Ethylene

What is Ethylene?

Ethylene is considered as one of the important plant metabolites and growth regulators. It is the only gaseous plant hormone that causes the ripening of fruit. It is commonly seen that unripe fruits kept with ripe banana ripen overnight. It is because of ethylene released from the ripe banana, which promotes the ripening of the other fruits.

Ethylene is a colorless, inflammable gas with a sweet musky odor. It is the smallest member of the alkene family. 

Ethylene is widely used in agricultural industries for the artificial ripening of fruits and vegetables. It is also used in the chemical and plastic industry for the production of several commonly used compounds like polyethylene.

Ethylene Formula

The IUPAC name for ethylene is ethene. It consists of two carbon atoms and four hydrogen atoms. The formula for ethene is C2H4 or H2C=CH2.

Ethylene Structure

Belonging to the alkene family, ethylene is considered to be an unsaturated hydrocarbon with a double bond between the two carbon atoms. The two carbon atoms and the four hydrogen atoms are coplanar. Two hydrogen atom forms bond with each carbon atom in an H-C-H manner at an angle of 117.4°, which is close to 120° required for sp2 hybridization. The π-bond between the two carbon atoms makes the molecule rigid; breaking the C-C bond requires high energy. The reactivity of ethylene is conferred by this π-bond. The double bond between the two carbon atoms has high electron density and is the main region for electrophilic reactions.

Ethylene As A Plant Hormone

Chinese used to burn incense to ripen fruits in ancient times. Similarly, Egyptians used gash figs for the same purpose. As said previously, ethylene hormone is the only gaseous hormone that plays an important role in:

• Hypocotyl elongation and root initiation.

• Promotes growth of root to increase the absorbing capacity for water and minerals.

• Promotion of Apo-geotropism during root development.

• Possible role in adventitious root development.

• Affect gravitropism.

• Stimulates stem growth.

• Plays an important role in bud outgrowth.

• Plays an essential role in stem branching.

• Stimulates bending of stem and branches in the direction of light.

• Promotes root and shoot differentiation in a triple response. 

• Interferes with the transport of other hormones like auxin.

• Prevents stem trichome formation in Arabidopsis.

• Promotes leaf growth.

• Promotes leaf area expansion.

• Involved in leaf development and leaf senescence.

• Dismantling of leaf cells and nutrient recycling from senescent leaves to other plant parts.

• Regulatory role in the transition from vegetative to reproductive phase of some plants.

• Sex determination of flower and production of female flower in male plants 

• Promotes flowering in pineapples.

• Induces flowering n Bromeliad.

• Induces the opening of flowers.

• Stimulates flower senescence.

• Breaks seed dormancy and promote seed germination.

• Induces epinasty.

• Rise in the respiration of some climacteric fruits.

• Fruit ripening.

• Change in color of the fruit during the ripening process.

• Promotes fruit senescence.

• It helps plants to respond to environmental conditions.

Function Of Ethylene

Ethylene has several defined functions at different stages of plant development. The roles of ethylene are studied at the cellular and tissue level in detail. Some of the functions of ethylene are defined below:

• Ethylene in cell division

Ethylene has conflicting roles in the cell division process, depending on the type of tissues. It promotes cell division in the sub-epidermal layers leading to apical hook development, where it works in conjunction with auxin. However, it inhibits cell division in the root apical meristem. It also controls cell division rate and, subsequently, the differentiation of vascular tissue. It inhibits the mitotic cell cycle in the abaxial surface of the leaf petiole, and in a way, contributes to the hyponastic response.

• Ethylene in cell elongation

The role of ethylene in the cell elongation process is also conflicting, depending on the cell type. It inhibits cell elongation in light-grown and dark-grown seedlings. However, it stimulates cell elongation in the growing hypocotyl of light-grown seedlings.

• Ethylene and cell death

Ethylene has also been found to play a critical role in cell death. During xylogenesis (formation of xylem cells), ethylene production is high. Xylogenesis is a part of programmed cell death. Ethylene production is also enhanced during bending of branches due to gravitation, due to modifications of the xylem morphology. 

Aerenchyma formation, which promotes gas exchange in hypoxic conditions, is also a form of PCD. Ethylene production is found to be enhanced during aerenchyma formation. 

• Response to environmental condition and competition

Any plant has to face several environmental challenges. They have to avoid shade regions and compete with other plants for sunlight. Upward leaf movement (hyponasty) is an adaptive response in this process, and ethylene plays an important role in it. Additionally, root hair formation is enhanced in poor nutrient conditions. The external application of ethylene promotes root hair formation.

Thigmomorphogenesis is the plant response to mechanical stress like touch and wind.  These responses include reduced root area, shoot elongation, and dry weight accumulation. Ethylene plays a crucial role in thigomomorphogenetic responses.

• Ethylene in plant growth 

Ethylene promotes horizontal growth of plants and swelling of the plant axis. It inhibits longitudinal growth.

• Ethylene in response to gravity

Ethylene desensitizes the plant towards the gravitational pull. It makes the stems positively geotropic, and the flowers and leaves droop due to ethylene production. 

• Senescence and abscission

Ethylene stimulates senescence and abscission in flowers, leaves, and fruits. It also promotes hydrolase formation.

• Apical Dominance

Apical dominance is enhanced, and lateral bud dormancy is prolonged during ethylene treatment.

• Flowering and fruit ripening

Ethylene promotes flowering and fruit ripening, mainly in climacteric fruits. It also induces flower fading in some plants.

Use Of Ethylene

• Ethylene is used as an artificial fruit ripener.

• Ethylene is used to develop female flowers in male plants.

• Ethylene is used to promote sprouting in rhizomes.

• Ethylene is used to thin tough fruits like walnut and prevent excess flowering.

• Used in the manufacturing of polyethylene bags.