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Plant Physiol. (1 987) 84, 1-2
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Review
The Extensins
Received for publication February 19, 1987
MARY L. TIERNEY* AND JOSEPH E. VARNER
Plant Biology Program, Washington University, St. Louis, Missouri 63130
ABSTRACIr
The plant cell wall includes a matrix which is species specific and
which chages in composition during growth and development. Characterization of the protein component of the wall matrix has resulted in the
purification of extensin and the genes which encode it. Analysis of the
protein sequences for the extensins has provided clues about the types of
interactions which may occur as the chemistry and architecture of the
cell wall accommodate growth and development.
SerHypHypHypHyp
is a conserved repetitive sequence within all three sequences.
However longer repetitive sequences which are rich in other
amino acid residues with chemically reactive side chains between
the Ser(Hyp)1 "spacer sequences" such as
SerHypHypHypHypValTyrLys
and
SerHypHypHypHypValLysProTyrHisProThrHypValTyrLys
can be used to distinguish carrot extensin from each of the two
The cell walls of higher plants are dynamic structures which
undergo changes in composition and structure in relation to cell
age, tissue type, and environmental stimuli. Individual components of the cell wall respond to physical damage (wounding),
hormones, pathogen infection, and treatment with fungal elicitors (for review, see Showalter and Varner [11]). Moreover,
endogenous cell wall oligosaccharins have been identified which
when released affect the synthesis of defense related plant enzymes as well as serve to initiate a variety of developmental
programs (3, 14). Thus, an understanding of cell wall components and their interactions is a prerequisite toward elucidating
the roles cell wall chemistry and architecture play in plant growth
and development.
Analysis of the protein composition of the plant cell wall
initially focused on hydroxyproline-rich glycoproteins following
the discovery by Lamport that hydroxyproline was a major
amino acid constituent in cell wall hydrolysates. HRGPs' have
subsequently been shown to fall into three classes: hydroxyproline-rich lectins, arabinogalactans, and extensins. The hydroxyproline-rich lectins are a group of hemagglutinating glycoproteins
which appear to be limited to the Solanaceae. These proteins
contain both a hydroxyproline/serine-rich and a cysteine-rich
domain and have been shown to increase upon wounding (11).
Arabinogalactan proteins are acidic glycoproteins which are present in most plant tissues and are loosely associated with the cell
wall. These proteins are rich in serine, alanine, and hydroxyproline and many function in cell-cell recognition (1 1). Extensins
are HRGPs which become insolubilized within the primary cell
wall. Soluble forms ofthis protein have been purified from carrot
storage roots, soybean seed coats, and cultured tomato cells (1 1),
and in all cases the protein is rich in hydroxyproline, serine,
tyrosine, and lysine. A genomic clone encoding the carrot extensin gene has also been cloned and sequenced (1 1).
The predicted amino acid sequence for carrot extensin from
the extensin genomic clone has been compared with tryptic
fragments of two species of tomato extensin. The sequence
Abbreviation: HRGP, hydroxyproline-rich glycoprotein.
tomato extensin species. Therefore it seems that the extensin
protein genes have evolved to allow for a variety of functional
sequences which may interact uniquely with the wall matrix to
be spaced between a common backbone of Ser(Hyp)l spacer
sequences.
In any guessing game about how the extensins function we
need to keep in mind that recently several plant genes have been
isolated which may code for structural cell wall proteins unrelated
to the HRGPs. cDNA and genomic clones encoding proline/
hydroxyproline-rich proteins have been isolated from both
wounded carrot roots and from soybean cell cultures treated with
auxin (7, 11). In the case of carrot, these cDNAs have been
shown to code for a protein which accumulates in the cell wall
after wounding (13). The cDNAs from both carrot and soybean
code for proteins with biased amino acid compositions and
contain the sequences
XYProPro
and
ProProValTyrLys
throughout the coding region. A genomic clone encoding a
glycine-rich protein has been isolated from petunia which consists mainly of alternating GlyX residues in which X is frequently
Gly, His, or Phe (2). While the protein encoded by this genomic
clone has not yet been identified in petunia, a glycine-rich cell
wall protein fraction has been identified in both pumpkin and
soybean seed coats (1) indicating that the glycine-rich gene
sequence may indeed code for another class of celLwall protein.
Questions about the function of these proteins must await the
determination oftheir compositions, sequences, and localization.
The extensins appear to be encoded by small multi-gene
families. This leads to the suggestion that different members of
the family are regulated by different environmental or developmental signals. In carrots, different extensin mRNA transcripts
have been found to accumulate in response to wounding and
ethylene treatment (4). Similar types of differential expression
have been observed when extensin mRNA accumulation patterns have been analyzed after fungal infection of Phaseolus
vulgaris (10). Also, while extensin mRNA transcripts increase in
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2
Plant Physiol. Vol. 84, 1987
TIERNEY AND VARNER
response to wounding in carrot storage roots and tomato stems
and roots, very little extensin mRNA can be detected in leaves.
While these data indicate that the extensin gene family may be
differentially regulated in various tissues and in response to
several external stimuli, more information is needed to elucidate
both the mechanisms involved in mediating extensin gene
expression and the function of the various extensin species within
the cell wall.
Once the architecture of the primary cell wall has been established during cell division, the cell wall must still be able to
"loosen" or alter this structural matrix during the process of cell
elongation. The effect of auxin on cell elongation has been well
studied and models involving both obligate new gene expression
and wall acidification have been proposed to explain its action.
It is well established that treatment of plant tissue with auxin
results in the synthesis of new gene products, one of which may
be localized in the cell wall (7). However, the contribution that
this induced gene expression makes to the mechanism of auxin
regulated cell wall elongation has not yet been determined. The
observation that there are two separate elongation responses to
auxin in higher plants, one which may ready the wall for the
process of elongation and a second which requires the action of
newly synthesized gene products, has led to the suggestion that
the distinctly different models of gene expression and wall
acidification initially proposed are not incompatible (15).
How might extensin ( 1-15% of the dry weight of the wall of
dicotyledonous plants) interact with the other polymer systems
(cellulose, 20-30%; xyloglucan, 20-30%; xylans, approximately
5%; pectins, 25-30%) in the wall? First let us look at how the
polysaccharide components of the wall are thought to interact.
It seems generally agreed that cellulose microfibrils are the principal load-bearing structures of the wall and that these microfibrils are coated with xyloglucans. These xyloglucans may be the
primary substrate for the auxin regulated ,B- 1 ,4-glucanase activity
which has been implicated in the control of cell wall elongation
(6). Thus, with only these two wall components we can model a
functional wall made up of load-bearing members embedded in
a gel of adjustable properties. Starting from the facts (a) that
pectic substances constitute a major fraction of the gel matrix of
the wall, (b) that these can be secreted as methyl esters and
deesterified later by wall-localized pectin methyl esterase, (c) that
the resulting polyanionic matrix will have a higher concentration
of protons than non-polyanionic portion of the wall, and (d) the
properties of this matrix change markedly with changes in pH
and [Ca2+], Nari et al. (9) have proposed that the change in
polyanionic character caused by pectin methyl esterase controls
the enzymes-e.g. endo-f3-1,4-glucanases-that allow the wall
loosening necessary for cell growth. Critical parameters of this
model include the density and distribution of the negative charges
and the ability of the cell to control these parameters-for
example, by pumping Ca2" and/or protons.
The theory and properties of polyelectrolyte (polyacrylamidepolyacrylic acid) gels have been studied in detail (12). In char-
acterizing the physical properties of gels Tanaka has shown that
the gel properties of a highly polymerized network change markedly with small changes in the composition, pH, and ionic
strength of the solvent in which the gel is immersed. It seems to
us that the theory described by Tanaka (12) is applicable to the
proposals of Nari et al. (9) and deserve examination. The physical
nature of the oligosaccharide/protein matrix present in the cell
wall may indeed resemble a gel in its response to changes in pH,
ion concentration, and oligosaccharide/enzyme interactions
which have been characterized as a function of growth and
development.
What then is the function of extensin in determining cell wall
structure? Do its positive charges align with the negative charges
of pectin to make an electrovalent zipper with the degree of
closure controlled by pH and [Ca2+]? Is it co-localized with pectin
or does it form its own discrete polycationic matrix? Is extensin
polymerized to form a net (5, 8)? Is extensin covalently crosslinked to carbohydrate components of the wall? If so, what is the
chemistry of the cross-links? Are we asking the right questions?
LITERATURE CITED
1. CASSAB GI, JE VARNER 1986 A new protein in petunia. Nature 323: 110
2. CONDIT CM, RB MEAGHER 1986 A gene encoding a novel glycine-rich structural protein of petunia. Nature 323: 178-181
3. DAvis KR, AG DARVILL, P ALBERSHEIM, A DELL 1986 Host pathogen interactions. XXIX. Oligogalacturonides released from sodium polypectate by
endopolygalacturonic acid lyase are elicitors of phytoalexin in soybean. Plant
Physiol 80: 568-577
4. ECKER JR, RW DAVIS 1987 Plant defense genes are regulated by ethylene. Proc
Natl Acad Sci USA. In press
5. FRY SC 1986 Cross-linking of matrix polymers in growing cell walls of
angiosperms. Annu Rev Plant Physiol 37: 165-186
6. HAYASHI T, Y-S WONG, G MACLACHLAN 1984 Pea xyloglucan and cellulose.
II. Hydrolysis by pea endo-1,4--yglucanases. Plant Physiol 75: 596-604
7. HONG JC, RT NAGAO, JL KEY 1985 Sequence analysis of an auxin-responsive
developmentally regulated soybean gene. In GA Galamed, ed, First International Congress of Plant Molecular Biology. University of Georgia Center
for Continued Education, Athens, p 91
8. LAMPORT DTA, L EPSTEIN 1983 A new model for the primary cell wall: a
concatenated extensin-cellulose network. In DD Randall, DG Blevins, RS
Larson, BS Rapp, eds, Current Topics in Plant Biochemistry and Physiology,
Vol. II. University of Missouri, Columbia, pp 73-83
9. NARI J, G NOAT, G DIAMANTIDIS, M WOUDSTRA, J RICARD 1986 Electrostatic
effects and the dynamics of enzyme reactions at the surface of plant cells. 3.
Interplay between limited cell-wall autolysis, pectin methyl esterase activity
and electrostatic effects in soybean walls. Eur J Biochem 155: 199-202
10. SHOWALTER AM, JN BELL, CL CRAMER, JA BAILEY, JE VARNER, CJ LAMB
1985 Accumulation of hydroxyproline-rich glycoprotein mRNAs in response
to fungal elicitor and infection. Proc Natl Acad Sci USA 82: 6551-6555
11. SHOWALTER AM, JE VARNER 1987 Plant hydroxyproline-rich glycoproteins.
In A Marcus, ed, The Biochemistry of Plants: A Comprehensive Treatise;
Vol XI. Molecular Biology. Academic Press, New York. In press
12. TANAKA T 1980 Phase transitions in ionic gels. Physiol Rev Lett 45: 1636-
1639
13. TIERNEY ML 1986 Accumulation of a new proline-rich protein in carrot root
cell walls after wounding. Plant Physiol 80: S-70
14. TRAN THANH VAN K, P TOUBART, A COUSSON, AG DARVILL, DJ GOLLIN, P
CHELF, P ALBERSHEIM 1985 Manipulation of morphogenetic pathways of
tobacco explants by oligosaccharins. Nature 314: 615-617
15. VANDERHOEF LN 1980 Auxin regulated cell enlargement: is there action at the
level of gene expression? In CJ Leaver, ed, Genome Organization and
Expression in Plants. Plenum Press, New York, pp 159-173
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Copyright © 1987 American Society of Plant Biologists. All rights reserved.