AUXIN

Auxins are a class of plant hormones (or plant growth substances) with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by the Dutch scientist Frits Went.[1]Kenneth V. Thimann isolated this phytohormone and determined its chemical structure as indole-3-acetic acid. Went and Thiman then co-authored a book on plant hormones, Phytohormones, in 1937. Native auxins indole-3-acetic acid (IAA) is the most abundant and the basic auxin natively occurring and functioning in plants. It generates the majority of auxin effects in intact plants, and is the most potent native auxin. There are three more native — endogenous auxins [2] All auxins are compounds with aromatic ring and a carboxylic acid group [3]: 4-chloroindole-3-acetic acid (4-CI-IAA) 2-phenylacetic acid (PAA) Indole-3-butyric acid (IBA) For representatives of synthetic auxins see chapter Synthetic auxins Overview Auxins derive their name from the Greek word αυξειν (auxein - "to grow/increase"). They were the first of the major plant hormones to be discovered. The (dynamic and to 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 [4][5] (such as leaves or flowers ). It is achieved through very complex and well coordinated active transport of auxin molecules from cell to cell throughout the plant body — by the so- called polar auxin transport .[4] 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. [3] The most important member of the auxin family is indole-3- acetic acid (IAA). [2] IAA generates the majority of auxin effects in intact plants, and is the most potent native auxin. And as native auxin, its stability 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. However, molecules of IAA are chemically labile in aqueous solution, so it is not used commercially as a plant growth regulator. The four naturally occurring (endogenous) auxins are IAA, 4- chloroindole-3-acetic acid, phenylacetic acid and indole-3-butyric acid ; only these four were found to be synthesized by plants. [2] However, most of the knowledge described so far in auxin biology and as described in the article below, apply basically to IAA; the other three endogenous auxins seems to have rather marginal importance for intact plants in natural environments. Alongside endogenous auxins, scientists and manufacturers have developed many synthetic compounds with auxinic activity. Synthetic auxin analogs include 1-naphthaleneacetic acid, 2,4- dichlorophenoxyacetic acid (2,4-D),[2] and many others. Some synthetic auxins, such as 2,4-D and 2,4,5- trichlorophenoxyacetic acid (2,4,5-T), are used also as herbicides. Broad-leaf plants (dicots), such as dandelions, are much more susceptible to auxins than narrow-leaf plants (monocots) such as grasses and cereal crops, so these synthetic auxins are valuable as synthetic herbicides. Auxins are also 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 and fruit set, and to prevent premature fruit drop. Hormonal activity Auxins coordinate development at all levels in plants, from the cellular level, through organs, and ultimately to the whole plant. The plant cell wall is made up of cellulose, proteins, and, in many cases, lignin. It is very firm and prevents any sudden expansion of cell volume (and, without the contribution of auxins, any expansion at all). Molecular mechanisms Auxin molecules present in cells may trigger responses directly through stimulation or inhibition of the expression of sets of certain genes.[6] or by means independent of gene expression. One of the pathways leading to the changes of gene expression involves the reception of auxin by TIR1 protein. In 2005, the F-box protein TIR1, which is part of the ubiquitin ligase complex SCFTIR1, was demonstrated to be an auxin receptor. [7] Upon binding of auxin, TIR1 recruits specific transcriptional repressors (the Aux/IAA repressors) for ubiquitination by the SCF complex. This marking process leads to the degradation of the Aux/ IAAs repressors by the proteasome. The degradation of the repressors leads, in turn, to potentiation of auxin response factor-mediated transcription of specific genes in response to auxins. [8]) Another protein, auxin- binding protein 1 (ABP1), is a putative receptor for different signaling pathway, but its role is as yet unclear. Electrophysiological experiments with protoplasts and anti-ABP1 antibodies suggest ABP1 may have a function at the plasma membrane, and cells can possibly use ABP1 proteins to respond to auxin through means faster and independent of gene expression. On a cellular level On the cellular level, auxin is essential for cell growth , affecting both cell division and cellular expansion. Auxin concentration level, together with other local factors, contributes to cell differentiation and specification of the cell fate. 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, auxins and other plant hormones nearly always appear to interact to determine patterns of plant development. 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, unevenly 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 originated because cells on one side grow faster than the cells on the other side of the organ. So, precise control of auxin distribution between different cells has paramount importance to the resulting form of plant growth and organization. Uneven distribution of auxin To cause growth in the required domains, auxins must of necessity be active preferentially in them. Auxins are not synthesized in all cells (even if cells retain the potential ability to do so, only under specific conditions will auxin synthesis be activated in them). For that purpose, auxins have to be not only translocated toward those sites where they are needed, but also they must have an established mechanism to detect those sites. For that purpose, auxins have to be translocated toward those sites where they are needed. Translocation is driven throughout the plant body, primarily from peaks of shoots to peaks of roots (from up to down). 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 morphogenetic properties. This process, the 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. Pin-formed (PIN) proteins are vital in transporting auxin. [5][9] 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). [5] Proper and timely auxin maxima within developing roots and shoots are necessary to organise the development of the organ. [4] [10][11] 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.