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Plant Tissues
Introductory article
Article Contents
Arp Schnittger, University of Tübingen, Tübingen, Germany
Martin Hülskamp, University of Tübingen, Tübingen, Germany
. Introduction
. The Dermal Tissue
Higher plants are organized into three tissue layers: the epidermis, the ground tissue and
the vascular system. The separation of the three layers is established in the meristems
and they are largely maintained as separate tissues, reflecting their separate cell lineages.
. The Ground Tissue
. The Vascular Tissue
. Origin and Maintenance of Tissue Layers
. Tissue Layers and Cell Differentiation
. Developmental Roles and Interactions Between Tissue
Layers
Introduction
In vascular plants, most organs are organized into three
tissue layers. Each tissue layer fulfils distinct functions and
is composed of a specific set of specialized cell types. The
outermost tissue layer, the epidermis, serves to protect the
plant body and to mediate the exchange of nutrients, water
and gas with the plant’s environment. The second tissue
layer is called the ground tissue and may adopt various
functions including photosynthesis, the storage of materials and wound healing. The third tissue layer is the vascular
tissue and is composed of two conducting systems: the
xylem and the phloem.
The Dermal Tissue
The dermal tissue is the outermost cell layer that mediates
the interactions of a plant with its environment. In most
plants, the dermal tissue consists of a single cell layer. The
various functions of the epidermis are executed by several
specialized cell types.
In the aerial parts of a plant, the majority of the
epidermal cells are small and compact with a cuticle
consisting of cutin and wax. This cuticle effectively protects
the plant from water loss and functions as a barrier against
pathogens. The exchange of water vapour and gases is
regulated by stomata, small gated pores that are formed by
the guard cells and their subsidiary cells. Turgor changes in
guard cells control stomatal opening and closing and
thereby regulate the optimal uptake of carbon dioxide for
photosynthesis and the overall water economy of the plant.
Leaf hairs, called trichomes, represent specialized epidermal cell types that play a role in the protection against
insects, against intense solar radiation and in reducing the
air exchange on the leaf surface thereby reducing water loss
through the stomata. In the root, the epidermis is
important for efficient water and nutrient uptake. This is
facilitated by root hairs that greatly enlarge the root
surface and help to anchor the plant in the soil.
The Ground Tissue
The ground tissue has a variety of functions including
photosynthesis, storage, reproduction and mechanical
support. Three specific tissue types are distinguished that
are classified according to their cell wall structure and
thickness and their role in mechanical support within the
ground tissue: parenchyma cells, collenchyma cells and
sclerenchyma cells.
Parenchyma cells are thin-walled living cells that remain
competent to grow and divide. They represent the majority
of the inner primary plant body and are found in all organs
including the cortex of stems and roots, the pith of the
stems, in leaves and in fruits. Parenchyma cells play
important roles in photosynthesis (e.g. in the mesophyll of
leaves), storage of nutrients (in fruits) and water (in
succulent plants). In addition, because of their capability to
divide and to redifferentiate parenchyma cells are important for regeneration and wound healing.
Collenchyma cells can be regarded as parenchyma cells
that have specialized as supporting and strengthening
tissues. Like parenchyma cells, collenchyma cells are alive
at maturity. They are found predominantly beneath the
epidermis in stems and petioles where they form strands or
whole cylinders. Collenchyma cells have an unevenly
thickened nonlignified primary cell wall which provides
excellent mechanical support to young and expanding
organs. At the same time, collenchyma cell walls are
flexible and can easily be modified during cell expansion to
allow the growth of the organ.
Sclerenchyma cells have an important function in
strengthening and supporting older plant parts that have
stopped expanding. They have thick, frequently lignified
secondary walls. Often sclerenchyma cells die during
maturation. Sclerenchyma cells may be organized as
bundles of fibres and well-known examples are the fibres
in hemp or jute. They may also occur as single sclereids or
groups of sclereids, the latter of which is found in seed coats
or the shells of nuts.
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net
1
Plant Tissues
The Vascular Tissue
The vascular tissue is a water- and food-conducting tissue
that forms a continuous system throughout the entire plant
body. It is also important for the distribution of plant
hormones (e.g. auxin) and other signalling molecules. The
vascular system consists of two different tissues: the xylem
and the phloem.
The xylem serves mainly as a water- and mineralconducting tissue but also plays a role in food storage and
as a supporting tissue. The main conducting cells of the
xylem tissue are the tracheary cells, of which two different
types can be distinguished: the vessel elements and the
more primitive tracheids. Tracheary cells have characteristic pits in their walls and may form secondary walls.
During maturation tracheary cells die and lose their
protoplast. They are interconnected through perforations
in their end walls, fairly large holes in the primary and
secondary cell wall that allow a relatively unimpeded water
flow from one tracheary cell to the other.
The phloem represents the major nutrient-conducting
tissue. Two cell types are distinguished: the sieve-tube cells
that are only found in angiosperms and the more primitive
sieve cells that are found in seedless vascular plants and
gymnosperms. Unlike tracheary cells, both cell types
remain alive at maturity. However, during maturation
the nucleus degenerates and is eventually lost. In addition
vacuoles, ribosomes and microtubules disappear. Sieve
cells and sieve-tube members form long cell rows that are
interconnected through the sieve plates. In sieve cells, the
connecting pores are narrow and show the same structure
on all walls. In sieve-tube member cells, a sieve plate is
found between the overlapping ends. Here, the pores are
much larger than on other wall areas of the same cell. Both
cell types are intimately connected by numerous plasmodesmatal connections with specialized parenchyma cells
that are thought to support them. Sieve-tube cells are
associated with so-called companion cells that originate
from an unequal cell division producing a companion cell
and a sieve-tube cell. Sieve cells are associated with
parenchyma cells called albuminous cells which are not
derived from the same mother cell as their associated sieve
cell.
Origin and Maintenance of Tissue
Layers
The existence of three discrete tissue layers was first
recognized by the analysis of cell lineages. In this type of
analysis, single cells are genetically marked (e.g. cell size or
colour) early in development so that their contribution to
adult structures can be followed. This type of analysis
revealed that cell lineages generally fell into three discrete
cell lineages with respect to the tissue layers. These findings
2
Figure 1 Origin of tissue layers. Tissue layers originate from the three cell
layers of the meristem: the L1, L2 and the L3. (a) Section of an apical
meristem. Modified after Raven (1992). (b) Schematic drawing of (a) with
the L1, L2 and L3 cell layers marked. (c) Schematic drawing of an apical
meristem with the general direction of cell divisions of the three
meristematic cell layers during further development indicated by arrows.
While L1 cells almost always divide by periclinal divisions, L2 and L3 cells
also frequently divide anticlinally. Modified after Sitte et al. (1991).
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net
Plant Tissues
indicated a different origin of the three tissue layers and
suggested a mechanism for keeping the three layers mostly
separated throughout development.
This separation of the three tissue layers is laid down in
the shoot and root meristems that already have a layered
structure. Each meristem layer, the L1, L2 and L3,
contributes to new tissues and organs by anticlinal cell
divisions (divisions occuring perpendicular to the surface
of the shoot) thus generating the three different tissue
layers (Figure 1).
Due to the regularity of the cell division pattern it is
possible to generate and propagate plants where one tissue
layer is genetically different and marked with a trait such as
albinism. Such plants are called periclinal chimaeras.
Using this approach, it was possible to study the relative
contribution of the meristematic L1, L2 and L3 layers to
the adult structures of the plant. The L1 layer divides
anticlinally and is destined to form the monolayered
epidermis. The L2 layer divides anticlinally in the
(a)
L1 L2 L3
meristem, but also periclinally during organ development
and produces the palisade layer, the lower spongy
mesophyll layer and the entire mesophyll at the leaf
margin. In addition, both the female and the male
gametophytes are derived from the L2 layer. The L3 layer
divides anticlinally and periclinally in the meristem and
during organ development and gives rise to the centre of
the stem and the core of leaves.
Tissue Layers and Cell Differentiation
The analysis of periclinal chimaeras also enabled the study
of rare events where cell divisions result in an invasion of
cells into an adjacent layer. Generally these cells adopt a
cell fate and differentiate according to their new position.
Thus, although the division patterns are fairly regular, the
fate of a cell is determined by its position and not by its
lineage.
(b)
L1 L2 L3
(c)
L1 L2 L3
Figure 2 Role of tissue layers in the regulation of morphogenesis. Periclinal chimaeras revealed that leaf shape in tomato is controlled by the L2 layer
whereas trichome development depends on the L1 layer. (a) Lycopersicum esculentum has compound leaves and is hairy. Below, a schematic presentation of
the three-layered meristem is shown in grey for the genotype Lycopersicum esculentum. (b) Solanum luteum has simple leaves and few hairs. Below, the
three-layered meristem is shown in black for the Solanum luteum genotype. (c) Periclinal chimaera with a Lycopersicum esculentum L1 and a Solanum luteum
L2 and L3 layer. The epidermis is hairy as in Lycopersicum esculentum and the leaf form is simple as in Solanum luteum. (a–c) Modified after CA Jorgensen and
MB Crane (1928) Formation and morphology of Solanum chimaeras. Journal of Genetics 18: 247–273.
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net
3
Plant Tissues
Developmental Roles and Interactions
Between Tissue Layers
Although the clonal origin of tissue layers implies that cell
and tissue differentiation is determined by their origin, the
finding that the relative contribution of meristematic L2
and L3 cells to inner tissues is highly variable indicates that
intimate interactions between tissue layers are very
important for development. One example where localized
inductive interactions control the formation of specific cell
types is represented in the formation of air spaces in
mesophyll tissues in the leaf. Here it was demonstrated that
the air spaces are induced by the presence of stomata in the
epidermal layer.
The coordination of growth orientation and growth
rates for proper morphogenesis of the respective organs
requires additional interactions between tissue layers. In
order to understand these interactions the roles of tissue
layers in the regulation of morphogenesis were studied. For
the control of leaf form, periclinal chimaeras between
Solanum luteum (simple leaves) and Lycopersicum esculentum (compound leaves) revealed an important role of
the L2 layer (Figure 2). In all combinations tested leaf form
was determined by the genotype of the L2 layer while the
genotype of the L1 and L3 layer had little influence on the
final leaf form. By contrast, trichome development in the
epidermis was controlled autonomously by the genotype of
the epidermal tissue layer.
Patterning during flower development seems to be
controlled mainly by the L3 layer. This was shown using
4
periclinal tomato chimaeras with one layer carrying the
mutation fasciated (f), which results in an increase in the
number of organs in each floral whorl. Only chimaeras
carrying the f mutation in the L3 layer exhibited an
increased number of floral organs.
The molecular basis for cell–cell interactions between
tissue layers is largely unknown. In one case, the shoot
meristem, it has been shown that cells of different cell layers
communicate via a receptor–ligand-based system similar
to those identified in animals. In addition, plants have
evolved a unique mechanism in which regulatory proteins
can travel from cell to cell through connections called
plasmodesmata, in a regulated manner. This is best shown
for the induction of epidermal differentiation by the
Knotted-1 gene in maize. The Knotted-1 gene encodes a
homeodomain transcription factor and it was shown that
epidermal differentiation can be controlled even if Knotted1 is expressed in subepidermal tissue layers. A comparison
of the RNA and protein distribution provided evidence
that Knotted-1 protein moved from subepidermal cells to
epidermal cells.
Further Reading
Howell SH (1998) Molecular Genetics of Plant Development. Cambridge,
UK: Cambridge University Press.
Raven PH (1992) Biology of Plants. New York: Worth Publishers.
Sitte P, Ziegler H, Ehrendorfer F and Bresinsky A (1991) Strasburger
Lehrbuch der Botanik, 33rd edn. Stuttgart, Jena, New York: GustavFischer-Verlag.
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net