Auxin

The auxins (Greek αὐξάνω " Auxano " - " I grow " ) are a group of natural and synthetic growth regulators with multiple effects on growth and differentiation processes in vascular plants and a specific effect in the protonema of mosses. The naturally occurring plant auxins are among the phytohormones. Opponent of auxins are the Blastokoline. The auxins are in technical language because of their action under the name stretching hormones known. In plants they are essential.

• 4.1 Concentration-dependent effect
• 4.2 Physiological effects 4.2.1 elongation growth
• 4.2.2 Apical dominance
• 4.2.3 Cell division and differentiation
• 4.2.4 senescence

Structure

The molecular structure of the various auxins is quite different. All together is a planar aromatic ring system, and a hydrophobic residual transition region and a terminal carboxyl group. The aromatic may, however, have quite different structures. Indoles are effective but also phenyls and a naphthyl radical. In addition, the length of the hydrophobic transition region varies (one to three CH2 groups ).

Natural auxins

Indole - 3-acetic acid (IAA, β - indoleacetic acid, indole - 3-acetic acid, heteroauxin ) is the most important representatives of the auxins. It comes in all higher plants in a small amount (1 to 100 micrograms per kg plant material ) in front, and is also present in lower plants and bacteria. It is the most common and auxin and therefore responsible for the majority of natural Auxineffekte. Commercial IAA is not used because it is relatively unstable in aqueous solution.

Indole- 3-acetic acid can be found in plant tissues, either in the free form and the carboxyl group by ester of myo -inositol, glucose or galactose, or a peptide-like amino acids such as aspartic acid or tryptophan, bound before. These indole-3- acetic acid derivatives are referred to as Auxinkonjugate and discriminated as glycosyl, or myo- Inosityl Peptidylkonjugate. Auxinkonjugate are all biologically inactive. They play an important role in the regulation of Auxinstoffwechsels. In different plants, there are other structural analogues of indole - 3-acetic acid such as 4- Chlorindolylessigsäure, Indolylethanol, Indolylacetamid, indolylacetonitrile and Indolylacetaldehyd. These are used partly as biosynthetic precursors ( Auxinvorstufen ).

Further natural auxins are phenylacetic acid (PAA ), 4- chlorindole - 3-acetic acid ( 4-Cl - IAA) and 4 - ( indol-3- yl ) butyric acid (IBA ). IBA has long been considered purely synthetic auxin, but now also from corn and other plants ( for example mustard plants ) was isolated.

Synthetic auxins

(Indol -3 -yl ) butyric acid ( β - indolobutyric IBA) and Indolylpropionsäure, phenyl and 1- naphthyl acetic acid and phenoxy and Naphthoxyessigsäuren of practical importance - Of the synthetic compounds with auxin activity are mainly 4. There are also 2,4-dichlorophenoxyacetic acid and dicamba.

They are produced artificially in the laboratory. Here, an alcoholic solution is stirred ( in % range ) at a carrier ( talc or activated carbon). After drying, a dust, for example in the cuttings bases can be immersed. Less often the solution is sprayed directly on the plants. Also rarely the growth substance is added directly to the irrigating water.

Products available commercially, for example: PhytoBoost ® ( active ingredients: indole - 3-acetic acid, vitamins) as the only product with the natural auxin IAA, also SuperThrive ® ( active ingredients: 1- naphthalene acetic acid and many others, unpublished components), Seradix ® ( active ingredient: 4 - (indol -3-yl ) butyric acid ), which is available in three different concentrations ( 0.2%, 0.4% and 0.8 %) or Rhizopon ® / Chryzopon ® (active substances: 4 - (indol -3-yl ) butyric acid or naphthylacetic acid in various concentrations).

Biosynthesis

The formation of indole - 3-acetic acid (IAA ) occurs in young, rapidly dividing and growing tissues, especially in the shoot, coleoptile and shoot tips, young leaves, developing seeds and the active cambium. Also in the apical root meristem IAA is formed. IAA is structurally related to tryptophan ( Trp). In fact, there is a tryptophan -dependent and independent pathway Trp, in which the biosynthesis of a precursor is carried out tryptophan.

The tryptophan - dependent pathway four metabolic pathways are known. There are the tryptamine pathway ( TAM), the indole - 3-pyruvate pathway ( IPA), indole -3 -acetonitrile pathway ( IAN) and a pathway that is found only in bacteria (A. tumefaciens ). In plants the first two paths are the most common. The discovery of mutants that tryptophan could not produce themselves, but still contained IAA, raised questions about a tryptophan - independent pathway. This mutant, moreover, were not able to produce by Trp administered in excess of this auxin. It has been determined by isotope-labeled feeding experiments that the tryptophan precursor indole -3 -glycerol phosphate serves as a precursor of auxin synthesis. More detailed studies have shown, however, that it is at the IAA formation is a chemical decomposition of indole-3 -glycerol phosphate, which is not catalyzed by enzymes.

Transport

The transport of auxin is mainly from the shoot to the root tip. The inactivation of auxins done by enzymatically catalyzed oxidative degradation or by conjugation for storage.

Auxin is the only polar transported phytohormone. The transport takes place either parenchymatous or via the vascular tissue ( the scion basipetally in the root acropetally or basipetally over short distances ). For transport in the phloem of a chemical modification is necessary. Here, there will be a covalent bond with glucose, myo -inositol or aspartate. This IAA conjugates are physiologically inactive and it is done at the target tissue cleavage of the covalent bond.

The transport can be divided into two groups:

• Over long distances: the phloem, mainly basipetally, about 10 to 20 cm / h
• Over shorter distances: polar transport from cell to cell in the parenchyma, also by means of two basipetally anion transporters, an auxin influx carrier (for example, AUX1 in Arabidopsis ), and an auxin efflux carrier (PIN proteins). Influx carriers located primarily in the apical cell membrane of a cell, while the efflux carriers opposite in the basal membrane occur. AUX1 functions as a proton symporter (secondary active transport ), i.e. deprotonated auxin (anionic form) is transported together with two protons into the cell. Auxin also may diffuse due to the low pH ( about 5.5 ) in the cell wall in the protonated form through the membrane. Through the neutral pH ( about 7.0 ) in the cell deprotonated auxin and can no longer simply diffuse through the membrane from the cell. It is funneled through PIN proteins at the basal end of the cell active again out of the cell. The same process is repeated in the next lower cell, and as a polar transport occurs at a rate of about 1 cm / h about.

In Arabidopsis, there are a gene family of AtPIN proteins. The name is derived from Arabidopsis thaliana and the leafless (English pin - needle ) phenotype corresponding mutants from and they are now quite well studied. The directional transport of cell to cell can be modified by changes in expression of the carrier pin. This allows the IAA current are deflected in the plant. Based on this effect, for example, phototropism and gravitropism of.

Effect

Auxins have a diverse, generally promoting effect on the overall development of higher plant species in complex interaction with other phytohormones. Auxins are particularly on cell elongation, especially of coleoptiles and in the shoot axes. This is the classic Auxineffekt. They stimulate the Kambiumtätigkeit that affect cell division, apical dominance, abscission, phototropism and gravitropism and other growth and developmental processes.

The controversial plant biology € writes auxin addition, a neurotransmitter- like effect to.

Concentration-dependent effect

In high doses Auxins are surprisingly potent inhibitory action. The reason for this lies in organ-specific concentration optima. This lower doses of the hormone to act in a certain concentration promotes the cell elongation growth, while at high concentrations inhibit the elongation growth. At too high a concentration of indole-3 -acetic acid synthesis is supported by gaseous ethylene, a stress " hormone ", which for example, has a negative effect on longitudinal growth of the roots. In the shoot axis, the optimum concentration is usually higher than in the root, which is why it is already there at lower auxin for growth inhibition. These may play an important role in Gravitropism.

The auxin is utilized in auxin herbicides (such as 2,4-dichlorophenoxyacetic acid 2,4-D or short, and 2,4,5- trichlorophenoxyacetic acid, 2,4,5 - T or short ). The herbicide 2,4-D acts selectively on dicotyledonous weeds by encouraging them to excessive growth and so exhausted their Biosynthesekapazität. Monocotyledonous plants ( such as crop plants ) do not respond to 2,4-D.

Physiological effects

Elongation growth

The principal mode of action of auxin is to promote cellular elongation growth. This is due to two effects:

Characterized in that the transport takes place judged by auxin, and the cell elongation is carried out by a corresponding pattern. Be based on several Auxinwirkungen:

• Phototropism plants can perceive the lighting conditions of their environment by so-called photoreceptors and align their growth and their development on it. The growth towards light is called phototropism. In higher plants, the corresponding photoreceptors, phototropins. For unequal exposure of the shoot there is, among others, the failure of PIN proteins on the light side. Thus, the auxin flow in the plant will be redirected to the dark side. There, reinforcing cell elongation, so that the plant bends toward the light. It is said that the shoot growth occurs positively phototropic.
• Gravitropism: Certain sensor systems ( to be discussed especially amyloplasts and Golgi vesicles) in the root of gravity can be perceived. Tilts to a plant for the page, the auxin current is diverted through PIN shift to the underside of the root. Due to the high sensitivity of the root of this auxin auxin current has an inhibitory effect on cell extension of the underside, so that the root is curved downward. The growth takes place in the root gravitropism so positive. Conversely, it can be observed that the scion of the plant to curve after tilting upward; shoot growth is therefore negative gravitropism.

This is probably due to the above-mentioned higher optimum concentration of auxin in the shoot, which here has a promoting effect on cell division.

Apical dominance

Another important effect of auxin is the apical dominance, that is more pronounced, for example, while the black run. It inhibits the auxin Sprosspitze formed the expulsion of lateral side buds. Opponent is here cytokinin, which (eg after removal of the shoot tip ) promotes the expulsion of lateral buds. However, the exact mechanism of the apical dominance is still controversial. An influence of the induction of ethylene biosynthesis auxinbedingten is discussed.

Cell division and differentiation

Together with the plant hormone cytokinin and auxin promotes the division growth and differentiation of cells.

• Cell division: Cell division is controlled by regulation of the cell cycle. Auxins, cytokinins, gibberellins and brassinosteroids be a facilitator, abscisic acid and jasmonic acid -resistant. The effect of promoting phytohormones based on the gene activation of cyclins and cyclin- dependent kinases (CDKs ).

Specifically, it is cyclin D and CDK A for the transition from the G1 to the S phase and the cyclins A and B and CDK A and B for the transition from the G2 to the M phase.

• Cell differentiation: For the differentiation of the ratio of auxin to cytokinin is crucial. At a high auxin: cytokinin ratio to root tissue forms, with a low auxin: cytokinin ratio is a scion forms. This effect is exploited for example for organogenesis in the Herbal tissue culture. In the plant, it comes at a high auxin concentration to increased formation of adventitious and lateral roots. Also, is formed in the cambium at an auxin - cytokinin ratio of approximately 1:1 xylem tissue. This plays a role, especially in plant development and after injury.

Auxin controls the fruiting and development. After pollination IAA stimulates pollen from the cell divisions for the fruit set, the subsequent elongation growth in fruit tissue is triggered by IAA from the developing seeds. Externally supplied auxin leads to synchronize the fruiting or to achieve, for example seedless fruits such as tomatoes, cucumbers, etc. in many plants to parthenocarpy, which is used for example in agriculture. In the embryo, the concentration gradient of auxin leads to pattern formation and thus determines which part developed into root, shoot and cotyledons.

In the protonema of mosses such as Physcomitrella patens, auxins induce specifically the transition from chloronema to caulonema. This causes a change resulting in cell cycle control.

Senescence

Auxin delays senescence and prevents the dropping of leaves, flowers and fruits, by inhibiting the formation of abscission tissues. Opponents are abscisic acid, and especially ethylene. Higher concentrations of IAA, however, promote ethylene biosynthesis.

Molecular effect

The molecular effect of auxin is not yet fully understood. First, a Auxinrezeptor was discovered with the so-called Auxinbindeprotein 1 ( ABP1 ) in the 1980s, which specifically binds auxins. ABP1 interacts with an as yet unidentified protein docking at the plasma membrane. The forwarding of the signal is unknown, however, causes the modulation of membrane transport proteins (in particular the proton pump). For the auxin -induced changes in the expression of certain genes auxininduzierter ABP1 was rejected.

The auxininduzierte gene expression can be divided into a fast, direct and subdivide a slightly slower, more indirect effect. At the direct gene expression include so-called " auxin response factors" ( ARFs ) involved, to the " auxin response elements " ( Aux -Res; TGTCTC sequence ) bind the DNA and control gene expression. This auxin causes basically the reversal of a gene inhibition. This is also the reason for the rapid effect. In the normal state auxinregulierter genes ARF is tied together with a repressor ( "AUX / IAA " ) as a heterodimer to the AUX -Res. The gene is not expressed. If auxin is added, it binds to and activates the so-called " SCF complex", a ubiquitin protein ligase ( with TIR1 ), which ubiquitinates the repressor and thus marked for degradation. The repressor is removed from the proteasome, and the gene can be transcribed. For example, cell wall components for cell elongation are formed. Many direct auxin - controlled genes come from, among others, from the gene families AUX / IAA, SAUR (Small Auxin Up RNAs) and GH3. A potassium channel ( ZMK1 ) was identified as a growth- relevant auxininduziertes protein recently. The indirect gene expression via the just-described direct induction of transcription factors. These in turn allow the expression of other genes.

Reduction

Auxins are broken down by enzymes ( peroxidases ) and UV - rays, which, however, plays only a very minor role. The exact course of the reaction of enzymatic degradation is still unknown. Discussed IAA oxidases IAA degrade the side chain ago and degradation by the cleavage of the indole nucleus.

Proof

The detection and quantitation of auxins was formerly carried out mostly by specific bioassay systems, for example the Haferkoleoptilen writhing assay. Nowadays, gas chromatography or gas chromatography / mass spectrometry and immunoassays are used for auxin analysis.

Use

Indole -3 -acetic acid and especially some synthetic auxins such as 2,4- D have been used as growth regulators in agriculture and in horticulture and fruit ( fruit thinning, promote fruit set ) wide application. Examples here are the rooting of cuttings or acting as selective herbicides in cereal crops, cotton, soybean and sugar beet crops. Militarily, the butyl ester of 2,4,5- trichlorophenoxyacetic acid in the Vietnam War was used as "Agent Orange" to defoliation. The damage to people on the ground and in aircraft crews were based on the contamination by polychlorinated dibenzodioxins and dibenzofurans.

Auxins play an important role in cotton fiber development. Researchers at the Southwest University (Chongqing) was accomplished by genetic engineering an increase in IAA production in the epidermis of the plant at the beginning of fiber growth. This leads to an increase in the number and length of useful fibers ( lint ), and a decrease in the number of non- textiles can be processed to fibers ( linter ). Field trials of four years showed that the linter support in the transgenic plants was consistently higher by more than 15 % than that of the conventional controls. In addition, the fineness of the fibers improved.