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標題: 低溫處理對不同時期冰花葉片抗氧化機制的影響
The effects of low temperature on antioxidative mechanisms in ice plant (Mesembryanthemum crystallinum L.) leaves at different growth stages
作者: 黃文伶
Wen-Ling Huang
關鍵字: 低溫逆境;抗氧化;活性氧;冰花;antioxidant;ROS;low temperature;ice plant
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冰花(Mesembryanthemum crystallinem L.)為原生於南非沙漠的耐旱耐鹽植物,具有C3-CAM (Crassulacean Acid Metabolism)光合作用轉變機制,來提高水分利用效率,並且具有滲透調節、區隔鈉離子以及許多相關機制來適應缺水逆境,但是目前冰花對於低溫的適應性並無文獻報導,無法得知會使冰花產生低溫逆境的溫度。低溫逆境會誘導氧化逆境,當ROS (reactive oxygen species)不能夠正常代謝或過量生產造成ROS過度累積時,即造成氧化逆境。避免氧化逆境對植物造成傷害,負責代謝ROS的抗氧化系統包含酵素型及非酵素型機制,ascorbate-glutathione cycle在其中扮演重要的角色。本論文利用市售蔬果常用的4˚C及18˚C進行低溫處理,觀察低溫以及低溫所誘導的氧化逆境對不同時期冰花的影響。低溫處理下植株會有延緩生長的情形,也會抑制開花,顯示低溫會影響冰花的生長發育。低溫會誘導氧化逆境,因此追蹤葉片中ROS的累積發現primary leaves及secondary leaves在18˚C累積較多的超氧陰離子,4˚C則累積較少,而4˚C處理下primary leaves累積較多的過氧化氫,而secondary leaves累積較少,發現低溫確實影響ROS的累積。測定清除ROS的抗氧化酵素活性顯示,幼年期的植株在進行低溫處理後APX (ascorbate peroxidase)、DHAR (dehydroascorbate reductase)、MDHAR (monodehydroascorbate reductase)及GR (glutathione reductse)的酵素活性皆有降低的趨勢,成熟期的植株在進行低溫處理後APX、DHAR及MDHAR有上升的趨勢,開花期的植株在進行低溫處理後APX及GR的活性有降低的趨勢。針對APX基因群進行進一步的表現分析,發現位於膜系的APX與低溫處理有趨勢性的相關,在primary leaves中APX3顯示溫度越低且時間越長則表現量較低,在secondary leaves中發現APX5在4˚C處理下會大量的表現。而冰花為可食用性的多肉植物,富含一般蔬菜中少有的特殊營養物質,如糖醇類中的pinitol,因此檢測植株中糖類及糖醇類的含量在低溫處理下的變化,發現在植物體內糖類主要運輸形式的蔗糖,在低溫處理下含量會下降;而果糖及糖醇類pinitol和myo-inositol在長期低溫處理下,在secondary leaves中含量會上升。以切離葉進行低溫處理觀察葉片鮮重變化情形,發現在8天4˚C處理下secondary leaves水分散失較少,然而將全株進行低溫處理後採取葉片進行電解質滲漏率測量,則顯示不受低溫影響。綜合以上結果發現,低溫造成冰花有延緩生長的情形,在4˚C下甚至有停止生長的情形,因此認為4˚C會造成冰花低溫逆境,雖然低溫可降低切離葉葉片水分散失率。低溫誘導氧化逆境對於不同時期的冰花有不同的ROS累積情形、抗氧化酵素活性反應、特定APX基因表現以及糖類的累積。經由此研究開始了解冰花對低溫逆境的生理反應,期望將來從中找出關鍵影響因子,做為後續改良低溫保存冰花切離葉的依據。

Ice plant (Mesembryanthemum crystallinum L.) is a halophyte native to the desert in South Africa. Ice plant has high water use efficiency by the shift from C3 photosynthesis to Crassulacean acid metabolism (CAM) and adapts water-deficit environment by osmotic adjustment, Na+ sequestration, and other related mechanisms. However, up to date, we have little knowledge of how low temperature affects the growth of ice plant, and there is no report discussing the adaptation to chilling stress of ice plant. Cold stress causes oxidative stress from accelerated production or abnormal metabolism of ROS (reactive oxygen species). ROS scavenging including enzymatic and non-enzymatic systems are responsible for avoiding the damage of oxidative stress to plants. Among them, the ascorbate-glutathione cycle plays a vital role. We chose 4˚C and 18˚C to study the effects of low temperature and oxidative stress in ice plant at different growth stages. Low temperature retarded vegetative growth and inhibited flowering suggesting that cold affcted the growth and development of ice plant. Cold stress induces oxidative stress. There was more superoxide anion in primary and secondary leaves at 18˚C and less at 4˚C. Primary leaves accumulated more H2O2 at 4˚C while secondary leaves accumulated less. These results showed that low temperature affected the accumulation of ROS. The results of measuring antioxidative enzyme activities showed that the activities of APX (ascorbate peroxidase), DHAR (dehydroascorbate reductase), MDHAR (monodehydroascorbate reductase), and GR (glutathione reductase) in juvenile plants decreased after 8-day low temperature treatments. In adult plants, the activities of APX, DHAR, and MDHAR increased and there was a trend of decreasing of APX and GR activities in flowering plants after low temperature treatments. Further analysis of APXs gene expression revealed that microsomal APXs were related to cold. The expression of APX3 in primary leaves showed the lower the temperature and the longer the chilling time, the lower the gene expressed. The expression of APX5 showed significant higher expression at 4˚C in secondary leaves than other temperatures. Ice plant is an edible succulent containing rich special nutrition such as pinitol, a derivative of sugar alcohol. The soluble sugars and sugar alcohols contents detected by high performance liquid chromatography showed that the content of sucrose, as a major transport form of sugars in plants, decreased in low temperature. The contents of fructose, pinitol, and myo-inositol increased in long-term low temperature treated secondary leaves. Cut leaves of ice plant were treated with 4˚C and 18˚C to monitor the loss of water content throughout a week. The 8-day 4˚C treated secondary leaves had the least water loss. However, low temperature has no effect on electrolyte leakage of leaves collected from whole plant treated with low temperature. In conclusion, low temperature delays the growth and development of ice plant, and 4˚C treatment even stops growing of the plant. I suspected that 4˚C is the temperature that causes cold stress to ice plant, although it could lower the water loss of cut leaves. Low temperature induces oxidative stress as shown by different ROS accumulation, antioxidative enzyme activities, specific APX gene expression, and sugar accumulation at different stages ice plant. Through this study, the physiological responses of ice plant to low temperature are starting to be uncovered. The goal is to identify the key factors affecting low temperature responses of ice plant and will be the basis for improvement of the storage in low temperature of ice plant cut leaves.
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