Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Ethylene

• the Chinese may have been the first to observe the effects of ethylene when they noted that burning incense increased fruit ripening
• in 1864 leaks in gas lights in street lamps were reported to stunt plant growth and defoliate trees
• in 1901, D. Neljubow realized that his dark-grown pea seedlings were short, fat and negatively gravitropic (the triple response) because of a component in "laboratory air" which he subsequently identified as ethylene
• Cousins (1910) first reported that ethylene occurred in plants.

• single compound (like ABA) and is not a family of related ones (i.e., gibberellins)
• CH₂=CH₂ 
• ethylene (MW 28) is similar in size/shape as water
• a gaseous plant hormone

III. Biosynthesis

A. General:

• made by most plants including angiosperms, gymnosperms, ferns, mosses, liverworts
• also synthesized by fungi and bacteria
• made by all parts of the plant
• meristematic regions (shoot apex) and senescing tissues are rich sources
• nodes make more ethylene than internodes
• ethylene production is stimulated by physiological stresses including wounding, anaerobic conditions, flooding, chilling, disease and drought.
• during the climacteric � which is the sudden surge of respiratory activity that occurs at the peak of ripening in many fruits - lots of ethylene is made

B. Pathway of synthesis

• the first precursor is methionine (one of the protein amino acids)
• methionine + ATP � S-adenosyl-methionine (SAM) � ACC synthase � amino cyclopropane carboxylic acid (ACC) � ACC oxidase � CH₂=CH₂ + HCN (hydrogen cyanide) + CO₂.   To summarize the highlights:

1. ATP reacts with methionine to form SAM
2. SAM is essentially a carrier form of methionine (it is involved in other reactions in the cell)
3. ACC synthase is the most crucial enzyme in the pathway. It is the rate limiting step and induced by: (a) fruit ripening; (b) flower senescence; (c) in response to IAA; (d) under wounding; (e) chilling injury; (f) drought; (g) flooding; (h) in response to ethylene ("one bad apple spoils the whole bunch"). ACC synthase is cytosolic and coded by a multi-gene family.
4. methionine is re-claimed via the Yang cycle.
5. ACC oxidase was formerly called "ethylene forming enzyme, abbreviated EFE."   It requires Fe²⁺ and ascorbate for activity � which explains why it took awhile to characterize this enzyme.  It is also the product of a multigene family.
6. One of the products of ACC oxidase activity is HCN � which explains why most plants have enzyme systems for detoxifying/metabolizing cyanide. ACC oxidase is inhibited by anaerobic conditions and cobalt ions but stimulated by ripening.

IV. Inhibitors

• silver ions (Ag⁺), CO₂ and KMnO₄ inhibit ethylene actions. These bind to ethylene receptors or otherwise interfere with the mechanism of ethylene action.
• Aminovinylglycine (AVG) and aminooxyacetic acid (AOA) block the action of ACC synthase.  Since these compounds are knows to block pyridoxal enzymes, it suggested that they participate in the process.

V. Disposal

• probably not a concern since it is a gas
• in addition, malonate will bind to ACC forming N-malonyl ACC.  Unlike most conjugates that can be hydrolyzed to produce the original compound, this one is NOT converted back to ethylene.
• a conjugate with glutamic acid (GACC) may be an important regulator of ethylene levels

VI.Actions

A. Fruit ripening
    Ethylene triggers fruit ripening (i.e., climacteric) - Flavr-Savr tomato.

B. Abscission
    This is the shedding of plant parts. Occurs at a specialized layer of cells � the abscission layers. Auxin apparently prevents leaf abscission by maintaining cells in the abscission zone insensitive to ethylene. When auxin levels in the leaf decline, the tissues become sensitive to ethylene that promotes abscission by producing and secreting cellulases, etc.

C. Epinasty
    Downward bending of leaves - common response to flooding or waterlogged soils.

D. Triple Response
    Pea seedlings treated with ethylene are short (inhibits internode elongation), fat (increase stem thickness) and "stupid" (horizontal growth, no positive gravitropism). Further, they show little leaf expansion and possess an apical hook.

E. Thigmomorphogenesis
    The change in growth form in response to a mechanical stimulation such as touch.

F. Stimulates germination in cereals, peanuts; promotes sprouting in potato tubers and other bulbs.

G. Flower senescence
    Stimulated by ethylene. Adding AVG to carnation can keep them fresh for weeks.

VII. Commercial applications
    Ethrel (Ethephon) liquid sprayed onto plants. It contains a dilute solution of 2-chloroethylphosphonic acid that breaks down to give off ethylene. Among others things, it is used to synchronize flowering and fruit set in pineapples. Commercial fruits are usually stored in low O2 to inhibit ethylene biosynthesis or under high CO2 to prevent ethylene action as a ripening promoter.