Ethylene is the simplest plant hormone.

Ethylene (or Ethene) functions as a plant hormone in plants. It stimulates the ripening of fruit, the opening of flowers, and the abscission of leaves. Its biosynthesis starts from methionine with 1-aminocyclopropane-1-carboxylic acid (ACC) as a key intermediate.

Ethylene was discovered when the byproducts of gas burning street lamps were shown to cause plant senescence in a greenhouse.

Location, Characteristics and Occasions for Synthesis Induction

• Directly induced by high levels of Auxin
• Found in germinating seeds
• Induced by root flooding
• Induced by drought
• Synthesized in nodes of stems
• Synthesized in tissues of ripening fruits
• Synthesized in response to shoot environmental, pest, or disease stress
• Synthesized in senescent leaves and flowers
• Rapidly diffuses
• Inhibiting effects of Ethylene on shoot growth (more specifically on stem elongation) reduced in the presence of light. Also Ethylene levels are decreased by light
• Released in mature cells when they do not have enough minerals and water to support both themselves and any dependent cells
• Released by all cells when they are experiencing conditions which would normally cause a mature shoot cell to produce Ethylene

Effects

• Stimulates leaf and flower senescence
• Induces leaf abscission mainly in older versus younger leaves.
• Induces seed germination
• Induces root hair growth – this increases the efficiency of water and mineral absorption
• Stimulates Epinasty – leaf petiole grows out, leaf hangs down and curls into itself
• Stimulates fruit ripening
• Induces the growth of adventitious roots during flooding
• Usually inhibits growth - just shoot growth
• Affects neighboring individuals
• Disease/wounding resistance
• Triple response when applied to seedlings – root ? and shoot growth inhibition and pronounced hypocotyl hook bending
• Inhibits stem swelling ? (Contradictory to the finding below – contradictory sources)
• Stimulates cell broadening (and lateral root growth)
• Interference with Auxin transport (when hormone levels are increasing)
• Directly or indirectly induces Auxin at high levels
• (From Theory II of plant hormones) Inhibits the rate of metabolism of cells in the shoot (who are not already at their lowest metabolism rates) in response to an decrease in the levels minerals and/or water