What is Ethylene Definition Structure Biosynthesis and Functions
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.
FAQs on Ethylene as a Plant Growth Regulating Hormone
1. What is ethylene in plants?
Ethylene is a gaseous plant hormone that regulates growth, development, and stress responses in plants. It is a simple hydrocarbon gas (C2H4) that acts as a phytohormone and diffuses easily between plant cells. Ethylene influences processes such as:
- Fruit ripening in climacteric fruits like banana and tomato
- Leaf and flower senescence
- Abscission (dropping of leaves and fruits)
- Response to mechanical stress and flooding
Because it is a gas, ethylene can act both within the plant and between nearby plants.
2. What is the function of ethylene in plants?
The main function of ethylene is to regulate fruit ripening and coordinate plant responses to stress. It acts as a signaling molecule that controls:
- Fruit ripening by activating ripening genes
- Senescence of leaves and flowers
- Abscission layer formation
- The triple response in seedlings
- Defense responses during pathogen attack
Ethylene ensures that developmental changes and stress adaptations occur at the right time.
3. How is ethylene produced in plants?
Ethylene is produced from the amino acid methionine through a specific biochemical pathway. The steps of ethylene biosynthesis are:
- Methionine is converted to S-adenosyl methionine (SAM)
- SAM is converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase
- ACC is oxidized to ethylene by ACC oxidase
This pathway is active in ripening fruits, aging tissues, and plants under stress conditions.
4. How does ethylene cause fruit ripening?
Ethylene causes fruit ripening by activating genes that control color, texture, flavor, and aroma changes. During climacteric fruit ripening, ethylene:
- Increases respiration rate
- Breaks down chlorophyll and promotes pigment formation (e.g., carotenoids)
- Softens fruit by degrading cell wall pectin
- Converts starch into sugars
This coordinated process makes fruits sweeter, softer, and more attractive for seed dispersal.
5. What is the triple response of ethylene?
The triple response of ethylene is a characteristic growth pattern seen in dark-grown seedlings exposed to ethylene. It includes three main effects:
- Inhibition of stem elongation
- Thickening of the hypocotyl
- Horizontal growth due to exaggerated apical hook formation
This response helps seedlings push through soil while protecting the shoot tip from mechanical damage.
6. What is the role of ethylene in leaf abscission?
Ethylene promotes leaf abscission by stimulating the formation of an abscission layer at the base of the leaf stalk. It acts by:
- Activating enzymes that break down cell walls in the abscission zone
- Reducing the effect of auxin, which normally delays leaf drop
- Facilitating separation of cells in the petiole
This process allows plants to shed old, damaged, or diseased leaves efficiently.
7. Is ethylene a growth inhibitor or promoter?
Ethylene can act as both a growth inhibitor and promoter depending on the tissue and conditions. For example:
- It inhibits stem elongation in the triple response
- It promotes fruit ripening and flower opening in some species
- It stimulates adventitious root formation in certain plants
Therefore, ethylene is considered a multifunctional plant hormone rather than purely inhibitory.
8. What is the difference between ethylene and auxin?
Ethylene and auxin are both plant hormones, but they differ in structure and primary function. The key differences are:
- Ethylene is a gaseous hormone mainly involved in fruit ripening and stress responses
- Auxin is an organic acid (like IAA) that primarily promotes cell elongation
- Ethylene often promotes abscission, while auxin delays it
- Auxin can stimulate ethylene production in some tissues
Both hormones interact closely to regulate plant growth and development.
9. How does ethylene help plants during stress?
Ethylene helps plants cope with stress by activating defense and adaptive responses. Under conditions like flooding, drought, or pathogen attack, ethylene:
- Triggers defense gene expression
- Promotes leaf senescence to conserve resources
- Induces aerenchyma formation during flooding
- Enhances resistance to certain pathogens
This makes ethylene a key hormone in plant stress physiology and survival.
10. Which fruits are affected by ethylene?
Ethylene mainly affects climacteric fruits that rely on it for ripening. Examples include:
- Banana
- Tomato
- Apple
- Mango
- Avocado
Non-climacteric fruits like grapes and citrus do not depend strongly on ethylene for ripening, which is an important distinction in post-harvest biology.
