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Encyclopedia > Auxin
IAA appears to be the most active auxin in plant growth.
IAA appears to be the most active auxin in plant growth.

Auxins are a class of plant growth substance (often called phytohormone or plant hormone). Auxins play an essential role in coordination of many growth and behavioral processes in the plant life cycle. Plant hormones (or plant growth regulators, or PGRs) are internally secreted chemicals in plants that are used for regulating their growth. ... Plant hormones (or plant growth regulators, or PGRs) are internally secreted chemicals in plants that are used for regulating their growth. ... Plant hormones (also known as plant growth regulators (PGRs) and phytohormones) are chemicals that regulate a plants growth. ...

Contents

Overview

Auxins derive their name from the greek word auxano (to grow). They were the first of the major plant hormones to be discovered and are a major coordinating signal in plant development. Their pattern of active transport through the plant is complex. They typically act in concert with (or opposition to) other plant hormones. For example, the ratio of auxin to cytokinin in certain plant tissues determines initiation of root versus shoot buds. Thus a plant can (as a whole) react on external conditions and adjust to them, without requiring a nervous system. On a molecular level, Auxins have an aromatic ring and a carboxylic acid group (Taiz and Zeiger, 1998). Plant hormones (also known as plant growth regulators (PGRs) and phytohormones) are chemicals that regulate a plants growth. ... Zeatin is named after the genera of corn, Zea as it was first discovered in corn. ... The nervous system is a highly specialized network whose principal components are nerves called neurons. ...


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. However, molecules of IAA are chemically labile in aqueous solution, so IAA is not used commercially as a plant growth regulator. Indole-3-acetic acid, also known as IAA, is a member of the group of phytohormones called auxins. ...

  • Naturally-occurring auxins include 4-chloro-indoleacetic acid, phenylacetic acid (PAA) and indole-3-butyric acid (IBA).
  • Synthetic auxin analogs include 1-naphthaleneacetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), and others.

Auxins are often used to promote initiation of adventitious roots and are the active ingredient of the commercial preparations used in horticulture to root stem cuttings. They can also be used to promote uniform flowering, to promote fruit set, and to prevent premature fruit drop. For other uses, see Root (disambiguation). ... Horticulture (Latin: hortus (garden plant) + cultura (culture)) are classically defined as the culture or growing of garden plants. ... Plant cuttings are a technique for vegetatively (asexually) propagating plants in which a piece of the source plant containing at least one stem cell is placed in a suitable medium such as moist soil, potting mix, coir or rock wool. ... For other uses, see Flower (disambiguation). ... For other uses, see Fruit (disambiguation). ...


Used in high doses, auxin stimulates the production of ethylene. Excess ethylene can inhibit elongation growth, cause leaves to fall (leaf abscission), and even kill the plant. Some synthetic auxins such as 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) have been used as herbicides. Broad-leaf plants (dicots) such as dandelions are much more susceptible to auxins than narrow-leaf plants (monocots) like grass and cereal crops. These synthetic auxin were the active agents in Agent Orange, a defoliant used extensively by American forces in the Vietnam War. Ethylene (or IUPAC name ethene) is the chemical compound with the formula C2H4. ... Look up foliage in Wiktionary, the free dictionary. ... Abscission (from ab- away from, and scission cutting or severing) is the shedding of a body part. ... 2,4,5- trichlorophenoxyacetic acid is a herbicide used to defoliate broad leafed plants. ... An herbicide is used to kill unwanted plants. ... Orders see text Dicotyledons or dicots are flowering plants whose seed contains two embryonic leaves or cotyledons. ... For other uses, see Dandelion (disambiguation). ... Orders Base Monocots: Acorus Alismatales Asparagales Dioscoreales Liliales Pandanales Family Petrosaviaceae Commelinids: Arecales Commelinales Poales Zingiberales Family Dasypogonaceae Monocotyledons or monocots are a group of flowering plants usually ranked as a class and once called the Monocotyledoneae. ... For other uses, see Grass (disambiguation). ... Cereal crops are mostly grasses cultivated for their edible seeds (actually a fruit called a grain, technically a caryopsis). ... For other uses, see Agent Orange (disambiguation). ... Combatants Republic of Vietnam United States Republic of Korea Thailand Australia New Zealand The Philippines National Front for the Liberation of South Vietnam Democratic Republic of Vietnam People’s Republic of China Democratic Peoples Republic of Korea Strength US 1,000,000 South Korea 300,000 Australia 48,000...


Hormonal activity

Auxins coordinate development at all levels of plants, from the cellular level to organs and ultimately the whole plant. Drawing of the structure of cork as it appeared under the microscope to Robert Hooke from Micrographia which is the origin of the word cell being used to describe the smallest unit of a living organism Cells in culture, stained for keratin (red) and DNA (green) The cell is the...

The plant cell wall is made up of cellulose and protein, and, in many cases, lignin. It is very firm and prevents any sudden expansion of cell volume, and, without contribution of auxins, any expansion at all.
The plant cell wall is made up of cellulose and protein, and, in many cases, lignin. It is very firm and prevents any sudden expansion of cell volume, and, without contribution of auxins, any expansion at all.

Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ...

Molecular mechanisms

Auxins directly stimulate or inhibit the expression of specific genes. Auxin induces transcription by targeting for degradation members of the Aux/IAA family of transcriptional repressor proteins, The degradation of the Aux/IAAs leads to the derepression of ARF-mediated transcription. Aux/IAAs are targeted for degradation by ubiquitination, catalysed by an SCF-type ubiquitin-protein ligase. Gene expression, or simply expression, is the process by which the inheritable information which comprises a gene, such as the DNA sequence, is made manifest as a physical and biologically functional gene product, such as protein or RNA. Several steps in the gene expression process may be modulated, including the... For other uses, see Gene (disambiguation). ... A repressor is a DNA-binding protein that regulates the expression of one or more genes by decreasing the rate of transcription. ... Ubiquitin is a very conserved small regulatory protein that is ubiquitous in eukaryotes. ...


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 auxin binding TIR1 recruits specific transcriptional repressors (the Aux/IAA repressors) for ubiquitination by the SCF complex. This marking process leads to the degradation of the repressors by the proteasome, alleviating repression and leading to specific gene expression in response to auxins. The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. ... Ubiquitin is a very conserved small regulatory protein that is ubiquitous in eukaryotes. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... A proteasome is a barrel-shaped multi-protein complex that can digest other proteins into short polypeptides and amino acids in an ATP-driven reaction. ...


Another protein called ABP1 (Auxin Binding Protein 1) is a putative receptor, but its role is unclear. Electrophysiological experiments with protoplasts and anti-ABP1 antibodies suggest that ABP1 may have a function at the plasma membrane. Protoplast, from the ancient Greek πρωτον (first) + πλασσειν (to mould), initially referred to the first organized body of a species. ... Drawing of a cell membrane A component of every biological cell, the cell membrane (or plasma membrane) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ...


On a cellular level

On the cellular level, auxin is essential for cell growth, affecting both cell division and cellular expansion. 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. The term cell growth is used in two different ways in biology. ... This does not adequately cite its references or sources. ...


According to the acid growth hypothesis for auxin action, auxins may directly stimulate the early phases of cell elongation by causing responsive cells to actively transport hydrogen ions out of the cell, thus lowering the pH around cells. This acidification of the cell wall region activates wall-loosening proteins known as expansins, which allow slippage of cellulose microfibrils in the cell wall, making the cell wall less rigid. When the cell wall is loosened by the action of auxins, this now-less-rigid wall is expanded by cell turgor pressure, which presses against the cell wall Acid growth refers to the ability of plant cells to quickly stretch. ... For other uses, see PH (disambiguation). ... Plant cells separated by transparent cell walls. ... This article or section does not cite its references or sources. ...


However, the acid growth hypothesis does not by itself account for the increased synthesis and transport of cell wall precursors and secretory activity in the Golgi system that accompany and sustain auxin-promoted cell expansion.


Organ patterns

Growth and division of plant cells together result in growth of tissue, and specific tissue growth contributes to the development of plant organs. Growth of cells contributes to the plant's size, but uneven 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 tropisms. Biological tissue is a collection of interconnected cells that perform a similar function within an organism. ... This article is about the biological unit. ... The Thale Cress (Arabidopsis thaliana) is regulated by blue to UV light (plantphys. ... Gravitropism [or geotropism] is a turning or growth movement by a plant or fungi in response to gravity. ... It has been suggested that chemotropism be merged into this article or section. ...


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. Throughout the plant's life, auxin helps the plant maintain the polarity of growth and recognize where it has its branches (or any organ) connected.


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 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 suppressive hormone 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. This behavior is used in pruning by horticulturists. Many conifers show particularly strong apical dominance, strongest of all in the family Araucariaceae, showing a single erect central trunk with strongly differentiated horizontal branching. ... In microeconomics, pruning taken as a metaphor from gardening, refers to the removal of excess items from a budget. ...


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, but each cell retains the potential ability to do so, and only under specific conditions will auxin synthesis be activated. For that purpose, not only do auxins have to be translocated toward those sites where they are needed but there has to be an established mechanism to detect those sites. Translocation is driven throughout the plant body primarily from peaks of shoots to peaks of roots. 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 process of 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. In vascular plants, phloem is the living tissue that carries organic nutrients, particularly sucrose, a sugar, to all parts of the plant where needed. ... Polar auxin transport is the regulated transport of the plant hormone, auxin, in plants. ...


A 2006 study showed plant-specific pin-formed (PIN) proteins are vital in transporting auxin. PINs also regulate auxin efflux from mammalian and yeast cells.[1]


Locations

  • In shoot (and root) meristematic tissue
  • In young leaves
  • In mature leaves in very tiny amounts
  • In mature root cells in even smaller amounts
  • Transported throughout the plant more prominently downward from the shoot apices

This article is about the plant section. ... For other uses, see Root (disambiguation). ... Tunica-Corpus model of the apical meristem. ... Biological tissue is a collection of interconnected cells that perform a similar function within an organism. ... Look up foliage in Wiktionary, the free dictionary. ...

Effects

A healthy Arabidopsis thaliana plant (left) next to an auxin signal-transduction mutant
A healthy Arabidopsis thaliana plant (left) next to an auxin signal-transduction mutant
Crown galls are caused by Agrobacterium tumefaciens bacteria; they produce and excrete auxin and cytokinin, which interfere with normal cell division and cause tumors
Crown galls are caused by Agrobacterium tumefaciens bacteria; they produce and excrete auxin and cytokinin, which interfere with normal cell division and cause tumors

The plant hormone stimulates cell elongation. It stimulates the Wall Loosening Factors, for example, elastins, to loosen the cell walls. If gibberellins are also present, the effect is stronger. It 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 while xylem tissues can be generated when the auxin concentration is equal to the cytokinins. Image File history File links Metadata Size of this preview: 376 × 598 pixelsFull resolution (884 × 1407 pixel, file size: 105 KB, MIME type: image/jpeg) File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Metadata Size of this preview: 376 × 598 pixelsFull resolution (884 × 1407 pixel, file size: 105 KB, MIME type: image/jpeg) File historyClick on a date/time to view the file as it appeared at that time. ... Binomial name Arabidopsis thaliana (L.) Heynh. ... Agrobacterium tumefaciens causes gall fomration on roots. ... Agrobacterium tumefaciens causes gall fomration on roots. ... Kalanchoë infected with crown-gall using Agrobacterium tumefaciens. ... Binomial name Agrobacterium tumefaciens Smith & Townsend, 1907 Synonyms Bacterium tumefaciens Smith and Townsend 1907 Pseudomonas tumefaciens (Smith and Townsend 1907) Duggar 1909 Phytomonas tumefaciens (Smith and Townsend 1907) Bergey et al. ... Phyla Actinobacteria Aquificae Chlamydiae Bacteroidetes/Chlorobi Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Lentisphaerae Nitrospirae Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Verrucomicrobia Bacteria (singular: bacterium) are unicellular microorganisms. ... Zeatin is named after the genera of corn, Zea as it was first discovered in corn. ... This does not adequately cite its references or sources. ... For malignant tumors specifically, see cancer. ... Gibberellins are a plant growth substance (phytohormone) involved in promotion of stem elongation, mobilisation of food reserves in seeds and other processes. ... Cytokinins are a class of plant growth substances (plant hormones) active in promoting cell division. ... This article is about calluses and corns of human skin. ...


It participates in phototropism, geotropism, hydrotropism and other developmental changes. The uneven distribution of auxin, due to environmental cues (for example, unidirectional light and gravity force), results in uneven plant tissue growth. The Thale Cress (Arabidopsis thaliana) is regulated by blue to UV light (plantphys. ... // Gravitropism (or geotropism) is a turning or growth movement by a plant in response to gravity. ... Hydrotropism is the tendency for an organism to grow towards (or away from) water. ...


It also induces sugar and mineral accumulation at the site of application.


Wounding response

It induces formation and organization of phloem and xylem. When the plant is wounded, the auxin can induce the Cell differentiation and regeneration of the vascular tissues. In vascular plants, phloem is the living tissue that carries organic nutrients, particularly sucrose, a sugar, to all parts of the plant where needed. ... In vascular plants, xylem is one of the two types of transport tissue, phloem being the other one. ... Cellular differentiation is a concept from developmental biology describing the process by which cells acquire a type. The morphology of a cell may change dramatically during differentiation, but the genetic material remains the same, with few exceptions. ...


Root growth and development

Auxin induces new root formation by breaking root apical dominance induced by cytokinins. In horticulture, auxins, especially NAA and IBA, are commonly applied to stimulate root growth 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. 1-Naphthaleneacetic acid (2-(1-naphthyl)acetic acid, NAA) is a white powder, with the molecular formula C12H10O2. ... Indole-3-butyric acid (1H-Indole-3-butanoic acid, IBA) is a white to light-yellow crystalline solid, with the molecular formula C12H13NO2. ... Plant cuttings are a technique for vegetatively (asexually) propagating plants in which a piece of the source plant containing at least one stem cell is placed in a suitable medium such as moist soil, potting mix, coir or rock wool. ...


Apical dominance

It induces shoot apical dominance; 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 causes inhibition of its growth and potentiation of apical dominance. Many conifers show particularly strong apical dominance, strongest of all in the family Araucariaceae, showing a single erect central trunk with strongly differentiated horizontal branching. ... Ethylene (or IUPAC name ethene) is the chemical compound with the formula C2H4. ...


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.[citation needed]


It inhibits abscission prior to formation of abscission layer and thus inhibits senescence of leaves. Abscission (from ab- away from, and scission cutting or severing) is the shedding of a body part. ...


Fruit growth

Auxin delays fruit senescence.


It is required for fruit growth. 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). In botany and horticulture, parthenocarpic literally means virgin fruit; the fruit develops without fertilization of ovules, therefore it is seedless. ...


Flowering

Auxin plays a minor role in the initiation of flowering. It can delay the senescence of flowers in low concentrations.


Herbicide manufacture

The defoliant Agent Orange was a mix of 2,4-D and 2,4,5-T. 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 leukaemia, miscarriages, birth defects, liver damage, and other diseases. Agent Orange was sprayed in Vietnam as a defoliant to deny ground cover to the Vietnamese army. A defoliant is any chemical sprayed or dusted on plants to cause its leaves to fall off. ... For other uses, see Agent Orange (disambiguation). ... 2,4-Dichlorophenoxyacetic acid (2,4-D) is a common systemic herbicide used in the control of broadleaf weeds. ... 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) is an herbicide used to defoliate broad leafed plants. ... EPA redirects here. ... Also: 1979 by Smashing Pumpkins. ... Dioxin is the common name for the group of compounds classified as polychlorinated dibenzodioxins (PCDDs). ... TCDD may refer to any of the following: 2,3,7,8-Tetrachlorodibenzo-p-Dioxin — a type of dioxin. ... Leukemia (leukaemia in Commonwealth English) is a group of blood diseases characterized by malignancies (cancer) of the blood-forming tissues. ... Miscarriage or spontaneous abortion is the natural or spontaneous end of a pregnancy at a stage where the embryo or the fetus is incapable of surviving, generally defined in humans at a gestation of prior to 20 weeks. ... A congenital disorder is a medical condition or defect that is present at or before birth (for example, congenital heart disease). ... The liver is the largest internal organ in the human body, and is an organ present in vertebrates and some other animals. ... This article is about the medical term. ... For other uses, see Agent Orange (disambiguation). ...


"Killex" is another herbicide, composed of 2,4-D and Dicamba.


See also

An herbicide is used to kill unwanted plants. ... Pruning is a technique that is employed by gardeners in order to control growth, remove dead or diseased wood or stimulate the formation of flowers and fruit buds. ... There are very few or no other articles that link to this one. ...

References

  • Plant Physiology Online - Chapter 19: Auxin: The Growth Hormone
  • Plant Physiology

Taiz, L. & Zeiger, E. (1998). Plant Physiology. 2nd edition. Massachusetts: Sinauer Associates, Inc. 792 p.

Plant hormones (also known as plant growth regulators (PGRs) and phytohormones) are chemicals that regulate a plants growth. ... Abscisic Acid (ABA), also known as abscisin II and dormin, is a plant hormone. ... Zeatin is named after the genera of corn, Zea as it was first discovered in corn. ... Ethylene (or IUPAC name ethene) is the chemical compound with the formula C2H4. ... GA1 GA3 ent-Gibberellane ent-Kauren Gibberellins (GAs) are plant hormones involved in promotion of stem elongation, mobilization of food reserves in seeds and other processes. ... Brassinolide appears to be the most common Brassinosteroid. ... Jasmonic Acid is released by plants when wounded and helps organize healing and defense. ... The polyamines are organic compounds having two or more primary amino groups - such as putrescine, cadaverine, spermidine, and spermine - that are growth factors in both eucaryotic and procaryotic cells. ... Salicylic acid (from the Latin word for the willow tree, Salix, from whose bark it can be obtained) is a beta hydroxy acid (BHA) with the formula C6H4(OH)CO2H, where the OH group is adjacent to the carboxyl group. ...

  Results from FactBites:
 
Auxin - Wikipedia, the free encyclopedia (1421 words)
Auxins are often used to promote root growth as a main compound of rooting stimulators (beneficial mainly in horticulture for treating of stem cuttings).
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.
Since Auxin attracts nutrients to the cell where it is, this transport of auxin away from sugar synthesis may partly explain the transport of sugar in the phloem to the roots.
Auxin (1290 words)
An auxin transporter — one of the PIN proteins — is inserted in the plasma membrane at the lateral face of cells of the shoot.
The localized accumulation of auxin in epidermal cells of the root initiates the formation of lateral or secondary roots.
Auxin (IAA) is actively transported into cells by a transmembrane transporter and leaves the cells by facilitated diffusion through a different transporter.
  More results at FactBites »

 
 

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