Annals of Botany 78 : 3–13, 1996
Structure and Function of the Root Cap of Lolium temulentum L. (Poaceae) :
Parallels with the Ligule
N. J. C H A F F E Y*
Botany Department, UniŠersity of Durham, Science Site, South Road, Durham DH1 3LE, UK
Received : 21 August 1995
Accepted : 10 December 1995
Structure of the root cap of Lolium temulentum L. and of the chemistry of its secretory product were investigated with
conventional transmission electron microscopy, zinc iodide–osmium tetroxide impregnation, and use of protein and
polysaccharide localization techniques at the ultrastructural level. Structural and functional information concerning
the root cap is compared with that previously obtained for the membranous ligule of this species. Similarities between
root cap cells and adaxial epidermal cells of the ligule are described and discussed. It is concluded that both systems
are secretory and represent a similar response to the common problem of easing the passage of growing organs against
the soil or other plant parts.
# 1996 Annals of Botany Company
Key words : Darnel, Gramineae, grasses, ligule, Lolium temulentum L., Poaceae, root cap, secretion, ultracytochemistry, ultrastructure, zinc iodide-osmium tetroxide impregnation.
INTRODUCTION
There are a number of secretory structures associated with
the vegetative organs of the grass plant : e.g. bicellular hairs
of the Chloridoideae, Arundinoideae, Panicoideae and
Bambusoideae (Amarasinghe and Watson, 1988 ; Amarasinghe, 1990), multicellular glandular hairs on Panicum spp.
(Kabuye and Wood, 1968), salt glands in Spartina (Skelding
and Winterbotham, 1939 ; Levering and Thomson, 1971),
Cynodon (Oross and Thomson, 1984 ; Oross, Leonard and
Thomson, 1985) and Leptochloa (Wieneke, Sarwar and
Roeb, 1987), and extrafloral nectaries on Andropogon
gayanus (Bowden, 1971).
However, by far the most-studied of these secretory
structures is the root cap (for reviews see Barlow, 1975 ;
Rougier, 1981 ; Battey and Blackbourn, 1993) which
occupies the most apical region of the root. It is the site of
perception of gravity by the root (Juniper et al., 1966) and
is a region of intense cell division and synthetic activity—the
root cap of maize is replaced daily (Clowes, 1971). During
this rapid growth, cells are produced in a proximal
calyptrogen and displaced towards the periphery before
eventually being sloughed off and lost into the soil.
Pronounced changes in both ultrastructure and function
take place as the cells ‘ migrate ’. Cells of the central region,
the statocytes (Barlow, 1982), are the sites of geoperception,
and are characterized by the presence of numerous
amyloplasts. As the cells near the periphery they assume a
secretory ro# le as their dictyosomes become hypersecretory
(e.g. Jones, Morre! and Mollenhauer, 1966).
Another grass organ for which a secretory capacity has
recently been proposed is the membranous ligule of leaves
of darnel (Lolium temulentum L.) (Chaffey, 1985 d ) and of
several other agriculturally-important grass species, including wheat and meadow fescue (Chaffey, 1994). The
ligules of these species are devoid of vascular tissue and
comprise a tripartite structure of two uniseriate epidermides
enclosing a chlorenchymatous mesophyll of variable thickness. The adaxial epidermis (close-appressed against the
enclosed leaf or culm) has the ultrastructural characteristics
associated with a secretory tissue : hypertrophied dictyosomes, abundant strands of rough endoplasmic reticulum
(RER), numerous highly-cristate mitochondria, and accumulations of extracellular product within the periplasmic
space and on the surface of the cuticle (Chaffey, 1985 a, c, d,
1994, 1995).
The root cap has long been recognized as a secretory
organ producing a mucilage. An apparent secretory capacity
of the ligule of Lolium temulentum L., on the other hand, is
a comparatively recent suggestion and not widely-known.
In an attempt, therefore, to provide further evidence for the
ligule’s secretory activity and clues to the function of the
secretory product, the root cap of darnel was examined and
comparisons made with the data already obtained for its
ligule. The study has identified a number of features
common to both root cap and ligule and strengthens the
view that the latter is a secretory organ, and lends support
to the suggestion that both act in a lubricant capacity.
Aspects of root cap structure, ultrastructure and cytochemistry were examined using a variety of techniques
previously used on the ligule of this species.
MATERIALS AND METHODS
Plant material
* For correspondence at : IACR—Long Ashton Research Station,
Department of Agricultural Sciences, University of Bristol, Long
Ashton, Bristol, BS18 9AF, UK.
0305–7364}96}070003­11 $18.00}0
Plants of Lolium temulentum L. strain Ba 3081 (Poaceae)
were grown as described previously (Chaffey, 1985 a).
# 1996 Annals of Botany Company
4
Chaffey—Root Cap and Ligule of Darnel
Transmission electron microscopy (TEM)
ConŠentional. Apical portions of root tissue were fixed for
2±5 h in 2±5 % glutaraldehyde and 1±5 % paraformaldehyde
in 50 m sodium cacodylate buffer, pH 7±0. Following a
cacodylate buffer wash for 2¬15 min, material was postfixed in 1 % aqueous OsO for 2 h, dehydrated in a graded
%
water}ethanol series and embedded in Spurr resin. Ultrathin
transverse sections were collected on uncoated copper grids
and sequentially stained with saturated aqueous uranyl
acetate and alkaline lead citrate before viewing in a Philips
EM400 electron microscope at 80 kV.
Zinc iodide-osmium tetroxide (ZIO) impregnation.
Material was fixed and processed for conventional TEM
except that the osmication step was replaced with a ZIOimpregnation stage. The ZnI solution was freshly prepared
#
according to the recipe of Harris (1978), and material
processed as described previously (Chaffey, 1995). Transverse sections were cut 0±5–1 µm thick using glass knives
and viewed unstained in the TEM operating at 100 kV.
greatest contrast with a densely-staining region, generally
against the inner tangential wall, and a much lighter region
which occupied the greater part of the cell. In addition, the
stretched walls of destroyed cells were frequently seen at the
periphery of the root cap (Fig. 1A). Starch granules within
amyloplasts were readily identified using Sudan Black B ;
their numbers decreasing from cells of Type 1 to 2 to 3 (not
shown).
Fluorescence microscopy
Incubation of root tips in Calcofluor revealed a longitudinal staining pattern in which three distinct zones could
be recognized (Fig. 3A, B) : Zone (i) : the extreme tip of the
root ; exhibited fluorescence chiefly associated with cell walls
and with cell contents ; Zone (ii) : adjacent to Zone (i),
showed fluorescence of cell contents and cross-walls (arrow
heads in Fig. 3B) ; Zone (iii) (not shown) : fluorescence
apparently confined to cell walls.
Transmission electron microscopy
Polysaccharide cytochemistry
Phosphotungstic acid (PTA). Material was processed using
the method of Syrop and Beckett (1972), as described
previously (Chaffey, 1985 d).
Periodic
acid-thiocarbohydrazide-silŠer
proteinate
(PATAg). Material was processed using the method of
Thie! ry (1967), as described previously (Chaffey, 1985 d).
Ruthenium red (RR). Material was stained using the
method of Dexheimer (1976). Tissue was processed as for
conventional TEM but with the addition of 0±1 % (w}v) RR
to both fixation stages and the intervening buffer-wash.
Ultrathin sections were collected on uncoated copper grids
and viewed in the TEM without further staining.
Fluorescence microscopy
Fresh material was stained with 0±1 % (v}v) Calcofluor
White M2R New and examined using epifluorescence
illumination, as described previously (Chaffey, 1985 a, 1994).
Light microscopy
Semi-thin sections of material processed for conventional
TEM were stained with toluidine blue [1 % (w}v) in 1 %
(w}v) sodium tetraborate] and examined with transmitted
light. Other sections were treated with Sudan Black B
[saturated solution in 70 % (v}v) ethanol], for demonstration
of starch-bearing plastids (Bronner, 1975).
RESULTS
Light microscopy
Three cell types could be distinguished, after staining with
toluidine blue (Fig. 1A) : Type 1 cells : the whole of the cell
contents stained uniformly and lightly ; Type 2 cells : there
was a distinction between a central, densely-staining region
and a much lighter peripheral region ; Type 3 cells : exhibited
General ultrastructure. Type 1 cells. These cells were
characterized by the presence of numerous amyloplasts,
dictyosomes and highly-cristate mitochondria (Figs 1A and
2A), strands of rough endoplasmic reticulum (RER) and
numerous free ribosomes and polysomes. The dictyosomes
were not ‘ hypersecretory ’ (Fig. 2A) although synthetic
activity (presence of dictyosome-derived vesicles) and
secretory activity [presence of periplasmic accumulations
(dart in Fig. 1B)] was evident. Frequently the dictyosomes
were seen sectioned near the edge of the stack (d« in Fig.
2A). The vacuome apparently consisted of a few small
vacuoles scattered throughout the cell. These cells appeared
to correspond to the statocytes referred to in the Introduction.
Type 2 cells. The organelle complement of these cells was
qualitatively similar to that of Type 1 cells, but amyloplasts
were fewer and larger numbers of hypertrophied
dictyosome-derived vesicles were present within the cytoplasm (Figs 1C and 2B, C). These cells appeared to be highly
active in mucilage synthesis and secretion ; material could be
seen within the periplasmic space right around the cell (darts
in Fig. 1C). The contents of the dictyosome-derived vesicles
(Fig. 2B, C) and the periplasmic space (Figs 2C and 3D) were
similar and appeared to have an amorphous-fibrillar nature.
The evident reduction in cytoplasm of these cells with
concomitant increase in the volume occupied by the
periplasmic space presumably explains the toluidine bluestaining image referred to above—the lightly-staining
peripheral region corresponding to the contents of the
periplasmic space.
Type 3 cells. Organelle complement was similar to that of
the other two cells types, but the numbers of all organelles
appeared to be lower. Secretory activity—as judged by the
size of the hypertrophied dictyosome-derived vesicle
population—appeared to have greatly diminished relative
to Type 2 cells ; most of the cell volume was occupied by
periplasmic space (Fig. 1D). In contrast to the situation in
Type 2 cells, here the periplasmic space was largely absent
Chaffey—Root Cap and Ligule of Darnel
5
F. 1. Structure and ultrastructure of the root cap of Lolium temulentum as seen in transverse section. A, Light micrograph showing the
distribution of the three cell types (1, 2, 3) within the root cap ; darts indicate stretched cell walls. B, C, Ultrastructure of part of a Type 1 cell
(B) and a Type 2 cells (C) ; dart indicates periplasmic space. D, Ultrastructure of part of a Type 3 cell ; amy, amyloplast ; cyto, cytoplasm ; d,
dictyosome ; dv, dictyosome-derived vesicle ; ext, exterior ; mt, mitochondrion ; n, nucleus ; peri, periplasmic space ; rer, rough endoplasmic
reticulum ; vac, vacuole ; w, cell wall.
from the inner tangential wall. By reference to the toluidine
blue-staining pattern of these cells, the densely-staining
region corresponds to the cytoplasm, and the lightly-
staining region to the periplasmic space. Accumulations of
fibrillar material—assumed to be mucilage—were often seen
in the intercellular spaces between these cells (Fig. 2D).
6
Chaffey—Root Cap and Ligule of Darnel
F. 2. Ultrastructure of the root cap of Lolium temulentum as seen in transverse sections. A, Part of the cytoplasm of a Type 1 cell. B, Part of
the cytoplasm of a Type 2 cell. C, Part of the cytoplasm and cell wall of a Type 2 cell. D, Parts of two adjacent Type 3 cells ; cyto, cytoplasm ;
d,d«, dictyosome ; dv, dictyosome-derived vesicle ; ext, exterior ; fib, fibrils ; mt, mitochondrion ; peri, periplasmic space ; po, polysomes ; rer, rough
endoplasmic reticulum ; w, cell wall.
Chaffey—Root Cap and Ligule of Darnel
7
F. 3. Aspects of the polysaccharide cytochemistry and biology of the root cap of Lolium temulentum. All are transverse sections except for (A)
and (B). A, Fluorescence micrograph of the surface of the root tip (Calcofluor-staining). B, Higher power view of (A) ; arrow-heads indicate crosswalls. C, Ultrastructure of part of the cytoplasm and cell wall of a Type 2 cell (RR-staining). D, Ultrastructure of the periplasmic space of a Type
2 cell (conventional TEM). E, Ultrastructure of fibrillar material external to the root cap (RR-staining). F, Region similar to (E) above (PTAstaining) ; cyto, cytoplasm ; fib, fibrils ; i, root cap ; ii, epidermis ; peri, periplasmic space ; w, cell wall.
8
Chaffey—Root Cap and Ligule of Darnel
F. 4. Polysaccharide cytochemistry and ZIO-staining of the root cap of Lolium temulentum as seen in transverse section. A, Parts of several Type
2 cells (PATAg-staining) ; darts indicate periplasmic space. B, Part of the cytoplasm of a Type 2 cell (PATAg-staining). C, Parts of two adjacent
Type 3 cells (PATAg-staining). D, Part of the cytoplasm of a Type 2 cell (ZIO-staining) ; darts indicate nuclear pores ; the star indicates a possible
intermediate form of tubular endoplasmic reticulum ; arrows indicate internal membranes of amyloplasts. E, Part of the cytoplasm of a Type 2
cell (ZIO-staining). F, Putative paramural body in the periplasmic space of a Type 2 cell (ZIO-staining) ; amy, amyloplast ; d, dictyosome ; dv,
dictyosome-derived vesicle ; ext, exterior ; fib, fibrils ; mt, mitochondrion ; n, nucleus ; peri, periplasmic space ; pmb, paramural body ; rer, rough
endoplasmic reticulum ; st, starch grain ; ter, tubular endoplasmic reticulum ; vac, vacuole ; w, cell wall.
Chaffey—Root Cap and Ligule of Darnel
Plasmodesmatal connections between all cells, regardless of
Type, were few.
Polysaccharide cytochemistry
Mucilage external to the root cap gave the same staining
pattern with all three procedures—RR (Fig. 3E), PTA (Fig.
3F), PATAg (Fig. 4C)—and was similar to that observed
with conventional TEM of double-stained sections (Fig.
2D). The mucilage appeared to be composed of fibrillar
material radiating from amorphous loci in a non-staining
matrix. RR-staining of material in the periplasmic space
revealed the presence of dense, amorphous deposits (Fig.
3C). PATAg-staining of material in dictyosome-derived
vesicles and the periplasmic space (Fig. 4A) also exhibited a
more amorphous nature compared to that observed in the
external mucilage. Of the three polysaccharide stains used,
only PATAg stained the dictyosomes. Figure 4B shows the
staining pattern observed at the edge of the cisternal stack ;
the nascent vesicles were still attached to the thread-like
stained cisternae. RER appeared to stain lightly with PTA
(not shown), but not with PATAg (Fig. 4B). Amyloplast
starch grains stained uniformly only with PATAg (Fig. 4A) ;
PTA treatment did, however, result in a densely-staining
periphery to the starch grains (not shown).
Zinc iodide-osmium tetroxide impregnation
As expected, the stain largely accumulated within elements
of the endomembrane system, mitochondria (Fig. 4E), and,
to some extent, internal membranes of amyloplasts (Fig.
4D) (cf. also Barlow, Hawes and Horne, 1984). Despite the
abundance of dictyosomes and tubular ER, no direct
connections were observed between them, although close
associations could be found. Close associations were also
observed between the nuclear envelope and cisternal ER.
The distribution and shape of the nuclear pores was clearly
visible in suitably-oriented sections (darts in Fig. 4D). The
starred object in Fig. 4D is interpreted as a form of tubular
ER and may represent a transitional form between nuclear
envelope and more conventional tubular ER. The frequent
aggregations of material of vesicular nature seen in ZIOtreated tissue, adjacent to the cell wall (Fig. 4F), are
interpreted as paramural bodies.
DISCUSSION
The observations presented above were largely in accord
with those recorded in the literature for root caps of other
grasses. Differences and similarities between the root cap
and the adaxial epidermal cells of the membranous ligule of
Lolium temulentum will be considered below.
DeŠelopment
The three cell types of the root cap appeared to represent
stages in the course of development of a single cell from the
central region to the periphery. The different organelle
assemblages observed thus represent stages in the differentiation of structure and function within the cell, as it
9
changes from statocytic to secretory. We know the structure
of the newly-initiated ligule (Chaffey, 1985 b) and of the
emerged, mature ligule (Chaffey, 1985 a) ; we do not yet
know the various stages through which cells of the adaxial
epidermis pass to reach the secretory state. Mature adaxial
epidermal cells correspond to Type 2 cells of the root cap.
Polysaccharide cytochemistry
All four techniques used (conventional TEM, PATAg-,
PTA-, and RR-staining) gave the same staining image for
the externally-located root cap mucilage ; it appeared to be
composed of fibrils radiating from amorphous loci within a
non-staining matrix. In the ligule, the extracuticular
material appeared as amorphous aggregates with all
techniques, except RR [which was, however, considered to
be an unreliable technique in this material (Chaffey, 1983)].
PATAg- and PTA-staining of the root cap periplasmic
mucilage and external mucilage gave the same image. RR,
however, stained the periplasmic material as dense amorphous aggregates, which may indicate a high pectic content
(e.g. also Wright and Northcote, 1974 for maize). In the
ligule, paramural bodies and fibrils within the periplasmic
space stained with PATAg and PTA (Chaffey, 1985 d ) ; only
the former appeared to stain with RR (Chaffey, 1983).
Spatial relationships within highly-hydrated materials
become altered during dehydration stages prior to embedment ; ligule and root cap materials may have been
differently-hydrated to start with. Further, the observed
differences may be related to differential loss of low
molecular weight polysaccharides during TEM-processing,
or alteration of the secretory product during its passage
through the wall. The differences in staining patterns of the
secretory products between the two organs may, therefore,
be more apparent than real. The best way of resolving this
problem would be to extract the ligule and root cap
products from the two sides of the wall and chemically
analyse them.
Apparent staining of the periplasmic material in the ligule
with Calcofluor White M2R New (Chaffey, 1985 a) and
Uvitex BOPT (Chaffey, 1983) suggested that it had, at least
in part, a β-linked hexapyranose composition (Chaffey,
1985 a). Comparison of the observed fluorescence image of
Calcofluor-stained roots with light micrographs of longitudinal sections of other roots (e.g. Clowes, 1959) suggests
that Zone (i) is the root cap and that Zones (ii) and (iii) are
in the epidermis. Zone (i), therefore, represents cells on the
periphery of the cap which are in the process of being
sloughed off ; loss of most of their mucilage has resulted in
staining being confined largely to cell walls although the
‘ halo ’ of fluorescence associated with the tip is inferred to
represent mucilage-fluorescence. Epidermal cells in Zone (ii)
were dividing and showed staining of newly-made cross
walls and cytoplasmic (periplasmic ?) material. It is perhaps
a moot point whether the cytoplasmic fluorescence results
from newly-made cell wall precursors, e.g. β-linked cellulose,
or a mucilaginous material [which such cells can make (e.g.
Miki, Clarke and McCully, 1980 ; Rougier, 1981)]. Epidermal cells in Zone (iii) were apparently no longer dividing
or producing mucilaginous material, consequently only the
10
Chaffey—Root Cap and Ligule of Darnel
walls were stained. Thus it appears that the root cap
mucilage also stained with Calcofluor indicating a β-linked
hexapyranose content. This is in agreement with Wright and
Northcote (1976) who demonstrated the presence of a
central β-1,4-glucan in maize root cap slime, which stained
with Calcofluor.
On the basis of polysaccharide ultracytochemistry and
Calcofluor staining, ligule and root cap secretory products
are similar. A glycoprotein nature was suggested for the
former (Chaffey, 1983, 1985 d ). PTA is ‘ not a stain for
polysaccharide ’ (Glick and Scott, 1970), but is used as a
protein stain at the ultrastructural level (e.g. Lewis and
Knight, 1977) ; PATAg is a well-established TEM procedure
for localization of polysaccharides (e.g. Lewis and Knight,
1977). Staining of the root cap material with both PTA and
PATAg is, therefore, evidence for it too having a glycoprotein nature [as demonstrated for maize root cap slime
by Green and Northcote (1978)]. Root cap mucilage also
appears to have a high pectic content by reference to its
staining with RR ; the RR-staining results for the ligule are
considered ambiguous (Chaffey, 1983).
Energy and substrate sources
Glycosylation and phosphorylation reactions are needed
in the synthesis and elaboration of polysaccharides (e.g.
Northcote, 1979). These reactions require a source of
substrates, both to generate the requisite ATP and act as
precursors for polysaccharide synthesis. In the case of a
glycoprotein, more energy-requiring reactions are involved
and the demand for substrates is correspondingly greater.
Both the root cap cells and the ligule have large numbers of
highly-cristate mitochondria (this study and Chaffey,
1985 c). In the ligule, enzyme cytochemistry has demonstrated the presence of mitochondrial succinate dehydrogenase and cytochrome c oxidase (Chaffey, 1985 c), and
hence the probability that the mitochondria are active. In
the root cap such direct evidence is lacking ; the mitochondria
do, however, look like those of the ligule and are, therefore,
assumed to be active. Thus potential ATP sources exist in
both secretory systems.
The decrease in root cap amyloplast starch and concomitant increase in hypertrophy of dictyosome-derived
vesicles has been interpreted as evidence that the starch may
act as a source of precursors for polysaccharide synthesis
(e.g. Juniper and Pask, 1973). The apparent rarity of
plasmodesmata between cells of the darnel root cap reported
here favours such an intracellular source of polysaccharide
precursors and respirable substrates. For the ligule, substrates might be made within the mesophyll chloroplasts
and transferred to the adaxial epidermal cells via the
numerous plasmodesmata between these two tissues
(Chaffey, 1985 a). The abundance of RER in the ligule and
of free polysomes in the root cap suggests that any protein
component of the secretory products may be made within
the secretory cells themselves.
It would thus appear that root cap cells use intracellular
sources of substrates for ATP, polysaccharide and protein
synthesis ; both intra- and extra-cellular sources may be
utilized in the ligule.
Pathway of synthesis, elaboration and secretion of the
product
Work of others, largely upon maize root cap, has
demonstrated the probable involvement of both RER and
dictyosomes in the synthesis and elaboration of the mucilage
(e.g. Bowles and Northcote, 1972 ; Green and Northcote,
1978). Direct connections between these two endomembrane
components have been observed in maize root cells by
Mollenhauer, Morre! and vanderWoude (1975) and
Mollenhauer and Morre! (1976), and in wheat by Marty
(1980).
Using ZIO in this study, direct connections between ER
and dictyosomes were not seen ; close associations, however,
were common. PTA-staining of RER and PATAg-staining
of dictyosomes and dictyosome-derived vesicles indicates
involvement of these endomembrane components in synthesis of the mucilage. The pathway of secretion from the
dictyosomes to the plasma membrane (PL) is assumed to be
via the smooth, dictyosome-derived vesicles which discharge
their contents into the periplasmic space (e.g. Dumas et al.,
1974). Paramural bodies were not apparent in ultrathin
sections but were observed in ZIO-stained material. Sections
of the latter are three to four times thicker than ultrathin
sections and hence presence of paramural bodies only in
ZIO-stained material may be related to the small size of the
vesicles and their rarity. In view of the large size of
the hypertrophied dictyosome-derived vesicles, which are
assumed to fuse with the PL, paramural bodies may not
form from these vesicles. The paramural bodies observed in
ZIO-stained material may, therefore, be derived from
smaller vesicles not involved in mucilage secretion. This
latter suggestion has relevance to the work of Roy and Vian
(1991) with maize root caps, where at least two populations
of Golgi vesicles were identified.
As previously observed by Juniper, Gilchrist and Robins
(1977), secretion appeared to take place all around the cell
but the periplasmic material eventually accumulated next to
what was—or was destined to be—the outer tangential wall
(cf. cell Type 2 and 3 in Fig. 1A). The outer tangential walls
were neither cutinized nor cuticularized—as judged by
Sudan Black B ‘ staining ’—and are therefore assumed not
to present a barrier to secretion of the mucilage (e.g. also
Paull and Jones, 1976). According to Paull and Jones
(1976), the mucilage may be released from the cells through
the wall which has been ‘ loosened ’ by enzyme action.
A pathway was proposed for the ligule which involved
direct RER}dictyosome connections, transport of smooth
dictyosome-derived vesicles to the PL and the formation of
paramural bodies (Chaffey, 1995). Paramural bodies, and
hence evidence of secretion, were commonest against the
outer tangential walls of ligule adaxial epidermal cells, as
was the periplasmic material (as in the root cap). Passage
through the wall was assumed to take place culminating in
release of the secretory product from the ligule via cuticular
gaps (Chaffey, 1985 d ). Postulated formation of these latter
discontinuities by turgor and accumulation of product
between the wall and cuticle is akin to the mode of secretion
of root cap mucilage described by Morre! , Jones and
Mollenhauer (1967).
11
Chaffey—Root Cap and Ligule of Darnel
Thus the pathways of synthesis and secretion of the
products proposed for the two systems are similar. The
main differences appear to be the much reduced frequency
of paramural bodies and absence of cuticle in the root cap.
Function
The root cap slime or mucilage has generally been
considered to provide lubrication and protection to the root
as it grows through the soil (e.g. Fritsch and Salisbury,
1938 ; Juniper and Pask, 1973), although recent work by
Guinel and McCully (1986) stresses that this view may have
limited applicability. A variation on this theme was
suggested by Esau (1953) who proposed that the mucilage
might aid separation between the root cap and sloughing
cells. However, in view of the prominence of root caps in
hydrophytes, she further suggested that here the cap might
perform other functions. The aerial roots of Cattleya were
studied by Mollenhauer (1967) who suggested that the root
cap slime might act as a water-absorbing material since a
lubricant ro# le was considered unlikely here. Additional
involvement in influencing the availability of soil-borne
chemical elements has also been suggested for root exudates,
such as mucilage (e.g. Jauregui and Reisenauer, 1982 ;
Mench, Morel and Guckert, 1987), and in interactions
between micro-organisms and the root surface (reviewed by
Rougier and Chaboud, 1985). The mucilage is also believed
to have a ro# le in gravitropism (e.g. Miller and Moore, 1990).
By analogy with the principal function proposed for the
root cap, it has been suggested that the ligule secretory
produce might act as a ‘ lubricant ’ which eases the exsertion
of the enclosed leaf or culm (Chaffey, 1983, 1994).
CONCLUSION
The root cap is probably the most thoroughly studied
secretory system of the grass plant. Comparison of the
anatomy and chemistry of the secretory product of this
organ with the ligule has provided additional evidence for
considering the latter to be a secretory organ (see Table 1).
The root is an organ essentially of indeterminate growth,
by contrast with the well-defined cycle of birth and death of
the leaf (e.g. Chaffey, 1983). In the former, therefore, there
is a requirement for continual replenishment of the secretory
cells of the root cap which is constantly rubbed against the
rooting medium as the root continues to grow and ramify
throughout the soil (see Introduction), and is evidenced by
the transformation of cells from Type 1 to Type 3. The
ligule, on the other hand, is an integral component of the
determinate grass leaf, which is apparently only active for
the limited lifetime of the leaf which bears it (unpublished
observations). It is essentially a static structure against
which the enclosed leaf or culm move during their exsertion,
and which does not, therefore, require the continual renewal
which is such a characteristic feature of the root cap.
In considering ‘ lubricant ro# les ’ for the root cap and
ligule, we may be witnessing an example of the ‘ principle of
parsimony ’ (Ockam’s razor) as both the root and leaf
employ the same solution to the common problem of
permitting movement of a plant part relative to another
T     1. Comparison of selected features of the anatomy
and secretory product chemistry between root cap and adaxial
epidermal cells of Lolium temulentum L.
Anatomy
Plasmodesmata
Mitochondria
ER
Dictyosomes
Paramural bodies
Peri. accumulations
Amyloplasts
Cuticle
Chemistry
PTA-staining
PATAg-staining
RR-staining
Calc.-staining
Composition
Root cap
Ligule
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β-1,4-linked
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­­
­}®
­­
β-1,4-linked
glycoprotein(?)
­, present (relative amount indicated by ­­ or ­­­) ; ®,
absent ; ­}® inconclusive ; calc., calcofluor-fluorescent probe for βlinked polysaccharides ; ER, endoplasmic reticulum ; PATAg, periodic
acid-thiocarbohydrazide-silver proteinate stain for α- or β-linked
polysaccharides ; peri., periplasmic space ; PTA, phosphotungstic acid
stain for proteins ; RR, ruthenium red stain for ‘ pectins ’.
structure (soil or enclosed leaf ). Indeed, it is not considered
too far-fetched to suggest that the ligule’s adaxial epidermis
may be an aerial equivalent of the subterranean root cap.
Any lingering doubts as to the secretory nature of the
ligule will be removed by using techniques successfully
applied to the study of the root cap and other secretory
systems, such as product extraction and analysis (Wright
and Northcote, 1976), autoradiography (Dauwalder and
Whaley, 1982), use of chemical perturbation e.g. brefeldin
A (Satiat-Jeunemaitre and Hawes, 1992, 1993), and immunolocalization of product (e.g. Staehelin et al., 1991 ; Horsley
et al., 1993).
A C K N O W L E D G E M E N TS
It is a pleasure to thank Dr J. A. Pearson for discussions of
all aspects of ligule biology, Dr P. W. Barlow for commenting upon an earlier draft and the former Agricultural
and Food Research Council (UK) for a Research Assistantship during the tenure of which the work was performed.
I am also grateful to Drs B. E. Juniper and M. B. Jackson
and an anonymous referee for their suggestions and
comments.
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