Review Article
Expansins in Plant Growth and Development:
an Update on an Emerging Topic
S. J. McQueen-Mason and F. Rochange
The Plant Laboratory, Department of Biology, University of York, York, UK
Received: August 3, 1998; Accepted: October 23, 1998
Abstract: Expansins are a class of proteins identified by their
ability to induce the extension of isolated plant cell walls. Expansins are encoded by an extensive multigene family in higher
plants, several members of which have been shown to be
expressed in a tissue-specific manner. Besides playing an appar-
ently key role in wall expansion, and hence in cell growth, expansins have been implicated in an increasing number of processes during plant growth and development. These include:
leaf organogenesis, fruit softening, and wall disassembly. A second class of closely related proteins (referred to as -expansins)
has been identified. Other recent advances in expansin research
include the recovery of transgenic plants with altered level of
expansins, and the production of recombinant expansins in heterologous expression systems.
Key words: Expansin, cell wall, plant cell enlargement, plant
growth.
Discovery of Expansins
In 1970, Rayle et al.128 demonstrated that isolated growing cell
walls were highly extensible at acidic pH values but not at neutral pH. This so-called acid-induced extension process is only
seen in walls isolated from growing cells, and the degree to
which walls can undergo this process is closely correlated with
growth rate (McQueen-Mason, 1995121]). Acid-induced extension is eliminated in isolated walls by treatments, such as heat
or proteolysis, likely to inhibit enzyme mediated processes
(Rayle and Cleland, 1972129]; Cosgrove, 198914]). This observa-
tion led to the development of a simple reconstitution assay
which was used to identify the proteins responsible for the
acid-induced extension of etiolated cucumber hypocotyl walls
(McQueen-Mason et al., 1992]22]). Fig.1 summarises data from
these experiments. The method used is sometimes referred to
as creep measurement and involves monitoring the extension
of material under constant mechanical stress. A unidirectional
extensive force is applied to isolated wall material to repro-
Introduction
The study of expansins in plant growth and development is a
rapidly developing area. Much of the early work on expansins
has been reviewed in depth in other publications (McQueenMason, 1995121]; Cosgrove, 1996'; Shieh and Cosgrove,
1998 a134]). The aim of this current review is to bring readers
up to date with some of the latest developments in this field.
The cell wall plays a central role in many aspects of growth and
development in plants. In particular, the role of the wall in cell
expansion has received considerable attention. A typical plant
cell will increase its volume by between a hundred and a thousand fold during development. Because plant cells are encased
in a strong fibre-composite wall, cell growth is dependent on
the ability of this wall to expand. The walls of non-growing
cells are incapable of plastic extension, whereas those of grow-
ing cells are more plastic. Factors such as light, gravity, and
hormones, have been shown to rapidly modulate plant growth,
and achieve this principally by modulating wall extensibility.
It is now generally accepted that plant cell growth is ultimately
controlled by the extensibility of the walls.
Plant biol. 1(1999)19—25
© Georg Thieme Verlag Stuttgart. New York
ISSN 1435-8603
duce the effects of turgor-induced stress. The first trace shows
the acid-induced extension of isolated walls from the growing
region of an etiolated hypocotyl. When the walls are bathed at
neutral pH values, a very low rate of extension is apparent.
After about 20 minutes the pH is switched to an acidic value
inducing a rapid increase in extension rate which can be sus-
tained over a period of hours often leading to increases in
length in excess of 60%. This acid-induced extension is elimin-
ated by heat treatment (shown in second trace). In the third
trace, the ability of heat treated walls to extend at acid pH is
reconstituted by the addition of proteins extracted from growing walls.
Standard protein chromatographic techniques were used in
conjunction with this reconstitution assay to isolate two very
similar enzymes capable of restoring wall extension from
growing hypocotyls. This class of proteins became known as
expansins and their importance in various aspects of plant development is becoming increasingly apparent.
Expansin Sequences
Following partial protein sequencing, cDNAs encoding the two
cucumber hypocotyl expansins were isolated by a combination
of RT-PCR and library screening methods (Shcherban et al.,
1995133]). Sequencing of these cDNAs revealed expansins to
have a distinct and unique structure. Database searches re-
19
20 Plant biol. 1 (1999)
S. J. McQueen-Mason and F. Rochange
Native
sins and pollen allergens show about 20—25% amino acid similarity. Short conserved regions are distributed along the pro-
tein backbone and both groups of proteins have a similar
predicted secondary structure. Furthermore, group I allergen
from maize pollen has expansin-like activity. Based on these
pH 6.8 to 4.5
observations, Cosgrove et al. (1997161) designated these allergens, and related sequences, as 3-expansins, referring to the
Boiled
Boiled
first-described class as a-expansins. The allergen-like 3-expansins have closely related homologues in vegetative tissues,
the functions of which are unclear. A notable difference between these two groups is the extensive glycosylation of 3-expansins which appears to be absent in the a-expansins.
Expansin
Conserved features in exponsins
0.0
0.5
1.0
1.5
2.0
2.5
Time (h)
Fig.1
Extension of native and reconstituted cell wall specimens
(adapted from McQueen-Mason et al., 1992122]). Apical sections cut
from etiolated cucumber hypocotyls were frozen, thawed, abraded
and pressed prior to suspension in the extensometer. Samples were
clamped under an applied force of 20g. and extension was recorded
using a linear voltage displacement transducer. The specimen length
between the clamps was 5 mm. From top to bottom: native walls
were suspended in 50mM Hepes, pH 6.8, for 20mm after which
the bathing solution was replaced by 50mM sodium acetate, pH
4.5. To inactivate the walls, specimens were treated for 15 sec with
boiling water prior to clamping; as shown in the second line, this
treatment eliminated acid-induced extension. For reconstitution ex-
Fig. 2 shows the protein sequence of cucumber expansin Cs
Exp-1 which was the first to be cloned and exemplifies a-expansin structure. At the N-terminus, expansins have a hydrophobic signal peptide that is cleaved during uptake into the
ER, setting it on course for secretion to the wall. The N-terminal half of the mature protein contains eight conserved cysteinyl
residues which may form disulfide linkages within the protein.
The C-terminal half contains 4 conserved tryptophan residues,
the spacing between which is similar to that seen in the cellulose binding domains of bacterial cellulases. As these residues
seem to be involved in cellulose binding of these enzymes,
they might also be important for expansirl binding to cellulose
and related wall glycans. The residues shown in italics are regions of high homology between a- and 3-expansins. Some of
these italicised homologies (which include conserved cysteines) are also shared with fungal family-45 glycosyl hydrolases (Cosgrove, 199716]).
periments (bottom line), inactivated walls were suspended in
50mM sodium acetate, pH 4.5, for approximately 30 mm, at which
point the bathing solution was replaced by 0.5 mL of fresh solution
containing 2 to 3 mg of proteins extracted from growing cell walls.
vealed close homologies to EST sequences from the Arabidopsis
and rice collections. Sequence information from these ESTs,
and an ever growing number of new expansin sequences, have
indicated a number of distinct structural characteristics associated with expansin proteins.
Two classes of exponsins
Expansins show a high level of protein sequence conservation
with similarities in the range of 70 to 90%. The most closely
related group of proteins, based on sequence similarity to expansins, are the group I pollen allergens from grasses. Expan-
Exponsins comprise an extensive multigene family
in higher plants
Expansin sequences have now been characterised from a wide
range of higher plants and organs including: cucumber hypocotyls, Arabidopsis (Shcherban et al., 1995[]); rice internodes
(Cho and Kende, 1997 b]3]); tomato, melon, and strawberry
fruit (Rose et a!., 19971301); tomato meristems (Fleming et al.,
1997114!); tomato vegetative tissues (Rochange and McQueen-
Mason, unpublished data); pine hypocotyls (Huchison et a!.
Genbank entry, accession number U64895); pea petals (Michael, 1996]251); soybean cell culture (Downes and Crowell,
19981131); flax stems (van Amstel and McQueen-Mason, unpublished data); cotton fibres (Shimizu et al., 1997136]); and Eucalyptus stems (Cambridge et al., unpublished data), and it has
become clear that these proteins are represented by a substan-
tial multigene family. For example, in Arabidopsis more than
—
DYGGWQSGHATFYfGGDASGTMGGACLYGNLYSQGY
G I N TV A L S TA L F N N
L S C i A C LE M T C TN D P K W L P G T I
RV TA TNFCPEN FALPNNNGGWNPP L QHFDMAEPA..EL
QIAQYRAIVPV SFRJLEPCMKKGVRFTINGHS YFNLVL
ITN
A GD VH S S IKGS RT GWQAM S RNW.G QN .WQ SNN
YLNGQGLSFQVTL SDRTLTAYNLV?SNH'QFGQTYEG
PQF
Fig.2 Amino-acid sequence of cucumber
expansin Cs Exp-1. Regions of high conserva-
tion between a- and 3-expansins are indicated in italics. Conserved cysteines and tryptophans are shown in boldface.
Plant biol. 1 (1999) 21
Expansins in Plant Growth and Development
20 expansin open reading frames have been identified (Cos-
detectable changes in the covalent structure of cell walls
grove, 1998171).
(McQueen-Mason and Cosgrove, 19951241).
This diversity of expansin forms in a plant may represent a
need for tissue-specific expression of expansins at different
The lack of hydrolytic activity associated with expansin action
led to the hypothesis that they acted by disrupting non-covalent interactions between wall polymers. This idea was given
stages of a plant's life cycle. Various pieces of data are presently emerging which support this notion. For example Cho and
Kende (1997b'3]) have shown that in deepwater rice different
expansin transcripts show distinct expression patterns. Repor-
ter gene fusions to expansin promoter regions in transgenic
Arabidopsis have shown clear and distinctive expression patterns (Cosgrove et al., 1998 b112l). Similarly transcripts expressed in tomato during fruit softening (Rose et al., 19971301), or leaf
primordia initiation (Fleming et al., 19971141), show specific ex-
pression patterns. The significance of expansin sequence diver-
sity with regards to different biochemical properties will be
discussed in the next section.
Biochemistry of Expansin Action
Wall structure
The major components and structure of primary plant cell
walls are summarised below, for comprehensive reviews of
this topic two papers (Carpita and Gibeaut, 199311]; and
McCann and Roberts, 1994120]) are recommended. Plant cell
walls are complex composites of a range of polymeric substances. In growing cell walls the majority of the material is carbohydrate with low levels of structural and enzymatic proteins,
whereas the network of structural glycoproteins is more important in older, non-growing walls. In growing cells the major
load-bearing network is composed of cellulose microfibrils,
coated by, and held to one another by hemicellulose polymers.
Cellulose microfibrils are composed of linear J3(1-4) glucan
polymers which hydrogen bond tightly to one another giving
rise to long crystalline rods which may contain between 20 to
100 polymers in cross section. In walls of dicots the major
hemicelluloses are xyloglucans which are also p(1-4) glucans,
but in this case the polymers are decorated with side chains of
xylose, galactose and fucose. Although the glucan backbone of
xyloglucans can hydrogen bond with the microfibrils, the presence of these side chains is believed to prevent crystallisation
of the polymers. Co-extensive with this is another network
composed of pectic polymers. Pectins are a mix of complex
polysaccharides rich in galacturonic acid or esters thereof. It
is worthwhile noting that in growing cell walls there are far
fewer covalent associations between polymers than in mature
walls. This suggests that growing walls may largely be held together by non-covalent interactions between polymers.
Expansins and elongation
Because the
cellulose/hemicellulose network is considered the
load bearing (in mechanical terms) component of the walls, it
was predicted that enzymes that influence extensibility would
do so by alterations to this network. The favoured candidates
for wall loosening enzymes were for many years hydrolases of
the hemicellulose network and considerable circumstantial
support when it was shown that expansins could markedly
weaken cellulose-based filter paper without detectable hydrolysis of the cellulose fibres (McQueen-Mason and Cosgrove,
1994123]). Because the mechanical properties of paper depend
upon the interactions between the fibres, and these are mostly
hydrogen bonds, it appears that in this case expansins act by
weakening these bonds. The situation in the cell wall is rather
different as the cellulose microfibrils do not contact each other
directly. Binding studies have shown that expansins have low
affinity for pure cellulose, and similarly low affinity for xyloglucans in solution (McQueen-Mason and Cosgrove, 19951241).
In contrast, expansins have high affinity for composites made
from cellulose and hemicelluloses such as xyloglucan. This indicates that expansins probably bind at the interface between
these polymer groups in the cell wall, and induce extension by
disrupting the hydrogen bonds between them. In support of
this hypothesis, it was shown that expansin-induced extension
could be synergised by the addition of molar concentrations of
urea, a chaotrope which generally weakens hydrogen bonds
(McQueen-Mason and Cosgrove, 1994123]).
More recently we have been studying the effects of expansins
on carbohydrate matrices produced by cultures of the cellulose-secreting bacteria Acetobacterxylinus. These bacteria produce mats of cellulose fibrils referred to as pellicles. The fibrils
within these pellicles are greatly entangled giving rise to a
strong, relatively iriextensible, material. It was recently shown
that when these bacteria are grown in media containing xylo-
glucans (from tamarind seed), the resulting composites are
markedly different to those normally produced. Electron microscopy studies revealed differences in the ultrastructure of
these (Whitney et al., 199511). Clear inter-fibril connections
were apparent in the xyloglucan-containing material producing structures similar to those seen in pectin-extracted wall
material.
Pellicles produced in the presence of soluble xyloglucans have
a lower breaking stress than do control pellicles, but are also
more extensible (Whitney, Gothard, Mitchell, McQueen-Mason and Gidley; unpublished data). That is to say that the xyloglucan-containing pellicles can extend to a far greater degree
before the material breaks than can cellulose-only pellicles.
Expansins have dramatic effects on this xyloglucan-cellulose
composite inducing extension in a manner similar to that seen
in cell walls (Whitney et al., unpublished data). Not only do
these data indicate that this composite material has some mechanical characteristics in common with growing walls, they
also confirm that cucumber expansins probably work at the interface between xyloglucans and cellulose microfibrils.
Expansins and wall hydrolysis
Hoson, 19931181). However, initial studies found no trace of hy-
is a recent report that expansins exhibit extremely low
levels of hydrolytic activity on soluble mixed link 3-glucan
drolytic activity associated with expansin wall-loosening action. Indeed, studies using stress relaxation data indicated that
prolonged incubation of cell walls with expansins led to no
from barley, as shown using a sensitive Congo Red assay (Cosgrove et al., 1998 a1101). This activity is extremely low, about
1,000 times less than that of a typical cellulase. As mentioned
support for this hypothesis exists (reviewed in Fry, 19891151;
There
22 Plant biol. 1 (1999)
-____________________
earlier, there is some structural similarity between expansins
and the family-45 endoglucanases (for a review on glucanase
classification see Henrissat and Davies, 19971171) and this similarity is probably related to the low level hydrolytic activity of
expansins. It is possible that these two classes of proteins have
diverged from some common ancestral form. The significance
of this limited, and probably vestigial, hydrolytic activity in expansins is unclear. Although they cannot themselves hydrolyse
crystalline cellulose, it was also found that expansins could
stimulate the hydrolytic activity of fungal cellulase on this
substrate by up to 50%. This suggests that expansins may be
able to solubilise 3-glucans from the crystal structure of the
fibrils, making them more susceptible to hydrolases.
Expansin diversity
In addition to the need for differential expression of expansins
during development, the diversity of expansin genes in plants
may also represent differences in substrate specificity in the
proteins that they encode. For example, the biochemical composition of primary walls from a typical dicot plant is quite dif-
ferent from that found in grasses, particularly in terms of
hemicellulose composition (Carpita and Gibeaut, 1993111). It
would seem likely, then, that expansins from these different
types of plants would be active on different substrates to one
another. In line with this, 1-expansins from maize pollen
showed no activity on walls from cucumber hypocotyls but
were active on maize silks and coleoptiles (Cosgrove et al.,
1997t1). However, it has also been observed that cz-expansins
from a variety of sources can induce extension in walls of unrelated species. For example, expansins from oat coleoptiles
were actually more active in assays on cucumber hypocotyls
than in assays on the hypocotyl walls from which they were
extracted (Li et al., l993]9I). Similarly cucumber hypocotyl expansins have been shown to be active on walls from a variety
of different plants (McQueen-Mason et al., 1992]22]).
S. j. McQueen-Mason and F. Rochange
could be extracted from rapidly growing cell walls but not
from the walls of non-growing cells (McQueen-Mason et al.,
1992]22]). A more critical comparison of growth rate, wall extensibility and extractable expansin activity revealed that the
decline in extractable expansin activity, as walls mature and
stop growing, lags behind the decline in growth and wall extensibility (McQueen-Mason, 1995121]). It was also shown that
as the walls reach maturity they become less sensitive to expansin action. It appears that a combination of declining expansin levels and increased crosslinking of the walls lead to
the cessation of cell expansion in this system.
The association of expansin activity and growth rates has also
been examined in oat coleoptiles (Cosgrove and Li, 1993111]),
and
in internodes of deepwater rice (Cho and Kende,
1997 a]2]). In both these cases expansin activities were highest
in rapidly growing walls but declined more slowly than did
growth rate. As in the cucumber hypocotyls, changes in wall
sensitivity to expansin action appears to play a major role in
regulating wall expansion. A similar situation was also revealed in a study of the role of expansin in the growth of maize
primary roots under drought conditions (Wu et al,, 19961381).
Root growth is maintained under low water potentials by increased wall extensibility in the extreme growing tip of the
roots. This was shown to be accompanied by both increased
expansin activity and increased sensitivity to expansins.
These changes in wall sensitivity to expansins highlight the
importance of other wall enzymes in the growth process. It is
likely that expansins work in concert with enzymes such as
xyloglucan endotransglycosylases (Fry, 1995116]), and 3-glucanases (Nicol and Hofte, 1998125]) which can influence the struc-
tural character of the cellulose/hemicellulose network. Similarly, enzymes, such as peroxidases (Schopfer, 1996132]) and
pectin esterases (Quentin et al., 1997]27]), which crosslink wall
polymers to one another are likely to play a role in rigidifying
the cell wall.
Expansins with different properties can also be found within a
tissue. For example, in stress relaxation assays using two different expansins purified from cucumber hypocotyls, these
two proteins clearly affected very different regions of the relaxation spectrum (McQueen-Mason and Cosgrove, 19951241).
This suggests that these two proteins may work on different
components within the walls, or perhaps in the walls of different cell types.
Finally, it has recently become evident that some expansins
are probably involved in wall disassembly rather than elongation (discussed below). It is possible that expansins involved in
wall breakdown and those involved in expansion may work on
different wall components.
Roles of Expansins
Expansins and cell expansion
In historical terms of our understanding, cell expansion is the
primary role of expansins, as they were isolated and named
based on their ability to induce wall extension. This aspect of
expansin activity has, therefore, also received the most attention. The role and importance of expansins in inducing wall
extension during growth is now fairly well established. In
cucumber hypocotyls it was reported that expansin activity
Expansins and wall disassembly
The
extremely limited apparent hydrolytic activity of expansins, and their ability to synergise the digestion of cellulose
by increasing glucan solubilisation have already been discussed. This indicates that expansins may well play a role in cell
wall disassembly at certain stages in a plant's life cycle. The
characterisation of expansin transcripts that are only apparent
during the softening of fruit in tomato, strawberry and melon
(Rose et al., 1997130]) supports this idea. No role for cell expan-
sion during fruit softening is evident. Wall disassembly is,
however, a major activity during this process. These expansin
isoforms might then be involved in releasing glucan polymers
from the microfibrils for digestion by hydrolases. Similarly, expansin-like activity and epitope have been detected in protein
extracts from the digestive tract of snails (Cosgrove and Durachko, 199411). As with the fruit softening expansin, it is likely
that an expansin in the digestive tract of an animal would be
involved in wall disassembly rather than expansion.
In a separate line of evidence, promoter-reporter fusion studies have indicated that an isoform of expansins from Arabidopsis is expressed in abscission zones in leaf petioles (Cosgrove et
al., 1998 b112]). This again is a region where wall disassembly is
the predominant physiological activity. From these observa-
Plant biol. 1 (1999) 23
Expansins in Plant Growth and Development
tions one might predict that expansins may be found in asso-
ciation with other areas in which wall breakdown is important.
Such tissues as abscission zones, regions of lateral root formation, airspace formation etc. may prove fruitful areas to look for
expansin roles.
Expansins and plant development
Given the role of expansins in cell enlargement, it seems likely
that expansins may play an important role during plant devel-
opment and organogenesis. In a recent report it was shown
that topical applications of expansin coated beads to the flanks
of the vegetative meristem of tomato plants could induce the
production of leaf primordia-like organs (Fleming et al.,
1997114]). In fact, the early steps of primordium initiation involve tissue bulging around the meristem. Expansins applied
locally would increase cell wall extensibility in the outer cells,
hence allowing the formation of a bulge. In the same report it
was shown, using in-situ hybridisations, that an expansin transcript (isolated by RT-PCR from meristematic tissues) accumulated in normal developing leaf primordia in the meristem.
More recent in-situ hybridisation studies show that the appearance of this expansin transcript precedes any detectable
bulging out of a new primordium (Reinhardt et al., 1998131]) indicating an early role of expansins in primordium initiation.
These results suggest that expansins may be important in morphogenesis in this system. They may also be involved in other
aspects of development and organogenesis.
\ III)'
ILt '''L
Expression of Expansin Sequences
Recombinant expansins
Several groups have tried to express recombinant expansins in
transgenic bacterial and fungal cultures. In several experiments we successfully produced recombinant expansin protein in both E. coli and Pichia pastoris but this was accompanied by little or no expansin activity (Filatov and McQueenMason, unpublished data). More recently, two groups have
produced active recombinant expansins in cultured insect
Fig. 3 Transgenic tomato plant expressing cucumber expansin. Tomato plants were transformed with a construct containing a cucumber expansin gene (Cs Exp-1) under the control of the Cauliflower
Mosaic Virus 355 promoter. One of the Fl transformants (Cu-i) is
shown with a wild-type plant of the same age.
cells transfected with Baculovirus vectors containing expansin
coding sequences (Filatov and McQueen-Mason, unpublished
data; Shieh and Cosgrove, 1998 b]351). We have also obtained
high levels of expansin activity in transgenic tomato plants ex-
treme cases plant growth is actually retarded (Fig. 3) and this
pressing the coding sequence of a cucumber expansin (Rochange and McQueen-Mason, unpublished data). These experiments are the first to confirm that expression of a presumed expansin coding sequence gives rise to expansin activ-
overall impression is that abnormally high expansin levels necessitate the plant to modify its walls so as to limit extensibil-
ity.
Expansins in trcinsgenic plants
In our group we have been investigating the effects of altering
levels of expansin activity in transgenic plants. Two approaches have been adopted. In the first (mentioned above) we have
introduced the coding sequence for a cucumber expansin into
transgenic tomato plants under the control of the Cauliflower
Mosaic Virus 35S promoter (Rochange and McQueen-Mason,
unpublished data). Transgenic lines with various levels of
transgene expression have been produced. A number of phenotypic effects are associated with these plants. Some of these
effects appear to reflect modifications of the wall which enable
the plant to cope with abnormally high expansin levels. In ex-
is associated with decreased wall extensibility. It appears that
this decrease in extensibility may result from a decrease in
sensitivity to expansins of cell walls in growing tissues. The
ity. A second approach is the use of antisense constructs to
lower the levels of specific expansin isoforms in tomato plants.
So far three expansins have received this treatment and we are
currently assessing the phenotypic effects which include
dwarfing and other alterations of various organs (Rochange
and McQueen-Mason, unpublished data).
Future Directions
Expansin research has gained considerable momentum in the
last two or three years as more groups have become involved.
Much recent work has focused on the characterisation of expansin transcripts from a wide variety of plants and tissues.
This has suggested expansin involvement in an increasing
number of developmental processes. In many cases the biochemical activity of the enzymes encoded by these transcripts
remains unknown. There are good reasons for this. Most tis-
24 Plant biol. 1 (1999)
S. j. McQueen-Mason and F. Rochange
sues contain more than one expansin isoform and these need
separation before definitive biochemical observations can be
made. In addition, low levels of expansins are normally available for extraction from a given tissue. This is compounded by
the fact that the actual tissue from which the expansin can be
isolated is often very small. Current expansin assays also use
quite large amounts of protein and are rather time-consuming.
Cosgrove, D. J. and Durachko, D. M. (1994) Autolysis and extension
of isolated walls from growing cucumber hypocotyls. J. Exp. Bot.
45, 1711 —1719.
10
have cryptic endoglucanase activity and can synergize the breakdown of cellulose by fungal cellulases. ASPP meeting, Abstract 171.
Cosgrove, D.J. and Li, Z. C. (1993) Role of expansins in developmen-
tal and light control of growth and wall extension in oat coleop-
This situation should be alleviated by the ability to produce active recombinant expansins which will make the production of
12
purified expansin isoforms a far simpler task. This facility
should allow the biochemistry of expansin action to be more
carefully explored. It will also allow the use of site-directed
13
mutagenesis to determine expansin epitopes involved in binding and activity. It is hoped that sufficient protein will become
available to undertake studies of expansin three-dimensional
structure using crystallographic methods, should protein crystallisation prove possible. Alternatively, solution structure determination using NMR techniques may be possible. The availability of novel expansin substrates such as composite materials containing different glycans should also help our understanding of substrate specificity and activity among various
isoforms.
There is commercial potential in some aspects of expansin research. The important role of expansins in growth processes
suggests that modifying expansin levels in specific organs in a
plant might prove useful in various areas of agriculture and
horticulture. The ability of expansins to enhance the digestibility of cellulose suggests they might be used to increase the digestibility of forage products. Similarly, the effects of exparlsins in weakening inter-fibre associations in paper and other
cellulosic products could be of interest to the paper and fabrics
industries. Now that active recombinant expansins are available, large enough quantities of expansins can be produced to
investigate some of these areas.
Cosgrove, D. J., Durachko, D. M., and Li, L. C. (1998 a) Expansins
tiles. Plant Physiol. 103, 1321 —1328.
Cosgrove, D. J., Shcherban, 1. Y., Durachko, D. M., and Altmann, T.
(1998b) Highly specific and distinct expression patterns for two
alpha-expansin genes in Arabidopsis. ASPP meeting, Abstract 175.
Downes, B. P. and Crowell, D. N. (1998) Cytokinin regulates the ex-
pression of a soybean beta-expansin gene by a post-transcription14
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growth: a possible relationship. Physiol. Plant. 75, 532—536.
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Fry, S. C. (1995) Polysaccharide modifying enzymes in the plant
17
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Henrissat, B. and Davies, G. (1997) Structural and sequence-based
classification of glycoside hydrolases. Current Biology 7,637—644.
Hoson, 1. (1993) Regulation of polysaccharide breakdown during
auxin-induced cell wall loosening. J. Plant Res. 103, 369—381.
18
Li, Z. C., Durachko, D. M., and Cosgrove, D. J. (1993) An oat coleoptile wall protein that induces wall extension in vitro and is antigenically related to a similar protein from cucumber hypocotyls.
Planta 191, 349—356.
20
McCann, M. C. and Roberts, 1<. (1994) Changes in cell wall architec-
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