Download Auxin and other signals on the move in plants

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Auxin and other signals on the move in plants
© 2009 Nature America, Inc. All rights reserved.
Hélène S Robert & Jiří Friml
As multicellular organisms, plants, like animals, use endogenous signaling molecules to coordinate their own physiology and
development. To compensate for the absence of a cardiovascular system, plants have evolved specialized transport pathways to
distribute signals and nutrients. The main transport streams include the xylem flow of the nutrients from the root to the shoot and
the phloem flow of materials from the photosynthetic active tissues. These long-distance transport processes are complemented
by several intercellular transport mechanisms (apoplastic, symplastic and transcellular transport). A prominent example of
transcellular flow is transport of the phytohormone auxin within tissues. The process is mediated by influx and efflux carriers,
whose polar localization in the plasma membrane determines the directionality of the flow. This polar auxin transport generates
auxin maxima and gradients within tissues that are instrumental in the diverse regulation of various plant developmental processes,
including embryogenesis, organogenesis, vascular tissue formation and tropisms.
Plants differ from most animals in having a sessile lifestyles. They are
thus limited in their capacity to fight or rapidly escape from adverse
environmental situations. To overcome these restrictions, plants have
evolved multiple mechanisms to flexibly adapt their growth to environmental conditions and nutrient supplies. The most apparent examples
of this life strategy include specific developmental responses to various
external stimuli, such as regulation of plant size and architecture1 in
response to light availability (shade avoidance, photomorphogenesis and
phototropism)2–4. Plant hormones, which are small signaling molecules,
play a crucial role in regulating and coordinating plant growth and are
involved in all developmental processes, including directional growth
responses (tropisms)4, control of plant architecture5–7, abiotic and biotic
stress responses8,9 and flower and embryo development10–12. The phytohormone auxin plays a prominent role by acting as a versatile signal
for spatial-temporal coordination of development4,10–15. Directional,
intercellular auxin transport, a distribution mechanism unique to auxin,
generates differential distribution of auxin within plant tissues. The
resulting auxin concentration gradients, or localized areas with high
auxin concentration (auxin maxima), provide positional information
in the course of many developmental processes.
In plants, the distribution of substances can pose complications
because the absence of blood and lymphatic streams limits the effective distribution of hormones, nutrients and metabolites. Therefore,
plants rely on other transport mechanisms such as a vascular system
and intercellular transport pathways. Two vascular networks, the phloem
and the xylem, serve as the primary conduits for long-distance transport
but with opposite directions of flow (from and to the source tissues,
respectively)16 (Box 1, Fig. 1). Short-range intercellular transport can be
realized through three main mechanisms (Box 1, Fig. 2): (i) apoplastic
transport through the extracellular space, (ii) symplastic transport aided
by the plasmodesmata, which are plant-specific cytoplasmic tunnels17,18,
Department of Plant Systems Biology, Flanders Institute for Biotechnology (VIB) and
Department of Plant Biotechnology and Genetics, Ghent University, Gent, Belgium.
Correspondence should be addressed to J.F. ([email protected]).
Box 1 Key terms
Xylem Part of the vasculature; transports water, nutrients and
some hormones from the root to the aerial, photosynthetically active tissues.
Phloem The other part of the vasculature; carries products of
the photosynthesis (mainly sucrose) and some hormones from the
source tissues to the sink tissues.
Symplastic transport Plant-specific type of transport through
the symplasm, which is composed of a network of connected cell
cytoplasms. This transport uses plasmodesmata, specific tunnellike connections between the cytoplasm of two cells.
Apoplastic transport Extracellular transport mechanism in which
molecules diffuse through the extracellular matrix (cell wall).
Transcellular transport A cell-to-cell transport mechanism
that involves crossing both the apoplast and the symplast and
requires activity of transporters or channels to cross the plasma
membrane.
Plasmodesmata Plant-specific connections between cytoplasm
of two neighboring cells, allowing targeted and nontargeted transport of molecules through networks of connected cells called
symplasm.
Cell wall Complex, cellulose-based structure that surrounds
plant cells.
Casparian band Located at the endodermis cell layer, the
Casparian band is made of the wax-like hydrophobic substances
suberin and lignin and forms a barrier that limits apoplastic
transport between the outer cell layers and the vasculature.
Stomata A pore in the epidermis of aerial tissues, whose regulated opening and closing mediates gas exchange with the surrounding environment and regulates water transport throughout
the plant.
Guard cells Cells that form the pore of the stomata. Their
turgescence, regulated by temperature, humidity, light, ABA and
calcium, controls the opening and the closing of the stomata.
Published online 17 April 2009; doi:10.1038/nchembio.170
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a
Xylem
pathway
Transpiration
b
Phloem
pathway
Epidermis
Cortex
Endodermis
Interfascicular region
(fibers)
Xylem
}
Phloem
© 2009 Nature America, Inc. All rights reserved.
Phloem
Xylem
Import from the soil
c
Vascular
bundle
Epidermis
Cortex
Endodermis
Phloem
Xylem
}
Stele
Figure 1 Phloem and xylem transport in plants. (a) Routes of long-distance transport in
plants. The xylem transport (blue) conducts water and nutrients from the root to the shoot. The
phloem flow (red) redistributes the photosynthetic products from the leaves to roots and other
sink tissues. (b,c) Shoot stem (b) and root (c) section schemes showing the disposition of the
vascular tissues.
and (iii) transcellular transport crossing both the intra- and intercellular
space19 and requiring repeated transport through plasma membranes.
The goal of this review is to provide a general overview of the different
types of signaling molecule transport in plants, with a particular focus
on the directional, transcellular transport that is unique to the plant
hormone auxin. This so-called ‘polar auxin transport’ has been intensively studied for more than a century and is important in generating
the differential auxin distribution within tissues that is crucial to the
regulation of many essential aspects of plant development.
Transported substances
Water, nutrients, biomolecules and organisms such as viruses and fungi
are all examples of cargos that get transported throughout the plant
by one or several different transport mechanisms. Mineral nutrients,
such as nitrogen, sulfur, phosphorus, magnesium, calcium and potassium, are taken up from the soil as inorganic materials, assimilated into
organic compounds and transported through the plant depending on
the balance between nutrient availability and the concentrations of the
assimilated forms. The transport of nutrients, and thus this balance, is
regulated by both environmental (nutrients, carbon, water availability)
and internal (hormonal, metabolic, osmotic, energetic signals) cues
mainly via regulation of the activity or expression of specific transporters20. Most macronutrients are used to manufacture amino acids,
proteins, nucleic acids, phospholipids, chlorophyll or the cell wall.
Phosphorus and magnesium, mainly in the form of Mg-ATP, have an
important function in bioenergetics. Potassium and calcium ions are
crucial to maintaining the electrical potential of cellular membranes.
Potassium plays a specific role in guard cells (Box 1), influencing the
opening and closing of stomata in response to external factors such
326
as temperature, CO2 and light and helping to
regulate the water balance of the plant. Calcium,
in plants as in other eukaryotes, is a major cytoplasmic secondary messenger in many intracellular signaling pathways21. Carbon is distributed
mainly in the form of the photosynthetic product
sucrose, which is transported from the photosynthetic source tissues to sink tissues (growing
parts at the shoot and the root apices) where it is
required for starch formation and storage during
fruit and seed development22. Phytohormones,
transcription factors and mRNA molecules are
mostly locally synthesized and transported as
short-range signaling molecules to neighboring
cells, but in some instances they undergo vasculature-based long-distance transport as well.
Pathogenic viruses and fungi have adapted their
infection strategies to hijack the plant transport
pathways so as to efficiently spread through plant
tissues23,24.
Types of transport
Plants possess different transport systems that
can be classified according to the distance and
the direction of transport. To assure that the substances are effectively distributed, higher plants
combine a long-distance transport system with a
short-range cell-to-cell distribution network.
Long-distance transport: xylem and phloem.
Xylem and phloem constitute the vasculaturebased transport systems. They both consist of
conductive elements that form continuous tubular columns. Xylem is
made of tracheary elements, which are dead, thick-walled cells that are
depleted of their cellular content (including the nucleus) and are perforated at both ends. The rigid composition of xylem also provides an
essential structural support, helping the plant to maintain its stature.
Phloem, on the other hand, is formed by sieve elements connected into
a long sieve tube. In contrast to mature tracheary elements, sieve elements are living cells but depleted of most of the cellular content. In
shoot stems of the model plant Arabidopsis thaliana, xylem and phloem
are located in collateral vascular bundles, with phloem at the periphery
and xylem in the center. In Arabidopsis roots, xylem and phloem are
arranged in bilateral symmetry, with the xylem in a medial axis flanked
by two phloem poles16 (Fig. 1).
Xylem transports and stores water, nutrients and hormones from the
roots to the aboveground tissues (Fig. 1). The hormones transported by
xylem include abscisic acid (ABA)25, cytokinins26 and strigolactones—
newly identified hormones that regulate shoot branching5,6,27. Phloem
distributes products of photosynthesis (mainly carbohydrates), as well
as proteins, mRNAs and hormones such as auxin28, ABA25 and cytokinins26, from the source tissues (photosynthetically active leaves and
young buds) to sink tissues29 (Fig. 1). Transport of signals through the
phloem has been shown to be crucial for development and defense
against pathogens. A classical example of phloem’s developmental
role is the transition from vegetative development to flowering that is
induced by day length–dependent photoperiodic stimuli generated in
leaves. One component of this so-called ‘florigen’ signal is the protein
FLOWERING LOCUS T, which is produced in the leaves and moves
through the phloem to the shoot apex, where it activates the flowering
signaling pathway29,30.
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© 2009 Nature America, Inc. All rights reserved.
Short-range transport. Membranes of plant cells are not in direct contact with each other because of the presence of a cellulose-based cell
wall. Thus, molecules have three possibilities for short-range movement:
(i) transport inside the cell wall, without entering the cell (apoplastic transport); (ii) direct transport between the cytoplasm of two cells
through the specialized, plant-specific structures known as plasmodesmata (symplastic transport); and (iii) transcellular transport involving
entering and exiting the cells across the plasma membrane and crossing
the apoplastic space between them (Box 1, Fig. 2).
Intercellular radial transport, combining the different types of cellto-cell transport pathways, often contributes crucially to the loading
of vascular stream. Water, nutrients and ions (boron, iron, magnesium, nitrates and others) are absorbed from the soil by the root
epidermis and are further transported through the internal cell layers
to the xylem (Fig. 3).
Apoplastic transport uses diffusion of transported molecules through
the cell wall all the way from the root surface to the endodermis cell
sheet that surrounds the vascular bundle. There, the apoplastic flow
is limited by the Casparian strip, a massive deposit of suberin and/or
lignin, which are wax-like hydrophobic substances (Fig. 3). This structure creates a tight barrier that prevents the invasion of pathogens and
restricts the apoplastic flow of water and nutrients into and out from
the vasculature19,36,37. To reach the vasculature, molecules have to cross
the membranes of the endodermal cells and enter the symplasm. For
example, ABA, the drought stress hormone, as well as water and solutes
move in the apoplast, but before reaching the vascular flow, they enter
the endodermis cells to pass the Casparian barrier19. The apoplast is also
the main location for short-distance transport between the cells that
secrete a signal and the target cells, where the signal interacts typically
with a cell surface–localized receptor, as is the case for hormones such as
brassinosteroids38 and cytokinins26 and for peptide-based signals such
as the CLAVATA peptide that is important for maintaining stem cell
population at the shoot apex39.
Symplastic transport connects the cytoplasm of two neighboring
cells directly, through the plasmodesmata18(Fig. 2). Plasmodesmata
are tunnels in the plant cell wall that interconnect the cytoplasms of
a group of cells and create symplastic domains that vary in size during development40,41. The plasmodesmata-mediated transport of sugars42, RNAs43, peptides, signal proteins44–47, ribonucleoproteins, plant
viruses23 and fungi24 is important in non-cell-autonomous signaling,
signal relay, defense against pathogens and distribution of nutrients.
Symplastic transport is regulated by the exclusion limit (‘aperture’)
of the plasmodesmata pores, which control the passage of materials
through plasmodesmata based on molecular size40,41,44. Some proteins
targeted for this type of transport can specifically adapt the exclusion
limit to allow their own flow48.
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Symplastic
transport
Transcellular
transport
Apoplastic
transport
Cell wall
Plant hormones not only are transported through the vasculature but
also have major effects on its formation and differentiation31. This is particularly true for auxin, which is a well-known player in determining the
complex pattern of vasculature in leaves13, forming the vasculature connecting newly formed organs and regenerating vasculature after wounding32. During these processes, the initial auxin flow is reinforced through a
positive feedback regulation by auxin itself, which controls the throughput
and directionality of the auxin flow. This gradual canalization of auxin
flow leads to the formation of auxin-transporting cell files (canals) that
demarcate the position of the future vascular strands. This self-organizing property of auxin transport is referred to as the “auxin canalization
model”13,32,33. Other hormones are also involved in vasculature formation.
For instance, cytokinins act as negative regulators of xylem specification34,
whereas brassinosteroids promote xylem differentiation35.
Diffusion
ER complex/plasmodesmata
Receptors
Transporter/permease/carrier
Vesicular-mediated
secretion
Transcellular transport
Symplastic transport
Apoplastic transport
Figure 2 Three pathways for intercellular transport. Symplastic
transport (left) uses plasmodesmata that interconnect the cytoplasms
of neighboring cells. A modified endoplasmic reticulum (ER) forms part
of the plasmodesmata structure. Apoplastic transport (right) involves
passive diffusion of molecules in the extracellular space, the cell wall.
Transcellular transport (center) combines apoplastic transport with a
secretion- and endocytosis-based or channel- and carrier-based transport
pathway to cross plasma membranes.
Passage through the plasmodesmata not only is a means of transport but may also serve as a mechanism to regulate protein activity. An
example is the plasmodesmata-based movement of transcription factors such as SHORT-ROOT or CAPRICE, which are involved in radial
root and epidermis patterning, respectively. These proteins are cytosolic
and inactive in the cells in which they are produced, but after they have
moved to neighboring cells through plasmodesmata, they are targeted to
the nucleus and become active in promoting transcription of patterning
factors44,47. These observations imply that proteins can be modified by
their passage through the plasmodesmata, but the mechanism underlying this effect is unknown.
Transcellular transport is the movement of molecules from cell to cell
without the use of direct symplastic connections, which implies transport through plasma membranes by import-export mechanisms such as
membrane diffusion, secretion and receptor- or transporter-mediated
systems (Fig. 2). Such transport is typically slower and more energetically demanding but allows more elaborate regulation and integration
of various signals at the level of each transporting cell. For example,
nutrients have specific plasma membrane transporters that are regulated
by plant nutrient status and external cues20. This regulation can occur at
several levels, such as transcription49,50, degradation51,52, trafficking53 or
activity of the transporters. Signals regulating the expression or activity
of the transporters include nutrient availability54, photosynthetic activity, and the level of sucrose55,56 and cross-talk between nutrients57,58.
Some molecules—such as boric acid, the transported form of boron
or auxin—use a transcellular transport mechanism. Boric acid, as
a weak acid, is radially transported by passive diffusion in the apoplast or by a transporter-mediated mechanism from the root surface
to the xylem (Fig. 3). There are two main transmembrane transport
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roots, leaves or flowers, thus selecting these
cells from the field of similar cells to undergo
B B
Apoplast B flux
the fate change15,65,66. In other instances,
B
B
B
such as in gynoecium development67 or root
DetoxiB
meristems68,69, the auxin distribution can be
B
B
fication
B B
B
B
described in terms of a gradient, whereby the
B
B
B
B
Uptake
B
auxin concentration changes more gradually
across the tissue. Localized auxin minima
B
B
Parenchyma
Epidermis
Cortex
Endodermis Pericycle
can also regulate tissue patterning, as shown,
Xylem
Phloem
for example, in fruit development and seed
BOR1
NIP5;1
NIP6;1
BOR4
dispersal70.
Stele
Differential auxin accumulation is perceived
and interpreted at the level of individual cells
Figure 3 Boron transport as an example that combines different transport mechanisms. Radial
by the nuclear auxin signaling pathway, which
transport of boron uses several plasma membrane–localized transporters: the BOR exporters and the
regulates gene expression and reprogramming
53
NIP influx carriers. BOR1 localizes to the endodermis and pericycle . BOR4 assists in the boron
of cell fates71. Differential auxin distribution
61
detoxification of the plant by secreting boron from the root . NIP5;1 localizes to the epidermal,
is essential for many developmental processes,
cortical and endodermal cells59, whereas NIP6;1 is required for transport between the xylem and the
and thus manipulation of auxin distribution
phloem flows60. The apoplastic distribution of boron is limited by the Casparian barrier in endodermis.
or cellular auxin signaling strongly disrupts the
associated developmental process12,14,66,68,72.
One important question concerns how this differential auxin distrimechanisms: import into epidermal, cortical or endodermal cells, and
export from endodermal, pericycle or xylem parenchyma cells into bution is generated and maintained. Both polar auxin transport and
the xylem flow (xylem loading). Polarly localized plasma membrane local auxin biosynthesis contribute to differential auxin distribution.
transporter proteins have been identified and found to be involved in Two tryptophan-dependent auxin biosynthesis pathways, which are
the cellular import and export of boric acid (Fig. 3)53,59–61. Similarly, dependent on the YUCCA73,74 and TAA1 (ref. 75) protein families, are
auxin transport uses a transcellular, cell-to-cell pathway, characterized required for differential auxin distribution during some developmental
by influx and efflux transporters. This transport mechanism will be dis- processes72. Mutants defective in either of these pathways have aberrant
cussed in detail because of its unique aspects and prominent implica- DR5 activity patterns and related developmental defects74,75.
tions for plant development.
Notably, different internal and external signals have been shown to
modulate auxin biosynthesis (at the level of transcription of the key
Auxin transport and differential auxin distribution
enzymes) and polar auxin transport (at the level of transcription, cell
The principal naturally occurring form of auxin is indole-3-acetic acid surface abundance and polar localization of transport components)76.
(IAA), a weak acid derivative of the amino acid tryptophan. Auxin is As a result, transport-dependent differential auxin distribution reprea small signaling molecule that acts as a versatile trigger in multiple sents an additional level of regulation and signal integration in auxin
developmental processes including embryogenesis12, organogenesis15, signaling.
vascular development13 and tropic responses4,12. Its distribution
throughout the plant combines rapid, long-distance movement through The chemiosmotic model. Attempts to explain the mechanism underlythe phloem28 with a slow cell-to-cell transport that is highly regulated ing polar auxin transport originally relied on biochemical and physiand occurs strictly in a given direction within tissue (Fig. 4). This trans- ological studies that led to the formulation of the chemiosmotic model
cellular-type of transport is termed ‘polar auxin transport’. It is unique in the 1970s77,78. Auxin—IAA—is a weak acid, and its transporter-based
to auxin and was discovered by Darwin while studying the phototropic transcellular pathway takes advantage of the pH difference between the
growth responses of grasses to unidirectional light62. The actual chemi- extracellular space and the cytoplasm that is generated by plasma memcal substance behind the growth-inducing (‘auxein’ in Greek) signal was brane H+-ATPases. The apoplast has a more acidic pH (5.5), in which
isolated by Went and Thimann several decades later63.
IAA is partly protonated and can diffuse passively through the plasma
The important recent discovery in the auxin field concerns the find- membrane into the cell. This passive influx is aided by the action of
ing that in the course of many auxin-dependent processes, the activity auxin uptake carriers. In the more basic environment of the cytoplasm,
of auxin shows differential distribution between cells. Auxin activ- IAA exists in its charged, membrane-impermeable form and is trapped
ity can be visualized indirectly in vivo by monitoring the activity of inside the cell, so that its transport out of the cell requires auxin efflux
the synthetic auxin-responsive promoter DR5 (ref. 64) (Fig. 5a–c). In carriers (Fig. 4). Transport experiments using radioactively labeled auxin
theory, DR5 activity is proportional to the throughput of nuclear auxin derivatives established that auxin transport is directional (polar), with
signaling in a given cell and should reflect levels of free, active auxin the main transport route running from the apex of the shoot toward that
in the cell. Despite the obvious limitations of this approach, measured of the root. To mechanistically explain this directionality, the chemiosDR5 activity has been shown to correlate well with auxin accumulation motic model postulated that an asymmetric (polar) plasma membrane
patterns (as inferred from anti-IAA immunolocalization studies) dur- localization of the efflux carriers would determine the direction of the
ing embryogenesis and organogenesis and in the root meristem12,15. In intercellular auxin flow77,78. This assumes that the polarity and polar
some instances, the differential auxin distribution can best be described targeting of auxin carriers at the level of a single cell determines the
in terms of local auxin maxima, in which one or a small group of cells directional transport of a signal between cells and, thus, mediates the
shows high auxin activity in sharp contrast with that of the surround- development of the whole tissue. The chemiosmotic hypothesis gained
ing cells. Such local auxin accumulations or maxima can be seen dur- strong molecular support after the auxin efflux carriers were identified
ing various organogenic processes. For example, auxin accumulates and the importance of their polar subcellular localization for the direcin founder cells that will give rise to new organ primordia for lateral tionality of auxin flow was demonstrated12,69,79–82.
© 2009 Nature America, Inc. All rights reserved.
Casparian band
328
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review
The auxin transport proteins. The chemiosmotic hypothesis originally postulated the
existence of auxin influx and efflux carriers in
OH
the absence of information about their molecc
b
a
Cell wall
AUX1/LAX
ular identity. Since then, largely thanks to the
pH 5.5
establishment of Arabidopsis as a successful
O
model organism for plant molecular genetCytoplasm
pH 7
ics, the identity of the auxin transporters has
N
been revealed. These comprise (i) the AUX1/
IAA–
PGP
H
LAX influx proteins, which mediate auxin
IAAH
entry to the cell; (ii) the ABCB/PGP proteins,
Cell-to-cell
PIN
which mediate ATP-dependent auxin transport
polar
auxin
–
+
through the plasma membrane; and (iii) the
IAAH
IAA + H
transport
AUX1/LAX
PIN exporters, which determine the directionPhloem
Phloem
ality of auxin transport by means of their polar
auxin
Xylem
subcellular localization.
IAAH
distribution
PGP transporters
The AUX1/LAX influx transporter protein
PGP
AUX1/LAX auxin
family was identified through the isolation
IAA– + H+
influx carriers
of the Arabidopsis auxin1 (aux1) mutant.
PIN auxin efflux
aux1 roots are resistant to the membranePIN
carriers
IAAH
impermeable synthetic auxin derivative 2,4dichlorophenoxyacetic acid (2,4-D) and show
agravitropic growth—both consequences of a
defect in cellular auxin uptake. The AUX1 gene
Figure 4 Phloem-based transport and chemiosmotic model for polar auxin transport. (a) Auxin
codes for a protein homologous to amino acids
distribution via the phloem from source tissues—young leaves and floral buds—to root and shoot
83
permeases and has been shown to mediate tips. (b) The chemiosmotic model, based on the pH difference between the apoplast (pH 5.5) and
auxin import when heterologously expressed in the cytoplasm (pH 7.0). Protonated auxin—undissociated indole-3-acetic acid (IAAH)—can diffuse
Xenopus laevis oocytes84. AUX1 localizes to the through the lipidic plasma membrane or be transported by the AUX1/LAX influx carriers into the cell.
plasma membrane of various cell types; nota- In the cytosol, it dissociates and gets trapped inside the cell in its deprotonated form (IAA–). IAA– can
bly, this includes asymmetric localization at the exit cells by the action of PGP- or PIN-type efflux carriers. The polar cellular localization of the carriers
upper side of young phloem (protophloem) determines the directionality of the intercellular auxin flow. (c) Structure of protonated IAAH.
cells of the root, where AUX1 presumably
unloads auxin from the phloem flow into the
short-range transcellular transport pathway in the root meristem85. In mediate directional auxin fluxes and ensure the fine regulation of the
Arabidopsis, three LIKE AUX1 (LAX) proteins are present and also seem auxin distribution in many developmental processes.
to function as auxin influx carriers. For example, they act redundantly
PIN proteins are crucial factors in determining the directionality of
to regulate phyllotaxis86, and LAX3 has a unique role in communication auxin flow. They have been identified based on the auxin transport–
between the growing lateral root primordium and surrounding tissues87. related shoot and flower phenotypes of pin-formed1 (pin1) mutants
In summary, there is compelling biochemical and genetic evidence that and the agravitropic root growth of pin2 mutants80,81. The function
the AUX1/LAX proteins act as influx carriers and have important roles of several Arabidopsis PIN proteins in cellular auxin efflux has been
in root growth, tropisms and organogenesis.
demonstrated in tobacco BY-2 cultured cells, yeast and mammaAnother class of transporters implicated in auxin transport are the lian HeLa cells88. Setting aside the PIN5, PIN6 and PIN8 subclade,
phosphoglycoproteins (PGPs), which are members of the B-type ATP- whose function is unclear, the developmental roles of the other five
binding cassette (ABCB) transporter family. They were originally identi- Arabidopsis PIN proteins are well documented94,95 and include actions
fied as proteins binding to the synthetic inhibitors of auxin efflux. Some during embryogenesis12,80, organogenesis14,15, root meristem patternof these proteins—PGP1, PGP19 and PGP4—mediate auxin efflux88–90 ing95, vascular tissue differentiation13 and tropic responses79 (Fig. 5).
and show complex, multilevel functional interaction with other efflux Notably, PIN proteins typically show asymmetric plasma membrane
transporters from the PIN family91–93. Combinations between pgp localization within cells, as predicted by the chemiosmotic hypothesis
mutants and pin loss-of-function mutants indicate that these two types for auxin efflux carriers. This polarity at the cellular level correlates with
of auxin exporters can act both synergistically and antagonistically in and determines (at least in some instances) the directionality of the
different developmental processes91,92. Although their functional rela- auxin flow within tissues72,82,96.
tionship is far from clear, the PIN and PGP proteins probably comprise
Particularly important are changes in PIN subcellular polarity,
two distinct auxin efflux mechanisms. At the polar domains where the because by this mechanism different signals can redirect the auxin
PGP and PIN proteins colocalize and interact, PGPs might act syner- fluxes and trigger developmental reprogramming12,79. For example,
gistically with PIN proteins in auxin efflux, possibly by regulating PIN at early stages of Arabidopsis embryo development, PIN7 is polarized
stability at plasma membrane microdomains93. In addition, PGPs can at the upper side of suspensor cells and mediates auxin flow to the
also act at the nonpolar domains, independently of the PIN proteins; young embryo, where auxin accumulates. However, at later stages of
in such a scenario, PGPs would mediate apolar auxin efflux and control embryo development, an as-yet-unknown signal causes polarization
the amount of auxin available in PIN-expressing cells for the directional of PIN1 and PIN7 to the lower side of cells, and thus auxin is redistribPIN-driven transport91. Thus, complex interactions of directional PIN- uted from the embryo to the root pole where a new auxin maximum
and ATP-dependent PGP-dependent transport systems at different levels is established12 (Fig. 5a–f). PIN polarization and the resulting auxin
nature chemical biology volume 5 number 5 MAY 2009
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a
b
c
d
e
f
root tip—translocates to the new bottom side of these gravity-perceiving cells79,98. This PIN3 relocalization (presumably together with
the redundant action of coexpressed PIN4 and PIN7) redirects auxin
flow toward the bottom side of the root, where auxin is further transported, by a concerted action of AUX1 and PIN2, to the elongation
zone85,99. Here auxin accumulation inhibits growth, ultimately leading to the downward bending of the root (Fig. 5g). These examples of
PIN polarity switches demonstrate that this mechanism can be used
to integrate diverse signals and to trigger developmental changes by
redirecting auxin fluxes.
g
v
en
c
e
lrc
Gravity
Gravity
PIN1
PIN2
PIN3
PIN4
PIN7
Auxin
Figure 5 Dynamic PIN polar localization during embryo and root
development. (a–f) Embryo development: DR5 auxin response activity
(a–c) and PIN1 localization (d–f). Illustrated is the 8-cell stage (a,d),
during which auxin accumulates in the proembryo and PIN1 shows
no pronounced polarity; the globular stage (b,e), during which PIN1
polarizes toward the root pole to transport auxin to this region, where
auxin accumulation serves as a signal for room specification; and the
(early) heart stage (c,f), during which auxin response persists at the root
pole. (g) Root gravitropism: auxin distribution (green) and polarity of the
PIN localization and auxin fluxes (depicted by arrows) in the root tip (left)
and effect of a gravistimulation on the auxin distribution and auxin fluxes
(right). Gravity induces PIN3 relocation to the bottom side of gravityperceiving root cap cells. Auxin flow is thus redirected to the lower side of
the root, where auxin inhibits elongation and causes root bending. Panel
a is reproduced with permission from ref. 12 and panel g is adapted from
ref. 95 with kind permission from Springer Science and Business Media.
Scale bars in a–f represent 20 µm.
accumulation at the root pole are key triggers for the specification of
the future root meristem, as demonstrated by the fact that mutants
with defects in these processes do not develop a functional root72,96.
Moreover, if PIN polarization fails, the apical part of the embryo ectopically accumulates auxin even at later stages, giving a false signal for
root initiation and leading to the development of root-like structures
that are derived from embryonic leaf tissue97.
In addition to developmental signals, environmental signals also
have been shown to trigger PIN polarity changes. During the root
gravitropic response, even within in a matter of minutes after gravistimulation, PIN3—previously localized uniformly in the cells of the
330
Conclusions
The transport mechanisms in higher plants, as exemplified by the
model plant Arabidopsis, rely on two main streams. First, the longdistance transport system depends mainly on the specialized conductive tissue—the vasculature. The xylem serves as a means to transport
water, nutrients and some hormones from the root, whereas the
phloem is used predominantly for the distribution of the photosynthetic products and signals from the source tissues to the rest of the
plant. Second, short-distance cell-to-cell transport often complements
vasculature-based translocation and is used to load and unload substances from the vasculature as well as to distribute short-range signals within tissues. Transported signals comprise signaling peptides,
proteins such as transcription factors, mRNAs and siRNAs that use an
apoplastic, symplastic or transcellular transport mechanisms to move
through tissues and mediate a non-cell-autonomous signaling. In
addition, plant pathogens, including viruses, have adapted themselves
to hijack existing transport systems to efficiently infect plants. A special case of signal distribution is the cell-to-cell directional movement
of the phytohormone auxin, a versatile spatial-temporal signal. Polar
auxin transport can integrate various signals at the level of individual
transporting cells and generate local auxin maxima and gradients
that are instrumental for many developmental processes. This unique
signal-molecule transport mechanism to a large extent underlies the
remarkable developmental plasticity of plants that allows their growth
and architecture to adjust to changing environments.
Unraveling the mechanisms of transport and their associated regulation is of great economical and ecological importance, for example
to reduce the use of fertilizers or to engineer plants more suitable for
dry or other suboptimal growth conditions. However, there are still
many open questions in our understanding of transport processes
in plants. The powerful approaches of molecular genetics have been
helpful, but they are limited by the potential lethality of mutations
affecting essential transport processes and by the extensive functional
redundancy present in the genome. A combination of classical forward
genetics, chemical genomics and biochemical approaches may help to
identify yet-undiscovered transport components. Furthermore, transcriptome and metabolite profiling under various stress conditions
have hinted at extensive cross-talk between transport for different
nutrients58,100. The mechanisms and molecular components of such
cross-talk regulation are still largely unknown and will also be an
important topic for future studies.
ACKNOWLEDGMENTS
We thank E. Meyerowitz for providing seeds of the DR5rev:N7:VENUS, S.
Vanneste for providing material for Figure 5 and M. De Cock for help in
preparing the manuscript. The authors are supported by the FWO (Fonds voor
Wetenschappelijk Onderzoek).
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