 1991; Reid et al., 1995). The transferase activities
responsible are integral to Golgi membranes; few such
enzymes have been solubilized in active form. However, partial fractionation of the cis-, medial- and
trans-cisternae has indicated that different steps in
polysaccharide assembly occur in different cisternae.
6
2. Immunocytochemical studies by electron microscopy
have demonstrated the presence of epitopes characteristic of these polysaccharides within the Golgi cisternae and associated vesicles. Simultaneous labelling
with two distinguishable antibodies confirms that
different epitopes first appear in different cisternae
(Zhang and Staehelin, 1992).
Polymer Assembly at the Plasma
Membrane
Cellulose and callose are not synthesized within the
endomembrane system but at the plasma membrane.
Callose
Callose synthase is readily demonstrated in isolated plasma
membranes in vitro (reaction [X]) and can be solubilized in
an active form.
UDP-Glc 1 (Glc)n!(Glc)n+1 1 UDP
[X]
Callose synthase activity is immediately promoted by
various stimuli, e.g. Ca2 1 , perhaps explaining the dramatic induction of callose as a wound response.
Cellulose
The mechanism of plant cellulose synthesis is poorly
understood. This is particularly galling because cellulose is
the world’s most abundant organic chemical! Quantitative
metabolic studies in vivo showed that UDP-Glc is a
precursor of cellulose. However, cellulose synthesis has
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Plant Cell Wall Biosynthesis
never been demonstrated with plant enzymes in vitro at
rates approaching those observed in vivo.
In vivo, new cellulose chains are almost instantly
corralled into microfibrils, which appear by electron
microscopy as strands issuing from ‘terminal complexes’
(rosettes) in the plasma membrane. Each terminal complex
presumably contains several cellulose synthase molecules
(equal to the number of cellulose chains in a microfibril
cross-section). Since microfibrils are firmly embedded in
the wall, the terminal complexes must move about in the
plasma membrane as the microfibrils elongate. This
movement, and hence the orientation of new microfibrils,
may be guided by microtubules in the cytosol.
Since plants have unique cell walls, inhibitors of wall
biosynthesis could be highly specific herbicides. Cellulose
synthesis in vivo is specifically inihibited by the herbicides
dichlobenil (2,6-dichlorobenzonitrile) and isoxaben. The
interaction of these compounds with cellulose synthase
may assist identification of the enzyme or a closely
associated protein.
Recent research has identified genes that may encode
plant cellulose synthases. In cotton seed hairs, a gene called
celA1 is massively upregulated when synthesis of the
cellulose-rich secondary wall begins, 17 days after anthesis.
The celA1 gene resembles a bacterial cellulose synthase
gene (celA) and celA1 protein binds UDP-Glc. These
observations suggest that celA1 encodes a cotton cellulose
synthase (Pear et al., 1996). An Arabidopsis mutant (rsw1)
exhibiting ‘radial swelling’ may be defective in cellulose
synthase. The mutant fails to synthesize crystalline
cellulose and the terminal complexes disorganize, suggesting that the wild-type allele encodes a protein involved in
microfibril assembly (Arioli et al., 1998). Thus, although
there are still severe difficulties in studying plant cellulose
synthases, rapid progress is being made in the related
genetics.
Polymer Assembly in the Cell Wall
Polysaccharide and glycoprotein crosslinking
The final integration of polysaccharides and proteins,
synthesized by the protoplast, occurs in the wall itself. For
example:
. XET may attach new segments of xyloglucans to the
ends of existing wall-bound xyloglucans (Thompson
et al., 1997);
. enzymic removal of methyl-esters allows pectins to
become crosslinked by Ca2 1 bridges;
. oxidative crosslinking of ferulate and tyrosine sidechains of some polysaccharides and proteins, respectively, may occur in the wall by peroxidase action (cf.
lignin synthesis).
Cutin and suberin
Cutin and suberin are water-insoluble; they are synthesized
at their final destination in the wall and/or cuticle. HFA–
CoAs (see above) are probably the major precursors, as
indicated by the ability of a particulate fraction from bean
leaf epidermis to incorporate [14C]HFAs into insoluble
[14C]polyesters in the presence of CoA and ATP (Croteau
and Kolattukudy, 1974). It is unclear how HFA–CoAs
reach the cutinizing or suberizing part of the wall or how
CoA, released during polymerization, is recycled.
Lignin
Lignin is synthesized in the wall by polymerization of
secreted monolignols. In contrast to other major biopolymers, lignin forms from free radical intermediates. Each
monolignol (e.g. coniferyl alcohol, C10H12O3) loses 1H,
thus becoming a free radical, (C10H11O3).. The major
hydrogen acceptor is probably H2O2, as in reaction [XI]
(catalyst: peroxidase).
2 C10H12O3 1 H2O2!2 (C10H11O3). 1 2 H2O [XI]
However O2 may also contribute, as in reaction [XII]
(catalyst:oxidase).
4 C10H12O3 1 O2!4 (C10H11O3). 1 2 H2O
[XII]
The free radicals formed undergo rapid, nonenzymic
interconversions (tautomerization; Figure 2), before eventually coupling, as in reaction [XIII], to form a dimer
(C20H22O6).
2 (C10H11O3).!(C10H11O3)–(C10H11O3)
[XIII]
The new bond formed can be C–C or C–O, depending on
which tautomers participate (Figure 2). Coupling is widely
assumed to be stochastic (semirandom); however, recent
evidence suggests that ‘dirigent’ proteins can favour
specific couplings, at least in related intraprotoplasmic
reactions (Davin et al., 1997). Some of the dimers (quinone
methides) undergo nonenzymic intramolecular rearrangements or bonding to external nucleophiles (e.g. polysaccharides, generating ‘lignin–carbohydrate complexes’).
A dimer can itself lose an H to become a new free radical,
(C20H21O6)., which can couple with another free radical. If
the latter is also a dimer, as in reaction [XIV], then the
product is a tetramer (C40H42O12).
2 (C20H21O6).!(C20H21O6)–(C20H21O6)
[XIV]
Continuation of such reactions ( 5 oxidative polymerization) generates the lignin polymer.
The source of H2O2 for lignin synthesis remains
uncertain. It may arise from O2 by the action of
wall-bound oxidases on polyamines (reaction [XV]; R– is
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Plant Cell Wall Biosynthesis
NH2 –(CH2)4 –NH–(CH2)2 – in the case of spermidine) or
some other reducing agent.
R–CH2 –NH2 1 O2 1 H2O!R–CHO 1 H2O2 1 NH3
[XV]
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8
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Further Reading
Bolwell GP (1988) Synthesis of cell wall components: aspects of control.
Phytochemistry 27: 1235–1253.
Boudet AM, Lapierre C and Grima-Pettanati J (1995) Biochemistry and
molecular biology of lignification. New Phytologist 129: 203–236.
Delmer DP and Amor Y (1995) Cellulose biosynthesis. Plant Cell 7: 987–
1000.
Delmer DP and Stone BA (1988) Biosynthesis of plant cell walls. In:
Preiss J (ed.) Biochemistry of Plants: a Comprehensive Treatise, vol. 14,
pp. 373–419. New York: Academic Press.
Feingold DS and Barber GA (1990) Nucleotide sugars. In: Dey PM (ed.)
Methods in Plant Biochemistry, vol. 2, pp. 39–78. London: Academic
Press.
Fry SC (1988) The Growing Plant Cell Wall: Chemical and Metabolic
Analysis. Harlow, Essex: Longman.
Kolattukudy PE (1996) Biosynthetic pathways of cutin and waxes, and
their sensitivity to environmental stresses. In: Kerstiens G (ed.) Plant
Cuticles – An Integrated and Functional Approach, pp. 83–108. Oxford:
Bios.
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