Development 120, 2113-2120 (1994)
Printed in Great Britain © The Company of Biologists Limited 1994
2113
Early event in maize leaf epidermis formation as revealed by cell lineage
studies
Sergio Cerioli1, Adriano Marocco1,†, Massimo Maddaloni2, Mario Motto2 and Francesco Salamini2,*
1Istituto
2Istituto
di Botanica e Genetica Vegetale, Università Cattolica del S. Cuore, I-29100 Piacenza, Italy
Sperimentale per la Cerealicoltura, I-24100 Bergamo, Italy
*Author for correspondence at his present address: Max-Planck-Institut für Züchtungsforschung, Carl-von-Linné-Weg 10, D-50829 Köln, Germany
†Present Address: Istituto di Agronomia, Università di Padova, I-35100 Padova, Italy
SUMMARY
The epidermal cells of the juvenile leaves of maize are
covered by a wax layer. glossy mutants are known which
reduce drastically wax deposition. We have used the somatically unstable glossy-1 mutable 8 allele to study the distribution on the epidermis of spontaneous revertant sectors
of wild-type tissues. Sectors tend to start and end at
positions that correlate with the location on the epidermis
of the long costal cells of ribs. It is concluded that in the
protoderm only a few cells have a role and position in the
generation of each of the developmental modules located
between leaf midrib and margin. The module consists of an
epidermal strip of cells bordered by two lateral ribs. The
module originates from at least 4 cells, with one cell being
the progenitor of the other three. Data are provided
describing the mode of longitudinal anticlinal epidermal
cell divisions within the module that are responsible for the
increase in leaf width. The results suggest the existence of
a clonal type of development during early leaf epidermis
formation.
INTRODUCTION
long cells (Freeling and Lane, 1993), which constitute the
structures known as ribs.
The epidermis of the young maize shoot is covered by the
juvenile wax layer (Salamini, 1963). Mutations mapping to at
least 8 different genetic loci modify the shape and drastically
reduce the wax layer (summarized by Bianchi et al., 1985).
Wax extrusion is cell autonomous, as is evident from the variegation pattern of leaves of somatically unstable mutants
(Maddaloni et al., 1990). Mutable alleles of the glossy-1 locus
revert both somatically and germinally to the wild-type
phenotype (Maddaloni et al., 1990; Bossinger et al., 1992).
We have used the mutant glossy-1 mutable 8 (gl1-m8) to
study the width and distribution of revertant sectors. The leaf
epidermis is quite suited to such analyses because it originates
from a single meristematic layer (Sharman, 1942), and
because the start and end of revertant sectors can be defined
with respect to ‘landmark’ signals represented by vein-rib
boundaries. When using transposon-induced sectors in cell
lineage analysis, a requisite is that the excision of the transposon is not developmentally regulated. To avoid this, only
sectors that originate in the apical meristem before leaf
promordia are formed should be studied. The inception of leaf
primordia, in fact, induces differentiation between cells. Steffensen (1968) has shown that sectors with a size from 5 to
34% of the midrib to margin space – fully comparable with
those studied by us – always appear in more than one leaf, and
are assumed to have been generated by cells present on the
shoulder of the meristem before leaf primordia inception.
The epidermis of the grass leaf is a single cell layer organized
as longitudinally oriented parallel rows of cells that are
elongated in the direction base to leaf tip. Starting from the
primordium, the cells of the epidermal layer divide according
to planes that allow the epidermis to grow in the transverse
direction, via longitudinal anticlinal divisions, or in the
longitudinal direction, via transverse anticlinal divisions.
Both anatomical studies (Esau, 1977) and the type of leaf
sectoring reported in grass species (Tilney-Basset, 1986;
Klekowski, 1988) support the view that longitudinal anticlinal divisions are mainly restricted to early stages of leaf
development.
The organization of cells in longitudinal rows is typical also
for the internal layers of the leaf. In maize, for example, the
leaf is divided into longitudinal units by parallel veins located
in the mesophyll layer (Sharman, 1942; Esau, 1943; Russell
and Evert, 1985; reviewed by Langdale et al., 1989 and
Freeling, 1992). Veins have been classified as mid, lateral,
intermediate and small (Sharman, 1942). They derive from the
central layer of the leaf primordium (Langdale et al., 1989).
The adaxial and the abaxial epidermis and the middle
mesophyll layer show coordinate development (Freeling,
1992; Freeling and Lane, 1993): cells with a particular shape
are present on the epidermis in positions corresponding to the
location of veins. In short, when maize leaves are viewed from
above, vein positions are marked on the epidermis by costal
Key words: clonal development, sectors analysis, leaf epidermis,
maize, glossy
2114 S. Cerioli and others
Additional proof that in our system excisions are random in
position is given by Maddaloni et al. (1990), who noted that
single cell sectors were spread randomly over the epidermal
surface, while sectors as large as those studied in this paper
appeared in more than one leaf, as was the case for the sectors
described by Steffensen (1968).
Three developmental problems are addressed. The first
concerns the existence of leaf epidermal compartments. The
concept of compartment (Garcia-Bellido and Merriam, 1971;
Garcia-Bellido et al., 1973; 1976) refers to the observation that
cellular clones do not cross a line that defines the border
between morphologically distinct domains. It is accepted that
epidermal segments of Drosophila are subdivided into compartments, developmental units expressing a specific set of
homeotic genes (Brower, 1985; Lawrence and Morata, 1993).
In our system candidates for compartment boundaries are the
epidermal ribs.
The second question addresses the possibility of recognizing the number of cells that have a founder role during the early
development of the leaf epidermis. This role can be clarified
because of the particular position these cells occupy with
respect to the ribs in the primordia or in adult leaves; as
founders they generate groups of cells that are clonally related.
Thirdly, we have attempted to gain information on the
polarity of longitudinal anticlinal cell divisions needed to add,
in the transverse direction of the leaf, cells to leaf width.
Starting with one cell the problem is to establish which out of
several models of cell division (see Fig. 1), fits best the distribution and width of revertant sectors observed.
MATERIALS AND METHODS
The maize mutant gl1-m8 was isolated in an attempt to tag the Glossy1 locus by transposon mutagenesis (Maddaloni et al., 1990). The stock
waxy mutable 7 (wx-m7), where an active copy of the activator (Ac)
transposon (McClintock, 1951) is inserted into the Wx gene, was the
transposon donor. The allele gl1-m8, however, was not generated by
the insertion of Ac but, nevertheless, it behaved autonomously, i.e.
another self-excising element was present at the locus (Maddaloni et
al., 1990). The gl1-m8 mutant is characterized by its somatic instability: reversions to the wild-type phenotype (longitudinal ‘sectors’)
are present on both the leaf sheath and blade (Bossinger et al., 1992).
Four hundred seedlings of the gl1-m8 strain were grown at 25°C
under natural light conditions supplemented with 16 hours per day of
artificial illumination (1900 µE m−2 second−1). Revertant sectors were
studied in 344 leaves, of which 88, 156 and 100 were, respectively,
on the first, second and third leaves of the young shoot. Sectors were
localized on both the adaxial and abaxial leaf surfaces; when, as usual,
both surfaces of a leaf were sectored in corresponding positions, only
the adaxial sector was studied.
Leaves were dissected from the shoot as soon as the tip of the fourth
leaf appeared. Each leaf was inspected under water to determine
whether revertant sectors extended along the whole blade length
(Bossinger et al., 1992). A 1 cm strip was cut transversely where the
leaf blade width was at its maximum. Samples were mounted on a
stub, coated with gold and studied with a Hitachi S-2300 scanning
electron microscope (SEM). Under these conditions, the epidermal
cells tended to collapse, with the exception of those constituting the
mid, lateral and intermediate ribs. Revertant epidermal sectors
covered by the wax layer were easily recognized and classified. Where
necessary, more precise details of the leaf surface (see Fig. 3) were
obtained from leaf samples infiltrated with 7.4% formaldehyde, 5%
acetic acid and 50% ethanol, fixed overnight at 4°C, dehydrated with
Fig. 1. Different models to describe how a series of longitudinal
anticlinal cell divisions add cells to the width of the leaf, starting
from the founder cell numbered 1. Later, all cells will divide
according to transverse anticlinal plans to follow the elongation of
the leaf lamina. Numbers in large type: cells that will divide.
Numbers in smaller type: cells not dividing. a to e: successive
divisions. During this process, when a cell changes genetically to
produce a different phenotype, a sector is generated. This cell is
colored in black. The position and width of derived sectors are
represented by horizontal lines below the last cell files. In the models
the founder cell is always proximal with respect to a landmark signal
(the midrib).
Polarized. Each cell divides only once. The dividing cell is always
located distally with reference to the landmark signal. Reverted
sectors decrease in width when they start further to the right.
Stem-cell-like (I). Only the founder cell divides. Derived cells can
revert. The sectors generated are narrow and all have a similar width.
Stem-cell-like (II). Only the founder cell divides. The founder cell
can revert after each division. The width distribution of sectors is as
in the polarized type of cell division.
Exponential. All cells can divide and revert. The frequency of sector
width should peak for sectors composed of 1, 2, 4, 8, 16 cells.
dimethoxymethane (DMM) for 24 hours, dried in liquid carbon
dioxide, mounted on a stub and coated with gold.
The leaf epidermis developmental module
The maize leaf blade possesses mid, lateral and intermediate ribs. The
midrib consists of 8-12 rows of cells more elongated in the direction
base to leaf tip than the adjacent leaf blade cells. These are the costal
long cells. Between the midrib and each leaf margin, 5 lateral ribs are
present (Fig. 2A). These lateral ribs consist of two rows of costal long
cells. The space between two consecutive lateral ribs is defined as a
leaf developmental module. The module (Fig. 2B) is divided into parts
α and β by the intermediate rib, represented by a single row of costal
long cells.
In young leaf blades, 12 modules make up the epidermis. The
nomenclature concerning leaf modules is given in Fig. 2A and a
module is detailed in Fig. 2B. Leaf modules with revertant sectors on
the adaxial or abaxial leaf surface were studied according to Fig. 2B:
leaves sectored on the adaxial side, right from the midrib (R), or on
the abaxial side, left from the midrib (L), were positioned with their
tips pointing upwards; leaves sectored in L on the adaxial side or in
R on the abaxial side were positioned with the tip downwards.
Development of the maize leaf epidermis 2115
The leaf developmental module has a mean width of 53 cells. Cell
numbering was assingned starting in the half module α with 1α for
the first cell following the lateral rib; the other 25 cells were given
the numbers 2α to 26α. The 27 cells of β were numbered from 1β
(the first cell after the intermediate rib) to 27β. The two costal long
cells of the lateral rib distal to the midrib were marked 26β and 27β;
the intermediate costal long cell was given the number 26α.
Revertant sectors
Epidermal sectors are clones of cells covered by the juvenile wax
layer. Two types of sectors (A and B) were studied: sectors of type
A extended transversally for G, 1 or more modules; sectors of type B
were smaller than a G module. Because sectors not starting at the
precise borders of the module were commonly observed, and because
between modules differences in width were noted, a sector was
defined as type A when its minimum width was of 22 cells.
The data recorded for the A and B sectors were, beside width, the
cell number at the start and end, and the position of the module(s) on
the leaf surface (right (R) or left (L) with respect to the midrib; Fig.
2A). In assigning cell position of the start, counting took place from
the left in both half-modules; the cell number at the end of sectors
was assigned in a similar way, with the exceptions of sectors ending
at 22α to 26α or at 23β to 27β: for these, cell numbering was from
the right.
RESULTS
The borders of epidermal
developmental modules
The ribs divide the juvenile leaf
blade into repetitive modules. The
module contains the cells betwen
two lateral ribs, the lateral rib distal
with respect to the midrib, and the
intermediate rib cell (Fig. 3A).
Intermediate and lateral ribs consist
of rows of 1 and 2 costal long cells,
respectively (Fig. 3B, C). In lateral
ribs nearer to the midrib, the
number of rows of long costal cells
is, in rare cases, larger than two.
Six developmental modules are
present on both the right and the left
side of the leaf blade (Fig. 2A). The
modules near the leaf borders are
not flanked by lateral ribs. Hairs
mark the cell files at the leaf border;
these hairs are oriented in the
direction of the tip of the leaf blade,
a useful feature in microscopic
analysis. The module width
(number of cells in the transverse
direction) was counted for each of
the 12 units present on the abaxial
and adaxial surfaces of the first
three leaves. Because adaxial and
abaxial module width were not significantly different, only the mean
width of modules is reported (Table
1). Module width varied between
40 and 53 cells. RA and LA (see
Fig. 2) were exceptional because of
their reduced width of about 20 cells. In these two modules, in
a few cases, the intermediate rib was not precisely positioned
in the centre. The presence of intermediate ribs with more than
one long costal cell, or of more than one intermediate rib, was
also noted in RA and LA. Sectors present on such exceptional
modules were not considered. Compared with other modules,
the modules at the border of the leaf had a slightly reduced and
more variable width. The width of module LC, third leaf (53
cells), was taken as standard.
Revertant sectors: type and frequency
A sector type A is a somatic clone derived from a cell that
experienced a reversion event from gl1-m8 to Gl1, and that
occupies at least half a module (see also footnotes of Table 2).
B sectors occupy less than half a module.
In total, 292 A sectors were considered and Fig. 4 shows the
types most commonly found. A sector covering precisely one
module (from 1α to 27β) is shown in Fig. 4A. Sectors precisely
limited to the half-modules α or β were also noted (Fig. 4B).
Fig. 4C shows a sector that started at 1α but ended within β.
An A sector crossing a lateral rib is illustrated in Fig. 4D.
A classification of A sectors is presented in Table 2. Preliminary analyses of the width and distribution of A sectors
revealed only marginal differences among leaves or between
Fig. 2. Schematic representation of the
distribution of ribs on the surface of young
maize leaves (A), and details of a single
developmental module (B). This consists of
the two half-modules α and β, with α
proximal and β distal with respect to the
midrib. The developmental module is made
up of 53 cells, numbered as follows. 1α: the
first cell after a lateral rib in the direction
midrib to border; 2α to 26α: cell following
1α; 1β: the first cell after the intermediate
rib in the direction midrib to border; 2β to
27β: cells following 1β.
2116 S. Cerioli and others
Table 1. Number of epidermal cells in the 12 developmental modules of the first three leaves of the maize seedling
Leaf
number
Width of epidermal developmental modules*
LF
LE
LD
LC
LB
LA
RA
RB
RC
RD
RE
RF
1
36
(11.30)†
41
(4.65)
50
(6.33)
47
(6.56)
44
(8.53)
20
(7.53)
(X)
19
(7.38)
45
(5.25)
47
(5.82)
48
(6.86)
40
(7.65)
35
(10.85)
2
35
(9.35)
44
(6.83)
50
(7.03)
50
(7.01)
47
(7.46)
17
(8.35)
12
(4.66)
46
(8.22)
49
(7.02)
52
(7.79)
44
(6.75)
37
(10.09)
3
33
(9.28)
43
(8.29)
51
(8.06)
53
(6.16)
48
(7.57)
14
(5.28)
17
(8.68)
47
(7.62)
51
(7.96)
51
(7.35)
44
(7.03)
34
(8.87)
These mean values have been calculated from 6 to 23 developmental modules.
*L=left part of the leaf blade; R=right (nomenclature also in Fig. 2).
†Standard erros in brackets.
X, position of the main midrib.
their abaxial and adaxial surfaces (results not shown). Sectors
of type A were progressively larger when starting nearer to the
midrib. Sectors with a width corresponding to G, 1 or 1+G
modules were frequent; also frequent were those with a width
Fig. 3. (A) The developmental module as seen on the adaxial surface
of the second leaf of a maize seedling. In the transverse direction of
the leaf (see Fig. 2) the module covers all cells between two lateral
ribs, including the intermediate rib and the lateral rib on the right.
(B) Close-up of an intermediate rib with 1 costal long cell.
(C) Close-up of a lateral rib consisting of two rows of costal long
cells Magnification (A) ×40, (B,C) ×200.
between G and 1, 1 and 1+G and 1+G and 2 modules. It was noteworthy that out of the 276 A sectors with a width from G to 4
modules, 100 had a width corresponding to G a module or to
multiples thereof (see, however, footnote 1 of Table 2).
One hundred and twenty two type B sectors were scored.
Representative types are reproduced in Fig. 4, covering half of
α (Fig. 4E), or crossing the intermediate rib (Fig. 4F), or
ending precisely at the intermediate or lateral ribs (Fig. 4G,H).
Where sectors start and end
In the last two columns of Table 2, A sectors are classified
based on their origin, either in α or in β. As a rule, type A
sectors showed a strong tendency to start within the α module.
Exceptions were the G class and, in part, the G-1 class. The
precise position of start and end of the 292 A sectors within
the developmental module is summarized in Fig. 5. The figure
consists of a reproduction of the standard module (with the
midrib at the left), and of histograms representing the number
of sectors starting (upper) or ending (lower) at a given cell
number.
The preferred start position of A sectors was cell 1α (90
cases out of 292). Other preferential start cells were 26β, 27β,
2α, 26α and 1β (respectively, 13, 11, 9, 9 and 10 sectors).
These are all cell positions flanking cell 1α, or correspond to
the lateral or intermediate rib cells, or are located near the
initiation of the half-module β. Sectors type A most frequently
ended at cells 25β, 26β and 24α, 25α, 26α. Among the 74
sectors whose ends were positioned at 27β, 56 were those that
extended up to the leaf border. The number of A sectors
starting around 1β was significantly lower than the number of
those ending in the proximity of this cell; in the module, this
cell follows the costal long cell of the intermediate rib.
Fig. 6 presents start and end points for the 122 B sectors.
Start points were most frequently around positions 1α and 1β.
Moreover, in α, start points between 2α and 26α were less
numerous than in β at the homologous positions 2β to 27β. B
sectors ended preferentially before cell 1α and 1β, with minor
peaks around cells 9α, 12α, 2β, 9β, and 22β.
The data available for B sectors were used to study the relationship between the width and position of the start of sectors
(Fig. 7A). In α, a gradual decrease of width was evident when
the starting point of sectors moved from 1α to 26α. This was
less evident in β. The width of B sectors was also plotted
against their observed number (Fig. 7B). If the exponential
type of cell division shown in Fig. 1 were to operate during
epidermis formation, the preferred sector widths should be of
Development of the maize leaf epidermis 2117
1, 2, 4, 8, 16 cells. The data in Fig. 7B contradict this interpretation.
DISCUSSION
Sussex (1989) and Walbot (1985) have summarized the relevant
developmental differences that exist
between plants and animals. Among
these, the lack of a sequestered germline, the continuous embryogenic state
of meristems and the developmentally
late commitment of cells to a specific
fate, are characteristics of plant development. However, perhaps the unique
cellular feature, which has the most
profound implications on plant cell
growth and differentiation, is that they
are encapsulated by a rigid cell wall.
This particular condition prevents cell
rotation and migration and increases
the morphogenetic importance of
early cell divisions which already contribute to defining adult cell arrangements. Our results define the role
played by early longitudinal anticlinal
divisions in shaping the modular
nature of leaf lamina epidermis.
Module founder cells
In the introduction to this paper we
have stressed that a very large proportion of the sectors considered
originate from cells present in the
meristem before leaf primordia are
initiated. This allows us to conclude
that early transposon excision is
apparently not influenced by a
specific state of cell differentiation.
The data provided, which demonstrate that sectors preferentially start
and end at cellular positions corresponding to the borders of developmental modules, support the conclusion that cells, which by longitudinal
anticlinal divisions originate a clone
of reverted tissue, have a defined role
in the formation of the developing
module already at an early stage of
primordium differentiation. These
cells are considered ‘module founder
cells’ and alternative possibilities for
their early positioning in the module
are described in Fig. 8A. The start
positions of sectors, together with
sector widths (Fig. 8B), can be
explained by the existence of at least
4 founder cells, all derived from one
progenitor. With one exception, these
cells should arise from divisions
adding cells to the width of the leaf in
the direction from the midrib towards the leaf margin. Sectors
of type A larger than G module have, in fact, a strong tendency
to start in the midrib proximal half-module α (Table 2): β
seems, in a sense, to be hierarchically dependent on α as the
site for the origin of sectors. This suggests a clonal derivation
of the cell(s) founding β from the one(s) starting the module in
α, supporting a polarity of development in the direction α to β.
Fig. 4. Type of reversion sectors found on the surface of the first three leaves of gl1-m8 plants,
viewed with the scanning electron microscope. The sectors consist of cells covered by waxes and
appear as whitish stripes (genotype Gl1/gl1-m8) flanked by darker mutant tissue (genotype gl1m8 gl1-m8). A-C, E-H reproduce developmental modules with lateral ribs visible near the left and
right borders of the figures. Midrib is at left. In D only a single lateral rib is seen is the center of
the figure. (A) Sector type A covering a complete module from cell 1α to 27β. (B) Sector of type
A extending in α from the lateral to the intermediate rib. (C) Sector of type A starting at 1α and
ending within β. (D) Sector of type A crossing a lateral rib. (E) B sector covering half α, or (F)
crossing an intermediate rib, or (G) ending precisely at the intermediate rib, or (H) at the lateral
rib Magnification (A) ×50, (B-H) ×60.
2118 S. Cerioli and others
Table 2. Width, number and location on the leaf of type A sectors
Number of sectors starting in the module indicated
Type of
sectors*
g
g-1
1
1-1g
1g
1g-2
2
2-2g
2g
2g-3
3
3-3g
3g
3g-4
4
>4
Total
LF
5
LE
1
6
1
8
LD
LC
LB
5
8
1
5
2
4
2
6
1
2
6
1
5
3
1
6
1
2
2
3
2
3
1
3
12
1
3
4
1
4
LA
(X)
RA
RB
RC
RD
RE
3
8
7
2
6
1
3
1
2
2
9
4
8
3
9
1
4
1
1
4
1
3
2
4
1
1
5
6
3
2
2
1
2
21
30
1
2
2
31
2
5
5
2
4
1
1
5
9
1
1
3
RF
2
2
3
1
4
5
30
18
31
1
2
32
39
33
17
5
TOT
Sectors
starting in†
α
β
32
68
25
48
12
32
12
19
3
6
9
0
3
3
4
16
18
26
23
16
9
11
12
3
2
1
7
0
1
2
4
16
14
15
2
5
3
1
0
2
1
0
2
0
2
1
0
0
298
152
48
*g=sectors including the cells between a lateral and an intermediate rib. They have a width of 22-32 cells. The variation in sector size depends on their
position on the leaf blade and on leaf number (1, 2 or 3; see method). Due to this, and while the precise points of start and end of A sectors are given in Fig. 5, in
this class sectors may have been included which have not perfectly filled the space between lateral and intermediate ribs.
1=sectors including from 48 to 58 cells between 2 lateral ribs;
g-1=sectors including from 32 to 48 cells;
2, 2g, 3, 3g, 4, >4=sectors that are multiple of type g;
1-1g, 1g-2, 2-2g, 2g-3, 3-3g, 3g-4=sectors that are multiple of type g-1.
†Sectors were considered as starting in α or β when the size of their first half module obeyed the rules described in *. Based on this, all sectors defined as g, 1,
1g, 2, 2g, 3, 3g and 4 were assigned to either α or β. For the other types of sector (g-1, 1-1g, etc), assignment was made only when the first midrib-proximal
half module occupied by them agreed with the rules.
X, position of the midrib.
Fig. 5. Cellular positions of the start and end of 292 sectors of type
A. Cell numbering is according to Fig. 2. On the top of each bar the
number of sectors starting or ending at a specific cell position is
given.
Fig. 6. Cellular position of the start and end of 122 sectors of type B.
Details as in Fig. 5.
Development of the maize leaf epidermis 2119
Fig. 7. (A) Starting points of 122 B sectors plotted against sector width. (B) Distribution of B sectors according to their width.
A partial exception to the rule is the
behaviour of the G-1 A sectors.
Our findings are consistent with
the notion that leaf blade expansion
proceeds from midrib to margin
(Sharman, 1942; Esau, 1943;
Freeling, 1992). Becraft and
Freeling (1991) demonstrate that
putative morphogens involved in
blade differentiation move in the
direction midrib to margin. The
inception of leaf primordia in the
apical meristem starts also where
the future midrib will differentiate
(position marked by the presence of
a major procambial strand) and
proceeds around the meristem dome
in two, opposite, directions
(Sharman, 1942); the same applies
to ligule differentiation, which
proceeds from the midrib toward
the margins (Hake et al., 1985).
Cell divisions within the
module
Data concerning B sectors help to
elucidate the types of longitudinal
anticlinal cell divisions that are
taking place within half-modules α
and β. The available data indicate
that in the α-sector, the sector width
decreases when the starting point
moves from 1α to 26α. In β this
relationship holds true particularly
for the right part of the half-module.
Such behaviour is compatible with
both the polarized and the stem-celllike type II models of cell division
shown in Fig. 1. Experiments based
on the use of two epidermal markers
may indicate, in the future, which
gl1-m8→Gl
Fig. 8. Hypothesized sequence of early longitudinal anticlinal cell divisions occurring in the
epidermis of young maize leaves. The postulated hierarchical relations among module founder
2120 S. Cerioli and others
model is more correct. The stem-cell-like type I and the exponential models are excluded, the first because it predicts a
similar width for all sectors, and the second because the sectors
should preferentially have a width of 1, 2, 4, 8, and 16 cells, a
prediction contradicted by the data of Fig. 7B.
Ribs as compartment boundaries
In this paper we provide results in favour of a clonal type of
development during early leaf epidermis formation. The
sectors studied extend longitudinally along the whole leaf
blade and reflect events occurring early in leaf primordia development, often prior to the formation of epidermal ribs
(Langdale et al., 1989; Poethig, 1984). Because type A sectors
frequently occupy precisely half of the entire module, this
allows the ribs to be considered as compartment boundaries.
The clonal relationship between α and β half modules restricts
the consideration of compartment boundaries to lateral ribs.
These boundaries are frequently respected by epidermal
clones; for instance, the location where the proximal border of
a sector is positioned has a high probability of being the first
cell after a lateral rib. In sectors covering more than 1 module,
however, lateral ribs do not represent effective boundaries.
This can be explained by proposing that large epidermal
sectors reflect a mutated state of a large fraction of the apical
meristem (Bossinger et al., 1992).
The identification of the vein-rib as a line separating clonally
unrelated cells is not new. Sectored maize seedlings with
bilateral symmetry, where the boundary between the contrasting
phenotypes coincides with the leaf midrib, have been described
(Steffensen, 1962; Coe and Neuffer, 1978; Bossinger et al.,
1992). Christianson (1986) has also provided data in favour of
the existence of compartments in cotton cotyledons. In his experiment the boundaries between two tissues different in genotype
corresponded to veins, or were halfway between. Although in
50% of cases the sectors failed to exactly fill a putative clonal
compartment – because of either a deficiency or an excess of
small fan-shaped subclones – Christianson’s results can be
accepted as proof for the existence of compartments.
In Drosophila, the final proof of the existence of clonal compartments was obtained by using mutations that gave a growth
advantage to somatic sectors (Garcia-Bellido et al., 1976). For
the dorsal-ventral boundary of the wing margin of Drosophila,
exceptions have, however, been described where the compartment boundaries are crossed by cellular clones (Garcia-Bellido
et al., 1976; Morata and Lawrence, 1979). The case of maize
leaf epidermis discussed here shows more flexible relationships between boundaries and cellular clones. A particular case
of flexibility is the positioning of sector borders around cells
1α and 1β, a positioning ‘almost’ precise but not restricted to
the two cells mentioned. Such cases suggest that lateral ribs
may differentiate after the maturation of veins has determined
the positioning in the meristem of the module founder cells.
Alternatively, the ribs should be considered as the end product
of a differentiation process stimulated by the vein and capable
by itself of preventing further cell divisions. In the maize leaf,
the position in the mesophyll of veins seems, in any case, to
have a critical morphogenetic role in the development of the
epidermis. This observation favours the possibility that veins
already have their initials topographically imprinted in the
meristem at the time when leaf primordia are in the process of
being formed (discussed by Freeling et al., 1988).
REFERENCES
Becraft, P. W. and Freeling, M. (1991). Sectors of liguleless-1 tissue interrupt
an inductive signal during maize leaf development. Plant Cell 3, 801-807.
Bianchi, A., Bianchi, G., Avato, P. and Salamini, F. (1985). Biosynthetic
pathways of epicuticular wax of maize as assessed by mutation, light, plant
age and inhibitor studies. Maydica 30, 179-198.
Bossinger, G., Maddaloni, M., Motto, M. and Salamini, F. (1992).
Formation and cell lineage patterns of the shoot apex of maize. Plant J. 2,
311-320.
Brower, D. L. (1985). The sequential compartmentalization of Drosophila
segments revisited. Cell 41, 361-364.
Christianson, M. L. (1986). Fate map of the organizing shoot apex of
Gossypium. Amer. J. Bot. 73, 947-958.
Coe, E. H. and Neuffer, M. G. (1978). Embryo cells and their destinies in the
corn plant. In The Clonal Basis of Developments (eds. S. Subterby and I. M.
Sussex). pp. 113-129. New York: Academic Press.
Esau, K. (1943). Ontogeny of the vascular bundle in Zea mays. Hilgardia 15,
327-368.
Esau, K. (1977). Anatomy of Seed plants. New York: John Wiley & Sons.
Freeling, M., Bongard-Pierce, D. K., Harberd, N., Lane, B. and Hake, S.
(1988). Genes involved in the patterns of maize leaf cell division. In Plant
Gene Research: Temporal and Spacial Regulation of Plant Genes (eds. D. P.
S. Verma and R. B. Goldberg), pp. 41-62. New York: Springer Verlag.
Freeling, M. (1992). A conceptual framework for maize leaf development.
Dev. Biol. 153, 44-58.
Freeling, M. and Lane, B. (1993). The maize leaf. In The Maize Handbook
(eds. M. Freeling and V. Walbot). New York: Springer Verlag.
Garcia-Bellido, A. and Merriam, J. (1971). Parameters of the wing
immaginal disc development in Drosophila melanogaster. Dev. Biol. 24, 6187.
Garcia-Bellido, A., Ripoll, P. and Morata, G. (1973). Developmental
compartmentalization of the wing disc of Drosophila. Nature 245, 251-253.
Garcia-Bellido, A., Ripoll, P. and Morata, G. (1976). Developmental
compartmentalization in the dorsal mesothoracic disc of Drosophila. Dev.
Biol. 48, 132-147.
Hake, S., Bird, R., Neuffer, M. and Freeling, M. (1985). The maize ligule and
mutants that affect it. In Plant Genetics (ed. M. Freeling). pp. 61-72. New
York: Alan R. Liss.
Klekowski, E. J. (1988). Mutation, Developmental Selection and Plant
Evolution. New York: Columbia University Press.
Langdale, J. A., Lane, B., Freeling, M. and Nelson, T. (1989). Cell lineage
analysis of maize bundle sheath and mesophyll cells. Dev. Biol. 133, 128139.
Lawrence, P. A. and Morata, G. (1993). A no-wing situation. Nature 366,
305-306.
Maddaloni, M., Bossinger, G., Di Fonzo, N., Motto, M., Salamini, F. and
Bianchi, A. (1990). Unstable alleles of Glossy-1 of maize show a lightdependent variation in the pattern of somatic reversion. Maydica 35, 409420.
McClintock, B. (1951). Chromosome organization and gene expression. Cold
Spring Harb. Symp. Quant. Biol. 16, 13-40.
Morata, G. and Lawrence, P. A. (1979). Development of the eye-antenna
marginal disc of Drosophila. Dev. Biol. 70, 355-371.
Poethig, R. S. (1984). Cellular parameters of leaf morphogenesis in maize and
tobacco. In Contemporary Problems in Plant Anatomy (eds. R. A. White and
W. C. Dickenson), pp. 235-259. New York: Academic Press.
Russell, S. H. and Evert, R. F. (1985). Leaf vasculature in Zea mays L. Planta
164, 448-458.
Salamini, F. (1963). La superficie fogliare del mais. Maydica 8, 67-72.
Sharman, B. C. (1942). Developmental anatomy of the shoot of Zea mays L.
Ann. Bot. 6, 245-283.
Steffensen, D. M. (1962), Patterns of sectoring in seedling with reference to
early embryonic development. Maize Genet. Coop. Newsletter 36, 24.
Steffensen, D. M. (1968). A reconstruction of cell development in the shoot
apex of maize. Am. J. Bot. 55, 354-369.
Sussex, I. M. (1989). Developmental programming of the shoot meristem. Cell
56, 225-229.
Tilney-Bassett, R. A. E. (1986). Plant Chimeras. London: Edward Arnold.
Walbot, V. (1985). On the life strategies of plants and animals. Trends Genet.
1, 165-169.
(Accepted 21 April 1994)