Biogenesis of Ethylene

Abstract

Summary 1. The main characteristics of the biosynthetic system forming ethylene in plant tissues have been reviewed. The dependence of synthesis on a liberal supply of oxygen is clearly indicated by the fact that atmospheres containing 3–5% oxygen prevent the synthesis in fruits. There is no close connexion between respiratory activity and synthesis. Ripening of fruits and the changes associated with it may be initiated by ethylene; under such conditions the progress of formation of the hydrocarbon is autocatalytic. 2. Synthesis appears to be dependent on some degree of cell organization, since it responds acutely to changes in toxcity, tissue wounding and tissue destruction. Homogenates of many plant tissues do not produce ethylene in vitro, and the inability to use such extracts has imposed serious restrictions on biochemical studies which have in the past been mainly concerned with tracer studies and the use of tissue slices. 3. The chief difficulty associated with tracer studies aimed at determining the nature of the precursor stems from the fact that the synthesis of ethylene is only a minor pathway on the general metabolism of the cell. Thus the ratio of CO2 to ethylene production is of the order of 164 in the case of the apple and as high as 18,000 in the case of less vigorous producers of ethylene. The incorporation of label from labelled substrates which enter the general metabolism of the cell is thus usually very low, and this makes it difficult to determine whether the incorporation observed has any real physiological significance. In fact only where incorporation into ethylene relative to that into CO2 is high, as is the case with methionine, can one conclude that the substance can be considered to be an immediate precursor. 4. Because of the difficulty of obtaining clear‐cut results with tracer techniques, attention has been devoted to the production of ethylene by model systems from substances of physiological interest. The studies have revealed that many substances found in plant tissue can be decomposed to yield ethylene in model systems functioning under physiological conditions. Two such substances, which have received most attention, are methionine and linolenic acid, and conditions under which ethylene is formed from them have been described. 5. Such developments have stimulated research to obtain evidence for or against the operation of such model systems in vivo. Using tissue‐slice techniques, methionine and linolenic acid have both been found to stimulate ethylene formation in tissue slices. 6. The first demonstration of the synthesis of ethylene in vitro by enzymes isolated from the florets of the cauliflower has now been reported. The system involves the intermediate formation of methional from methionine by enzymes contained in the mitochondria, and the subsequent enzymic decomposition of methional into ethylene by non‐particulate enzymes. These latter consist of a glucose oxidase and a peroxidase. The glucose oxidase in the presence of its substrate generates hydrogen peroxide, and per...

Plant Hormones in the Control of Abscission

“…As the second stage progresses, pectinase activity develops (MorrC, 1968), cellulase activity increases (Horton & Osborne, 1967;Abeles, 1969), and there is a rapid decline in break-strength of the abscission zone (Cracker & Abeles, (3) Mode of action of ethylene Consideration of the mode of action of E T H must take into account a sizeable body of evidence in addition to that already presented in this section. Especially important is the fact that E T H release is frequently autocatalytic; that is, a small amount of E T H can stimulate the release of much larger amounts of E T H by tissues such as fruits, leaves and explants (Mapson, 1969). Thus the question arises, does the hormonal role of E T H end with this relatively massive release of ETH or does it have further biochemical effects more fundamental to the process of abscission?…”

Ethylene Biosynthesis in Fruit Tissues

“…Another characteristic feature of ethylene formation in vivo is its oxygen dependence. When air is replaced by nitrogen, ethylene production by plant tissues declines rapidly (4,6,10). Table II illustrates the absolute requirement of oxygen for the conversion of methionine or KMB to ethylene by apple tissue.…”

“…It is well known that in fruits the dramatic increase in ethylene production is closely associated with the increase in respiration and with ripening (3,4,10,14). Oxygen is essential for this endogenous production of ethylene (4, 10).…”

“…The ability of methionine and its keto analogue (a-keto-,y-methylthiobutyric acid) to serve as ethylene precursors by apple tissues was compared. Chemical and kinetic evidence support the view that methionine is a closer precursor of ethylene than its keto analogue.It is well known that in fruits the dramatic increase in ethylene production is closely associated with the increase in respiration and with ripening (3,4,10,14). Oxygen is essential for this endogenous production of ethylene (4, 10).…”

“…Oxygen is essential for this endogenous production of ethylene (4,10). Evidence from tracer studies in a number of plant tissues has shown that methionine serves as a precursor of ethylene in vivo (1,2,5,9,11,18).…”