Auxin 496667 223339582 2008-07-03T17:21:55Z Amirobot 7390477 robot Adding: [[fa:اکسین]] [[Image:Indol-3-ylacetic acid2.svg|thumb|right|175px|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. ==Overview== Auxins derive their name from the [[Greek (language)|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 hormone]]s. 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). 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. * ''Naturally-occurring auxins'' include [[4-chloro-indoleacetic acid]], [[phenylacetic acid]] (PAA) and [[indole-3-butyric acid]] (IBA). * ''Synthetic auxin analogs'' include [[1-Naphthaleneacetic acid|1-naphthaleneacetic acid]] (NAA), [[2,4-dichlorophenoxyacetic acid]] (2,4-D), and others. <gallery caption="Gallery of native auxins"> Image:Indol-3-ylacetic acid2.svg|[[indole-3-acetic acid]] (IAA) Image:Indole-3-butyric acid structure.svg|[[Indole-3-butyric acid]] (IBA) Image:4-chloroindole-3-acetic acid.png|4-chloroindole-3-acetic acid (4-CI-IAA) Image:2-phenylacetic acid.png|2-[[phenylacetic acid]] (PAA) </gallery> <gallery caption="Gallery of synthetic auxins"> Image:2,4-Dichlorophenoxyacetic acid structure.svg|[[2,4-Dichlorophenoxyacetic acid]] (2,4-D) Image:1-Naphthaleneacetic acid.png|[[1-Naphthaleneacetic acid|α-Naphthalene acetic acid]] (α-NAA) Image:Dicamba.png|[[2-Methoxy-3,6-dichlorobenzoic acid]] (dicamba) Image:Picloram.png|[[4-Amino-3,5,6-trichloropicolinic acid]] (tordon or picloram) Image:2,4,5-T.svg|[[2,4,5-Trichlorophenoxyacetic acid]] (2,4,5-T) Image:α-(p-Chlorophenoxy)isobutyric acid.png|α-(p-Chlorophenoxy)isobutyric acid (PCIB, an antiauxin) </gallery> Auxins are often used to promote initiation of adventitious [[root]]s and are the active ingredient of the commercial preparations used in [[horticulture]] to root [[Cutting (plant)|stem cuttings]]. They can also be used to promote uniform [[flower]]ing, to promote [[fruit]] set, and to prevent premature fruit drop. Used in high doses, auxin stimulates the production of [[ethylene]]. Excess ethylene can inhibit elongation growth, cause [[leaf|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 [[herbicide]]s. Broad-leaf plants ([[dicots]]) such as [[dandelion]]s are much more susceptible to auxins than narrow-leaf plants ([[monocots]]) like [[grass]] and [[cereals|cereal]] crops. These synthetic auxins were the active agents in [[Agent Orange]], a defoliant used extensively by American forces in the [[Vietnam War]]. ==Hormonal activity== Auxins coordinate development at all levels in plants, from the [[Cell (biology)|cell]]ular level to organs and ultimately the whole plant. [[Image:Plant cell structure svg.svg|thumb|right|400px|The plant cell wall is made up of [[cellulose]], [[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.]] === Molecular mechanisms === Auxins directly stimulate or inhibit the [[gene expression|expression]] of specific [[gene]]s. 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 '''A'''uxin '''R'''espose '''F'''actors [[B3_domain#B3_families|ARF]]-mediated transcription. Aux/IAAs are targeted for degradation by [[ubiquitin]]ation, catalysed by an SCF-type ubiquitin-protein ligase. In 2005, it was demonstrated that the [[F-box protein]] TIR1, which is part of the ubiquitin ligase complex [[SCF complex|SCFTIR1]], is an auxin receptor. Upon auxin binding TIR1 recruits specific [[Transcription (genetics)|transcriptional]] repressors (the Aux/IAA repressors) for [[ubiquitin]]ation 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 (reviewed in [http://www.springerlink.com/content/g71q260r72q47546/]). 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]]. === On a cellular level === {{Unreferencedsection|date=June 2008}} 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. 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 [[expansin]]s, 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.{{Fact|date=June 2008}} 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.{{Fact|date=June 2008}} ===Organ patterns=== Growth and division of plant cells together result in growth of [[Biological tissue|tissue]], and specific tissue growth contributes to the development of plant [[organ (anatomy)#Plant organs|organ]]s. 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 [[tropism]]s. ===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. '''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. <!--parallelism, PLEASE: "not only...but" requires that SAME part of speech after each element of the correlative-conjuntive phrase; INCORRECT as written: "For that purpose, auxins have to be not only translocated toward those sites where they are needed, but there has to be an established mechanism to detect those sites.--> 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. 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.[http://www.sciencemag.org/cgi/content/short/312/5775/914] ==Locations== *In [[shoot]] (and [[root]]) [[meristem]]atic [[Biological tissue|tissue]] *In young [[leaf|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 ==Effects==<!--the cleanup required to make this section even legible - never mind intelligible - was WORK--> [[Image:Auxin.jpg|thumb|250px|A healthy ''[[Arabidopsis thaliana]]'' plant (left) next to an auxin signal-transduction mutant]] [[Image:Agrobacteriumgall.jpg|thumb|250px|[[Gall|Crown galls]] are caused by ''[[Agrobacterium tumefaciens]]'' [[bacteria]]; they produce and excrete auxin and [[cytokinin]], which interfere with normal [[cell division]] and cause [[tumor]]s]] 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. 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. 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 may induce the [[Cell differentiation]] and regeneration of the vascular tissues. ===Root growth and development=== Auxin induces new root formation by breaking root apical dominance induced by cytokinins. In horticulture, auxins, especially [[1-Naphthaleneacetic acid|NAA]] and [[Indole-3-butyric acid|IBA]], are commonly applied to stimulate root growth when taking [[Cutting (plant)|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=== 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. ===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. {{Fact|date=April 2007}} It inhibits [[abscission]] prior to formation of abscission layer and thus inhibits senescence of leaves. ===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). ===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-dichlorophenoxyacetic acid|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 [[United States Environmental Protection Agency|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]], [[miscarriage]]s, [[birth defect]]s, [[liver]] damage, and other [[disease]]s. [[Agent Orange]] was sprayed in [[Vietnam]] as a defoliant to deny ground cover to the Vietnamese army. ==See also== * [[Herbicide]] * [[Pruning fruit trees]] * [[Fusicoccin]] ==References== *[http://www.plantphys.net/chapter.php?ch=19 Plant Physiology Online - Chapter 19: Auxin: The Growth Hormone] * [http://www.iephb.ru/biolibrary/Biolib4/Plant%20Physiology/ Plant Physiology] Taiz, L. & Zeiger, E. (1998). Plant Physiology. 2nd edition. Massachusetts: Sinauer Associates, Inc. 792 p. {{Plant_hormones}} [[Category:Plant physiology]] [[Category:Auxins| ]] [[ca:Auxina]] [[cs:Auxiny]] [[de:Auxine]] [[es:Auxina]] [[fa:اکسین]] [[fr:Auxine]] [[gl:Auxina]] [[it:Auxina]] [[he:אוקסין]] [[lt:Auksinai]] [[nl:Auxine]] [[ja:オーキシン]] [[pl:Auksyny]] [[pt:Auxina]] [[ru:Ауксины]] [[sr:Ауксин]] [[fi:Auksiini]] [[uk:Ауксини]] [[zh:生長素]]