Plant Molecular Biology 49: 273­284, 2002.
Perrot-Rechenmann and Hagen (Eds.), Auxin Molecular Biology.
 2002 Kluwer Academic Publishers. Printed in the Netherlands.
273
Polar auxin transport ­ old questions and new concepts?
Jiˇr´i Friml1,2, and Klaus Palme3
1Zentrum für Molekularbiologie der Pflanzen, Universität Tübingen, Auf der Morgenstelle 3, 72076 Tübingen,
Germany (author for correspondence; e-mail jiri.friml@zmbp.uni-tuebingen.de); 2Department of Biochemistry,
Faculty of Science, Masaryk University, Kotl´aˇrsk´a 2, 611 37 Brno, Czech Republic (e-mail friml@chemi.muni.cz);
3Max-Delbrück-Laboratorium in der Max-Planck-Gesellschaft, Carl-von-Linn´e-Weg 10, 50829 Köln, Germany
Received 13 September 2001; accepted 22 October 2001
Key words: auxin, polar auxin transport
Abstract
Polar auxin transport controls multiple aspects of plant development including differential growth, embryo and root
patterning and vascular tissue differentiation. Identification of proteins involved in this process and availability of
new tools enabling `visualization' of auxin and auxin routes in planta largely contributed to the significant progress
that has recently been made. New data support classical concepts, but several recent findings are likely to challenge
our view on the mechanism of auxin transport. The aim of this review is to provide a comprehensive overview of
the polar auxin transport field. It starts with classical models resulting from physiological studies, describes the
genetic contributions and discusses the molecular basis of auxin influx and efflux. Finally, selected questions are
presented in the context of developmental biology, integrating available data from different fields.
Abbreviations: AEI, auxin efflux inhibitors; agr, agravitropic; ARF, auxin response factor; ARF GEF, guaninenucleotide
exchange factor on ADP-ribosylation factor G protein; BFA, Brefeldin A; eir, ethylene-insensitive root;
IAA, indole-3-acetic acid; 1-NAA, 1-naphtylacetic acid; NPA, 1-N-naphthylphthalamic acid; PAT, polar auxin
transport; rcn1, roots curl in NPA; TIBA, 2,3,5 -triiodobenzoic acid; tir, transport inhibitor response
Historical concepts
Polar auxin transport represents a unique process specific
to plants and the phytohormone auxin. Already
in the 19th century classical experiments studying
the phototropism of canary grass coleoptiles indicated
the existence of a transmissible signal, which was
later identified and termed auxin (Darwin and Darwin,
1881). Further research in this field led to the formulation
of the Cholodny-Went hypothesis, which proposed
that differential auxin distribution represents the
mechanism for tropic growth (summarized in Went,
1974; Firn et al., 2000). Auxin transport experiments
using chemical inhibitors revealed the physiological
importance of auxin transport and led to the formulation
of the chemiosmotic model that explains auxin
transport at the cellular level through the action of
specific influx and efflux carriers (Rubery and Sheldrake,
1974; Raven, 1975). Joint efforts of genetics
and molecular biology resulted in identification of
genes encoding putative auxin influx and efflux carrier
proteins represented by AUX and PIN gene families
respectively (Palme and Gälweiler, 2000). Although
direct biochemical evidence establishing their function
as carriers is still lacking, it is undisputable that
AUX and PIN proteins participate in the auxin transport
process. Today the availability of new tools for
localization of these proteins (Palme and Gälweiler,
2000; Friml et al., 2001b; Swarup et al., 2001) and
in situ monitoring of auxin accumulation (Sabatini
et al., 1999) as well as availability of mutants impaired
in auxin transport (Palme and Gälweiler, 2000;
Friml et al., 2001b) enable us to give the old physiological
theories new molecular meaning and precisely
address the role of auxin transport in many developmental
processes. So far these studies largely support
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the widely accepted classical concepts and give a fresh
input to old discussions about auxin as a plant morphogen
(Sabatini et al., 1999; Friml et al., 2001b).
However recent data suggest that auxin efflux carriers
may not solely act at the plasma membrane. Based on
these data a new view on the mode of auxin transport
inhibitors action is proposed (Geldner et al., 2001).
These findings are likely to change our long-held
views about auxin transport and, therefore, further
advances in this field are eagerly awaited.
Physiology of auxin transport
Auxin is thought to be synthesized in plants locally,
in young growing regions, predominantly in the shoot
apex, young leaves and developing seeds (Normanly
et al., 1991; Ljung et al., 2001), but it seems that
almost any plant tissue can at certain times be responsive
for auxin (Davies, 1995). Already in the 1920s,
Cholodny and Went were independently trying to hypothesize
how auxin moves from the apex into the
elongation zone (Went, 1974).
Two main pathways describe the transport of
auxin, a fast, non-directional transport in the phloem
and a slower, directional, so-called polar auxin transport
(PAT) in various tissues. The evidence for the
existence of phloem transport was established through
experiments with radioactively labelled auxin (Morris
and Thomas, 1978). This transport occurs in both
basipetal and acropetal directions, proceeds relatively
fast (5­20 cm/h) and seems to correlate well with the
transport of assimilates and inactive auxin conjugates
(Nowacki and Bandurski, 1980). Direct auxin analysis
revealed physiological relevant amounts of free
indole-3-acetic acids (IAA) within the phloem exudate
(Baker, 2000). Experiments in pea showed that the labeled
auxin transported within the phloem was later
detected in the PAT system indicating that both transport
pathways may be linked (Cambridge and Morris,
1996).
In contrast to phloem transport, PAT is specific
for active free auxins, occurs in a cell-to-cell manner
and has a strictly unidirectional character. The main
PAT stream runs from the apex basipetally with a velocity
of 5­20 mm/h towards the base of the plant
(Lomax et al., 1995). Using radioactively labelled
auxin this kind of transport was mainly detected in the
cambium and adjacent, partially differentiated xylem
elements (Morris and Thomas, 1978). In roots the
auxin stream continues acropetally towards the root
tip, where part of the auxin is redirected backwards
and transported basipetally through the root epidermis
to the elongation zone (Rashotte et al., 2000).
In contrast, PAT in shoots occurs in lateral direction
(Morris and Thomas, 1978). Auxin transport assays
also revealed that PAT requires energy, is saturable and
sensitive to protein synthesis inhibitors. These results
suggested the existence of specific auxin transport proteins
and led in the middle of the 1970s to the formulation
of a coherent model for auxin transport, termed
the chemiosmotic hypothesis (Rubery and Sheldrake,
1974; Raven, 1975). In the relatively acidic environment
of the cell wall (pH around 5.5) about 15% of
IAA exists in its protonated form (IAAH). This noncharged,
lipophilic molecule passes easily through the
plasma membrane by diffusion. In the more basic cytoplasm
(pH around 7) IAAH dissociates and hence
the resulting IAA-anion is `trapped' in the cell due
to its poor membrane permeability. Therefore, a specific
efflux carrier was postulated and the polarity
of the auxin flux was explained by its asymmetric
distribution in cells. In addition, the existence of
specific auxin influx carriers was hypothesized (Goldsmith,
1977) and later saturable auxin influx, probably
working as an IAA-/2H+ co-transporter, was physiologically
demonstrated (Lomax et al., 1985; Benning,
1986). Auxin influx and efflux pathways can
be physiologically distinguished using auxin efflux
inhibitors (AEI). These pharmacological tools arose
from correlative exploration of structure-activity profiles
of chemicals with auxin-like activity (Katekar
and Geissler, 1977) and were demonstrated to inhibit
efflux of auxin from cells and hypocotyl segments (reviewed
in Rubery, 1990). Most widely recognized AEI
such as 1-N-naphthylphthalamic acid (NPA) share a
benzoic acid ortho-linked with an aromatic ring system.
However other very effective inhibitors such as
2,3,5-triiodobenzoic acid (TIBA) do not share this
structural pattern. In order to explain the mechanism
of inhibition of auxin efflux by AEI, the existence
of an NPA-binding protein forming part of the auxin
efflux carrier complex was postulated. Additional experiments
with auxin efflux and NPA binding studies
in the presence of inhibitors of protein synthesis suggested
the existence of a third unstable component of
the auxin efflux carrier complex likely coupling the
NPA-binding protein to auxin transport protein (Morris
et al., 1991). Attempts to localize NPA-binding
protein within the cell through NPA binding studies
led to controversial results favouring either association
with cytoskeleton (Cox and Mudday, 1994) or inte[26
]
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gral membrane localization (Bernasconi et al., 1996).
Despite very limited data on the molecular mechanism
of AEI function, they have shown to be valuable
tools for the establishment of the role of the auxin efflux
carrier in plant development. Due to the lack of
similar inhibitors of auxin influx the characterization
of the corresponding protein complex and elucidation
of the role of auxin influx in plant development
was retarded. Nevertheless, the recent isolation of
compounds such as 1-naphthoxyacetic acid (1-NOA)
and 3-chloro-4-hydroxyphenylacetic acid (CHPAA)
specifically inhibiting auxin uptake in tobacco culture
cells has re-established the balance (Imhoff et al.,
2000).
Genetic and molecular analysis of auxin transport
The biochemical attempts to isolate the components
of auxin influx and efflux carrier complexes and other
proteins involved in PAT have been so far not fully
successful. The main contributions concerning their
identity come from the genetic field, especially from
the analysis of Arabidopsis mutants. Different screening
strategies have been successfully applied to identify
mutants affected in PAT. Some mutants have been
selected on the basis of abnormal responses to auxin
transport inhibitors or were identified fortuitously in
screens for developmental alterations and only later
the connection to PAT was discovered. A mutant called
rcn1 was isolated, whose roots curl in the presence of
NPA in contrast to straight root growth in wild type
(Garbers et al., 1996). This mutant shows a reduction
in root and hypocotyl elongation and is also defective
for apical hook formation. The RCN1 gene was
cloned and shown to encode a protein phosphatase II
A subunit. RCN1 may control the level of phosphorylation
and thereby the activity of a component involved
in PAT (Garbers et al., 1996). This hypothesis was
corroborated by recent findings that the rcn mutant
displays enhanced basipetal auxin transport, a phenotype
feature, which was also observed in plants treated
with the phosphatase inhibitor cantharidin (Rashotte
et al., 2001). The pinoid (pid) mutants display defects
in the formation of flowers and cotyledons resembling
plants grown on AEI. Moreover, auxin transport in
pid inflorescences is reduced (Okada et al., 1991).
The PID gene has been recently cloned and shown to
encode a serine-threonine protein kinase, which was
proposed to act as a negative regulator of auxin signaling
(Christensen et al., 2000). However, another
analyses demonstrate that PID action is sensitive to
AEI thus favouring the hypothesis that PID functions
as a positive regulator of auxin transport (Benjamins
et al., 2001). Thus analysis of both RCN and PID
highlights a role of phosphorylation in the regulation
of PAT. The tir (transport inhibitor response) mutants
were isolated on their ability to allow root elongation
in the presence of AEI (Ruegger et al., 1997). The
mutant called tir3 displays a variety of morphological
defects including reduced elongation of root and inflorescences,
decreased apical dominance and reduced
lateral root formation. Both auxin transport and NPAbinding
activity are reduced in tir3 mutants (Ruegger
et al., 1997), suggesting that the TIR3 gene may encode
the NPA-binding protein or some closely related
protein (Hobbie, 1998). The corresponding gene designated
as BIG has been recently cloned (Gil et al.,
2001) and shown to encode a protein with several putative
Zn-finger domains homologous to the Drosophila
Calossin (CalO)/Pushover protein. A defect in CalO
interferes with neurotransmitter release in Drosophila.
The big mutation interferes with an effect of auxin efflux
inhibition on putative auxin efflux barrier AtPIN1
cycling (Gil et al., 2001) supporting the function of
BIG in vesicle trafficking, although the mechanism of
action has not yet been not clarified.
AUX1 protein: an influx carrier?
Another mutant called aux1, which confers a root
agravitropic and auxin-resistant phenotype, was instructive
for identification of a gene possibly encoding
an auxin influx carrier (Bennett et al., 1996). The aux1
phenotype is consistent with a defect in auxin influx,
but similar phenotypes have been observed also in mutants
defective in auxin response (Lincoln et al., 1990).
The AUX1 gene encodes a 485 amino acid protein
sharing significant similarity with plant amino acid
permeases favouring the role for AUX1 in the uptake
of the tryptophan-like IAA (Bennett et al., 1996). Despite
the fact that final biochemical proof of AUX1
function as an auxin uptake carrier is still lacking, several
lines of evidence strongly support the involvement
of AUX1 in auxin influx. The strongest support came
from a detailed analysis of the aux1 phenotype. It has
been demonstrated that the membrane permeable 1NAA
rescues the aux1 root agravitropic phenotype
much more efficiently than membrane less permeable
IAA or 2,4-D and that this rescue coincides with
restoration of basipetal auxin transport (Yamamoto
and Yamamoto, 1998; Marchant et al., 1999). More[27
]
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over, this phenotype including its specific NAA rescue
can be mimicked by growing seedlings on recently
isolated inhibitors of auxin influx (Parry et al., 2001).
The most direct support that AUX1 participates in
auxin influx came from the comparison of auxin transport
properties of aux1 and wild-type roots. Uptake
assays using radioactively labelled auxins and auxin
analogues revealed that aux1 roots accumulated significantly
less 2,4-D than wild-type roots (Marchant
et al., 1999). Interestingly, such a difference was
not found when the membrane-permeable1-NAA or
the IAA-like amino acid tryptophan were assayed.
Recently the AUX1 protein was localized within Arabidopsis
root tissue by an epitope tagging approach
(Swarup et al., 2001). The AUX1 protein was detected
in a remarkable pattern in a subset of stele,
columella, lateral root cap and epidermal cells exclusively
in the root tips (Figure 1). Disruption of
AUX1 causes changes in cell-specific auxin accumulation
associated with tissues mediating basipetal auxin
transport (Swarup et al., 2001; Rashotte et al., 2001).
However, aux1 mutants are also defective in auxin
supply to the root tip, since mutant root tips contain
less free auxin than those of wild type. This paradox
taken together with localization of AUX1 at the upper
side of protophloem cells suggest a role of AUX1 protein
in unloading of the bulk flow via the protophloem
to the root apical meristem (Swarup et al., 2001).
Thus AUX1 would appear to provide the first molecular
connection between polar and non-polar auxin
transport routes.
PIN proteins: efflux carriers?
The knitting needle-like pin1 mutant phenotype
strongly resembles plants treated with inhibitors of
auxin efflux. In addition, this mutant displays a strong
reduction in basipetal auxin transport (Okada et al.,
1991). The AtPIN1 gene was cloned by transposon
tagging and found to encode a 622 amino acid
protein with up to 12 putative transmembrane segments.
AtPIN1 similarity to a group of transporters
from bacteria supports a role of this protein as an
auxin efflux carrier (Gälweiler et al., 1998). Alternatively
and equally likely on the basis of currently
available genetic evidence, it could act as a regulator
of auxin transport. Almost simultaneously a homologous
gene was found by several laboratories, AtPIN2/EIR1/AGR1
(Chen et al., 1998; Luschnig et al.,
1998; Müller et al., 1998; Utsuno et al., 1998), and
cloning and analysis of additional sequences (AtPIN3
Figure 1. The AUX1 protein localizes to the apical surface of
protophloem cells, to the columella and to the lateral root cap.
and AtPIN4) followed (Friml et al., 2001a, b). In total,
the Arabidopis PIN gene family consists of eight
members and homologous genes were found in other
plant species such as maize, rice and soybean. As for
the role of AUX1 in auxin influx, the proposed function
for PIN proteins as efflux carriers has not been
biochemically demonstrated to date. However, there
are several lines of evidence strongly supporting a role
of PIN proteins in auxin transport and particularly in
auxin efflux.
AtPIN protein sequences and topology suggest
transport function
The AtPIN proteins share more than 70% similarity
and have an identical topology ­ two highly hydrophobic
domains with five to six transmembrane segments
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277
linked by a hydrophilic region. Transporters of the
major facilitator class display similar topology. Moreover,
PIN proteins were demonstrated to share limited
sequence similarity with prokaryotic and eukaryotic
transporters (Palme and Gälweiler, 1999, Müller et al.,
1998; Luschnig et al., 1998; Chen et al., 1998; Utsuno
et al., 1998) supporting a proposed transport function
for PIN proteins.
Yeast over-expressing AtPIN2 are resistant to
fluoroindole
To date, the only experimental system used for to
address AtPIN transport activity were yeast cells carrying
a mutation in the GEF1 gene resulting in altered
ion homeostasis (Gaxiola et al., 1998; Luschnig et
al., 1998). Yeast cells defective in this gene and overexpressing
AtPIN2 (in these experiments designated
EIR1 and AGR1) showed enhanced resistance to the
yeast toxin fluoroindole, a substance with some albeit
limited structural similarity to auxin (Luschnig et al.,
1998). Other experiments demonstrated that AtPIN2
over-expressing yeast retain less radioactively labelled
auxin than control yeast (Chen et al., 1998). The decreased
accumulation of labelled auxins or toxic auxin
analogues may indicate an auxin efflux function for
AtPIN2 protein in yeast. Nevertheless, direct measurements
of auxin efflux instead of auxin retention have
not been demonstrated, leaving this issue unsolved.
AtPIN proteins are localized in a polar manner in
auxin transport competent cells
The classical concept of chemiosmotic hypothesis predicted
that auxin efflux carriers adopt a polar localization
(Rubery and Sheldrake, 1974; Raven, 1975).
Such a remarkable localization in accordance with the
known direction of auxin flux has been demonstrated
for several AtPIN proteins in PAT-competent cells. AtPIN1
protein is localized at the lower side of elongated
parenchymatousxylem and cambial cells of Arabidopsis
inflorescence axes in accordance with the basipetal
direction of PAT (Gälweiler et al., 1998; Palme and
Gälweiler, 2000). In contrast, the AtPIN2 protein was
polarly localized at the upper side of the lateral root
cap and epidermis cells (Müller et al., 1998). The
AtPIN3 protein localizes predominantly to the lateral
side of shoot endodermis cells (Friml et al., 2002a)
and the polar localization of AtPIN4 in root directs
towards columella initials, the site of auxin accumulation
in the root apex (Sabatini et al., 1999; Friml et al.,
2002b).
Atpin mutants are defective in PAT
One of the strongest arguments for the involvement
of PIN proteins in auxin transport is a reduction of
PAT in Atpin mutants, which directly correlates with
loss of AtPIN expression in corresponding tissue. This
was demonstrated for basipetal auxin transport in stem
of Atpin1 mutant or in root of Atpin2 mutant (Okada
et al., 1991; Rashotte et al., 2000).
Disruption of AtPIN function cause changes in cellspecific
auxin accumulation
Auxin accumulation has been indirectly monitored
by the activity of an auxin-responsive construct (e.g.
DR5::GUS; Sabatini et al., 1999), which seems to
correlate very well with direct free IAA measurements
(Casimiro et al., 2001; Friml et al., 2002b). Using this
approach changes in cell-specific auxin accumulation
were found in several Atpin mutants which correlated
with loss of AtPIN expression in corresponding tissues.
These changes were demonstrated for Atpin2
(in these experiments called eir1) mutant roots after
changes in gravistimulation (Luschnig et al., 1998)
and for Atpin4 mutant roots and embryos where the
defects in establishment and maintenance of the characteristic
auxin response maximum were observed
(Friml et al., 2002b). The use of radioactively labelled
auxins enables more direct determination of auxin
content than the visualization by auxin-responsivepromotors,
but does not provide the cellular resolution.
These experiments in root tips of wild type and Atpin2
(in these experiments called agr1) mutants revealed
that with radioactive IAA-preloaded Atpin2 root tips
retain more radioactivity than similarly treated wildtype
roots (Chen et al., 1998).
Atpin mutants are hypersensitive to 1-NAA
Loss of auxin efflux carrier activity would be predicted
to result in an accumulation of its substrate 1-NAA
in plant cells, leading to greater sensitivity towards
this auxin analogue. Indeed, root growth assays have
revealed that several Atpin2 mutant alleles exhibit an
1-NAA-hypersensitive phenotype, consistent with our
predictions (Müller et al., 1998; Parry and Bennett,
personal communication).
Phenotypes of Atpin mutants can be phenocopied by
auxin efflux inhibitors
Most of the defects observed in Atpin mutants are in
processes known to be regulated by PAT and can be
phenocopied by AEI. These include:
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­ the Atpin1 aerial phenotype with defects in stem
vasculature (Gälweiler et al., 1998);
­ the affected root and shoot gravitropism in Atpin2
and Atpin3 mutants, respectively (Maher and
Martindale, 1980, Friml et al., 2002a);
­ the defect in hypocotyland root elongation in light,
in apical hook opening and in lateral root initiation,
which have been reported for Atpin3 mutants
(our unpublished data);
­ the Atpin4 root meristem pattern aberrations,
which can be also found in seedlings germinated
on low concentrations of AEI (Friml et al., 2002b).
The data accumulated so far, especially from Atpin
mutants analyses, provides an extensive body of
evidence to argue that AtPIN proteins are involved in
some important aspects of auxin transport. However,
the central question still remains whether PIN proteins
represent either transport or a regulatory components.
To answer this question, auxin transport assays have to
be developed to establish directly carrier functions of
different PIN proteins and to determine their substrate
specificities, affinities and kinetic properties.
PIN protein dynamics - turning over the concept
To date the identification of polarly localized PIN
proteins as putative efflux carriers and the analysis
of corresponding knockout mutants appear to agree
with the chemiosmotic hypothesis (Rubery and Sheldrake,
1974; Raven, 1975). However, recent studies
focused on cell biological requirements of the polar localization
of AtPIN1 protein revealed surprising facts,
which are difficult to reconcile with this model. These
studies even question the specificity of AEI, which
represents one of the most valuable tools to study
PAT which have been developed in the past decades.
Previous studies have revealed that interference with
vesicle trafficking either genetically by disrupting the
GNOM gene or chemically by Brefeldin A (BFA), a
fungal toxin, interferes with correct plasma membrane
localization of the AtPIN1 protein (Steinmann et al.,
1999). More detailed studies in Arabidopsis roots have
shown that in the presence of BFA the AtPIN1 protein
is rapidly and reversibly internalized from the plasma
membrane into the endosomal compartment (Figure 2;
Geldner et al., 2001). This also occurred in the presence
of a protein synthesis inhibitor thereby demonstrating
that internalized AtPIN1 originated from the
plasma membrane and that AtPIN1 is rapidly cycling
between an endosomal compartment and the plasma
membrane. The combinative treatment with BFA and
Cytochalasin D or Latrunculin B, which are known to
disrupt actin, led to the inhibition of this cycling, suggesting
that AtPIN1 vesicles are transported along the
actin cytoskeleton. The analysis of effects of AEI on
AtPIN1 localization revealed that they leave AtPIN1
at the plasma membrane, but block AtPIN1 cycling.
Surprisingly, AEI block also the cycling of other functionally
unrelated membrane proteins. This suggested
a much more general mode for AEI, namely inhibition
of membrane protein trafficking.
It is reasonable to assume that the rapidly cycling
AtPIN proteins are components very much sensitive to
trafficking inhibition. This taken together with importance
of AtPIN-dependent PAT in plant development
would explain seemingly specific physiological effects
of AEI. Most interestingly, these studies show that
the very rapid cycling of the AtPIN1 protein seems
to be an essential part of the PAT process and AtPIN1
function, since vesicle trafficking inhibitor BFA is also
known to rapidly interfere with auxin efflux and could
phenocopy the effect of AEI (Morris and Robinson
1998; Delbarre et al., 1998; Geldner et al., 2001). This
surprising finding can be hardly incorporated in the old
static models in which influx and efflux carrier complexes
sit at the plasma membrane and mediate their
transport function. The remaining crucial question is
the functional relevance of cycling? Different scenarios
can be conceived: (1) a high turnover of auxin
efflux complexes would provide important flexibility
for rapid changes in polarity of plasma membrane localization
and thereby for redirection of an auxin flux;
(2) if the hypothesis proposed by Hertel would be
true and a component of auxin efflux has a dual receptor/transporter
function as was proposed for sugar
carriers (Lalonde et al., 1999), the cycling might be
part of signal transduction and receptor regeneration,
as is known for other kinds of receptors (Knutson,
1991); (3) the most exciting scenario would be that the
vesicle trafficking itself is part of the auxin transport
machinery and that, in analogy with neurotransmitter
release, auxin would be a vesicle cargo, released
from cells by polar exocytosis. Regardless whether
one or more of these scenarios are true, uncovering
of cell mechanisms controlling the subcellular dynamics
of the auxin carriers seems to be central for our
understanding of auxin transport.
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Figure 2. AtPIN1 cycling. The reversible internalization of AtPIN1 label upon Brefeldin A (BFA) treatment suggests that AtPIN1 cycles
between the plasma membrane and an endosomal compartment. Polar auxin transport inhibitors (AEI) such as TIBA block AtPIN1 cycling.
The role of polar auxin transport in plant
development
The central role of PAT in plant development, especially
in tropic growth, vascular development, lateral
root initiation, embryo and root patterning as well as
apical dominance was initially established by physiological
studies using chemical inhibitors of PAT
(Lomax et al., 1995). The recent availability of genetic
and molecular tools now allows to address more
specifically the role of auxin transport in each of these
developmental processes.
Vascular tissue development and canalization
hypothesis
The manipulation of plants with auxin transport inhibitors
drew attention to the role of auxin in vascular
development (Camus, 1949). Early grafting experiments
demonstrated that buds induced the differentiation
of new vascular tissues and the replacement of
buds by auxin indicated its importance as the major
inductive signal (Wetmore and Rier, 1963). Several
reports discussed auxin as a correlative and morphogenetic
signal, which influences vascular maturation
through a partially self-organizing mechanism (Newcomb
et al., 1970; Sachs, 1989). This so-called
canalization hypothesis assumes that centres with elevated
auxin levels gradually gain more competency
for auxin conduction and narrowing auxin flow which
finally results in differentiation of vascular strands.
Auxin transport inhibition experiments have shown
that vascular development in leaves and cotyledons
of Arabidopsis plants grown on different concentration
of NPA was severely affected (Mattsson et al.,
1999): secondary veins were not developed and, using
higher NPA concentrations, the differentiation of
the whole leaf vasculature was completely abolished.
These studies together with previous studies on vascular
regeneration after wounding (Wetmore and Rier,
1963) seemed to be in very good agreement with the
canalization hypothesis and provided a strong link
between PAT and vascular development. However, recent
results of extensive genetic screens in Arabidopsis
for mutants defective in vascular patterning led to isolation
of seven different loci designated van1­van7
(Koizumi et al., 2000). Most of these mutations led to
fragmentation of otherwise fully differentiated cotyledon
vasculature without largely affecting the overall
architecture of the vascular network thereby favouring
a pre-patterning mechanism for vasculature differentiation
(Koizumi et al., 2000). The molecular characterization
of these mutants and use of molecular markers
for visualization of auxin transport routes and early
vascular fate identity will certainly contribute to the
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further clarification of the role of auxin canalization
versus prepatterning in vascular tissue differentiation.
Differential growth in shoot: a role for
AtPIN3-regulated lateral auxin transport?
Differential growth in shoot (gravitropism, phototropism
and apical hook formation) have been proposed
to result from the asymmetric redistribution of
auxin, which subsequently promotes or inhibits cell
growth and elongation resulting in bending (Went,
1974). This model gained strong support by visualization
of differentially distributed auxin response reporters
in gravity-stimulated tobacco shoots (Li et al.,
1991), in Arabidopsis light and gravity stimulated
hypocotyls and newly formed apical hooks (Friml
et al., 2002a). In this system PAT inhibitors blocked
both the asymmetric distribution of auxin response and
differential growth, implicating PAT as the process
underlying differential auxin distribution and thereby
regulating differential growth (Friml et al., 2002a;
Lehman et al., 1996). The lateral auxin transport system
has been proposed to facilitate the exchange of
auxin between the main basipetal stream in vasculature
and peripheral regions of shoot, where control
of elongation occurs (Epel et al., 1992). The
recent characterization of the putative auxin efflux carrier,
AtPIN3, recently provided experimental data for
this concept (Friml et al., 2002a). The Atpin3 mutants
display defects in hypocotyl differential growth.
Moreover, AtPIN3 expression and localization studies
revealed that the AtPIN3 protein is predominantly localized
at the lateral side of endodermis cells further
supporting the model of lateral auxin transport.
Root gravitropism: an influx and efflux matter
In the root the sites of gravity perception are columella
cells and the site of response is the elongation zone,
where elevated auxin levels on the lower side lead to
the inhibition of growth and thereby downward bending
of the root (for an overview, see Chen et al., this
issue). The main contribution of PAT to the root gravitropic
response was supposed to be the basipetal transport
from columella region through epidermis towards
the elongation zone. The identification of several
root agravitropic mutants (agr1/eir1/wav6/Atpin2) in
a gene encoding the root-specific putative auxin efflux
carrier AtPIN2 supports this hypothesis (Chen
et al., 1998; Müller et al., 1998; Luschnig et al.,
1998; Utsuno et al., 1998). The AtPIN2 protein was
localized to the cortex, epidermis and lateral root cap,
predominantly at the upper side of cells, consistent
with the model (Figure 3; Müller et al., 1998). The
role of AtPIN2 in basipetal auxin transport was further
supported by the finding that the auxin response
maximum at the lower side of the root after a change
of gravity vector is not established and basipetal auxin
transport is reduced in Atpin2 mutants (Luschnig et al.,
1998; Rashotte et al., 2000). AUX1 is also involved in
both regulation of root gravitropism and auxin transport.
Aux1 mutants display defects in basipetal auxin
transport correlating with root agravitropic phenotype
(Swarup et al., 2001). The aux1 mutant phenotype was
rationalized by the visualization of AUX1 protein at
the plasma membrane of lateral root cap and epidermis
cells (Figure 1; Swarup et al., 2001). Thus it seems
that both auxin influx and efflux as well as its molecular
components are required for root gravitropism.
Nonetheless it is still not clear if PAT plays a permissive
or regulatory role in root gravitropism and where
and how the asymmetry of the auxin distribution is initially
established. These questions have recently been
addressed by visualization of auxin distribution and
auxin carriers in gravistimulated roots. The presence
of both putative auxin influx and efflux carriers in columella
cells (AUX1, Figure 1, Swarup et al., 2001;
AtPIN3,4, Figure 3; Friml et al., 2002a,b) suggests
that the asymmetry of auxin distribution may be established
there and not during the basipetal transport
to the responsive tissue. Moreover, the actin dependent
cycling of AtPIN3 and its rapid lateral relocation
after gravity stimulation in columella cells provide a
likely mechanism for establishment of this asymmetry
(Friml et al., 2002a). In this model the sedimentation
of statoliths upon gravity stimulation leads to
rearrangement of the actin cytoskeleton resulting in relocation
of the efflux carrier components. This causes
the asymmetric flux of auxin from columella cells
and auxin is translocated by basipetal transport to the
responsive tissue, where it elicits differential growth.
Root meristem patterning: climbing to the sink
The role of auxin and PAT in roots is not restricted to
growth responses. Exogenous manipulation of auxin
levels as well as analysis of mutants impaired in auxin
signalling demonstrated a role for auxin in regulating
the pattern of cell division and differentiation
(Kerk et al., 1994; Ruegger et al., 1997) and raised
a debate about a possible role of auxin as a morphogen
(Sabatini et al., 1999). Similarly to animal
morphogens, auxin concentration gradients exist as
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Figure 3. Immunolocalization of AtPIN proteins with presumptive
auxin routes in the Arabidopsis root tip. AtPIN1 localization
suggests auxin flux from apical tissues towards the central root
meristem (red arrows), AtPIN4 regulates the focusing of auxin flux
into columella establishing the auxin maximum (blue arrows). Part
of the auxin is rerouted through outer cell layers backward to the
elongation zone regulating root bending (AtPIN2, green arrows).
has been demonstrated by direct auxin measurements
in Scots pine (Uggla et al., 1998) as well as by indirect
visualization using auxin responsive promoter
DR5 in Arabidopsis roots and embryos (Sabatini et al.,
1999). In the Arabidopsis root the maximum of auxin
response was detected in the columella initial cells
and first columella layer (Sabatini et al., 1999). Localization
studies of the recently characterized AtPIN4
(Figure 3) revealed polar localization pointing towards
the same area, suggesting involvement of AtPIN4
in establishment of the auxin gradient with a maximum
in columella initials and columella cells. Both
chemical inhibition of PAT and the Atpin4 mutation
change the auxin gradient and relocalize its maximum.
These changes in auxin gradients were accompanied
with various patterning defects and correlated well
with changes in cell fate specification thus providing a
genetic link between PAT, auxin gradients and patterning.
Morphogen gradients known from animal systems
are source-driven. Surprisingly, it appears that the data
emerging from an analysis of the role of AtPIN4 and
PAT in the root meristem can be best integrated into a
model involving an active AtPIN4-dependent sink for
auxin in the columella region, implying that auxin gradients
are sink-driven. According to this model the AtPIN4
protein promotes auxin transport within the central
root meristem tissues and actively increases auxin
concentration thereby creating maximum in columella
cells. The establishing and proper positioning of this
maximum seems to be necessary for local downregulation
of auxin, since both NPA-treated roots and
Atpin4 mutant roots display elevated auxin levels in
the root tip and fail to canalize exogenously applied
auxin properly (Friml et al., 2002b). However, which
mechanisms are involved in auxin down-regulation is
unclear. Experiments in maize demonstrated high levels
of ascorbate oxidase, an enzyme degrading auxin
in vitro, in the root tip (Kerk et al., 2000). However,
there are no data available favouring oxidative decarboxylation
as a means for auxin degradation in vivo.
Another possibility to remove auxin would be active
export or passive leaking of the auxin from the root or
by pealing off columella cells loaded with auxin. An
interesting mechanism would be the continual trafficking
of auxin in the apoplast between columella initial
and columella cells which is suggested by AtPIN proteins
localization. (Friml et al., 2002b). The longer
presence of auxin in the oxidative environment of the
apoplast would greatly facilitate auxin degradation (G.
Sandberg, personal communication).
Embryonic development: establishment of axis
Exogenous manipulations of auxin levels and PAT in
carrot callus-derived somatic embryos as well as in
vitro cultured Indian mustard (Brassica juncea L.)
zygotic embryos established a role for PAT in the
initiation and maintenance of polarized growth in developing
embryos (Schiavone et al., 1987; Hadfi et al.,
1998). AEI interfered with cotyledon separation highlighting
the important role of auxin and its transport
in the progression from the radially symmetric to the
bilaterally symmetric embryo in analogy to the regulation
of radial position of lateral organs in later development
(Reinhardt et al., 2000). Genetic studies using
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Arabidopsis mutants support a role for PAT in embryo
development and suggest a possible role in the establishment
of the apical basal axis. The monopteros (mp)
mutant was isolated as an embryo mutant with defects
in apical-basal pattern formation (Berleth and Jürgens,
1993), but adult mp plants display a strong reduction
in PAT (Przemeck et al., 1996). The MP gene encodes
an auxin-regulated transcription factor from the
ARF family raising the possibility that the MP protein
could regulate the expression of components of the
PAT machinery (Hardtke et al., 1998). Another mutant
called gnom (gn) displays defects strongly resembling
AEI-treated embryos (Mayer et al., 1993). The GN
gene encodes a guanine nucleotide exchange factor of
ADP-ribosylation factors (ARF GEF), a protein regulating
vesicle trafficking to the plasma membrane
(Steinmann et al., 1999). It has been demonstrated that
gradual polarity establishment of AtPIN1 localization
in gn embryos is disturbed suggesting an involvement
of GN in proper localization of AtPIN1 (Steinmann
et al., 1999). However, in view of recent reports suggesting
that vesicle trafficking represents an essential
part of PAT (Geldner et al., 2001), gn may by disrupting
vesicle trafficking directly interfere with this
process. Despite the physiologically and genetically
well established role of PAT in embryogenesis, little
is known about auxin transport routes and auxin accumulation
in embryos. Due to technical difficulties
especially in tiny Arabidopsis embryos, we can deduce
these only indirectly from the polarity of AtPIN
protein localizations and activity of auxin response reporters.
Two AtPIN genes, AtPIN1 and AtPIN4, are
expressed during embryogenesis and can be found
localized in both polar and non-polar fashion (Steinmann
et al., 1999; Friml et al., 2002b). AtPIN1 was
detected already in the 8-cell stage at the inner cell
boundaries. The polarity of the AtPIN1 localization
is then gradually established at the globular stage at
the basal side of provascular cells, where also AtPIN4
protein is localized. The establishment of AtPIN1
and AtPIN4 polarity in provascular initials coincides
with that establishment of a basally positioned DR5
auxin response maximum (Friml et al., 2002b). At
the late globular and triangular stage the polar localization
of AtPIN1 is also established in epidermis
cells towards the position of future outgrowing cotyledons.
During further progression towards the torpedo
stage the vascular precursor cells become marked at
their basal sides forming a typical Y-shaped pattern.
These AtPIN1 and AtPIN4 localization patterns suggest
auxin flux towards the quiescent centre, below
which an auxin response maximum is formed. The
AtPIN4 protein was also detected at the basal side of
upper suspensor cells (Friml et al., 2002b), suggesting
that auxin homeostasis may be regulated via the
suspensor. However, such deduced auxin routes are
hypothetical and there are no data available yet on a
role of auxin influx carriers during this early stage of
development. Thus, the role of PAT in embryogenesis
remains still far from clarified. It seems that the
auxin homeostasis regulation system during embryogenesis
is rather robust, since mutations in AtPIN1
or AtPIN4 genes results only in less pronounced and
not fully penetrating defects in embryo development
(Okada et al., 1991; Friml et al., 2002b).
Outlook
It seems that PAT is now a process very well characterized
from a physiological point of view. The
use of genetic approaches resulted in an extensive
collection of mutants defective in PAT. The main recent
breakthrough in the field happened when some
of these mutants were characterized at the molecular
level, and AUX1 and PIN gene families encoding putative
auxin influx and efflux carrier components have
been identified. The main task for the near future is
the biochemical establishment of auxin transport function
and transport properties of these proteins and the
identification of their interaction partners. Also the
isolation and characterization of remaining members
of AUX and PIN families in Arabidopsis is awaited in
order to describe the whole network of these carriers in
plants, their possible functional redundance and their
expression and activity regulation in a developmental
context. However, one of the most interesting areas
of PAT research lies probably in their cellular analysis.
Here the polarly localized PIN and AUX proteins
provide ideal tools to study cellular polarity establishment,
a process poorly understood in higher plants.
For PAT research, however, we are awaiting with curiosity
further studies on the relationship between the
vesicle trafficking pathway and auxin efflux.
Acknowledgements
We are very grateful to Eva Benková, Malcolm
Bennett, Niko Geldner, Matthias Godde, Thorsten
Hamann and Gerd Jürgens for helpful discussions
and critical reading of the manuscript. We
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also acknowledge support by DAAD (J.F.) and
the Schwerpunktprogramme `Phytohormone' by the
Deutsche Forschungsgemeinschaft.
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