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. (jiri.friml@psb.vib-ugent.be).

Published online 17 April 2009; doi:10.1038/nchembio.170

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 environ-mental 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 phy-tohormone 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 effec-tive 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,

Auxin and other signals on the move in plantsHélène 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.

Box 1 Key termsXylem Part of the vasculature; transports water, nutrients and some hormones from the root to the aerial, photosynthetically ac-tive 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 tunnel-like 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 trans-port 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 regu-lated opening and closing mediates gas exchange with the sur-rounding 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.

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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 cyto-plasmic secondary messenger in many intracel-lular signaling pathways21. Carbon is distributed mainly in the form of the photosynthetic product sucrose, which is transported from the photo-synthetic 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 vas-culature-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 transportPlants possess different transport systems that can be classified according to the distance and the direction of transport. To assure that the sub-stances 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 vasculature-based 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 per-forated 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 ele-ments 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 cyto-kinins26, 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.

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 inten-sively 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 substancesWater, 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 potas-sium, 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 trans-porters20. 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

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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.

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Passage through the plasmodesmata not only is a means of trans-port but may also serve as a mechanism to regulate protein activity. An example is the plasmodesmata-based movement of transcription fac-tors 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 underly-ing this effect is unknown.

Transcellular transport is the movement of molecules from cell to cell without the use of direct symplastic connections, which implies trans-port 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 energeti-cally 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 activ-ity, 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 apo-plast or by a transporter-mediated mechanism from the root surface to the xylem (Fig. 3). There are two main transmembrane transport

Plant hormones not only are transported through the vasculature but also have major effects on its formation and differentiation31. This is par-ticularly true for auxin, which is a well-known player in determining the complex pattern of vasculature in leaves13, forming the vasculature con-necting newly formed organs and regenerating vasculature after wound-ing32. 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-organiz-ing 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.

Short-range transport. Membranes of plant cells are not in direct con-tact 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 (apoplas-tic transport); (ii) direct transport between the cytoplasm of two cells through the specialized, plant-specific structures known as plasmodes-mata (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 cell-to-cell transport pathways, often contributes crucially to the loading of vascular stream. Water, nutrients and ions (boron, iron, magne-sium, 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 struc-ture 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 dur-ing development40,41. The plasmodesmata-mediated transport of sug-ars42, 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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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.

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roots, leaves or flowers, thus selecting these cells from the field of similar cells to undergo the fate change15,65,66. In other instances, such as in gynoecium development67 or root meristems68,69, the auxin distribution can be described in terms of a gradient, whereby the auxin concentration changes more gradually across the tissue. Localized auxin minima can also regulate tissue patterning, as shown, for example, in fruit development and seed dispersal70.

Differential auxin accumulation is perceived and interpreted at the level of individual cells by the nuclear auxin signaling pathway, which regulates gene expression and reprogramming of cell fates71. Differential auxin distribution is essential for many developmental processes, and thus manipulation of auxin distribution or cellular auxin signaling strongly disrupts the associated developmental process12,14,66,68,72.

One important question concerns how this differential auxin distri-bution is generated and maintained. Both polar auxin transport and local auxin biosynthesis contribute to differential auxin distribution. Two tryptophan-dependent auxin biosynthesis pathways, which are dependent on the YUCCA73,74 and TAA1 (ref. 75) protein families, are required for differential auxin distribution during some developmental processes72. Mutants defective in either of these pathways have aberrant DR5 activity patterns and related developmental defects74,75.

Notably, different internal and external signals have been shown to modulate auxin biosynthesis (at the level of transcription of the key enzymes) and polar auxin transport (at the level of transcription, cell surface abundance and polar localization of transport components)76. As a result, transport-dependent differential auxin distribution repre-sents an additional level of regulation and signal integration in auxin signaling.

The chemiosmotic model. Attempts to explain the mechanism underly-ing polar auxin transport originally relied on biochemical and physi-ological studies that led to the formulation of the chemiosmotic model in the 1970s77,78. Auxin—IAA—is a weak acid, and its transporter-based transcellular pathway takes advantage of the pH difference between the extracellular space and the cytoplasm that is generated by plasma mem-brane H+-ATPases. The apoplast has a more acidic pH (5.5), in which IAA is partly protonated and can diffuse passively through the plasma membrane into the cell. This passive influx is aided by the action of auxin uptake carriers. In the more basic environment of the cytoplasm, IAA exists in its charged, membrane-impermeable form and is trapped inside the cell, so that its transport out of the cell requires auxin efflux carriers (Fig. 4). Transport experiments using radioactively labeled auxin derivatives established that auxin transport is directional (polar), with the main transport route running from the apex of the shoot toward that of the root. To mechanistically explain this directionality, the chemios-motic model postulated that an asymmetric (polar) plasma membrane localization of the efflux carriers would determine the direction of the intercellular auxin flow77,78. This assumes that the polarity and polar targeting of auxin carriers at the level of a single cell determines the directional transport of a signal between cells and, thus, mediates the development of the whole tissue. The chemiosmotic hypothesis gained strong molecular support after the auxin efflux carriers were identified and the importance of their polar subcellular localization for the direc-tionality of auxin flow was demonstrated12,69,79–82.

mechanisms: import into epidermal, cortical or endodermal cells, and export from endodermal, pericycle or xylem parenchyma cells into the xylem flow (xylem loading). Polarly localized plasma membrane transporter proteins have been identified and found to be involved in the cellular import and export of boric acid (Fig. 3)53,59–61. Similarly, auxin transport uses a transcellular, cell-to-cell pathway, characterized by influx and efflux transporters. This transport mechanism will be dis-cussed in detail because of its unique aspects and prominent implica-tions for plant development.

Auxin transport and differential auxin distributionThe principal naturally occurring form of auxin is indole-3-acetic acid (IAA), a weak acid derivative of the amino acid tryptophan. Auxin is a small signaling molecule that acts as a versatile trigger in multiple developmental processes including embryogenesis12, organogenesis15, vascular development13 and tropic responses4,12. Its distribution throughout the plant combines rapid, long-distance movement through the phloem28 with a slow cell-to-cell transport that is highly regulated and occurs strictly in a given direction within tissue (Fig. 4). This trans-cellular-type of transport is termed ‘polar auxin transport’. It is unique to auxin and was discovered by Darwin while studying the phototropic growth responses of grasses to unidirectional light62. The actual chemi-cal substance behind the growth-inducing (‘auxein’ in Greek) signal was isolated by Went and Thimann several decades later63.

The important recent discovery in the auxin field concerns the find-ing that in the course of many auxin-dependent processes, the activity of auxin shows differential distribution between cells. Auxin activ-ity can be visualized indirectly in vivo by monitoring the activity of the synthetic auxin-responsive promoter DR5 (ref. 64) (Fig. 5a–c). In theory, DR5 activity is proportional to the throughput of nuclear auxin signaling in a given cell and should reflect levels of free, active auxin in the cell. Despite the obvious limitations of this approach, measured DR5 activity has been shown to correlate well with auxin accumulation patterns (as inferred from anti-IAA immunolocalization studies) dur-ing embryogenesis and organogenesis and in the root meristem12,15. In some instances, the differential auxin distribution can best be described in terms of local auxin maxima, in which one or a small group of cells shows high auxin activity in sharp contrast with that of the surround-ing cells. Such local auxin accumulations or maxima can be seen dur-ing various organogenic processes. For example, auxin accumulates in founder cells that will give rise to new organ primordia for lateral

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Figure 3 Boron transport as an example that combines different transport mechanisms. Radial transport of boron uses several plasma membrane–localized transporters: the BOR exporters and the NIP influx carriers. BOR1 localizes to the endodermis and pericycle53. BOR4 assists in the boron detoxification of the plant by secreting boron from the root61. NIP5;1 localizes to the epidermal, cortical and endodermal cells59, whereas NIP6;1 is required for transport between the xylem and the phloem flows60. The apoplastic distribution of boron is limited by the Casparian barrier in endodermis.

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mediate directional auxin fluxes and ensure the fine regulation of the auxin distribution in many developmental processes.

PIN proteins are crucial factors in determining the directionality of auxin flow. They have been identified based on the auxin transport–related shoot and flower phenotypes of pin-formed1 (pin1) mutants and the agravitropic root growth of pin2 mutants80,81. The function of several Arabidopsis PIN proteins in cellular auxin efflux has been demonstrated in tobacco BY-2 cultured cells, yeast and mamma-lian HeLa cells88. Setting aside the PIN5, PIN6 and PIN8 subclade, whose function is unclear, the developmental roles of the other five Arabidopsis PIN proteins are well documented94,95 and include actions during embryogenesis12,80, organogenesis14,15, root meristem pattern-ing95, vascular tissue differentiation13 and tropic responses79 (Fig. 5). Notably, PIN proteins typically show asymmetric plasma membrane localization within cells, as predicted by the chemiosmotic hypothesis for auxin efflux carriers. This polarity at the cellular level correlates with and determines (at least in some instances) the directionality of the auxin flow within tissues72,82,96.

Particularly important are changes in PIN subcellular polarity, because by this mechanism different signals can redirect the auxin fluxes and trigger developmental reprogramming12,79. For example, at early stages of Arabidopsis embryo development, PIN7 is polarized at the upper side of suspensor cells and mediates auxin flow to the young embryo, where auxin accumulates. However, at later stages of embryo development, an as-yet-unknown signal causes polarization of PIN1 and PIN7 to the lower side of cells, and thus auxin is redistrib-uted from the embryo to the root pole where a new auxin maximum is established12 (Fig. 5a–f). PIN polarization and the resulting auxin

The auxin transport proteins. The chemios-motic hypothesis originally postulated the existence of auxin influx and efflux carriers in the absence of information about their molec-ular identity. Since then, largely thanks to the establishment of Arabidopsis as a successful model organism for plant molecular genet-ics, the identity of the auxin transporters has been revealed. These comprise (i) the AUX1/LAX influx proteins, which mediate auxin entry to the cell; (ii) the ABCB/PGP proteins, which mediate ATP-dependent auxin transport through the plasma membrane; and (iii) the PIN exporters, which determine the direction-ality of auxin transport by means of their polar subcellular localization.

The AUX1/LAX influx transporter protein family was identified through the isolation of the Arabidopsis auxin1 (aux1) mutant. aux1 roots are resistant to the membrane-impermeable synthetic auxin derivative 2,4-dichlorophenoxyacetic acid (2,4-D) and show agravitropic growth—both consequences of a defect in cellular auxin uptake. The AUX1 gene codes for a protein homologous to amino acids permeases83 and has been shown to mediate auxin import when heterologously expressed in Xenopus laevis oocytes84. AUX1 localizes to the plasma membrane of various cell types; nota-bly, this includes asymmetric localization at the upper side of young phloem (protophloem) 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 Arabidopsis, three LIKE AUX1 (LAX) proteins are present and also seem to function as auxin influx carriers. For example, they act redundantly to regulate phyllotaxis86, and LAX3 has a unique role in communication between the growing lateral root primordium and surrounding tissues87. In summary, there is compelling biochemical and genetic evidence that the AUX1/LAX proteins act as influx carriers and have important roles in root growth, tropisms and organogenesis.

Another class of transporters implicated in auxin transport are the phosphoglycoproteins (PGPs), which are members of the B-type ATP-binding cassette (ABCB) transporter family. They were originally identi-fied as proteins binding to the synthetic inhibitors of auxin efflux. Some of these proteins—PGP1, PGP19 and PGP4—mediate auxin efflux88–90 and show complex, multilevel functional interaction with other efflux transporters from the PIN family91–93. Combinations between pgp mutants and pin loss-of-function mutants indicate that these two types of auxin exporters can act both synergistically and antagonistically in different developmental processes91,92. Although their functional rela-tionship is far from clear, the PIN and PGP proteins probably comprise two distinct auxin efflux mechanisms. At the polar domains where the PGP and PIN proteins colocalize and interact, PGPs might act syner-gistically with PIN proteins in auxin efflux, possibly by regulating PIN stability at plasma membrane microdomains93. In addition, PGPs can also act at the nonpolar domains, independently of the PIN proteins; in such a scenario, PGPs would mediate apolar auxin efflux and control the amount of auxin available in PIN-expressing cells for the directional PIN-driven transport91. Thus, complex interactions of directional PIN- and ATP-dependent PGP-dependent transport systems at different levels

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Figure 4 Phloem-based transport and chemiosmotic model for polar auxin transport. (a) Auxin distribution via the phloem from source tissues—young leaves and floral buds—to root and shoot tips. (b) The chemiosmotic model, based on the pH difference between the apoplast (pH 5.5) and the cytoplasm (pH 7.0). Protonated auxin—undissociated indole-3-acetic acid (IAAH)—can diffuse through the lipidic plasma membrane or be transported by the AUX1/LAX influx carriers into the cell. In the cytosol, it dissociates and gets trapped inside the cell in its deprotonated form (IAA–). IAA– can exit cells by the action of PGP- or PIN-type efflux carriers. The polar cellular localization of the carriers determines the directionality of the intercellular auxin flow. (c) Structure of protonated IAAH.

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root tip—translocates to the new bottom side of these gravity-per-ceiving 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 trans-ported, by a concerted action of AUX1 and PIN2, to the elongation zone85,99. Here auxin accumulation inhibits growth, ultimately lead-ing 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.

ConclusionsThe transport mechanisms in higher plants, as exemplified by the model plant Arabidopsis, rely on two main streams. First, the long-distance transport system depends mainly on the specialized conduc-tive 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 photosyn-thetic 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 sub-stances from the vasculature as well as to distribute short-range sig-nals 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 spe-cial 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 regu-lation 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, tran-scriptome 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.

ACKNOWLEDGMENTSWe 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).

1. Valladares, F., Gianoli, E. & Gómez, J. Ecological limits to plant phenotypic plasticity. New Phytol. 176, 749–763 (2007).

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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 ectop-ically 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 gravis-timulation, PIN3—previously localized uniformly in the cells of the

PIN1PIN2PIN3PIN4PIN7Auxin

vence

lrcGravity Gravity

g

a b c

d e f

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 gravity-perceiving 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.

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