Radzio Jessica A, Lorence Argelia, Chevone Boris I, Nessler Craig L
Department of Plant Pathology, Physiology and Weed Science, Virginia Polytechnic Institute and State University, 413 Price Hall, Blacksburg, VA 24061-0331, USA.
Plant Mol Biol. 2003 Dec;53(6):837-44. doi: 10.1023/B:PLAN.0000023671.99451.1d.
Vitamin C (L-ascorbic acid) has important antioxidant and metabolic functions in both plants and animals, humans have lost the ability to synthesize it. Fresh produce is the major source of vitamin C in the human diet yet only limited information is available concerning its route(s) of synthesis in plants. In contrast, the animal vitamin C biosynthetic pathway has been elucidated since the 1960s. Two biosynthetic pathways for vitamin C in plants are presently known. The D-mannose pathway appears to be predominant in leaf tissue, but a D-galacturonic acid pathway operates in developing fruits. Our group has previously shown that transforming lettuce and tobacco with a cDNA encoding the terminal enzyme of the animal pathway, L-gulono-1,4-lactone oxidase (GLOase, EC 1.1.3.8), increased the vitamin C leaf content between 4- and 7-fold. Additionally, we found that wild-type (wt) tobacco plants had elevated vitamin C levels when fed L-gulono-1,4-lactone, the animal precursor. These data suggest that at least part of the animal pathway may be present in plants. To further investigate this possibility, wild-type and vitamin-C-deficient Arabidopsis thaliana (L.) Heynh (vtc) plants were transformed with a 35S: GLOase construct, homozygous lines were developed, and vitamin C levels were compared to those in untransformed controls. Wild-type plants transformed with the construct showed up to a 2-fold increase in vitamin C leaf content compared to controls. All five vtc mutant lines expressing GLOase had a rescued vitamin C leaf content equal or higher (up to 3-fold) than wt leaves. These data and the current knowledge about the identity of genes mutated in the vtc lines suggest that an alternative pathway is present in plants, which can bypass the deficiency of GDP-mannose production of the vtc1-1 mutant and possibly circumvent other steps in the D-mannose pathway to synthesize vitamin C.
维生素C（L-抗坏血酸）在植物和动物体内都具有重要的抗氧化和代谢功能，但人类已丧失合成它的能力。新鲜农产品是人类饮食中维生素C的主要来源，然而关于其在植物中的合成途径的信息却很有限。相比之下，自20世纪60年代以来，动物体内维生素C的生物合成途径已被阐明。目前已知植物中维生素C有两条生物合成途径。D-甘露糖途径似乎在叶片组织中占主导地位，但D-半乳糖醛酸途径在发育中的果实中起作用。我们的研究小组之前已经表明，用编码动物途径末端酶L-古洛糖酸-1,4-内酯氧化酶（GLOase，EC 1.1.3.8）的cDNA转化生菜和烟草，可使叶片中维生素C的含量增加4至7倍。此外，我们发现野生型烟草植株在饲喂动物前体L-古洛糖酸-1,4-内酯时，其维生素C水平会升高。这些数据表明植物中可能存在动物途径的至少一部分。为了进一步研究这种可能性，用35S:GLOase构建体转化野生型和维生素C缺陷型拟南芥（L.）Heynh（vtc）植株，培育出纯合系，并将维生素C水平与未转化的对照植株进行比较。用该构建体转化的野生型植株与对照相比，叶片中维生素C含量最多增加了2倍。所有五个表达GLOase的vtc突变系的叶片维生素C含量都得到了挽救，与野生型叶片相当或更高（最多3倍）。这些数据以及目前关于vtc系中突变基因身份的知识表明，植物中存在一条替代途径，该途径可以绕过vtc1-1突变体中GDP-甘露糖生产的缺陷，并可能绕过D-甘露糖途径中的其他步骤来合成维生素C。
