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.     

Physiology and firmness determination of ripening tomato fruit

Tomato ( Lycopersicon esculentum Mill.) genotypes varying in intrinsic firmness were examined to determine the quantitative relationships between polygalacturonase (EC 3.2.1.15) activity, firmness and other ripening parameters including rate (days from mature-green to full red) and intensity (rate of ethylene production at climacteric peak) of ripening. Texture, respiration and ethylene production were monitored in the immature-green through the red (ripe) stages of development. Polygalacturonase activity was measured by direct assay of salt-extractable wall protein or by monitoring the release of pectins from isolated, enzymically active wall. In all fruit, polygalacturonase activity was highly correlated with pericarp softening, but only moderately correlated with softening of whole fruit (r = 0.920 and 0.757, respectively). Polygalacturonase activity was positively correlated with cell-wall autolytic activity in pink (r = 0.969) and red (r = 0.900) fruit. Firmer genotypes exhibited lower rates of respiration and ethylene production during ripening. Polygalacturonase activity in isolates prepared from fruit at the climacteric peak was positively correlated with ethylene production and respiration, and negatively correlated with days to ripening (r = 0.929, 0.805, and -0.791, respectively). The data demonstrate the importance of selecting the appropriate method of firmness determination and are consistent with the hypothesis that pectin fragments released by polygalacturonase contribute to the production of autocatalytic (system II) ethylene.     

Molecular and genetic characterization of a non-climacteric phenotype in melon reveals two loci conferring altered ethylene response in fruit

Fruit ripening and abscission are associated with an ethylene burst in several melon (Cucumis melo) genotypes. In cantaloupe as in other climacteric fruit, exogenous ethylene can prematurely induce abscission, ethylene production, and ripening. Melon genotypes without fruit abscission or without ethylene burst also exist and are, therefore, non-climacteric. In the nonabscising melon fruit PI 161375, exogenous ethylene failed to stimulate abscission, loss of firmness, ethylene production, and expression of all target genes tested. However, the PI 161375 etiolated seedlings displayed the usual ethylene-induced triple response. Genetic analysis on a population of recombinant cantaloupe Charentais x PI 161375 inbred lines in segregation for fruit abscission and ethylene production indicated that both characters are controlled by two independent loci, abscission layer (Al)-3 and Al-4. The non-climacteric phenotype in fruit tissues is attributable to ethylene insensitivity conferred by the recessive allelic forms from PI 161375. Five candidate genes (two ACO, two ACS, and ERS) that were localized on the melon genetic map did not exhibit colocalization with Al-3 or Al-4.     

Role of Brassinosteroids, Ethylene, Abscisic Acid, and Indole-3-Acetic Acid in Mango Fruit Ripening

Rapid ripening of mango fruit limits its distribution to distant markets. To better understand and perhaps manipulate this process, we investigated the role of plant hormones in modulating climacteric ripening of ??Kensington Pride?? mango fruits. Changes in endogenous levels of brassinosteroids (BRs), abscisic acid (ABA), indole-3-acetic acid (IAA), and ethylene and the respiration rate, pulp firmness, and skin color were determined at 2-day intervals during an 8-day ripening period at ambient temperature (21?±?1°C). We also investigated the effects of exogenously applied epibrassinolide (Epi-BL), (+)-cis, trans-abscisic acid (ABA), and an inhibitor of ABA biosynthesis, nordihydroguaiaretic acid (NDGA), on fruit-ripening parameters such as respiration, ethylene production, fruit softening, and color. Climacteric ethylene production and the respiration peak occurred on the fourth day of ripening. Castasterone and brassinolide were present in only trace amounts in fruit pulp throughout the ripening period. However, the exogenous application of Epi-BL (45 and 60?ng?g^?1 FW) advanced the onset of the climacteric peaks of ethylene production and respiration rate by 2 and 1?day, respectively, and accelerated fruit color development and softening during the fruit-ripening period. The endogenous level of ABA rose during the climacteric rise stage on the second day of ripening and peaked on the fourth day of ripening. Exogenous ABA promoted fruit color development and softening during ripening compared with the control and the trend was reversed in NDGA-treated fruit. The endogenous IAA level in the fruit pulp was higher during the preclimacteric minimum stage and declined during the climacteric and postclimacteric stages. We speculate that higher levels of endogenous IAA in fruit pulp during the preclimacteric stage and the accumulation of ABA prior to the climacteric stage might switch on ethylene production that triggers fruit ripening. Whilst exogenous Epi-BL promoted fruit ripening, endogenous measurements suggest that changes in BRs levels are unlikely to modulate mango fruit ripening.     

Effect of low oxygen and high carbon dioxide on the levels of ethylene and 1-aminocyclopropane-1-carboxylic acid in ripening apple fruits

The association of the level of ACC and the ethylene concentration in ripening apple fruit (Malus sylvestris Mill, var. Ben Davis) was studied. Preclimacteric apple contained small amounts of ACC and ethylene. With the onset of the climacteric and a concomitant decrease in flesh firmness, the level of ACC and ethylene concentration both increased markedly. During the postclimacteric period, ethylene concentration started to decline, but the level of ACC continued to increase. Ethylene production and loss of flesh firmness of fruits during ripening were greatly suppressed by treatments with low O₂ (O₂ 1–3%, CO₂ O%) or high CO₂ (CO₂ 20–30%, O₂ 15–20%) at the preclimacteric stage. However, after 4 weeks an accumulation of ACC was observed in treated fruits when control fruit was at the postclimacteric stage. Treatment of fruit with either low O₂ or high CO₂ at the climacteric stage resulted in a decrease of ethylene production. However, the ACC level in fruit treated with low O₂ was much higher than both control and high CO₂ treated fruit; it appears that low O₂ inhibits only the conversion of ACC to ethylene, resulting in an accumulation of ACC. Since CO₂ inhibits ethylene production but does not result in an accumulation of ACC, it appears that high CO₂ inhibits both the conversion of ACC to ethylene and the formation of ACC.     

乙酰水杨酸和乙烯处理对猕猴桃果实后熟软化的影响

以猕猴桃(Actinidia deliciosa(A.Chev.)C.F.Liang et A.R.Ferguson cv.Bruno)果实为试材,研究乙酰水杨酸(ASA)与乙烯处理对果实内源水杨酸(SA)含量变化以及后熟软化相关因子的影响,探讨SA在果实成熟衰老进程的作用.研究结果表明:果实后熟软化进程中,内源SA水平呈下降变化,组织中SA水平与果实硬度变化呈极显著正相关关系(r=0.969 4**),ASA处理可显著地维持组织中较高的SA水平,抑制脂氧合酶(LOX)和丙二烯氧合酶(AOS)活性增加,减低O-.2生成速率,维持细胞膜稳定性,进而抑制了乙烯生物合成或推迟乙烯跃变的到来,延缓了果实后熟软化进程,这些效应主要表现在乙烯跃变之前或乙烯跃变前期;相反,外源乙烯处理则显著降低果实组织中内源SA水平,促进LOX和AOS活性的增加,促使O-.2积累,增加了细胞膜透性,促使乙烯跃变的提前到来,加速了果实的后熟软化.推测组织中的内源SA水平与细胞膜脂过氧化作用密切相关,外源ASA可能作为一种O-.2等自由基的清除剂或是细胞膜稳定剂在组织成熟衰老过程中起作用.     

The involvement of 1-aminocyclopropane-1-carboxylic acid synthase isogene, Pp-ACS1, in peach fruit softening

Ethylene promotes fruit ripening, including softening. The fruit of melting-flesh peach (Prunus persica (L). Batsch) cultivar 'Akatsuki' produces increasing levels of ethylene, and the flesh firmness softens rapidly during the ripening stage. On the other hand, the fruit of stony hard peach cultivars 'Yumyeong', 'Odoroki', and 'Manami' does not soften and produces little ethylene during fruit ripening and storage. To clarify the mechanism of suppression of ethylene production in stony hard peaches, the expression patterns of four ethylene biosynthesis enzymes were examined: ACC synthases (Pp-ACS1, Pp-ACS2, and Pp-ACS3) and ACC oxidase (Pp-ACO1). In the melting-flesh cultivar 'Akatsuki', Pp-ACS1 mRNA was dramatically induced after harvesting, and a large amount of ethylene was produced. On the other hand, in stony hard peaches, Pp-ACS1 mRNA was not induced during the ripening stage, and ethylene production was inhibited. Since Pp-ACS1 mRNA was induced normally in senescing flowers, wounded leaves, and wounded immature fruit of 'Yumyeong', Pp-ACS1 was suppressed only at the ripening stage, and was not a defect in Pp-ACS1. These results indicate that the suppression of fruit softening in stony hard peach cultivars was caused by a low level of ethylene production, which depends on the suppressed expression of Pp-ACS1.     

挥发性香气物质和乙烯生产在"湖景蜜露"桃果实发育过程中的变化

以"湖景蜜露"水蜜桃(Prunus persica L.)为试材,检测了果实从未成熟到成熟发育过程中乙烯生成、呼吸速率及挥发性香气性物质的变化;同时对果实大小、果皮色泽、果肉硬度、可溶性固形物、可滴定酸进行了测定;对与果实乙烯产生密切相关的1-氨基环丙烷-1-羧酸(ACC)含量、ACC合成酶活性、ACC氧化酶活性也进行了测定.结果表明,随果实成熟度的增加,果实大小、果皮L*值、可溶性固形物含量增加,而果实硬度、果皮h°值、可滴定酸含量减少.在未成熟的果实中,C6的醛类(反式-2-己烯醛)和醇类(顺式-3-己烯醇)是主要的成分;乙烯生成量很低;呼吸速率较高.到跃变阶段C6～C12的内酯类物质明显增加,尤其是γ和δ-内酯类成为果实主要的香气挥发性物质.推测果实乙烯、呼吸作用等基本的生理变化可能调节着内酯类物质的生成.在乙烯跃变上升时果肉中ACC氧化酶的活性下降,ACC含量和ACC合成酶活力的变化与乙烯生成量变化的趋势一致.根据以上结果可以认为桃果实主要的香气挥发性物质的形成与乙烯、呼吸跃变的开始密切相关.香气物质形成速率动态变化可能是桃果实发育过程中成熟度的另一个生理学指标.     

Inhibition of the ethylene response by 1-MCP in tomato suggests that polyamines are not involved in delaying ripening, but may moderate the rate of ripening or over-ripening

Ethylene initiates the ripening and senescence of climacteric fruit, whereas polyamines have been considered as senescence inhibitors. Ethylene and polyamine biosynthetic pathways share S-adenosylmethionine as a common intermediate. The effects of 1-methylcyclopropene (1-MCP), an inhibitor of ethylene perception, on ethylene and polyamine metabolism and associated gene expression was investigated during ripening of the model climacteric fruit, tomato (Solanum lycopersicum L.), to determine whether its effect could be via polyamines as well as through a direct effect on ethylene. 1-MCP delayed ripening for 8 d compared with control fruit, similarly delaying ethylene production and the expression of 1-aminocyclopropane-1-carboxylic acid (ACC)-synthase and some ethylene receptor genes, but not that of ACC oxidase. The expression of ethylene receptor genes returned as ripening was reinitiated. Free putrescine contents remained low while ripening was inhibited by 1-MCP, but increased when the fruit started to ripen; bound putrescine contents were lower. The activity of the putrescine biosynthetic enzyme, arginine decarboxylase, was higher in 1-MCP-treated fruit. Activity of S-adenosylmethionine-decarboxylase peaked at the same time as putrescine levels in control and treated fruit. Gene expression for arginine decarboxylase peaked early in non-treated fruit and coincident with the delayed peak in putrescine in treated fruit. A coincident peak in the gene expression for arginase, S-adenosylmethionine-decarboxylase, and spermidine and spermine synthases was also seen in treated fruit. No effect of treatment on ornithine decarboxylase activity was detected. Polyamines are thus not directly associated with a delay in tomato fruit ripening, but may prolong the fully-ripe stage before the fruit tissues undergo senescence.     

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.