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Auxins (plural of auxin ) are a class of (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essential for plant body development. The Dutch biologist Frits Warmolt Went first described auxins and their role in plant growth in the 1920s. Frits Warmolt Went Kenneth V. Thimann became the first to isolate one of these phytohormones and to determine its chemical structure as indole-3-acetic acid (IAA). Went and Thimann co-authored a book on plant hormones, Phytohormones, in 1937.
The (dynamic and 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 leaf or ). It is achieved through very complex and well-coordinated active transport 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 an aromatic ring and a carboxylic acid group. The most important member of the auxin family is indole-3-acetic acid (IAA), which generates the majority of auxin effects in intact plants, and is the most potent native auxin. And as native auxin, its equilibrium is controlled in many ways in plants, from synthesis, through possible conjugation to degradation of its molecules, always according to the requirements of the situation. Auxin can act in a heat sensitive manner in many situations, which will in turn effect a plant's phenotypic development.(Jones, Daniel S.et al,2021)
Some synthetic auxins, such as 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), are sold as . Broad-leaf plants (dicots), such as , are much more susceptible to auxins than narrow-leaf plants (monocots) such as and cereals crops, making these synthetic auxins valuable as herbicides.
In 1911, Boysen Jensen concluded from his experimental results that the transmission of the phototropic stimulus was not a physical effect (for example due to a change in pressure) but serait dû à une migration de substance ou d’ions (was caused by the transport of a substance or of ions). These results were fundamental for further work on the auxin theory of .
When the growth-promoting chemical was distributed evenly the coleoptile grew straight. If the chemical was distributed unevenly, the coleoptile curved away from the side with the cube, as if growing towards the light, even though it was grown in the dark. Went later proposed that the messenger substance is a growth-promoting hormone, which he named auxin, that becomes asymmetrically distributed in the bending region. Went concluded that auxin is at a higher concentration on the shaded side, promoting cell elongation, which results in coleoptiles bending towards the light.
Another auxin-binding protein, ABP1 is now often regarded as an auxin receptor (at the apoplast), but it is generally considered to have a much more minor role than the TIR1/AFB signaling pathway, and much less is known about ABP1 signaling.
The binding of auxin to TIR1/AFBs allows them to bind to Aux/IAAs. When bound by TIR1/AFBs, Aux/IAAs are marked for degradation. The degradation of Aux/IAA frees ARF proteins, which are then able to activate or repress genes at whose promoters they are bound.
The large number of Aux/IAA and ARF binding pairs possible, and their different distributions between cell types and across developmental age are thought to account for the astonishingly diverse responses that auxin produces.
In June 2018, it was demonstrated that plant tissues can respond to auxin in a TIR1-dependent manner extremely quickly (probably too quickly to be explained by changes in gene expression). This has led some scientists to suggest that there is an as yet unidentified TIR1-dependent auxin-signalling pathway that differs from the well-known transcriptional response.
Depending on the specific tissue, auxin may promote axial elongation (as in shoots), lateral expansion (as in root swelling), or iso-diametric 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, auxins and other plant hormones nearly always appear to interact to determine patterns of plant development.
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 properties.
This process, polar auxin transport, is directional, very strictly regulated, and based in uneven distribution of auxin efflux carriers on the plasma membrane, which send auxins in the proper direction. While PIN-FORMED (PIN) proteins are vital in transporting auxin in a polar manner, the family of AUXIN1/LIKE-AUX1 (AUX/LAX) genes encodes for non-polar auxin influx carriers.
The regulation of PIN protein localisation in a cell determines the direction of auxin transport from cell, and concentrated effort of many cells creates peaks of auxin, or auxin maxima (regions having cells with higher auxin – a maximum). Proper and timely auxin maxima within developing roots and shoots are necessary to organise the development of the organ. PINs are regulated by multiple pathways, at both the transcriptional and the post-translational levels. PIN proteins can be phosphorylated by PINOID, which determines their apicobasal polarity and thereby the directionality of auxin fluxes. In addition, other AGC kinases, such as D6PK, phosphorylate and activate PIN transporters. AGC kinases, including PINOID and D6PK, target to the plasma membrane via binding to phospholipids. Upstream of D6PK, 3'-phosphoinositide dependent protein kinase 1 (PDK1) acts as a master regulator. PDK1 phosphorylates and activates D6PK at the basal side of plasma membrane, executing the activity of PIN-mediated polar auxin transport and subsequent plant development. 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.
Auxin has a significant effect on spatial and temporal gene expressions during the growth of apical meristems. These interactions depend both on the concentration of Auxin as well as the spatial orientation during primordial positioning. Auxin relies on PIN1 which works as an auxin efflux carrier. PIN1 positioning upon membranes determines the directional flow of the hormone from higher to lower concentrations. Initiation of primordia in apical meristems is correlated to heightened auxin levels. Genes required to specify the identity of cells arrange and express based on levels of auxin. STM (SHOOT MERISTEMLESS), which helps maintain undifferentiated cells, is down-regulated in the presence of auxin. This allows growing cells to differentiate into various plant tissues. The CUC (CUP-SHAPED COTYLEDON) genes set the boundaries for growing tissues and promote growth. They are upregulated via auxin influx. Experiments making use of GFP (GREEN FLUORESCENCE PROTEIN) visualization in Arabidopsis have supported these claims.
An important principle of plant organization based upon auxin distribution is apical dominance, which means 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 or cytokinins) in the process on various positions along the growth axis in plant body to achieve this phenomenon. This plant behavior is used 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, the plant does not react just by the outgrowth of lateral buds — which are supposed to replace to original lead. It also follows that smaller amount of auxin arriving at 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 to grow faster.
Auxin stimulates cell elongation by stimulating wall-loosening factors, such as , to loosen . 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 with higher auxin to cytokinin ratios, shoot growth is induced by lower auxin to cytokinin ratios, and a callus is formed with intermediate ratios, with the exact threshold ratios depending on the species and the original tissue. Auxin also induces sugar and mineral accumulation at the site of application.
In horticulture, auxins, especially NAA and IBA, are commonly applied to stimulate root initiation when rooting 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.
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).
The evolutionary transition from diploid to triploid - and the production of - may have occurred due to a shift in gametophyte development which produced a new interaction with an auxin-dependent mechanism originating in the earliest .
Auxin inhibits abscission prior to the formation of the abscission layer, and thus inhibits senescence of leaves.
Auxins are toxic to plants in large concentrations; they are most toxic to and less so to . Because of this property, synthetic auxin herbicides, including 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), have been developed and used for weed control.
However, some exogenously synthesized 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 for different agricultural purposes such as the prevention of fruit drop in .
Used in high doses, auxin stimulates the production of ethylene, also a native plant hormone. Excess ethylene can inhibit elongation growth, cause leaf to fall (abscission), and even kill the plant. Some synthetic auxins, such as 2,4-D and 2,4,5-T are marketed also as . Dicots, such as , are much more susceptible to auxins than monocots, such as and cereals crops. So these synthetic auxins are valuable as synthetic herbicides. 2,4-D was the first widely used herbicide, and it is still in use. The Industry Task Force II on 2,4-D Research Data 2,4-D was first commercialized by the Sherwin-Williams company and saw use in the late 1940s. It is easy and inexpensive to manufacture.
Triclopyr (3,5,6-TPA), while known as an herbicide, has also been shown to increase the size of fruit in plants. At increased concentrations, the hormone can be lethal. Dosing down to the correct concentration has been shown to alter photosynthetic pathways. This hindrance to the plant causes a response that increases carbohydrate production, leading to larger fruit.
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