Mitalo Oscar W, Asiche William O, Kang Seung W, Ezura Hiroshi, Akagi Takashi, Kubo Yasutaka, Ushijima Koichiro
Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan.
Graduate School of Environmental and Life Science, Okayama University, Okayama, Japan.
Front Plant Sci. 2022 Jul 13;13:918226. doi: 10.3389/fpls.2022.918226. eCollection 2022.
Peel degreening is the most conspicuous aspect of fruit ripening in many citrus fruits because of its importance for marketability. In this study, peel degreening in response to propylene (an ethylene analog) and at varying storage temperatures was characterized in Satsuma mandarin ( Marc.) fruit. Propylene treatment triggered rapid peel degreening (within 4-6 days), indicated by an increase in the citrus color index (CCI) and chlorophyll loss. Peel degreening was also observed in fruit at 10°C and 15°C after 28-42 days, with gradual CCI increase and chlorophyll reduction. However, fruit at 5°C, 20°C, and 25°C remained green, and no substantial changes in peel CCI and chlorophyll content were recorded during the 42-day storage duration. The transcriptomes of peels of fruit treated with propylene for 4 days and those stored at varying temperatures for 28 days were then analyzed by RNA-Seq. We identified three categories of differentially expressed genes that were regulated by (i) propylene (and by analogy, ethylene) alone, (ii) low temperature (5°C, 10°C, or 15°C vs. 25°C) alone, and (iii) either propylene or low temperature. Gene-encoding proteins associated with chlorophyll degradation (such as , and ) and a transcription factor () were differentially expressed by propylene or low temperature. To further examine temperature-induced pathways, we also monitored gene expression during on-tree fruit maturation vs. postharvest. The onset of on-tree peel degreening coincided with autumnal drops in field temperatures, and it was accompanied by differential expression of low temperature-regulated genes. On the contrary, genes that were exclusively regulated by propylene (such as and ) displayed insignificant expression changes during on-tree peel degreening. These findings indicate that low temperatures could be involved in the fruit ripening-related peel degreening independently of ethylene.
果皮褪绿是许多柑橘类水果成熟过程中最显著的特征，因为它对果实的市场适销性很重要。在本研究中，对温州蜜柑（Marc.）果实响应丙烯（一种乙烯类似物）和不同贮藏温度时的果皮褪绿情况进行了表征。丙烯处理引发了快速的果皮褪绿（在4 - 6天内），这通过柑橘颜色指数（CCI）的增加和叶绿素损失来表明。在10°C和15°C下贮藏28 - 42天的果实中也观察到了果皮褪绿，伴随着CCI的逐渐增加和叶绿素的减少。然而，在5°C、20°C和25°C下贮藏的果实保持绿色，在42天的贮藏期内果皮CCI和叶绿素含量没有显著变化。然后通过RNA测序分析了用丙烯处理4天的果实以及在不同温度下贮藏28天的果实的果皮转录组。我们鉴定出了三类差异表达基因，它们分别受（i）单独的丙烯（类推为乙烯）、（ii）单独的低温（5°C、10°C或15°C与25°C相比）以及（iii）丙烯或低温调控。与叶绿素降解相关的基因编码蛋白（如 、 和 ）以及一个转录因子（ ）受丙烯或低温差异表达调控。为了进一步研究温度诱导的途径，我们还监测了树上果实成熟与采后果实成熟过程中的基因表达。树上果皮褪绿的开始与田间温度的秋季下降同时发生，并且伴随着低温调控基因的差异表达。相反，仅受丙烯调控的基因（如 和 ）在树上果皮褪绿过程中表达变化不显著。这些发现表明，低温可能独立于乙烯参与果实成熟相关的果皮褪绿过程。
