BIOGENESIS OF ETHYLENE

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.     

EFFECT OF ETHYLENE ON AERENCHYMA DEVELOPMENT

When applied to a part of stem or basal part of stem and root system, 1 ppm ethylene induced lysigenous aerenchyma development in the stem cortex of Helianthus annuus, Lycopersicon esculentum, and Phaseolus vulgaris. Local application of ethylene to a part of stem significantly increased the activity of cellulase in that part of stem in the above three species. Pretreatment of a part of stem with 100 ppm AgNO₃ counteracted the effects of ethylene which was subsequently applied to the part of stem, completely suppressing aerenchyma development and highly significantly reducing cellulase activity in Helianthus annuus. These results support the earlier proposal that the deficiency of oxygen in waterlogged plants triggers the anaerobic stimulation of ethylene production, which in turn increases the cellulase activity leading to aerenchyma development.     

GROWTH REGULATION BY ETHYLENE IN FERN GAMETOPHYTES. V. ETHYLENE AND THE EARLY EVENTS OF SPORE GERMINATION

Early events during the germination of spores of the fern Onoclea sensibilis were studied to determine the time during germination when ethylene had its greatest inhibiting effect. Water imbibition by dry spores was rapid and did not appear to be inhibited by ethylene. During normal germination DNA synthesis occurred about four hours before the nucleus moved from a central position to the spore periphery. Following nuclear movement, mitosis and cell division occurred, partitioning the spore into a small rhizoid cell and a large protonemal cell. Cell division was complete approximately six hours after nuclear movement. Ethylene treatment of the spores blocked DNA synthesis, nuclear movement, and cell division. The earliest DNA replication in uninhibited spores was observed after 14 hours of germination, and the maximal rate of spore labeling with ³H-thymidine was between 16 and 20 hours. Spores were most sensitive to ethylene, however, during the stages of germination prior to DNA synthesis, and it was concluded that ethylene did not directly inhibit DNA replication but blocked germination at some earlier fundamental step. The effects of ethylene were reversible. since complete recovery from inhibition of germination was possible if ethylene was released and the spores were kept in light. Recovery was much slower in darkness. It was hypothesized that light acted photosynthetically to overcome the ethylene inhibition of germination. Consistent with this, it was shown that spores exhibit net photosynthesis after only two hours of germination.     

GROWTH REGULATION BY ETHYLENE IN FERN GAMETOPHYTES. IV. INVOLVEMENT OF PHOTOSYNTHESIS IN OVERCOMING ETHYLENE INHIBITION OF SPORE GERMINATION

The inhibitory effects of ethylene on spore germination were investigated. In darkness spore germination was completely inhibited by 10 μ1 · 1⁻¹ ethylene. Light partially overcame this inhibition, and the effect of continuous irradiation with white fluorescent light saturated at about 450 μW · cm⁻². Monochromatic red, blue and far-red light were effective in overcoming ethylene inhibition, whereas green was not. Short periodic exposures to red or far-red light were not sufficient to overcome ethylene inhibition. This suggested that phytochrome was not involved. The photosynthetic inhibitor DCMU blocked the effect of light. Infrared gas analysis showed that photosynthesis saturated at about 450 μW · cm⁻² in white light. Red, blue and far-red light were more efficient photosynthetically than green light; DCMU blocked photosynthesis. Normalized curves of photosynthesis and germination vs. light intensity showed a similar dependence on light energy. It was concluded that light appears to overcome the inhibitory effects of ethylene through some process dependent on photosynthesis.     

ANAEROBIC ELEVATION OF ETHYLENE CONCENTRATION IN WATERLOGGED PLANTS

Anaerobic elevation of ethylene concentration in waterlogged and non-waterlogged Helianthus annuus L. and Lycopersicon esculentum Mill. was studied. A balloon method was devised to provide an anaerobic atmosphere around the intact sunflower stem. Anaerobic conditions were also produced by bubbling nitrogen into the floodwater. Ethylene concentration in the stem of waterlogged plants was higher when nitrogen was bubbled through the floodwater than when aerated, the effect being greater for the soil-grown plants than for the sand-cultured plants. Ethylene concentration in the stem of waterlogged plants was highest in the region exposed to anaerobiosis, and less with increasing distance or height on the non-waterlogged part of the stems. Intact sunflower stems increased their ethylene concentration in that part of the stem which was maintained in an oxygen-free atmosphere. The results suggest that enhanced ethylene production in waterlogged plants primarily occurs in the waterlogged part of roots and stems.     

水稻铁害和应激乙烯释放的关系

在热带地区的水稻栽培中,常遇到水稻青铜病（ｂｒｏｎｚｉｎｇ）的危害．已知它是由水田中高浓度的亚铁离子所引起,故又叫铁害．但至今没有可靠的生理诊断指标用于抗性品种的筛选．本文研究了铁害与应激乙烯释放的关系,试图以应激乙烯的释放作为铁害的生理诊断指标．试验用两种方法模拟水稻致病．第一种方法是将水稻离体叶片的剪口端浸入ＦｅＳＯ４溶液中,靠叶片蒸腾作用吸收Ｆｅ＋＋而致病．另一种方法是在水培培养液中加入ＦｅＳＯ４通过水稻根系吸收Ｆｅ＋＋而致病．研究结果表明,当处理离体叶片时,发病强度和应激乙烯释放量呈显著相关,但叶片内铁含量的增加与发病强度和应激乙烯释放都没有相关性．而处理完全植株时,叶片中乙烯释放几乎不受影响．当部分或全部切除根时,叶片中乙烯释放则可被亚铁离子激发。表明水稻根系限制了Ｆｅ＋＋的吸收速率,而Ｆｅ＋＋进入叶组织的速率又决定应激乙烯的释放和组织的伤害程度．因此,叶片应激乙烯的释放作为铁害的生理诊断指标只有在当根系受到某种伤害时才可能适用,譬如移栽和毒性土壤等因素造成的根系的伤害．     

香石竹花瓣对乙烯的敏感性与蛋白质合成

基因转录抑制剂α-amanitin和蛋白质合成抑制剂cycloheximide完全抑制了香石竹(Dianthuscaryophyllus L.cvs.White Sim and Sandrosa)花瓣对乙烯反应的症状,包括花瓣卷曲和细胞膜离子渗漏增加。观察到花中蛋白质合成能力随着花的衰老而降低,花对乙烯的敏感性随花的衰老而增加。但是用乙烯合成抑制剂aminooxyacetic acid(AOA)预处理切花,则改变了花对乙烯敏感性的变化趋势。常用的香石竹品种D.caryophyllus L.cv.White Sim花经AOA处理后,对乙烯的敏感性随着花的衰老而下降。这些结果揭示花对乙烯的敏感性可能受蛋白质合成能力影响。