auxins

Auxins (plural of auxin ) are a class of plant hormones (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.
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.

Overview

Auxins were the first of the major plant hormones to be discovered. They derive their name from the Greek word αυξειν (''auxein'' – "to grow/increase"). Auxin is present in all parts of a plant, although in very different concentrations. The concentration in each position is crucial developmental information, so it is subject to tight regulation through both metabolism and transport. The result is the auxin creates "patterns" of auxin concentration maxima and minima in the plant body, which in turn guide further development of respective cells, and ultimately of the plant as a whole.

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 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. 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.

* Five naturally occurring (endogenous) auxins in plants include indole-3-acetic acid, 4-chloroindole-3-acetic acid, phenylacetic acid, indole-3-butyric acid, and indole-3-propionic acid. However, most of the knowledge described so far in auxin biology and as described in the sections which follow, apply basically to IAA; the other three endogenous auxins seems to have 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), and many others.

Some synthetic auxins, such as 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), are sold 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, making these synthetic auxins valuable as herbicides.

Discovery of auxin

Charles Darwin

In 1881, Charles Darwin and his son Francis performed experiments on coleoptiles, the sheaths enclosing young leaves in germinating grass seedlings. The experiment exposed the coleoptile to light from a unidirectional source, and observed that they bend towards the light. By covering various parts of the coleoptiles with a light-impermeable opaque cap, the Darwins discovered that light is detected by the coleoptile tip, but that bending occurs in the hypocotyl. However the seedlings showed no signs of development towards light if the tip was covered with an opaque cap, or if the tip was removed. The Darwins concluded that the tip of the coleoptile was responsible for sensing light, and proposed that a messenger is transmitted in a downward direction from the tip of the coleoptile, causing it to bend.

Peter Boysen Jensen

In 1910, Danish scientist demonstrated that the phototropic stimulus in the oat coleoptile could propagate through an incision. These experiments were extended and published in greater detail in 1911 and 1913. He found that the tip could be cut off and put back on, and that a subsequent one-sided illumination was still able to produce a positive phototropic curvature in the basal part of the coleoptile. He demonstrated that the transmission could take place through a thin layer of gelatin separating the unilaterally illuminated tip from the shaded stump. By inserting a piece of mica he could block transmission in the illuminated and non-illuminated side of the tip, respectively, which allowed him to show that the transmission took place in the shaded part of the tip. Thus, the longitudinal half of the coleoptile that exhibits the greater rate of elongation during the phototropic curvature, was the tissue to receive the growth stimulus.

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 tropisms.

Frits Went

In 1928, the Dutch botanist Frits Warmolt Went showed that a chemical messenger diffuses from coleoptile tips. Went's experiment identified how a growth promoting chemical causes a coleoptile to grow towards the light. Went cut the tips of the coleoptiles and placed them in the dark, putting a few tips on agar blocks that he predicted would absorb the growth-promoting chemical. On control coleoptiles, he placed a block that lacked the chemical. On others, he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side.

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.

Hormonal activity

Auxins help
development at all levels in plants, from the cellular level, through organs, and ultimately to the whole plant.

Molecular mechanisms

When a plant cell comes into contact with auxin, it causes dramatic changes in gene expression