BIOGENESIS OF ETHYLENE

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 peroxidase, in the presence of two co-factors, ^-coumaric acid esters and methane sulphinic acid, utilizes the peroxide to produce ethylene from methional. Although all components of this system have been isolated from extracts of floret tissue, proof that this is the actual or only process in vivo for this or other plant tissue has not as yet been achieved. The more recent demonstration of the possible involvement of linolenic acid underlines the necessity for further work. 7. Whilst much work still remains to be done to establish the mechanism of synthesis, which may not be identical in different plants, the related question of the nature of the events which stimulate the tissue to produce ethylene remains to be answered. Recent work has suggested that these events, induced by ageing of the tissue, are associated with the synthesis of new enzyme proteins, which are themselves the cause of the rapid onset of synthesis of ethylene, observed in most fruits, at the climacteric. 8. Much more information on the nature of events leading to and changes associated with the ripening syndrome in fruits and onset of senescence in vegetable tissues is needed before authoritative answers can be given to any of the questions raised in this review.