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Plant Cell Differentiation
Secondary article
Article Contents
Martin Hülskamp, University of Tübingen, Tübingen, Germany
Hilmar Ilgenfritz, University of Tübingen, Tübingen, Germany
. Differentiated Cell Types
. Epidermis
During plant cell development cells become specialized for a particular function. This
process is called plant cell differentiation.
. Trichomes
. Root Hairs
. Xylem
. Phloem
Differentiated Cell Types
During plant development cells adopt a cell fate according
to their developmental context. Subsequently, cells begin
to differentiate and acquire distinct morphological, biochemical and physiological properties. Although many
differentiated plant cells can dedifferentiate and may
regenerate into whole plants under appropriate conditions,
some cell types are less flexible or unable to change cell fate.
These cells exit mitosis and start endoreduplication, which
usually locks the cells in a nonmitotic state. Other cells
undergo programmed cell death as part of their differentiation programme. To illustrate different aspects of
plant cell differentiation, a few well-studied cell types are
described with emphasis on the experimental approaches
used. In addition, the role of the cytoskeleton and the cell
wall in cell differentiation is discussed.
Epidermis
The plant epidermis is the single-celled outermost layer. It
is established early and is maintained separate from
underlying tissue types throughout development. It
protects the plant against insects, the invasion of microorganisms and environmental conditions. The epidermis is
important for the control of gas exchange (aerial part) and
nutrient uptake (root). These different functions are
accommodated for by the division of labour between cells.
In aerial tissues three cell types are formed: stomata,
trichomes and epidermal pavement cells. Stomata control
the gas exchange through pores formed by guard cell pairs.
Trichomes are hairs that are thought to serve to protect the
plant against insects, intensive light and dehydration.
Epidermal pavement cells represent the majority of the
epidermal cells and are characterized by a thick apical wall
that secretes an impermeable wax cuticle. In Arabidopsis it
has been shown that the shape of pavement cells is
regulated by several genes. During wild-type development
of leaves in Arabidopsis, epidermal cells undergo extensive
changes in cell form from initially regular round or
elongated cells to puzzle-like pavement cells. In mutants
affecting leaf shape, such as angustifolia (narrow leaves) or
rotundifolia (wide leaves), the phenotype is expressed at the
. Cell Shaping and the Cytoskeleton
. Cell Walls and Differentiation
cellular level such that the average length–width ratio of
individual cells is altered.
Trichomes
Trichomes in higher plants show a wide range of
morphological features. They may be unbranched or
stellate, unicellular or multicellular, and often plant hairs
differentiate into specialized secreting cells. In Arabidopsis,
trichomes are single large cells with a DNA content that is
16-fold increased as compared to normal diploid cells. The
first indication of trichome differentiation is an increase in
nuclear DNA content and cell size. The incipient trichome
cell extends out of the epidermal surface and initiates two
successive branching events. While the morphological
description does not allow us to draw conclusions about
the underlying mechanisms, a genetic analysis has enabled
the dissection of trichome development into five processes
(Hülskamp et al., 1998): (1) regulation of endoreduplication, (2) local outgrowth, (3) branching, (4) extension
growth and (5) wall maturation (Figure 1a).
1. The initial switch from mitosis to endoreduplication
and the first three endoreduplication cycles are an
immediate consequence of trichome initiation and are
probably regulated by two genes involved in trichome
patterning: GLABRA1 (GL1), which encodes a mybrelated transcription factor, and TRIPTYCHON
(TRY). Further endoreduplication cycles require
GLABRA3 (GL3) function. The total number of
endoreduplication cycles is negatively regulated by
TRY and three KAK genes. The KAK genes appear to
represent components of a gibberellic acid-dependent
pathway indicating that the number of endoreduplication cycles is controlled by plant hormones.
2. Local outgrowth requires the function of the GLABRA2 (GL2) gene, which encodes a homeodomain
transcription factor. In gl2 mutants trichomes frequently do not extend out of the epidermal surface and
remain flat.
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Plant Cell Differentiation
(a) Trichomes
(b) Root hairs
Endoreduplication
Local outgrowth
GL1, TRY
GL2
Primary branching
STA, STI
Endoreduplication
Secondary branching
Extension growth
Cell wall maturation
GL3
KAK, RFI, PYC, TRY
AN, STI
NOK
Distorted genes
UDT, TBR, CHA,
CDO, RTS
(c) Xylem
Stage I
Site
selection
RHD6
Local
outgrowth
RHD1
Tip growth
RHD2
Growth
direction
TIP1, RHD3,
RHD4
(d) Phloem
Precursor
cell
Dedifferentiation
Asymmetric
cell division
TED expression
Stage II
Accumulation of ER,
microtubules,
mitochondria, etc.
PC=phloem cell
CC=companion cell
Callose
deposition
Branched
plasmodesmata
Rupture of tonoplast
PC CC
Mature
sieve pores
Loss of cell contents
Organelle
lysis
Cell wall rigidification
PC CC
Figure 1 Schematic drawings of cell differentiation in selected cell types.
(a) Trichome cell differentiation, (b) root hair cell differentiation, (c) xylem
cell differentiation, (d) phloem cell differentiation. Abbreviations: KAK,
KAKTUS; RFI, RASTAFARI; PYC, POLYCHOME; STI, STICHEL; NOK, NOECK;
UDT, UNDER DEVELOPED TRICHOME; TBR, TRICHOME BIREFRINGENCE;
CHA, CHABLIS; CRD, CHARDONNAY; RTS, RETSINA
3. Trichome branching involves two qualitatively different branching events that depend on STACHEL
(STA) activity and ANGUSTIFOLIA (AN) functions,
respectively. Additional genes are involved in the
regulation of both branching events, for example the
ZWICHEL (ZWI) gene is required as a general factor
for branching and growth. ZWI encodes a member of
the kinesin-like family of microtubule motor proteins
that contain a calmodulin-binding site. This indicates
that the directional transport of cellular components
to spatially defined regions is important for branching.
Branch number strongly correlates with the DNA
2
The five processes generally reflect the temporal sequence
of morphological changes, implying that each step is a
prerequisite for the next to be initiated. The genetic
analysis of the various mutants reveals that this is the case
for some processes, e.g. local outgrowth is necessary for all
subsequent events. However, other processes take place
independently of each other, e.g. extension growth is
independent of branching.
PC CC
Secondary cell wall
formation
Stage III
content such that trichomes with a reduced DNA
content (as in gl3 mutants) produce fewer branches
and trichomes exhibiting an increased DNA content
(e.g in kak mutants) produce more branches. This
suggests that branch initiation is regulated by cell size
or cell growth. In addition, branching is regulated by
genes that act independently of the DNA content.
4. After branching is initiated, the trichome cell undergoes extension growth. Directionality of the extension
growth requires the activity of eight genes of the
DISTORTED group. Mutations in these genes cause a
very similar phenotype: mature trichomes display
irregular and twisted growth.
5. During trichome maturation the outer cell wall
thickens and is strengthened by encrustation. To date,
five genes are known to be involved in this process.
Root Hairs
Root hairs are single elongated epidermal cells that
produce a tubular projection perpendicular to the cell
axis. In Arabidopsis, the analysis of mutants affecting root
hair differentiation revealed distinct regulatory steps
(Aeschbacher et al., 1994) (Figure 1b). Root hairs develop
from trichoblasts that are shorter and less vacuolated than
atrichoblasts. Initiation of root hair differentiation requires GL2. Further outgrowth involves two processes: site
selection and elongation. Root hairs grow out at the basal
end (relative to the root axis) of the root hair cell. In ROOT
HAIR DEFECTIVE6 (RHD6) mutants the site of emergence is frequently shifted to apical positions. Site selection
also involves the two plant hormones auxin and ethylene.
After site selection the cell starts to grow out locally. In
rhd1 mutants the initial growth is not locally restricted,
although subsequent root hair elongation is normal. Root
hairs elongate by tip growth. This is characterized by a tiphigh gradient of calcium and the polarized secretion of
plasma membrane and cell wall material into the growing
tip. Four genes are known to be required during root hair
elongation: RHD2, TIP1, RHD3 and RHD4. RHD2 is
required for tip growth. tip1 mutants display branched
root hairs, suggesting that TIP1 is required to maintain
stable tip growth. RHD3 and RHD4 are required for the
directionality of tip growth based on the phenotypes of
rhd3 and rhd4 mutants in which root hairs have an
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Plant Cell Differentiation
abnormal, corkscrew-like appearance. In rhd3 mutants it
has been shown that irregular tip growth is correlated with
an asymmetric distribution of vesicles. RHD3 encodes a
member of a new class of GTP-binding proteins and thus
represents the first identified component of a molecular
pathway for localized secretion during root tip growth.
Xylem
the companion cell become branched on the companion
cell side. Plasmodesmata between neighbouring phloem
cells differentiate to sieve pores. This process involves the
deposition of callose (a complex polymer of glucose)
around the plasmodesmata, which is thought to replace the
cellulose. Hydrolysis of callose in later stages results in the
formation of wide pores. Later differentiation stages of the
phloem include the breakdown of the nucleus, the ER and
organelles. The plasmalemma, however, remains intact
and becomes covered by an elaborate system of membranes known as the sieve element reticulum.
Xylem cells are highly specialized cells of the vascular
system required for the regulated transport of water and
nutrients. They develop from procambial or cambial cells
(i.e cells of the secondary meristem surrounding the
vascular tissue) or can be induced to form from parenchymal cells by wound stress. Transdifferentiation from
parenchymal cells into xylem cells can also be induced in
vitro in Zinnia cell cultures by the appropriate combination
of plant hormones. This has enabled three differentiation
stages to be defined (Fukuda, 1997) (Figure 1c). During the
initial stage, parenchymal cells dedifferentiate without
undergoing cell division, differentiating into a xylem
precursor cell. These events resemble wound-induced
xylem differentiation and involve the expression of
wound-induced genes. The second stage is defined by three
genes (TED2, TED3, TED4) expressed prior to secondary
wall thickening. This phase is characterized by a general
increase of transcriptional and physiological activities
including an increase of the levels of the endoplasmic
reticulum (ER), vesicle formation, mitochondria and
tubulin. During the third stage, xylem cells form secondary
cell walls and enter programmed cell death. The spatially
ordered deposition of secondary cell walls depends on the
coordinated organization of actin and microtubules which
are thought to guide the movement of cellulose-synthesizing complexes in the plasma membrane. Rigidification of
secondary cell walls involves several proteins that are
specifically expressed during stage III including extensinlike proteins, arabinogalactan proteins, glycoproteins and
enzymes involved in lignin biosynthesis. In late stage III,
programmed cell death of the xylem cell occurs: the nucleus
degenerates, organelles disappear, and the Golgi apparatus and the ER ruptures, resulting in a mature tracheary
element without any cell content.
The cytoskeleton is important for the spatial organization
of intracellular processes and hence plays a crucial role in
the regulation of cell shape, which is best understood for
two different growth modes: tip growth and cell expansion.
The role of actin microfilaments in targeted transport
during tip growth, for example in pollen tubes, is evident
from the application of inhibitor drugs. Cytochalasin, an
inhibitor of actin-based transport processes, abolishes
pollen tube growth. By contrast, colchicine, an inhibitor of
microtubule-based transport processes, does not inhibit
pollen tube growth.
Cell expansion appears to be spatially controlled by the
arrangement of microtubules. This is supported by a strong
correlation between the orientation of cortical microtubules and growth direction. This is particularly evident
when the directionality of growth, longitudinal versus
radial expansion, is experimentally manipulated by the
application of the plant hormone ethylene. In pea stem
segments treated with ethylene reorientation of microtubules from a transverse to longitudinal orientation
precedes the transition from longitudinal to radial growth.
In other cell types this link has been further established by
demonstrating that cellulose microfibrils coalign with the
cortical microtubules. This suggests that enzymes involved
in the synthesis of extracellular microfibrils are targeted by
a microtubule-based system. Genetic evidence comes from
the analysis of ton1 and ton2 mutants in Arabidopsis. In
these mutants, irregular cell expansion is correlated with
disorganized interphase cortical microtubules.
Phloem
Cell Walls and Differentiation
The phloem consists of a series of connected sieve cells that
form a syncytium and functions to transport assimilate
molecules in the plant. Phloem cell differentiation starts
with an unequal division that gives rise to a cambial initial
cell and a companion cell (Figure 1d). During early
differentiation stages, plasmodesmata undergo marked
changes. Plasmodesmata connecting the phloem cell with
Cell morphogenesis involves the spatially controlled
remodelling of the cell wall (Roberts, 1994). Cell expansion
is thought to be regulated by a balance between turgor
(providing the drive for expansion) and localized wall
loosening, i.e. the loosening of existing cellulose/pectic
polysaccharide network and synthesis and intercalation of
new wall material. Two classes of proteins appear to be
Cell Shaping and the Cytoskeleton
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Plant Cell Differentiation
involved in this process. Xyloglucan-endotransglycosylases cleave the glycan backbone and transfer the cut end to
an acceptor molecule, thus enabling controlled rearrangements of the cellulose network. In contrast, expansins
appear to act nonenzymatically by loosening hydrogen
bonds between cellulose/hemicellulose fibres.
The extracellular matrix is not only subject to regulation
as part of the differentiation process, but also regulates cell
differentiation itself. This is most evident from experiments
with the brown alga Fucus. During embryo development
the first division gives rise to a thallus and a rhizoid cell. If
the rhizoid cell is destroyed, the fate of the dividing thallus
cell can be redirected to a rhizoid cell fate by contact to the
remnants of the former rhizoid cell wall. A second example
is provided by a particular class of extracellular matrix
4
proteins, the arabinogalactan proteins (AGPs). It has been
shown that secreted AGPs from embryogenic tissues of
carrots trigger somatic embryogenesis in tissue culture.
References
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molecular basis of root development. Annual Review of Plant
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Fukuda H (1997) Tracheary element differentiation. Plant Cell 9: 1147–
1156.
Hülskamp M, Folkers U and Grini P (1998) Cell morphogenesis in
Arabidopsis. BioEssays 20: 20–29.
Roberts K (1994) The plant extracellular matrix: in a new expansive
mood. Current Opinion in Cell Biology 6: 688–694.
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net