2675
The differential distribution of the plant signaling molecule
auxin is required for many aspects of plant development. Local
auxin maxima and gradients arise as a result of local auxin
metabolism and, predominantly, from directional cell-to-cell
transport. In this primer, we discuss how the coordinated activity
of several auxin influx and efflux systems, which transport auxin
across the plasma membrane, mediates directional auxin flow.
This activity crucially contributes to the correct setting of
developmental cues in embryogenesis, organogenesis, vascular
tissue formation and directional growth in response to
environmental stimuli.
Introduction
The plant hormone auxin (the predominant form of which is indole-
3-acetic acid; IAA) is a major coordinating signal in the regulation
of plant development. Many aspects of auxin action depend on its
differential distribution within plant tissues, where it forms local
maxima or gradients between cells. Besides local biosynthesis and
the release of active forms from inactive precursors, the major
determinant of differential auxin distribution is its directional
transport between cells. This regulated polar auxin transport (PAT)
within plant tissues appears to be unique to auxin, as it has not been
detected for any other signaling molecule. Molecular biology and
genetics approaches in the model system Arabidopsis thaliana have
contributed fundamentally to our understanding of the mechanisms
of auxin transport. Currently, a large body of evidence supports the
concept that intercellular auxin movement depends on several auxintransporting
mechanisms, which include both passive and active
processes that transport auxin over long and short distances. Of
these, the major mechanism for controlling auxin distribution during
plant development appears to be the active directional cell-to-cell
movement of auxin that is mediated by plasma membrane-based
influx and efflux carriers (see Glossary, Box 1). Here, we summarize
the present state of knowledge on how the various auxin transport
mechanisms cooperate during plant development to fine-tune auxin
distribution. We describe the basic pathways of auxin transport and
discuss auxin transport routes during diverse developmental
processes, such as embryogenesis, root and shoot organogenesis,
vascular tissue formation and tropisms (see Glossary, Box 1).
Auxin transport systems in plants
In plants, auxin is generally transported by two distinct pathways.
Throughout the plant, most IAA is probably transported away from
the source tissues (young leaves and flowers) by an unregulated bulk
flow in the mature phloem (see Glossary, Box 1). In addition, a
slower, regulated, carrier-mediated cell-to-cell directional transport
moves auxin in the vascular cambium from the shoot towards the
root apex (Goldsmith, 1977), and also mediates short-range auxin
movement in different tissues. These two pathways seem to be
connected at the level of phloem loading in leaves (Marchant et al.,
2002) and phloem unloading in roots (Swarup et al., 2001).
Aseries of classical physiological experiments (Box 2) predicted
the existence of carrier-type auxin influx and efflux components that
mediate PAT. The asymmetric cellular localization of these
transporters has been proposed to determine the direction of auxin
flow. During the past two decades, candidates for auxin carrier
proteins and for the relevant regulatory mechanisms have been
identified (Fig. 1). Heterologous expression experiments in cultured
plant cells, yeast, Xenopus laevis oocytes and mammalian cells have
demonstrated the auxin-transporting capacity of these carrier
proteins (Vieten et al., 2007). Expression and localization studies of
auxin carrier proteins, as well as specific defects in differential auxin
distribution (Box 3) in plants that lack the function of these carriers,
established that carrier-dependent PAT is absolutely required for the
generation and maintenance of local auxin maxima and gradients.
Influx carriers
For auxin influx, the characterization of an agravitropic (see
Glossary, Box 1) auxin resistant 1 mutant (aux1) of Arabidopsis that
shows resistance to an exogenous synthetic auxin, 2,4-D, led to the
identification of the AUX1/LIKE AUX1 (AUX1/LAX) family of
transmembrane proteins, which are similar to amino acid permeases,
a group of proton-gradient-driven transporters (Bennett et al., 1996;
Swarup et al., 2008). To date, four auxin influx carriers with specific
functions have been described in Arabidopsis, and the functions of
some homologs in other plants have also been studied (Table 1).
Recently, AUX1 and LAX3 has been shown to mediate IAAuptake
when heterologously expressed in Xenopus oocytes (Yang et al.,
2006; Swarup et al., 2008), which provides biochemical evidence
for their role as auxin influx carriers.
Efflux carriers
The investigation of several Arabidopsis mutants, namely of the
allelic root mutants agravitropic 1 (agr1), wavy roots 6 (wav6)
(Bell and Maher, 1990; Okada and Shimura, 1990) and ethylene
insensitive root 1 (eir1) (Roman et al., 1995), and the floral
mutant pin-formed1 (pin1) (Okada et al., 1991), resulted in the
identification of auxin efflux carrier candidates. The root
agravitropic phenotypes, as well as the pin1 phenotype with
defects in organ initiation and phyllotaxy (see Glossary, Box 1),
can be phenocopied by the pharmacological inhibition of auxin
efflux. Additionally, these mutants display decreased PAT in
shoots and roots. The corresponding PIN1 gene encodes a plantspecific
protein with two transmembrane regions separated by a
hydrophilic loop (Gälweiler et al., 1998). Concomitantly, the
agr1, wav6 and eir1 mutants have been shown to be allelic with a
mutant that carries a mutation in another PIN family member,
PIN2. The AGR1, WAV6, EIR1 and PIN2 genes encode a
homologous protein designated PIN2 (Chen et al., 1998; Luschnig
Development 136, 2675-2688 (2009) doi:10.1242/dev.030353
Auxin transport routes in plant development
Jan Petrášek1,2
and Jirˇí Friml3,4,
*
1
Institute of Experimental Botany, ASCR, 165 02 Prague 6, Czech Republic.
2
Department of Plant Physiology, Faculty of Science, Charles University, Vinicná 5,
128 44 Prague 2, Czech Republic. 3
Department of Plant Systems Biology, VIB and
Department of Plant Biotechnology and Genetics, Ghent University, Technologiepark
927, 9052 Gent, Belgium. 4
Department of Functional Genomics and Proteomics,
Faculty of Science, Masaryk University, 625 00 Brno, Czech Republic.
*Author for correspondence (e-mail: jiri.friml@psb.vib-ugent.be)
PRIMER
DEVELOPMENTDEVELOPMENT
2676
et al., 1998; Müller et al., 1998; Utsuno et al., 1998). Until now,
eight members of the PIN protein family have been isolated in
Arabidopsis and are commonly referred to as PIN1 to PIN8
(Vieten et al., 2007; Zazímalová et al., 2007). A subgroup
comprising PIN5, PIN6 and PIN8 has a reduced middle
hydrophilic loop and presumably regulates the auxin exchange
between the endoplasmic reticulum and the cytosol (Mravec et al.,
2009). The PIN1, PIN2, PIN3, PIN4 and PIN7 proteins, by
contrast, are localized at the plasma membrane, where they act as
auxin efflux carriers (Mravec et al., 2008; Petrásek et al., 2006).
PIN homologs in other plants have also been identified
(Zazímalová et al., 2007), and some of them have been
functionally characterized (Table 1).
Other proteins that play a role in auxin efflux are plant orthologs
of the mammalian ATP-binding cassette subfamily B (ABCB)-type
transporters of the multidrug resistance/phosphoglycoprotein
(ABCB/MDR/PGP) protein family (Noh et al., 2001; Verrier et al.,
2008). Some of these (ABCB1, ABCB4 and ABCB19) have been
identified as proteins with binding affinity to the auxin transport
inhibitor 1-naphthylphthalamic acid (NPA) (Murphy et al., 2002;
Noh et al., 2001). The biochemical evidence for these ABCB
proteins having a role in auxin transport has been provided by
heterologously expressing them in tobacco cells, HeLa cells and
yeast (Geisler et al., 2005; Petrásek et al., 2006; Santelia et al., 2005;
Terasaka et al., 2005). The importance of the ABCB proteins for
auxin transport-related development has been also documented in
other higher plants (Table 1).
Recently, a system for comparative analyses of transport activities
and the structure of all three groups of auxin transporters
(AUX1/LAX, PIN and ABCB) has been established in
Schizosaccharomyces pombe (Yang and Murphy, 2009). It
represents a valuable tool for testing the cooperation between these
transporters, as well as with other regulatory proteins.
Other auxin transporter candidates exist, for example the
members of a group of aromatic and neutral amino acid transporters
in Arabidopsis (Chen et al., 2001) or the transmembrane protein
TM20 in maize (Zea mays) (Jahrmann et al., 2005). However, their
contribution to the intercellular transport of auxin is still unclear.
Auxin transport regulation
Various aspects of plant development are mediated by transportdependent
differential auxin distribution within tissues.
Conceptually, multiple signals can be integrated to modulate auxindependent
development, which highlights the importance of
regulating each auxin-transporting system individually. Auxin itself
seems to be one of the most important regulators of its own
transport. Earlier physiological observations on the role of auxin in
PRIMER Development 136 (16)
Box 1. Glossary
Acropetal transport: Transport of various compounds (including
auxin) towards the tip of a particular organ (stem or root).
Agravitropic: Having defects in response to gravity. This defect
might be a result of a specific mutation. Horizontally placed
agravitropic roots are unable to grow downwards, agravitropic stems
are unable to grow upwards.
Anticlinal division: Cell division in a layer of cells that occurs
perpendicular to the plane of the cell layer.
Apical: ‘Upper’ side of the cell, facing the shoot apical meristem.
Auxin influx and efflux carriers: Integral plasma membrane
proteins that transport auxin molecules into and out of the cell,
respectively.
Basal: ‘Lower’ side of the cell, facing the root tip.
Basipetal transport: Transport of various compounds (including
auxin) from the tip towards the basis of the particular organ (stem or
root).
Columella: Group of cells in the central root cap, which contain
plastids with starch; the site of root gravity perception.
Cotyledons: Embryonic leaves formed during embryonic
development. In dicotyledon plants, cotyledons are typically
positioned symmetrically.
Flavonoids: Plant secondary polyphenolic metabolites. Besides
playing a role in defense responses to environmental impact, they
have been shown to modulate auxin transport by their preferential
effect on ABCB auxin transporters.
Hypophysis: The most apical cell of the suspensor, which forms the
attachment between the suspensor and the developing embryo. It
gives rise to the embryonic root of a plant, the radicle, which
develops into the primary root.
Periclinal division: Cell division in a layer of cells that occurs parallel
to the plane of the cell layer.
Phloem: Part of the vasculature that transports metabolites from the
source tissues (leaves) to other tissues.
Phyllotaxy: The typically regular arrangement of leaves or floral
organs, which initiates at the shoot apical meristem.
Primary root: The first root that develops from the embryonic root
of a plant embryo, the radicle.
Stele: The central part of the root or stem that contains the vascular
tissue.
Suspensor: Single cell file formed from the zygote daughter basal
cell by transverse divisions. This cell file connects the embryo with
mother tissues and later degenerates.
Tropisms: Directional plant growth responses to various
environmental stimuli, such as light (phototropism) or gravity
(gravitropism). The response always depends on the direction of the
stimulus and could therefore be positive or negative (towards or
away from the stimulus, respectively).
Vasculature: Complex conductive tissue that consists of specialized
cells that transport water and nutrients from roots (xylem), cells that
transport products of photosynthesis and other metabolites from
source tissue (phloem) and several other cell types that form
supporting tissues. In both xylem and phloem, various plant
hormones have been detected.
Box 2. Physiology-based models of auxin transport
across the plasma membrane
A model for the mechanism that underlies the directionality of cellto-cell
auxin transport was proposed simultaneously by Rubery and
Sheldrake (Rubery and Sheldrake, 1974) and Raven (Raven, 1975),
and is known as the chemiosmotic polar diffusion model (Goldsmith,
1977). According to this model, an undissociated lipophilic form of
the native auxin molecule (IAA) can easily enter the cell cytoplasm
from a slightly acidic extracellular environment (pH 5.5) by passive
diffusion. As the pH of the cytoplasm is more alkaline (pH 7) than the
extracellular environment, a difference that is maintained by the
plasma membrane-located H+
-ATPase, more IAA molecules
dissociate after entering the cells, and the resulting hydrophilic auxin
anions (IAA–
) are trapped in the cytosol. The exit of IAA–
was
therefore proposed to be assisted by active auxin anion efflux carriers
that constitute the limiting step and the major control units of auxin
transport. The directionality of auxin transport was postulated to be
attributable to the asymmetric distribution of such carriers at a
particular side of the cell (Goldsmith, 1977), which would steer auxin
flow in the direction of the predominant localization of the
transporters. Early experiments with suspension-cultured crown gall
cells of Parthenocissus tricuspidata (Rubery and Sheldrake, 1974) also
suggested the existence of active auxin anion uptake carriers that
probably act as 2H+
cotransporters.
DEVELOPMENTDEVELOPMENT
2677PRIMERDevelopment 136 (16)
the formation and regeneration of vascular tissues led to the
formulation of the canalization hypothesis, which postulates that
auxin acts to polarize its own transport (Sachs, 1981). This theory
proposes that the initial diffusion of auxin away from a source
positively reinforces its own transport, which ultimately leads to the
distribution of auxin into narrow canals, and that this canalization is
an important part of the mechanism that underlies coordinated tissue
polarization.
In general, the carrier-mediated transport of auxin can be
regulated at three levels: by the regulation of (1) the abundance of a
carrier (by regulating its transcription, translation and degradation);
(2) subcellular trafficking and targeting of auxin carriers to a specific
position on the plasma membrane; and (3) transport activity (e.g.
through the post-translational modification of carriers, the levels and
activity of endogenous inhibitors, the regulation of the plasma
membrane pH gradient, the composition of the plasma membrane
and the interactions among individual transporters or transport
systems).
Indeed, the transcription of all known carrier proteins (PIN,
ABCB and AUX1/LAX) is influenced by auxin triggering a
signaling cascade that involves the F-box protein TRANSPORT
INHIBITOR RESPONSE 1 (TIR1) auxin receptor (Fig. 1) (Geisler
et al., 2005; Noh et al., 2001; Terasaka et al., 2005; Vanneste et al.,
2005; Vieten et al., 2005). Variable timing of the transcriptional
response, as well as its modulation by the developmental context,
has been reported. In the case of the PIN proteins, the auxindependent
regulation of transcription might play an important role
in the extensive functional redundancy within the PIN family, which
becomes apparent in the specific upregulation of other PINs in the
expression domain of a PIN gene that is affected by a mutation
(Blilou et al., 2005; Vieten et al., 2005). Other plant hormones, such
as ethylene or cytokinins, might also modulate the expression of PIN
and AUX1 proteins (Dello Ioio et al., 2008; Pernisová et al., 2009;
Ru˚ zicka et al., 2007; Ru˚ zicka et al., 2009).
In addition, the abundance of some PIN proteins is further
controlled by degradation via the vacuolar targeting pathway
(Kleine-Vehn et al., 2008b; Laxmi et al., 2008), which requires
proteasome-mediated steps (Abas et al., 2006) and is regulated by
the MODULATOR OF PIN (MOP) proteins (Malenica et al.,
2007).
Transport can also be controlled by the incidence of transporters
at the plasma membrane (Box 4). This mode of regulation has been
demonstrated for some PIN proteins that undergo constitutive
internalization and recycling back to the cell surface (Dhonukshe et
al., 2007; Geldner et al., 2001). It is probably important for the
establishment of (Dhonukshe et al., 2008b), and for dynamic
changes in (Kleine-Vehn et al., 2008a), PIN subcellular localization.
Importantly, auxin inhibits PIN internalization by an unknown
mechanism, thus increasing the amount and the activity of PIN
proteins at the cell surface (Paciorek et al., 2005). This constitutes
another, possibly non-transcriptional, mechanism for the feedback
regulation of auxin transport.
The regulation of PIN subcellular targeting is an effective way to
modulate auxin distribution because, consistent with classical
predictions (Box 2), the polar subcellular localization of the PIN
auxin efflux carriers has been shown to be important for the
directionality of auxin fluxes (Wisniewska et al., 2006). Little is
known about the mechanisms that control cell polarity in plants;
nonetheless, the phosphorylation of PIN is important for decisions
on PIN polar targeting. Analyses of Arabidopsis mutants that have
phenotypes typical for altered auxin transport, namely roots curl in
NPA 1 (rcn1) and pinoid (pid), have led to the identification of the
A Auxin transport
B Auxin-regulated gene expression
Directionofauxinmovement
PIN1,2,3,4,7
ABCB1,4,19
IAA IAA
–
H++
AUX1/LAX
SCFTIR1
Aux/IAA
Ub
ARFs
AuxRE
ARFs
Aux/IAA
AuxRE
No gene expression Gene expression
AUX1/LAX1, 2, 3
ABCB1, 4, 19
PIN1, 2, 3, 4, 7
IAA
SCF
TIR1
ABCB1,4,19
Ub
Ub
Proteasome
(Aux/IAA
degradation)
Directionofauxinmovement
IAA
PIN1,2,3,4,7
ABCB1,4,19
Passive
diffusion
pH ~ 7
pH ~ 5.5
IAA IAA
–
H+
+
IAA– 2H+
AUX1/LAXIAA–
IAA
–
ATP ADP
IAA
–
IAA–
Cytoplasm
Cell wall, extracellular space
H+
ATPase H+
ATP
ADP
ABCB1,4,19IAA–
ATP
ADP
IAA
–
IAA–
H+
+
IAA–
(a) (b)
(c)
(d)
(c)
(c)
(e)
Influx carriers
Efflux carriers
(a) (b)
(d)
(c)
Cytoplasm
Cell wall, extracellular space
Nucleus
PIN5
IAA–
IAA–
ERNucleus
Fig. 1. Auxin transport across the plasma membrane and
auxin-regulated gene expression. (A) Schematic depiction of
auxin transport across the plasma membrane. Both passive diffusion
and specific auxin influx and efflux carriers are involved in the
transport of auxin (IAA) across the plasma membrane. Undissociated
IAA molecules enter cells by passive diffusion (a), whereas the less
lipophilic, and therefore less permeable, dissociated auxin anions
(IAA–
) are transported inside via auxin influx 2H+
cotransporters of
the AUX1/LAX family (b). In the more basic intracellular environment
(c), IAA dissociates and requires active transport through the PIN or
ABCB efflux transporter proteins to exit the cell. Some cytosolic IAA
is transported by PIN5 into the lumen of the endoplasmic reticulum
(ER). This compartmentalization presumably serves to regulate auxin
metabolism (Mravec et al., 2009). Whereas PIN transporter activity is
supposed to use a H+
gradient that is maintained by the action of
the plasma membrane H+
-ATPase (d), and possibly also the vacuolar
H+
pyrophosphatase (Li et al., 2005), ABCB transporters have ATPase
activity (e). (B) Schematic depiction of auxin-regulated gene
expression. Intracellular auxin binds to its nuclear receptor from the
TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX
(TIR1/AFB) family of F-box proteins, which are subunits of the SCF
E3-ligase protein complex (a). This leads to the ubiquitylation and the
proteasome-mediated specific degradation of auxin Aux/IAA
transcriptional repressors (b). Subsequently, the auxin response
factors (ARFs) are derepressed and activate auxin-inducible gene
expression (c) (Dharmasiri et al., 2005; Kepinski and Leyser, 2005).
Among other auxin-responsive genes, all known auxin transporters
are regulated by this feedback mechanism (d). Ub, ubiquitin.
DEVELOPMENTDEVELOPMENT
2678
regulatory subunit of protein phosphatase 2A (PP2A) (Deruére et
al., 1999) and the serine/threonine protein kinase PID (Christensen
et al., 2000) as factors that are important for PIN targeting. The
current model is that PID phosphorylates PIN proteins, thus
supporting their apical targeting, and that PP2A antagonizes this
action, thus promoting basal PIN delivery (Friml et al., 2004;
Michniewicz et al., 2007). Moreover, the Arabidopsis 3PHOSPHOINOSITIDE-DEPENDENT
PROTEIN KINASE 1
(PDK1) has been shown to stimulate the activity of PID kinase,
which provides evidence for a role of upstream phospholipid
signaling in the control of auxin transport (Zegzouti et al., 2006).
Similarly, the transcription factor INDEHISCENT (IND) regulates
PID expression, thus mediating auxin distribution-dependent fruit
development (Sorefan et al., 2009). Conceptually, one can imagine
that any signaling pathway upstream of PID/PP2Ahas the capacity
to modulate the transport-dependent distribution of auxin by
changing the balance between phosphorylation and
dephosphorylation. Interestingly, auxin itself regulates PID
expression (Benjamins et al., 2001) and PIN polarity through TIR1mediated
signaling (Sauer et al., 2006).
The composition of the plasma membrane provides the
appropriate environment for protein-protein interactions and can
thereby determine how effective the auxin flux across the membrane
will be. Indeed, the sterol composition of membranes, which
depends on the activity of the enzymes STEROL METHYL
TRANFERASE 1 (SMT1) and CYCLOPROPYL ISOMERASE 1
(CPI1) has been shown to be crucial for the positioning of certain
PIN proteins in the plasma membrane (Men et al., 2008; Willemsen
et al., 2003). Plasma membrane composition is also important for
the localization of ABCB19, which has been found to be present in
PRIMER Development 136 (16)
Box 3. Tracking auxin distribution and transport in plants
Despite the fact that auxin (indole-3-acetic acid; IAA) distribution plays an important morphoregulatory role in plants, scientists still have no direct
method for tracking it in vivo at the cellular level and, instead, have to rely on a set of more or less indirect techniques. For directly measuring the
endogenous IAA content, even in very small samples of plant tissue, gas chromatography-mass spectrometry (GC-MS) is the most frequently
employed method (panel A) (Ljung et al., 2005), but this technique lacks cellular resolution. To track auxin distribution at the cellular level, antibodies
against auxin carriers (panel B) or IAA (panel C) are used (Benková et al., 2003; Friml et al., 2003a). However, immunohistochemical staining
procedures often suffer from technical problems connected with the fixation of the rather diffusive IAA molecules, as well as with the specificity of
anti-IAA antibodies. Therefore, for noninvasive in vivo tracking of auxin activity, synthetic promoters based on auxin-inducible genes are employed
(panel C) (Ulmasov et al., 1997). These consist of multiple TGTCTC repeats of the auxin-responsive element (designated DR5 or DR5rev in reverse
orientation) and can be coupled to markers, such as Escherichia coli β-D-glucuronidase (GUS) (Sabatini et al., 1999), endoplasmic reticulum-localized
Aequorea victoria green fluorescent protein (GFP) (Friml et al., 2003b), and a nucleus-localized version of GFP or the modified yellow fluorescent
protein (YFP) version VENUS-N7 (Heisler et al., 2005), to track their activity in plant tissues. Auxin-responsive reporter constructs are widely used to
get a preliminary impression of the distribution of auxin activity, but their efficiency is limited by their dependence on a comparable availability of
the auxin signaling machinery in all cells, nonlinear signal output, a relatively narrow concentration range for detection, the time requirements of
the transcription and protein folding process, as well as the stability of the reporter molecules. For measurements of auxin flow in plants, microscale
assays with radiolabeled IAA have been successfully adapted for Arabidopsis stem and root segments, and even for whole seedlings (Lewis and
Muday, 2009; Murphy et al., 2000). More detailed information on the kinetic parameters of auxin transporters can be obtained with the same
technique in plant suspension cultures (panel D) (Delbarre et al., 1996; Petrášek et al., 2006). An alternative, but yet not well established, approach
for measuring the actual flow of IAA at the tissue level utilizes vibrating IAA-selective microelectrodes (Mancuso et al., 2005), which offer the
advantage of noninvasive and continual recording of auxin flow. Images are reproduced, with permission, from (A) Ljung et al. (Ljung et al., 2001),
(B) Mravec et al. (Mravec et al., 2008), (C) Benková et al. (Benková et al., 2003) and (D) Petrášek et al. (Petrášek et al., 2006).
0 5 10 15 20 25 30
0
20
40
60
80
100
Control
Time (min)
Accumulationof[H]NAA3
(%ofcontrol)
PIN7 - higher efflux of auxin
decreases net accumulation of
radioactively labeled auxin marker
Control PIN7
D Auxin transport assay in a simplified cell culture model. Tobacco cells expressing the
PIN7 auxin efflux carrier from Arabidopsis (Petrášek et al., 2006)
Late globular Arabidopsis
embryo
Auxin detected
with anti-IAA
antibody
(arrowheads)
Auxin-driven gene
expression detected
with DR5::GUS
(arrowheads)
Two stages of lateral root development (I,IV)
A GC-MS (gas chromatography-mass
spectrometry) determination of
endogenous auxin (IAA) levels in
Arabidopsis leaf (Ljung et al., 2001)
B Vizualization of auxin carriers
using antibodies or GFP-tagged
fusion proteins (Mravec et al., 2008)
C Vizualization of auxin using
antibodies and auxin-inducible gene
expression (Benková et al., 2003)
DEVELOPMENTDEVELOPMENT
2679PRIMERDevelopment 136 (16)
the detergent-resistant microsomal protein fractions of Arabidopsis
seedling tissue lysates (Titapiwatanakun et al., 2009). Such steroland
sphingolipid-rich plasma membrane microdomains presumably
constitute important specialized sites at which ABCB19 and PIN1
might interact physically (Blakeslee et al., 2007). Moreover,
ABCB19 stabilizes PIN1 in these domains, and presumably
influences the rate of PIN1 endocytosis and thus its incidence at the
plasma membrane (Titapiwatanakun et al., 2009) (Box 4).
Table 1. Selected auxin carriers with established developmental roles
Gene Role in development Key references
Auxin influx carrier
AtAUX1 Root gravitropism, lateral root formation, phloem loading Bainbridge et al., 2008; Bennett et al., 1996; Jones
in leaves and unloading in roots, root hair development, et al., 2009; Marchant et al., 2002; Stone et al.,
phyllotaxis, hypocotyl phototropism 2008; Swarup et al., 2001
AtLAX1 Phyllotaxis Bainbridge et al., 2008
AtLAX2 Phyllotaxis Bainbridge et al., 2008
AtLAX3 Phyllotaxis, lateral roots emergence Bainbridge et al., 2008; Swarup et al., 2008
PttLAX1-3 Vascular cambium development in wood-forming tissues Schrader et al., 2003
PaLAX Root gravitropism Hoyerová et al., 2008
MtLAX1-5 Early nodule development de Billy et al., 2001; Schnabel and Frugoli, 2004
CsAUX1 Root gravitropism Kamada et al., 2003
LaAUX1 Etiolated hypocotyl growth Oliveros-Valenzuela et al., 2007
CgAUX1, CgLAX3 Actinorhizal nodule formation after Frankia infection Peret et al., 2007
ZmAUX1 Root development Hochholdinger et al., 2000
Auxin efflux carrier
AtPIN1 Vascular development, phyllotaxis, vein formation, Benková et al., 2003; Gälweiler et al., 1998;
embryogenesis, lateral organ formation Reinhardt et al., 2003; Scarpella et al., 2006;
Weijers et al., 2005
AtPIN2 (EIR1,
AGR1, WAV6) Root gravitropism, lateral organ development Benková et al., 2003; Chen et al., 1998; Luschnig
et al., 1998; Müller et al., 1998; Utsuno et al., 1998
AtPIN3 Shoot and root gravitropism and phototropism, lateral Benková et al., 2003; Friml et al., 2002b
organ development
AtPIN4 Embryogenesis, root patterning Benková et al., 2003; Friml et al., 2002a;
Friml et al., 2003b; Weijers et al., 2005
AtPIN5 Regulation of the intracellular auxin homeostasis and Mravec et al., 2009
metabolism
AtPIN6 Transport activity demonstrated in tobacco cells, in planta Benková et al., 2003; Petrášek et al., 2006
function unknown
AtPIN7 Embryogenesis, root development Benková et al., 2003; Friml et al., 2003b;
Blilou et al., 2005
PttPIN1-3 Vascular cambium development in wood-forming tissues Schrader et al., 2003
CsPIN1 Gravitropism Kamada et al., 2003
LaPIN1,3 Etiolated hypocotyl growth Oliveros-Valenzuela et al., 2007
ZmPIN1 Inflorescence branching Carraro et al., 2006
BjPIN1-3 Differential expression in various tissues, vascular Ni et al., 2002a; Ni et al., 2002b
development
OsPIN1 Adventitious root emergence Xu et al., 2005
AtPGP1 (ABCB1) Embryogenesis, lateral root organogenesis, hypocotyl Geisler et al., 2005; Lin and Wang, 2005; Mravec
and plant growth et al., 2008; Noh et al., 2001
ZmPGP1 (br2; Elongation growth Multani et al., 2003
brachytic) SbPGP1
(dw3; dwarf)
AtABCB19 (MDR1, Embryogenesis, lateral root formation, root gravitropism, Lewis et al., 2007; Mravec et al., 2008; Nagashima
MDR11, PGP19) hypocotyl phototropism and gravitropism, leaf shape et al., 2008a; Nagashima et al., 2008b; Noh et al.,
2001; Petrášek et al., 2006; Wu et al., 2007
AtABCB4 (MDR4, Basipetal transport in root epidermis, lateral root and root Cho et al., 2007; Lewis et al., 2007; Santelia et al.,
PGP4) hair development, gravitropism 2005; Terasaka et al., 2005; Wu et al., 2007
ZmTM20 Vasculature development Jahrmann et al., 2005; Stiefel et al., 1999
At, Arabidopsis thaliana; Bj, Brassica juncea; Cg, Casuarina glauca; Cs, Cucumis sativus; La, Lupinus albus; Mt, Medicago truncatula; Os, Oryza sativa; Pa,
Prunus avium; Ptt, Populus tremula x tremuloides; Sb, Sorghum bicolor; Zm, Zea mays.
DEVELOPMENTDEVELOPMENT
2680
Little is known about the mechanisms that might regulate the
activity of auxin transporters directly. It is possible that an additional
phosphorylation of PIN, distinct from PID-dependent action and
mediated by D6 protein kinases, controls PIN auxin efflux activity
(Zourelidou et al., 2009). Alternatively, PIN auxin transport activity
might be regulated by chemical inhibitors. These exogenous
compounds, which have been known for decades, have been valuable
tools in physiological studies on auxin transport and include a wellknown
inhibitor of auxin efflux, NPA (Rubery, 1990), as well as a
well-known inhibitor of auxin influx, 1-naphthoxyacetic acid (1NOA)
(Parry et al., 2001). Detailed knowledge about the mechanisms
by which NPA and similar compounds inhibit auxin efflux is still
lacking. NPA has a high affinity for binding ABCB-type auxin
carriers, but low-affinity binding sites have also been found (Murphy
et al., 2002). This low-affinity binding might be related to the more
general inhibitory effects of some efflux inhibitors on actin
cytoskeleton dynamics and PIN trafficking processes (Dhonukshe et
al., 2008a; Geldner et al., 2001). A group of naturally occurring
substances that might act analogously to auxin transport inhibitors are
the flavonoids (see Glossary, Box 1), endogenous polyphenolic
compounds that modulate auxin transport and tropic responses
(Murphy et al., 2000; Santelia et al., 2008). Both NPAand flavonoids
regulate the activity of ABCB1 and ABCB19 (Bailly et al., 2008;
Geisler et al., 2003; Murphy et al., 2002; Noh et al., 2001; RojasPierce
et al., 2007), possibly through influencing interaction with the
peripheral plasma membrane protein TWISTED DWARF 1 (TWD1)
(Geisler et al., 2003; Bailly et al., 2008).
The above-mentioned examples only constitute glimpses into
how the auxin distribution network might be regulated at different
levels. Nonetheless, they demonstrate the potential for various
internal and external signals to influence the throughput and the
direction of intercellular auxin fluxes, and thus to regulate auxindependent
development.
Auxin transport routes during embryogenesis
Auxin and auxin transport is already important at the earliest
stages of plant development. The analysis of Arabidopsis mutants,
combined with the visualization of the auxin response by means
PRIMER Development 136 (16)
Box 4. Intracellular trafficking of auxin transporters
The developmentally regulated formation of auxin gradients depends largely on the fine-tuning of auxin flow polarity by means of the differential
subcellular trafficking and targeting of the AUX1/LAX, PIN and ABCB auxin transporters. As all of these transporters are integral plasma membrane
(PM) proteins, they are distributed by the general mechanisms of vesicle trafficking. All auxin transporters have been shown to be constitutively
recycled between the plasma membrane and endosomal compartments (shown in blue). The endocytosis step of PIN1 and PIN2 recycling depends
on clathrin (Dhonukshe et al., 2007) and on the sterol composition of the plasma membrane (Men et al., 2008; Willemsen et al., 2003), which
also influences AUX1 trafficking (Kleine-Vehn et al., 2006). It is also crucial for the interaction between ABCB19 and PIN1 proteins (Titapiwatanakun
et al., 2009). This interaction seems to be important for the stabilization of PIN1 at the plasma membrane sterol-rich microdomain (SRM), with the
subsequent enhancement of its auxin transporting activity. Compared with ABCB1, ABCB19 is a rather stable plasma membrane protein, and its
trafficking requires the activity of the GNOM-LIKE 1 (GNL1) guanine nucleotide exchange factor for ADP-ribosylation factors (ARF-GEF)
(Titapiwatanakun et al., 2009). PIN1 targeting to the basal plasma membrane is regulated by GNOM, another ARF-GEF (Geldner et al., 2003),
whereas one or more additional ARF-GEFs mediate PIN targeting to the apical plasma membranes (Kleine-Vehn et al., 2008a). The apical localization
of AUX1 is maintained by the activity of another ARF-GEF and is assisted by the endoplasmic reticulum accessory protein AXR4 (Dharmasiri et al.,
2006). ARF-GEF-dependent endosomal sorting is also involved in the trafficking of PIN2 to the lytic vacuolar pathway through the prevacuolar
compartment (PVC), from which PIN proteins might be retrieved again into the trans-Golgi network through the assistance of the retromer complex
subunits SORTING NEXIN 1 (SNX1) and VACUOLAR PROTEIN SORTING 29 (VPS29) (Jaillais et al., 2006; Kleine-Vehn et al., 2008b). The ubiquitylation
of PIN2 potentially plays a role in the subcellular trafficking of PIN2 and further regulates the amount of PIN2 at the plasma membrane (Abas et
al., 2006). Additionally, PIN proteins are targets of phosphorylation by PINOID (PID) kinase and of dephosphorylation by protein phosphatase 2A
(PP2A) (Michniewicz et al., 2007); their phosphorylation state might be crucial for determining PIN recruitment into the apical or basal targeting
pathways.
AUX1
ABCB
ABCB
PIN
GEF
ARF
GNOM
ARF
Transcytosis
Recycling
PIN
AXR4
GEF
ARF
P
ARF-GEF-
dependent
PM deposition
Endoplasmic
reticulum
Trans-Golgi
Vacuole
PVC
PID
SNX1
VPS29
Retromer-
dependent
retrieval and
sorting
Golgi
ARF-GEF-
dependent
PM deposition
GNOM-
dependent
PM deposition
PIN
GNL1
Endoplasmic
reticulum
Golgi
Clathrinand
sterol-
dependent
endocytosis
Basal
Apical
Clathrinand
sterol-
dependent
endocytosis
PIN
Sterol-
dependent
endocytosis
Degradation
SRMSRM
ABCB stabilization of PIN in
sterol-rich microdomain (SRM)
ARF-GEF-dependent
PM deposition
Phosphorylation
ARF-GEF-
dependent
vacuolar targeting
Dephosphorylation
PP2A
P
P
P
P
P
P
GEF
ARF
GEF
ARF
ARF-GEF-
dependent
vacuolar targeting
Recycling
DEVELOPMENTDEVELOPMENT
2681PRIMERDevelopment 136 (16)
of auxin-inducible promoters, demonstrated that differential auxin
distribution mediates important steps during embryogenesis, such
as apical-basal axis specification and embryonic leaf formation.
The concerted action of PIN1, PIN4 and PIN7 efflux carriers
(Friml et al., 2002a; Friml et al., 2003b) is required for the
differential auxin distribution in embryogenesis (Fig. 2).
Individual PIN proteins act redundantly, given that single pin
mutants can still complete embryogenesis, whereas pin1 pin3 pin4
pin7 quadruple mutants are strongly defective in the overall
establishment of apical-basal polarity (Benková et al., 2003;
Friml et al., 2003b). In contrast to pin mutants, mutants in other
auxin transport components, such as the abcb and aux1/lax
mutants, are not defective in embryogenesis, which suggests a
major role for PIN-dependent auxin transport in patterning the
embryo.
Soon after the first anticlinal division (see Glossary, Box 1) of a
fertilized zygote, increased auxin accumulation can be detected in
the apical cell by the activity of the auxin-inducible element DR5 or
by IAA immunolocalization (Box 3). This differential distribution
results from the activity of PIN7 that is localized apically in the
adjacent suspensor cells. At this stage, PIN1 presumably mediates
the uniform distribution of auxin between cells of the forming proembryo
(Fig. 2, Box 4). Both ABCB1 and ABCB19 contribute to
auxin transport during the early stages of pro-embryo formation
(Mravec et al., 2008). ABCB1 is localized to all suspensor cells (see
Glossary, Box 1) and pro-embryonal cells, and ABCB19
localization is restricted to the suspensor-forming cells. Both
proteins are localized without obvious polarity. Later, during the
early globular stage, PIN1 gradually relocalizes to the bottom
plasma membranes of the embryo cells that face the uppermost
suspensor cell, the hypophysis (Kleine-Vehn et al., 2008a).
Simultaneously, the polarity of PIN7 shifts from apical to basal in
the suspensor cells (Fig. 2, Box 4). These coordinated PIN polarity
rearrangements, which are later also supported by the action of
PIN4, lead to an apical-to-basal flow of auxin and to auxin
accumulation in the hypophysis. At this stage, the auxin distribution
and response are crucial for the specification of the hypophysis as
the precursor of the root meristem. Accordingly, mutants of the
auxin-binding F-box proteins TIR1 and AFB (Dharmasiri et al.,
2005), and of the downstream transcriptional regulators
MONOPTEROS (MP, also known as ARF5) and BODENLOS
(BDL, also known as IAA12) (Hamann et al., 2002; Hardtke and
Berleth, 1998), show pronounced defects in embryonic root
formation.
Afterwards, during the development of the heart stage of the
Arabidopsis embryo, additional auxin maxima are formed at the
positions of the two initiating cotyledons (see Glossary, Box 1),
mainly through the action of PIN1 (Benková et al., 2003). At this
stage, the ABCB19 expression pattern is largely complementary
to that of PIN1 and shows the highest expression in endodermal
and cortical tissues (Fig. 2). The pin1 abcb1 abcb19 triple
mutants, in contrast to the single pin1 or double abcb1 abcb19
mutants, are severely defective in establishing auxin maxima and
show fused cotyledons, which hints at a synergistic genetic
interaction between PIN1 and ABCB proteins (Mravec et al.,
2008). These results indicate a role for both the ABCB-mediated
1-cell 2-cell Globular Mid heartTriangular
Auxin concentration gradient
(low-high)
PIN4
PIN1
ABCB19
ABCB1
Octant
a
Apical cell
Suspensor cells
h
Hypophysis
PIN7
SAM
Future shoot apical meristem
c c
Cotyledon
Future vasculature
s
Key
h
a
s
c
SAM
Fig. 2. Auxin gradients and auxin transporters during embryogenesis. Schematic depiction of the auxin distribution and the localization of
auxin transporters during early plant embryonic development. Auxin distribution (depicted as a green gradient) has been inferred from DR5 activity
and IAA immunolocalization (Benková et al., 2003; Friml et al., 2002a; Friml et al., 2003a). The localization of the efflux transporters PIN1, PIN4 and
PIN7, as well as that of ABCB1 and ABCB19, is based on immunolocalization studies and on in vivo observations of green fluorescent protein (GFP)tagged
proteins (Dhonukshe et al., 2008b; Friml et al., 2003b; Mravec et al., 2008). Arrows indicate auxin flow mediated by a particular
transporter; dotted lines indicate the cell type-specific localization of particular auxin transporters with no obvious polarity. PIN7, localized at the
apical sides of the suspensor cells (s), transports auxin towards the apical cell (a) that forms the pro-embryo; there, PIN1, which is localized at all
inner cell sides, distributes auxin homogenously. ABCB1 and ABCB19 cooperate during this initial stage and are localized apolarly in all cells or only
in the uppermost suspensor cell, respectively. The crucial moment in the setting of the basal end of the apical-basal embryonic axis occurs during
the early globular stage, when PIN1 starts to be localized basally in the pro-embryonal cells, and PIN7 is simultaneously shifted from the apical to
the basal plasma membrane of suspensor cells. These PIN polarity rearrangements reverse the auxin flow downwards and, with the aid of PIN4,
lead to auxin accumulation in the forming hypophysis (h) (see Glossary, Box 1). At this stage, ABCB19 helps to maintain the auxin distribution in the
outer layers of the embryo. In triangular- and heart-stage embryos, bilateral symmetry is established through auxin maxima at the incipient
cotyledon (c) primordia. These auxin maxima are generated by PIN1 activity in the epidermis; in the inner cells of cotyledon primordia, however,
PIN1 mediates basipetal auxin transport towards the root pole. SAM, future shoot apical meristem.
DEVELOPMENTDEVELOPMENT
2682
and PIN-dependent auxin transport pathways in the generation
of differential auxin distribution at different stages of
embryogenesis.
Auxin and postembryonic root and shoot
development
Auxin plays an important role in the patterning of both shoot and
root apices, as well as in the initiation and the subsequent
development of root and shoot organs. Increased auxin levels at the
incipient positions of the primary root and the cotyledons (see
Glossary, Box 1) during embryogenesis are reflected in
postembryonic development. Auxin maxima always mark the
positions of organ initiation and, later, of the tips of developing
organ primordia (Benková et al., 2003). Correspondingly, the local
application and production of auxin triggers the formation of leaves
or flowers (Reinhardt et al., 2000) and of lateral roots (Dubrovsky
et al., 2008). Auxin fluxes and maxima in root- and shoot-derived
organ primordia are similar and can be described in terms of
fountain and reverse fountain models, respectively (Benková et al.,
2003) (Fig. 3A). In general, all three auxin transport systems,
PRIMER Development 136 (16)
A B
Shoot
Root
P1
LR
SAM
lrc
ep
c
en
s
lrc
ep
c
en
p
en
c
ep
p
en
c
ep
en c ep
C
s
P2
L1
D
L2
L3
P0
P1
SAM
PIN1
ABCB4
ABCB19
ABCB1
SAM
P1
P2
LR
Auxin concentration
gradient (low-high)
Shoot apical
meristem
Leaf primordium 1
(the youngest)
Lateral root
Leaf primordium 2
P0 Primordium
initiation site
L1-L3 Epidermal
cell layers 1-3
Key
Auxin transport
lrc
ep
en
s
p
Lateral
root cap
Epidermis
Cortex
Endodermis
Stele
Pericycle
PIN2
PIN3
PIN4
PIN7
AUX1
LAX3 Basipetal
Acropetal
c
Fig. 3. Auxin gradients and auxin transporters in root and shoot morphogenesis. (A) Schematic overview of the directional flow of auxin in
the shoot and root of Arabidopsis thaliana. Auxin maxima in shoot- and root-derived primordia and the root apex (green) are maintained by auxin
flow towards the root and shoot apices (solid arrows) and reverse flow towards the root and shoot basis (dashed arrows). In the shoot and in shootderived
organs (leaf primordia P1 and P2), auxin is transported towards the tip in the epidermal layers and refluxed back through inner tissues
(future vasculature). In the root and in root-derived organs (lateral root, LR), auxin is transported towards the tip through the interior of the
primordium and refluxed back through the epidermis. (B,C) Auxin transporters in the root tip (B) and developing lateral roots (C). (D) Auxin
transport in the shoot apical meristem (SAM) and during phyllotaxis. See the main text for details on the role of each individual transporter. Auxin
distribution (depicted as a green gradient) has been inferred from DR5 activity and IAA immunolocalization. The localization of auxin transporters is
based on immunolocalization studies and on in vivo observations of GFP-tagged proteins (Benková et al., 2003; Blakeslee et al., 2007; Friml et al.,
2002a,b; Friml et al., 2003b; Heisler et al., 2005; Lewis et al., 2007; Reinhardt et al., 2003; Swarup et al., 2008; Swarup et al., 2001; Wu et al.,
2007). Arrows indicate auxin flow mediated by a particular transporter; dotted lines depict the cell type-specific localization of particular auxin
transporters with no obvious polarity.
DEVELOPMENTDEVELOPMENT
2683PRIMERDevelopment 136 (16)
using PIN, ABCB and AUX1/LAX proteins, contribute to
postembryogenic auxin transport, although the exact contribution of
each of these cooperating transport systems to total auxin transport
remains unresolved.
Auxin transport routes during root development
In the primary root, auxin is transported acropetally (see Glossary,
Box 1) towards the root tip by a PIN-dependent route through the
vascular parenchyma and through the phloem, with subsequent
AUX1-dependent unloading into protophloem cells (Friml et al.,
2002a; Swarup et al., 2001). Auxin flow towards the tip is
maintained by the action of basally localized PIN1, PIN3 and PIN7
in the stele (see Glossary, Box 1) (Blilou et al., 2005; Friml et al.,
2002a). In the columella (see Glossary, Box 1), the action of PIN3
and PIN7 redirects auxin flow laterally to the lateral root cap and the
epidermis, where the apically localized PIN2 mediates the upward
flow of auxin to the elongation zone (Friml et al., 2003a; Müller et
al., 1998) (Fig. 3B). The PIN2-based epidermal auxin flow is further
supported by the action of AUX1 (Swarup et al., 2001) and ABCB4
(Terasaka et al., 2005; Wu et al., 2007), whereas PIN1, PIN3 and
PIN7 recycle some auxin from the epidermis back to the vasculature
(Blilou et al., 2005). The concerted action of the PIN auxin efflux
carriers is one of the major determinants of pattern formation in root
tips (Fig. 3B). By concentrating auxin in the quiescent center, the
columella initiates, whereas surrounding stem cells (Sabatini et al.,
1999) restrict, the expression domain of the auxin-inducible
PLETHORA (PLT) transcription factors. PLTs are the master
regulators of root fate and, in turn, are required for PIN transcription
(Blilou et al., 2005). The ABCB1 and ABCB19 auxin transporters
seem to play a supportive role in controlling how much auxin is
available for each PIN-based transport flow. ABCB1 is expressed in
all root cells, except for the columella (Mravec et al., 2008), whereas
ABCB19 expression is restricted to the endodermis and the
pericycle, which might help to separate the acropetal and basipetal
auxin fluxes in the stele and the epidermis, respectively (Blakeslee
et al., 2007; Mravec et al., 2008; Wu et al., 2007).
Auxin transport is also crucial for lateral root initiation and
development (Fig. 3C). In pericycle cells, auxin maxima specify the
founder cells for lateral root initiation (Dubrovsky et al., 2008).
Subsequent rounds of coordinated divisions form the lateral root
primordium, from which the lateral root emerges later. Indeed, the
functionally redundant network of PIN efflux carriers facilitates the
auxin transport that is needed for the correct development of lateral
root primordia (Benková et al., 2003). During the initiation phase,
PIN1 is localized at the anticlinal membranes. The switch of the
pericycle cell division plane from anticlinal to periclinal (see
Glossary, Box 1) is accompanied by the redistribution of PIN1 to the
outer lateral plasma membranes of inner cells (Benková et al., 2003).
This guanine nucleotide exchange factor for ADP-ribosylation
factors (ARF-GEF)-dependent, transcytosis-like PIN1 polarity
switch (Kleine-Vehn et al., 2008a) mediates the auxin flow towards
the primordium tip, where an auxin maximum is formed. At later
stages, the PIN2-mediated auxin transport away from the tip through
the outer layers is established.
AUX1 significantly contributes to lateral root formation, probably
by controlling the overall auxin levels in the root tip (by unloading
auxin from the phloem) and its availability in the region of lateral
root initiation (by basipetal transport from the tip) (Marchant et al.,
2002). An interesting role is reserved for LAX3, which is induced
in cells around the developing primordium, where it establishes the
auxin maxima needed for the specific production of cell-wallremodeling
enzymes, which is necessary for lateral root emergence
(Swarup et al., 2008). The ABCB1 and ABCB19 proteins are also
expressed and required for lateral root formation, as indicated by the
defects in the abcb and pin abcb mutants (Mravec et al., 2008;
Petrášek et al., 2006).
Auxin transport routes during shoot development
In the shoot apical meristem (SAM), the main source of auxin is
unclear, but auxin is probably partly supplied by the phloem (as
in the case of roots) and by young developing organs in the
vicinity. Auxin fluxes are largely reversed in shoots when
compared with roots. Auxin arrives at the organ initiation sites
through the epidermis layer L1 and is canalized through the
interior of developing primordia into the basipetal stream of the
main shoot (Fig. 3D). This stream is mostly maintained by the
activities of PIN1, localized basally in xylem parenchyma cells
(Gälweiler et al., 1998), and of ABCB19 (Noh et al., 2001),
which, together with ABCB1, helps to concentrate auxin flux in
the vascular parenchyma (Blakeslee et al., 2007; Geisler et al.,
2005).
Shoot lateral organs (leaves and flowers) are generated from the
SAM in a highly periodic phyllotactic pattern. In Arabidopsis
phyllotaxis, the 137° angle between developing primordia is marked
by auxin maxima at the position of incipient primordia (Benková et
al., 2003; Heisler et al., 2005). This highly organized auxin
distribution is maintained by the cooperative action of AUX1,
LAX1, LAX2 and LAX3 (Bainbridge et al., 2008), as well as that of
PIN1. PIN1 polarities in the L1 layer, which also undergo complex
rearrangements relative to auxin maxima, appear to be responsible
for generating the phyllotactic pattern of auxin distribution, whereas
auxin influx activities largely restrict auxin to the L1 layer
(Reinhardt et al., 2003). Not only the positioning, but also the
development of shoot lateral organs is regulated by auxin
distribution, with the maximum concentration at the primordium tip,
where it is maintained mainly by the activity of PIN1, which
transports auxin through the epidermis towards the tip. From there,
a new basipetal, PIN1-dependent, transport route is gradually
established through the interior of the primordium. This marks
future developing vascular tissues that will connect new organs with
the pre-existing vascular network (Benková et al., 2003; Heisler et
CP CP CP
CP CP CP CP
PIN1
CP Convergence point
Key
C
Auxin concentration
gradient (low-high)
BA
Fig. 4. Auxin distribution and localization of auxin transporters
in vascular tissue formation. Three developmental stages of leaf
primordia according to Scarpella et al. (Scarpella et al., 2006).
(A,B) Polarly localized PIN1 in the epidermis directs auxin flow towards
the convergence point (CP). (C) Gradual PIN1 polarization and
establishment of auxin channels away from the CPs determine the
future development of the venation pattern. Arrows indicate auxin flow
mediated by PIN1.
DEVELOPMENTDEVELOPMENT
2684 PRIMER Development 136 (16)
Gravity
vector
A
s
PIN1
PIN2
PIN3
PIN4
PIN7
ABCB4
AUX1
lrc
ep
c
en
s
ABCB19
Lateral root cap
Epidermis
Cortex
Endodermis
Stele
Auxin concentration
gradient (low-high)
s
Starch amyloplasts
B
en
c ep
s
Gravity
vector
en
c
ep
s
Key
Direction
of bending
lrc
ep
c
en
lrc
ep
c
en
lrc
ep
en
c
lrc
ep
en
c
Direction
of bending
Fig. 5. Auxin gradients and auxin transporters during gravitropic response. (A) Positive root gravitropism. In starch-containing, gravitysensing
root cap cells, PIN3 is relocalized from a symmetric distribution towards the newly established bottom side after gravistimulation (Friml et
al., 2002b) (left). The auxin that is redirected to the lower side of the root tip is further transported to the elongation zone by epidermal
PIN2/AUX1-mediated flow, where it inhibits cell growth and causes the downward bending of the root (Luschnig et al., 1998; Müller et al., 1998;
Bennett et al., 1996; Swarup et al., 2001; Swarup et al., 2005) (right). (B) Negative shoot gravitropism. Gravity is detected in starch-containing
endodermal cells, where PIN3 is supposed to redirect auxin laterally to the vasculature (stele) (left). After gravistimulation (right), PIN3 is presumably
(in analogy to the situation in roots) relocated to the new basal side of endodermal cells, and auxin flow is redirected to the outer cell layers along
the bottom side of the shoot, where auxin stimulates cell elongation and the subsequent upward bending of the stem (Friml et al., 2002b). Arrows
indicate auxin flow mediated by a particular transporter; dotted lines indicate the cell type-specific localization of particular auxin transporters with
no obvious polarity; black arrows indicate the gravity vector (left) and the direction of bending (right).
DEVELOPMENTDEVELOPMENT
2685PRIMERDevelopment 136 (16)
al., 2005). ABCB1 and ABCB19 also contribute to the
establishment of this auxin sink (Noh et al., 2001) (Fig. 3D).
Observations regarding the localization of the components of
different auxin transport systems, combined with the defects in the
corresponding mutants, show that all the transport systems that
depend on ABCB, AUX1/LAX and PIN proteins are involved in
shoot-derived organogenesis.
Auxin in vascular tissue development
As indicated already by the role of PIN1-dependent auxin flow in
the establishment of new vasculature from shoot-derived organs,
auxin and auxin transport are among the major determinants of the
organized development of vascular tissues, which serve as the
main distribution route for water and nutrients. Auxin seems to be
a major positional signal for vascular tissue formation, because
local auxin applications to responsive tissues are sufficient to
trigger the de novo formation of vasculature (Sachs, 1991). As
stated before, the canalization model of auxin flow predicts a
feedback regulation of the auxin transport rate and polarity by a
localized auxin source. Such a mechanism would be adequate to
gradually generate more concentrated auxin channels that would
determine the position of the new vasculature and explain the
vasculature formation seen in leaves after wounding or in newly
initiated organs. Indeed, multiple feedback regulatory loops of
PIN-dependent auxin transport have been identified. Auxin
modulates PIN transcription (Vieten et al., 2005), PIN incidence
at the plasma membrane (Paciorek et al., 2005) and also PIN polar
localization (Sauer et al., 2006). For example, during the formation
of vascular veins in leaves, PIN1 directs auxin towards a
convergence point in the leaf epidermis, from where veins are
being initiated and where PIN1 expression and polar localization
mark the position of all future veins (Scarpella et al., 2006) (Fig.
4). Similarly, after wounding, PIN1 is repolarized, and a new
transport route is set up that determines the position of the
regenerating vasculature. Importantly, local auxin application is
sufficient to induce PIN1 expression, polarization and the
subsequent establishment of PIN1-based auxin channels, thus
essentially specifying the future vasculature (Sauer et al., 2006).
These observations provide strong support for the canalization
hypothesis and suggest that the auxin-dependent polarization of
PIN1 is a key event in vascular tissue formation during a variety
of developmental processes.
The role of other auxin transport mechanisms in this process is
unclear, but they might have supporting functions. For example,
AUX1 presumably facilitates auxin loading into and out of the
phloem component of the vascular transport system (Marchant et al.,
2002) (Fig. 4). ABCB19 is mostly localized in the vascular bundle
sheet cells and potentially prevents auxin leakage from the vascular
flow (Blakeslee et al., 2007).
Auxin routes in tropisms
The role of auxin and auxin transport in the directional growth
responses of plants to light (phototropism) and to gravity
(gravitropism) played a major role in the discovery of auxin and in the
formulation of the concept of plant hormones (Darwin, 1880). The
negative gravitropism of stems, the positive gravitropism of roots and
the positive phototropic curvature of stems are characterized by the
uneven distribution of auxin at the different sides of stimulated organs.
This differential auxin distribution activates asymmetric growth and
subsequent organ bending (Went, 1974) in a context-specific manner:
whereas higher intracellular auxin concentrations trigger elongation
in shoots, they are inhibitory in roots.
In roots, gravity is detected in the starch-containing root cap cells,
in which PIN3 is relocalized from its originally uniform distribution
to the bottom plasma membranes after gravistimulation (Friml et al.,
2002b). Auxin flow is redirected towards the lower side of the root tip,
from where it is transported through the lateral root cap and epidermal
cells towards the elongation zone, where growth-inhibitory auxin
responses are induced (Swarup et al., 2005). This basipetal transport
route requires both the epidermally localized PIN2 (Luschnig et al.,
1998; Müller et al., 1998) and AUX1 (Bennett et al., 1996; Swarup et
al., 2001; Swarup et al., 2005) (Fig. 5A). The flow along the lower side
of the root is further enhanced by the vacuolar targeting of PIN2 and
its degradation on the upper root side (Abas et al., 2006; Kleine-Vehn
et al., 2008b). In addition, ABCB-dependent auxin transport might
regulate the gravitropic response, considering that abcb4 and abcb1
abcb19 mutants show an enhanced gravitropic response (Lewis et al.,
2007) and a genetic interaction with pin2 (Mravec et al., 2008) (Fig.
5A). Moreover, flavonoids, the putative endogenous modulators of
auxin transport, might contribute to root bending through their
influence on PIN and ABCB4 expression and activity (Santelia et al.,
2008; Lewis et al., 2007).
In shoots, gravity is detected in endodermal cells (starch sheath
cells), where PIN3 is localized at the inner plasma membrane. The
corresponding pin3 mutants are partially defective in hypocotyl
gravitropism (Friml et al., 2002b). It is likely, but has not been
conclusively demonstrated, that, similar to the root gravitropic
response, the PIN3 relocation to the bottom side of endodermis cells
triggers auxin accumulation in the lower side of the shoot, where the
auxin response promotes growth and upward bending (Fig. 5B).
The mechanisms that generate auxin asymmetry in response to light
remain unclear, but studies with mutants or inhibitors show that
phototropism also requires the activity of all auxin transport
components, such as PIN3 (Friml et al., 2002b), AUX1 (Stone et al.,
2008), ABCB1 (Lin and Wang, 2005) and ABCB19 (Lin and Wang,
2005; Nagashima et al., 2008a; Nagashima et al., 2008b; Noh et al.,
2003).
Conclusions
As discussed here, the polarized transport of auxin is crucial for plant
development. In addition to the passive diffusion of auxin molecules
across plasma membranes, three active and mutually cooperating
auxin-transporting systems have been described so far. Whereas the
PIN auxin transporters are the primary determinants of directionality,
AUX1/LAX and ABCB proteins mainly generate auxin sinks and
control auxin levels in the auxin channels. The open questions for
future studies include the identification of the core action of the
different auxin transporters, how exactly auxin is transported across
the plasma membrane, how this process is regulated and how
individual transporters cooperate. Furthermore, the analysis of the
regulatory sequences in promoters of genes that code for auxin
transporters, together with the study of crosstalk with other plant
hormones, will be crucial for understanding how this system is
controlled by other signaling pathways. The wealth of available
genetic tools will significantly contribute to answering these
questions; however, more biochemical and structural biology work
will also be needed, in particular to address the issues of the precise
mechanism of auxin movement across the plasma membrane.
Acknowledgements
We apologize to all authors whose work is not cited here owing to space
constraints. We thank Martine De Cock for help in preparing the manuscript.
This work was supported by the Grant Agency of the Academy of Sciences of
the Czech Republic (J.P. and J.F.) and by the Odysseus program of the Research
Foundation-Flanders (J.F.).
DEVELOPMENTDEVELOPMENT
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