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16.1: Auxin - Biology

16.1: Auxin - Biology


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Learning Objectives

  • Explain the mechanism of polar auxin transport.
  • Identify locations of synthesis and actions of auxin.
  • Define apical dominance and explain the role of auxin in maintaining it.
  • Describe the commercial applications of auxin.
  • Interpret and predict outcomes of experiments that demonstrate the action of auxin.

The term auxin is derived from the Greek word auxein, which means "to grow." While many synthetic auxins are used as herbicides, indole-3-acetic acid (IAA) is the only naturally occurring auxin that shows physiological activity (Figure (PageIndex{1})). Auxin is synthesized in apical meristems, young leaves, and developing seeds.

The Discovery of Auxin

Recall from the Tropisms section that the Boysen-Jensen experiment showed a chemical signal must be downward from the tip of the coleoptile tip along the shaded side, resulting in phototropism. Went extracted the chemical signal involved in the Boysen-Jensen experiment. He removed the tips of several coleoptiles of oat, Avena sativa, seedlings. He placed these on a block of agar for several hours. At the end of this time, the agar block itself was able to initiate resumption of growth of the decapitated coleoptile. The growth was vertical because the agar block was placed completely across the stump of the coleoptile and no light reached the plant from the side (Figure (PageIndex{2})). The unknown substance that had diffused from the agar block was named auxin. The amount of auxin in coleoptile tips was far too small to be purified and analyzed chemically. Therefore, a search was made for other sources of auxin activity.

This search was aided by a technique called the Avena test developed by Went for determining the relative amount of auxin activity in a preparation. The material to be assayed is incorporated into an agar block, and the block is placed on one edge of a decapitated Avena coleoptile. As the auxin diffuses into that side of the coleoptile, it stimulates cell elongation and the coleoptile bends away from the block (Figure (PageIndex{3})). The degree of curvature, measured after 1.5 hours in the dark, is proportional to the amount of auxin activity (e.g., number of coleoptile tips used). The use of living tissue to determine the amount of a substance, such as in the Avena test, is called a bioassay.

The Avena test soon revealed that substances with auxin activity occur widely in nature. One of the most potent was first isolated from human urine. It was indole-3-acetic acid (IAA) and turned out to be the auxin actually used by plants.

Auxin Transport

Auxin moves through the plant by two mechanisms, called polar and nonpolar transport.

Polar Transport

In contrast to the other major plant hormones, auxins can be transported in a specific direction (polar transport) through parenchyma cells. The cytoplasms of parenchyma cells are neutral (pH = 7), but the region outside the plasma membranes of adjacent cells (the apoplast) is acidic (pH = 5). When auxin is in the cytoplasm, it releases a proton and becomes an anion (IAA-). It cannot pass through hydrophobic portion of the plasma membrane as an anion, but it does pass through special auxin efflux transporters called PIN proteins. Eight different types of these transmembrane proteins have been identified so far. When IAA- enters the acidic environment of the apoplast, it is protonated, becoming IAAH. This uncharged molecule can then pass through the plasma membrane of adjacent cells through diffusion or via influx transporters. Once it enters the cytoplasm, it loses its proton, becoming IAA- again. PIN proteins can be unevenly distributed around the cell (for example, only occurring on the bottom of the cell), which directs the flow of auxin (Figure (PageIndex{4})).

Nonpolar Transport

Auxins can also be transported nondirectionally (nonpolar transport) through the phloem. It passes in the assimilate that translocates through the phloem from where it is synthesized (its "source", usually the shoot) to a "sink" (e.g., the root).

Actions of Auxin

Tropisms

Auxins are the main hormones responsible for phototropism and gravitropism. The auxin gradients that are required for these tropisms are facilitated by the movement of PIN proteins and the polar transport of auxin in response to environmental stimuli (light or gravity). Note that higher auxin concentration on one side of the stem typically causes that side of the stem to elongate; however, the effect is opposite in roots with higher auxin concentration inhibiting elongation (Figure (PageIndex{5})).

Growth and Development

Embryo Development

Auxins play a role in embryo development. From the very first mitotic division of the zygote, gradients of auxin guide the patterning of the embryo into the parts that will become the organs of the plant, including the shoot apex, primary leaves, cotyledon(s), stem, and root.

Vascular Tissue Differentiation

They also control cell differentiation of vascular tissue.

Leaf Development and Arrangement

The formation of new leaves in the apical meristem is initiated by the accumulation of auxin. Already-developing leaves deplete the surrounding cells of auxin so that the new leaves do not form too close to them. In this way, the characteristic pattern of leaves in the plant is established. Auxin also controls the precise patterning of the epidermal cells of the developing leaf.

Root Initiation and Development

The localized accumulation of auxin in epidermal cells of the root initiates the formation of lateral or secondary roots. Auxin also stimulates the formation of adventitious roots in many species. Adventitious roots grow from stems or leaves rather than from the regular root system of the plant. Once a root is formed, a gradient of auxin concentration develops highest at the tip promoting the production of new cells at the meristem, and lowest in the region of differentiation, thus promoting the elongation and differentiation of root cells. The drop in auxin activity in the regions of elongation and differentiation is mediated by cytokinin — an auxin antagonist.

Shade Avoidance

Auxins stimulate cell elongation parts of the plants that have access to light as part of the shade-avoidance response (see Etiolation and Shade Avoidance).

Interactions with Other Growth-Regulating Hormones

Auxin is required for the function of other growth-regulating hormones such as cytokinins; cytokinins promote cell division, but only in the presence of auxin.

Apical Dominance

Apical dominance—the inhibition of axillary bud (lateral bud) formation—is triggered by downward transport of auxins produced in the apical meristem. Many plants grow primarily at a single apical meristem and have limited axillary branches (Figure (PageIndex{6})). Growth of the shoot apical meristem (terminal shoot) usually inhibits the development of the lateral buds on the stem beneath. If the shoot apical meristem of a plant is removed, the inhibition is lifted, and axillary buds begin growth. However, if the apical meristem is removed and IAA applied to the stump, inhibition of the axillary buds is maintained (Figure (PageIndex{7})). Gardeners exploit this principle by pruning the terminal shoot of ornamental shrubs, etc. The release of apical dominance enables lateral branches to develop and the plant becomes bushier. The process usually must be repeated because one or two laterals will eventually outstrip the others and reimpose apical dominance.

The common white potato also illustrates the principle of apical dominance. Note that a potato is a tuber, which is an underground stem modified for starch storage. As with an ordinary shoot, the potato has a terminal bud (containing the shoot apical meristem) or "eye" and several axillary (lateral) buds. After a long period of storage, the terminal bud usually sprouts but the other buds do not. However, if the potato is sliced into sections, one bud to a section, the axillary buds develop just as quickly as the terminal bud (Figure (PageIndex{8})).

As will be discussed in the Abscisic Acid section, abscisic acid in the lateral buds inhibits production of auxin, and removal of the apical bud will release this inhibition of auxin, allowing the lateral buds to begin growing.

Flowering and Fruit Development

Auxins promote flowering and fruit setting and ripening. Pollination of the flowers of angiosperms initiates the formation of seeds. As the seeds mature, they release auxin to the surrounding flower parts, which develop into the fruit that covers the seeds.

Prevention of Abscission

Some plants drop leaves and fruits in response to changing seasons (based on temperatures, photoperiod, water, or other environmental conditions). This process is called abscission, and is regulated by interactions between auxin and ethylene. During the growing season, the young leaves and fruits produce high levels of auxin, which blocks activity of ethylene; they thus remain attached to the stem. As the seasons change, auxin levels decline and permit ethylene to initiate senescence, or aging (see Ethylene).

Figure (PageIndex{9}) demonstrates the role of auxin in abscission. If the blade of the leaf is removed, as shown in the figure, the petiole remains attached to the stem for a few more days. The removal of the blade seems to be the trigger as an undamaged leaf at the same node of the stem remains on the plant much longer, in fact, the normal length of time. If, however, auxin is applied to the cut end of the petiole, abscission of the petiole is greatly delayed.

Mechanisms of Auxin Action

Auxin effects are mediated by two different pathways: immediate, direct effects on the cell and turning on of new patterns of gene expression. The arrival of auxin in the cytosol initiates such immediate responses as changes in the concentration of and movement of ions in and out of the cell and reduction in the redistribution of PIN proteins. At the cellular level, auxin generally increases the rate of cell division and longitudinal cell expansion. Some of the direct effects of auxin may be mediated by its binding to a cell-surface receptor designated ABP1 ("Auxin-binding protein 1").

Many auxin effects are mediated by changes in the transcription of genes. Auxin enters the nucleus and binds to its receptor, a protein called TIR1 ("transport inhibitor response protein 1") which now can bind to proteins responsible for attaching ubiquitin to one or another of several Aux/IAA proteins. This triggers the destruction of the Aux/IAA proteins by proteasomes. Aux/IAA proteins normally bind transcription factors called auxin response factors (ARF) preventing them from activating the promoters and other control sequences of genes that are turned on (or off) by auxin. Destruction of the Aux/IAA proteins relieves this inhibition, and gene transcription begins.

This mechanism is another of the many cases in biology where a pathway is turned on by inhibiting the inhibitor of that pathway (a double-negative is a positive). The presence in the cell of many different Aux/IAA proteins (29 in Arabidopsis), many different ARFs (23 in Arabidopsis) and several (~4) TIR1-like proteins provides a logical basis for mediating the different auxin effects that are described here, but how this is done remains to be discovered.

Commercial Applications of Auxins

Commercial use of auxins is widespread in for propagation in nurseries, crop production, and killing weeds. Horticulturists may propagate desirable plants by cutting pieces of stem and placing them base down in moist soil. Eventually adventitious roots grow out at the base of the cutting. The process can often be hastened by treating the cuttings with a solution or powder containing a synthetic auxin.

Applying synthetic auxins to tomato plants in greenhouses promotes normal fruit development. Fruit growers often apply auxin sprays to cut down the loss of fruit from premature dropping. Additionally, outdoor application of auxin promotes synchronization of fruit setting and dropping to coordinate the harvesting season. Fruits such as seedless cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins.

Synthetic auxins are widely used as herbicides. Examples include 2,4-dichlorophenoxy acetic acid (2,4-D) and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T), shown in Figure (PageIndex{10}). 2,4-D and its many variants are popular because they are selective herbicides, killing broad-leaved eudicots but not narrow-leaved monocots. (No one knows the basis of this selectivity). Why should a synthetic auxin kill the plant? It turns out that the auxin influx transporter works fine for 2,4-D, but that 2,4-D cannot leave the cell through the efflux transporters. Perhaps it is the resulting accumulation of 2,4-D within the cell that kills it. A mixture of 2,4,-D and 2,4,5-T was the "agent orange" used by the U.S. military to defoliate the forest in parts of South Vietnam. Because of health concerns, 2,4,5-T is no longer used in the U.S.


Q&A: Auxin: the plant molecule that influences almost anything

Auxin is an essential molecule that controls almost every aspect of plant development. Although the core signaling components that control auxin response are well characterized, the precise mechanisms enabling specific responses are not yet fully understood. Considering the significance of auxin in plant growth and its potential applications, deciphering further aspects of its biology is an important and exciting challenge.


Developmental roles of Auxin Binding Protein 1 in Arabidopsis thaliana

Auxin is a major plant growth regulator, but current models on auxin perception and signaling cannot explain the whole plethora of auxin effects, in particular those associated with rapid responses. A possible candidate for a component of additional auxin perception mechanisms is the AUXIN BINDING PROTEIN 1 (ABP1), whose function in planta remains unclear. Here we combined expression analysis with gain- and loss-of-function approaches to analyze the role of ABP1 in plant development. ABP1 shows a broad expression largely overlapping with, but not regulated by, transcriptional auxin response activity. Furthermore, ABP1 activity is not essential for the transcriptional auxin signaling. Genetic in planta analysis revealed that abp1 loss-of-function mutants show largely normal development with minor defects in bolting. On the other hand, ABP1 gain-of-function alleles show a broad range of growth and developmental defects, including root and hypocotyl growth and bending, lateral root and leaf development, bolting, as well as response to heat stress. At the cellular level, ABP1 gain-of-function leads to impaired auxin effect on PIN polar distribution and affects BFA-sensitive PIN intracellular aggregation. The gain-of-function analysis suggests a broad, but still mechanistically unclear involvement of ABP1 in plant development, possibly masked in abp1 loss-of-function mutants by a functional redundancy.

Keywords: AUXIN BINDING PROTEIN 1 (ABP1) Auxin Auxin signaling Plant development.


X-ray structure of ILL2, an auxin-conjugate amidohydrolase from Arabidopsis thaliana

The plant hormone indole-3-acetic acid (IAA) is the most abundant natural auxin involved in many aspects of plant development and growth. The IAA levels in plants are modulated by a specific group of amidohydrolases from the peptidase M20D family that release the active hormone from its conjugated storage forms. Here, we describe the X-ray crystal structure of IAA-amino acid hydrolase IAA-leucine resistantlike gene 2 (ILL2) from Arabidopsis thaliana at 2.0 Å resolution. ILL2 preferentially hydrolyses the auxin-amino acid conjugate N-(indol-3-acetyl)-alanine. The overall structure of ILL2 is reminiscent of dinuclear metallopeptidases from the M20 peptidase family. The structure consists of two domains, a larger catalytic domain with three-layer αβα sandwich architecture and aminopeptidase topology and a smaller satellite domain with two-layer αβ-sandwich architecture and α–β-plaits topology. The metal-coordinating residues in the active site of ILL2 include a conserved cysteine that clearly distinguishes this protein from previously structurally characterized members of the M20 peptidase family. Modeling of N-(indol-3-acetyl)-alanine into the active site of ILL2 suggests that Leu175 serves as a key determinant for the amino acid side-chain specificity of this enzyme. Furthermore, a hydrophobic pocket nearby the catalytic dimetal center likely recognizes the indolyl moiety of the substrate. Finally, the active site of ILL2 harbors an absolutely conserved glutamate (Glu172), which is well positioned to act as a general acid-base residue. Overall, the structure of ILL2 suggests that this enzyme likely uses a catalytic mechanism that follows the paradigm established for the other enzymes of the M20 peptidase family. Proteins 2009. © 2008 Wiley-Liss, Inc.


The role of AUX1 gene and auxin content to the branching phenotype of Kenaf (Hibiscus cannabinus L.)

The objectives of this research were to identify auxin gene, AUX1, and to determine the plant auxin content and their role in conferring branching on Kenaf. PCR analysis using AUX1 primer capable to amplify the DNA of non branching (KR11) and branching kenaf mutant, resulting in 800 bp PCR product. The sequence of the PCR product showed high degree of homology with the sequence of AUX1 gene of other plants in the NCBI GenBank database, confirming kenaf possession of the gene AUX1. However, some variation on the DNA sequence was found between branching and non branching phenotype indicated allele differences of the same gene which were responsible for the variation in the type of branching. Identification of auxin content in the roots, apical shoot, and axillary branches using spectrophotometry method showed that the branching plant has higher auxin content in the apical shoot compared to the content in the branches. This indicate that AUX1 controls the formation of branches by controlling either the content of auxin in the apical shoot and branches, or the ratio of auxin content in the shoot and branches.

Keywords: AUX1 Hibiscus cannabinus Malvaceae PCR auxin branching.


Watch the video: A2 Biology - Plant growth and development OCR A Chapter (June 2022).


Comments:

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