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Growth Hormone: Abscisic Acid

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.

Abscisic Acid Discovery

Discovery of ABA took place between 1950 – 1960, scientists had a hunch that when a growth stimulating endogenous hormones are present in the plant cell, growth inhibiting hormones which causes the senescence or abscission of fruits must be governed by other hormones namely Abscisic Acid (ABA).

ABA does not cause abscission they just inhibit growth.

Functions of Abscisic Acid

1. Inhibits growth and metabolism based on the developmental stage

2. Growth and inhibition of root is variable when ABA is present in them

3. Increases stress tolerance

4. Seed development and maturation enables seed to withstand desiccation.

Abscisic Acid Biosynthesis

ABA are synthesised from Xanthophylls namely Violaxanthin and neoxanthin.

Epoxidation or the presence of epoxy – carotenoids is necessary for the synthesis of ABA.

The synthesis however initiates from IPP forming GGPP further leads to the formation of Zeaxanthin produces violaxanthin. Violaxanthin forms cis – neoxanthin followed by cis – xanthin produces ABA Aldehyde leads to ABA.

Site of Synthesis: In mature leaves and stems and in developing fruits, seeds, etc.,

Regulation of Abscisic Acid Levels

1. Synthesis

2. Conjugation

3. Oxidation to inactive forms

Abscisic Acid Citations

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