乙烯诱导西瓜果实的水渍化败坏

为了研究乙烯在西瓜(Citrullus lanatusThunb.Mansfeld)果实水渍化败坏过程中的作用,先将果实在5μL/L 1-甲基环丙烯(1-MCP)气体中处理18 h,然后在50 μL/L乙烯和20℃温度下贮藏.西瓜果实对乙烯处理的最初反应表现为胎座组织的电导率和游离汁液增加,同时出现组织软化和水渍化.水渍化的症状最初在靠近花萼端的内果皮中发生,在乙烯处理的第2天开始出现,ACC合成酶(ACS)和ACC氧化酶(ACO)的活性明显提高.1-MCP单独处理不产生任何明显的作用,但是会完全抑制外源乙烯诱导的水渍化败坏.没有经过乙烯处理的西瓜果实,贮藏2 d以后出现呼吸强度和乙烯释放量的高峰,10 d以后水渍化现象也零星出现.这些结果和1-MCP的预防效果说明,西瓜果实的水渍化败坏是一种由乙烯诱导的衰老现象.     

Expression of ACC synthase is enhanced earlier than that of ACC oxidase during fruit ripening of mume (Prunus mume)

Mume (Japanese apricot: Prunus mume Sieb. et Zucc.) is a climacteric fruit that produces large amounts of ethylene as it ripens. Ripening is accompanied by marked increases in the activities of two ethylene-biosynthetic enzymes, namely, 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase. To study the molecular aspects of ripening of mume, we isolated cDNA clones for proteins that we considered likely to be involved in the biosynthesis and perception of ethylene during ripening, namely, ACC synthase, ACC oxidase and the ethylene receptor. Northern blotting analysis revealed the markedly increased expression of ACC synthase prior to that of ACC oxidase and the increase in ethylene production during ripening. Overall, the levels of the mRNAs for the genes corresponded closely to the levels of activity of the ethylene-biosynthetic enzymes. Exposure of mature green mume fruit to ethylene for 12 h induced strong expression of ACC synthase, as well as of ACC oxidase. Wounding of the pericarp of mume fruit induced the expression of ACC synthase but not of ACC oxidase. The rate of ethylene production increased only slightly after wounding. These results suggest that expression of the genes for ACC synthase and ACC oxidase must be activated sequentially for maximum production of ethylene during ripening of mume fruit and that several mechanisms regulate the expression of ethylene-biosynthetic genes during ripening.     

柿果实ACC合成酶cDNA的克隆及其序列分析

根据其它植物ACC合成酶（1－aminocyclopropane-1-carboxylic acid synthase,ACS)氨基酸保守区,设计1组简并引物,用RT－PCR法,从柿（Diospyros kaki Thunb.)果实扩增出3个约1kb左右的cDNA片段,将其克隆至pGEM-T载体上,对这些重组克隆进行序列测定和氨基酸序列推导,DK－ACS1由1101个碱基组成,编码364个氨基酸;DK－ACS2是1086个碱基,编码359个氨基酸;DK－ACS3为1089个碱基,编码363个氨基酸。它们均具有其它植物ACS合成酶中存在的7个保守区和11个不变氨基酸残基,且在多肽水平上有较高的同源性。与番茄LE－ACS2的同源性DK－ACS1是60.5A%,DK-ACS2是70.7%,DK－ACS3为66．9％,与甜瓜CM－ACS1的同源性依次分别是60．4％,72．1％和64．4％。     

Ethylene and fruit ripening

The latest advances in our understanding of the relationship between ethylene and fruit ripening are reviewed. Considerable progress has been made in the characterisation of genes encoding the key ethylene biosynthetic enzymes, ACC synthase (ACS) and ACC oxidase (ACO) and in the isolation of genes involved in the ethylene signal transduction pathway, particularly those encoding ethylene receptors ( ETR ). These have allowed the generation of transgenic fruit with reduced ethylene production and the identification of the Nr tomato ripening mutant as an ethylene receptor mutant. Through these tools, a clearer picture of the role of ethylene in fruit ripening is now emerging. In climacteric fruit, the transition to autocatalytic ethylene production appears to result from a series of events where developmentally regulated ACO and ACS gene expression initiates a rise in ethylene production, setting in motion the activation of autocatalytic ethylene production. Differential expression of ACS and ACO gene family members is probably involved in such a transition. Finally, we discuss evidence suggesting that the NR ethylene perception and transduction pathway is specific to a defined set of genes expressed in ripening climacteric fruit and that a distinct ETR pathway regulates other ethylene-regulated genes in both immature and ripening climacteric fruit as well as in non-climacteric fruit. The emerging picture is one where both ethylene-dependent and -independent pathways coexist in both climacteric and non-climacteric fruits. Further work is needed in order to dissect the molecular events involved in individual ripening processes and to understand the regulation of the expression of both ethylene-dependent and -independent genes.     

Xyloglucan-derived oligosaccharides induce ethylene synthesis in persimmon (Diospyros khaki L.) fruit

In this work the effect of injection of xyloglucan-derived oligosaccharides (XGOs) into whole persimmon (Diospyros khaki L.) fruits on ethylene biosynthesis was investigated. Fruits collected during different ripening stages produced low levels of ethylene without a climacteric-like peak. Pretreatment of these fruits with 10 cm³ C2H4 m^-3 for 8 h stimulated little or no endogenous ethylene production. However, when persimmon fruits were injected with a mixture of XGOs a burst in ethylene production was observed compared with water-injected control fruits or fruits injected with different monosaccharide solutions. In order to study the influence of oligosaccharide structure and fruit ripening stage on the ability of XGOs to induce ethylene synthesis, fucosylated and non-fucosylated XGOs were injected into persimmon fruits harvested at two different ripening stages. Both oligosaccharide structures were able to induce ethylene production. Induction of ethylene by XGOs was much more evident in fruits harvested later in time, indicating that the process is developmentally regulated. The levels of 1-aminocyclopropane-1-carboxylic acid (ACC) in injected persimmon fruits were also examined. This study showed that the increase in the rate of ethylene biosynthesis induced by XGOs was accompanied by the accumulation of its metabolic precursor ACC.     

Ethylene regulation of fruit ripening: Molecular aspects

Progress in ethylene regulating fruit ripening concerning itsperception and signal transduction and expression of ACC synthaseand ACC oxidase genes is reviewed. ACC synthase and ACC oxidasehave been characterized and their genes cloned from various fruittissues. Both ACC synthase and ACC oxidase are encoded bymultigene families, and their activities are associated withfruit ripening. In climacteric fruit, the transition toautocatalytic ethylene production appears to be due to a seriesof events in which ACC sythase and ACC oxidase genes have beenexpressed developmentally. Differential expression of ACCsynthase and ACC oxidase gene family members is probably involvedin such a transition that ultimately controls the onset of fruitripening.In comparison to ACC synthase and ACC oxidase, less is knownabout ethylene perception and signal transduction because of thedifficulties in isolating and purifying ethylene receptors orethylene-binding proteins using biochemical methods. However, theidentification of the Nr tomato ripening mutant as anethylene receptor, the applications of new potent anti-ethylenecompounds and the generation of transgenic fruits with reducedethylene production have provided evidence that ethylenereceptors regulate a defined set of genes which are expressedduring fruit ripening. The properties and functions of ethylenereceptors, such as ETR1, are being elucidated.Application of molecular genetics, in combination withbiochemical approaches, will enable us to better understand theindividual steps leading from ethylene perception and signaltransduction and expression of ACC synthase and ACC oxidase genefamily member to the physiological responses.     

Recovery of ethylene biosynthesis in diazocyclopentadiene (DACP)-treated tomato fruit

Diazocyclopentadiene (DACP), a competitive ethylene action inhibitor binds irreversibly to the ethylene receptor to reduce tissue responses to ethylene. Tomato fruit (Lycopersicon esculentum Mill cv lsquo;Rondellorsquo;) were treated with DACP at the mature green stage. Ethylene biosynthesis and respiration rate were depressed. Color changes from green to red were delayed. Compared to the control, ACC content increased and ACC oxidase activity in vivo decreased in DACP-treated fruit. Thus, decrease of ethylene production caused by DACP treatment was due to the reduction of ACC oxidase activity. The decline in ripening subsequently recovered after DACP treatment. Results from the Northern analysis for gene expression of ACC synthase and ACC oxidase, showed that expression of both genes declined in DACP-treated fruit, and then recovered. Therefore the recovery of ethylene production was due to the recovery in gene expression and activity of ACC oxidase. We conclude that the effects of DACP on ethylene biosynthesis are on expression of ACC synthase and ACC oxidase genes, and/or regulation of ACC oxidase activity.     

ETHY. A theory of fruit climacteric ethylene emission

A theory of fruit climacteric ethylene emission was developed and used as the basis of a simulation model called ETHY. According to the theory, the biosynthetic pathway of ethylene is supplied by ATP and is regulated by 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase. The conjugation of ACC with malonate to form MACC was taken into account as a way to decrease the availability of ACC. Because of the seasonal increase of fruit volume, the dilution of biochemical compounds used in ETHY was taken into account. Finally, the ethylene diffusion across the skin was considered. The theory took into account the effect of temperature and O(2) and CO(2) internal concentrations on ethylene. The model was applied to peach (Prunus persica) fruit over 3 years, several leaf:fruit ratios, and irrigation conditions. An adequate ethylene increase was predicted without considering any increase in respiration during the ripening period, which suggests that the respiratory climacteric may not be required for ripening. Another important result of this study is the high sensitivity of ETHY to the parameters involved in the calculation of ACC oxidase and ACC synthase activities, ATP production, and skin surface and permeability. ETHY was also highly sensitive to changes in fruit growth and temperature.     

A role for jasmonates in climacteric fruit ripening

Jasmonates are a class of oxylipins that induce a wide variety of higher-plant responses. To determine if jasmonates play a role in the regulation of climacteric fruit ripening, the effects of exogenous jasmonates on ethylene biosynthesis and color, as well as the endogenous concentrations of jasmonates were determined during the onset of ripening of apple (Malus domestica Borkh. cv. Golden Delicious) and tomato (Lycopersicon esculentum Mill. cv. Cobra) fruit. Transient (12 h) treatment of pre-climacteric fruit discs with exogenous jasmonates at low concentration (1 or 10 μM) promoted ethylene biosynthesis and color change in a concentration-dependent fashion. Activities of both 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase and ACC synthase were stimulated by jasmonate treatments in this concentration range. The endogenous concentration of jasmonates increased transiently prior to the climacteric increase in ethylene biosynthesis during the onset of ripening of both apple and tomato fruit. The onset of tomato fruit ripening was also preceded by an increase in the percentage of the cis-isomer of jasmonic acid. Inhibition of ethylene action by diazocyclopentadiene negated the jasmonate-induced stimulation of ethylene biosynthesis, indicating jasmonates act at least in part via ethylene action. These results suggest jasmonates may play a role together with ethylene in regulating the early steps of climacteric fruit ripening. Received: 14 August 1997 / Accepted: 4 October 1997     

Indole-3-acetic acid, ethylene, and abscisic acid metabolism in developing muskmelon (Cucumis melo L.) fruit

Hormonal metabolism associated with fruit development in muskmelon was investigated by measuring IAA, ABA, and ACC levels in several tissues at various stages of development. In addition, levels of conjugated IAA and ABA were determined in the same tissues. Ethylene production, which is believed to signal the ripening and senescence of mature fruit, was also measured. Ethylene production was highest in the outer tissue near the rind and gradually declined during maturation, except for a dramatic increase in all fruit tissues at the climacteric. In contrast to ethylene production, ACC levels increased during maturation and remained equal throughout the fruit until the climacteric, when levels in the outer tissues increased nearly 5-fold over levels in the inner tissues. The consistent presence of ACC indicates that ACC oxidase rather than the availability of ACC regulates ethylene production in developing fruits. ABA and ABA esters generally declined during maturation, however an increase in ABA esters associated with the outer mesocarp tissue was observed in fully mature, climacteric fruit. IAA and IAA conjugates were only found in the outer tissue near the rind, and their levels remained low until the fruit was fully mature and entering the climacteric. At that time, increased levels of conjugates were detected. The late burst of hormonal metabolism in the outer mesocarp tissue appeared to signal its degeneration and the deterioration that typically occurs in ripening fruit. The tissue-specific conjugation of IAA and ABA, in addition to the production of climacteric ethylene, may represent part of the signaling mechanism initiating ripening and eventual deterioration of tissues in muskmelon fruits.Abbreviations ABA abscisic acid - ACC 1-aminocylopropane-1-carboxylic acid - DAP days after pollination - IAA indole-3-acetic acid     

Characterization of ethylene production in developing strawberry fruit

Ethylene production, ACC content, and ACC oxidase activity were determined in strawberry fruit harvested at different stages of development and in fruit harvested green and developed in vitro in solutions containing sucrose. In fruit harvested at progressive stages of development from green through full ripe, ethylene production and ACC oxidase activity decreased whereas ACC content increased between the white and pink stages. Fruit detached at the green stage and developed to full ripe by immersion of the cut pedicel in sucrose solutions exhibited an increase in ACC content, decreased ethylene production, and no change in ACC oxidase activity. Detached green fruit provided with sucrose containing 0.5 mM silver (STS) had elevated ethylene production and more ACC oxidase activity than did fruit incubated without the silver salt. Green fruit provided with sucrose containing 1 mM ACC showed markedly increased ACC content, ACC oxidase activity, and ethylene production. These increases were noted following 4 days incubation in ACC, and were more pronounced after 11 days, at which time fruit of all treatments had attained a full-ripe stage of development. Calyx tissue exhibited more ACC oxidase activity, less ACC content, and similar ethylene production compared with receptacle tissue. ACC synthase could not be detected in fruit harvested at different developmental stages or in fruit detached and developed in vitro.abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - HQS 8-hydroxyquinoline hemisulfate - SAM S-adenosyl methionine - STS silver thiosulfate     

The climacteric-like behaviour of young, mature and wounded citrus leaves

Although leaves and other vegetative tissues are generally considered as non-climacteric, citrus leaves show a climacteric system II behaviour after detachment. Upon harvest, young, fully expanded 'Valencia' orange (Citrus sinensis) leaves ( approximately 60-d-old) exhibited two phases of ethylene production. The first phase, up to 6 d after detachment, was characterized by a low and constant ethylene production (system I pathway), associated with a constitutive expression of ACC synthase 2 (CsACS2), CsERS1, and CsETR1. ACC synthase 1 (CsACS1) was not expressed during this phase and autoinhibition of ethylene production was apparent following treatment with exogenous ethylene or propylene. The second phase, 7-12 d after detachment, was characterized by a climacteric rise in ethylene production, preceded by the induction of CsACS1 and ACC oxidase 1 (CsACO1) gene expression in the system II pathway. This induction was accelerated and augmented by exogenous ethylene or propylene, indicating an autocatalytic system II ethylene biosynthesis. Mature leaves (6-8-months-old) behaved similarly, except that the climacteric peak in ethylene production occurred earlier (day 5). Young and mature leaves varied in the timing of the climacteric ethylene rise and CsACS1 and CsACO1 induction. The two phases of ethylene production, system I and system II, were also detected in wounded leaf discs of both young and mature leaves. The first phase peaked 15 min after excision and the second phase peaked after 6 h.