Auxins

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Chemical structure of indole-3-acetic acid , the most important auxin
Healthy Arabidopsis thaliana plant (left) next to a mutant of auxin signal transduction

The auxins ( old Greek αὐξάνω auxánō "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 the moss . The naturally occurring plant auxins belong to the phytohormones . Antagonists of the auxins are the blastocolins . The auxins are also known under the name of stretching hormones because of their effect. They are essential in plants.

Frits Warmolt Went discovered their role as plant hormones in the 1920s . The auxin indole-3-acetic acid was isolated by Kenneth V. Thimann (and independently Fritz Kögl and colleagues).

structure

The molecular structure of the various auxins is quite different. What they all have in common is a planar aromatic ring system and a residue with a hydrophobic transition region and a terminal carboxy group . The aromatic can, however, have quite different structures. Effective are indoles also phenyls and a naphthyl . In addition, the length of the hydrophobic transition region (one to three CH 2 groups) varies .

Natural auxins

Indole-3-acetic acid (IAA, β-indolyl acetic acid, heteroauxin) is the most important representative of the auxins. It occurs in small amounts in all higher plants (1 to 100 µg per kg of plant material) and is also represented in lower plants and bacteria . It is the strongest and most common auxin and is therefore responsible for most of the natural auxin effects. However, IAA is not used commercially because it is relatively unstable in aqueous solution.

Indole-3-acetic acid is found in plant tissue either in free form and bound via the carboxy group in an ester-like manner to myo- inositol, glucose or galactose or in a peptide-like manner to amino acids such as aspartic acid or tryptophan . These indole-3-acetic acid derivatives are called auxin conjugates and a distinction is made between glycosyl, myo- inosityl and peptidyl conjugates. Auxin conjugates are all biologically inactive. They play an important role in regulating the auxin metabolism. Other structurally related compounds of indole-3-acetic acid such as 4-chloroindolylacetic acid, indolylethanol, indolylacetamide, indolylacetonitrile and indolylacetaldehyde can be found in various plants. Some of these serve as biosynthetic precursors (auxin precursors).

Other natural auxins are phenylacetic acid (PAA), 4-chloroindole-3-acetic acid (4-Cl-IAA) and 4- (indol-3-yl) butyric acid (IBA). IBA was long regarded as a purely synthetic auxin, but has now also been isolated from maize and other plants (e.g. mustard plants).

Synthetic auxins

Of the synthetic compounds having auxin activity are especially 4- (indol-3-yl) butyric acid (β-indolobutyric, IBA) and Indolylpropionsäure , phenyl and 1-naphthylacetic acid as well as phenoxy and Naphthoxyessigsäuren of practical importance. There are also 2,4-dichlorophenoxyacetic acid and dicamba .

They are made artificially in the laboratory. An alcoholic solution (in the% range) is stirred under a carrier substance (talc or activated charcoal). After drying, a dust is created in which, for example, the cuttings bases can be immersed. More rarely, the solution is sprayed directly onto the plants. The growth substance is also rarely given directly into the irrigation water.

Commercially available products are for example: PhytoBoost ® (active ingredients: indole-3-acetic acid, vitamins) as the only product with the natural auxin IAA, also SuperThrive ® (active ingredients: 1-naphthylacetic acid and many other, unpublished ingredients), 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 ingredients: 4- (indol -3-yl) butyric acid or naphthylacetic acid in various concentrations).

biosynthesis

Plant galls are of Agrobacterium tumefaciens - bacteria caused. These produce and release auxin and cytokinin , which disrupt normal cell division at the infection site and lead to growths

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

In the tryptophan-dependent pathway, a total of four metabolic pathways are known, the end product of which is IAA. They are the tryptamine pathway (TAM), the indole-3-pyruvate pathway (IPA), the indole-3-acetonitrile pathway (IAN) and a metabolic pathway that occurs only in bacteria ( A. tumefaciens ). The first two ways are most common in plants. The discovery of mutants that could not produce tryptophan themselves, but still contained IAA, raised questions about a tryptophan-independent synthetic route. Furthermore, these mutants were also unable to produce auxin from this auxin by administering excess Trp. It has been established through isotope-marked feeding experiments that the tryptophan precursor indole-3-glycerol phosphate serves as a precursor for auxin synthesis. However, more detailed investigations showed that IAA formation is a chemical decomposition of indole-3-glycerol phosphate, which is not catalyzed by enzymes.

transport

Auxin is mainly transported from the shoot to the root tip. The auxins are inactivated by enzymatically catalyzed oxidative degradation or by conjugate formation for storage.

Auxin is the only polar transported phytohormone . The transport takes place either parenchymatically or via the vascular system (basipetal in the shoot, acropetal in the root or, over short distances, also basipetal). A chemical modification is necessary for transport in the phloem . A covalent bond with glucose , myo-inositol or aspartate takes place here. These IAA conjugates are physiologically inactive and the covalent bond is cleaved on the target tissue.

Transport can be divided into two groups:

  • over longer distances: in the phloem, predominantly basipetal, about 10 to 20 cm / h
  • Over shorter distances: polar transport from cell to cell in the parenchyma , also basipetal with the help of two anion transporters, an auxin influx carrier (for example AUX1 in Arabidopsis ) and an auxin efflux carrier (PIN proteins). Influx carriers are mainly located in the apical cell membrane of a cell, while the efflux carriers occur in the opposite direction in the basal membrane. AUX1 functions as a proton symporter (secondary active transport), i.e. H. Deprotonated auxin (anionic form) is transported into the cell together with two protons . In addition, due to the low pH value (around 5.5) in the cell wall , auxin can diffuse through the membrane in protonated form. Due to the neutral pH (about 7.0) in the cell, auxin deprotonates and can no longer simply diffuse out of the cell through the membrane. It is actively channeled back out of the cell by PIN proteins at the basal end of the cell. The same process is repeated in the next cell below, resulting in polar transport at a speed of about 1 cm / h.

There is a whole family of AtPIN proteins in Arabidopsis . Their name is derived from Arabidopsis thaliana and the leafless (English pin - needle ) phenotype of corresponding mutants and they are now quite well studied. The directed transport from cell to cell can be modified by changing the expression of the PIN carrier. This allows the IAA current to be diverted in the plant. For example, phototropism and gravitropism are based on this effect .

effect

Structural formula of 2,4-D

Auxins have a diverse, generally promoting effect on the overall development of higher plant species in complex interaction with other phytohormones. Auxins have a particular effect on cell elongation, especially of coleoptiles and in the shoot axes . This is the classic auxin effect. They stimulate the cambium activity , influence cell division , apical dominance , abscission , phototropism and gravitropism and other growth and development processes.

The controversial plant neurobiology also ascribes auxin a neurotransmitter-like effect.

Concentration-dependent effect

In high doses, surprisingly, auxins have a strong growth-inhibiting effect. The reason for this lies in organ-specific concentration optima. Lower doses of the hormone up to a certain concentration have a promoting effect on cell elongation, while concentrations that are too high inhibit elongation growth. If the concentration of indole-3-acetic acid is too high , the synthesis of gaseous ethylene is promoted, a stress "hormone" which, for example, has a negative effect on the growth in length of the roots. In the shoot axis, the optimal concentration is usually higher than in the root, which is why growth is inhibited there even with lower auxin concentrations. This may play an important role in gravitropism .

The auxin effect is used in growth herbicides (for example 2,4-dichlorophenoxyacetic acid or 2,4-D for short, and 2,4,5-trichlorophenoxyacetic acid or 2,4,5-T for short). The herbicide 2,4-D acts selectively on dicotyledon weeds by stimulating them to overgrow and thus exhausting their biosynthetic capacity. Monocot plants (such as the cereal plants) do not respond to 2,4-D.

Physiological effect

Elongation growth

The primary effect of auxin is to promote cellular elongation growth. This is due to two effects:

  1. Acid growth: The acid growth theory was discovered by Hager in 1971 and confirmed many times. IAA causes the activation of membrane-bound H + -ATPases as well as the induction of their new synthesis and the export of their building blocks into the cell membrane. There they cause the cell wall to become acidic by releasing protons. As a result, hydrogen bonds between cell wall fibers , namely the xyloglucans and cellulose microfibrils, are split with the help of expansins . As a result, the cell wall pressure is reduced and the water potential in the cell drops, which leads to an influx of water into the cell and thus into the vacuoles. As a result of the compressive tension that now prevails in the cells, the cell wall expands (especially across the direction of the fibrils) and the cells stretch. The auxin binding protein 1 (ABP1; see below) is probably involved in this auxin effect.
  2. Cell wall synthesis: In addition to the actual stretching process, IAA also induces the formation of new cell wall components and thus the expansion of the cell wall parallel to the stretching growth. This auxin effect occurs via direct gene induction (see below).

Because the transport of auxin takes place in a directed manner, the cells also stretch according to a corresponding pattern. Several auxin effects are based on this:

  • Phototropism : Plants can perceive the light conditions of their environment through so-called photoreceptors and adjust their growth and development accordingly. The growth towards light is called phototropism. In higher plants the corresponding photoreceptors are phototropins . If the shoot is unevenly exposed, PIN proteins will fail on the light side, among other things. This diverts the auxin current in the plant to the shadow side. There, the cells stretch more intensely, so that the plant curves towards the light. It is said that shoot growth is positive phototropic.
  • Gravitropism : Certain sensor systems ( amyloplasts and Golgi vesicles in particular are discussed ) in the root can be used to perceive gravity. If a plant is tilted to one side, the auxin current is redirected to the underside of the root by shifting the PIN. Due to the high auxin sensitivity of the root, this auxin current has an inhibiting effect on the cell stretching of the underside, so that the root curves downwards. The growth takes place positively gravitropically in the root. Conversely, it can be observed that the shoot of the plant curves upwards after tilting; shoot growth is negative gravitropic.

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 , which is more pronounced, for example, during the shadow escape. Auxin formed in the tip of the shoot inhibits the sprouting of lateral side buds. The antagonist here is cytokinin , which promotes the expulsion of side buds (for example after removing the tip of the shoot). However, the exact mechanism of apical dominance is still controversial. An influence of the auxin-related induction of ethylene biosynthesis is discussed.

Cell division and differentiation

Together with the phytohormone cytokinin , auxin also promotes the growth of division and the differentiation of cells.

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

  • Cell differentiation : The ratio of auxin to cytokinin is decisive for differentiation. With a high auxin: cytokinin ratio, root tissue forms; with a low auxin: cytokinin ratio, a shoot forms. This effect is used, for example, for organogenesis in plant tissue culture . When the auxin concentration is high in the plant, there is increased formation of adventitious and lateral roots. In addition, xylem tissue is formed in the cambium at an auxin-cytokinin ratio of approximately 1: 1. This plays a role above all in plant development and after being wounded.

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

In the protonema of the moss , such as Physcomitrella patens , auxins specifically induce the transition from chloronema to caulonema . This is accompanied by a change in cell cycle control .

Senescence

Auxin delays senescence and prevents leaves, flowers and fruits from shedding by inhibiting the formation of separating tissues. Opponents are abscisic acid and, above all, ethylene . However, higher concentrations of IAA promote ethylene biosynthesis .

Molecular Effect

The molecular effects of auxin are not yet fully understood. The first to be discovered in the 1980s was an auxin receptor with the so-called auxin binding protein 1 (ABP1), which specifically binds auxins. ABP1 interacts with an as yet unidentified docking protein on the plasma membrane . The transmission of the signal is unknown, but it causes the modulation of membrane transport proteins (especially the proton pump ). However , ABP1 was discarded for the auxin-induced change in the expression of certain auxin-induced genes.

The auxin-induced gene expression can be divided into a fast, direct and a somewhat slower, indirect effect. Among other things, so-called “auxin response factors” (ARFs) are involved in direct gene expression, which bind to “auxin response elements” (Aux-Res; sequence TGTCTC) of the DNA and control gene expression. Basically, auxin removes a gene inhibition. That is also the reason for the quick effect. In the normal state of auxin-regulated genes, ARF is bound to the AUX-Res as a heterodimer together with a repressor (“AUX / IAA”). The gene is not expressed. When auxin is added, it binds and activates the so-called "SCF complex", a ubiquitin protein ligase (with TIR1) that ubiquitinates the repressor and thus marks it for degradation. The repressor is broken down by the proteasome and the gene can be transcribed . For example, cell wall components are formed for cell elongation. Many directly auxin-controlled genes come from the gene families AUX / IAA , SAUR ( Small Auxin Up RNAs ) and GH3, among others . A potassium channel (ZMK1) was recently identified as a growth-relevant auxin-induced protein. The indirect gene expression takes place via the direct induction of transcription factors just described . These in turn enable the expression of further genes.

Inhibition of auxin-dependent growth

The mechanisms behind photo- and gravitropism are the subject of current research. For some years it has been assumed that not only the auxin effect is due to the curvature of the shoot axes , as z. B. occurs in phototropism, is responsible. Chemical compounds, so-called sesquiterpene lactones , most likely affect physiological processes in plant tissues and act as inhibitors of auxin-dependent elongation growth . It was previously assumed that an auxin gradient in the stem axis is responsible for the differential elongation of the cells. Today it is assumed that a gradient of sesquiterpene lactones, which inhibits the polar transport of auxin, contributes significantly to this. This thesis u. a. supported by experiments in which it was found that with a one-sided exposure of sunflower hypocotyls no differential distribution of auxin occurs. From this it was concluded that auxin inhibitors on the exposed side of the plant must be responsible for the curvature reaction in the stem axis. A downward diffusion of the sesquiterpene lactones was also found. In addition, these compounds have the property of being able to bind to thiol groups . This means that an interaction of the compounds with amino acids such as cysteine and proteins with free SH groups is not excluded. Such proteins could e.g. B. proteins of the AUXIN RESISTANT1 and / or the PIN family that are significantly involved in the polar auxin transport and thus in auxin-dependent elongation. The inhibition of the downward transport of auxin was shown in experiments by the application of dehydrocostus lactone to the hypocotyls of Raphanus . The sesquiterpene lactones are a characteristic feature of Asteraceae , but are also found in other plant families. They are potential candidates for plant hormones and have a variety of effects.

Dismantling

Auxins are broken down by enzymes ( peroxidases ) and UV rays , but this only plays a very minor role. The exact course of the enzymatic degradation reaction is still unknown. IAA oxidases, which break down IAA from the side chain, and breakdown via cleavage of the indole nucleus are discussed.

proof

The detection and quantitative determination of auxins used to be done mostly by specific bioassay systems , for example the oat coleoptile curvature test . Nowadays gas chromatography or gas chromatography / mass spectrometry as well as immunassays are used for auxin analysis.

use

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

Auxins play an important role in cotton fiber development . Using genetic engineering, researchers at the University of Southwest China in Chongqing succeeded in increasing IAA production in the plant's epidermis at the beginning of fiber growth. This leads to an increase in the number and length of usable fibers (lint) and a decrease in the number of fibers that cannot be processed into textiles (linters). Field trials over four years showed that the lint yield in the transgenic plants was consistently more than 15% higher than in the conventional control groups. In addition, the fineness of the fibers improved.

literature

Web links

Individual evidence

  1. ↑ Kiyoshi Mashiguchi, Keita Tanaka, Tatsuya Sakai, Satoko Sugawara, Hiroshi Kawaide: The main auxin biosynthesis pathway in Arabidopsis . In: Proceedings of the National Academy of Sciences . tape 108 , no. 45 , November 8, 2011, p. 18512-18517 , doi : 10.1073 / pnas.1108434108 , PMID 22025724 , PMC 3215075 (free full text).
  2. ↑ Jan Petrášek, Jozef Mravec, Rodolphe Bouchard, Joshua J. Blakeslee, Melinda Abas: PIN Proteins Perform a Rate-Limiting Function in Cellular Auxin Efflux . In: Science . tape 312 , no. 5775 , May 12, 2006, p. 914-918 , doi : 10.1126 / science.1123542 , PMID 16601150 .
  3. ↑ Kiyoshi Mashiguchi, Keita Tanaka, Tatsuya Sakai, Satoko Sugawara, Hiroshi Kawaide: The main auxin biosynthesis pathway in Arabidopsis . In: Proceedings of the National Academy of Sciences . tape 108 , no. 45 , November 8, 2011, p. 18512-18517 , doi : 10.1073 / pnas.1108434108 , PMID 22025724 , PMC 3215075 (free full text).
  4. ↑ Eric D. Brenner, Rainer Stahlberg et al .: Plant neurobiology: an integrated view of plant signaling. Trends in Plant Science 11 (8), 2006, pp. 413-419
  5. ^ Goeschl, JD, Rappaport, L., Pratt, HK (1966) Ethylene as a factor regulating the growth of pea epicotyls subjected to physical stress. Plant Physiol. 41, pp. 877-884.
  6. ^ Eva L. Decker, Wolfgang Frank, Eric Sarnighausen, Ralf Reski (2006): Moss systems biology en route: Phytohormones in Physcomitrella development. Plant Biology 8, 397-406. doi : 10.1055 / s-2006-923952
  7. ↑ Kaori Yokotani-Tomita, Jun Kato, Kosumi Yamada, Seiji Kosemura, Shosuke Yamamura: 8-Epixanthatin, a light-induced growth inhibitor, mediates the phototropic curvature in sunflower (Helianthus annuus) hypocotyls . In: Physiologia Plantarum . tape 106 , no. 3 , July 1999, ISSN  0031-9317 , p. 326-330 , doi : 10.1034 / j.1399-3054.1999.106310.x .
  8. ↑ Otmar Spring, Achim Hager: Inhibition of elongation growth by two sesquiterpene lactones isolated from Helianthus annuus L. In: Planta . tape 156 , no. 5 , December 1982, ISSN  0032-0935 , pp. 433-440 , doi : 10.1007 / bf00393314 .
  9. ↑ J. BRUINSMA, CM KARSSEN, M. BENSCHOP, JB VAN DORT: Hormonal Regulation of Phototropism in the Light-grown Sunflower Seedling, Helianthus annuusL .: Immobility of Endogenous Indoleacetic Acid and Inhibition of Hypocotyl Growth by Illuminated Cotyledons . In: Journal of Experimental Botany . tape 26 , no. 3 , 1975, ISSN  0022-0957 , pp. 411-418 , doi : 10.1093 / jxb / 26.3.411 .
  10. ↑ SHIBAOKA, HIROH: STUDIES ON THE MECHANISM OF GROWTH INHIBITING EFFECT OF LIGHT . In: Plant and Cell Physiology . tape 2 , no. May 2 , 1961, ISSN  1471-9053 , doi : 10.1093 / oxfordjournals.pcp.a077675 ( oup.com [accessed September 24, 2018]).
  11. ↑ Junichi Ueda, Yuta Toda, Kiyotaka Kato, Yuichi Kuroda, Tsukasa Arai: Identification of dehydrocostus lactone and 4-hydroxy-β-thujone as auxin polar transport inhibitors . In: Acta Physiologiae Plantarum . tape 35 , no. 7 , March 29, 2013, ISSN  0137-5881 , p. 2251-2258 , doi : 10.1007 / s11738-013-1261-6 .
  12. ↑ Junichi Ueda, Yuta Toda, Kiyotaka Kato, Yuichi Kuroda, Tsukasa Arai: Identification of dehydrocostus lactone and 4-hydroxy-β-thujone as auxin polar transport inhibitors . In: Acta Physiologiae Plantarum . tape 35 , no. 7 , March 29, 2013, ISSN  0137-5881 , p. 2251-2258 , doi : 10.1007 / s11738-013-1261-6 .
  13. ↑ Mi Zhang, Xuelian Zheng, Shuiqing Song, Qiwei Zeng, Lei Hou, Demou Li, Juan Zhao, Yuan Wei, Xianbi Li, Ming Luo, Yuehua Xiao, Xiaoying Luo, Jinfa Zhang, Chengbin Xiang & Yan Pei: Spatiotemporal manipulation of auxin biosynthesis in cotton ovule epidermal cells enhances fiber yield and quality. Nature Biotechnology , Vol. 29, pp. 453-458. doi : 10.1038 / nbt.1843
  14. ↑ Z Jeffrey Chen & Xueying Guan: Auxin boost for cotton. Nature Biotechnology, Vol. 29, pp. 407-409. doi : 10.1038 / nbt.1858