Ethylene (ethene) was discovered in the early 1900s as a gas that regulated fruit ripening. It had been realized that the close proximity of ripe fruit, such asoranges or apples, speeded up the ripening of other fruits, such as tomatoes andbananas. Regulating ripening, and therefore ethylene, has become an importantpart of the storage, transport and marketing of fruit worldwide. Ethylene has avariety of other roles in plants, including senescence of leaves and fruit, elongationof roots, and responses to waterlogging and other stresses. Although asimple molecule, its effects are highly specific.
Fruit ripening. Many ripening fruits show a rise in ethylene production that precedes the onset of ripening. Fruits that produce and respond to ethylene in ripening are the climacteric fruits (apples, tomatoes and bananas); the climacteric is a characteristic burst of respiration that occurs just before the final stages of ripening take place. Ethylene production in climacteric fruit is autocatalytic, i.e. ethylene stimulates its own production, the rapidly rising ethylene concentration then triggering the rapid burst of respiration.
The triple response. Ethylene-treated shoots (e.g. pea seedlings) show three characteristic growth responses simultaneously: epinasty (downward curvature of the leaves); decreased elongation and lateral cell expansion (i.e. increase in stem width) and loss of gravity response to give horizontal growth. Ethyleneinduced epinasty gives the apex of young dicot seedlings a ‘hook’ like appearance.
Ethylene and waterlogging responses. Whereas ethylene normally inhibits elongation growth, in some wet-land species, including rice, ethylene induces rapid elongation growth, allowing the plant to reach air. The formation of
aerenchyma (air spaces in the root cortex, formed by programmed cell death; Topic C1) is also induced by ethylene, which is synthesized in response to low oxygen and accumulates in waterlogged roots.
Other roles of ethylene. High concentrations of ethylene (>10 μl l–1) induce adventitious rooting and root hair formation. In leaf abscission, ethylene accelerates the synthesis of cell-wall degrading enzymes in the abscission layer, a specialized layer of cells at the leaf pulvinus which separate from adjacent cells permitting the leaf to fall from the plant.
Ethylene is produced from the amino acid methionine, via S-adenosyl methionine (SAM) and 1-aminocyclopropane-1-carboxylic acid (ACC). The enzymeproducing ACC is key in regulating ethylene production; it has a very short halflifeand the expression of its gene is stimulated by factors known to induceethylene responses. Ethylene is inactivated by oxidation (e.g. to ethylene oxide orCO2) or it can diffuse from the plant. Rates of ethylene production rise rapidly intissues subject to stress or wounding and subsequently decline to normal levels.
Ethylene is active at very low concentrations (around 1 ppm or 1 μl l–1).