|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; email@example.com|
B. Pathway of synthesis
- ATP reacts with methionine to form SAM
- SAM is essentially a carrier form of methionine (it is involved in other reactions in the cell)
- 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.
- methionine is re-claimed via the Yang cycle.
- 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.
- 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.
A. Fruit ripening
Ethylene triggers fruit ripening (i.e., climacteric) - Flavr-Savr tomato.
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.
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.
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.
01/07/2009 � Copyright by SG