Fruit ripening in response to propylene (an ethylene analog), 1-methylcyclopropene (1-MCP, an ethylene action inhibitor), and low temperature (5°C) treatments was characterized in “Kosui” kiwifruit ( Actinidia rufa × A. chinensis ). Propylene treatment induced ethylene production, with increased expression levels of 1-aminocyclopropane-1-carboxylic acid ( ACC ) synthase 1 ( AcACS1 ) and ACC oxidase 2 ( AcACO2 ), and rapid fruit softening together with increased expression levels of polygalacturonase ( AcPG ) and expansin 1 ( AcEXP1 ) within 5 days (d). Fruit soluble solids concentration (SSC) and contents of sucrose, glucose, and fructose together with the expression levels of β-amylase 1 ( Acβ-AMY1 ), Acβ-AMY2 , and invertase ( AcINV3-1 ) increased rapidly after 5 d exposure to propylene. Furthermore, propylene exposure for 5 d was sufficient to induce the production of key aroma volatile compounds, ethyl- and methyl butanoate, accompanied with increased expression levels of alcohol acyl transferase ( AcAAT ). Application of 1-MCP at the start of the experiment, followed by continuous exposure to propylene, significantly delayed fruit softening, changes in SSC and sugars, and strongly suppressed the production of ethylene, aroma volatiles, and expression of associated genes. During storage, fruit softening, SSC and sugar increase, and increased expression of genes associated with cell wall modification and carbohydrate metabolism were registered without detectable ethylene production; however, these changes occurred faster at 5°C compared to 22°C. Interestingly, ethyl and methyl butanoate as well as AcAAT expression were undetectable in kiwifruit during storage, while they were rescued by post-storage propylene exposure, indicating that the production of aroma volatile compounds is strongly ethylene-dependent. Transcript levels of a NAC-related transcription factor (TF), AcNAC3 , increased in response to both propylene and low temperature treatments, while AcNAC5 was exclusively up-regulated by propylene. By contrast, transcript levels of a MADS-box TF, AcMADS2 , exclusively increased in response to low temperature. The above findings indicate that kiwifruit ripening is inducible by either ethylene or low temperature signals. However, fruit ripened by low temperature were deficient in ethylene-dependent aroma volatiles, suggesting that ethylene signaling is non-functional during low temperature-modulated ripening in kiwifruit. These data provide further evidence that ethylene-dependent a...

BackgroundKiwifruit are classified as climacteric since exogenous ethylene (or its analogue propylene) induces rapid ripening accompanied by ethylene production under positive feedback regulation. However, most of the ripening–associated changes (Phase 1 ripening) in kiwifruit during storage and on–vine occur largely in the absence of any detectable ethylene. This ripening behavior is often attributed to basal levels of system I ethylene, although it is suggested to be modulated by low temperature.ResultsTo elucidate the mechanisms regulating Phase 1 ripening in kiwifruit, a comparative transcriptome analysis using fruit continuously exposed to propylene (at 20 °C), and during storage at 5 °C and 20 °C was conducted. Propylene exposure induced kiwifruit softening, reduction of titratable acidity (TA), increase in soluble solids content (SSC) and ethylene production within 5 days. During storage, softening and reduction of TA occurred faster in fruit at 5 °C compared to 20 °C although no endogenous ethylene production was detected. Transcriptome analysis revealed 3761 ripening–related differentially expressed genes (DEGs), of which 2742 were up–regulated by propylene while 1058 were up–regulated by low temperature. Propylene exclusively up–regulated 2112 DEGs including those associated with ethylene biosynthesis and ripening such as AcACS1, AcACO2, AcPL1, AcXET1, Acβ–GAL, AcAAT, AcERF6 and AcNAC7. Similarly, low temperature exclusively up–regulated 467 DEGS including AcACO3, AcPL2, AcPMEi, AcADH, Acβ–AMY2, AcGA2ox2, AcNAC5 and AcbZIP2 among others. A considerable number of DEGs such as AcPG, AcEXP1, AcXET2, Acβ–AMY1, AcGA2ox1, AcNAC6, AcMADS1 and AcbZIP1 were up–regulated by either propylene or low temperature. Frequent 1–MCP treatments failed to inhibit the accelerated ripening and up–regulation of associated DEGs by low temperature indicating that the changes were independent of ethylene. On–vine kiwifruit ripening proceeded in the absence of any detectable endogenous ethylene production, and coincided with increased expression of low temperature–responsive DEGs as well as the decrease in environmental temperature.ConclusionsThese results indicate that kiwifruit possess both ethylene−dependent and low temperature–modulated ripening mechanisms that are distinct and independent of each other. The current work provides a foundation for elaborating the control of these two ripening mechanisms in kiwifruit.Electronic supplementary materialThe online version of this article (10.1186/s12870-018-1264-y) contains supplementary material, which is available to authorized users.

The responses of three kiwifruit cultivars, Actinidia chinensis 'Sanuki Gold', A. chinensis 'Rainbow Red', and A. deliciosa 'Hayward' to various storage temperatures (0, 5, 10, 15, and 20°C) for 8 weeks were investigated. The rate of fruit which initiated ethylene production due to rot development increased with increases in storage temperature. Early-maturing cultivars, 'Rainbow Red' and 'Sanuki Gold' fruit stored at 5, 10, and 15°C showed drastic softening and a decrease in titratable acidity (TA) to an edible level within 4 weeks without detectable ethylene production, whereas fruit stored at 0 and 20°C maintained high firmness and TA even after 8 weeks unless they were infected with rot. A late-maturing cultivar, 'Hayward' fruit stored at 5 and 10°C softened more rapidly than when stored at 0, 15, or 20°C. Treatment with 1-Methylcyclopropene (1-MCP) did not suppress the low temperature modulated fruit ripening in any cultivars, indicating its independence from ethylene. These results suggest that 'Sanuki Gold' and 'Rainbow Red' are more sensitive to low temperatures compared to 'Hayward' and the sensitivity is involved in the determination of storage life and how early the fruit matures on the vine.

Peel degreening is an important aspect of fruit ripening in many citrus fruit, and previous studies have shown that it can be advanced by ethylene treatment or by low-temperature storage. However, the important regulators and pathways involved in natural peel degreening remain largely unknown. To determine how natural peel degreening is regulated in lemon fruit (Citrus limon), we studied transcriptome and physiochemical changes in the flavedo in response to ethylene treatment and low temperatures. Treatment with ethylene induced rapid peel degreening, which was strongly inhibited by the ethylene antagonist, 1-methylcyclopropene (1-MCP). Compared with 25 ºC, moderately low storage temperatures of 5–20 °C also triggered peel degreening. Surprisingly, repeated 1-MCP treatments failed to inhibit the peel degreening induced by low temperature. Transcriptome analysis revealed that low temperature and ethylene independently regulated genes associated with chlorophyll degradation, carotenoid metabolism, photosystem proteins, phytohormone biosynthesis and signalling, and transcription factors. Peel degreening of fruit on trees occurred in association with drops in ambient temperature, and it coincided with the differential expression of low temperature-regulated genes. In contrast, genes that were uniquely regulated by ethylene showed no significant expression changes during on-tree peel degreening. Based on these findings, we hypothesize that low temperature plays a prominent role in regulating natural peel degreening independently of ethylene in citrus fruit.

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In order to extend the "eating window", the optimum ripening phase suitable for eating, the combination of treatment with propylene (an ethylene analog) and 1-methylcyclopropene (1-MCP; an ethylene inhibitor) was assessed in three kiwifruit cultivars: 'Rainbow Red' Actinidia chinensis, 'Sanuki Gold' A. chinensis, and 'Hayward' A. deliciosa. Propylene treatment initiated the ripening process by inducing fruit softening, increasing soluble solid content (SSC), and decreasing titratable acids (TA), with or without endogenous ethylene production, depending on the duration of exposure. Once endogenous ethylene was induced, it accelerated fruit ripening, resulting in an over-ripening phase and shortening of the "eating window". 'Rainbow Red' and 'Sanuki Gold' fruit treated with propylene continuously or for 48 h initiated endogenous ethylene production that led to an "eating window" lasting only 2 days (range of 3-5 days after the start of treatment), whereas it lasted for 7 days (range 3-10 days) in 'Hayward' fruit. Limited propylene treatment of the three cultivars for 24 h induced ripening without the detection of ethylene production, suggesting that the optimum ripening phase suitable for eating can be attained without endogenous ethylene production, resulting in a longer "eating window". 'Rainbow Red' and 'Sanuki Gold' fruit treated with propylene for 48 h followed by 1-MCP treatment had extended "eating window" and shelf-life, with the suppression of endogenous ethylene. These results illustrate the practicability of different durations of propylene treatment in facilitating kiwifruit ripening and the additional benefit of 1-MCP treatment to the extend shelf-life of new high-quality kiwifruit cultivars, 'Rainbow Red' and 'Sanuki Gold'.

'Rainbow Red' kiwifruit have been reported to gradually ripen during low temperature storage and on the vine in the absence of detectable ethylene. This study was conducted to compare the expression of ripeningrelated genes during storage at different temperatures and on the vine. Fruit at 5°C and 10°C ripened faster to eating quality within four weeks accompanied with increased expression of ripening-related genes: AcACO3, AcXET2, AcEXP1, AcPG, AcPMEi, AcSUS, AcβAMY1, AcβAMY2, AcGA2ox2, AcNAC3, AcNAC4, and AcMADS2. Fruit at 15°C required a longer period of eight weeks to attain eating quality in concurrence with delayed accumulation of the ripening-related genes. Fruit at 22°C ripened at the slowest rate and did not attain eating quality even after eight weeks, with very minimal accumulation of ripening-related genes. Onvine ripening occurred slowly at the early stages when the average field temperature was ~20°C, but the rate increased as the temperature dropped to ≤15°C accompanied by increased expression of ripening-related genes. These results indicate that both ripening on-vine and during low temperature storage are modulated by low temperature independent of ethylene.