Graduate School of Environmental and Life Science, Okayama University, Okayama, 700-8530, Japan.
Meru University of Science and Technology, Meru, Kenya.
BMC Plant Biol. 2018 Mar 21;18(1):47. doi: 10.1186/s12870-018-1264-y.
Kiwifruit 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.
To 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.
These 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.
猕猴桃被归类为呼吸跃变型果实，因为外源乙烯（或其类似物丙烯）在正反馈调节下诱导快速成熟，并伴随着乙烯的产生。然而，猕猴桃在贮藏和挂树期间的大多数与成熟相关的变化（第一阶段成熟）主要发生在没有任何可检测到的乙烯的情况下。这种成熟行为通常归因于系统 I 乙烯的基础水平，尽管有人认为它受到低温的调节。
为了阐明调节猕猴桃第一阶段成熟的机制，我们使用果实连续暴露于丙烯（在 20°C 下）以及在 5°C 和 20°C 下贮藏时进行了比较转录组分析。丙烯处理在 5 天内诱导猕猴桃软化、滴定酸度（TA）降低、可溶固形物含量（SSC）增加和乙烯产生。在贮藏过程中，与 20°C 相比，5°C 下果实软化和 TA 降低更快，尽管未检测到内源乙烯的产生。转录组分析显示 3761 个与成熟相关的差异表达基因（DEGs），其中 2742 个被丙烯上调，1058 个被低温上调。丙烯仅上调了 2112 个 DEGs，包括与乙烯生物合成和成熟相关的基因，如 AcACS1、AcACO2、AcPL1、AcXET1、Acβ-GAL、AcAAT、AcERF6 和 AcNAC7。同样，低温仅上调了 467 个 DEGs，包括 AcACO3、AcPL2、AcPMEi、AcADH、Acβ-AMY2、AcGA2ox2、AcNAC5 和 AcbZIP2 等。相当数量的 DEGs，如 AcPG、AcEXP1、AcXET2、Acβ-AMY1、AcGA2ox1、AcNAC6、AcMADS1 和 AcbZIP1，被丙烯或低温上调。频繁的 1-MCP 处理未能抑制低温引起的加速成熟和相关 DEGs 的上调，表明这些变化与乙烯无关。挂树猕猴桃的成熟过程中没有检测到任何内源乙烯的产生，与低温响应 DEGs 的表达增加以及环境温度的降低同时发生。
这些结果表明，猕猴桃既有依赖乙烯的成熟机制，也有低温调节的成熟机制，两者是不同的且相互独立的。本研究为阐述猕猴桃这两种成熟机制的控制提供了基础。
