Auxins, gibberellins, cytokinins and brassinosteroids tend to be growth stimulators.  Ethylene and abscisic acid (ABA) tend to be growth inhibitors or antagonists. Both ethylene and ABA control processes in later stages of plant development (e.g., senescence, fruit ripening, absicission).  Also involved in environmental stresses (drought, flooding, wounding, etc.)

Discovery:

Trees near street gas lamps lost leaves - active component in coal gas found to be ethylene (1901).  In 1934 ethylene identified as natural plant product.  In 1950s recognized as important endogenous growth regulator (use of gas chromatography enabled researchers to detect low concentrations in plant samples).

Biosynthesis:      C₂H₄      See Fig 22.1.
 
Synthesized from amino acid methionine:     CH₃ - S - CH₂ - CH - COOH
                                                                                              |
                                                                                             NH₃

Methionine + ATP ---- S-adenosyl methionine (SAM) ----- ACC ------ ethylene.  But note that part of Yang cycle - Fig 22.2.

Biosynthesis is stimulated as fruits mature.  Stresses (drought, flooding, chilling, wounding) increase ethylene production.  Auxin promotes conversion of SAM to ACC (aminocyclopropane carboxylic acid), thereby enhancing ethylene synthesis.  Thus some ethylene responses were inaccurately attributed to auxin.

Aminooxyacetic acid (AOA) blocks the SAM to ACC step. Thus there are synthetic inhibitors of ethylene production.  CO₂ at 5-10% competitively inhibits many of ethylene responses (e.g., fruit ripening - used commercially).

Transport:

Diffusion in intercellular air spaces (but accumulates under flooded conditions).  Easily diffuses through waxy layers (cuticle) thus does not usually build up to high concentration in tissues.  Almost insoluble in water.  Axial transport is slow, because rapidly lost to lateral transport out of tissue.  Can also be transported as ‘inactive’ ACC which is then converted to ethylene.

Physiological Roles:

• Major agent controlling fruit ripening.  Climacteric vs. non-climateric fruits.  In former respiration and ethylene synthesis increase with ripening - thus they respond well to ethylene treatment (bananas, tomatoes)
• Abscission - leaves, fruits, flowers.  Ethylene regulates formation of abscission layer.  Auxin suppresses its effect.  Note that ABA is not the major abscission hormone
• Epinasty (downward curvature of leaves). High [auxin] triggers ethylene production which causes epinasty.  Buildup of ethylene in tissues during flooding causes epinasty of shoots.  The ACC to ethylene step needs O₂, but in water-logged soil there is little or no O₂.  Thus ACC transported from the root to the shoot where it can be turned to ethylene because of the presence of O₂
• Aerenchyma formation is stimulated in flood-tolerant (wetland) species so that O₂ can diffuse to roots and even oxidize sediments.  O₂ diffusing down stem and into roots allows roots to respire aerobically, instead of anaerobically (fermenting and forming toxic ethanol)
• Flowering is inhibited by ethylene  in many species but stimulated in few others (e.g., pineapple)
• Senescence is increased by ethylene (e.g., Chl loss). Thus opposite effect to cytokinin.

Mechanism of Action:  See pages 666-668.  Fig 22.11.

• transcription of genes (mRNA amounts) for cell-wall digesting enzymes (e.g., cellulose).  Fruit ripening, or acrenchyma formation, or absicission layer formation
• Ethylene binding sites on integral membrane proteins (part of signal transduction pathway)
• activation of ion channels.

Link to Abscisic Acid