Auxin
Encyclopedia
Auxins are a class of plant hormone
s (or plant growth substances) 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 the Dutch scientist Frits Went
. Kenneth V. Thimann
isolated this phytohormone and determined its chemical structure as indole-3-acetic acid
. Went and Thiman then co-authored a book on plant hormones, Phytohormones, in 1937.
s to be discovered.
The (dynamic and to environment responsive) pattern of auxin distribution within the plant is a key factor for plant growth, its reaction to its environment, and specifically for development of plant organs (such as leaves
or flower
s). It is achieved through very complex and well coordinated active
transport of auxin molecules from cell to cell throughout the plant body — by the so called polar auxin transport
. Thus a plant can (as a whole) react to external conditions and adjust to them, without requiring a nervous system
. Auxins typically act in concert with, or in opposition to other plant hormones. For example, the ratio of auxin to cytokinin
in certain plant tissues determines initiation of root versus shoot buds.
On the molecular level, all auxins are compounds with aromatic ring and a carboxylic acid group. The most important member of the auxin family is indole-3-acetic acid
(IAA). It generates the majority of auxin effects in intact plants, and is the most potent native auxin. And as native auxin its stability is controlled in many ways in plant, from synthesis, through possible conjugation to degradation of its molecules, always according to the requirements of the situation. However, molecules of IAA are chemically labile in aqueous solution, so IAA is not used commercially as a plant growth regulator.
However, most of the knowledge described so far in auxin biology and also the auxin biology described in this article bellow, apply basically to the IAA, the other three endogenous auxins seems to have rather marginal importance for intact plants in natural environments.
Alongside endogenous auxins, scientists and manufacturers have developed many synthetic compounds with auxinic activity.
Some synthetic auxins such as 2,4-D and 2,4,5-trichlorophenoxyacetic acid
(2,4,5-T) are used also as herbicide
s. Broad-leaf plants (dicots) such as dandelions are much more susceptible to auxins than narrow-leaf plants (monocots) like grass
and cereal crops. So these synthetic auxins are valuable as synthetic herbicides. See also chapter Auxinic herbicides.
Auxins are also often used to promote initiation of adventitious root
s and are the active ingredient of the commercial preparations used in horticulture
to root stem cuttings. They can also be used to promote uniform flower
ing, to promote fruit
set, and to prevent premature fruit drop.
level through organs and ultimately to the whole plant.
of set of certain gene
s. or by means independent on gene expression.
One of the pathways leading to the changes of gene expression involves the reception of auxin by TIR1 protein. In 2005, it was demonstrated that the F-box protein
TIR1, which is part of the ubiquitin ligase complex SCFTIR1
, is an auxin receptor. Upon binding of auxin, TIR1 recruits specific transcriptional
repressors (the Aux/IAA repressors) for ubiquitin
ation by the SCF complex
.
This marking process leads to the degradation of the Aux/IAAs repressors by the proteasome
. The degradation of the repressors leads in turn to potentiation of Auxin Respose Factors ARF-mediated transcription of specific genes in response to auxins (reviewed in).
Another protein called ABP1 (Auxin Binding Protein 1) is a putative receptor for different signaling pathway, but its role is as yet unclear. Electrophysiological experiments with protoplasts and anti-ABP1 antibodies suggest that ABP1 may have a function at the plasma membrane and cell can possibly utilize ABP1 proteins to respond to auxin through means faster and independent on gene expression.
, affecting both cell division
and cellular expansion. Auxin concentration level together with other local factors, contributes to cell differentiation and specification of the cell fate.
Depending on the specific tissue, auxin may promote axial elongation (as in shoots), lateral expansion (as in root swelling), or isodiametric expansion (as in fruit growth). In some cases (coleoptile growth) auxin-promoted cellular expansion occurs in the absence of cell division. In other cases, auxin-promoted cell division and cell expansion may be closely sequenced within the same tissue (root initiation, fruit growth). In a living plant it appears that auxins and other plant hormones nearly always interact to determine patterns of plant development.
, and specific tissue growth contributes to the development of plant organs.
Growth of cells contributes to the plant's size, unevenly localized growth produces bending, turning and directionalization of organs- for example, stems turning toward light sources (phototropism
), roots growing in response to gravity (gravitropism
), and other tropism
s originated from the fact that cells on one side grow faster than the cells on the other side of the organ. Because of that, precise control of auxin distribution between different cells has paramount importance for resulting form of plant growth and organization.
to peaks of roots (From up — down).
For long distances, relocation occurs via the stream of fluid in phloem
vessels, but, for short-distance transport, a unique system of coordinated polar transport directly from cell to cell is exploited. This short distance — active transport exhibits some morphogen
etic properties.
This process, the polar auxin transport
is directional and very strictly regulated. It is based in uneven distribution of auxin efflux carriers on the plasma membrane, which send auxins in the proper direction. Pin-formed (PIN) proteins are vital in transporting auxin.
The regulation of PIN protein localisation in a cell determines direction of auxin transport from cell, and concentrated effort of many cells create peaks of auxin, or auxin maxima (regions having cells with higher auxin). Proper and timely auxin maximas within developing roots and shoots are necessary to organise the development of the organ
Surrounding auxin maxima are cells with low auxin troughs, or auxin minima. For example, in the Arabidopsis fruit, auxin minima have been shown to be important for its tissue development.
An important principle of plant organization based upon auxin distribution is apical dominance
, which means that the auxin produced by the apical bud (or growing tip) diffuses (and is transported) downwards and inhibits the development of ulterior lateral bud growth, which would otherwise compete with the apical tip for light and nutrients. Removing the apical tip and its suppressively acting auxin allows the lower dormant lateral buds to develop, and the buds between the leaf stalk and stem produce new shoots which compete to become the lead growth. The process is actually quite complex, because auxin transported downwards from the lead shoot tip has to interact with several other plant hormones (such as strigolactone
s, cytokinins) in the process on various positions along the growth axis in plant body to achieve this phenomenon. This plant behavior is utilized in pruning
by horticulturists.
Finally, the sum of auxin arriving from stems to roots influences the degree of root growth. If shoot tips are removed, plant doesn't react just by outgrowth of lateral buds — which are supposed to replace to original lead. It also follows that smaller amount of auxin arriving to the roots results in slower growth of roots and the nutrients are subsequently in higher degree invested in the upper part of the plant, which hence starts grow faster.
Auxin participates in phototropism
, geotropism, hydrotropism
and other developmental changes. The uneven distribution of auxin, due to environmental cues such as unidirectional light or gravity force, results in uneven plant tissue growth. And generally auxin governs the form and shape of plant body, direction and strength of growth of all organs and their mutual interaction.
Auxin stimulates cell elongation by stimulating wall loosening factors, such as elastin
s, to loosen cell wall
s. The effect is stronger if gibberellins are also present. Auxin also stimulates cell division if cytokinins are present. When auxin and cytokinin are applied to callus
, rooting can be generated if the auxin concentration is higher than cytokinin concentration. Xylem
tissues can be generated when the auxin concentration is equal to the cytokinins.
Auxin also induces sugar and mineral accumulation at the site of application.
and xylem
. When the plant is wounded, the auxin may induce the Cell differentiation and regeneration of the vascular tissues.
, were the process is more complex).
In horticulture, auxins, especially NAA
and IBA
, are commonly applied to stimulate root initiation when taking cuttings of plants. However, high concentrations of auxin inhibit root elongation and instead enhance adventitious root formation. Removal of the root tip can lead to inhibition of secondary root formation.
; the axillary buds are inhibited by auxin. When the apex of the plant is removed, the inhibitory effect is removed and the growth of lateral buds is enhanced as a high concentration of auxin directly stimulates ethylene
synthesis in lateral buds causing inhibition of its growth and potentiation of apical dominance.
. When seeds are removed from strawberries, fruit growth is stopped; exogenous auxin stimulates the growth in seed removed fruits. For fruit with unfertilized seeds, exogenous auxin results in parthenocarpy
("virgin-fruit" growth).
Auxin is important for the correct development of fruit. Fruits form abnormal morphologies when auxin transport is disturbed. In Arabidopsis fruits auxin controls the release of seeds from the fruit (pod). The valve margins are a specialised tissue in pods that regulates when pod will open (dehiscence). Auxin must be removed from the valve margin cells to allow the valve margins to form. This process requires modification of the auxin transporters (PIN proteins).
of flowers.
Auxin inhibits abscission
prior to formation of abscission layer and thus inhibits senescence of leaves.
Auxins are toxic to plants in large concentrations; they are most toxic to dicots and less so to monocots. Because of this property, synthetic
auxin herbicides including 2,4-D and 2,4,5-T have been developed and used for weed
control.
However, synthetic auxins, especially 1-Naphthaleneacetic acid
(NAA) and Indole-3-butyric acid
(IBA), are also commonly applied to stimulate root growth when taking cuttings of plants or yet for different agricultural application as is the prevention of fruit drop in fruit orchard
s.
. Excess ethylene (also native plant hormone) can inhibit elongation growth, cause leaves
to fall (leaf abscission
), and even kill the plant. So some synthetic auxins such as 2,4-D and 2,4,5-trichlorophenoxyacetic acid
(2,4,5-T) started to marketed also as herbicide
s. Broad-leaf plants (dicots) such as dandelions are much more susceptible to auxins than narrow-leaf plants (monocots) like grass
and cereal crops. So these synthetic auxins are valuable as synthetic herbicides and as such, they were, for example also the active agents in Agent Orange
, a defoliant
used extensively by American forces in the Vietnam War
. The 2,4-dichlorophenoxyacetic acid was actually first widely used herbicide in the world and it actually is still the most widely used herbicide in the world since. 2,4-D was first commercialized by the Sherwin-Williams Paint company and saw use in the late 1940s. It is easy and inexpensive to manufacture.
Herbicide manufacture
The defoliant
Agent Orange
was a mix of 2,4-D
and 2,4,5-T. The compound 2,4-D is still in use and is thought to be safe, but 2,4,5-T was more or less banned by the EPA
in 1979. The dioxin TCDD
is an unavoidable contaminant produced in the manufacture of 2,4,5-T. As a result of the integral dioxin contamination, 2,4,5-T has been implicated in leukemia
, miscarriage
s, birth defects, liver
damage, and other disease
s. Agent Orange
was sprayed in Vietnam
as a defoliant to deny ground cover to the Vietnamese army.
Plant hormone
Plant hormones are chemicals that regulate plant growth, which, in the UK, are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, when moved...
s (or plant growth substances) with some 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...
-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 the Dutch scientist Frits Went
Frits Went
Friedrich August Ferdinand Christian Went ForMemRS was a Dutch botanist, professor of botany and director of the Botanical Garden at the University of Utrecht...
. Kenneth V. Thimann
Kenneth V. Thimann
Kenneth Vivian Thimann was an English-American plant physiologist and microbiologist known for his studies of plant hormones, which were widely influential in agriculture and horticulture. He isolated and determined the structure of auxin, the first known plant hormone...
isolated this phytohormone and determined its chemical structure as indole-3-acetic acid
Indole-3-acetic acid
Indole-3-acetic acid, also known as IAA, is a heterocyclic compound that is a phytohormone called auxin. This colourless solid is native plant compound, potent and the most important auxin...
. Went and Thiman then co-authored a book on plant hormones, Phytohormones, in 1937.
Overview
Auxins derive their name from the Greek word αυξειν ("auxein" -- "to grow/increase"). They were the first of the major plant hormonePlant hormone
Plant hormones are chemicals that regulate plant growth, which, in the UK, are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, when moved...
s to be discovered.
The (dynamic and to environment responsive) pattern of auxin distribution within the plant is a key factor for plant growth, its reaction to its environment, and specifically for development of plant organs (such as leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
or 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). It is achieved through very complex and well coordinated 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...
transport of auxin molecules from cell to cell throughout the plant body — by the so called polar auxin transport
Polar auxin transport
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...
. Thus a plant can (as a whole) react to external conditions and adjust to them, without requiring a nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
. Auxins typically act in concert with, or in opposition to other plant hormones. For example, the ratio of auxin to cytokinin
Cytokinin
Cytokinins are a class of plant growth substances 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 leaf senescence...
in certain plant tissues determines initiation of root versus shoot buds.
On the molecular level, all auxins are compounds with aromatic ring and a carboxylic acid group. The most important member of the auxin family is indole-3-acetic acid
Indole-3-acetic acid
Indole-3-acetic acid, also known as IAA, is a heterocyclic compound that is a phytohormone called auxin. This colourless solid is native plant compound, potent and the most important auxin...
(IAA). It generates the majority of auxin effects in intact plants, and is the most potent native auxin. And as native auxin its stability is controlled in many ways in plant, from synthesis, through possible conjugation to degradation of its molecules, always according to the requirements of the situation. However, molecules of IAA are chemically labile in aqueous solution, so IAA is not used commercially as a plant growth regulator.
- There are four naturally occurring — endogenous auxins. Apart from IAA, only 4-chloroindole-3-acetic acid4-Chloroindole-3-acetic acid4-Chloroindole-3-acetic acid is a natural plant hormone. It is a member of the class of compounds known as auxins and a chlorinated derivative of the more common auxin indole-3-acetic acid . 4-Cl-IAA is found in the seeds of a variety of plants, particularly legumes such as peas and broad beans...
(4-Cl-IAA), phenylacetic acidPhenylacetic acidPhenylacetic acid is an organic compound containing a phenyl functional group and a carboxylic acid functional group. It is a white solid with a disagreeable odor...
(PAA) and indole-3-butyric acidIndole-3-butyric acidIndole-3-butyric acid is a white to light-yellow crystalline solid, with the molecular formula C12H13NO2. It melts at 125 °C in atmospheric pressure and decomposes before boiling.-Plant hormone:...
(IBA) may qualify as such, because only those 4 were found to be synthesized by plants.
However, most of the knowledge described so far in auxin biology and also the auxin biology described in this article bellow, apply basically to the IAA, the other three endogenous auxins seems to have rather marginal importance for intact plants in natural environments.
Alongside endogenous auxins, scientists and manufacturers have developed many synthetic compounds with auxinic activity.
- Synthetic auxin analogs include 1-naphthaleneacetic acid1-Naphthaleneacetic acid1-Naphthaleneacetic acid, commonly abbreviated NAA, is an organic compound with the formula C10H7CH2CO2H. This colourless solid is soluble in organic solvents...
(NAA), 2,4-dichlorophenoxyacetic acid2,4-Dichlorophenoxyacetic acid2,4-Dichlorophenoxyacetic acid is a common systemic pesticide/herbicide used in the control of broadleaf weeds. It is the most widely used herbicide in the world, and the third most commonly used in North America...
(2,4-D), and many others. See also chapter Synthetic auxins.
Some synthetic auxins such as 2,4-D and 2,4,5-trichlorophenoxyacetic acid
2,4,5-Trichlorophenoxyacetic acid
2,4,5-Trichlorophenoxyacetic acid , a synthetic auxin, is a chlorophenoxy acetic acid herbicide used to defoliate broad-leafed plants. It was developed in the late 1940s and was widely used in the agricultural industry until being phased out, starting in the late 1970s due to toxicity concerns....
(2,4,5-T) are used also as herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
s. Broad-leaf plants (dicots) such as dandelions are much more susceptible to auxins than narrow-leaf plants (monocots) like grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
and cereal crops. So these synthetic auxins are valuable as synthetic herbicides. See also chapter Auxinic herbicides.
Auxins are also often used to promote initiation of adventitious 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 and are the active ingredient of the commercial preparations used in horticulture
Horticulture
Horticulture is the industry and science of plant cultivation including the process of preparing soil for the planting of seeds, tubers, or cuttings. Horticulturists work and conduct research in the disciplines of plant propagation and cultivation, crop production, plant breeding and genetic...
to root stem cuttings. They can also be used to promote uniform 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...
ing, to promote 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,...
set, and to prevent premature fruit drop.
Hormonal activity
Auxins coordinate development at all levels in plants, from the cellularCell (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....
level through organs and ultimately to the whole plant.
Molecular mechanisms
Auxin molecules present in cell may trigger response directly through stimulation or inhibition of the expressionGene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
of set of certain gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s. or by means independent on gene expression.
One of the pathways leading to the changes of gene expression involves the reception of auxin by TIR1 protein. In 2005, it was demonstrated that the F-box protein
F-box protein
F-box proteins are proteins containing at least one F-box domain. The first identified F-box protein is one of three components of the SCF complex, which mediates ubiquitination of proteins targeted for degradation by the proteasome....
TIR1, which is part of the ubiquitin ligase complex SCFTIR1
SCF complex
Skp, Cullin, F-box containing complex is a multi-protein E3 ubiquitin ligase complex catalyzing the ubiquitination of proteins destined for proteasomal degradation...
, is an auxin receptor. Upon binding of auxin, TIR1 recruits specific transcriptional
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
repressors (the Aux/IAA repressors) for ubiquitin
Ubiquitin
Ubiquitin is a small regulatory protein that has been found in almost all tissues of eukaryotic organisms. Among other functions, it directs protein recycling.Ubiquitin can be attached to proteins and label them for destruction...
ation by the SCF complex
SCF complex
Skp, Cullin, F-box containing complex is a multi-protein E3 ubiquitin ligase complex catalyzing the ubiquitination of proteins destined for proteasomal degradation...
.
This marking process leads to the degradation of the Aux/IAAs repressors by the proteasome
Proteasome
Proteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
. The degradation of the repressors leads in turn to potentiation of Auxin Respose Factors ARF-mediated transcription of specific genes in response to auxins (reviewed in).
Another protein called ABP1 (Auxin Binding Protein 1) is a putative receptor for different signaling pathway, but its role is as yet unclear. Electrophysiological experiments with protoplasts and anti-ABP1 antibodies suggest that ABP1 may have a function at the plasma membrane and cell can possibly utilize ABP1 proteins to respond to auxin through means faster and independent on gene expression.
On a cellular level
On the cellular level, auxin is essential for cell growthCell growth
The term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells"...
, affecting both cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
and cellular expansion. Auxin concentration level together with other local factors, contributes to cell differentiation and specification of the cell fate.
Depending on the specific tissue, auxin may promote axial elongation (as in shoots), lateral expansion (as in root swelling), or isodiametric expansion (as in fruit growth). In some cases (coleoptile growth) auxin-promoted cellular expansion occurs in the absence of cell division. In other cases, auxin-promoted cell division and cell expansion may be closely sequenced within the same tissue (root initiation, fruit growth). In a living plant it appears that auxins and other plant hormones nearly always interact to determine patterns of plant development.
Organ patterns
Growth and division of plant cells together result in growth of tissueBiological tissue
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...
, and specific tissue growth contributes to the development of plant organs.
Growth of cells contributes to the plant's size, unevenly localized growth produces bending, turning and directionalization of organs- for example, stems turning toward light sources (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...
), roots growing in response to gravity (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...
), and other 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 originated from the fact that cells on one side grow faster than the cells on the other side of the organ. Because of that, precise control of auxin distribution between different cells has paramount importance for resulting form of plant growth and organization.
Uneven distribution of auxin
To cause growth in the required domains, it is necessary that auxins be active preferentially in them. Auxins are not synthesized everywhere (even if each cell retains the potential ability to do so, only under specific conditions will auxin synthesis be activated in them). For that purpose auxins have to be translocated toward those sites where they are needed. Translocation is driven throughout the plant body, primarily from peaks of shootsMeristem
A meristem is the tissue in most plants consisting of undifferentiated cells , found in zones of the plant where growth can take place....
to peaks of roots (From up — down).
For long distances, relocation occurs via the stream of fluid in phloem
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
vessels, but, for short-distance transport, a unique system of coordinated polar transport directly from cell to cell is exploited. This short distance — active transport exhibits some 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...
etic properties.
This process, the polar auxin transport
Polar auxin transport
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...
is directional and very strictly regulated. It is based in uneven distribution of auxin efflux carriers on the plasma membrane, which send auxins in the proper direction. Pin-formed (PIN) proteins are vital in transporting auxin.
The regulation of PIN protein localisation in a cell determines direction of auxin transport from cell, and concentrated effort of many cells create peaks of auxin, or auxin maxima (regions having cells with higher auxin). Proper and timely auxin maximas within developing roots and shoots are necessary to organise the development of the organ
Surrounding auxin maxima are cells with low auxin troughs, or auxin minima. For example, in the Arabidopsis fruit, auxin minima have been shown to be important for its tissue development.
Organization of the plant
As auxins contribute to organ shaping, they are also fundamentally required for proper development of the plant itself. Without hormonal regulation and organization, plants would be merely proliferating heaps of similar cells. Auxin employment begins in the embryo of the plant, where directional distribution of auxin ushers in subsequent growth and development of primary growth poles, then forms buds of future organs. Next it helps to coordinate proper development of the arising organs, such as roots, cotyledons and leaves and mediates long distance signals between them, contributing so to the overal architecture of the plant. Throughout the plant's life, auxin helps the plant maintain the polarity of growth and actually "recognize" where it has its branches (or any organ) connected.An important principle of plant organization based upon auxin distribution is apical dominance
Apical dominance
In plant physiology, apical dominance is the phenomenon whereby the main central stem of the plant is dominant over other side stems; on a branch the main stem of the branch is further dominant over its own side branchlets....
, which means that the auxin produced by the apical bud (or growing tip) diffuses (and is transported) downwards and inhibits the development of ulterior lateral bud growth, which would otherwise compete with the apical tip for light and nutrients. Removing the apical tip and its suppressively acting auxin allows the lower dormant lateral buds to develop, and the buds between the leaf stalk and stem produce new shoots which compete to become the lead growth. The process is actually quite complex, because auxin transported downwards from the lead shoot tip has to interact with several other plant hormones (such as strigolactone
Strigolactone
Strigolactones are plant hormones that have been implicated in inhibition of shoot branching. Strigolactones are carotenoid-derived and trigger germination of parasitic plant seeds and stimulate symbiotic mycorrhizal fungi...
s, cytokinins) in the process on various positions along the growth axis in plant body to achieve this phenomenon. This plant behavior is utilized in pruning
Pruning
Pruning is a horticultural practice involving the selective removal of parts of a plant, such as branches, buds, or roots. Reasons to prune plants include deadwood removal, shaping , improving or maintaining health, reducing risk from falling branches, preparing nursery specimens for...
by horticulturists.
Finally, the sum of auxin arriving from stems to roots influences the degree of root growth. If shoot tips are removed, plant doesn't react just by outgrowth of lateral buds — which are supposed to replace to original lead. It also follows that smaller amount of auxin arriving to the roots results in slower growth of roots and the nutrients are subsequently in higher degree invested in the upper part of the plant, which hence starts grow faster.
Effects
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...
, geotropism, hydrotropism
Hydrotropism
Hydrotropism is a plant's growth response in which the direction of growth is determined by a stimulus or gradient in water concentration...
and other developmental changes. The uneven distribution of auxin, due to environmental cues such as unidirectional light or gravity force, results in uneven plant tissue growth. And generally auxin governs the form and shape of plant body, direction and strength of growth of all organs and their mutual interaction.
Auxin stimulates cell elongation by stimulating wall loosening factors, such as elastin
Elastin
Elastin is a protein in connective tissue that is elastic and allows many tissues in the body to resume their shape after stretching or contracting. Elastin helps skin to return to its original position when it is poked or pinched. Elastin is also an important load-bearing tissue in the bodies of...
s, to loosen 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...
s. The effect is stronger if gibberellins are also present. Auxin also stimulates cell division if cytokinins are present. When auxin and cytokinin are applied to callus
Callus
A callus is an especially toughened area of skin which has become relatively thick and hard in response to repeated friction, pressure, or other irritation. Rubbing that is too frequent or forceful will cause blisters rather than allow calluses to form. Since repeated contact is required, calluses...
, rooting can be generated if the auxin concentration is higher than cytokinin concentration. Xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
tissues can be generated when the auxin concentration is equal to the cytokinins.
Auxin also induces sugar and mineral accumulation at the site of application.
Wounding response
Auxin induces the formation and organization of phloemPhloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
and xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
. When the plant is wounded, the auxin may induce the Cell differentiation and regeneration of the vascular tissues.
Root growth and development
Auxins promote root initiation. Auxin induces both growth of preexisting roots and adventitious root formation i.e. branching of the roots. The more of native auxin is transported down the stem to the roots, the more is stimulated the overall development of the roots. If the source of auxin is removed, for example the tips of stems are trimmed, the roots are stimulated accordingly less and growth of stem is supported instead. (See also apical dominanceApical dominance
In plant physiology, apical dominance is the phenomenon whereby the main central stem of the plant is dominant over other side stems; on a branch the main stem of the branch is further dominant over its own side branchlets....
, were the process is more complex).
In horticulture, auxins, especially NAA
1-Naphthaleneacetic acid
1-Naphthaleneacetic acid, commonly abbreviated NAA, is an organic compound with the formula C10H7CH2CO2H. This colourless solid is soluble in organic solvents...
and IBA
Indole-3-butyric acid
Indole-3-butyric acid is a white to light-yellow crystalline solid, with the molecular formula C12H13NO2. It melts at 125 °C in atmospheric pressure and decomposes before boiling.-Plant hormone:...
, are commonly applied to stimulate root initiation when taking cuttings of plants. However, high concentrations of auxin inhibit root elongation and instead enhance adventitious root formation. Removal of the root tip can lead to inhibition of secondary root formation.
Apical dominance
Auxin induces shoot apical dominanceApical dominance
In plant physiology, apical dominance is the phenomenon whereby the main central stem of the plant is dominant over other side stems; on a branch the main stem of the branch is further dominant over its own side branchlets....
; the axillary buds are inhibited by auxin. When the apex of the plant is removed, the inhibitory effect is removed and the growth of lateral buds is enhanced as a high concentration of auxin directly stimulates ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
synthesis in lateral buds causing inhibition of its growth and potentiation of apical dominance.
Fruit growth and development
Auxin is required for fruit growth and development and delays fruit senescenceSenescence
Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
. When seeds are removed from strawberries, fruit growth is stopped; exogenous auxin stimulates the growth in seed removed fruits. For fruit with unfertilized seeds, exogenous auxin results in parthenocarpy
Parthenocarpy
In botany and horticulture, parthenocarpy is the natural or artificially induced production of fruit without fertilization of ovules. The fruit is therefore seedless. Stenospermocarpy may also produce apparently seedless fruit, but the seeds are actually aborted while still small...
("virgin-fruit" growth).
Auxin is important for the correct development of fruit. Fruits form abnormal morphologies when auxin transport is disturbed. In Arabidopsis fruits auxin controls the release of seeds from the fruit (pod). The valve margins are a specialised tissue in pods that regulates when pod will open (dehiscence). Auxin must be removed from the valve margin cells to allow the valve margins to form. This process requires modification of the auxin transporters (PIN proteins).
Flowering
Auxin plays also a minor role in the initiation of flowering and development of reproductive organs. In low concentrations it can delay the senescenceSenescence
Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
of flowers.
Ethylene biosynthesis
In low concentrations, auxin can inhibit ethylene formation and transport of precursor in plants; however, high concentrations of auxin can induce the synthesis of ethylene. Therefore, the high concentration can induce femaleness of flowers in some species.Auxin inhibits abscission
Abscission
Abscission is a term used in several areas of biology. In plant sciences it most commonly refers to the process by which a plant drops one or more of its parts, such as a leaf, fruit, flower or seed...
prior to formation of abscission layer and thus inhibits senescence of leaves.
Synthetic auxins
In the course of research on auxin biology, many compounds with noticeable auxin activity were synthesized. Many of them had been found to have economical potential for man-controlled growth and development of plants in agronomy. Synthetic auxins include the following compounds:Auxins are toxic to plants in large concentrations; they are most toxic to dicots and less so to monocots. Because of this property, synthetic
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
auxin herbicides including 2,4-D and 2,4,5-T have been developed and used for weed
Weed
A weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More specifically, the term is often used to...
control.
However, synthetic auxins, especially 1-Naphthaleneacetic acid
1-Naphthaleneacetic acid
1-Naphthaleneacetic acid, commonly abbreviated NAA, is an organic compound with the formula C10H7CH2CO2H. This colourless solid is soluble in organic solvents...
(NAA) and Indole-3-butyric acid
Indole-3-butyric acid
Indole-3-butyric acid is a white to light-yellow crystalline solid, with the molecular formula C12H13NO2. It melts at 125 °C in atmospheric pressure and decomposes before boiling.-Plant hormone:...
(IBA), are also commonly applied to stimulate root growth when taking cuttings of plants or yet for different agricultural application as is the prevention of fruit drop in fruit orchard
Orchard
An orchard is an intentional planting of trees or shrubs that is maintained for food production. Orchards comprise fruit or nut-producing trees which are grown for commercial production. Orchards are also sometimes a feature of large gardens, where they serve an aesthetic as well as a productive...
s.
Auxinic herbicides
Used in high doses, auxin stimulates the production of ethyleneEthylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
. Excess ethylene (also native plant hormone) can inhibit elongation growth, cause leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
to fall (leaf abscission
Abscission
Abscission is a term used in several areas of biology. In plant sciences it most commonly refers to the process by which a plant drops one or more of its parts, such as a leaf, fruit, flower or seed...
), and even kill the plant. So some synthetic auxins such as 2,4-D and 2,4,5-trichlorophenoxyacetic acid
2,4,5-Trichlorophenoxyacetic acid
2,4,5-Trichlorophenoxyacetic acid , a synthetic auxin, is a chlorophenoxy acetic acid herbicide used to defoliate broad-leafed plants. It was developed in the late 1940s and was widely used in the agricultural industry until being phased out, starting in the late 1970s due to toxicity concerns....
(2,4,5-T) started to marketed also as herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
s. Broad-leaf plants (dicots) such as dandelions are much more susceptible to auxins than narrow-leaf plants (monocots) like grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
and cereal crops. So these synthetic auxins are valuable as synthetic herbicides and as such, they were, for example also the active agents in Agent Orange
Agent Orange
Agent Orange is the code name for one of the herbicides and defoliants used by the U.S. military as part of its herbicidal warfare program, Operation Ranch Hand, during the Vietnam War from 1961 to 1971. Vietnam estimates 400,000 people were killed or maimed, and 500,000 children born with birth...
, a defoliant
Defoliant
A defoliant is any chemical sprayed or dusted on plants to cause its leaves to fall off. A classic example of a highly toxic defoliant is Agent Orange, which the United States armed forces used abundantly to defoliate regions of Vietnam during the Vietnam War from 1961 to 1970.Defoliants differ...
used extensively by American forces in the Vietnam War
Vietnam War
The Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...
. The 2,4-dichlorophenoxyacetic acid was actually first widely used herbicide in the world and it actually is still the most widely used herbicide in the world since. 2,4-D was first commercialized by the Sherwin-Williams Paint company and saw use in the late 1940s. It is easy and inexpensive to manufacture.
Herbicide manufacture
The defoliant
Defoliant
A defoliant is any chemical sprayed or dusted on plants to cause its leaves to fall off. A classic example of a highly toxic defoliant is Agent Orange, which the United States armed forces used abundantly to defoliate regions of Vietnam during the Vietnam War from 1961 to 1970.Defoliants differ...
Agent Orange
Agent Orange
Agent Orange is the code name for one of the herbicides and defoliants used by the U.S. military as part of its herbicidal warfare program, Operation Ranch Hand, during the Vietnam War from 1961 to 1971. Vietnam estimates 400,000 people were killed or maimed, and 500,000 children born with birth...
was a mix of 2,4-D
2,4-Dichlorophenoxyacetic acid
2,4-Dichlorophenoxyacetic acid is a common systemic pesticide/herbicide used in the control of broadleaf weeds. It is the most widely used herbicide in the world, and the third most commonly used in North America...
and 2,4,5-T. The compound 2,4-D is still in use and is thought to be safe, but 2,4,5-T was more or less banned by the EPA
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
in 1979. The dioxin TCDD
2,3,7,8-Tetrachlorodibenzodioxin
2,3,7,8-Tetrachlorodibenzo-p-dioxin is a polychlorinated dibenzo-p-dioxin . It is the most potent compound of the series and became known as a contaminant in Agent Orange, a herbicide used in the Vietnam War, as well as the Seveso disaster...
is an unavoidable contaminant produced in the manufacture of 2,4,5-T. As a result of the integral dioxin contamination, 2,4,5-T has been implicated in leukemia
Leukemia
Leukemia or leukaemia is a type of cancer of the blood or bone marrow characterized by an abnormal increase of immature white blood cells called "blasts". Leukemia is a broad term covering a spectrum of diseases...
, miscarriage
Miscarriage
Miscarriage or spontaneous abortion is the spontaneous end of a pregnancy at a stage where the embryo or fetus is incapable of surviving independently, generally defined in humans at prior to 20 weeks of gestation...
s, birth defects, liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
damage, and other disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
s. Agent Orange
Agent Orange
Agent Orange is the code name for one of the herbicides and defoliants used by the U.S. military as part of its herbicidal warfare program, Operation Ranch Hand, during the Vietnam War from 1961 to 1971. Vietnam estimates 400,000 people were killed or maimed, and 500,000 children born with birth...
was sprayed in Vietnam
Vietnam
Vietnam – sometimes spelled Viet Nam , officially the Socialist Republic of Vietnam – is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea –...
as a defoliant to deny ground cover to the Vietnamese army.
See also
- Auxin binding proteinAuxin binding proteinIn molecular biology, the auxin binding protein family is a family of proteins which bind auxin. They are located in the lumen of the endoplasmic reticulum . The primary structure of these proteins contains an N-terminal hydrophobic leader sequence of 30-40 amino acids, which could represent a...
- FusicoccinFusicoccinFusicoccin is an organic compound produced by a fungus. It has detrimental effect on plants and causes their death.Fusicoccin is synthesized by the fungus Fusicoccum amygdali, which is a parasite of mainly almond and peach trees...
- HerbicideHerbicideHerbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
; specifically, see the chapter: #History- Phenoxy herbicidePhenoxy herbicideA phenoxy herbicide is any member of a family of chemicals related to the growth hormone indoleacetic acid . When sprayed on broad-leaf plants they induce rapid, uncontrolled growth, eventually killing them. When sprayed on crops such as wheat or corn, it selectively kills just the broad-leaf...
- Phenoxy herbicide
- Polar auxin transportPolar auxin transportPolar 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...
- Pruning fruit treesPruning fruit treesPruning fruit trees is a technique that is employed by arboriculture to control growth, remove dead or diseased wood, and stimulate the formation of flowers and fruit buds. Pruning often means cutting branches back to laterals or back to the main stem using proper pruning techniques. It may also...
- TropismTropismA 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...
- Witch's broomWitch's broomA Witch's broom is a disease or deformity in a woody plant, typically a tree, where the natural structure of the plant is changed. A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a bird's nest....
- Toshio MurashigeToshio MurashigeToshio Murashige is a professor emeritus of University of California Riverside in plant biology.He is most widely known for his efforts in creating the plant tissue culture medium known as Murashige and Skoog medium.- External links :*...
- Rupert SheldrakeRupert SheldrakeRupert Sheldrake is an English scientist. He is known for having proposed an unorthodox account of morphogenesis and for his research into parapsychology. His books and papers stem from his theory of morphic resonance, and cover topics such as animal and plant development and behaviour, memory,...
- Folke K. Skoog
- Kenneth V. ThimannKenneth V. ThimannKenneth Vivian Thimann was an English-American plant physiologist and microbiologist known for his studies of plant hormones, which were widely influential in agriculture and horticulture. He isolated and determined the structure of auxin, the first known plant hormone...
- Frits Warmolt WentFrits Warmolt WentFrits Warmolt Went 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...