Download Emerging patterns of organization at the plant cell surface

COMMENTARY
Emerging patterns of organization at the plant cell surface
PAUL KNOX
Department of Cell Biology, John Innes Institute, Colney Lane, Noiwich NR4 7UH, UK
The processes involved in the coordinated development of
multicellular organisms are undoubtedly highly complex.
In animal systems extensive information has been obtained about molecules of the cell surface and extracellular matrix that are involved in cell interactions and
developmental processes (Edelman, 1986; Ekblom et al.
1986; Gallagher, 1989). In mature plant tissues, specific
cell surface changes are known and can be related to
specific functions and cell types, such as cutin at the outer
surface of plants, suberin in secondary protective tissue,
the thickened walls of collenchyma cells and the asymmetrically thickened walls of guard cells. However, knowledge of specific interactions or modulations at the surfaces
of plant cells during the primary stages of plant cell
organization, i.e. in meristems and during embryogenesis,
is lacking.
It is, of course, at this level of cell interactions and the
organization of cells into organisms that plant cells display some of their most obvious differences from animal
cells. A developing meristem, such as that of a plant root,
is a striking phenomenon in that the whole developmental
pathway can be encountered at one time in the maturing
files of cells occurring proximally to the meristematic
initials. However, the distinctive developmental features
of plants - cell immobility, rigid walls, growth dependent
upon the plane of cell division and cell expansion - have
not been conducive to the investigation of the molecular
properties of the plant cell surface in relation to the
organization and formation of the tissue pattern within
such a system. Little is known of the local variations in the
molecular condition of the cell wall that must be important
for the direction and nature of cell expansion, or of the
nature of molecular links of the plant cytoskeleton with
components of the plasma membrane and the cell surface.
Although the extracellular zone of plant tissues can be
viewed as a unified space with cell walls in intimate
contact, and the presence of plasma membrane-lined
plasmodesmata permits a correspondingly unified intracellular space, virtually nothing is known of the need for
or occurrence of interactions between neighbouring cells,
both within and between cell lineages, across the milieu of
the cell wall and their influence on cell development and
gene expression.
A useful starting point for any investigation of the
molecular mechanisms leading to the formation of complex structures, such as those of a plant, is the identification of molecules that display restricted patterns of
occurrence within the developing system. What follows is
a survey of the currently known instances in which the
Journal of Cell Science 96, 657-561 (1990)
Printed in Great Britain © The Company of Biologists Limited 1990
occurrence of plant cell surface molecules, predominantly
glycoproteins, have been correlated with developmental
stages, tissues or cell types.
The molecules of the plant cell surface
Often thought of as a relatively inert structural box within
which the protoplast resides, the plant cell wall can
perhaps more accurately be regarded as the major component of a dynamic extracellular matrix, albeit more
confining and displaying more rigidity than that of animal
cells (Roberts, 1989). The great complexity of polysaccharides that account for much of the wall has been, and
continues to be, elucidated by chemical means, but knowledge of any compositional differences relating to the early
events of morphogenesis is fragmentary (Bacic et al. 1988).
Cell wall proteins can contribute up to a tenth of all wall
material, but the nature of their interaction, attachment,
developmental or biochemical (other than enzymic) function and relation to overall cell wall architecture remains
uncertain (Cassab and Varner, 1988). A striking characteristic of many of the wall proteins is the presence of high
levels of hydroxyproline, a feature of collagens of the
animal extracellular matrix. In the thirty years since
hydroxyproline was shown to occur in plant cell walls
(Lamport and Northcote, 1960) various classes of hydroxyproline-rich glycoproteins (HRGPs) have been discerned,
but still remain broadly categorized into three groups the extensins, the arabinogalactan proteins and the Solanaceous lectins (Showalter and Varner, 1989). Some of the
defining characteristics of these and other developmentally regulated proteins of the plant extracellular matrix
are shown in Table 1 and the current knowledge of the
structure of the genes encoding these proteins is to be
found in the reviews by Varner and Lin (1989) and
Showalter and Varner (1989).
Developmental patterns
The extensins are the most studied of the HRGPs. They
have been characterized by a repetitive Ser-(Hyp)4 peptide
sequence, and have been shown to occur as rod-like
structures, stabilized by glycosylation (Showalter and
Varner, 1989). This may be related to a structural role in
strengthening walls at the completion of cell expansion by
the formation of a cross-linked insoluble matrix. Several
forms of extensin can occur in the same tissue, for example
up to four in tomato (Showalter and Varner, 1989), but
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Table 1. Structural characteristics of the major groups of developmentally regulated proteins of the plant cell surface
Peptide sequences
Extensins
Arabinogalactan proteins1"2
Proline-rich proteins34
Glycine-rich proteins
Ser-(Hyp)4
Ala-Hyp
Pro-Pro-Val-X-Y
(Giy-X)n
Carbohydrate (%)
-50
-90
0/?
0
Main protein-carbohydrate linkages
Hyp-Ara, Ser-Gal
Hyp-Gal, Ser-Gal, Hyp-Ara
Source references: ' Showalter and Vamer (1989); 3 Gleeson et al. (1989); 3 Hong et al. (1990); * Keller et al. (1989).
whether this reflects differences in location has not been
resolved. Antibodies generated to soybean coat extensin,
have been used to localize extensin specifically to the
mature sclerenchyma tissue of seed coats, which may
indicate a protective function (Cassab and Varner, 1987).
These same antibodies, utilized in a nitrocellulose printing technique possibly only allowing visualization of
soluble extensin and not the insolubilized form, is suggestive of tissue variation and association of extensin with the
vascular tissue of bean hypocotyl and the epidermal and
vascular tissue of pea epicotyls (Cassab and Varner, 1987;
Cassab et al. 1988). Although these observations have
been supported by immunolocalization studies using a
monoclonal antibody directed against extensin (Meyer et
al. 1988) the precise cell types reactive with the antibodies
have not been reported. In a complementary study antibodies, generated against a carrot extensin, have been
used to locate the antigen in the cell wall of phloem of the
carrot storage root, the source of the immunogen (Stafstrom and Staehelin, 1988). It is of interest in this study
that extensin appeared to occur throughout the cell wall,
but with significantly less in the region of the middle
lamella. A striking observation, also with the same antibodies, was that no antigen could be detected in the
primary root of the carrot seedling (Stafstrom and Staehelin, 1988). It is not clear from these two sets of studies
whether the observed patterns of localization indicate the
restricted occurrence of extensin or reflect tissue variation
in the antigenic components of extensins; both possibilities suggesting developmental regulation. Antibodies
to non-glycosylated epitopes are capable of the specific
recognition of distinct extensins (Kieliszewski and Lamport, 1986).
Although graminaceous monocots generally contain low
levels of HRGPs, a threonine-rich HEGP, homologous
with dicot extensins, has been isolated from maize cell
cultures (Kieliszewski and Lamport, 1987; Kieliszewski et
al. 1990) and evidence has accumulated that related
molecules are developmentally regulated within maize
tissues (Hood et al. 1988; Stiefel et al. 1988). Analysis of
mRNA has indicated abundant expression in developing
tissues such as the maize root tip and coleoptile node, but
not in mature root or shoot tissues (Stiefel et al. 1988). The
protein has recently been immunolocalized to the cell wall
of maize root tips, and evidence indicates that its occurrence correlates with cell division rather than elongation
(Ludevid et al. 1990). Further analyses of the protein
structure of extensins indicates that in sugar beet the Ser(Hyp)4 block appears to be split, indicating an unusually
high interspecific variability for a putative structural
molecule (Li et al. 1990).
Other than the extensins, two further classes of cell wall
protein genes have been studied. These are the prolinerich proteins (PRPs) and the glycine-rich proteins (GRPs).
The gene family encoding the PRPs of soybean show a
strikingly complex pattern of organ-specific and developmentally regulated expression (Hong et al. 1989; Datta et
558
P. Knox
al. 1989). PRP mRNAs were detected in all organs and also
in cultured cells, and SbPRPl and SbPRP2 display contrasting gradients of expression in the developing hypocotyl of the soybean seedling (Hong et al. 1989). Subsequent
analysis of the PRP gene family has indicated highly
conserved regions, perhaps related to the yet unknown
function of these molecules (Hong et al. 1990). PRPs are
thought to be non- or only slightly glycosylated (Hong et
al. 1990) and thus distinct from the extensins, although
the emerging knowledge of the structural variation of the
protein components of the extensins indicates similarities
(Li et al. 1990). A study of the specific cells expressing
these genes and their products will be of great interest.
The search for genes specifically expressed during legume
nodule formation has led to the isolation of the ENOD12
gene, encoding a proline-rich protein, similar to the
soybean PRPs discussed above (Scheres et al. 1990) and in
fact it has been observed that this gene is not nodulespecific, but is also expressed in stem tissue and that the
expression is restricted to a zone of cortical cells surrounding the vascular tissues (Scheres et al. 1990).
The genes encoding GRPs also display highly localized
expression, occuring only in the protoxylem cells of the
vascular system of the bean hypocotyl (Keller et al. 1989).
There is an indication that this glycoprotein is closely
associated with lignin deposition and may be insolubilized, by means of tyrosine linkages, later in development.
Arabinogalactan proteins (AGPs), as major components
of plant exudates and secretions, have been studied extensively in terms of their chemistry (Clarke et al. 1979;
Fincher et al. 1983), but are also known to occur in all
plant tissues and in organ-specific forms (Van Hoist and
Clarke, 1986). The recent generation of monoclonal antibodies to the plasma membrane of plant cells has led to
observations that glycoproteins associated with the
plasma membrane contain carbohydrate components that
also occur on soluble AGP proteoglycans (i.e. contain
common epitopes; Pennell et al. 1989; Knox et al. 1989).
The expression of these epitopes shows strict developmental regulation. In the root meristems of the Umbelliferae
the expression of the J1M4 epitope is a very early developmental event and reflects the position of certain developing lineages in relation to the overall emerging tissue
pattern of the root, rather than a specific cell type (see
Fig. 1A; and Knox et al. 1989). The MAC 207 epitope has
been shown to occur at the surface of all cells in pea other
than cell lineages in the developing flower leading to male
and female gametes and is only re-expressed at the surface
of cells during later stages of embryo development (Pennell and Roberts, 1990). Monoclonal antibodies to carbohydrate antigens, that may indeed be cell surface arabinogalactan proteins, indicate spatial distinctions in the
tobacco flower, where epitopes are restricted to small
discrete groups of cells in diverse floral tissues (Evans et
al. 1988). The current implication of these observations is
that carbohydrate elaboration or modification on a common glycoprotein core results in the restricted expressions
Fig. 1. The expression of a cell surface arabinogalactan protein epitope (J4e), recognized by monoclonal antibody JIM4, is
developmentally restricted. A. Expression of J4e by epidermal (e) and certain pericycle (p) and stele cells at the parsley root apex,
visualized by JIM4-immunofluore8cence. The transverse section is approximately 100 /on from the root initials. Bar, 100 jan. B. A
diagram of the cells comprising the root section seen in A in which the shape of cells characteristic of the developing tissues can be
seen and related to the JIM4 reactive cells. The arrow indicates the extent of the stele and the alignment of the band of developing
xylem. Heavy lines indicate emerging boundaries between epidermis (e) and cortex (c) and between cortex and stele. C. JIM4
binding to cells only at the periphery of a large proembryogenic mass of carrot callus cells as seen by immunofluorescence labelling
of a cross-section. Bar, 100 /an. D. The proembryogenic mass can be seen to be a solid ball of cells in a phase-contrast image of the
same section as shown in C.
of subclasses of cell surface glycoproteins of the AGP class
and that patterns of expression reflect aspects of plant cell
and lineage identity. There is very little information
available on the protein core of AGPs (Showalter and
Varner, 1989), although a recent report indicates Ala-Hyp
repeats in a ryegrass AGP (Gleeson et al. 1989). In contrast
to the extensins, current evidence indicates great variation in the carbohydrate components of AGPs, with a
diversity of saccharide linkage on the galactan backbone.
It appears to be modifications or differences in this arabinogalactan component that the monoclonal antibodies
recognize.
The observation that these cell surface AGPs are modified in relation to cell position in the relatively unorganized clumps of cultured cells (for example, JTM4 binds only
to certain cells at the surface of a clump of carrot callus, as
shown in Fig. 1C) further suggests a fundamental role in
plant cell organization for this class of glycoproteins
(Stacey et al. 1990) and suggests specific expression of this
set of glycoproteins in response to an unknown factor or
factors. These may be the variant physical tension within
tissues or gradients of metabolites, such as hormones, that
may also be involved in the formation of tissue patterns.
The precise function of these glycoproteins is unknown,
Plant cell surface
559
but the observed patterns of expression, their location at
the plasma membrane and the known ability of AGPs to
react with Yariv antigens (Fincher et al. 1983) may
indicate a role involving molecular recognition and cellcell interaction in relation to cell identity or position.
An extracellular glycoprotein, rich in aspartic acid,
serine, threonine and possessing N-linked oligosaccharide
chains, has been found in the conditioned medium of
auxin-supplied carrot cells and has been immunolocalized
to both the epidermis and endodermis of the carrot root
and to the epidermis of the carrot petiole (Satoh and Fujii,
1988). Significantly, it could not be located in the root apex
before differentiation of the vascular tissues.
A highly localized expression of a gene encoding a cell
wall HRGP in tobacco has been noted in mature pericycle
and endodermal cells in regions that will give rise to
lateral root meristems (Keller and Lamb, 1989). This
expression is transient; not being expressed when the root
has fully burst beyond the cortex and epidermis of the
main root axis or in the main root meristem itself (Keller
and Lamb, 1989). This wall glycoprotein may also be
involved in a structural strengthening of the cell wall,
required as the lateral root penetrates the outer tissues of
the main root.
presence of pectin, and its esterification, are regulated in
certain root apices in a manner that reflects the major
tissue boundaries, suggests that this important polysaccharide of the plant cell wall may also play some role in the
more complex processes that comprise plant cell development.
I thank Clive Lloyd and Keith Roberts for critical comments
that have influenced the final form of this essay.
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