Polar Auxin Transport
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Polar auxin transport is the regulated transport of the plant hormone auxin in plants. It is an
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process, the hormone is transported in cell-to-cell manner and one of the main features of the transport is its asymmetry and directionality (
polarity Polarity may refer to: Science * Electrical polarity, direction of electrical current * Polarity (mutual inductance), the relationship between components such as transformer windings * Polarity (projective geometry), in mathematics, a duality of o ...
). The polar auxin transport functions to coordinate plant development; the following spatial auxin distribution underpins most of plant growth responses to its environment and plant growth and developmental changes in general. In other words, the flow and relative concentrations of auxin informs each plant cell where it is located and therefore what it should do or become.


Chemiosmotic model

Polar auxin transport (PAT) is directional and
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flow of auxin molecules through the plant tissues. The flow of auxin molecules through the neighboring cells is driven by carriers (''type of
membrane transport protein A membrane transport protein (or simply transporter) is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral membrane prote ...
'') in the cell-to-cell fashion (from one cell to other cell and then to the next one) and the direction of the flow is determined by the localization of the carriers on the
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
in the concerned cells. The transport from cell to the neighboring one is achieved through relatively complex combination of several sub-processes. To explain the mechanism behind unique character of auxin transport through living cell files of the plant, the so-called chemiosmotic model was formulated. The mechanism was first proposed in the seventies by Ruberry and Sheldrake and this visionary prediction was finally proven in the 21st century. The mechanism below describes the process in which auxin is trapped in the cell by the so-called ''acid trap'' and how it can then leave the cell only by activity of specific carriers, which control the directionality of the flow from cells and generally the direction of auxin transport through the whole plant body.


Acid trap

As weak acids, the protonation state of auxins is dictated by the pH of the environment; a strongly acidic environment inhibits the forward reaction ( dissociation), whereas an alkaline environment strongly favors it (see Henderson-Hasselbalch equation): The export of auxins from cells is termed ''auxin efflux'' and the entry of auxin in to cells is called ''auxin influx''. The first step in polar transport is auxin influx. Auxin enters plant cells by two methods, first by
passive diffusion Passive transport is a type of membrane transport that does not require energy to move substances across cell membranes. Instead of using cellular energy, like active transport, passive transport relies on the second law of thermodynamics to dri ...
as non-ionized protonated
indole-3-acetic acid Indole-3-acetic acid (IAA, 3-IAA) is the most common naturally occurring plant hormone of the auxin class. It is the best known of the auxins, and has been the subject of extensive studies by plant physiologists. IAA is a derivative of indole, co ...
(IAAH) across the
phospholipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many ...
, or second by active co-transport in the anionic form IAA. As IAAH is lipophilic, it can easily cross the lipid bilayer. ::IAAH IAA + H+, where IAAH = indole-3-acetic acid; IAA = its conjugate base The inside of cells (pH ~ 7) is less acidic than the outside (the
apoplast Inside a plant, the apoplast can mean the space outside of cell membranes, where material can diffuse freely; that is, the extracellular spaces. ''Apoplast '' can also refer especially to the continuum of cell walls of adjacent cells; fluid and ma ...
; pH ~ 5.5). So outside the cell a significant portion (17%) of the IAA molecules remain un-
dissociated Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an ac ...
(proton-associated). This portion of auxin molecules is charge-neutral and therefore it is able to
diffuse Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
through the lipophilic
lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many ...
(lipid bilayer being constituent of
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (the ...
) into the cells. Once through the bilayer in the cell, the molecules are exposed to the more basic pH of the cell interior, and there they dissociate almost completely, producing anionic IAA. These chemically polar ions are unable to passively diffuse across the cell membrane and remain trapped inside the cell.


Polarity of auxin export

Once inside the cell, auxin cannot leave the cell on its own by crossing the lipid bilayer. Hence the export of auxin from the cell requires an
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transport component in the
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
- i.e. some
membrane transport protein A membrane transport protein (or simply transporter) is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral membrane prote ...
. Two protein families: The
PIN proteins In plants, the PIN proteins are integral membrane proteins that transport the anionic form of the phytohormone auxin across membranes. Most of the PIN proteins (e.g. PIN1/2/3/4/7 in the model plant ''Arabidopsis thaliana'') localize at the plas ...
and ABCB ( PGP proteins) transporters function as "auxin efflux carriers" and transport the anionic form of auxin out of the cell. While the PGP auxin efflux carriers are evenly distributed, the PIN proteins normally maintain polar (i.e. asymmetric) localisation on the plasma membrane. That is to say they are most concentrated on one side of the cell. Furthermore, the asymmetrical localisation of the PIN proteins is coordinated between neighbouring cells. As a result, the PIN proteins generate a directional flow of auxin at the tissue and organ scale. This PIN-generated flow is called auxin polar transport. For example, the cells located in the vasculature (at the center) of the root all show PIN1 proteins on their basal membrane only (i.e. on their lower side). As a result, in the root vasculature, auxin is transported directionally from the shoot to the root tip (i.e. downwards).


Role in plant development


Self-organisation of polar auxin transport

:''See also "Uneven distribution of auxin" and "Organization of the plant" in the main Auxin article'' Auxin plays a central role in PIN protein polarity establishment. The regulation of PIN localisation by auxin creates a
feedback loop Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
where PIN proteins control the directionality of auxin fluxes, and auxin in turn controls PIN proteins localisation. These interactions between auxin and its own transporters confer to the system self-organizing properties, which explains for instance
phyllotaxis In botany, phyllotaxis () or phyllotaxy is the arrangement of leaves on a plant stem. Phyllotactic spirals form a distinctive class of patterns in nature. Leaf arrangement The basic arrangements of leaves on a stem are opposite and alternat ...
(the regular and geometrical arrangements of lateral organ along the stem), the formation of leaf serrations, and the formation of vascular strands. This positive feedback regulation auxin on its own transport also plays an essential role in vascular development, which process is called canalization. PIN proteins are so named because
mutant In biology, and especially in genetics, a mutant is an organism or a new genetic character arising or resulting from an instance of mutation, which is generally an alteration of the DNA sequence of the genome or chromosome of an organism. It ...
plants lacking the founding member of this family, PIN1, cannot develop
flowers A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechani ...
. The formation of flowers is triggered by regularly spaced local auxin accumulation at the surface of the shoot apical meristem and, for this PIN1 is required. As a result, the pin1 mutant plants produce a "pin-like" inflorescence consisting only of a naked stem. This highlights the importance of polar auxin transport in plant development.


Tropisms

Other external and internal signals (e.g. blue light, mechanical stress, gravity or
cytokinins Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and le ...
) can interfere with PIN protein polarity and therefore with the directionality of auxin polar transport. Because auxin controls cell division and cell elongation, the change of PIN proteins localisation, and the subsequent change in auxin distribution, often lead to a change in the growth pattern. For instance, the regulation of polar auxin transport is central in a process such as
gravitropism Gravitropism (also known as geotropism) is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general feat ...
. This process, which ensures that the root grows downwards, relies on the redistribution of auxin by the columella cells (the cells located at the very tip of the root). These cells respond to gravity by special organelles, the statoliths, that redistribute auxin from the
vasculature The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
to the root
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the ...
and the lateral
root cap The root cap is a type of tissue at the tip of a plant root. It is also called calyptra. Root caps contain statocytes which are involved in gravity perception in plants. If the cap is carefully removed the root will grow randomly. The root cap ...
. These tissues (which form the external cell layers of the root) transport auxin back to the
elongation zone Elongation may refer to: * Elongation (astronomy) * Elongation (geometry) * Elongation (plasma physics) * Part of transcription of DNA into RNA of all types, including mRNA, tRNA, rRNA, etc. * Part of translation (biology) of mRNA into proteins ...
where it regulates cell elongation. When the gravitational gradient is not aligned with the axis of the columella cells (because the root is not vertical), the PIN proteins move to the side of the cell membrane that is gravitationally lowest. This causes more auxin to flow to the lower side of the root. Once in the elongation zone, the extra auxin inhibits cell elongation and cause the root to re-orient downwards. Similar mechanisms occur in other tropic responses, such as
phototropism Phototropism is the growth of an organism in response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light contain a hor ...
. The mechanisms were first described by the Cholodny-Went model, proposed in the 1920s by N. Cholodny and
Frits Warmolt Went Frits Warmolt Went (May 18, 1903 – May 1, 1990) was a Dutch biologist whose 1928 experiment demonstrated the existence of auxin in plants. Went's father was the prominent Dutch botanist F.A.F.C. Went. After graduating from the University o ...
.


Generation of morphogenetic gradients

Polar auxin transport is required for the generation of auxin gradients throughout the plant body. Those gradients have development significances akin to the gradients of
morphogen A morphogen is a substance whose non-uniform distribution governs the pattern of tissue development in the process of morphogenesis or pattern formation, one of the core processes of developmental biology, establishing positions of the various ...
s in animal bodies. They are necessary for development, growth and response of any plant organ (such as
cotyledon A cotyledon (; ; ; , gen. (), ) is a significant part of the embryo within the seed of a plant, and is defined as "the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germinating seed." The num ...
s, leaves,
root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...
s,
flower A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechanism ...
s or
fruit In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in partic ...
s) and response of plant to environmental stimuli known as
tropism A tropism is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the stimulus (as oppos ...
s.


Regulation

Although the detailed molecular mechanism of PIN proteins polarity establishment remains to be elucidated, many endogenous and exogenous regulators of PIN proteins localisation have been characterised.


Auxin

Most importantly, PIN proteins localisation on the plasma membrane is controlled by auxin. Several mathematical models making different assumptions on the way auxin influences PIN localisation explain different observations. Some models assume PIN proteins polarize towards the neighbouring cell containing the highest cytosolic auxin concentration. These models are called "up-the-gradient" models and explain for instance phyllotaxis. Other models assume that PIN proteins localise on the side of the cell where the efflux of auxin is the highest. These models are called "with-the-flux" models and explain the formation of vascular strands in leaves. The molecular mechanism responsible for these different behaviours of the system (with-the-flux and up-the-gradient) is not yet fully understood. Noticeably, an auxin receptor protein called ABP1 is thought to play a potentially significant role in the control of PIN proteins polarity by auxin.


Mechanical stress

Mechanical signals have been proposed to regulate PIN polarity.


Vesicle Trafficking

The
asymmetrical Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pr ...
localisation of PIN efflux carrier protein at the plasma membrane has been shown to involve the localized targeting of vesicles and the local regulation of endocytosis. The latter involves the
actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...
cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is comp ...
.


Inhibitors of the transport

In research, 1-N-Naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid (TIBA) are used as specific inhibitors of the auxin efflux.
Quercetin Quercetin is a plant flavonol from the flavonoid group of polyphenols. It is found in many fruits, vegetables, leaves, seeds, and grains; capers, red onions, and kale are common foods containing appreciable amounts of it. It has a bitter flavor ...
(a
flavonol Flavonols are a class of flavonoids that have the 3-hydroxyflavone backbone (IUPAC name : 3-hydroxy-2-phenylchromen-4-one). Their diversity stems from the different positions of the phenolic -OH groups. They are distinct from flavanols (with " ...
) and
Genistein Genistein (C15H10O5) is a naturally occurring compound that structurally belongs to a class of compounds known as isoflavones. It is described as an angiogenesis inhibitor and a phytoestrogen. It was first isolated in 1899 from the dyer's bro ...
are naturally-occurring auxin transport inhibitors.p.435 Plant Physiology Third Edition Taiz and Zeiger (2002) 9-Hydroxyfluorene-9-carboxylic acid (HFCA), TIBA, and trans-
cinnamic acid Cinnamic acid is an organic compound with the formula C6H5-CH=CH- COOH. It is a white crystalline compound that is slightly soluble in water, and freely soluble in many organic solvents. Classified as an unsaturated carboxylic acid, it occurs n ...
(TCA) are also example of Polar Auxin Transport Inhibitors. They prevent the development of the bilateral growth of the plant embryo during the globular stage. All 3 inhibitors induce the formation of fused cotyledons in globular but not heart-shaped embryo.


Phosphorylation

Polar auxin transport can be regulated by reversible protein
phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, ...
;
protein kinase A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them (phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a fu ...
s and
protein phosphatases A protein phosphatase is a phosphatase enzyme that removes a phosphate group from the phosphorylated amino acid residue of its substrate protein. Protein phosphorylation is one of the most common forms of reversible protein posttranslational modifi ...
mediate the phosphorylation and dephosphorylation, respectively. A study suggests that phosphatase inhibition can alter the activities of acropetal and basipetal auxin transport. With decades of studies, multiple kinases have been reported to phosphorylate PIN proteins, including PINOID, D6PK, PAX, MPK6 and CRK5; and phosphorylated PIN proteins can be oppositely dephosphorylated by Protein Phosphatase 2A (PP2A), Protein Phosphatase 1 (PP1) and PP6. The AGC family of kinases play essential roles in catalyzing PIN phosphorylation and in regulating PIN function. 3'-Phosphoinositide dependent protein kinase 1 (PDK1), also from the AGC family, is a critical activator of AGC kinases and is thus also involved in the regulation of PIN-mediated auxin transport.Gloria K Muday, Alison DeLong. (2001)''Polar auxin transport:controlling where and how much''. ''Trends in Plant Science'' 6(11):535-542 PINOID and D6PK share at least three phosphosites (P-sites) at the cytoplasmic loop (also called hydrophilic loop) of long PIN proteins, but their functions are not all the same. both of the two kinases can increase PIN activity via phosphorylation. However, PINOID (non-polar)-mediated phosphorylation also determines the apicobasal polar targeting of PIN proteins, i.e., more phosphorylation, more apical. D6PK and its homologs localize at the basal side of plasma membrane, modulating the rootward auxin fluxes and subsequent developmental processes.


References

{{Commonscat, Auxin transport Auxin action Plant organogenesis