Auxins | The first major plant hormone.

 Basically auxin is a growth promoting hormone present in plants which produce in the tops of the shoots and roots. Auxins stimulate cell elongation in stem and root. Due to this, the stem apex of plants turn towards light due to dissimilar cell elongation of stem.  -Thats why plants always bend and grow towards light. 

              Caption- A plant growing towards light. 

Question - what is the relationship between plant growth and Auxin hormone?

Auxins produced in the apex diffuse downwards. Therefore it speed up the growth of new cells and the shoot of the plant grow upwards. 

Question - What are the other functions rather than growth and stem elongation?

• Promotes formation of lateral and adeventitious roots. 
• Regulates development of fruit.
• Enhances apical dominance.

Note- Auxins concentrate in the parts of the plant where the light intensity is low. This is due to the the process of diffusion. Auxins move to the darker side of the plant, causing the cells there to grow longer than the corresponding cells on the other side of the plant. This causes curving of the plant stem tip towards light. Hence it is know as Positive phototropic movement. 

An outline from Wikipedia 

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.

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.^[8] 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.^[9]

Peter Boysen Jensen

In 1910, Danish scientist Peter Boysen Jensen [da] demonstrated that the phototropic stimulus in the oat coleoptile could propagate through an incision.^[10] These experiments were extended and published in greater detail in 1911 and 1913.^[11][12] 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.^[13][14]

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).^[11] These results were fundamental for further work on the auxin theory of tropisms.

Frits Went

Main article: Frits Warmolt 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.^[9]

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    Caption - (Inside)- cells diffuse due to auxins IAA.

                                Caption - Apex in the plant bend towards light.

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Written by

Nehan Dulvin       | This article is a property of NDK blog. 

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