Responses of banana fruit to treatment with 1-methylcyclopropene

Experiments were conducted to determine levels of 1-methylcyclopropene (1-MCP) exposure needed to prevent ethylene-stimulated banana fruit ripening, characterise responses of ethylene-treated fruit to subsequent treatment with 1-MCP, and to test effects of subsequent ethylene treatment on 1-MCP-treated fruit softening. Fruit softening was measured at 20°C and 90% relative humidity. One hour exposure at 20°C to 1000 nl 1-MCP/l essentially eliminated ethylene-stimulated ripening effects. Exposure for 12 h at 20°C to just 50 nl 1-MCP/l was similarly effective. Fruit ripening initiated by ethylene treatment could also be delayed with subsequent 1-MCP treatment. However, 1-MCP treatment only slowed down ripening of ethylene-treated fruit when applied at 1 day after ethylene and was ineffective when applied 3 or 5 days after ethylene treatment. The ripening response of fruit treated with 1-MCP and subsequently treated with ethylene varied with interval time between 1-MCP and ethylene treatments. As time increased, the response of 1-MCP-treated fruit to ethylene was enhanced. Responses to 0.1, 1, 10 or 100 µl ethylene/l concentrations were similar. Enzyme kinetic analysis applied to 1-MCP effects on ethylene-induced softening of banana fruit suggested that 1-MCP inhibition is by noncompetitive antagonism of ethylene binding.     

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.     

Differential expression of seven alpha-expansin genes during growth and ripening of pear fruit

Seven cDNAs, designated PcExp1 to PcExp7 , encoding expansin homologues, were isolated from mature pear fruit and their expression profiles were investigated in ripening fruit and other tissues, and in response to ethylene. Accumulation of PcExp2 , - 3, - 5 and - 6 mRNA increased markedly with fruit softening and then declined at the over-ripe stage. Treatment of fruit at an early ripening stage with 1-methylcyclopropene (MCP), an inhibitor of ethylene action, suppressed ethylene biosynthesis, fruit softening and the accumulation of the expansin mRNAs. Conversely, propylene treatment at the preclimacteric stage induced accumulation of the same four expansin genes, as well as ethylene production and fruit softening. The expression patterns correlated with alteration in the rate and extent of fruit softening. The abundance of PcExp1 mRNA increased at the late expanding phase of fruit development and further increased during ripening, whereas PcExp4 mRNA levels were constant throughout fruit growth and ripening. The MCP and propylene treatments had little effect on PcExp1 and PcExp4 expression. PcExp7 was expressed in young but not mature fruit. PcExp4 and PcExp6 mRNA was also detected in flowers. The accumulation of PcExp4, -5, -6 and - 7 mRNA was more abundant in young growing tissues, but not in fully expanded tissues, suggesting roles for these genes in cell expansion. These results demonstrate that characteristically, multiple expansin genes show differential expression and hormonal regulation during pear fruit development and at least six expansins show overlapping expression during ripening.     

Expression of multiple forms of polygalacturonase gene during ripening in banana fruit.

The activity of polygalacturonase (PG, E.C 3.2.1.15) during ripening in climacteric fruits has been positively correlated with softening of the fruit tissue and differential expression of its gene is suspected to be regulated by the plant hormone ethylene. We have cloned four partial cDNAs, MAPG1 (acc. no. AF311881), MAPG2 (acc. no. AF311882), MAPG3 (acc. no. AF542382) and MAPG4 (acc. no. AY603341) for PG genes and studied their differential expression during ripening in banana. MAPG3 and MAPG4 are believed to be ripening related and regulated by ethylene whereas MAPG2 is associated more with senescence. MAPG1 shows constitutive expression and is not significantly expressed in fruit tissue. The genomic clone MAGPG (acc. No. AY603340) includes the complete MAPG3 gene, which consists of four exons and three introns. The structure of the gene has more similarity to tomato abscission PG rather than tomato fruit PG. It is concluded that softening during ripening in banana fruit results from the concerted action of at least four PG genes, which are differentially expressed during ripening.     

The use of ethylene-suppressed lines to assess differential sensitivity to ethylene of the various ripening pathways in Cantaloupe melons

Physiological characterization of ethylene-suppressed Cantaloupe Charentais melons ( Cucumis melo var. cantalupensis Naud cv. Védrantais) revealed that some ripening-associated events, like degreening of the rind and cell separation in the peduncular abscission zone, are totally dependent on ethylene. By contrast, some other ripening events, like softening and membrane deterioration, depend only partially on ethylene and display some ethylene-independent components. Application of increasing levels of exogenous ethylene on these antisense 1-aminocyclopropane-1-carboxylic acid oxidase fruits enabled the determination of the gradual sensitivity of various ripening pathways to the hormone. The threshold level of ethylene capable of physiological activity varied from 1 ppm for degreening of the rind to 2.5 ppm for softening, membrane deterioration and cell separation in the peduncular abscission zone. Up to a saturating dose of 5 ppm, the extent of rind degreening was proportionally related to the level of applied ethylene. The saturating levels of ethylene for flesh softening (2.5 ppm) and for membrane deterioration and cell separation in the peduncular abscission zone (5 ppm) were much lower than the internal ethylene found at the climacteric peak of wild-type fruit (over 100 ppm). The cessation of ethylene treatment resulted in a complete arrest of the rind degreening and peduncular cell separation indicating that both ripening pathways are completely dependent on ethylene. On the contrary, softening and membrane deterioration, though significantly slowed upon removal of ethylene treatment, continued to proceed in the absence of the hormone, thereby unmasking the ethylene-independent component of softening and membrane deterioration. The presence of ethylene-independent components in the regulation of individual pathways indicates that the ripening of climacteric fruit involves a substantial portion of non-climacteric regulation.     

Ethylene regulation of fruit softening and cell wall disassembly in Charentais melon

Cell wall disassembly in ripening fruit is highly complex, involving the dismantling of multiple polysaccharide networks by diverse families of wall-modifying proteins. While it has been reported in several species that multiple members of each such family are expressed in the same fruit tissue, it is not clear whether this reflects functional redundancy, with protein isozymes from a single enzyme class performing similar roles and contributing equally to wall degradation, or whether they have discrete functions, with some isoforms playing a predominant role. Experiments reported here sought to distinguish between cell wall-related processes in ripening melon that were softening-associated and softening-independent. Cell wall polysaccharide depolymerization and the expression of wall metabolism-related genes were examined in transgenic melon (Cucumis melo var. cantalupensis Naud.) fruit with suppressed expression of the 1-aminocyclopropane-1-carboxylate oxidase (ACO) gene and fruits treated with ethylene and 1-methylcyclopropene (1-MCP). Softening was completely inhibited in the transgenic fruit but was restored by treatment with exogenous ethylene. Moreover, post-harvest application of 1-MCP after the onset of ripening completely halted subsequent softening, suggesting that melon fruit softening is ethylene-dependent. Size exclusion chromatography of cell wall polysaccharides, from the transgenic fruits, with or without exogenous ethylene, indicated that the depolymerization of both pectins and xyloglucans was also ethylene dependent. However, northern analyses of a diverse range of cell wall-related genes, including those for polygalacturonases, xyloglucan endotransglucosylase/hydrolases, expansin, and beta-galactosidases, identified specific genes within single families that could be categorized as ethylene-dependent, ethylene-independent, or partially ethylene-dependent. These results support the hypothesis that while individual cell wall-modifying proteins from each family contribute to cell wall disassembly that accompanies fruit softening, other closely related family members are regulated in an ethylene-independent manner and apparently do not directly participate in fruit softening.     

Hormonal regulation during plant fruit development

The modern concept of the hormonal regulation of fruit set, growth, maturation, and ripening is considered. Pollination and fertilization induce ovule activation by surmounting the blocking action of ethylene and ABA to be manifested in auxin accumulation. Active fruit growth by pericarp cell division and elongation is due to the syntheses of auxin in the developing seed and of gibberellins in the pericarp. In climacteric fleshy fruits, the maturation is controlled by ethylene via so-called System 1 combining the possibilities of autoinhibition and autocatalysis by ethylene of its own biosynthesis. Transition of tomato fruits from maturation to ripening is characterized by highly active synthesis of ethylene and its receptors due to the functioning of regulatory System 2 resulting in the up-regulation of much greater number of ethylene-inducible genes. In peach fruits, the hormonal regulation of ripening includes also an active auxin involvement in the ethylene biosynthesis, which is combined with the ethylene-induced expression of genes encoding both auxin biosynthesis and the response to auxin. Ethylene induces the expression of genes responsible for the fruit softening, its taste, color, and flavor. Nonclimacteric fleshy fruits produce very small amounts of ethylene; its evolution increases only by the very end of ripening and can be described by a reduced System 1. The ripening of nonclimacteric fruits only weakly depends on ethylene but is stimulated by abscisic acid.     

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.     

Ethylene is required for both the initiation and progression of softening in pear (Pyrus communis L.) fruit

In order to investigate the physiological role of ethylene in the initiation and subsequent progression of softening, pear fruit were treated with propylene, an analogue of ethylene or 1-methylcyclopropene (1-MCP), a gaseous inhibitor of ethylene action at the preclimacteric or ripening stages. The propylene treatment at the pre-ripe stage stimulated ethylene production and flesh softening while the 1-MCP treatment at the same stage markedly retarded the initiation of the ripening-related events. Moreover, 1-MCP treatment after the initiation of ripening markedly suppressed the subsequent flesh softening and ethylene production. These results clearly indicate that ethylene is not merely a by-product, but plays a crucial role in both the initiation and maintenance of regulating the softening process during ripening. The observations also suggest that ethylene in ripening is regulated entirely in an autocatalytic manner. The mRNA accumulation of pear polygalacturonases (PG) genes, PC-PG1 and PC-PG2, was in parallel with the pattern of fruit softening in both propylene and 1-MCP treatments. However, the expression pattern of pear endo-1,4-beta-D-glucanases (EGase) genes, PC-EG1 and PC-EG2, was not affected in both treatments. The results suggest that ethylene is required for PGs expression even in the late ripening stage, but not for EGases.     

1-甲基环丙烯采后处理对樱桃番茄果实成熟过程的影响

研究了不同浓度(0、0.035、0.07和0.11μL/L)的乙烯受体竞争性抑制剂1-甲基环丙烯(1-MCP)采后处理对绿熟期樱桃番茄的乙烯合成、果实软化、果实色素(叶绿素、茄红素、β-胡萝卜素)含量消长的影响.0.07 μL/L及其以上浓度的1-MCP降低了前期乙烯合成,同时推迟了乙烯释放高峰,但0.035 μL/L浓度的1-MCP处理并不能抑制内源乙烯合成.1-MCP显著延迟了果实软化和叶绿素降解,但并不影响这两个过程的启动.茄红素合成的启动和积累均受到了1-MCP抑制,而1-MCP并不推迟β-胡萝卜素合成的启动,只抑制其积累.这些结果提示了乙烯调节成熟生理过程的不同机制.对于绿熟期的樱桃番茄,0.07～0.11μL/L的1-MCP是实用的有效处理浓度.1-MCP有效浓度可能用于了解果实的乙烯受体水平和乙烯敏感性.