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Auxin: The First Major Phytohormone to be Discovered

Plantlet Lead, Physiology 2 Comments 5,916 Views

Growth: A process in which there is a change in the form and increase in size and weight by means of cell division, cell enlargement and cell differentiation.

Development: A process in which growth, differentiation of organ, maturation and senescence take place in a regular sequence in the life history of a plant.

Phytohormone: The organic compound produced within the plant which regulate or modify the different physiological functions at a very low concentration are called plant hormones. Plant hormones increase the rate of the growth or inhibit it. The action of plant hormone on physiological process is very specific.

Plant hormones

Plant hormones are also called: Growth hormones, growth regulator. growth bio-regulators etc.

Criteria of plant hormones

Are not nutrients but organic compounds that promote and influence the growth, development and differentiation of cells and tissues.
Produced within the plant.
Acts in extremely low concentration.
Action must be specific.
Transferred from the sight of synthesis to sight of action. Also termed as ‘Polar Transport’.

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Classification of Phytohormones

There are five major classes of phytohormones.

Auxins,
Gibberellins,
Cytokinin,
Abscisic acid,
Ethylene.

Auxins

The first of the major phytohormones to be discovered. The term ‘Auxin’ is derived from the Greek word which means ‘to grow’ or ‘to increase’.
All auxins are compounds with an aromatic ring and a carboxylic acid group.

Discovery of Auxins

Charles Darwin (1880)

Charles Darwin was among the first scientists to dabble in plant hormone research. In his book “The Power of Movement in Plants” presented in 1880, he first describes the outcomes of light on motion of canary grass (Phalaris canariensis) coleoptiles.

Darwin’s try out advised that the tip of the coleoptile was the tissue contributing for perceiving the light and raising some signal which was transported to the lower part of the coleoptile where the physiological response of bending occurred. He then cut off the tip of the coleoptile and exposed the rest of the coleoptile to unidirectional light to see if curving occurred. Curvature did not occur encouraging the results of his first experiment (Darwin, 1880).

Figure: Experiments of Darwin and Boysen-Jensen

Boysen-Jensen (1913, Denmark)

In 1913, Boysen-Jensen altered Fritting’s experimentation by inserting parts of mica to block the transfer of the signal and showed that transport of auxin toward the base occurs on the dark side of the plant as contradicted to the side open to the unidirectional light (Boysen-Jensen, 1913). In 1918, Paal supported Boysen-Jensen’s results by cutting off coleoptile tips in the dark, uncovering only the tips to the light, replacing the coleoptile tips on the plant but off centered to one side or the other. Results showed that whichever side was exposed to the coleoptile, curvature occurred toward the other side (Paal, 1918). Soding was the next scientist to extend auxin research by extending on Paal’s idea. He showed that if tips were cut off there was a reduction in growth but if they were cut off and then replaced growth continued to occur (Soding, 1925).

Fritz Went (Holland, 1926)

In 1926, a graduate student from Holland by the name of Fritz Went publicized a paper describing how he isolated a plant growth substance by placing agar blocks under coleoptile tips for a period of time then removing them and placing them on decapitated Avena stems (Went, 1926). After placement of the agar, the stems resumed growth (see below). In 1928, Went developed a method of quantifying this plant growth substance. His results suggested that the curvatures of stems were proportional to the amount of growth substance in the agar (Went, 1928). This test was called the avena curvature test.(see below)

Went called the hormone auxin (auxein: to grow). It took 20 years before this auxin was identified chemically as indole-3-acetic acid. Since then additional natural auxins have been identified.

Much of our current knowledge of auxin was obtained from its applications. Went’s work had a great influence in stimulating plant growth substance research. He is often credited with dubbing the term auxin but it was actually Kogl and Haagen-Smit who purified the compound auxentriolic acid (auxin A) from human urine in 1931 (Kogl and Haagen-Smit, 1931). Later Kogl isolated other compounds from urine which were similar in structure and function to auxin A, one of which was indole-3 acetic acid (IAA) initially discovered by Salkowski in 1985. In 1954 a committee of plant physiologists was set up to characterize the group auxins. The term comes from the Greek auxein meaning “to grow.” Compounds are generally considered auxins if they are synthesized by the plant and are substances which share similar activity to IAA (the first auxin to be isolated from plants) (Arteca, 1996; Davies, 1995).

Types of Auxins

Natural auxins

They are easily diffusable and endogenous in nature. Examples include:

IAA: Most potent native auxin.
Indole-3-acetyldehyde.
Indole-3-acetonitrile.
Indole-3-ethanol.
Cytrus auxins.

Synthetic auxins

They are not produced in plant. Usually produced in the lab.

indole-3-butyric acid.
2,4-dichlorophenoxyacetic acid (2, 4-D).
alpha-naphthalene acetic acid.
2, 4, 5-trichlorophenoxyacetic acid (2, 4,5-T) and many more.

Translocation of auxin

From apex to base (basipetal).
In free form.
Through the parenchyma cells, which are in contact with vascular bundle.
Translocation is slow but more than the ordinary diffusion rate.
Polar translocation, in stem-basipetal and in root-acropetal.
Translocation is blocked if O2 is not available and ATP synthesis is not taking.