Ethylene as a volatile hormone

Biosynthesis of Ethylene

Ethylene biosynthesis is a climacteric process, meaning it increases dramatically at specific stages of fruit ripening. It originates from methionine via two main pathways:

• S-adenosylmethionine (SAM) pathway: This is the primary pathway. Methionine is converted to SAM by methionine adenosyltransferase (MAT). SAM is then converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase (ACS), a key regulatory enzyme.
• ACC oxidase (ACO) pathway: ACC is finally converted to ethylene by ACC oxidase (ACO).

The expression of ACS and ACO genes is regulated by various developmental and environmental factors.

Physiological Effects of Ethylene

Ethylene exerts a diverse range of physiological effects on plants, often concentration-dependent:

• Fruit Ripening: Ethylene triggers a cascade of events leading to fruit ripening, including changes in color, texture, aroma, and sweetness.
• Senescence: It promotes leaf and flower senescence, leading to chlorophyll degradation and nutrient remobilization.
• Abscission: Ethylene induces the formation of an abscission layer, causing leaves, flowers, and fruits to detach from the plant.
• Root and Shoot Growth: At low concentrations, ethylene can inhibit root elongation but promote root hair formation. It can also inhibit shoot elongation.
• Seed Germination: In some species, ethylene promotes seed germination by overcoming dormancy.
• Stress Responses: Ethylene is induced by various stresses, such as wounding, flooding, and pathogen attack, and plays a role in adaptive responses.

Factors Influencing Ethylene Production

Several factors can influence ethylene production in plants:

• Developmental Stage: Ethylene production typically increases during fruit ripening and senescence.
• Wounding: Physical damage to plant tissues stimulates ethylene production.
• Flooding/Hypoxia: Low oxygen levels trigger ethylene synthesis, leading to adventitious root formation.
• Pathogen Attack: Infection by pathogens can induce ethylene production as part of the plant’s defense response.
• Temperature: Optimal temperature ranges exist for ethylene biosynthesis and action.
• Hormonal Interactions: Ethylene interacts with other plant hormones, such as auxin, abscisic acid (ABA), and jasmonic acid, to regulate plant responses.

Ethylene Signaling Pathway

Ethylene perception involves a family of ethylene receptors located in the endoplasmic reticulum membrane. In the absence of ethylene, the receptors activate a CTR1 kinase, which inhibits downstream signaling. When ethylene binds to the receptors, CTR1 is inactivated, allowing EIN2 (Ethylene Insensitive 2) to activate EIN3, a transcription factor that regulates the expression of ethylene-responsive genes.

Commercial Applications of Ethylene

Ethylene is widely used in agriculture and horticulture:

• Fruit Ripening: Ethylene is used to artificially ripen fruits like bananas, tomatoes, and mangoes during storage and transportation.
• Flower Induction: It can induce flowering in pineapple plants.
• Rooting of Cuttings: Ethylene promotes root formation in plant cuttings.
• Sex Determination: In some plants, ethylene influences flower sex expression.
• Deblossoming: Used to remove unwanted flower buds in crops like cotton.

Application	Crop Example	Mechanism
Fruit Ripening	Banana	Ethylene triggers enzymatic breakdown of cell walls and starch.
Flower Induction	Pineapple	Ethylene promotes synchronous flowering.
Rooting	Rose	Ethylene stimulates adventitious root formation.