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179
Plant cell walls as targets for biotechnology
Clint Chapple∗ and Nick Carpita†
Plants are the sources of major food, feed, and fiber products
that are used globally. This past year has seen advances
in our understanding of the enzymes that modify wall
architecture, the cloning of the first cellulose synthase gene,
and revisions to the lignin biosynthetic pathway. These
discoveries have facilitated the development of new strategies
to alter cell wall properties in transgenic plants.
corresponding genes, indicate that this may not be the
case, and that there is much yet to be learned about
all aspects of cell wall synthesis and structure. In this
brief review, we highlight a few of the advances in the
identification of the relevant genes and gene products that
either are being or could be manipulated to alter cell wall
structures in our crop plants and trees.
Addresses
∗Department of Biochemistry and †Department of Botany and Plant
Pathology, Purdue University, West Lafayette, Indiana 47907, USA
∗e-mail: [email protected]
†e-mail: [email protected]
Cell walls as food products
Current Opinion in Plant Biology 1998, 1:179–185
http://biomednet.com/elecref/1369526600100179
 Current Biology Ltd ISSN 1369-5266
Abbreviations
4CL
hydroxycinnamoyl CoA ligase
CAD
hydroxycinnamoyl alcohol dehydrogenase
CCR
hydroxycinnamoyl CoA reductase
C4H
cinnamate-4-hydroxylase
EST
expressed sequence tag
FSH
ferulate-5-hydroxylase
OMT
caffeic acid/5-hydroxyferulic acid O-methyltransferase
PAL
phenyl ammonia-lyase
PGase polygalacturonase
PME
pectin methylesterase
XET
xyloglucan endotransglycosylase
Introduction
Herbicide- and insect-resistant plants are now in mass
production, and the first genetically engineered plant oils
and other products are reaching the market-place. As the
range of products produced by transgenic plants continues
to broaden, plant cell walls have now become the targets
for engineering. Recently, many enzymes that function
in the assembly, modification, and turnover of wall
polysaccharides have been purified and their cDNAs have
been cloned but we have only recently begun to identify
the genes responsible for polysaccharide synthesis. A
major milestone just passed was the identification of a
gene encoding a catalytic subunit of cellulose synthase
[1••] and proof of its function by complementation of
a temperature-sensitive synthase mutant [2••]. These
discoveries have indicated that many previously unknown
and unrecognized genes are present in the Arabidopsis
expressed sequence tag (EST) database that may encode
synthases of many non-cellulosic polysaccharides [3]. In
comparison, it had been thought that the pathway of
lignin biosynthesis was better understood than those
that generate cell wall polysaccharides. The analysis
of plants that are downregulated in the expression
of lignification-associated enzymes, or mutated in their
Nutritionally, plant cell wall polysaccharides are important
dietary fibers. They are used widely in the food industry
as gelling and thickening agents. Along with storage
proteins, unique features of the cell walls in different
seed flours alter baking properties and crumb textures.
Cell walls are the principal textural components of fruits
and vegetables, and this texture changes markedly during
ripening and/or cooking. We are just now learning which
components are the important determinants of texture,
how these components are assembled, and how the many
wall enzymes change wall characteristics.
Pectins are the major gelling agent from higher plants.
Apple and orange pectins are the major commercial
sources of pectins but, as Thakur et al. [4] explain,
many other pectin-rich plant materials could become value
added by-products if undesirable chemical characteristics
could be eliminated with the use of transgenic plants.
Changes in the pectin matrix are regarded as the
obvious components of fruit cell walls that impact wall
texture during ripening which, to different degrees in
different plants, involves wall softening and cell separation. Prolonging the desirable texture during ripening
is the key to prolonging the shelf-life of the fruit.
Although activities of pectin methylesterase (PME) and
polygalacturonase (PGase) increase in ripening fruit and
near-elimination of PGase can increase tomato paste
thickness [5], antisense inhibition experiments conducted
a decade ago demonstrated that depolymerization of
pectins is not the only determinant of texture. Researchers
are now examining the roles other kinds of enzymes may
play in wall softening, such as pectate lyases [6], cellulases
and xyloglucan-degrading hydrolases [7,8]. For example,
antisense inhibition of pepper glucanase increases the
shelf-life of peppers (see Bedbrook, in [9]). In addition
to glycan hydrolases, attention has also turned to two
proteins that do not depolymerize the wall: expansins,
which are proteins capable of disrupting hydrogen bonds
between cellulose and non-cellulosic glycans [10], and
the xyloglucan endotransglycosylases (XETs) which catalyze interchain transfer among populations of xyloglucan
cross-linking glycans [11]. Expansins are encoded by a
large gene family, and Rose et al. [12•] discovered family
members the expression of which is restricted to ripening
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Plant biotechnology
stages of tomato, strawberry and melon fruit after growth
has ceased and the walls begin to soften.
In addition to large-scale changes in major cell wall
polysaccharides, alterations in some of the more minor
cell wall polysaccharides also have an impact on cell wall
properties. For example, in contrast to what happens in
the tomato pericarp, xylans and pectic polysaccharides
are extensively degraded and solubilized in the locule
during liquefaction [13]. Rhamnogalacturonan I (RG I)
is another major pectic polysaccharide of the fruit wall
and one that is highly branched with (1→4)-β-D-galactans
that can affect wall texture. These RG-I-decorating
galactans were immunolocalized to all cells of the pericarp
but not the locule or epidermis [14]. Similarly, the
de-esterification of methylated pectins catalyzed by PME
does not occur uniformly but occurs in distinct block-like
domains, indicating that PME activity is spatially restricted
[15]. Infrared microspectroscopy is able to non-invasively
map esterified and non-esterified pectin domains and
orientation of polysaccharides in 10 × 10 µm sections of
underivatized tissues [16•]. This technique will be brought
to bear on the problems of localized alterations in wall
metabolism that impact overall fruit quality.
Not all the architectural changes can be traced strictly
to the structure of individual polysaccharides. Waldron
et al. [17] have shown that diferulic acid crosslinks of
arabinoxylans are critical to cell wall structural rigidity and
methods to enhance cross-linking may enhance textural
qualities during processing. Another important processing
characteristic is wall softening, which is correlated with
wall swelling. Redgwell et al. [18•] showed that isolated
walls from ripe kiwi fruit swell almost ten-fold greater
than do walls from unripe fruit, and this enormous volume
change occurs without significant pectin depolymerization.
How to prevent breakage of polysaccharide cross-linkages
must be included in engineering strategies.
Cellulose biosynthesis and improvement of
fibers
Improvement of plant fibers for use in the textile industry
will require an understanding of how glucan chains are
packed into a para-crystalline cellulose microfibril, how the
microfibrils are oriented around the fiber cells and how, in
some instances, non-cellulosic polysaccharides space the
microfibrils apart and contribute to unique textures of the
fiber. Cellulose synthesis is associated with six-membered
particle rosettes located at the plasmamembrane. The full
pathway of cellulose biosynthesis, from translocated sugar
to the synthesis of a para-crystalline array of several dozen
(1→4)-β-D-glucan chains into a single microfibril, is still
unknown [19]. For over 30 years, investigators have been
stymied in their attempts to stabilize cellulose synthesis in
vitro.
Two principal fiber crops are cotton and flax. Despite
the lack of a stable cellulose synthase complex in vitro
from higher plants, two cotton genes termed CelAs and
thought to encode the catalytic subunit for cellulose
synthase were first described in the past year [1••]. A
bacterial cellulose synthase operon was first described
several years ago [20] but use of the bacterial clone as
probe for higher plant cellulose synthase proved fruitless.
The identification of four domains essential for binding
of the UDP-glucose substrate in many (1→4)-β-D-glucosyl
transferases [20] proved to be the breakthrough needed
to identify the plant homolog [1••] (Figure 1). Arioli
et al. [2••] have identified a temperature-sensitive cellulose
synthesis mutant of Arabidopsis that is defective in a
gene which is a homolog of the cotton CelA genes. In
this mutant, the rosette particle arrays were disoriented
at non-permissive temperatures and reintroduction of the
CelA gene into the mutant restored normal cellulose
synthesis at elevated temperatures.
The celA protein is probably only one of several proteins
comprising the complete cellulose synthase complex.
The finding of a Zn2+-binding domain near the amino
terminus of celA is consistent with the hypothesis that
the synthase interacts with at least one other protein
[21]. One candidate is a recently-discovered, extracellular,
membrane-associated endo-β-D-glucanase [22••]. A similar
membrane-associated glucanase gene is also found within
an operon essential for cellulose biosynthesis in Acetobacter
[23]; however, the role for such an enzyme in cellulose
synthesis is unknown. Hydrolases associated with the cellulose synthesis machinery may function in a proof-reading
capacity to excise mistake linkages and ensure formation
of only (1→4)-β-D-glucosyl units.
Not all the biotechnological advances that impact cell
walls need to be targeted to the wall by the living cell.
Beyond the efforts to understand how cellulose is made,
researchers are now considering how the cotton fiber can
be modified to impart special properties important to the
textile industry [24]. For example, transgenic cotton fibers
that make the thermoplastic polyhydroxyalkanoate, were
shown to have improved insulating properties [25•]. In
this instance, the plastic accumulated in the cytosol of the
cotton hair and was heat-pressed into the cellulose after
harvest of the mature fibers.
Whereas cotton fibers are trichomes of the seed coat and
easily harvested, flax fibers are in bundles encircling the
vascular cylinder and must be extracted from the plant
and combed apart. The recovery of high-quality flax fibers
is compromised by several production problems related
to extraction of the fibers from the pectin-rich cells that
surround them. Unlike cotton fibers, which are nearly pure
cellulose, mature flax fibers are ∼70% cellulose with the remainder comprising various glycans and pectic substances
and uncharacterized aromatic substances. Flax fibers are
stiffer and stronger than cotton fibers, partly because of
the greater crystalline nature of the cellulose, as revealed
by solid-state NMR, and partly because the fiber cells
Plant cell walls as targets for biotechnology Chapple and Carpita
181
Figure 1
Agrobacterium tumefaciens Cel1A
Acetobacter xylinum BcsA
H-1
P-1
H-2
HVR
H-3
Cotton CelA
U-1
U-2
U-3
U-4
Current Opinion in Plant Biology
Comparison of the bacterial cellulose synthase genes BcsA from Acetobacter xylinum and CelA from Agrobacterium tumifaciens with CelA1
from cotton. Three regions (H-1, H-2, and H-3) in the deduced amino acid sequences of the plant CelA gene products possess high similarity
with the proteins that are encoded by the bacterial genes [1••]. Within the conserved regions are four highly conserved subdomains (U-1, U-2,
U-3, and U-4) previously suggested by Saxena et al. [20] to be critical for catalysis and/or binding of the substrate UDP-Glucose. The plant
CelA genes also contain two internal insertions of sequence, one conserved among the plant CelA genes (P-1) and one hypervariable (HVR),
that are not found in the bacterial genes.
are cemented tightly together in late development [26].
Many of the polysaccharides surrounding the cellulose
microfibrils in flax are acetylated, a feature that must
impact fiber qualities [27]. Among the pectin substances
of the developing fiber is an unbranched β-D-galactan
that turns over during the maturation of the fiber [28].
This unbranched galactan may prevent the cementing of
the fiber bundles until after intrusive growth is complete.
Suppression of galactan degradation may facilitate the
combining of high-quality fibers used for linen. Flax
is readily transformed by Agrobacterium tumefaciens, and,
therefore, is particularly amenable to improvement by
genetic engineering. Efforts must be directed towards the
identification of fiber-specific genes.
Gene discovery through cell wall mutants
The discovery of a temperature-sensitive cellulose synthase mutant has been pivotal in the confirmation of the
celA gene by complementation [2••]. Cutler and Somerville
[3] reported that Arabidopsis contains several homologs
of cotton CelA. They also found that a number of other
sequences share significant identity to one or more of
the suspected UDP-glucose-binding domains. Why plants
have so many different CelA and related genes is now a
focus of investigation in a number of laboratories. The
glucan chains in cellulose microfibrils made by primary
and secondary walls are distinguished by differences in
their degree of polymerization, which may eventually be
traced to different modes of catalysis and organization
into para-crystalline arrays. A family of CelA genes may
represent the diversity of cellulose synthases needed
to build the different kinds of walls that plants make.
For example, another Arabidopsis mutant was described
whose xylem cells incompletely form secondary walls
[29], apparently due to a defect in cell-specific cellulose
deposition. In addition, some of the other related genes
may well encode synthases of β-D-xylans, mannans, the
backbone of xyloglucan, mixed-linkage β-D-glucans, or
callose (1→3β-D-glucan). Almost two dozen cell wall
mutants representing 11 loci were discovered by screening
a mutagenized Arabidopsis population [30•]. The first of
these mutations to be analyzed is the mur1 mutant,
which is characterized by the lack of fucose in any
polymer of the aerial portion of the plant. The mutation
has been traced to a defective gene encoding GDP-Dmannose-4,5-dehydratase, the first step towards formation
of GDP-fucose [31]. Interestingly, blockage at this step
shunts the carbon to GDP-L-galactose, which is added
to a few of the 2-linked D-galactose units of xyloglucan
instead of L-fucose [32•]. Whereas such a substitution
imparts the mutant with a brittle influorescence stem, the
oligosaccharide containing L-galactose in place of L-fucose
is able to inhibit auxin-induced growth.
The biotechnological modification of
lignification
Lignin is intimately associated with cellulose and other
polysaccharides of the secondary cell wall. It is an obstacle
in the production of pulp and paper, and it lowers the
nutritional value of agricultural crops used for animal
feed [33••]. Although insights into the enzymes and
genes involved in cell wall polysaccharide biosynthesis
are relatively recent, the enzymes of lignin biosynthesis
(Figure 2) have been studied for many years and several
of the genes of the pathway had been cloned by the early
part of this decade. It is somewhat surprising then that
recent attempts at engineering lignin biosynthesis have
demonstrated that our current models of the pathway are
incomplete and, in some instances, in error.
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Plant biotechnology
Figure 2
Phenylalanine
Cinnamic acid
p-Coumaric acid
p-Coumaroyl CoA
Caffeic acid
Caffeoyl CoA
Ferulic acid
Feruloyl CoA
5-Hydroxyferulic acid
Sinapic acid
5-Hydroxyferuloyl CoA
Sinapoyl CoA
Dihydroconiferyl alcohol
Coniferaldehyde
Coniferyl alcohol
5-Hydroxyconiferaldehyde
5-Hydroxyconiferyl alcohol
Sinapaldehyde
Sinapoyl alcohol
Current Opinion in Plant Biology
The biosynthetic pathway leading to the production of monolignols in dicotyledonous plants. The enzymatic steps required include phenylalanine
ammonia-lyase (PAL), cinnamate-4-hydroxylase (C4H), p-coumarate-3-hydroxylase (C3H), caffeic acid/5-hydroxyferulic acid O-methyltransferase
(OMT) ferulate-5-hydroxylase (F5H), hydroxycinnamoyl CoA ligase (4CL) p-coumaroyl CoA-3-hydroxylase (pCCoA3H), caffeoyl CoA
O-methyltransferase (CCoAOMT), hydroxycinnamoyl CoA reductase (CCR), and hydroxycinnamoyl alcohol dehydrogenase (CAD). Reactions
shown with question marks and dotted arrows represent reactions that may occur in plants, or appear to occur in transgenic plants that have
been altered in the expression of lignification-related genes. The activation of sinapic acid to sinapoyl CoA by 4CL is shown with a question
mark to reflect the recent finding of Lee et al. [36••]. The structure of the unusual lignin monomer dihydroconiferyl alcohol found in the pine cad
mutant [45••,46••] is illustrated in the inset, although the route of its biosynthesis is still unclear.
Many attempts at the biotechnological modification of
lignification have been aimed at decreasing the total
quantity of lignin in plant tissues by targeting the
enzymes required for the synthesis of all lignin precursors.
Other research has addressed the modification of lignin
quality by down-regulating or overexpressing enzymes
required for the synthesis of guaiacyl (derived from ferulic
acid) and syringyl (derived from sinapic acid) monomers.
These experiments, and the analysis of mutants defective
in lignification-associated genes, have revealed that the
Plant cell walls as targets for biotechnology Chapple and Carpita
makeup of the lignin polymer is far more chemically
plastic than was previously supposed.
Manipulation of lignin quantity
Tobacco plants with sense-suppressed levels of phenylalanine ammonia-lyase (PAL), the intial enzyme of
the phenylpropanoid pathway, were the first plants to
be generated with an engineered lignin [34]. These
plants were characterized in greater detail in combination
with plants downregulated in the second enzyme of
the phenylpropanoid pathway, cinnamate-4-hydroxylase
(C4H) [35•]. As expected, downregulation of PAL or
C4H decreased stem lignin content; however, these two
strategies had opposite effects with respect to lignin
monomer composition. It is not clear why suppression of
these two enzymatic activities led to different phenotypes;
however, the authors speculate that they may reflect the
perturbance of pathway channeling involved in the flux of
phenylalanine toward monolignols.
183
Hydroxycinnamoyl alcohol dehydrogenase (CAD) has
been downregulated in transgenic tobacco with little
effect on the amount of lignin accumulated but with
a striking change in lignin composition [44]. When
extractable CAD activity was depressed to <10% of
wild-type levels, a substantial amount of the lignin
polymer was derived from hydroxycinnamaldehydes. An
even more extreme example of CAD downregulation and
lignin chemistry perturbation was reported recently when
a CAD-deficient pine was identified and characterized
[45•,46••]. Homozygous cad mutants are relatively normal
in appearance but their wood is brown and contains large
amounts of ethanol-soluble coniferaldehyde and vanillin.
Surprisingly, dihydroconiferyl alcohol — which is normally
only a very minor component — was the most abundant
component of the lignin of the cad mutant [45•]. Although
it is clear that a certain amount of lignin is essential for
viability, the normal appearance of these CAD-suppressed
plants again indicates that, at least in certain respects,
lignin chemistry can be changed dramatically relative to
what is normally found in Nature.
Experiments aimed at downregulation of 4-coumarate
CoA ligase (4CL) activity have also been attempted in
several species [36••,37,38••,39]. Lee et al. [36••] found
that antisense suppression of 4CL in Arabidopsis decreased
lignin guaiacyl content, whereas syringyl content was
essentially unchanged. These observations are consistent
with the inability of Arabidopsis 4CL preparations to
activate sinapate to sinapoyl CoA and indicate that there
may be an alternative route for sinapate activation, at
least in some plants. Alternatively, there may be other
sinapate-utilizing CoA ligases that are sufficiently different
in sequence and enzymatic character that they have not
been identified to date. The activity of 4CL is also
necessary for the so-called ‘alternative’ pathway of lignin
biosynthesis. This pathway was originally described in the
context of the synthesis of compounds in plant/pathogen
interactions [39,40], but has recently been implicated in
the biosynthesis of lignin monomers [41]. Although no
mutants have been reported for genes of the alternative
pathway, and there have been no published reports of the
impact of antisensed or co-suppressed alternative pathway
gene expression, the pattern of expression of alternative
pathway genes is consistent with a role in lignification.
Modification of caffeic acid /5-hydroxyferulic acid Omethyltransferase (OMT) expression was aimed at modifying both total lignin content and composition. In those
cases where the greatest degree of endogenous OMT suppression was achieved [47,48], the lignin of the transgenic
plants had a decreased syringyl content and contained
novel 5-hydroxyguaiacyl units. In contrast, there was no
effect on lignin guaiacyl content, presumably because
the alternative pathway provides an OMT-independent
route for guaiacyl lignin synthesis. The vascular tissue
of the transgenic poplar trees produced in one of these
studies [48] was also wine-red in color, similar to that of
the brown-midrib-3 mutant of maize, which is defective
in the gene encoding OMT [49]. The fact that this
strategy did not modify lignin content demonstrates how
a thorough knowledge of the interconnecting pathways of
lignin biosynthesis will be required for the rational design
of metabolic engineering strategies.
The recent cloning of the gene encoding hydroxycinnamoyl CoA reductase (CCR) [42] has permitted the
examination of the utility of CCR-downregulation on
lignin quality and quantity [43]. The primary phenotype
of tobacco plants carrying antisense CCR constructs was a
decrease in total lignin content, although the incorporation
of guaiacyl units was somewhat more sensitive to CCR
downregulation than was the deposition of syringyl units.
The xylem of these plants had an orange-brown coloration,
which may be at least partly explained by the elevated
levels of ester-linked ferulic acid in the xylem cell walls.
Collapse of tracheary elements was observed in the most
severely downregulated lines, indicating that the walls of
these cells were not sufficiently lignified to withstand the
tension generated during transpiration.
The identification of a mutant of Arabidopsis deficient
in ferulate-5-hydroxylase (F5H) activity [50] led to the
cloning of the gene encoding F5H [51], a cytochrome
P450-dependent monooxygenase. As F5H catalyzes the
first committed step in the biosynthesis of syringyl lignin,
Meyer et al. (K Meyer, JC Cusumano, DA Bell-Lelong,
C Chapple, unpublished data) hypothesized that its
expression may regulate syringyl lignin accumulation.
Overexpression of the F5H gene under the control of
the CaMV 35S promoter in transgenic Arabidopsis led
to a 50% increase in the syringyl lignin content and
the accumulation of syringyl lignin in the xylem, which
normally deposits only guaiacyl lignin. These data indicate
that, at least in Arabidopsis, the lignin monomer is
controlled at the level of F5H expression. Surprisingly,
Manipulation of lignin quality
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Plant biotechnology
overexpression of F5H under the control of the Arabidopsis
C4H promoter [52] was much more efficacious and the
syringyl content of these transgenic plants was as high
as 95%. Considering that most previous antisense and
cosuppression efforts aimed at lignin modification have
employed the CaMV 35S promoter, it would be interesting
to revisit these strategies to determine how the use
of a lignification-associated promoter might improve the
degree of suppression achieved.
Conclusions
The biotechnological improvement of the yield and
quality of commercially useful cell wall derived products
is now drawing on diverse areas of research including
plant genome sequencing and mutant analysis as well
as more traditional biochemical approaches. Improvement
and maintenance of desirable characteristics, and the
introduction of novel traits will require a thorough understanding of the complement of enzymes that function
in the biosynthesis, modification and degradation of
plant wall components. We are experiencing a period of
gene discovery of synthetic enzymes for both lignin and
polysaccharides; discoveries that will pave the way for
improvement through metabolic engineering.
Acknowledgements
We would like to acknowledge support from the Department of Energy,
Division of Energy Biosciences (Contract FG02-94ER20138 to C Chapple,
and FG02-88ER13903 to N Carpita). Journal paper No. 15,623 of the Purdue
University Agricultural Experiment Station.
References and recommended reading
Papers of particular interest, published within the annual period of review,
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• of special interest
•• of outstanding interest
1.
••
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Higher plants contain homologs of the bacterial celA genes
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This article reports the cloning of the first cellulose synthase gene (CelA)
from a higher plant. The gene was recognized by deduced amino acid sequence identity of four domains critical for UDP–Glc binding. The higher
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2.
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Arioli T, Peng L, Betzner AS, Burn J, Wittke W, Herth W, Camilleri
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