College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Plant Physiol Biochem. 2010 Apr;48(4):216-24. doi: 10.1016/j.plaphy.2010.01.015. Epub 2010 Jan 28.
Ascorbic acid (AsA), as a unique antioxidant and enzyme cofactor, has multiple roles in plants. However, there is very limited information on the mechanism of AsA accumulation and controlling in leaves. In this study, we determined AsA accumulation levels, analyzed expression patterns of the genes involved in synthesizing via l-galactose pathway and recycling as well as enzyme activities in apple (Malus domestica Borkh) leaves with different age. AsA content was found to increase with leaf development, reaching the highest level in 20-day-old leaves. This level was maintained in mature leaves until the dropping in senescent leaves. Comparing with young and senescent leaves, mature leaves had higher capability for AsA synthesis with high expression levels and activity of l-galactose dehydrogenase and l-galactono-1,4-lactone dehydrogenase. The mRNA expression of genes involved in AsA synthesis also showed highest abundance in 20-day-old leaves, though GDP-mannose-3',5'-epimerase and l-galactose-1-phosphate phosphatase expression reached the highest levels before 20 days old. These results suggest that AsA accumulation in apple leaves mainly occurs during the transition phase from young to mature leaves with high rates of synthesis and recycling, and that l-galactose-1-phosphate phosphatase could play an important role in regulating AsA biosynthesis via the l-galactose pathway.
抗坏血酸（AsA）作为一种独特的抗氧化剂和酶辅因子，在植物中具有多种功能。然而，关于叶片中 AsA 积累和调控的机制的信息非常有限。在本研究中，我们测定了不同年龄苹果（Malus domestica Borkh）叶片中 AsA 积累水平，分析了通过 l-半乳糖途径合成和循环以及酶活性相关基因的表达模式。结果发现，AsA 含量随叶片发育而增加，在 20 日龄叶片中达到最高水平。在成熟叶片中，这一水平保持到衰老叶片脱落。与幼叶和衰老叶相比，成熟叶片具有更高的 AsA 合成能力，l-半乳糖脱氢酶和 l-半乳糖酸-1,4-内酯脱氢酶的表达水平和活性较高。参与 AsA 合成的基因的 mRNA 表达在 20 日龄叶片中也表现出最高丰度，尽管 GDP-甘露糖-3',5'-差向异构酶和 l-半乳糖-1-磷酸磷酸酶的表达在 20 日龄之前达到最高水平。这些结果表明，苹果叶片中 AsA 的积累主要发生在幼叶向成熟叶过渡的阶段，此时合成和循环的速度较高，l-半乳糖-1-磷酸磷酸酶可能通过 l-半乳糖途径在调节 AsA 生物合成中发挥重要作用。
