Consortium for
Educational
Communication
Module on
Shoot Apical Meristem:
Histological Organization
By
Dr. Shaista Qadir
Assistant Professor
Department of Botany, Govt. Degree
College for Women.
M. A. Road, Srinagar. University of
Kashmir, Srinagar India.
[email protected]
CELL NO: 9469106443
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Module 1(introduction)
Text
Meristems contain a population of cells with characteristics of stem
cells; cell division serves to constantly replenish the meristem and
to provide cells that will differentiate into plant organs and tissues.
Unlike most examples of stem cells in animals, where the potential
of the differentiating daughter is restricted, cells produced by
plant meristems have the capacity to differentiate as any cell
type. Thus some prefer to think of meristems as populations of
embryonic cells because the cells produced, go through the entire
process of organogenesis, pattern formation, histogenesis and
differentiation. During postembryonic development, all organs of
a plant are ultimately derived from a few pluripotent stem cells
found in specialized structures called apical meristems. Apical
meristems are the completely undifferentiated (indeterminate)
meristems in a plant. Usually shoot apex and root apex are
employed as synonyms for apical meristems-both root and
shoot. The overall similarity in the organization of shoot apices
allows terms such as shoot apical meristem and shoot apex to be
defined in a simple and functionally significant manner. The shoot
apical meristems are minute but complex structures that are
covered within new developing leaves or bracts. It is the distalmost portion of the shoot and comprises two groups of cells:
the initial or source cells and the cells that are the progenitors
for tissues and lateral organs (Wardlaw, 1957; Cutter, 1965).
By contrast, the shoot apex comprises several cell and tissue
types: the apical meristem itself, a region just proximal to the
meristem where lateral organ primordia are formed, a subapical
region where the shoot widens and primordia enlarge, and the
region of maturation, where differentiation becomes apparent
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(Wardlaw, 1957; Cutter, 1965). With these definitions, reference
to the shoot apex is restricted to a small portion of the shoot,
typically including only three to six leaf primordia. The shoot tip
can then be defined as that portion of the shoot comprising all
tissues and organs distal to the still-differentiating leaves. The
number of leaves in a shoot tip would vary, depending on the rate
of differentiation. The above definitions are based on decades of
studies and are used widely today (Steeves and Sussex, 1989).
Although meristems function as generic sources of cells for
differentiation into organs, each type of meristem is programmed
to produce only certain structures. The functions of the shoot
apical meristem (SAM) are the formation of the shoot axis and the
initiation of lateral organs, such as leaves, bracts, etc. To maintain
this ability, the SAM has to be self-perpetuated, i.e. general
features of the SAM geometry and size have to be maintained, or
change only slowly. The self-perpetuation takes place despite the
continuous flow of cells basipetally from the distal SAM portion
and the cyclical initiation of lateral organ primordia.
Module 2( dimensions of SAM and its growth pattern)
Shape: The shoot apex is radially symmetrical. There occur
great variations in shape and size of the shoot apices among the
spermatophytes. Usually the shoot apex in most of the plants
is more or less convex. In Anacharis, and some grasses it is a
narrow cone with a rounded tip. However, in some cases,e.g.
in Hibiscus syriacus, Drimys, etc, it is slightly concave (Tolbert,
1961, Gifford, 1950).
Size: The size of shoot apex varies in different plants or in different
branches of the same plant. The measurement of the diameter
is taken as the width of the apex immediately above the newly
formed primordium. It may be 90 µm in some grasses, 130-200
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µm in dicots, 280 µm in banana and 500-800 µm in palmae.
Clowes (1961) reported the size of Cycas revoluta shoot apex
at the level of youngest leaf primordium to be 3300 µm in width
(Foster, 1940).
Growth pattern: Growth in the shoot apex is not uniform. Two
fundamental processes contribute to the SAM growth. In the selfperpetuation process, the expansion of the apical dome is such
that its size and shape are generally maintained. At the same
time, lateral organs are cyclically initiated as bulges at the SAM
dome flanks, giving rise to phyllotactic patterns of fascinating
regularity. As a result, the shape and size of the SAM cyclically
change (Kwiatkowska, 2004). Changes in SAM shape and size can
occur also in seasonal cycles (Gifford, 1950; Romberger, 1963;
Steeves and Sussex, 1989). According to Owens and Molder
(1976) spectacular seasonal changes also are characteristic of
some conifers, such as spruce. In their apices, the two SAM
activities (the self-perpetuation and the leaf initiation) are to
some extent separated in time (Figs). The apical dome enlarges
greatly at first, but virtually no leaf primordia are initiated (Figs).
Afterwards, a large number of primordia are initiated using up
the undifferentiated surface produced earlier (Figs.).
Module 3 (Apical cell theory and histogen theory)
Theories regarding histological organisation of shoot
apical meristem
Shoot apex was first recognised by Wolff (1759) as an undeveloped
region, from which growth of plant proceeded. He termed this
region as ‘punctum vegetationis’. Since then, the concept has
undergone profound changes and different theories dealing with
the histological organisation of shoot apical meristem have been
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put forward.
1. The Apical cell theory
Apical cell theory was postulated by Nageli in 1878, while examining
the shoot apical meristems of most cryptograms. According to
this theory, a single apical cell is the structural and functional unit
of apical meristem, and it governs the whole process of growth.
Single apical cells occurs in many vascular cryptograms like
algae (Chara) ferns, horse tails and many species of Selaginella.
In many cases, the growth at the apex precedes from few apical
meristem cells. These cells are often conspicuous because of
their large size and relatively high degree of vacuolation. Most
single apical cells are pyramidal (tetrahedral) in shape, e.g. in
Equisetum and most leptosporangiate ferns. Single apical cells
may also be three sided, e.g. water ferns, Salvinia and Azolla.
In Lycopsida, single apical cells, as well as groups of initial cells,
have been described. However, recent studies on shoot apex using
colchicine have confirmed that shoot apex of spermatophytes
consists of a group of cells which constitute the meristem.
2. Histogen theory
This theory was postulated by Hanstein (1868). According to this
theory, the main body of the plant arises not from superficial
cells, but from a mass of meristems of considerable depth, and
this mass consist of three parts, called histogens (which may
be differentiated by their origin and course of development. The
histogens were: i) the dermatogen, a uniseriate, external layer
forming epidermis,ii) the periblem, which gives rise to cortex
and internal tissues; and iii) the plerome, a central core that fills
and constitutes the entire mass of the axis.
According to this theory, these three histogens develop from
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independent groups of initials and meristems are predestined to
produce specific tissues. The viewpoint and the terminology of the
histogen theory have long dominated anatomical interpretation
of tissues, regions, and even organs in vascular plants, but the
distinction of these histogens in an apex cannot be made in
some plants, and in others the regions have no morphological
significance. Furthermore, it is now accepted that all cells have
basically equal potential of differentiation, and that one zone
of the apical meristem may contribute cells to one another.
Hence this theory, which held a ground for a long time, was later
dropped.
Module 4 (Tunica-Corpus
hypothesis)
theory
and
Mantle-core
3. Tunica-Corpus theory
Another theory that explains the structure of shoot apical
meristem of angiosperms is the Tunica and the Corpus Theory
given by Schmidt (1924). According to this theory, different rates
and method of growth in the apex, set apart two regions of unlike
structure and appearance in the apical meristem, the tunica
and the corpus. The tunica consists of one or more peripheral
layers of cells, and the corpus of a mass of cells overarched by
the tunica. The layers of the tunica show anticlinal divisions;
that is, they are undergoing surface growth. The corpus cells
divide in various planes, and the whole mass grows in volume.
Each layer of the tunica arises from a small group of separate
initials, and the corpus has its own initials located beneath those
of the tunica. Although the epidermis usually arises from the
outermost layers of tunica layer, the rest of the tissues may
originate either from tunica or from both. Foster(1941) and
his students strongly believed in tunica-corpus theory and, as
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such it dominated studies on shoot apical meristems for about
two decades. As more and more plants were examined, the
concept went under some modification especially with regard to
the strictness of definition of the tunica. According to one view,
tunica should include only those layers that never show any
periclinal divisions, the other stratified layers which occasionally
show periclinal divisions being interpreted as part of corpus
(Jentsch, 1957). Other workers treat the tunica more loosely,
and describe it as fluctuating in number of layers, one or more
layers of the tunica may undergo periclinal divisions, and thus
become part of corpus (Clowes, 1961a).
The tunica-corpus concept was developed with reference to
the angiosperms; it proved to be largely unsuitable for the
characterization of the apical meristem of gymnosperms (Foster,
1949). Shoot apices of only a few gymnosperms have an
independently propagating layer that could be interpreted as
tunica; in others, the outermost layer divides periclinally and,
thus, is ontogenetically related to the subjacent tissue.
4. Mantle-core hypothesis
Mantle-core hypothesis has been proposed by Popham and Chan
(1950). They have strongly criticised the morphological aspects
of the tunica-corpus theory, because its total application leads to
variations in the number of layers belonging to tunica, even in the
same plant. Instead they have proposed mantle-core hypothesis
as applicable to shoot apices on a general layered pattern. They
did not laid much emphasis on planes of division. According to
this theory, shoot apical meristem consist of two histological
zones-i) the mantle, that includes all the outer layers of the
apex forming a dome, which can be distinguished histologically
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from the inner cell mass ii) the core, that shows an irregular
arrangement. Later on Popham (1952) further recognised two
main kinds of cores in angiosperms:
a) The usual angiosperm type, in which the core consist of
three zones:
i. Central mother cell zone, which lies just below the apical
initials,
ii. Rib meristem, which is in direct continuation with central
mother cell zone, and gives rise to vascular tissues, and
iii. Flank meristem, which lies in peripheral position, and gives
rise to cortex.
b) The opuntia type, in which the core consist of four zones. The
three are central mother cell zone, rib meristem, flank meristem,
and the additional one is cambium-like transition zone, which is
cup shaped, located between the central mother cells and the rib
and flank meristems. The cambium-like zone is not a universal
feature and its presence is reported only in some taxas such as
Opuntia cylindrica, Livistona, Bellis perennis, Chrysanthemum
morifolium, Xanthium pennsylvanicum, Liriodendron tulipfera,
Bougainvillea spectabilis,and Ricinus communis. This zone differs
from the other zones of apical meristem in that it is usually shortlived and develops during mid-Plastochron and disorganises,
soon after the Plastochron ends. Successive zones, however,
during next plastochrons and the remnants of the previous ones
can be seen in older parts. For example, a median longitudinal
section of the vegetative apex of Ficus carica shows a series of
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such zones, corresponding to the basal part of the developing
internode. Most of the recent workers do not consider cambiumlike zone as a part of the apical meristem, since neither it is
present in the proper meristematic head, nor it contributes to
the elongation of the apex. It seems therefore preferable not
to use this zone as a criterion for classifying shoot apices of the
angiosperms into two different types.
Module 5 (Histogenic layer Theory, Cytohistological
zonation theory, and Anneau Initial and Meristeme D’
attente theory
5. Histogenic layer Theory
According to Dermen (1947), the terms tunica and corpus do
not have fixed values. On the basis of his studies on naturally
occurring periclinal cytochimeras in apple (1951), and periclinal
cytochimeras in cranberry and peach (1947), induced by colchicine
treatment, he suggested that tunica-corpus designations in these
plants be replaced by primary histogenic layers. Three basic
histogenic layers recognised in all angiospermic plants were
referred as L-I, L-II and L-III layers. These layers exist in various
combinations of ploidy in the shoot apex. For example, the cells
of L-I may be 2x, those of L-II 2x, and those of L-III 4x. Expressed
in terms of ploidy, the constitution would be 2-2-4. Besides this,
there exists possibility of various other combinations, such as
2-2-4, 4-2-2, 2-4-4, 4-4-4 and 2-4-2.
(i) L-I gives rise to – epidermis of stem and leaves.
(ii) L-II forms – 1-3 layered hypodermis, cortex and in some
cases the vascular tissues of stems.
(iii) L- III forms – vascular tissue and pith.
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6. Cytohistological zonation theory
Most recent workers regard shoot apical meristem to be very
complex, not only structurally but also functionally. They have
shown a distinct cytohistological zonation pattern superimposed
on the tunica-corpus organisation, and recognised different
zones based on well-defined cytological characteristics, including
rates of growth (Clowes, 1961; Romberger et al., 1993). A
number of genes are involved to control the maintenance and
size of the SAM dome and its cytohistological zones (Bowman
and Eshed, 2000; Brand et al., 2001; Traas and Doonan, 2001).
Widely accepted cytohistological zonation was introduced by
Foster (1943) for the SAM of gymnosperms, e.g. Ginkgo biloba,
wherein he recognized four zones. This zonation is based not
only on planes of division, but also on cytological and histological
differentiation and degree of meristematic activity of component
cell complexes. The different zones recognized were as:
I. Apical-initial zone: The most distal portion of the dome is
the initiation or apical-initial zone. It includes a group of initials
along the apical surface, and the lateral and sub-adjacent
derivatives.
II. The central meristem/mother cell zone: It comprises of
slowly growing cells located below the apical initial group and
is derived from them, this entire distal zone usually takes a
light stain. Its cells are more vacuolated,larger and thick walled
than other SAM cells.
III.Rib meristem zone: It is the fast growing zone and is located
proximal to the central meristem zone. Because its cells grow
predominantly in an axial direction, they form characteristically
elongated ladder-like cell files.
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IV.The peripheral zone/outer zone: This zone is also called
flank meristem. It originates partly from the lateral derivatives
of apical initials and partly from the central mother zone.
This zone surrounds the central meristem zone and the rib
meristem. The peripheral meristem zone typically is the most
meristematic of all the zones, has the densest protoplasts and
smallest cell dimensions
Similar cyto-histologic zonation has since been observed in
many angiosperms (Cloves 1961a). The concept of zonation in
Foster’s sense has considerably advanced the understanding
of growth in shoot apices. It has also related the apical
organization to that of the underlying derivative shoot parts
without reintroducing a formalized concept of histogen initials.
But efforts to bring about such reintroduction are not lacking
7.Anneau Initial and Meristeme D’ attente theory
Since 1950, a theory of angiosperm apical zonation, developed
by French and Belgian botanists, has been gaining support.
This theory proposes that the central region of the apical dome
constitutes a mass of cells with relatively low division rates, the
méristème d’attente, or “waiting meristem.” Surrounding this
region is an annular zone of cells with higher division rates, the
anneau initial or “initiating ring.” Features other than division
rates characterize these zones: RNA and protein content are
lower in the méristème d’attente than in the anneau initial, and
the nucleoli are smaller. In longitudinal section, the differences
contribute to the patterns distinguishable in apices, some of
which have been used as bases for structural classification.
The main contention of the Franco-Belgian school, however,
is that the zonation represents a functional difference. The
méristème d’attente is regarded as a region mainly concerned
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with controlling the geometry of the apex. The cells have a
restricted metabolism, concerned primarily in maintaining a low
rate of increase in cell number, and they themselves, as well
as their immediate derivatives, take no part in organogenesis
or associated differentiation. The anneau initial, by contrast, is
that part of the apex that produces the beginnings, or primordia,
of lateral organs. Not only is the division rate higher, but the
tissue as a whole is involved in metabolic syntheses that precede
morphogenesis. Buvat (1955a) and his students made strong
efforts to obtain a unified concept of growth of shoot apical
meristem under the same theory. They recognized three distinct
zones or regions in the shoot apical meristem as follows:
i) anneau initial (peripheral active zone),
(ii) meristeme d’ attente (waiting meristem), and
(iii) meristeme medullaire ( central pith region).
This theory became controversial because the results of natural
and colchicine-induced polyploid chimeras favored the viewpoint
that divisions do occur in meristeme d’ attente zone also.
The investigations on variegated chimeras, carried by various
workers, like Thielke, 1954, Steward and Dermen, 1970, lead
to a similar conclusion. Ball (1960), and others, also arrived on
the conclusion that the central meristeme d’ attente is capable
of dividing frequently and actively. Also the application of radioactively labeled precursors of DNA synthesis and other methods
did not reveal the existence of an inactive zone in the shoot apex
(Cloves and MacDonald, 1987). Besides this, the supporters of
the Meristeme d’ attente theory itself, agreed that a few mitosis
(59) may occur in the central apical zone but claim that their role
is minimal in relation to that played by the anneau initial (Buvat,
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1955). Similarly, in young embryo shoots of Zea mays, Clowes
(1978) also found that the rates of division at the summit of the
apex do not differ greatly from those in the rest of the apex.
Thus, we can conclude, by saying that the most of the theories
regarding SAMs have undergone severe criticism from various
quarters from time to time, but the tunica-corpus concept has
stood the test of time and is currently regarded as the best
and a very adaptable one, at least for describing the SAMs of
angiosperms, but is not applicable to SAMs of gymnosperms.