Download Root Hairs as a Model System for Studying Plant

Annals of Botany 88: 1±7, 2001
doi:10.1006/anbo.2001.1430, available online at http://www.idealibrary.com on
B OTA N I CA L B R I E F I N G
Root Hairs as a Model System for Studying Plant Cell Growth
J U L I A FO R E M A N and L I A M D O L A N *
Department of Cell and Developmental Biology, John Innes Centre, Norwich, NR4 7UH, United Kingdom
Received: 10 January 2001 Returned for revision: 16 February 2001
Accepted: 20 March 2001
Root hairs are tip-growing projections that form on specialized epidermal cells. Physiological studies are identifying
key transporters required for hair growth, and drug studies have been instructive in de®ning the role of the cytoskeleton in cell morphogenesis. Genetic analysis is identifying proteins involved in cell growth and the phenotypes of
the mutants are instructive in de®ning the precise function of these proteins in cellular morphogenesis. Recent
progress in our understandings of cell growth using the arabidopsis root hair as a model system is reviewed.
# 2001 Annals of Botany Company
Key words: Arabidopsis, root hair, trichoblast, actin, microtubules, cell wall, genetics, calcium, potassium,
phosphorus.
I N T RO D U C T I O N
Root hairs are tip-growing tubular-shaped outgrowths that
emerge from the basal end of specialized cells called
trichoblasts (Dolan et al., 1994; Fig. 1). They are responsible
for anchorage of the root and the uptake of water and
nutrients. During the formation of Rhizobium symbioses
they are the site of interaction between the bacterium and the
legume host (Oldroyd, 2001). Arabidopsis is well established
as a model system for plant biology, and the recent
publication of its entire genome (Kaul et al., 2000) is
accelerating molecular-based research into this organism.
The structure of the arabidopsis root is simple and invariant,
and provides a useful model for studying plant development.
The epidermal layer is the outermost layer of the mature root
and consists of two cell types. Cells that overlie the anticlinal
walls between adjacent cortical cells di€erentiate into
trichoblasts (root hair producing cells). Cells that overlie
the periclinal walls di€erentiate into atrichoblasts (non-root
hair producing cells) (Dolan et al., 1993). Laser abalation
experiments have shown that it is positional information,
not cell lineage, that de®nes cell fate (Berger et al., 1998).
The root tip consists of three distinct organizational
zones (Fig. 2). The meristematic zone contains the initial
cells and dividing cells of the root. Trichoblasts are
morphologically distinct from atrichoblasts in this early
developmental stage. Proximal to this zone is the elongation
zoneÐit is at this stage that root hairs initiate. The next
zone is the di€erentiation zone in which elongated cells
mature into fully di€erentiated cells and where root hairs
grow and reach maturity (Dolan et al., 1993). Observations
using time-lapse video microscopy, cryo-SEM and light
microscopy have identi®ed three structural phases of hair
development. The ®rst stage is the appearance of a bulge at
* For correspondence. Fax ‡44 (0) 1603 450022, e-mail liam.
[email protected]
0305-7364/01/070001+07 $35.00/00
F I G . 1. Cryo-SEM image of an arabidopsis root hair. Tip-growing
tubular-shaped outgrowths are produced from the basal end of
trichoblast cells.
the basal end of the trichoblast. During the second stage,
slow tip growth occurs from a selected site on the bulge.
The third stage, which begins when the hair is between 20
and 40 mm in length, involves an increase in the rate of hair
growth to between 1 and 2 mm per minute (Dolan et al.,
1994).
The intracellular zonation of arabidopsis root hairs is
characteristic of tip-growing cells. The cytoplasm at the
extreme tip of the hair contains a large number of secretory
vesicles containing cell wall components. A variety of
vesicle types, including coated vesicles and pleiomorphic
vesicles, are present at the tip, showing that this region of
the cell is active in membrane tracking and exocytosis.
Below this is an organelle-rich region containing large
# 2001 Annals of Botany Company
2
Foreman and DolanÐArabidopsis Root Hairs
their membranes are incorporated into the plasma
membrane (Galway et al., 1997). Treatment of root hairs
with the cellulose synthase inhibitor, 2,6-dichlorobenzonitrile (DCB), results in cell rupture at the weakest
point of the cell, the tip. It is probable that DCB treatment
results in the inhibition of cell wall synthesis at the tip while
the protoplast of the cell continues to grow.
To identify mutations in genes encoding proteins
required for cell wall synthesis, mutants with phenotypes
resembling the DCB-treated roots were identi®ed. kojak
(kjk) mutants initiate root hairs that rupture soon after
initiation (Fig. 3G±I). KJK encodes a member of the D
subfamily of cellulose synthase-like (CSL) proteins
(AtCSLD3). The KJK/AtCSLD3 protein is similar in size
to several other plant cellulose synthase catalytic subunit
proteins (CESA). The four highly conserved sub-domains
of the plant CESA and the arabidopsis CSLD proteins that
characterize the b-glycosyl transferases are present in KJK/
AtCSLD3. Consistent with this view is the ®nding that the
KJK protein is located on the endoplasmic reticulum. This
suggests that it is required for the synthesis of non-cellulosic
wall polysaccharides. The Kjk ÿ phenotype and the pattern
of expression suggest that KJK acts early in the process of
root hair growth. This is the ®rst member of the AtCSLD
subfamily of proteins to be shown to have a function in cell
growth (Favery et al., 2001).
I O N I C R E G UL AT I O N O F RO OT H A I R
G ROW T H
F I G . 2. Cryo-SEM image of the elongation and di€erentiation zones of
an arabidopsis root. Root hairs begin to emerge at the proximal end of
the elongation zone of the root. Tip growth and cell di€erentiation
occur in the di€erentiation zone.
numbers of Golgi bodies and mitochondria, as well as some
vesicular bodies. These cell structures are involved in the
biosynthesis and transport of macromolecules. The area
between the apical and sub-apical regions contains smooth
and ribosome-coated endoplasmic reticulum. The vacuole
occupies the basal region of the hair, and grows in concert
with the expansion of the cell. Once growth has ceased, the
vacuole extends to the hair tip (Galway et al., 1997).
T H E C E L L WA L L
An integral aspect of plant cell growth is the expansion of
the cell wall in concert with the protoplast. The cell wall is a
complex network of cellulose, hemicelluloses, pectins and
structural proteins. During cell growth, some cell wall
components, such as cellulose, are synthesized at the
plasma membrane whilst others, such as pectin and
xyloglucan, are synthesized in the endomembrane system
and transported to the cell surface in vesicles (Favery et al.,
2001). At the very tip of the hair, exocytosis of vesicles
containing cell wall material causes the release of their
contents into the matrix of the developing cell wall and
It is well established that an internal hydrostatic pressure
(turgor) is required for cellular expansion (Cosgrove, 1986).
The plasma membrane H ‡ -ATPase has been shown to play
a key role in the generation of turgor in root hairs by
generating an electrochemical H ‡ gradient that drives the
uptake of ions (Lew, 1991). There is also evidence that the
H ‡ -ATPase causes acidi®cation of the cell wall, leading to
cell wall loosening and thus allowing growth to occur (Taiz,
1984). In arabidopsis root hairs, it has been shown that a
local acidi®cation of the cell wall at the site of the initial
bulge is required for root hair initiation. The acidi®cation of
the cell wall coincides with a localized cytoplasmic
alkalization. This suggests that the acidi®cation of the
wall is due to the movement of protons out of the cell and
into the wall at the hair tip. However, this alkalization per
se is not required for growth since dissipation of the
alkaline region by treatment with weak acids such as
propionic acid or butyric acid does not inhibit root hair
initiation (Bibikova et al., 1998). This suggests that
alkalization is merely the result of local depletion of
protons due to the activity of the H ‡ -ATPase.
Potassium
Potassium is the major osmotically active cation in most
plant cells. A net inward potassium current has been shown
to exist in growing root hairs that is sucient to drive
cellular expansion of the root hair (Lew, 1991). However, it
has long been established that two K ‡ uptake mechanisms
with distinct kinetic parameters reside in parallel on the
Foreman and DolanÐArabidopsis Root Hairs
3
F I G . 3. Morphology of root hair mutants. A±I. Stereomicroscope images of arabidopsis root hair mutants. Bars ˆ 100 mm. H±I, Cryo-SEM
images of kjk mutants. A, rhd1 mutantÐhairs have a wide bulbous swelling at the base. B, rhd2 mutantÐroot hair initiation occurs but tip
growth is arrested, causing a stubby hair phenotype. C, rhd6 mutants have fewer root hairs than the wild-type. D, Wild-type. E, trh1 mutantÐ
root hairs do not elongate causing a short hair phenotype. F, tip1 mutantÐhairs are initiated from a large bulge on the epidermal cell, from which
one to four branches emerge, some of which undergo further branching. G, kjk mutantÐroot hairs initiate but rupture causing the trichoblast
cells to die. H, Scanning electron micrograph of kjk root hairs soon after rupture. I, SEM of a kjk root hair after rupture.
plasma membrane of roots (Maathuis and Sanders, 1993).
One of these is a low anity pathway that has the
characteristics of a channel-mediated transporter. The
other is a high anity pathway that moves K ‡ into the
cytosol against its electrochemical gradient. Studies into the
mechanism of the high anity transport system by patchclamping root protoplasts suggest that it is independent of
ATP and imply an H ‡ -coupled K ‡ transport with a ratio of
1 H ‡ : 1 K ‡ (Maathuis and Sanders, 1994). Furthermore,
there is molecular evidence for at least six potassium
transport systems in the arabidopsis root, AtKT, AKT1,
SKOR1, AtKUP1, AtKT2 and AtHKT1 (Debrosses et al.,
2001).
Mutants with defects in potassium transport are providing insight into the role of potassium during root hair
elongation. tiny root hair (trh1) mutants form short root
hairs and frequently initiate more than one root hair per
trichoblast (Fig. 3E). trh1 encodes a member of the AtKT
(Arabidopsis thailana K ‡ transport) family of potassium
carriers, previously designated AtKUP4 or AtKT2. The
trh1 phenotype cannot be suppressed by high external KCl
concentrations indicating an absolute requirement for
TRH1/AtKT2 in root hair growth. Another potassium
channel present in the arabidopsis root is the inward
rectifying potassium channel AKT1. akt1 mutants develop
longer root hairs than the wild-type when grown in the
absence of external potassium, but develop shorter root
hairs than the wild-type when grown in potassium
concentrations above 10 mM (Debrosses et al., 2001). This
suggests that TRH1/AtKT2 and AKT1 have di€erent
functions in the growing hair cell.
Calcium
Calcium in¯ux through plasma membrane calcium
channels is required for normal root-hair tip growth
(Schiefelbein et al., 1992). Patch-clamping studies of root
hair apices have recently identi®ed hyperpolarizationactivated calcium channels that are most active at the
apex of root hairs. It is suggested that these channels are
involved in the apical in¯ux of growing root hairs (VeÂry and
Davies, 2000). A localized increase in the cytoplasmic
calcium concentration, [Ca2‡ ]c , is formed as soon as hair
growth initiates. root hair defective2 (rhd2) mutants have
short root hairs (Fig. 3B) and lack the localized elevation in
[Ca2‡ ]c at the hair tip (Wymer et al., 1997). This suggests
that RHD2 activity is required either directly or indirectly
to transport Ca2‡ from outside the cell into the cytoplasm
at the hair tip. Evidence suggests Ca2‡ is not the primary
factor in determining the direction of elongation, since
arti®cially created [Ca2‡ ]c gradients and mechanical
4
Foreman and DolanÐArabidopsis Root Hairs
obstacles only cause transient re-orientation of growth
(Bibikova et al., 1997).
T H E C Y TO S K E L E TO N
The plant cytoskeleton consists of two components, actin
micro®laments and microtubules. Together they form a
dynamic cytoplasmic network that is involved with many
aspects of cell growth and polarity. The cytoskeleton
stabilizes structural features and is involved with cellular
morphogenesis through controlling the directionality of
growth.
Axial bundles of ®lamentous actin are present in the root
hairs of most plant species. The bundles are present
throughout the hair and penetrate the apical dome (Lloyd
et al., 1987). Actin ®laments (AFs) are composed of
identical actin monomers (G-actin) arranged in a polar
helical polymer. They play a key role as tracks for
intracellular transport of organelles and vesicles and are
involved in cytoplasmic streaming. The direction of
cytoplasmic streaming is determined in part by the polarity
of the AFs. In root hairs of the aquatic monocotyledon
Hydrocharis, AF bundles are longitudinally orientated in
the transvacuolar strand and the sub-cortical region. The
AFs in each bundle are of uniform polarity and actin
bundles in the transvacuolar strand and the sub-cortical
region have opposite polarities. Since the polarity of the AFs
determines the direction of cytoplasmic streaming, cytoplasmic streaming moves basipetally in the transvacuolar
strand and is acropetal in the sub-cortical region. This
suggests organelles and vesicles move towards the tip in the
transvacuolar strand and away from the tip in the subcortical region (Tominaga et al., 2000). Consistent with this
view is the observation that actin ®laments have their barbed
( fast growing) ends oriented towards the tip in the transvacuolar strands and are arranged with the pointed ends to
the tip in the sub-cortical regions (Tominaga et al., 2000).
The ®mbrin/villin family of proteins is involved in the
bundling of actin ®laments with uniform polarity in animal
cells. A 135 kDa actin bundling protein (135-ABP) from
Hydrocharis, homologous to the arabidopsis actin bundling
protein villin, has been shown to co-localize with AF
bundles of Hydrocharis. 135-ABP is involved in the
bundling of actin ®laments in vivo since microinjection of
antiserum against 135-ABP into living root hair cells causes
the transvacuolar strand to disappear. The thick ®lamentous actin bundles in the transvacuolar strand disperse into
thin bundles that are displaced to the sub-cortical region.
Displacement of the thin AF bundles suggests that a
minimum thickness of actin bundles is required to maintain
the structure of the transvacuolar strand (Tominaga et al.,
2000).
A role for actin micro®laments in the growth of root
hairs has been shown using drugs that depolymerize actin
micro®laments. The application of the actin antagonist
latrunculin B causes inhibition of growth rates in arabidopsis and maize root hairs (Bibikova et al., 1999; Baluska
et al., 2000).
A complex microtubular network has been shown to be
present in the root hairs of many species. Microtubules are
composed of tubulin and are arranged in ®lamentous
structures consisting of 13 proto®laments in a cylindrical
array. Two networks of microtubules exist in root hairs:
plasma membrane associated microtubules run from tip to
base; internal bundles of microtubules are present in the
cytoplasmic strands between the nucleus and the hair tip.
These two systems are continuous, the endoplasmic bundles
of microtubules branch within the apical dome and assume
a cortical location (Lloyd et al., 1987). Using microtubule
disrupting drugs, the microtubule cytoskeleton has been
shown to play a role in controlling the directionality of root
hair growth and morphology. The application of low levels
(51 mM) of taxol, a microtubule-stabilizing drug, or
oryzalin, a microtubule-depolymerizing drug, causes a
waving of the root hair as it elongates. Increased levels
(10 mM) of taxol or oryzalin cause root hairs to form several
elongating branches on a single root hair. Growth rates are
not a€ected by these drugs, suggesting an intact microtubule cytoskeleton is not required for tip growth but is
required to maintain growth polarity and to stabilize a
single growth point (Bibikova et al., 1999). Microtubulestabilizing and -depolymerizing drugs have similar e€ects,
suggesting that it is microtubule dynamics that are
important for controlling the directionality of growth.
There is evidence that microtubules are involved in
restricting the movement of growth machinery to the apex
of an elongating root hair by restricting the movement of
the tip-focused [Ca2‡ ]c gradient. In root hairs, an arti®cial
[Ca2‡ ]c causes transient reorientation of growth only if
applied across the apical, cytoplasm-rich region of the tip
where growth is already occurring. In root hairs in which
microtubules have been stabilized by treatment with taxol, a
locally induced elevated [Ca2‡ ]c is sucient to form a new
growth point at the site of the new gradient, even when this
gradient is generated in the vacuolar region of the hair more
than 10 mm away from the established tip-growing point
(Bibikova et al., 1999). This suggests that microtubule
dynamics are required to limit growth to a single point and
to limit the active region of growth. Furthermore, root hairs
show only a transient reorientation of growth and the
[Ca2‡ ]c gradient in response to touch. Taxol-treated root
hairs show redirected growth for an extended time in
response to touch and a new growth point forms further
away from the site of touch stimulation (Bibikova et al.,
1999). This suggests that microtubules are involved in
controlling the direction of growth.
NUTRIENT DEFICIENCIES
Phosphorus de®ciency substantially increases root hair
length by increasing the growth rate and duration of hair
growth. This response is local and occurs in response to
phosphorus availability in the immediate environment of
the root and is not related to the overall phosphorus status
of the plant (Bates and Lynch, 1996). The greater length of
root hairs is likely to increase phosphorus uptake since hair
length is positively correlated with phosphorus uptake in a
low phosphorus environment. For example, wild-type
plants can acquire more phosphorus under limiting
conditions than a short root hair mutant (rhd2; Fig. 3B)
Foreman and DolanÐArabidopsis Root Hairs
or a mutant with few root hairs (rhd6; Fig. 3C) (Bates and
Lynch, 2000). There is also evidence that low phosphorus
stress and high auxin concentrations elicit similar responses
(Bates and Lynch, 1996).
G E N E T I C A N A LY S I S O F RO OT H A I R
G ROW T H
Genetic analysis of root hair growth has led to the
identi®cation of key proteins involved in di€erent aspects
of cell elongation. Mutants with no root hairs de®ne genes
involved in the earliest stages of hair outgrowth. rhl1, rhl2
and rhl3 have similar phenotypes: mutants have few hairs
on their primary root. The outer epidermal walls of the
mutants are perfectly smooth, with no sign of root hair
initiation. All three mutants have pleiotropic phenotypes
and plants are extreme dwarfs (Schneider et al., 1997).
RHL1 encodes a small hydrophilic protein containing a
nuclear localization signal, which targets the protein to the
nucleus (Schneider et al., 1998). Its precise function in cell
growth is unclear at present.
RHD6 is involved in the assembly of the cellular
components at the site of root hair initiation. rhd6 mutants
have fewer hairs than wild-type roots, exhibit a basal shift
of root hair emergence, and often have multiple root hairs
originating from a single cell (Fig. 3C). However, the rhd6
mutation does not a€ect the growth of the root hairs: the
few root hairs that form on rhd6 mutants are normal in all
respects. The phenotype suggests that RHD6 is active at an
early stage of root hair development and is not required
later, during the growth of the root hair (Masucci and
Schiefelbein, 1994). Double mutant analysis is consistent
with this view, suggesting RHD6 acts before the genes
required for root hair growth (Parker et al., 2000).
The ROOT HAIR DEVELOPMENT1 (RHD1) protein
is required for correct expansion of the hair cell. rhd1
mutant root hairs are similar in length to those of the wildtype, but the hairs have a wide bulbous region at their base
(Fig. 3A). In some cases the entire epidermal wall is forced
outward to form the basal portion of the hair. It is thought
that the RHD1 gene product is involved in regulating the
degree of epidermal cell wall loosening during root hair
initiation (Schiefelbein and Somerville, 1990).
A group of mutants has been identi®ed that have very
short root hair phenotypes, similar to kjk, although hair tip
rupture has not been reported for any of these mutants.
rhd2, shaven1 (shv1), shv2 and shv3 mutant root hairs begin
to emerge from the epidermal cells, but do not elongate,
causing a `stubby' hair phenotype (Schiefelbein and Somerville, 1990; Parker et al., 2000). The phenotype suggests that
the genes de®ned by these mutations are needed for the
establishment of tip growth (Parker et al., 2000).
rhd3 mutants have short root hairs that have a wavy
appearance and are occasionally branched. Rather than
elongating in a single direction, perpendicular to the root
axis, the hairs elongate in a corkscrew fashion (Schiefelbein
and Somerville, 1990). Experiments monitoring root hair
growth by microbead labelling revealed that the wavy
appearance of the hairs is due to irregular changes in the
direction of root hair growth associated with di€erential
5
expansion about the root hair tip. The average volume of
rhd3 root hairs is less than one-third that of wild-type hairs,
suggesting that the RHD3 mutation is required for cell
expansion. The vacuole is also smaller in rhd3 mutants than
in wild-type plants. The RHD3 gene product is a novel
89 kD polypeptide containing a putative GTP-binding
motif near its amino terminus (Wang et al., 1997). The
biochemical function of the protein is unknown.
rhd4 mutants produce short root hairs. The hairs vary in
diameter along their length, forming bulges and constrictions during the elongation process. Occasionally root hairs
are also branched. It has been suggested that the RHD4
protein is involved in cell wall deposition at the root hair tip
(Schiefelbein and Somerville, 1990).
Treatment of root hairs with taxol and oryzalin results in
the formation of root hairs with branches and occasional
swellings. A similar phenotype is found in can of worms1
(cow1) and tip growth defective 1 (tip1) mutants, suggesting
that the respective wild-type genes may encode proteins
involved in microtubule-mediated activities. cow1 mutants
have shorter, wider root hairs than the wild-type, and some
root hairs emerge from the same initiation site (Grierson
et al., 1997). tip1 mutants also have short root hairs, approx.
one-tenth the length of wild-type hairs (Fig. 3F) (Schiefelbein et al., 1993). Hairs are initiated from a large bulge on
the epidermal cells, from which one to four branches
emerge. Some of these branches undergo further branching.
Tip growth of pollen tubes is also defective in tip1 mutants
indicating that its activity is also required during tip growth
in this cell type (Schiefelbein et al., 1993; Ryan et al., 1998).
Several other mutants that have short root hairs and a
range of di€erent shapes have been identi®ed: bristled1
(bst1), centipede1 (cen1), cen2, cen3 and supercentipede1
(scn1). These genes are required to control the shape of the
root hair during tip growth. All of these mutants, with the
exception of cen1, also produce multiple hairs, suggesting
that these genes have a role in restricting the number of root
hairs produced by tip growth from each hair-forming site
(Parker et al., 2000).
A large double mutant analysis has suggested that there
is a complicated genetic network controlling root hair
formation, with many of the genes having several functions
(Parker et al., 2000). Few double mutant combinations
displayed clear epistasis, suggesting that many of the genes
act in parallel or independently of each other. Six epistatic
interactions have been described for root hair morphogenesis genes. Double mutant analysis suggests that RHD6
acts before RHD2 and SHV1; RHD2 acts before COW1;
COW1 acts before RHD4; and SHV1 acts before SHV2 and
SHV3. In this genetic-based study, four main groups of
genes were assigned to subsequent stages of root hair
growth. The ®rst group of genes is those required for the
beginning of root hair formation, this includes SHV3,
CEN2, RHD3, SCN1 and TIP1 (Fig. 4). The main evidence
for these genes acting at this stage is the synergistic e€ects
between pairs of these genes that prevent root hair
formation. The second group of genes is those required
for swelling formation. There are only two genes in this
class, TIP1 and RHD1 (Fig. 4). There is evidence that
RHD1 and TIP1 act in parallel, since the rhd1 : tip1 double
6
Foreman and DolanÐArabidopsis Root Hairs
TIP GROWTH
TIP1, SCN1, COW1, RHD3,
CEN1, CEN2, CEN3, BST1
TRANSITION TO TIP GROWTH
RHD2, SHV1, SHV2, SHV3,
TIP1, BST1, RHD3, CEN1,
CEN2, CEN3, SCN1
SWELLING FORMATION
TIP1, RHD1
HAIR INITIATION
SHV3, CEN2, RHD3, SCN1,
TIP1
F I G . 4. SEM image of the di€erentiation zone of an arabidopsis root showing the four groups of root hair morphogenesis genes, as classi®ed by
Parker et al. (2000). Double mutant analysis has suggested that there is a complicated genetic network controlling root hair formation, with many
genes having several functions.
mutant does not have the characteristic increased swelling
conferred by the tip1 mutation. Double mutant combinations with RHD1 and other root hair morphogenesis
genes have shown additive interactions, suggesting RHD1
acts in parallel with these genes. The third class of genes is
those required for the transition from swelling formation to
tip growth. There are 11 genes in this class: RHD2, SHV1,
SHV2, SHV3, TIP1, BST1, RHD3, CEN1, CEN2, CEN3
and SCN1 (Fig. 4). Two epistatic interactions have been
identi®ed in this class of genes, suggesting that SHV2 acts
after SHV1, and that SHV3 acts after RHD2. Other interactions are additive, suggesting that the genes work
independently. The fourth class of genes is those required
for root hair elongation by tip growth. This group of genes
includes TIP1, SCN1, COW1, RHD3, CEN1, CEN2, CEN3
and BST1 (Fig. 4). The only epistatic interaction is the
previously reported interaction between COW1 and RHD4,
implying COW1 acts before RHD4. This suggests that most
genes involved with tip growth act at least partly
independently (Parker et al., 2000). This study has led to
a complex model of the genetic contributions to root hair
development, which will act as a working model for
subsequent research.
CO N C L U S I O N S
The large range of arabidopsis root mutants are providing
insights into the physiology of root hair growth and
revealing the genetic basis of root hair morphogenesis.
Although many are not yet cloned, they will provide an
overwhelming wealth of information in the near future.
Current research is bringing together physiological, cellular
and genetically based studies of root hair growth to provide
an extensive knowledge of how plant cells grow.
AC K N OW L E D G E M E N T S
J.F. is funded by the BBSRC on a Quota studentship. L.D.
acknowledges receipt of funding from the BBSRC, European Union and the Gatsby Foundation. We are grateful to
Kim Findlay and Paul Linstead for images in Figs 1, 2 and
4. We thank Georg Seifert for detailed discussions and
suggestions that have immensely improved our manuscript.
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