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Xylem Structure and
Function
Introductory article
Article Contents
. Introduction
Alexander A Myburg, North Carolina State University, Raleigh, North Carolina, USA
Ronald R Sederoff, North Carolina State University, Raleigh, North Carolina, USA
. Xylem Structure and Variability
. Xylem Functions: Water Transport and Structural
Support
. Xylem Differentiation and Cell Wall Biosynthesis
Vascular plants have evolved a highly specialized tissue, called xylem, which provides
mechanical support and transports water, mineral nutrients and phytohormonal signals in
the plant. Although it is the most abundant biological tissue on earth, much remains to be
learned about the structure, function, development and evolution of xylem and of the
genes that regulate the processes.
Introduction
The earliest land plants were short, herbaceous plants that
evolved from primitive, water-living ancestors. For these
plants, the change from a predominantly aquatic to a
terrestrial environment was accompanied by the need for
additional structural support to keep the plants upright
and the need for more efficient transport of water to the
aboveground parts of the plants. Larger plant sizes also
increased the need for co-ordination between remote plant
parts. The development of specialized vascular tissues to
fulfil these requirements played an important role in the
evolution and adaptation of plants to the terrestrial
environment.
As the early land plants filled more and more terrestrial
niches, the selective advantage of increased propagule
dispersal associated with increase in height, and later
competition for sunlight, increased the selection pressure
for plants that could grow taller than other plants. The
most successful plants were able to support more weight,
transport water further and sustain growth for more than
one season. The dramatic result of this evolutionary
process is evident in the rapid increase of plants with
secondary vascular tissues and arborescent growth form in
a rather short evolutionary timespan (380–350 million
years ago).
The stems and roots of modern plants are highly
specialized conductive organs that can transport water,
nutrients, photosynthetic products and chemical regulatory signals. These organs contain two types of conductive
tissue: phloem and xylem. Phloem is the tissue that
transports photosynthetic products and plant growth
regulators (phytohormones) mainly from the leaves to
the rest of the plant. Xylem is the tissue that transports
water, mineral nutrients and phytohormones from the
roots to the leaves and other plant organs. While
herbaceous plants do contain xylem, it is a tissue that is
most prominent in woody plants, especially trees. Most of
our knowledge of xylem structure and function is based on
woody plants. The most important functions of xylem
. Origin and Evolution of Xylem in Plants
. Genetic Manipulation of Xylem Formation
include: (1) transport of water and mineral nutrients, (2)
mechanical support and (3) storage of nutrients and water.
Xylem Structure and Variability
The cell types that make up xylem tissue show great
variability across different plant groups, from species to
species and even within the same plant. This section will
focus on the structure and variability of xylem produced
during primary and secondary growth in different plant
groups.
Xylem cell types
The structural features of xylem are determined by the size,
shape and distribution of xylem cell types and, in
particular, by the shape and thickness of their cell walls.
Cell wall structure affects cell type and characteristics
Almost all plant cells produce primary cell walls. The
major component of most primary walls in xylem is a
disorganized network of cellulose fibrils, which allows the
wall to stretch and expand as the cell grows. The secondary
wall is deposited on the inner side of the primary wall
during and after the cell has elongated or enlarged. The
cellulose fibrils in the secondary wall are arranged in a
regular fashion with alternating layers at fixed angles to the
main axis of the cell (Figure 1). This reinforces the plant cell,
while preserving the elastic nature of the primary wall.
Most of the cell types in xylem can be distinguished based
on the shape and features of the secondary cell wall.
Xylem parenchyma cells store water, mineral nutrients
and carbohydrates, and respond to wounding
The cells responsible for most of the storage function of
xylem are called parenchyma cells. Many xylem parenchyma cells have secondary lignified walls, particularly in
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Xylem Structure and Function
S3 (60°–90°)
S2 (10°–30°)
Secondary
wall
S1 (50°–70°)
Primary wall
Middle lamella
Figure 1 Drawing of the secondary thickened wall of a mature tracheary
element showing the orientation of cellulose microfibrils in the different
layers of the wall. Note the designation of the secondary wall layers and the
average microfibril angle of each layer: S1 is the outermost layer, S2 is the
middle layer and S3 the innermost layer. Most of the wall thickness is
determined by the thickness of the S2 layer (the relative thicknesses are:
primary wall, 1%; S1,10 to 20%; S2, 40 to 90% and S3, 2 to 8%) . Modified
after Côté WA (1967) Wood Ultrastructure: An Atlas of Electron Micrographs.
Seattle: University of Washington Press.
wooden plants. In other cases, these cells have thin,
primary walls with areas of plasmodesmata, called primary
pit fields, through which cell-to-cell movement of water
and mineral nutrients can take place. Mature xylem
parenchyma cells in active xylem tissue retain a functional
protoplasm and can store carbohydrates in the form of
starch. These cells also play an important role in wound
healing by forming callus and can differentiate to
regenerate functional xylem cells.
Sclerenchyma cells provide mechanical support, defence
and water transport
The cells involved in mechanical support and defence are
specialized sclerenchyma cells. Fibres are long, narrow
sclerenchyma cells, mostly with thick secondary walls
(Figure 2b and c). They are mainly involved in the
mechanical support function of xylem and defence against
pathogens and herbivores.
The conducting cells of xylem are called tracheary
elements. There are two types of tracheary elements:
tracheids (Figure 2a) and vessel elements (Figure 2d and
e). Vessel elements are connected end-to-end through large
perforations in their end walls to form a vessel. Tracheids
are connected through large, circular bordered pits that are
concentrated at the tapered ends (in the radial walls) of the
cells (Figure 2a). Mature vessel elements and tracheids have
no cellular contents and consist mainly of thickened
secondary walls.
2
In most tracheary elements, almost the entire inner
surface of the primary wall is covered by secondary wall,
except for small areas called pits. In the lateral walls of such
vessel elements, and the walls of tracheids (mostly radial
walls), the pits occur in pit-pairs with the pits of
neighbouring cells precisely aligned (Figure 3). A pit
membrane, comprised of the primary walls of adjacent
cells, separates the pits of each pit-pair. The inner aperture
of the pit is often narrow and reinforced by extra secondary
wall material to form a border. The outer aperture of each
pit, which is bounded by the pit membrane, is usually wider
to allow maximum conductance of water across the pit
membrane. In most conifers, the central part of the pit
membrane is thickened and lignified to form a torus
(Figure 3). The torus is usually slightly larger than the
aperture of the pit border and is impermeable to water. The
outer part of the membrane (the margo) is digested to leave
a porous network of cellulose fibrils through which water
can move easily. Under certain circumstances, the torus
can block one of the two inner apertures of the pit-pair and
prevent the movement of water and air through the pit. In
tracheids, this may serve to isolate cavitated tracheids and
prevent the spread of embolisms.
The end walls of vessel elements are modified into
perforation plates (Figure 2d and e). Most vessel elements
possess simple transverse perforation plates with only one
large perforation, but compound perforation plates with
two or more perforations occur. Simple perforations
provide the least amount of resistance to water flow and,
therefore, maximum conductance. Some primitive angiosperm families have slanted scalariform perforation plates.
Primary growth
Primary xylem occurs in separate vascular bundles
Primary growth refers to the primary plant body that is
formed through cell production by the apical meristems of
the plant. In most but not all monocots (monocotyledons)
and herbaceous dicots (dicotyledons), almost the entire
plant body is the product of primary growth. In woody
plants, this represents the innermost layers of xylem along
the stem, branches and roots. The xylem tissue of young,
unthickened stems and roots usually occurs in separate
primary vascular bundles along with the phloem tissue. In
dicots, the primary vascular bundles are typically arranged
in a peripheral cylinder, while in monocots, the vascular
bundles are scattered throughout the parenchymatous
ground tissue of the plant body. The primary xylem in
stems usually consists of early differentiating protoxylem,
located on the inner side of the xylem, and late
differentiating metaxylem on the outer side of the xylem.
In most dicot and gymnosperm stems, a lateral
meristem, called the vascular cambium, separates the
primary xylem and phloem of each vascular bundle. This
layer of cells develops as an extension of the procambium,
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Xylem Structure and Function
Bordered
pits
Perforation plate
Perforation
Simple pit
(e)
(d)
(c)
(b)
(a)
Figure 2 Drawing showing the relative sizes and shapes of some xylem cell types: (a) conifer tracheid with circular bordered pits, (b) fibre tracheid with
bordered pits, (c) libriform fibre with simple pitting, (d) vessel element with scalariform perforations and (e) vessel element with a simple perforation. Note
that conifer tracheids (3 to 5 mm) are usually much longer in relationship to fibres (0.8 to 2.3 mm) and vessel elements (0.2 to 1.3 mm).
Secondary wall of
adjacent cell
Middle lamella
Secondary wall
strands of meristematic cells beginning just below the
growth tip of the stem (and root). The vascular cambium
will later give rise to secondary xylem and secondary
phloem. Although some do have thickening meristems, the
vascular cambium is absent in monocots.
Primary wall
Border
Inner aperture
Torus
Margo
Secondary growth
The secondary xylem of woody plants constitutes the
major part of the stem, i.e. the wood. This section will focus
on various aspects of wood structure that are directly
related to the development and organization of secondary
xylem.
The development of secondary xylem
Secondary xylem is formed by the vascular cambium
Figure 3 Structure of a bordered pit in the secondary wall of a conifer
tracheid showing the modification of the pit membrane to a torus and
margo. Note the loose network of cellulose fibrils that forms the margo and
the secondary thickening of the central region to form the torus. In
angiosperms, the pit membranes of bordered pits are usually not modified.
All gymnosperms and woody dicots undergo secondary
growth, which results in an increase in the diameter of the
stem, branches and roots. The onset of secondary growth is
characterized by the activation of cell division in the
fascicular vascular cambium, i.e. the meristematic layer
inside the vascular bundles. These cell divisions are
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Xylem Structure and Function
co-ordinated with cell divisions in the adjacent interfascicular region to produce a continuous cylinder of vascular
cambium. Usually, secondary xylem is formed on the inner
side and secondary phloem on the outer side of the
cambium (Figure 4).
Fusiform and ray initials give rise to the axial and radial
components of xylem
these circles are true annual rings and the age of the stem
can be deduced from the number of rings. In many regions
of the world, particularly the tropics, growth rings do not
always represent annual increments. More than one (or less
than one) growth ring can be formed per year, for example
when several dry and wet periods occur within a year.
Heartwood and sapwood
The vascular cambium consists of fusiform and ray initials.
Fusiform initials divide longitudinally to give rise to the
axial components of secondary growth, i.e. tracheary
elements, fibres and axial parenchyma towards the inside
and phloem cells towards the outside of the stem. Ray
initials divide to form ray cells that run radially across the
secondary vascular tissue. Rays serve to transport water,
dissolved gases and organic nutrients radially in wood. As
secondary growth proceeds, the cambial cylinder increases
in diameter through lateral division of fusiform initials.
Earlywood, latewood and growth rings
The cambium of many woody plants exhibits periodic
activity. In the spring and early summer (in temperate
regions), conditions are conducive to active growth and
relatively wide tracheary elements with thin walls are
produced. Later in the summer and autumn, relatively
narrow tracheary elements with thick walls are formed
(Figure 4). These two types of xylem, called earlywood and
latewood, are most commonly observed as concentric
circles on the transverse section of the stem and are formed
as a result of changes in the activity of the vascular
cambium. When the activity of the vascular cambium is
controlled by annual seasons (one ring is formed per year),
Wood cells have a limited lifetime in which they can
actively transport water. After a variable number of years,
cavitation occurs in most of the vessels and tracheids and
the rest of the xylem cells in the growth ring die. These cells
are then filled with resinous materials and polyphenols,
and constitute the inner, often darker part of the woody
stem called heartwood. The outer, water-conducting part
of the stem is called sapwood. In many species, as sapwood
is converted to heartwood, air-filled vessels in the sapwood
are often sealed off by the intrusive growth of surrounding
parenchyma cells. These intrusions are called tyloses and,
together with the resinous materials, serve to prevent
fungal growth in the empty vessel lumens. The outer,
conducting part of the stem is called sapwood.
Dicot versus conifer wood
Woods are commonly classified as either hardwoods or
softwoods. Hardwoods are angiosperm (dicotyledonous)
trees, while softwoods are gymnosperm (conifer) trees.
These two terms do not accurately express differences in
the hardness or density of the wood, but are useful for the
description of the basic structural differences between dicot
and conifer wood.
Latewood vessel
Earlywood vessel
Ray
(a)
Earlywood
Pith
Primary xylem
Secondary xylem
(b)
Latewood M M C M
Cambial
zone
Phloem
Figure 4 Drawing of cross sections of young woody stems showing the cambial zone and secondary xylem development. (a) Dicot wood. (b)
Conifer wood. Note the abrupt change in the size of tracheids from earlywood to latewood. M, differentiating xylem and phloem mother cells;
C, cambial initial.
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Xylem Structure and Function
Dicot wood contains vessels, fibres, parenchyma and
tracheids
Dicot wood contains a greater number of cell types than
conifer wood and the structure of dicot wood, therefore,
varies more than that of conifer wood. The cell types of
dicot wood include vessel elements, fibres and parenchyma
cells. Tracheids are rare in dicots, but occur in some species
such as oaks and chestnuts. The cell types in dicot wood are
also more diversified in function; vessels and tracheids
transport water, fibres provide structural support and
parenchyma cells perform storage and regeneration functions. The feature that most distinguishes dicot wood from
conifer wood is the presence of large-diameter vessel
elements that disrupt the regular organization of the radial
cell files derived from the cambial initials (Figure 4a). Two
types of fibres are common in dicot wood: fibre tracheids
and libriform fibres (Figure 2b and c). Fibre tracheids have
thick walls with bordered pits. Libriform fibres have simple
pits. Dicot wood generally contains larger rays than conifer
wood and in most dicot species the rays consist only of ray
parenchyma.
Conifer wood consists mostly of regular files of tracheids
Conifer wood is relatively simple in structure. The most
distinctive features of conifer wood include: the regular
organization of the radial files of tracheids, the absence of
vessels and fibres and the small amount of wood
parenchyma (Figure 4b). The long, tapered tracheids form
the predominant cell type and fulfil both the mechanical
and conductive functions of conifer wood. The majority of
parenchyma cells in conifer wood are present in rays and, in
some conifers such as the Pinaceae, in axial and radial resin
ducts. Conifer rays consist primarily of ray parenchyma
and, in some conifers, a smaller amount of ray tracheids.
Resin ducts are large intercellular spaces surrounded by
thin-walled parenchyma cells that excrete resin into the
duct. The resin is believed to seal wounds and protect the
plant against fungi and herbivores.
Reaction wood
Woody plants respond to bending induced by external
forces, such as wind and gravity, by making reaction wood.
Conifers produce reaction wood on the side of the branch
or stem where the tissues are compressed (usually the
underside) and it is therefore called compression wood. In
dicots, reaction wood forms on the side under tension
(usually the upper side) and it is called tension wood.
Compression wood has thicker cell walls, higher lignin
content and is darker than normal conifer wood. Tension
wood is characterized by the presence of gelatinous fibres,
low lignin content and high cellulose content. The purpose
of reaction wood is to reorient bent stems and branches to
allow optimal light exposure of the tree canopy.
Secondary thickening in monocots
The majority of monocots are herbaceous, which means
that the primary xylem has to fulfil all the requirements of
water transport that the plant may encounter throughout
its lifetime. However, some monocots do undergo thickening of the primary stem. In bamboos and other monocot
species with wide stems, a broad region of mitotic activity,
called the primary thickening meristem, is responsible for
radial and tangential expansion of the primary stem. Very
few examples exist of truly woody monocots. In woody
monocot genera such as Yucca and Dracaena, the activity
of a secondary thickening meristem in the outer cortex of
the stem is responsible for anomalous secondary growth.
Arborescent monocots such as palms undergo diffuse
secondary growth through the division of cells in the
ground parenchyma of the stem.
Xylem Functions: Water Transport and
Structural Support
Water transport
A gradient of water potential drives water transport
Despite a large amount of research on this topic, the precise
mechanism of water transport in plants is still debated. The
experimental evidence strongly suggests that water transport in plants is driven by a gradient of water potential that
exists between the air surrounding the leaves at one end and
the water that surrounds the roots at the other. These two
extremes are connected by the xylem, which supports a
water column that extends from the roots to the leaves. Air
usually has a very negative water potential (even when the
humidity is very high). As the leaves of the plant lose water
to the air, the water potential becomes more negative inside
leaf cells. This causes water to gradually move from xylem
cells to leaf cells. The water molecules inside the water
columns of the capillary xylem elements are pulled
upwards by cohesion forces when water molecules at the
top of the columns move out into the aerial parts of the
plants. This is known as the cohesion–tension theory of sap
ascent.
Adhesion, cohesion and tension forces act on the water
column
The upward movement of the water column is counteracted by three forces: (1) the weight of the water column,
(2) adhesion of water to the cell walls of tracheary elements
and (3) adhesion of the water to soil particles. The upward
movement of the water molecules in each tracheary
element will cause tension in the water column, causing it
to become narrower. During times of high transpiration,
the negative pressure inside tracheary elements can become
strong enough to cause these cells to collapse inward.
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Xylem Structure and Function
Vessel elements and tracheids possess secondary thickened
walls that serve to reinforce the walls and prevent inward
collapse under the tremendous forces produced inside the
tracheary element.
Water columns can break
The ability of tracheary elements to allow movement of
water is called conductance. The conductance of a
tracheary element is related to the fourth power of the
radius of the element (known as the Hagen–Poiseuille
Law). This means that a slight increase in diameter of the
element will significantly increase the conductance. Indeed,
this seems to have been a major driving force in the
evolution of tracheary elements. However, under certain
circumstances it is beneficial to possess tracheary elements
with small diameters. In such elements, hydrogen bonding
of water molecules to the wall (adhesion force) serves to
reinforce and strengthen the water column. If the tension in
the water column becomes sufficiently strong, however, the
water column can break (cavitate) and an embolism (‘air
bubble’) will form in the element. This problem is more
serious in large-diameter elements than small-diameter
elements. In tracheids, the embolism can expand to fill the
whole cell, but the surface tension of water will prevent it
from passing through the pit membrane. In vessels,
embolisms can spread from element to element through
the perforations that link consecutive vessel elements. The
whole vessel will then become dysfunctional for water
transport.
Tracheids and vessel elements are adapted for optimal
conductance
Tracheids are generally much longer than vessel elements.
This reduces the number of pit membranes that a water
molecule has to cross on its way to the leaves. Tracheids
also have long-tapered ends to allow the maximum number
of pit-pairs between consecutive cells. Vessel elements are
much shorter than tracheids, but they are connected endto-end to form long vessels. Gymnosperm tracheids tend to
be wider than those of angiosperms, where most of the
water transport occurs through large-diameter vessel
elements. Angiosperms combine the structural and
water-conducting benefits of small-diameter and largediameter tracheary elements. Most of the water volume is
transported by large-diameter vessels when water is readily
available, while small-diameter vessels and tracheids (in
some dicots) are used when the water column is under great
tension and greater protection against cavitation is
required.
Structural support
The aerial parts of all terrestrial plants require mechanical
support. This is provided in large part by xylem tissue in the
stem and branches. The mechanical support function of
xylem is most prominent in the stems of trees, which
include some of the largest living organisms on earth.
Cell walls form the basic unit of structural support
The basic unit of structural support in plants is the
mechanical support provided by the cell wall of each cell in
the plant body. Cell walls consist mostly of cellulose
microfibrils. Cellulose fibrils can be very strong; stronger
than steel, silk or nylon. This makes cell walls strong
enough to resist internal forces (turgor) as well as
externally applied forces (tension). Additional rigidity
and compressive strength is provided by lignin, especially
in tissues (such as xylem) that accumulate lignin.
Xylem contains several cell types with structural support
functions
Fibres provide most of the mechanical support in dicot
xylem. The structure of the fibre walls allows this cell type
to support weights of up to 15–20 kg mm 2 2. More
importantly, fibres are elastic enough to retain their
original length after subjection to tension forces of this
magnitude. Vessels and tracheids also contain secondary
thickened walls and therefore contribute to structural
support in xylem. In conifer wood, all the structural
support is provided by tracheids.
Wood is a complex material
The woody stems of large trees provide the most
spectacular examples of structural support in plants.
Wood in living tree stems is structurally complex, with
several levels of organization. At the molecular level, wood
is comprised of crystalline cellulose embedded in a matrix
of hemicellulose and lignin, a highly crosslinked phenolic
polymer. The cellulose fibrils in the secondary wall are
deposited in layers, each layer with a different preferred
microfibril angle (Figure 1). At the cellular level, the xylem
cells in wood are arranged in cylinders parallel to the long
axis of the stem. Finally, above the cellular level, the
growth rings form concentric layers of wood tissue with
different wall and lumen dimensions. This makes wood a
layered structural composite, which is much more complex
than reinforced concrete.
Xylem Differentiation and Cell Wall
Biosynthesis
The developmental process in which procambial and
cambial initials differentiate into mature xylem cells is
called xylogenesis. This process can be as short as four days
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Xylem Structure and Function
in primary xylem and from 14–21 days in secondary xylem.
Xylogenesis typically includes the following phases: (1) cell
division and enlargement, (2) cell wall thickening, (3)
lignification and (4) programmed cell death. Cell wall
biosynthesis is an integral part of xylem formation. The
basic chemical components and organization of xylem cell
walls are known, but little is known of the mechanisms by
which they are synthesized and organized to form the
highly complex cell wall. This section will outline the
phases of xylem differentiation and the formation of xylem
cell walls, which form the major part of mature xylem cells.
Xylem cells are derived from apical and lateral
meristems
Tracheary elements are derived from either the procambium (primary xylem) or vascular cambium (secondary
xylem). The differentiation of cambial initials into xylem
elements is thought to be initiated by plant hormones. The
immature xylem cells have dense protoplasm, small
vacuoles and thin primary walls (Figure 4). Soon after cell
division, these cells undergo cell elongation and an increase
in the size of the vacuole and nuclei.
Most xylem cell walls undergo secondary
thickening
The deposition of secondary walls begins sometime before
tracheary elements and fibres reach their full size. The
cellulose, lignin, hemicellulose and protein components of
the secondary wall are synthesized and deposited cooperatively during secondary wall thickening. The onset of
secondary wall thickening is associated with the formation
of arrays of microtubules under those regions of the plasma
membrane where active secondary wall deposition will
take place. Microtubules may play a role in defining the
pattern of secondary walls by guiding dictyosome-derived
vesicles with cell wall material to the sites of deposition on
the cell membrane. Cellulose microfibrils are produced at
the membrane surface of the cell by complex rosette
structures, which consist of several different proteins. The
movement of these rosette complexes in the plasma
membrane may also be directed by microtubules.
The cell walls and intercellular regions of
xylem cells are lignified
Following secondary thickening of the xylem cell walls,
lignin is deposited between the newly formed tracheary
elements and within their walls. The area between the cells,
called the middle lamella, and the primary walls are rapidly
lignified, followed by a more gradual lignification of the
secondary walls. Lignin is a very complex, crosslinked,
three-dimensional polymer of aromatic phenolic monomers, called cinnamyl alcohols. The lignin monomers are
delivered to the cell wall via Golgi and endoplasmic
reticulum-derived vesicles and polymerized into lignin by
wall-bound enzymes. The aromatic nature of the lignin
monomers makes lignin hydrophobic. Lignin, therefore,
provides a hydrophobic inner surface to the cell wall that
facilitates water transport. The three-dimensional nature
of the lignin polymer provides rigidity and compressive
strength to the cell wall, while the chemical stability of
lignin provides protection against pathogens.
Tracheary elements undergo programmed
cell death
At the completion of secondary wall deposition and
lignification, tracheary elements undergo autolysis, an
example of programmed cell death in higher plants. Soon
after the initiation of secondary thickening, hydrolytic
enzymes (DNAases, RNAases and proteases) start accumulating in the vacuole. The autolytic process is initiated
when the tonoplast ruptures, causing the hydrolytic
enzymes to spill out into the cytoplasm. This leads to the
complete degradation of the cell contents and partial
digestion of the unprotected regions of the primary wall.
Only regions covered by lignified secondary wall material
are protected from degradation. The end walls of
differentiating vessel elements are degraded at the perforation sites to allow direct cell-to-cell movement of water and
nutrients. Only regions covered by lignified secondary wall
material are protected from degradation. Pit membranes
are often partially degraded to leave mats of cellulose fibrils
(Figure 3). This enhances the movement of water through
pit-pairs, which is the only way water can enter and leave
tracheids.
Origin and Evolution of Xylem in Plants
Vascular plants (Tracheophyta) are characterized by the
presence of xylem tissue with lignified cell walls. Modern
vascular plants are ferns, gymnosperms and angiosperms.
Mosses, liverworts and hornworts (Bryophyta) do not
contain xylem. Tracheid-like cells, called hydroids, are
present in certain bryophytes, but lignified cell wall
thickenings are absent in these plants. This section will
outline the major trends of xylem evolution in vascular
plants.
Evolution of primary xylem
Tracheids were present in the first vascular land plants
It is widely accepted that the first land plants evolved from
green algae (Chlorophyta) and that these plants were
adapted to aquatic or semiaquatic environments. The
evolution of conducting tissue was closely associated with
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Xylem Structure and Function
the adaptation of plants to fully terrestrial environments.
The acquisition of xylem tissue allowed plants to supply
water and mineral nutrients to those parts of the plant body
exposed to the desiccating environment of the air. One of
the earliest known fossilized land plants, Cooksonia
(present as early as 420 million years ago in the MidSilurian Period), had tracheids with annular secondary
thickenings.
Vessel elements and fibre tracheids evolved from
tracheids
Vessel elements evolved independently from tracheids in
several groups of flowering plants, i.e. the conversion of
tracheid end walls to perforation plates had a polypheletic
origin. Fossil evidence of the early evolving vessel elements
is very scarce. It is assumed that vessel elements evolved
from tracheids with scalariformly reinforced walls and that
these cells gave rise to the short vessel elements with
transverse, simple perforation plates and wide lumens.
Fibres also evolved independently in many angiosperm
families. Fibre tracheids evolved very early in angiosperm
history, while libriform fibres with simple pits appeared
later.
Evolution of secondary xylem
Woody plants appeared early in the history of land plants
The ability to produce secondary vascular tissues evolved
soon after the appearance of the first vascular land plants.
Bifacial cambium was present in the Progymnospermopsida in the Devonian Period (approximately 370 million
years ago). It is still highly debatable whether the first
angiosperms that evolved from the progymnosperms were
woody or herbaceous plants. It appears however that most
present day herbaceous angiosperms are able to form
secondary tissues, although most usually flower and die
early, precluding much secondary growth.
Secondary xylem increased the lifespan of plants
The ability to produce secondary xylem had profound
consequences for early vascular plants. It greatly increased
the lifespan of plants by allowing plants to essentially form
a new water-conducting system each year that replaced the
non-functional xylem elements from previous years. The
increase in lifespan enabled the existence of taller plants
and increased the need for long-distance conductance and
mechanical support. The major trends of xylem evolution
(the shift towards vessel elements, simple perforations and
libriform fibres) are thought to be associated with the
increased efficiency of water transport in xylem and, to a
lesser degree, the increased demand for mechanical
support in plants.
8
Genetic Manipulation of Xylem
Formation
The content and composition of xylem cell walls affect the
commercial value of many biological materials, such as
wood and plant fibres, as well as many food crops, such as
fodder, cereals, fruits and vegetables. The potential to
improve the properties of these plants has motivated
studies dedicated to the modification of xylem cell walls.
Xylem properties are specified by a large
number of genes and proteins
The properties of wood and the xylem in herbaceous plants
result from the content, composition and location of xylem
cells and their walls. Except for the wall, tracheary elements
retain little or no material of the living cells from which
they are derived. The composition and structure of xylem
cell walls are determined by the coordinated expression of a
large number of genes and proteins during xylogenesis.
Variation in the developmental programme and levels of
expression of individual genes determine the variation in
cell wall architecture within and between different species.
Therefore, knowledge of the genes involved in this process
and the mechanisms by which they are controlled could
lead to the ability to manipulate the properties of xylem.
Although the general composition and structure of
xylem cell walls is known, very little is known of the
organization and biosynthesis of cell wall components.
Xylem cell walls contain hundreds of proteins and enzymes
involved in the formation of the primary cell wall, which
provide the framework for the synthesis of the secondary
wall. Biosynthesis of the secondary wall involves precisely
regulated formation of cellulose microfibrils, assembly of
hemicellulose–cellulose complexes and polymerization of
a network of the phenolic polymer lignin. Work is
progressing rapidly to identify important genes and
proteins in these processes, but only a few genes have been
studied sufficiently to establish their specific roles.
New technologies allow rapid progress in the
genetic manipulation of xylem
Studies of model plant systems, such as Arabidopsis,
Zinnia, tobacco and maize have been important in
identifying specific genes and proteins involved in cell wall
formation. Genetic and biochemical studies of cotton and
forest trees have identified some important genes for the
formation of cellulose and lignin. Most recently, many
laboratories have decided to use high-throughput automated techniques to identify all of the expressed genes of
higher plants and to learn their function. This approach,
called genomics, is expected to rapidly advance the
knowledge of the genes and proteins forming the primary
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
Xylem Structure and Function
and secondary cell walls of xylem. With this knowledge, the
modification of xylem in commercially important plants
will become a process of rational design.
Further Reading
Boudet AM, Lapierre C and Grima-Pettenati J (1995) Tansley Review
no 80: Biochemistry and molecular biology of lignification. New
Phytologist 129: 203–236.
Carlquist JS (1975) Ecological Strategies of Xylem Evolution. Berkeley,
CA: University of California Press.
Delmer DP and Amor Y (1995) Cellulose biosynthesis. Plant Cell 7: 987–
1000.
Fahn A (1990) Plant Anatomy, 4th ed. New York: Pergamon Press.
Fukuda H (1996) Xylogenesis: initiation, progression and cell death.
Annual Review of Plant Physiology and Molecular Biology 47: 299–325.
Higuchi T (1997) Biochemistry and Molecular Biology of Wood. Berlin:
Springer-Verlag.
Ingrouille M (1992) Diversity and Evolution of Land Plants. London:
Chapman & Hall.
Mauseth JD (1988) Plant Anatomy. Menlo Park, CA: Benjamin/
Cummings.
Whetten RW, MacKay JJ and Sederoff RR (1998) Recent advances in
understanding lignin biosynthesis. Annual Review of Plant Physiology
and Plant Molecular Biology 49: 585–609.
Zimmermann MH (1983) Xylem Structure and the Ascent of Sap. Berlin:
Springer-Verlag.
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
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