Plant Growth Hormone Auxin: Definition, Mechanism, and Function

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What is Auxin?

Growth and development of an organism depends upon the internal and external factors supporting the growth of organism.

The external environmental cues are essential for the stimulation of growth and development for reproduction and survival efficiency of these environmental cues (i.e.) stimulus process the internal response of the organism.

The response is manifested and cell functions corresponding to the response is recorded. Such stimulus and response are well coordinated in the plant body by the chemical messengers – the hormones.

Hormones acts as a mediator for carrying and transferring information for the coordination of physiological, metabolic and chemical activity.

For example: the growth of coleoptile of Phalaris canariensis towards the light is the coordinated movement for external stimulus light (i.e.) the phototropism is mediated by the hormone auxin. This was the initial discovery for the presence of phytohormones in plant system regulating the function of whole plant body.

These chemical compounds enhance cell communication and integrate the multicellular organism to organized as a single unit.

Further studies were undergoing for the deducing the mechanism and variety of hormones coordinating plant body.

Between 1950 – 1960 a group of five hormones were identified to maintain the plant homeostasis. These hormones were combinedly termed as ” Classical Five” they are: AUXIN, CYTOKININ, ETHYLENE, GIBBERELLIN, ABSCISIC ACID.

Along with classical five there are brassanosteroids and jasmonic acid. These set of hormones are termed as “Plant Growth Regulators” as they have an active role in regulating growth and development rather than a broad action spectrum.

Hormones are sensitive, specific, low concentration action and are naturally occurring in plant species. These important characteristic makes it an ideal small molecule chemical messengers and regulators.

The mode of action is receptor mediated and are transported to different regions by vascular tissues(i.e.) xylem and phloem. Hormones like ethylene are volatile, hence they are diffused throughout the plant body.

Auxin Discovery

Auxins are naturally occurring Indole 3 – Acetic Acid that are abundant and can exist as non-indole component too.

Auxin in Greek means – “to grow” or “to increase”.

The main function of auxin is cell division.

The Dutch biologist Frits Warmolt Went was the one who first described auxins and their role in plant growth.

In the 1920s Kenneth V. Thimann (1904-1997) became the first to isolate one of these phytohormones and to determine its chemical structure as indole-3-acetic acid (IAA).

Auxin Functions

1. Growth from embryo to adult

2. Cell division

3. Stem elongation

4. Apical dominance

5. Fruit development

6. Tropic responses

Auxin Biosynthesis

Conclusive research analysis or findings are not available for the biosynthesis of IAA because of minimal availability of hormones.

Many studies have concluded that Tryptophan to be the precursor for IAA. The synthesis initiates from Erythrose – 4 phosphate of pentose phosphate pathway which on further degradation provides tryptophan.

Tryptophan has many precursors from which many mechanisms are followed to produce Tryptophan.

Tryptophan deaminates to Indole – 3 -pyruvic acid which on decarboxylation produce indole – 3 – acetaldehyde on oxidation yields Indole Acetic Acid.

Though the synthesis and pathways deduced for tryptophan’s to be the precursor for IAA many bioassays and other techniques has proved that there are precursors other than Tryptophan from which IAA is synthesized.

A conclusion can be laid that the IAA can be synthesized by tryptophan dependent process and Tryptophan Independent process.

Site of biosynthesis: site of biosynthesis is dependent on endogenous precursor pools.

The precursors are not well defined for IAA. But, based on the existing analysis it can be detected that tryptophan are rich in plastids and it also has a cytoplasmic pool in plant cell.

Hence plastids and cytoplasm of plant cell can be assumed as site of biosynthesis.

An affirmation regarding the site of synthesis of auxin can be provided considering the presence of Auxin in the Apical meristematic zones and are absent in mature cell.

Regulation of Auxin Levels

Hormones are regulated by variety of mechanisms. They are – Synthesis, Conjugation, Irreversible modification breakdown, transportation and compartmentation.

Lesser the knowledge of biosynthesis of Auxin lesser the role of regulation identified for auxins. It is regulated by formation of conjugates and irreversible modification by breakdown of IAA.

Conjugation: IAA forms conjugates with ester linkage to sugar or alcohol or to amino acids by amide linkage.

Breakdown: IAA is decarboxylated with products: oxindole – 3 – methanol, 3 – methylene oxindole, 3 – methyl oxindole, indole – 3 aldehyde.

Auxin Inhibitors

IAA are inhibited by synthetic compounds. These compounds inhibit IAA and not the synthesis.

These inhibiting molecules are called antiauxins. They inhibit by binding to the receptors of IAA.

Auxin Citations


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