Polar auxin transport
Encyclopedia
Polar auxin transport is the regulated transport of the plant hormone auxin
in plants. It is an active
process, the hormone is transported in cell-to-cell manner and one of the main features of the transport is its directionality (polarity). The polar auxin transport has coordinative function in plant development, the following spatial auxin distribution underpins most of plant growth responses to its environment and plant growth and developmental changes in general.
flow of auxin
molecules through the plant tissues. The flow of auxin molecules through the neighboring cell
s is driven by carriers
(type of membrane transport protein) 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 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 already in the seventies by Ruberry and Shelldrake and this visionary prediction was finely fully accepted to be proven only in the coming new century.
In sum, mechanism bellow describes that auxin is first 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 whole plant body.
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-Hasselbach equation):
The inside of cells (pH ~ 7) is less acidic than the outside (the apoplast
; pH ~ 5.5). So outside the cell, significant portion (17%) of the IAA molecules remain un-dissociated
(proton-associated). This portion of auxin molecules is charge
-neutral and therefore it is able to diffuse
through the lipophilic lipid bilayer
(lipid bilayer being constituent of cell membrane
) into the cell
s. Once through the bilayer in the cell, the molecules are now exposed to the more basic pH of the cell interior, and there they dissociate almost completely, to give anionic IAA−, which being chemically polar
are therefore unable to cross the lipid bilayer back again. So the auxin molecules are trapped inside the cell.
Because the auxin can not leave the cell on its own anymore, the transport of anionic IAA out of the cell requires an active
component in plasma membrane - some membrane transport protein. Membrane transport protein would allow the auxin molecules cross the membrane and leave so the cell. Two protein families: The PIN proteins and ABCB (PGP proteins
) transporters are functioning as "auxin efflux carriers
" and transport auxins from the cell.
Of those two, the PIN proteins are maintaining asymetric localisation on plasma membrane: they are mostly located on the basipetal (i.e., root-ward) side of the cell and as consequence auxin is flowing from cells in the direction toward roots and root tips. The PIN proteins on membrane control directionality of the auxin flow.
s (in direction towards the roots).
The location of PIN proteins is linked to the polarity of the transport. In most of the plant tissues
, they are present on the rootward side of plant cells, meaning that they contribute to the general trend of moving IAA from the shoots to the roots. But on some locations the positioning of PIN proteins is sensitive to environmental stimuli and the transporter proteins can be relocated to different side of cells in response to it. For example, the direction of the light source and gravity triggers gravitropic
and phototropic
responses. This is also thought to be controlled by PIN proteins, as, when the stimuli reach the target cells, PIN proteins relocate to the sides and starts to pump auxin to one side of the root or stem respectively. PIN proteins fused with green fluorescent protein
can visualize the localisation and its change of the PIN proteins on membranes. As auxin is pumped asymmetrically to the side of root of stem, it results in asymmetric growth (one side growths faster) and bending of the root or stem in response to the stimuli.
PIN proteins are so named because mutant
plants lacking them cannot develop leaves (the formation of leaves is dependent on transported auxin), and so their seedling produces only a pin-like structure growing upwards.
Some experiments suggest that the asymmetrical development of efflux carrier protein requires the localized targeting of vesicles
and the interaction with actin
or the cytoskeleton
.
The process involves interaction of plasma membrane, plasma membrane proteins, the components of the cytoskeleton
and cell wall
.
The polar auxin transport is required for generation of pattern of auxin gradients throughout the plant body. Those gradients have development significances akin to the gradients of morphogen
s in animal bodies. They are necessary for development, growth and response of any plant organ (such as cotyledon
s, leaves
, root
s, flower
s or fruit
s) and response of plant to environmental stimuli known as tropism
s.
For example in process known as gravitropism
, roots bend in response to gravity due to the regulated relocation of the plant hormone auxin inside the root tip. That is, if a root is not oriented vertically, - so the bending downwards is actually developmentally required, gravity sensing mechanism inside root columella
will reorient direction of auxin flow toward root down-side. (Under normal situation, auxin flowing to columella by the centre of the root is redistributed to all sides from it and because it this is in the very tip of the root tip, the flow of auxin bend and flow backwards . For visualisation it is sometimes described as reverse fountain, or umbrella shape flow). Mechanistically it is provided by the reorientation of the auxin efflux carriers described above. More precisely, after the gravity perception occurs, it is followed by reorientation of the PIN3 proteins on the plasma membranes in the cells of columella in such a way, that the PIN proteins are oriented down, toward the gravity.
As a result one side of the root will be enriched by auxin more then the other one. The increase in the concentration of auxin will inhibit cell expansion of the targeted cells (those on down side of the root), while cells on the other side of the root will continue to grow. Because upper side of the root became to grow faster than the other one, it will eventually outgrowth around its position and turn downward. Therefore, the redistribution of auxin in the root can initiate differential growth in the elongation zone of the root, resulting in root curvature.
Similar mechanisms are working in other tropic responses, such as phototropism
or
Quercetin
(a flavonol) and Genistein
are naturally-occurring auxin transport inhibitors.
9-Hydroxyfluorene-9-carboxylic acid (HFCA), TIBA
, and trans-cinnamic acid (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.
; protein kinase
s and protein phosphatases mediate the phosphorylation and dephosphorylation, respectively. A study suggests that phosphatase inhibition can alter the activities of acropetal and basipetal auxin transport.
Auxin
Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...
in plants. It is an active
Active transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...
process, the hormone is transported in cell-to-cell manner and one of the main features of the transport is its directionality (polarity). The polar auxin transport has coordinative function in plant development, the following spatial auxin distribution underpins most of plant growth responses to its environment and plant growth and developmental changes in general.
Chemiosmotic model
Polar auxin transport (PAT) is directional and activeActive transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...
flow of auxin
Auxin
Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...
molecules through the plant tissues. The flow of auxin molecules through the neighboring cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s is driven by carriers
Carrier protein
Carrier proteins are proteins involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane. Carrier proteins are integral membrane proteins; that is they exist within and span the membrane across which they transport substances. The...
(type of membrane transport protein) 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 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 already in the seventies by Ruberry and Shelldrake and this visionary prediction was finely fully accepted to be proven only in the coming new century.
In sum, mechanism bellow describes that auxin is first 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 whole plant body.
Acid trap
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of the environment; a strongly acidic environment inhibits the forward reaction (dissociation
Dissociation (chemistry)
Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...
), whereas an alkaline environment strongly favors it (see Henderson-Hasselbach equation):
-
IAAHIAA− + H+, whereIAAH= indole-3-acetic acid;IAA−= its conjugate base
The inside of cells (pH ~ 7) is less acidic than the outside (the apoplast
Apoplast
Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant....
; pH ~ 5.5). So outside the cell, significant portion (17%) of the IAA molecules remain un-dissociated
Dissociation (chemistry)
Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...
(proton-associated). This portion of auxin molecules is charge
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
-neutral and therefore it is able to diffuse
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
through the lipophilic lipid bilayer
Lipid bilayer
The lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
(lipid bilayer being constituent of cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
) into the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s. Once through the bilayer in the cell, the molecules are now exposed to the more basic pH of the cell interior, and there they dissociate almost completely, to give anionic IAA−, which being chemically polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
are therefore unable to cross the lipid bilayer back again. So the auxin molecules are trapped inside the cell.
Because the auxin can not leave the cell on its own anymore, the transport of anionic IAA out of the cell requires an active
Active transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...
component in plasma membrane - some membrane transport protein. Membrane transport protein would allow the auxin molecules cross the membrane and leave so the cell. Two protein families: The PIN proteins and ABCB (PGP proteins
P-glycoprotein
P-glycoprotein 1 also known as multidrug resistance protein 1 or ATP-binding cassette sub-family B member 1 or cluster of differentiation 243 is a glycoprotein that in humans is encoded by the ABCB1 gene...
) transporters are functioning as "auxin efflux carriers
Carrier protein
Carrier proteins are proteins involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane. Carrier proteins are integral membrane proteins; that is they exist within and span the membrane across which they transport substances. The...
" and transport auxins from the cell.
Of those two, the PIN proteins are maintaining asymetric localisation on plasma membrane: they are mostly located on the basipetal (i.e., root-ward) side of the cell and as consequence auxin is flowing from cells in the direction toward roots and root tips. The PIN proteins on membrane control directionality of the auxin flow.
Polarity of auxin export
Polarity is set up in the cell, as the efflux carriers are positioned asymmetricaly on the plasma membrane. They are located normally only on the base of the plant cellPlant cell
Plant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:...
s (in direction towards the roots).
The location of PIN proteins is linked to the polarity of the transport. In most of the plant tissues
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
, they are present on the rootward side of plant cells, meaning that they contribute to the general trend of moving IAA from the shoots to the roots. But on some locations the positioning of PIN proteins is sensitive to environmental stimuli and the transporter proteins can be relocated to different side of cells in response to it. For example, the direction of the light source and gravity triggers gravitropic
Gravitropism
Gravitropism is a turning or growth movement by a plant or fungus in response to gravity. Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull and stems...
and phototropic
Phototropism
Phototropism is directional growth in which the direction of growth is determined by the direction of the light source. In other words, it is the growth and response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi...
responses. This is also thought to be controlled by PIN proteins, as, when the stimuli reach the target cells, PIN proteins relocate to the sides and starts to pump auxin to one side of the root or stem respectively. PIN proteins fused with green fluorescent protein
Green fluorescent protein
The green fluorescent protein is a protein composed of 238 amino acid residues that exhibits bright green fluorescence when exposed to blue light. Although many other marine organisms have similar green fluorescent proteins, GFP traditionally refers to the protein first isolated from the...
can visualize the localisation and its change of the PIN proteins on membranes. As auxin is pumped asymmetrically to the side of root of stem, it results in asymmetric growth (one side growths faster) and bending of the root or stem in response to the stimuli.
PIN proteins are so named because mutant
Mutant
In biology and especially genetics, a mutant is an individual, organism, or new genetic character, arising or resulting from an instance of mutation, which is a base-pair sequence change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character or trait not...
plants lacking them cannot develop leaves (the formation of leaves is dependent on transported auxin), and so their seedling produces only a pin-like structure growing upwards.
Some experiments suggest that the asymmetrical development of efflux carrier protein requires the localized targeting of vesicles
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
and the interaction with actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
or the cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
.
The process involves interaction of plasma membrane, plasma membrane proteins, the components of the cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
and cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
.
Effects of polar auxin transport
- See also chapters "Uneven distribution of auxin" and "Organization of the plant" in the main AuxinAuxinAuxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...
article
The polar auxin transport is required for generation of pattern of auxin gradients throughout the plant body. Those gradients have development significances akin to the gradients of morphogen
Morphogen
A morphogen is a substance governing the pattern of tissue development, and the positions of the various specialized cell types within a tissue...
s in animal bodies. They are necessary for development, growth and response of any plant organ (such as cotyledon
Cotyledon
A cotyledon , is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaves of a seedling. The number of cotyledons present is one characteristic used by botanists to classify the flowering plants...
s, leaves
Leaves
-History:Vocalist Arnar Gudjonsson was formerly the guitarist with Mower, and he was joined by Hallur Hallsson , Arnar Ólafsson , Bjarni Grímsson , and Andri Ásgrímsson . Late in 2001 they played with Emiliana Torrini and drew early praise from the New York Times...
, root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s, flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
s or fruit
Fruit
In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
s) and response of plant to environmental stimuli known as tropism
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...
s.
For example in process known as gravitropism
Gravitropism
Gravitropism is a turning or growth movement by a plant or fungus in response to gravity. Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull and stems...
, roots bend in response to gravity due to the regulated relocation of the plant hormone auxin inside the root tip. That is, if a root is not oriented vertically, - so the bending downwards is actually developmentally required, gravity sensing mechanism inside root columella
Columella (disambiguation)
Columella is Latin meaning "little column". It may refer to:*Columella , a Roman writer-Biology:*Columella , an anatomical feature of a coiled snail shell or gastropod shell...
will reorient direction of auxin flow toward root down-side. (Under normal situation, auxin flowing to columella by the centre of the root is redistributed to all sides from it and because it this is in the very tip of the root tip, the flow of auxin bend and flow backwards . For visualisation it is sometimes described as reverse fountain, or umbrella shape flow). Mechanistically it is provided by the reorientation of the auxin efflux carriers described above. More precisely, after the gravity perception occurs, it is followed by reorientation of the PIN3 proteins on the plasma membranes in the cells of columella in such a way, that the PIN proteins are oriented down, toward the gravity.
As a result one side of the root will be enriched by auxin more then the other one. The increase in the concentration of auxin will inhibit cell expansion of the targeted cells (those on down side of the root), while cells on the other side of the root will continue to grow. Because upper side of the root became to grow faster than the other one, it will eventually outgrowth around its position and turn downward. Therefore, the redistribution of auxin in the root can initiate differential growth in the elongation zone of the root, resulting in root curvature.
Similar mechanisms are working in other tropic responses, such as phototropism
Phototropism
Phototropism is directional growth in which the direction of growth is determined by the direction of the light source. In other words, it is the growth and response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi...
or
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
Quercetin , a flavonol, is a plant-derived flavonoid found in fruits, vegetables, leaves and grains. It also may be used as an ingredient in supplements, beverages or foods.-Occurrence:...
(a flavonol) and Genistein
Genistein
Genistein is one of several known isoflavones. Isoflavones, such as genistein and daidzein, are found in a number of plants including lupin, fava beans, soybeans, kudzu, and psoralea being the primary food source, also in the medicinal plant, Flemingia vestita and coffee Besides functioning as...
are naturally-occurring auxin transport inhibitors.
9-Hydroxyfluorene-9-carboxylic acid (HFCA), TIBA
Tiba
Tiba may refer to:* Chiba, a Japanese city* Chiba Prefecture, a Japanese prefecture* TIBÁ* Tiba, an Iranian car made by Saipa* Traffic Information Broadcasts by Aircraft, a contingency procedure used by pilots when there is a major disruption to air traffic service....
, and trans-cinnamic acid (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 phosphorylationPhosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
; protein kinase
Protein kinase
A protein kinase is a kinase enzyme that modifies other proteins by chemically adding phosphate groups to them . Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins...
s and protein phosphatases mediate the phosphorylation and dephosphorylation, respectively. A study suggests that phosphatase inhibition can alter the activities of acropetal and basipetal auxin transport.