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504
Cell walls: structures and signals
Roger Pennell
Cell walls harbor proteins and polysaccharides able to
condition the development of a plant. In the past year, genes
and enzymes modulating the composition and physical
properties of walls have been characterized, and wall
composition has been linked to the way a cell interacts with
another cell, and to the way in which it differentiates. The sum
of the signaling and physical activities of a cell wall may explain
much about the control of development.
Addresses
Ceres Inc., 3008 Malibu Canyon, Malibu, California 90265, USA;
e-mail: rpennell@ceres-inc.com
Current Opinion in Plant Biology 1998, 1:504–510
http://biomednet.com/elecref/1369526600100504
© Current Biology Ltd ISSN 1369-5266
Abbreviations
AGP
arabinogalactan-protein
HRGP hydroxyproline-rich glycoprotein
Mab
monoclonal antibody
Introduction
Most cell divisions in plants take place in meristems, and
new cells choose from a variety of potential fate. The divisions generate the correct numbers of cells, and pattern
formation sets up the anatomy. Progression through the
cell cycle seems to require the accurate assembly of a cell
wall. Signaling between newly divided cells then directs
the choice of developmental pathway and some of these
signals are thought to arise from the wall itself [1].
A cell differentiates some time after it has progressed
through a developmental pathway. This differentiation
process usually involves an up to one thousand-fold
increase in cell volume. The enlargement is brought about
by the relaxation of the wall and turgor-driven wall expansion, followed by the re-rigidification and changes in wall
structure. Eventually, walls can be irreversibly reinforced,
either by the addition of new components to the wall or
further cross-linking of existing components [2•].
Thus, the function of a cell is intimately linked to the
composition and structure of its wall. A comprehensive
understanding of the functions of a wall hinges on analysis
of wall assembly and metabolism. To this end, analysis of
wall polymers has revealed novel aspects of wall structure,
and these have been complemented by screens for plants
defective in wall biosynthesis. Experiments have shown
that a cell identified by its wall can control the fate of
another cell, and that walls are involved in cell–cell interactions; these and other studies suggest that signal
molecules released from walls are involved with these
developmental processes in planta. The localization of
genes and proteins affecting wall relaxation and the chemical analysis of wall cross-linking have helped to resolve
the mechanism of cell enlargement and rigidification.
The control of the cell cycle is a key regulator of plant
morphogenesis [3] and it is well documented that cells in
different stages of the cell cycle have characteristic walls.
For example, higher levels of unesterified pectins have
been localised to non-dividing cells than to dividing cells
in the root tip of Arabidopsis [4•], and the expression of two
classes of hydroxyproline-rich glycoproteins (HRGPs),
extensins and arabinogalactan-proteins (AGPs), were
shown to be dependent on the stage of the cell cycle and
on cell proliferation in Catharanthus and carrot [5•,6•].
Their cell cycle functions are unknown; however, the
tomato pectin methylesterase gene PMEU1 was cloned
and characterized [7•], and gain-of-function and loss-offunction experiments with this gene may help to address
these functions directly. It was previously known that prolyl hydroxylation inhibitors caused the disappearance of
HRGP from the wall and also blocked cell division. Now,
the cloning of PsHRGP1, which encodes an extensin
repressed in dividing pea cells [8•], may prove useful in
studying the role of HRGPs in cell division more directly.
These findings have further demonstrated that walls play
diverse and essential roles in plant cell development.
Here, I summarize recent work on the assembly, structure,
and function of cell wall components.
Wall assembly and structure
Polysaccharides and proteins are deposited in a new wall,
and together provide the wall with its properties.
Figure 1 shows some pictures of Arabidopsis leaf cells and
their walls. Cellulose is the major wall polymer, but
genes active in its synthesis were only recently identified
[9]. This is due in part to difficulty in designing genetic
screens for plants deficient in cellulose. In Arabidopsis,
rsw1 mutations cause a reduction in cellulose, the accumulation of a less-ordered β-1,4-glucan, and widespread
morphological abnormalities [10]. RSW1 was cloned, and
found to encode the catalytic subunit of cellulose synthase [11••]. This confirmed the central importance of
cellulose in plant development. The demonstration that
cellulose is absent from xylem walls in Arabidopsis irx
mutants, and that these xylem cells collapse at normal
water pressure, further suggests that cellulose may serve
as a template for the assembly of lignin [12••], and have
a dual role in morphogenesis. The role of an E-type
endo-1,4-β-D-glucanase, which catalyses the breakdown
of cellulose rather than its assembly, active during the
same process in tomato [13•], is less clear. One possibility is that this glucanase is involved with the cutting and
spacing of the cellulose.
Cell walls: structures and signals Roger Pennell
505
Figure 1
(a)
(b)
(c)
CW
CW
CW
CW
Electron micrographs of Arabidopsis leaf cells. A thin cell wall (CW) encloses each cell. Walls are thought to be involved with cell division, cell–cell
interaction, and cell differentiation. The bar in (a) = 50 µm, in (b) = 10 µm, and in (c) = 0.1 µm.
Cellulose is hydrogen-bonded to xyloglucan. Incubation of
pea microsomes with UDP-[14C] galactose and tamarind
seed xyloglucan allowed the identification of a galactosyl
transferase involved with xyloglucan biosynthesis [14•].
The tobacco polygalacturonate-4-α-galacturonosyltransferase that catalyses the transfer of UDP-[14C] galacturonic
acid to homogalacturonan acceptor substrates was also isolated and characterized [15•]. Xyloglucan and pectin play
important roles in morphogenesis, and the manipulation of
these enzymes may facilitate an understanding of their
function. The characterization of some novel wall proteins
[16•] could also be useful in this respect.
In the Arabidopsis mutant fdh1, the shoot epidermis is modified such that it can allow pollen grains to geminate and
enable organs that come into contact to fuse. This suggests
that a developmental program normally operating only in
carpels is activated ectopically. Wall and cuticular permeability were found to be disturbed in fdh1 mutants,
apparently as a result of altered lipid composition [19••].
Local signaling events, therefore, could be conditioned by
wall lipid. Pollen germination on stigmaless Petunia styles
can be enabled by trilinolein [20••], a triacylglyceride lipid
component of wet stigma exudates, suggesting that these
triacylglycerides could be extracellular signaling molecules.
In a genetic approach to the analysis of wall polysaccharide function, mutagenized Arabidopsis plants were used
to identify 11 loci involved with wall biosynthesis [17••].
Mutations causing the absence of fucose (mur1),
decreased levels of fucose (mur2, mur3), arabinose (mur4,
mur5, mur6, mur7), or rhamnose (mur8), or complex alterations in the proportions of several monosaccharides
including fucose, arabinose and rhamnose (mur9, mur10,
mur11), were identified [17••]. Some mutations (mur1,
mur9, mur10) resulted in dwarfism [17••]. This showed
that cell walls are amenable to genetic analysis, and also
that changes in certain monosaccharides are associated
with alterations in development.
Unusual polysaccharide profiles seem to be characteristics
of cell walls in embryonic suspension cultures. Walls in
such cultures of pine were found to contain higher levels
of xylans, fucoxylans, and arabinogalactans than in nonembryogenic lines [21]. Soluble signals including soluble
AGPs help to control somatic embryogenesis, so one possibility is that the wall contributes to a pool of such signals.
In an embryogenic culture of carrot, some cells label at the
wall with the monoclonal antibody (Mab) JIM8, which recognizes a rhamnose-containing epitope present in certain
AGPs. JIM8-reactive cells were found to control the development of JIM8-unreactive embryo initial cells, and
conditioned growth medium from cultures of JIM8-reactive cells could substitute for the cells themselves [22••].
This showed that cells identifiable by their wall composition release diffusible signals, possibly from their walls, to
control a developmental process [22••]. The transformation of Zinnia mesophyll cells into metaxylem-like cells
was also found to be stimulated by soluble signal molecules that were small and heat-resistant [23 •]. Two
tetrasaccharides of rhamnogalacturonan, differing only in
an O-3 acetyl group at an internal galacturonosyl residue,
activated D-glycohydrolases in cultured Rubus cells in
Signaling by walls
Local interactions between the cells in a meristem play an
important role in pattern formation in plants. Laser ablation
studies were used to demonstrate that non-dividing cells in
the quiescent centers of Arabidopsis roots inhibit the differentiation of surrounding cells; the signals achieving this
inhibition operate within the range of a single cell [18••].
Walls form the point of contact between cells; therefore,
one possibility is that the wall dictates this local signaling.
506
Cell biology
Figure 2
Processes involving cells walls. Following cell
division, a new wall can undergo relaxation,
shown by the loosening of the small horizontal
lines, and turgor-driven expansion, shown by
the increase in size of the wall. Cross-links
can be re-established in expanded walls,
shown as the replacement of small horizontal
lines, or the wall can be irreversibly modified,
shown as the shading of the wall. There is
also evidence that walls can release small
signaling molecules, shown as small dots, that
can diffuse from cell to cell and influence cell
development. The boxes represent parts of
cell walls, and the small vertical lines
represent wall structural elements.
Signaling
Division
Relaxation and
expansion
Modification
Cross-linking
Current Opinion in Plant Biology
what could be a related process [24•]. Walls and soluble
oligosaccharide signals released from them may, therefore,
be active in diverse aspects of cell development.
Labeling studies with Mabs reinforced the view that signals from walls are involved with vascular development
and cell patterning in planta [25•]. For example, epitopes
carried by some AGPs were shown to occur in root apices
in patterns that were mutually exclusive, and also reflecting the distribution of extensin epitopes, and both AGP
and extensin epitopes were associated with future cell
anatomy [26•]. The epitope recognized by JIM15 was only
found on an oxidatively coupled sugar beet AGP [27•].
Oxidative cross-linking of molecules in cells is usually
brought about by H2O2, suggesting that H2O2 has a role in
AGP function [27•]. β-glucosyl Yariv reagent, which binds
to AGPs, caused the appearance of shootless embryos
when added to a carrot suspension culture [28•], and inhibited root elongation when added to Arabidopsis seedlings
[29•]. Although β-glucosyl Yariv reagent was found not to
be specific for AGPs in certain circumstances [30•], these
experiments with carrot and Arabidopsis featured controls
with α-glucosyl reagent, and support the idea that certain
AGPs have mechanical functions such as cell expansion
functions. NaPRP5, which codes for the core protein of a
Nicotiana style AGP [31•], was cloned and gain-of-function
and loss-of-function experiments with this gene could help
to address this putative role. It had previously been postulated that a stylar AGP may act as a pollen attractant, but
this idea has since been questioned. The role of the stylar
protein itself could now be re-examined by using molecular genetics [32].
Wall differentation
When a cell has chosen from its range of potential fates, it
differentiates. Expansin is thought to loosen noncovalent
associations between wall polysaccharides, allowing turgor
pressure to drive wall creep. Four expansin genes
(OsEXP1 – OsEXP4) were shown to be expressed in elongating rice tissues [33••], and an antiserum to a cucumber
homolog of one of them was used to show that the protein
itself occurs at sites of elongation [34•]. Immunoblotting
also revealed that submerged and elongating internodes
contained more expansin than internodes grown in air
[35•], linking expansin accumulation in the wall to cell
and tissue elongation. Interestingly, expansin-coated
beads applied to tomato shoot apices generated small
bulges, some of which developed into leaf-like structures
bearing trichomes and expressing the gene encoding the
small subunit of ribulose bisphosphate carboxylase rbcS
[36••]. These ectopic structures reversed the phyllotaxy
[36••], suggesting that expansin, by generating wall relaxation, can have direct and profound effects on
plant morphogenesis.
The tomato expansin Le-EXP1 was found to be regulated
by ethylene, and cDNAs related by sequence homology to
Le-EXP1 were shown to be upregulated during ethyleneinduced ripening in melon and strawberry [37•]. Ethylene
may, therefore, influence plant morphogenesis in part by
controlling expansin activity. Grass pollen group I allergens
were also found to be expansins, likely facilitating pollen
penetration of the stigma [38•]. Thus, expansins are regulated during development and by environmental factors,
and play diverse role in plant growth.
Cell walls: structures and signals Roger Pennell
Some HRGPs [39] are also thought to condition cell
expansion. The tomato extensins encoded by DIF10 and
DIF54 are preferentially expressed in root hairs, and inhibition of tomato prolyl hydroxylation with
3,4-dehydro-L-proline caused the appearance of short
root hairs [40•]. Also, the soybean SbHRGP3 extensin
was found to accumulate in maturing roots at a time when
root hairs are growing [41•]. These experiments link
extensins with cell elongation, and it would be interesting to see if the Arabidopsis mutant cow1, which correctly
initiates root hairs but fails to allow root hair elongation
[42••], is affected in an extensin gene. Interestingly, treatment of Arabidopsis seedlings with β-glucosyl Yariv
reagent resulted in bulging epidermal cells similar to the
epidermal cells of AGP-deficient reb1 mutants [43•], suggesting that different kinds of HRGPs could have
separate but related roles in cell enlargement.
Xyloglucan endotransglycosylases are believed to allow
slippage in the wall by cutting adjacent xyloglucan chains
and rejoining the chains with one another [44].
Recombinant TOUCH 4 (TCH4) uses xyloglucan polymers as donor and acceptor molecules in the
transglycosylation reaction it catalyses [45••]. The fucosyl
content of either the donor or acceptor xyloglucan affects
the rate of the reaction [45••], suggesting that control of
xyloglucan fucosylation is part of an endogenous mechanism for regulating changes in the mechanical properties
of a wall. Also, tomato xyloglucan endotransglycosylase
(LeEXT) expression was found in elongating regions of
tomato hypocotyls [46•], and antisera to the TCH4
revealed accumulation in expanding cells in Arabidopsis
leaf bases, hypocotyls, and vascular tissues [47•]. Thus,
these studies support the view that xyloglucan endotransglycosylases play a role in cell expansion.
Expanded cells can undergo wall re-rigidification and
modification. Oxidative cross-linking of tyrosine
residues in wall proteins, which is also brought about by
H2O2, is thought to be one such rigidification mechanism. An important early step in this reaction is the
generation of di-isodityrosine, and pulcherosine (a trimer
of tyrosine and isodityrosine residues coupled by a
biphenyl linkage) was identified in a wall hydrosylate of
tomato [48••]. Steric considerations suggested that only
inter-polypeptide cross-links and/or wide intrapolypeptide loops are tolerated in the oxidative reaction
involving tyrosine and isodityrosine, suggesting that pulcherosine bridges wall proteins and acts as an
intermediate in wall cross-linking. Levels of H2O2 in
plants are regulated by control over oxidases and catalases, so it may be that oxidative cross-linking is subject
to developmental control.
An antiserum localized a glycine-rich protein in only the
primary walls of the xylem tracheary elements, demonstrating that the walls of these cells are actively modified
rather than degraded [49••]. These proteins probably
507
provide the xylem cell walls with special physical properties. The disposition of 1,4-β-D-galactose residues in
tomato fruit pectin was localized with an antiserum and
a Mab. These were present in the walls other than epidermal and sub-epidermal walls [50•]. 1,4-β-D-galactans
are known to be present in some of the most flexible wall
polysaccharides, so the absence of these galactans from
the epidermal cell walls may also contribute to the crosslinking of these walls. Modification of wall protein and
polysaccharide may, therefore, occur in concert during
wall specification.
Conclusions
Numerous reports over the past year have thrown new
light on the fundamental role of the cell wall in plant morphogenesis. Structural glycoproteins are involved with the
division of a cell, and structural and catalytic proteins are
necessary for expansion during cell differentiation.
Polysaccharides are among the targets for these proteins
and modifications of polysaccharides are involved with
wall loosing, rigidification, and modification. Cell–cell
interactions can be defined by cell walls, and soluble signals probably released from walls can control cell
metabolism and development.
These advances offer important new insight into the functions of cell walls, and it is with function that research into
walls should remain. Additional and refined genetic
screens for Arabidopsis plants defective in wall carbohydrate or in the perception of a signal released from a wall
will be invaluable in this respect. Forward and reverse
genetic approaches utilizing genes coding for structural
proteins and enzymes involved with wall biosynthesis and
turnover will complement this. With thorough chemical
and immunochemical analysis of such plants, wall structures and signals can be linked to functions.
Acknowledgements
I thank The Royal Society of London for a University Research Fellowship.
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
1.
Brownlee C, Berger F: Extracellular matrix and pattern in plant
embryos: On the lookout for developmental information. Trends
Genet 1995, 11:344-348.
2. Cosgrove DJ: Assembly and enlargement of the primary cell wall
•
in plants. Ann Rev Cell Biol 1997, 13:171-120.
Control of cell expansion is essential for cell and plant morphogenesis. The
different ways that a wall can become loosened are discussed in this article.
There is detailed coverage of the role of expansin proteins in wall loosening.
3.
Doerner P: Radicle development(s). Curr Biol 1995, 5:110-112.
Dolan L, Linstead P, Roberts K: Developmental regulation of pectic
polysaccharides in the root meristem of Arabidopsis. J Exp Bot
1997, 48:713-720.
Labelling studies which show that a relatively unesterified pectin epitope
identifies non-dividing cells in the root apical meristem of Arabidopsis.
Why such cells contain higher levels of non-esterified pectin than dividing cells in not known. However, a striking and potentially very
interesting observation.
4.
•
508
Cell biology
Ito M, Kodama H, Komamine A, Watanabe A: Expression of extensin
genes is dependent on the stage of the cell cycle and cell
proliferation in suspension-cultured Catharanthus roseus cells.
Plant Mol Biol 1998, 36:343-351.
The expression of two extensin genes is shown to occur during G2 and S
phases of the cell cycle. It would be interesting if a feedback control, from
wall to nucleus, were operating to drive the cycle in plants.
5.
•
Langhan KJ, Nothnagel EA: Cell surface arabinogalactan-proteins
and their relation to cell viability and proliferation. Protoplasma
1997, 196:87-98.
The authors demonstrate a relationship between the AGP composition of
the cell wall and responses to staining with Yariv reagent. Complexing AGPs
with Yariv reagent blocks cell division, and the reversibiity of this block
depends on the composition of the AGP. It may turn out that HRGPs play
an important role in cell cycle progression.
6.
•
Gaffe J, Tiznado ME, Handa AK: Characterization and functional
expression of a ubiquitously expressed tomato pectin
methylesterase. Plant Physiol 1997, 114:1547-1556.
Pectin methylesterification is known to be developmentally regulated, and
the level of esterification can reveal nondividing cells and the incipient anatomy of a root. Experiments with this gene could alter the patterns of pectin
esterification, and thereby enable questions about the role of the esterification in plant development.
7.
•
Woo HH, Hawes MC: Cloning of genes whose expression is
correlated with mitosis and localized in dividing cells in root caps
of Pisum sativum L. Plant Mol Biol 1997, 35:1045-1051.
This paper reports a gene encoding an extensin that is expressed in nondividing root cells. Related genes may be expressed at certain stages of the
cell cycle, and overall there may be a role for extensins in cell division.
Manipulation of such genes will be interesting.
8.
•
9.
Delmer D, Amor Y: Cellulose biosynthesis. Plant Cell 1995,
7:987-1000.
10. Baskin TI, Berzner AA, Hoggart R, Cork A, Williamson RE: Root
morphology mutants in Arabidopsis thaliana. Aust J Plant Physiol
1992, 19:427-437.
11. Arioli T, Peng L, Betzner AS, Burn J, Wittke W, Herth W, Camilleri C,
•• Höfte H, Plazinski J, Birch R, Cork A, Glover J, Redmond J, Williamson
RE: Molecular analysis of cellulose synthesis in Arabidopsis.
Science 1998, 279:717-720.
Chemical and structural analysis and map-based cloning identified RSW1
as the gene encoding the catalytic subunit of cellulose synthase. This paper
represents the latest in a series of advances in the molecular biology of cellulose synthesis, and also elegantly demonstrates that the correct deposition
of cellulose is essential for cell and plant morphogenesis.
12. Turner SR, Somerville CR: Collapsed xylem phenotype of
•• Arabidopsis identifies mutants deficient in cellulose deposition in
the secondary cell wall. Plant Cell 1997, 9:689-701.
A normal pattern of cellulose deposition may be necessary for lignin deposition and wall reinforcement. This has important implications for wall
assembly and development, but also shows that it is possible to screen EMS
populations by using anatomical methods, and to recover interesting and
important mutants in this way.
13. Brummel DA, Catala C, Lashbrook CC, Bennett AB: A membrane•
anchored E-type endo-1,4-b-glucanase is localized on Golgi and
plasma membranes of higher plants. Proc Natl Acad Sci USA
1997, 94:4794-4799.
Glucanases are interesting proteins with numerous potential functions in
plant development and defense. The function of this one is not known, but a
role in cellulose synthesis would be interesting and perhaps feed into a
growing picture of the mechanism of cellulose metabolism.
16. Robertson D, Mitchell GP, Gilroy JS, Gerrish C, Bolwell GP, Slabas
•
AR: Differential extraction and protein sequencing reveals major
differences in patterns of primary cell wall proteins from plants.
J Biol Chem 1997, 272:15841-15848.
The authors describe clear species-differences in extractable cell wall proteins, some of which are novel proteins. These species used in this study
were not closely related, and it would be interesting to perform similar comparisons on genera from a single family, or species from a single genus.
Reiter WD, Chapple C, Somerville CR: Mutants of Arabidopsis
thaliana with altered cell wall polysaccharide composition. Plant J
1997, 12:335-345.
The identification of 11 mutants in which the levels of specific monosaccharides are reduced shows that a biochemical screening procedure can
identify mutants altered in different kinds of cell wall polysaccharides and/or
glycoproteins. As some of the mutants were dwarfs, cloning, sequencing,
and manipulating the affected genes may be useful in relating wall composition and function.
17.
••
18. van den Berg C, Willemsen V, Hendriks G, Weisbeek P, Scheres B:
•• Short-range control of cell differentiation in the Arabidopsis root
meristem. Nature 1997, 390:287-289.
Laser ablation and genetic studies indicate that short-range signals originating in the quiescent center inhibit the differentiation of surrounding meristem
cells. The signals are not known; one possibility is that they originate in cell
walls. A great paper.
19. Lolle SJ, Berlyn GP, Engstrom EM, Krolikowski KA, Reiter WD, Pruitt
•• RE: Developmental regulation of cell interactions in the
Arabidopsis fiddlehead-1 mutant: a role for the epidermal cell wall
and cuticle. Dev Biol 1997, 189:311-321.
One possibily arising from fdh1 is that cell walls serve as molecular filters,
and refine the profile of the signaling molecules that move from one cell
to another during patterning and cell differentiation. It would be interesting to see if fdh1 walls contained higher than nomal levels of the any of
the lipids that promote pollen germination on stigmaless Petunia styles. A
fascinating paper.
20. Wolters-Arts M, Lush WM, Mariani C: Lipids are required for
•• directional pollen-tube growth. Nature 1998, 392:818-821.
This demonstration that trilinolein, an unsaturated fatty acid present in the
lipid fraction of Petunia stigmatic exudate, allows pollen to penetrate stigmaless pistils is a novel and interesting discovery, positioning fatty acids as
extracellular signaling compounds. This report deals with the interaction
between stigma and pollen; it will be intersting to see if other lipids are
involved with other cell–cell interactions in the vegetative parts of plants.
21. Mollard A, Domon JM, David H, Joseleau JP: Xylose-rich
polysaccharides from the primary walls of embryogenic cell lines
of Pinus caribaea. Int J Biol Macromol 1997, 21:189-194.
22. McCabe PF, Valentine TA, Forsberg LS, Pennell RI: Soluble signals
•• from cells identified at the cell wall establish a developmental
pathway in carrot. Plant Cell 1997, 9:2225-2241.
Cell sorting with Mabs was used to show that cells identifiable at their walls
control the progression of other cells into somatic embryos. Wall structures
may also identify interacting cells in whole plants. A molecular analysis of
these wall-derived cells could turn up some interesting and useful genes.
23. Roberts AW, Donovan SG, Haigler CH: A secreted factor induces
•
cell expansion and formation of metaxylem-like tracheary
elements in xylogenic suspension cultures of Zinnia. Plant Physiol
1997, 115:683-692.
The transdifferentiation of Zinnia mesophyll cells in tracheary elements
may be useful in understanding cell communication and committment.
This paper reports a soluble factor, probably an oligosaccahride, that is
relased from such cells and that affects several aspects of cell development. It will be interesting to obtain the strucutre of this factor, and to test
its specific activities.
14. Faik A, Chilsehe C, Sterling J, Maclachlan G: Xyloglucan galactosyl•
and fucosyltransferase activities from pea epicotyl microsomes.
Plant Physiol 1997, 114:245-254.
This paper advances our understanding of xyloglucan biosynthesis by showing that the side-chain galactose is attached at the xyloglucose next the the
xyloglucose at the reducing end of xyloglucan. Detailed studies on polysaccharide biosynthesis may reveal how wall compounds such as xyloglucan
conditions stage-specific effects in plant development.
24. Dinand E, Excoffier G, Liénart Y, Vignon MR: Two
•
rhamnogalacturonide tetrasaccharides isolated from semi-retted
flax fibers are signaling molecules in Rubus fruticosus L. cells.
Plant Physiol 1997, 115:793-801.
A water extraction yields a mixture of pectic oligosaccharides including two
rhamnogalacturonide tetrasaccharides able to activate D-glycohydrolases in
Rubus fructicosus cells. This shows that the tetrasaccharides are likely to
diffuse from cell wall in vivo, and may therefore represent signal molecules
with natural functions.
15. Doong RL, Mohnen D: Solubilization and charaterization of a
•
galacturonosyltransferase that synthesises the pectic
polysaccharide homoglacturonan. Plant J 1998, 13:363-374.
The characterization of the glycosyltransferase that synthesizes homogalacturonan should aid analysis of homogalacturonan biosynthesis, and
may help to reveal how this wall pectin conditions stage-specific effects
in morphogenesis.
25. Knox JP: The use of antibodies to study the architecture and
•
developmental regulation of plant cell walls. Int Rev Cytol 1997,
171:19-120.
Labeling sections of roots with Mabs to HRGPs has revealed unexpected
epitope expression patterns. Many of these patterns reflect the future anatomy of the root. One interpretation of the labeling patterns is that certain
HRGPs play a role in cell-cell interaction and pattern formation in plants.
Cell walls: structures and signals Roger Pennell
509
26. Casero PJ, Casimiro I, Knox JP: Occurrence of cell surface
•
arabinogalactan-protein and extensin epitopes in relation to
pericycle and vascular tissue development in the root apex of
four species. Planta 1998, 204:252-259.
That the expression of HRGP epitopes in the pericycles of pea, radish, carrot and onion reflects the pattern of vascular development suggests that the
HRGPs are involved with this patterning. What is needed is a functional
analysis of the significance of these epitope expression patterns.
Rose JKC, Lee HH, Bennett AB: Expression of a divergent
expansin gene is fruit-specific and ripening regulated. Proc Natl
Acad Sci USA 1997, 27:5955-5960.
It is shown that tomato expansin is specifically expressed in ripening fruits,
suggesting that it has a role in the ripening process. Evidence for expansin
function in cell elongation, pollen penetration and here ripening suggests
that expansin proteins play numerous and important roles in development,
morphogenesis and reproduction.
27.
•
38. Cosgrove DJ, Bedinger P, Durachko DM: Group I allergens of grass
•
pollen as cell wall-loosening agents. Proc Natl Acad Sci USA
1997, 94:6559-6564.
The presence of expansins in the walls of wheat pollen grains suggest that
pollen expansins might act on the walls of the stigma and style to aid penetration. It is also shown that pollen expansins can be used to identify mRNAs
coding for expansins in the vegetative parts of other species. Pollen could
be used as a rich source of expansins.
Kjellbom P, Snogerup L, Stöhr C, Reuzeau C, McCabe PF, Pennell RI:
Oxidative cross-linking of plasma membrane arabinogalactan
proteins. Plant J 1997, 12:1189-1196.
This paper adds AGPs to the list of cell wall molecules that can be crosslinked by H2O2. The significance is not clear, but the finding that certain
epitopes that are developmentally regulated in roots occur only on crosslinked AGPs suggests that H2O2 may play a role in pattern formation.
28. Thompson HJM, Knox JP: Stage-specific responses of
•
embryogenic carrot cell suspension cultures to arabinogalactan
protein-binding b-glucosyl Yariv reagent. Planta 1998, 205:32-38.
The addition of β-glucosyl Yariv reagent to an embryo culture can cause the
appearance of shoot-less embryos, suggesting that the AGPs at the future
shoot pole play a role in shoot morphogenesis. Difficult to interpret, but an
interesting paper and potentially important.
29. Ding L, Zhu JK: A role for arabinogalactan-proteins in root
•
epidermal cell expansion. Planta 1997, 203:289-294.
There is considerable evidence that HRGPs such as extensins play an
important role in cell expansion. This paper suggests that wall AGPs also
help to condition the physical properties of epidermal cell walls. Exactly how
they might do this is not known, but this and other studies do argue in favor
of this idea.
30. Triplett BA, Timpa JD: b-glucosyl and a-galactosyl Yariv reagents
•
bind to cellulose and other glucans. J Agric Food Chem 1997,
45:4650-4654.
The authors show β-glucosyl Yariv reagent is not specific for AGPs, and
can bind to other cell wall polysaccharides in certain circumstances. The
important point from this paper is that studies involving Yariv reagent
should always feature controls with the α-glucosyl Yariv reagent or Mabs
to AGPs.
31. Schultz CJ, Hauser K, Lind JL, Atkinson AH, Pu ZY, Anderson MA,
•
Clarke AE: Molecular characterization of a cDNA sequence
encoding the backbone of a style-specific 120kDA glycoprotein
which has features of both extensins and arbinogalactan proteins.
Plant Mol Biol 1997, 35:833-845.
The core protein encoded by NaPRP5 is soluble and expressed in styles.
The encoded protein contains three domains, one of which (at the carboxyterminal) is similar to a part of a galactose-rich style glycoprotein. The sharing
of different domains between distinct HRGPs shows further complexity in
this class of wall molecules.
32. Sommer-Knudsen J, Lush WM, Bacic, A, Clarke AE: Re-evaluation of
the role of a transmitting tract-specific glycoprotein on pollen
tube growth. Plant J 1998, 13:529-535.
33. Cho H-T, Kende H: Expression of expansin genes is correlated
•• with growth in deepwater rice. Plant Cell 1997, 9:1661-1671.
Pleasing demonstration at the RNA level that expression of three rice
expansin genes occurs in the elongating parts of the coleoptile, root, leaf and
internode of deepwater rice, and also correlates with acid-induced wall
extensibility in vitro. An elegant and convincing correlation between expansin
and elongation.
34. Cho H-T, Kende H: Expansins in deepwater rice internodes. Plant
•
Physiol 1997, 113:1137-1143.
The authors demonstrate at the protein level that two expansins occur in
the walls of rice internodes, and that they may mediate acid-induced wall
extensibility. Clear correlative evidence for a role for expansin in internode elongation.
35. Cho H-T, Kende H: Expansins and internodal growth of deepwater
•
rice. Plant Physiol 1997, 113:1145-1151.
An antiserum raised to a cucumber expansin was used as a probe to show
that rice expansin occurs primarily in the internode elongation zone. This
study links expansin accumulation to extension of the internode, and goes on
to localize these expansins to the walls of the vascular bundles and the inner
epidermal cells surrounding the internodal cavity.
36. Fleming AJ, McQueen-Mason S, Mandel T, Kuhlemeier C: Induction
•• of leaf primordia by the cell wall protein expansin. Science 1997,
276:1415-1418.
Beads loaded with expansin induce leaf-like primordia on the shoot apex of
tomato, and can reverse the direction of the phyllotaxy. These data suggest
a scheme for plant morphogenesis that is broader than the cause-and-effect
view of signal, signal transduction, and gene activation in the control plant
form. A fascinating paper.
37.
•
39. Kieliszewski MJ, Lamport DTA: Extensin: Repetitive motifs,
functional sites, post-tranlational codes, and phylogeny. Plant J
1994, 5:157-172.
40. Bucher M, Schroeer B, Willmitzer L, Riesmeier JW: Two genes
•
encoding extensin-like proteins are predominantly expressed in
tomato root hair cells. Plant Mol Biol 1997, 35:497-508.
The identification of two extensin genes which are preferentially expressed in
root hair cells, and the blockage of root hair growth by the prolyl hydroxylse
inhibitor 3,4-dehydro-L-proline, suggests a function for extensins in root hair
development. A clear and well-performed study.
41. Hoon AJ, Choi K, Kim SG, Myung KY, Do CY, Seob LJ: Expression of
•
a soybean hydroxyproline-rich glycoprotein gene is correlated
with maturation of roots. Plant Physiol 1998, 116:671-679.
The expression of SbHRGP3 increases during root development, showing
that expression is developementally-regulated. The function of the HRGP is
not known, but one possibility is that it is involved with modifying the expansibility of the cell walls. The gene may be useful for studies targeted to
resolving this function.
42. Grierson CS, Roberts K, Feldmann KA, Dolan L: The COW1 locus of
•• Arabidopsis acts after RHD2, and in parallel with RHD3 and TIP1,
to determine the shape, rate of elongation, and number of root
hairs produced from each site of hair formation. Plant Physiol
1997, 115:981-990.
Root hair abnormalities characteristic of COW1 are consistent with a defect in
an extensin gene. It would be interesting to see if these plants fail to express
or misexpress any of the extensins that are known to occur in Arabidopsis.
Conceivably, overexpression of such genes might make longer root hairs.
43. Ding L, Zhu JK: A role for arabinogalactan-proteins in root
•
epidermal cell expansion. Planta 1997, 203:289-294.
Treatment of Arabidopsis roots with Yariv reagent causes the appearance of
balloon-like swellings in the epidermal cells, thereby phenocopying the
mutant reb1. Yariv reagent binds AGPs, so this paper lends weight to the
view that certain AGPs play a mechanical role in cell expansion control.
Further analysis of reb1 will be interesting.
44. Xu W, Purugganan MM, Polisensky DH, Antosiewic DM, Fry SC,
Braam J: Arabidopsis TCH4, regulated by hormones and the
environment, encodes a xyloglucan endotransglycosylase. Plant
Cell 1995, 7:1555-1567.
45. Purugganan MM, Braam J, Fry S: The Arabidopsis TCH4 xyloglucan
•• endotransglycosylase. Substrate specificity, pH optimum, and
cold tolerance. Plant Physiol 1997, 115:181-190.
Purification has allowed the authors to detail the substrate requirements of
the Arabidopsis XET. A detailed understanding of this enzyme is likely to be
important for a complete picture of wall loosening and responses to endogenous and environmental signals.
46. Catalá C, Rose JKC, Bennett AB: Auxin regulation and spatial
•
localization of an endo-1,4-b-D-glucanase and a xyloglucan
endotransglycosylase in expanding tomato hypocotyls. Plant J
1997, 12:417-426.
The authors show unique and overlapping expression patterns an endo-1,4β-glucanase and an XET. These genes have the potential to promote wall
loosing and disassembly during with growth. One interesting issue raised by
these data is that they may act co-operatively to achieve this.
Antosiewicz DM, Purugganan MM, Polisensky DH, Braam J: Cellular
localization of Arabidopsis xyloglucan endotransglycosylaserelated proteins during development and after wind stimulation.
Plant Physiol 1997, 115:1319-1328.
Immunocytochemistry shows that TCH4 accumulates in expanding cells,
suggesting that it may modify walls to allow cell expansion and reinforcement. In so doing, it may play an important role in morphogenesis. Exactly
how this protein achieves these changes to a wall will represent an important advance in our understanding of development.
47.
•
510
Cell biology
48. Brady JD, Sadler IH, Fry SC: Pulcherosine, an oxidatively coupled
•• trimer of tyrosine in plant cell walls: its role in cross-link
formation. Phytochemistry 1998, 47:349-353.
This paper completes a series of publications establishing the chemistry of
one class of interpolypeptide cell wall cross-links thought to be modifiers of
the wall. When it is known how the polypeptid or the H2O2 can be regulated, it will be possible to make important predictions on the role of
pulcherosine in development and morphogenesis.
49. Ryser U, Schorderet M, Zhao GF, Studer D, Ruel K, Hauf G, Keller B:
•• Structural cell-wall proteins in protoxylem development: evidence
for a repair process mediated by a glycine-rich protein. Plant J
1997, 12:79-111.
A proline-rich protein and a glycine-rich protein are shown to occur in different parts of protoxylem cell walls. Along with cellulose, glycine-rich
proteins such as this and pectic galactans may function as a scaffold for
lignin assembly.
50. Jones L, Seymore, GB, Knox JP: Localization of pectic galactan in
•
tomato cell walls using a monoclonal antibody specific to (1-4)-bD-galactan. Plant Physiol 1997, 113:1405-1412.
Immunofluorescence shows that galactan side-chains composed of at least
3 (1-4)-β-galactosyl residues are absent from the walls of the epidermal and
subepidermal cells of tomato fruits. These absence of these side-chains may
promote homogalacturonan cross-linking and rigidification of the cell walls,
and in so doing provide a toughened barrier.
Cell walls: structures and signals
Roger Pennell
Cell walls harbor proteins and polysaccharides able to
condition the development of a plant. In the past year, genes
and enzymes modulating the composition and physical
properties of walls have been characterized, and wall
composition has been linked to the way a cell interacts with
another cell, and to the way in which it differentiates. The sum
of the signaling and physical activities of a cell wall may explain
much about the control of development.
Addresses
Ceres Inc., 3008 Malibu Canyon, Malibu, California 90265, USA;
e-mail: rpennell@ceres-inc.com
Current Opinion in Plant Biology 1998, 1:504–510
http://biomednet.com/elecref/1369526600100504
© Current Biology Ltd ISSN 1369-5266
Abbreviations
AGP
arabinogalactan-protein
HRGP hydroxyproline-rich glycoprotein
Mab
monoclonal antibody
Introduction
Most cell divisions in plants take place in meristems, and
new cells choose from a variety of potential fate. The divisions generate the correct numbers of cells, and pattern
formation sets up the anatomy. Progression through the
cell cycle seems to require the accurate assembly of a cell
wall. Signaling between newly divided cells then directs
the choice of developmental pathway and some of these
signals are thought to arise from the wall itself [1].
A cell differentiates some time after it has progressed
through a developmental pathway. This differentiation
process usually involves an up to one thousand-fold
increase in cell volume. The enlargement is brought about
by the relaxation of the wall and turgor-driven wall expansion, followed by the re-rigidification and changes in wall
structure. Eventually, walls can be irreversibly reinforced,
either by the addition of new components to the wall or
further cross-linking of existing components [2•].
Thus, the function of a cell is intimately linked to the
composition and structure of its wall. A comprehensive
understanding of the functions of a wall hinges on analysis
of wall assembly and metabolism. To this end, analysis of
wall polymers has revealed novel aspects of wall structure,
and these have been complemented by screens for plants
defective in wall biosynthesis. Experiments have shown
that a cell identified by its wall can control the fate of
another cell, and that walls are involved in cell–cell interactions; these and other studies suggest that signal
molecules released from walls are involved with these
developmental processes in planta. The localization of
genes and proteins affecting wall relaxation and the chemical analysis of wall cross-linking have helped to resolve
the mechanism of cell enlargement and rigidification.
The control of the cell cycle is a key regulator of plant
morphogenesis [3] and it is well documented that cells in
different stages of the cell cycle have characteristic walls.
For example, higher levels of unesterified pectins have
been localised to non-dividing cells than to dividing cells
in the root tip of Arabidopsis [4•], and the expression of two
classes of hydroxyproline-rich glycoproteins (HRGPs),
extensins and arabinogalactan-proteins (AGPs), were
shown to be dependent on the stage of the cell cycle and
on cell proliferation in Catharanthus and carrot [5•,6•].
Their cell cycle functions are unknown; however, the
tomato pectin methylesterase gene PMEU1 was cloned
and characterized [7•], and gain-of-function and loss-offunction experiments with this gene may help to address
these functions directly. It was previously known that prolyl hydroxylation inhibitors caused the disappearance of
HRGP from the wall and also blocked cell division. Now,
the cloning of PsHRGP1, which encodes an extensin
repressed in dividing pea cells [8•], may prove useful in
studying the role of HRGPs in cell division more directly.
These findings have further demonstrated that walls play
diverse and essential roles in plant cell development.
Here, I summarize recent work on the assembly, structure,
and function of cell wall components.
Wall assembly and structure
Polysaccharides and proteins are deposited in a new wall,
and together provide the wall with its properties.
Figure 1 shows some pictures of Arabidopsis leaf cells and
their walls. Cellulose is the major wall polymer, but
genes active in its synthesis were only recently identified
[9]. This is due in part to difficulty in designing genetic
screens for plants deficient in cellulose. In Arabidopsis,
rsw1 mutations cause a reduction in cellulose, the accumulation of a less-ordered β-1,4-glucan, and widespread
morphological abnormalities [10]. RSW1 was cloned, and
found to encode the catalytic subunit of cellulose synthase [11••]. This confirmed the central importance of
cellulose in plant development. The demonstration that
cellulose is absent from xylem walls in Arabidopsis irx
mutants, and that these xylem cells collapse at normal
water pressure, further suggests that cellulose may serve
as a template for the assembly of lignin [12••], and have
a dual role in morphogenesis. The role of an E-type
endo-1,4-β-D-glucanase, which catalyses the breakdown
of cellulose rather than its assembly, active during the
same process in tomato [13•], is less clear. One possibility is that this glucanase is involved with the cutting and
spacing of the cellulose.
Cell walls: structures and signals Roger Pennell
505
Figure 1
(a)
(b)
(c)
CW
CW
CW
CW
Electron micrographs of Arabidopsis leaf cells. A thin cell wall (CW) encloses each cell. Walls are thought to be involved with cell division, cell–cell
interaction, and cell differentiation. The bar in (a) = 50 µm, in (b) = 10 µm, and in (c) = 0.1 µm.
Cellulose is hydrogen-bonded to xyloglucan. Incubation of
pea microsomes with UDP-[14C] galactose and tamarind
seed xyloglucan allowed the identification of a galactosyl
transferase involved with xyloglucan biosynthesis [14•].
The tobacco polygalacturonate-4-α-galacturonosyltransferase that catalyses the transfer of UDP-[14C] galacturonic
acid to homogalacturonan acceptor substrates was also isolated and characterized [15•]. Xyloglucan and pectin play
important roles in morphogenesis, and the manipulation of
these enzymes may facilitate an understanding of their
function. The characterization of some novel wall proteins
[16•] could also be useful in this respect.
In the Arabidopsis mutant fdh1, the shoot epidermis is modified such that it can allow pollen grains to geminate and
enable organs that come into contact to fuse. This suggests
that a developmental program normally operating only in
carpels is activated ectopically. Wall and cuticular permeability were found to be disturbed in fdh1 mutants,
apparently as a result of altered lipid composition [19••].
Local signaling events, therefore, could be conditioned by
wall lipid. Pollen germination on stigmaless Petunia styles
can be enabled by trilinolein [20••], a triacylglyceride lipid
component of wet stigma exudates, suggesting that these
triacylglycerides could be extracellular signaling molecules.
In a genetic approach to the analysis of wall polysaccharide function, mutagenized Arabidopsis plants were used
to identify 11 loci involved with wall biosynthesis [17••].
Mutations causing the absence of fucose (mur1),
decreased levels of fucose (mur2, mur3), arabinose (mur4,
mur5, mur6, mur7), or rhamnose (mur8), or complex alterations in the proportions of several monosaccharides
including fucose, arabinose and rhamnose (mur9, mur10,
mur11), were identified [17••]. Some mutations (mur1,
mur9, mur10) resulted in dwarfism [17••]. This showed
that cell walls are amenable to genetic analysis, and also
that changes in certain monosaccharides are associated
with alterations in development.
Unusual polysaccharide profiles seem to be characteristics
of cell walls in embryonic suspension cultures. Walls in
such cultures of pine were found to contain higher levels
of xylans, fucoxylans, and arabinogalactans than in nonembryogenic lines [21]. Soluble signals including soluble
AGPs help to control somatic embryogenesis, so one possibility is that the wall contributes to a pool of such signals.
In an embryogenic culture of carrot, some cells label at the
wall with the monoclonal antibody (Mab) JIM8, which recognizes a rhamnose-containing epitope present in certain
AGPs. JIM8-reactive cells were found to control the development of JIM8-unreactive embryo initial cells, and
conditioned growth medium from cultures of JIM8-reactive cells could substitute for the cells themselves [22••].
This showed that cells identifiable by their wall composition release diffusible signals, possibly from their walls, to
control a developmental process [22••]. The transformation of Zinnia mesophyll cells into metaxylem-like cells
was also found to be stimulated by soluble signal molecules that were small and heat-resistant [23 •]. Two
tetrasaccharides of rhamnogalacturonan, differing only in
an O-3 acetyl group at an internal galacturonosyl residue,
activated D-glycohydrolases in cultured Rubus cells in
Signaling by walls
Local interactions between the cells in a meristem play an
important role in pattern formation in plants. Laser ablation
studies were used to demonstrate that non-dividing cells in
the quiescent centers of Arabidopsis roots inhibit the differentiation of surrounding cells; the signals achieving this
inhibition operate within the range of a single cell [18••].
Walls form the point of contact between cells; therefore,
one possibility is that the wall dictates this local signaling.
506
Cell biology
Figure 2
Processes involving cells walls. Following cell
division, a new wall can undergo relaxation,
shown by the loosening of the small horizontal
lines, and turgor-driven expansion, shown by
the increase in size of the wall. Cross-links
can be re-established in expanded walls,
shown as the replacement of small horizontal
lines, or the wall can be irreversibly modified,
shown as the shading of the wall. There is
also evidence that walls can release small
signaling molecules, shown as small dots, that
can diffuse from cell to cell and influence cell
development. The boxes represent parts of
cell walls, and the small vertical lines
represent wall structural elements.
Signaling
Division
Relaxation and
expansion
Modification
Cross-linking
Current Opinion in Plant Biology
what could be a related process [24•]. Walls and soluble
oligosaccharide signals released from them may, therefore,
be active in diverse aspects of cell development.
Labeling studies with Mabs reinforced the view that signals from walls are involved with vascular development
and cell patterning in planta [25•]. For example, epitopes
carried by some AGPs were shown to occur in root apices
in patterns that were mutually exclusive, and also reflecting the distribution of extensin epitopes, and both AGP
and extensin epitopes were associated with future cell
anatomy [26•]. The epitope recognized by JIM15 was only
found on an oxidatively coupled sugar beet AGP [27•].
Oxidative cross-linking of molecules in cells is usually
brought about by H2O2, suggesting that H2O2 has a role in
AGP function [27•]. β-glucosyl Yariv reagent, which binds
to AGPs, caused the appearance of shootless embryos
when added to a carrot suspension culture [28•], and inhibited root elongation when added to Arabidopsis seedlings
[29•]. Although β-glucosyl Yariv reagent was found not to
be specific for AGPs in certain circumstances [30•], these
experiments with carrot and Arabidopsis featured controls
with α-glucosyl reagent, and support the idea that certain
AGPs have mechanical functions such as cell expansion
functions. NaPRP5, which codes for the core protein of a
Nicotiana style AGP [31•], was cloned and gain-of-function
and loss-of-function experiments with this gene could help
to address this putative role. It had previously been postulated that a stylar AGP may act as a pollen attractant, but
this idea has since been questioned. The role of the stylar
protein itself could now be re-examined by using molecular genetics [32].
Wall differentation
When a cell has chosen from its range of potential fates, it
differentiates. Expansin is thought to loosen noncovalent
associations between wall polysaccharides, allowing turgor
pressure to drive wall creep. Four expansin genes
(OsEXP1 – OsEXP4) were shown to be expressed in elongating rice tissues [33••], and an antiserum to a cucumber
homolog of one of them was used to show that the protein
itself occurs at sites of elongation [34•]. Immunoblotting
also revealed that submerged and elongating internodes
contained more expansin than internodes grown in air
[35•], linking expansin accumulation in the wall to cell
and tissue elongation. Interestingly, expansin-coated
beads applied to tomato shoot apices generated small
bulges, some of which developed into leaf-like structures
bearing trichomes and expressing the gene encoding the
small subunit of ribulose bisphosphate carboxylase rbcS
[36••]. These ectopic structures reversed the phyllotaxy
[36••], suggesting that expansin, by generating wall relaxation, can have direct and profound effects on
plant morphogenesis.
The tomato expansin Le-EXP1 was found to be regulated
by ethylene, and cDNAs related by sequence homology to
Le-EXP1 were shown to be upregulated during ethyleneinduced ripening in melon and strawberry [37•]. Ethylene
may, therefore, influence plant morphogenesis in part by
controlling expansin activity. Grass pollen group I allergens
were also found to be expansins, likely facilitating pollen
penetration of the stigma [38•]. Thus, expansins are regulated during development and by environmental factors,
and play diverse role in plant growth.
Cell walls: structures and signals Roger Pennell
Some HRGPs [39] are also thought to condition cell
expansion. The tomato extensins encoded by DIF10 and
DIF54 are preferentially expressed in root hairs, and inhibition of tomato prolyl hydroxylation with
3,4-dehydro-L-proline caused the appearance of short
root hairs [40•]. Also, the soybean SbHRGP3 extensin
was found to accumulate in maturing roots at a time when
root hairs are growing [41•]. These experiments link
extensins with cell elongation, and it would be interesting to see if the Arabidopsis mutant cow1, which correctly
initiates root hairs but fails to allow root hair elongation
[42••], is affected in an extensin gene. Interestingly, treatment of Arabidopsis seedlings with β-glucosyl Yariv
reagent resulted in bulging epidermal cells similar to the
epidermal cells of AGP-deficient reb1 mutants [43•], suggesting that different kinds of HRGPs could have
separate but related roles in cell enlargement.
Xyloglucan endotransglycosylases are believed to allow
slippage in the wall by cutting adjacent xyloglucan chains
and rejoining the chains with one another [44].
Recombinant TOUCH 4 (TCH4) uses xyloglucan polymers as donor and acceptor molecules in the
transglycosylation reaction it catalyses [45••]. The fucosyl
content of either the donor or acceptor xyloglucan affects
the rate of the reaction [45••], suggesting that control of
xyloglucan fucosylation is part of an endogenous mechanism for regulating changes in the mechanical properties
of a wall. Also, tomato xyloglucan endotransglycosylase
(LeEXT) expression was found in elongating regions of
tomato hypocotyls [46•], and antisera to the TCH4
revealed accumulation in expanding cells in Arabidopsis
leaf bases, hypocotyls, and vascular tissues [47•]. Thus,
these studies support the view that xyloglucan endotransglycosylases play a role in cell expansion.
Expanded cells can undergo wall re-rigidification and
modification. Oxidative cross-linking of tyrosine
residues in wall proteins, which is also brought about by
H2O2, is thought to be one such rigidification mechanism. An important early step in this reaction is the
generation of di-isodityrosine, and pulcherosine (a trimer
of tyrosine and isodityrosine residues coupled by a
biphenyl linkage) was identified in a wall hydrosylate of
tomato [48••]. Steric considerations suggested that only
inter-polypeptide cross-links and/or wide intrapolypeptide loops are tolerated in the oxidative reaction
involving tyrosine and isodityrosine, suggesting that pulcherosine bridges wall proteins and acts as an
intermediate in wall cross-linking. Levels of H2O2 in
plants are regulated by control over oxidases and catalases, so it may be that oxidative cross-linking is subject
to developmental control.
An antiserum localized a glycine-rich protein in only the
primary walls of the xylem tracheary elements, demonstrating that the walls of these cells are actively modified
rather than degraded [49••]. These proteins probably
507
provide the xylem cell walls with special physical properties. The disposition of 1,4-β-D-galactose residues in
tomato fruit pectin was localized with an antiserum and
a Mab. These were present in the walls other than epidermal and sub-epidermal walls [50•]. 1,4-β-D-galactans
are known to be present in some of the most flexible wall
polysaccharides, so the absence of these galactans from
the epidermal cell walls may also contribute to the crosslinking of these walls. Modification of wall protein and
polysaccharide may, therefore, occur in concert during
wall specification.
Conclusions
Numerous reports over the past year have thrown new
light on the fundamental role of the cell wall in plant morphogenesis. Structural glycoproteins are involved with the
division of a cell, and structural and catalytic proteins are
necessary for expansion during cell differentiation.
Polysaccharides are among the targets for these proteins
and modifications of polysaccharides are involved with
wall loosing, rigidification, and modification. Cell–cell
interactions can be defined by cell walls, and soluble signals probably released from walls can control cell
metabolism and development.
These advances offer important new insight into the functions of cell walls, and it is with function that research into
walls should remain. Additional and refined genetic
screens for Arabidopsis plants defective in wall carbohydrate or in the perception of a signal released from a wall
will be invaluable in this respect. Forward and reverse
genetic approaches utilizing genes coding for structural
proteins and enzymes involved with wall biosynthesis and
turnover will complement this. With thorough chemical
and immunochemical analysis of such plants, wall structures and signals can be linked to functions.
Acknowledgements
I thank The Royal Society of London for a University Research Fellowship.
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
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•
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4.
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Cell biology
Ito M, Kodama H, Komamine A, Watanabe A: Expression of extensin
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Langhan KJ, Nothnagel EA: Cell surface arabinogalactan-proteins
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•
Gaffe J, Tiznado ME, Handa AK: Characterization and functional
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Woo HH, Hawes MC: Cloning of genes whose expression is
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Science 1998, 279:717-720.
Chemical and structural analysis and map-based cloning identified RSW1
as the gene encoding the catalytic subunit of cellulose synthase. This paper
represents the latest in a series of advances in the molecular biology of cellulose synthesis, and also elegantly demonstrates that the correct deposition
of cellulose is essential for cell and plant morphogenesis.
12. Turner SR, Somerville CR: Collapsed xylem phenotype of
•• Arabidopsis identifies mutants deficient in cellulose deposition in
the secondary cell wall. Plant Cell 1997, 9:689-701.
A normal pattern of cellulose deposition may be necessary for lignin deposition and wall reinforcement. This has important implications for wall
assembly and development, but also shows that it is possible to screen EMS
populations by using anatomical methods, and to recover interesting and
important mutants in this way.
13. Brummel DA, Catala C, Lashbrook CC, Bennett AB: A membrane•
anchored E-type endo-1,4-b-glucanase is localized on Golgi and
plasma membranes of higher plants. Proc Natl Acad Sci USA
1997, 94:4794-4799.
Glucanases are interesting proteins with numerous potential functions in
plant development and defense. The function of this one is not known, but a
role in cellulose synthesis would be interesting and perhaps feed into a
growing picture of the mechanism of cellulose metabolism.
16. Robertson D, Mitchell GP, Gilroy JS, Gerrish C, Bolwell GP, Slabas
•
AR: Differential extraction and protein sequencing reveals major
differences in patterns of primary cell wall proteins from plants.
J Biol Chem 1997, 272:15841-15848.
The authors describe clear species-differences in extractable cell wall proteins, some of which are novel proteins. These species used in this study
were not closely related, and it would be interesting to perform similar comparisons on genera from a single family, or species from a single genus.
Reiter WD, Chapple C, Somerville CR: Mutants of Arabidopsis
thaliana with altered cell wall polysaccharide composition. Plant J
1997, 12:335-345.
The identification of 11 mutants in which the levels of specific monosaccharides are reduced shows that a biochemical screening procedure can
identify mutants altered in different kinds of cell wall polysaccharides and/or
glycoproteins. As some of the mutants were dwarfs, cloning, sequencing,
and manipulating the affected genes may be useful in relating wall composition and function.
17.
••
18. van den Berg C, Willemsen V, Hendriks G, Weisbeek P, Scheres B:
•• Short-range control of cell differentiation in the Arabidopsis root
meristem. Nature 1997, 390:287-289.
Laser ablation and genetic studies indicate that short-range signals originating in the quiescent center inhibit the differentiation of surrounding meristem
cells. The signals are not known; one possibility is that they originate in cell
walls. A great paper.
19. Lolle SJ, Berlyn GP, Engstrom EM, Krolikowski KA, Reiter WD, Pruitt
•• RE: Developmental regulation of cell interactions in the
Arabidopsis fiddlehead-1 mutant: a role for the epidermal cell wall
and cuticle. Dev Biol 1997, 189:311-321.
One possibily arising from fdh1 is that cell walls serve as molecular filters,
and refine the profile of the signaling molecules that move from one cell
to another during patterning and cell differentiation. It would be interesting to see if fdh1 walls contained higher than nomal levels of the any of
the lipids that promote pollen germination on stigmaless Petunia styles. A
fascinating paper.
20. Wolters-Arts M, Lush WM, Mariani C: Lipids are required for
•• directional pollen-tube growth. Nature 1998, 392:818-821.
This demonstration that trilinolein, an unsaturated fatty acid present in the
lipid fraction of Petunia stigmatic exudate, allows pollen to penetrate stigmaless pistils is a novel and interesting discovery, positioning fatty acids as
extracellular signaling compounds. This report deals with the interaction
between stigma and pollen; it will be intersting to see if other lipids are
involved with other cell–cell interactions in the vegetative parts of plants.
21. Mollard A, Domon JM, David H, Joseleau JP: Xylose-rich
polysaccharides from the primary walls of embryogenic cell lines
of Pinus caribaea. Int J Biol Macromol 1997, 21:189-194.
22. McCabe PF, Valentine TA, Forsberg LS, Pennell RI: Soluble signals
•• from cells identified at the cell wall establish a developmental
pathway in carrot. Plant Cell 1997, 9:2225-2241.
Cell sorting with Mabs was used to show that cells identifiable at their walls
control the progression of other cells into somatic embryos. Wall structures
may also identify interacting cells in whole plants. A molecular analysis of
these wall-derived cells could turn up some interesting and useful genes.
23. Roberts AW, Donovan SG, Haigler CH: A secreted factor induces
•
cell expansion and formation of metaxylem-like tracheary
elements in xylogenic suspension cultures of Zinnia. Plant Physiol
1997, 115:683-692.
The transdifferentiation of Zinnia mesophyll cells in tracheary elements
may be useful in understanding cell communication and committment.
This paper reports a soluble factor, probably an oligosaccahride, that is
relased from such cells and that affects several aspects of cell development. It will be interesting to obtain the strucutre of this factor, and to test
its specific activities.
14. Faik A, Chilsehe C, Sterling J, Maclachlan G: Xyloglucan galactosyl•
and fucosyltransferase activities from pea epicotyl microsomes.
Plant Physiol 1997, 114:245-254.
This paper advances our understanding of xyloglucan biosynthesis by showing that the side-chain galactose is attached at the xyloglucose next the the
xyloglucose at the reducing end of xyloglucan. Detailed studies on polysaccharide biosynthesis may reveal how wall compounds such as xyloglucan
conditions stage-specific effects in plant development.
24. Dinand E, Excoffier G, Liénart Y, Vignon MR: Two
•
rhamnogalacturonide tetrasaccharides isolated from semi-retted
flax fibers are signaling molecules in Rubus fruticosus L. cells.
Plant Physiol 1997, 115:793-801.
A water extraction yields a mixture of pectic oligosaccharides including two
rhamnogalacturonide tetrasaccharides able to activate D-glycohydrolases in
Rubus fructicosus cells. This shows that the tetrasaccharides are likely to
diffuse from cell wall in vivo, and may therefore represent signal molecules
with natural functions.
15. Doong RL, Mohnen D: Solubilization and charaterization of a
•
galacturonosyltransferase that synthesises the pectic
polysaccharide homoglacturonan. Plant J 1998, 13:363-374.
The characterization of the glycosyltransferase that synthesizes homogalacturonan should aid analysis of homogalacturonan biosynthesis, and
may help to reveal how this wall pectin conditions stage-specific effects
in morphogenesis.
25. Knox JP: The use of antibodies to study the architecture and
•
developmental regulation of plant cell walls. Int Rev Cytol 1997,
171:19-120.
Labeling sections of roots with Mabs to HRGPs has revealed unexpected
epitope expression patterns. Many of these patterns reflect the future anatomy of the root. One interpretation of the labeling patterns is that certain
HRGPs play a role in cell-cell interaction and pattern formation in plants.
Cell walls: structures and signals Roger Pennell
509
26. Casero PJ, Casimiro I, Knox JP: Occurrence of cell surface
•
arabinogalactan-protein and extensin epitopes in relation to
pericycle and vascular tissue development in the root apex of
four species. Planta 1998, 204:252-259.
That the expression of HRGP epitopes in the pericycles of pea, radish, carrot and onion reflects the pattern of vascular development suggests that the
HRGPs are involved with this patterning. What is needed is a functional
analysis of the significance of these epitope expression patterns.
Rose JKC, Lee HH, Bennett AB: Expression of a divergent
expansin gene is fruit-specific and ripening regulated. Proc Natl
Acad Sci USA 1997, 27:5955-5960.
It is shown that tomato expansin is specifically expressed in ripening fruits,
suggesting that it has a role in the ripening process. Evidence for expansin
function in cell elongation, pollen penetration and here ripening suggests
that expansin proteins play numerous and important roles in development,
morphogenesis and reproduction.
27.
•
38. Cosgrove DJ, Bedinger P, Durachko DM: Group I allergens of grass
•
pollen as cell wall-loosening agents. Proc Natl Acad Sci USA
1997, 94:6559-6564.
The presence of expansins in the walls of wheat pollen grains suggest that
pollen expansins might act on the walls of the stigma and style to aid penetration. It is also shown that pollen expansins can be used to identify mRNAs
coding for expansins in the vegetative parts of other species. Pollen could
be used as a rich source of expansins.
Kjellbom P, Snogerup L, Stöhr C, Reuzeau C, McCabe PF, Pennell RI:
Oxidative cross-linking of plasma membrane arabinogalactan
proteins. Plant J 1997, 12:1189-1196.
This paper adds AGPs to the list of cell wall molecules that can be crosslinked by H2O2. The significance is not clear, but the finding that certain
epitopes that are developmentally regulated in roots occur only on crosslinked AGPs suggests that H2O2 may play a role in pattern formation.
28. Thompson HJM, Knox JP: Stage-specific responses of
•
embryogenic carrot cell suspension cultures to arabinogalactan
protein-binding b-glucosyl Yariv reagent. Planta 1998, 205:32-38.
The addition of β-glucosyl Yariv reagent to an embryo culture can cause the
appearance of shoot-less embryos, suggesting that the AGPs at the future
shoot pole play a role in shoot morphogenesis. Difficult to interpret, but an
interesting paper and potentially important.
29. Ding L, Zhu JK: A role for arabinogalactan-proteins in root
•
epidermal cell expansion. Planta 1997, 203:289-294.
There is considerable evidence that HRGPs such as extensins play an
important role in cell expansion. This paper suggests that wall AGPs also
help to condition the physical properties of epidermal cell walls. Exactly how
they might do this is not known, but this and other studies do argue in favor
of this idea.
30. Triplett BA, Timpa JD: b-glucosyl and a-galactosyl Yariv reagents
•
bind to cellulose and other glucans. J Agric Food Chem 1997,
45:4650-4654.
The authors show β-glucosyl Yariv reagent is not specific for AGPs, and
can bind to other cell wall polysaccharides in certain circumstances. The
important point from this paper is that studies involving Yariv reagent
should always feature controls with the α-glucosyl Yariv reagent or Mabs
to AGPs.
31. Schultz CJ, Hauser K, Lind JL, Atkinson AH, Pu ZY, Anderson MA,
•
Clarke AE: Molecular characterization of a cDNA sequence
encoding the backbone of a style-specific 120kDA glycoprotein
which has features of both extensins and arbinogalactan proteins.
Plant Mol Biol 1997, 35:833-845.
The core protein encoded by NaPRP5 is soluble and expressed in styles.
The encoded protein contains three domains, one of which (at the carboxyterminal) is similar to a part of a galactose-rich style glycoprotein. The sharing
of different domains between distinct HRGPs shows further complexity in
this class of wall molecules.
32. Sommer-Knudsen J, Lush WM, Bacic, A, Clarke AE: Re-evaluation of
the role of a transmitting tract-specific glycoprotein on pollen
tube growth. Plant J 1998, 13:529-535.
33. Cho H-T, Kende H: Expression of expansin genes is correlated
•• with growth in deepwater rice. Plant Cell 1997, 9:1661-1671.
Pleasing demonstration at the RNA level that expression of three rice
expansin genes occurs in the elongating parts of the coleoptile, root, leaf and
internode of deepwater rice, and also correlates with acid-induced wall
extensibility in vitro. An elegant and convincing correlation between expansin
and elongation.
34. Cho H-T, Kende H: Expansins in deepwater rice internodes. Plant
•
Physiol 1997, 113:1137-1143.
The authors demonstrate at the protein level that two expansins occur in
the walls of rice internodes, and that they may mediate acid-induced wall
extensibility. Clear correlative evidence for a role for expansin in internode elongation.
35. Cho H-T, Kende H: Expansins and internodal growth of deepwater
•
rice. Plant Physiol 1997, 113:1145-1151.
An antiserum raised to a cucumber expansin was used as a probe to show
that rice expansin occurs primarily in the internode elongation zone. This
study links expansin accumulation to extension of the internode, and goes on
to localize these expansins to the walls of the vascular bundles and the inner
epidermal cells surrounding the internodal cavity.
36. Fleming AJ, McQueen-Mason S, Mandel T, Kuhlemeier C: Induction
•• of leaf primordia by the cell wall protein expansin. Science 1997,
276:1415-1418.
Beads loaded with expansin induce leaf-like primordia on the shoot apex of
tomato, and can reverse the direction of the phyllotaxy. These data suggest
a scheme for plant morphogenesis that is broader than the cause-and-effect
view of signal, signal transduction, and gene activation in the control plant
form. A fascinating paper.
37.
•
39. Kieliszewski MJ, Lamport DTA: Extensin: Repetitive motifs,
functional sites, post-tranlational codes, and phylogeny. Plant J
1994, 5:157-172.
40. Bucher M, Schroeer B, Willmitzer L, Riesmeier JW: Two genes
•
encoding extensin-like proteins are predominantly expressed in
tomato root hair cells. Plant Mol Biol 1997, 35:497-508.
The identification of two extensin genes which are preferentially expressed in
root hair cells, and the blockage of root hair growth by the prolyl hydroxylse
inhibitor 3,4-dehydro-L-proline, suggests a function for extensins in root hair
development. A clear and well-performed study.
41. Hoon AJ, Choi K, Kim SG, Myung KY, Do CY, Seob LJ: Expression of
•
a soybean hydroxyproline-rich glycoprotein gene is correlated
with maturation of roots. Plant Physiol 1998, 116:671-679.
The expression of SbHRGP3 increases during root development, showing
that expression is developementally-regulated. The function of the HRGP is
not known, but one possibility is that it is involved with modifying the expansibility of the cell walls. The gene may be useful for studies targeted to
resolving this function.
42. Grierson CS, Roberts K, Feldmann KA, Dolan L: The COW1 locus of
•• Arabidopsis acts after RHD2, and in parallel with RHD3 and TIP1,
to determine the shape, rate of elongation, and number of root
hairs produced from each site of hair formation. Plant Physiol
1997, 115:981-990.
Root hair abnormalities characteristic of COW1 are consistent with a defect in
an extensin gene. It would be interesting to see if these plants fail to express
or misexpress any of the extensins that are known to occur in Arabidopsis.
Conceivably, overexpression of such genes might make longer root hairs.
43. Ding L, Zhu JK: A role for arabinogalactan-proteins in root
•
epidermal cell expansion. Planta 1997, 203:289-294.
Treatment of Arabidopsis roots with Yariv reagent causes the appearance of
balloon-like swellings in the epidermal cells, thereby phenocopying the
mutant reb1. Yariv reagent binds AGPs, so this paper lends weight to the
view that certain AGPs play a mechanical role in cell expansion control.
Further analysis of reb1 will be interesting.
44. Xu W, Purugganan MM, Polisensky DH, Antosiewic DM, Fry SC,
Braam J: Arabidopsis TCH4, regulated by hormones and the
environment, encodes a xyloglucan endotransglycosylase. Plant
Cell 1995, 7:1555-1567.
45. Purugganan MM, Braam J, Fry S: The Arabidopsis TCH4 xyloglucan
•• endotransglycosylase. Substrate specificity, pH optimum, and
cold tolerance. Plant Physiol 1997, 115:181-190.
Purification has allowed the authors to detail the substrate requirements of
the Arabidopsis XET. A detailed understanding of this enzyme is likely to be
important for a complete picture of wall loosening and responses to endogenous and environmental signals.
46. Catalá C, Rose JKC, Bennett AB: Auxin regulation and spatial
•
localization of an endo-1,4-b-D-glucanase and a xyloglucan
endotransglycosylase in expanding tomato hypocotyls. Plant J
1997, 12:417-426.
The authors show unique and overlapping expression patterns an endo-1,4β-glucanase and an XET. These genes have the potential to promote wall
loosing and disassembly during with growth. One interesting issue raised by
these data is that they may act co-operatively to achieve this.
Antosiewicz DM, Purugganan MM, Polisensky DH, Braam J: Cellular
localization of Arabidopsis xyloglucan endotransglycosylaserelated proteins during development and after wind stimulation.
Plant Physiol 1997, 115:1319-1328.
Immunocytochemistry shows that TCH4 accumulates in expanding cells,
suggesting that it may modify walls to allow cell expansion and reinforcement. In so doing, it may play an important role in morphogenesis. Exactly
how this protein achieves these changes to a wall will represent an important advance in our understanding of development.
47.
•
510
Cell biology
48. Brady JD, Sadler IH, Fry SC: Pulcherosine, an oxidatively coupled
•• trimer of tyrosine in plant cell walls: its role in cross-link
formation. Phytochemistry 1998, 47:349-353.
This paper completes a series of publications establishing the chemistry of
one class of interpolypeptide cell wall cross-links thought to be modifiers of
the wall. When it is known how the polypeptid or the H2O2 can be regulated, it will be possible to make important predictions on the role of
pulcherosine in development and morphogenesis.
49. Ryser U, Schorderet M, Zhao GF, Studer D, Ruel K, Hauf G, Keller B:
•• Structural cell-wall proteins in protoxylem development: evidence
for a repair process mediated by a glycine-rich protein. Plant J
1997, 12:79-111.
A proline-rich protein and a glycine-rich protein are shown to occur in different parts of protoxylem cell walls. Along with cellulose, glycine-rich
proteins such as this and pectic galactans may function as a scaffold for
lignin assembly.
50. Jones L, Seymore, GB, Knox JP: Localization of pectic galactan in
•
tomato cell walls using a monoclonal antibody specific to (1-4)-bD-galactan. Plant Physiol 1997, 113:1405-1412.
Immunofluorescence shows that galactan side-chains composed of at least
3 (1-4)-β-galactosyl residues are absent from the walls of the epidermal and
subepidermal cells of tomato fruits. These absence of these side-chains may
promote homogalacturonan cross-linking and rigidification of the cell walls,
and in so doing provide a toughened barrier.