Ethylene: a Plant Growth Hormone

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.

Ethylene Discovery

Discovery of ethylene was done by Dimitry Neljubov. He was growing pea seedling in dark and the laboratory was gas lit, instead of elongating apical stem, the stem is shortened and had stunted growth. the gas lit lab had an ethylene leak which stunted the plant growth by inhibiting it.

From the experiment, ethylene was identified to be reducing the growth rate of plants.

In 1930 endogenous ethylene pools were identified in plant material especially in fruits that are ripened.

Functions of Ethylene

1. Fruit ripening

2. Promotes senescence and abscission

3. Root hair production

4. Seed germination

5. Sprout lateral buds

Ethylene Biosynthesis

Methionine is the precursor for ethylene on addition of Adenosine group from ATP forms Ado Met. Ado Met cleaves to produce Aminocyclopropanecarboxylic acid and Methylthioadenosine.

ACC on enzyme catalyses yields ethylene. MTA is cleaved by adenosine and give rise to adenine and methylthioribose.

MTR on further steps yields methionine for the cycle to proceed.

Site of Synthesis: Ripening regions and in senescing leaves and other parts.