高等植物维生素C和维生素E代谢调控

维生素C和维生素E是植物自身合成的抗氧化剂,对植物发育具有重要调控作用。本文对近年来高等植物维生素C和维生素E合成途径、代谢调控、关键酶基因的克隆和转化进行了论述,分析两种维生素之间的相互作用,对该领域未来的研究方向进行了展望。     

高等植物中维生素C的功能、合成及代谢研究进展

植物体内合成的维生素C在植物抗氧化和自由基清除、光合作用和光保护、细胞生长和分裂以及一些重要次生代谢物和乙烯的合成等方面具有非常重要的生理功能.维生素C的生物合成途径及其代谢调控的基因工程研究最近取得了突破.     

辣椒维生素C生物合成及代谢研究进展

维生素C(抗坏血酸,AsA)是高等动物和少数生物必须的营养素,在动物和植物体内都有重要的生理功能。随着研究发现,AsA对抗氧化胁迫的病症具有良好的防护作用。这也正是人们关心和研究植物合成和积累AsA的主要原因。近年来,对其合成、代谢、功能、调控、利用等方面研究也日益广泛和深入。本研究就辣椒维生素C合成途径、代谢途径和相关酶的研究进行介绍,并进一步对其研究方向进行展望,从而较全面地概述维生素C生理代谢,有助于进一步了解维生素C的作用机理,为以后的研究提供参考。     

植物果实中维生素C合成代谢及其基因表达研究

维生素C(Vitamin C),又名抗坏血酸（Ascorbate acid,AsA）,是生物体中具有多效功能的必需化合物。整理了植物果实中维生素C的生理功能以及其中参与维生素C合成的L-半乳糖途径、D-半乳糖醛酸途径、L-古洛糖途径、肌醇途径和维生素C循环途径,并分析了相关基因的功能和表达。对参与维生素C合成与循环的16个基因：PMI、PMM、GMP、GME、GGP、GPP、GalDH、GLDH、GalUR、MIOX、GuLDH、MDHAR、DHAR、AO、APX、GR,及其在维生素C代谢途径中的作用进行了综述。植物果实中维生素C合成代谢途径多样,关键基因的表达会在不同程度上影响其合成代谢,了解植物果实中维生素C合成代谢及相关基因的功能和表达,可为生产富含维生素C的转基因水果和蔬菜奠定基础。     

植物维生素E合成及其生物技术改良

维生素E是一种抗氧化剂 ,对植物、动物和人类自身都具有十分重要的作用 ,而植物则是人类维生素E的主要来源 ,因此克隆植物中维生素E合成的相关酶基因 ,对维生素E含量进行改良 ,具有重要意义。对植物中维生素E的合成途径 ,相关酶基因的克隆以及用生物技术的方法对维生素E含量进行遗传改良进行了综述。     

植物维生素生物强化进展

维生素是动植物生长发育所必需的微量营养素,包括A、B族(B1、B2、B3、B5、B6、B7、B9、B12)、C、D、E和K等。这些物质由于人体内不能合成或合成量不足,所以虽然需要量很少,但必须从膳食特别是植物性食品中获得。因此,利用生物强化技术来提高植物合成的维生素含量可以有效地应对全球性维生素缺乏的问题,对人类的生存与健康具有重要意义。综述了近年来国内外植物中维生素代谢和生物强化的主要研究成果,并对利用分子设计育种进行维生素生物强化的未来发展方向进行了展望。     

拟南芥和作物中维生素C 生物合成与代谢研究进展

维生素C(vitamin C, Vc)是动植物体内含量较为丰富且发挥着重要功能的小分子物质。该文综述了近年来以模式植物拟南芥为实验材料研究Vc生物合成和代谢取得的进展, 并对作物中类似的研究进行了概述。总结的信息对于在作物中进一步 开展Vc合成与代谢研究并通过分子育种提高作物的抗逆性和营养价值具有参考意义。     

植物维生素E生物强化研究进展

维生素E是一种只能在光合组织中合成的脂溶性小分子有机化合物,是人体和动物营养不可缺少的重要维生素。由于植物中维生素E含量较低,人类大多处于慢性缺乏维生素E--“隐性饥饿”的状态,而动物饲料中则需要添加外源合成的维生素E以满足其营养需求。因此,提高植物中维生素E的含量是改善维生素E缺乏的重要途径之一。从维生素E的合成途径入手,详细地综述了维生素E合成关键酶基因的表达变化以及前体物质的含量变化对维生素E合成的影响,发现三烯生育酚和α-生育酚的生物强化效果较好,而生育酚总量提高受限;进而从遗传的角度探讨了维生素E合成受限的原因以及遗传上可能影响维生素E合成的其他代谢途径;最后结合可能影响维生素E合成的调控因子以及其前体物质的转运等方面为今后维生素E的生物强化提出了新的思路。     

利用基因工程提高植物维生素E营养品质的策略

维生素E是一种对植物、动物和人类都具有重要作用的脂溶性维生素。而植物则是人类维生素E的主要来源。对维生素E的结构和合成途径进行了简单的介绍,并重点综述了利用基因工程提高植物维生素E营养品质的策略。这些策略主要包括导入编码影响维生素E总量相关酶的基因来提高维生素E的总量;导入编码影响维生素E组成相关酶的基因,提高α-生育酚在总生育酚中所占的比例,从而提高维生素E的活性。     

动物维生素C的合成能力及其影响因素

维生素C是动物体内不可或缺的微量元素之一,对动物生长发育、代谢及维持正常生理机能具有重要作用。综述了近年来对动物维生素C合成能力及其影响因素的研究进展,为人们了解动物维生素C营养需求提供参考资料。     

L-Ascorbic acid is accumulated in source leaf phloem and transported to sink tissues in plants

L-Ascorbic acid (AsA) was found to be loaded into phloem of source leaves and transported to sink tissues. When L-[(14)C]AsA was applied to leaves of intact plants of three different species, autoradiographs and HPLC analysis demonstrated that AsA was accumulated into phloem and transported to root tips, shoots, and floral organs, but not to mature leaves. AsA was also directly detected in Arabidopsis sieve tube sap collected from an English green aphid (Sitobion avenae) stylet. Feeding a single leaf of intact Arabidopsis or Medicago sativa with 10 or 20 mM L-galactono-1,4-lactone (GAL-L), the immediate precursor of AsA, lead to a 7- to 8-fold increase in AsA in the treated leaf and a 2- to 3-fold increase of AsA in untreated sink tissues of the same plant. The amount of AsA produced in treated leaves and accumulated in sink tissues was proportional to the amount of GAL-L applied. Studies of the ability of organs to produce AsA from GAL-L showed mature leaves have a 3- to 10-fold higher biosynthetic capacity and much lower AsA turnover rate than sink tissues. The results indicate AsA transporters reside in the phloem, and that AsA translocation is likely required to meet AsA demands of rapidly growing non-photosynthetic tissues. This study also demonstrates that source leaf AsA biosynthesis is limited by substrate availability rather than biosynthetic capacity, and sink AsA levels may be limited to some extent by source production. Phloem translocation of AsA may be one factor regulating sink development because AsA is critical to cell division/growth.     

Senescence-induced increases in intracellular oxidative stress and enhancement of the need for ascorbic acid in human fibroblasts

Many studies have suggested that there is a close correlation among declines in internal ascorbic acid (AsA) levels, various disorders, and senescence. To clarify the relationships between age-associated changes in intracellular AsA levels and the effects of AsA administration on intracellular reactive oxygen species (ROS) levels, we investigated aging-related changes in AsA uptake, ROS levels, and the effects of AsA administration on intracellular ROS levels in young and old (senescent) human fibroblasts. Our results demonstrated that AsA uptake was increased in old cells compared with young cells, although mRNA and protein expression of sodium-dependent vitamin C transporter 2 was barely altered between the young and old cells. We also demonstrated that the intracellular superoxide anion level was higher in young cells, whereas the level of intracellular peroxides was significantly increased in old cells under both normal and oxidative stress conditions. Moreover, AsA administration markedly decreased the augmentation of intracellular peroxides in old cells, whereas there was no effect of AsA treatment in young cells under both normal and oxidative stress conditions. Therefore, our results also indicate that AsA could play an important role in regulating the intracellular ROS levels in senescent cells and that the need for AsA is enhanced by cellular senescence.     

Recent advances in the role and biosynthesis of ascorbic acid in plants

The past few years have provided many advances in the role and biosynthesis of L -ascorbic acid (AsA) in plants. There is an increasing body of evidence confirming that AsA plays an important role in the detoxification of reactive oxygen species. The role of AsA in photoprotection has been confirmed in vivo with the use of Arabidopsis mutants. A player in the defence against reactive oxygen species, AsA peroxidase, has been extensively studied at the molecular level, and regulation of this key enzymatic activity appears to occur at several levels. As a cofactor in the hydroxylation of prolyl and lysl-residues by peptidyl-prolyl and -lysyl hydroxylases, AsA plays a part in cell wall synthesis, defence, and possibly cell division. The maintenance of reduced levels of AsA appears to be highly regulated, involving the interplay of both monodehydroascorbate and dehydroascorbate reductases and possibly auxin. A major breakthrough in plant AsA biosynthesis has been made recently, and strong biochemical and genetic evidence suggest that GDP-mannose and L -galactose are key substrates. In addition, evidence for an alternative AsA biosynthetic pathway(s) exists and awaits additional scrutiny. Finally, newly described Arabidopsis mutants deficient in AsA will further increase our understanding of AsA biosynthesis     

Biosynthesis and Catabolism of L-Ascorbic Acid in Plants

L-Ascorbic acid (AsA) is a vital antioxidant compound that plays a critical role in the cellular metabolism of plants and animals. Research on plant AsA metabolism experienced a significant resurgence after 1998 following the identification of AsA-deficient Arabidopsis mutants and the elucidation of a biosynthetic pathway accepted by the overwhelming majority of the plant science community. The identification and cloning of novel biosynthetic genes and the ensuing metabolic engineering of plant AsA content has however revealed a more complex picture. Additional biosynthetic routes have been identified and unexpected biochemical phenotypes were observed upon expression of animal AsA biosynthetic genes. The isolation of novel AsA conjugates from plant tissues and the evidence for long distance transport of AsA in plants have provided additional facets to its functionality. Although some progress has been made regarding the impact of AsA recycling on pool size, we still do not have a clear picture of the biochemistry of AsA degradation. This communication comprehensively reviews new developments in the AsA metabolic system and prompts directions for future research.     

Effects of L-ascorbic acid on lysyl oxidase in the formation of collagen cross-links

To clarify the role of L-ascorbic acid (AsA) in the formation of pyridinoline, we examined the effects of AsA in vitro using soluble collagen and partially purified lysyl oxidase from bovine aorta. The concentration of dehydrodihydroxylysinonorleucine decreased when AsA was added in the early stage of pyridinoline formation. However, when AsA was added in a later stage of pyridinoline formation, the concentration of pyridinoline was not affected. These findings indicated that AsA was involved in the initial enzymatic reaction in pyridinoline synthesis. We purified lysyl oxidase to confirm its association of AsA. AsA inhibited the enzyme activity. Erythorbic acid and 3,4-dihydroxybenzoate suppressed the enzyme activity as well as AsA did. The inhibition by AsA of the lysyl oxidase activity arose from characteristics of AsA structure. AsA might be important in the regulation of the oxidative reaction of lysine.     

Gene expression of ascorbic acid-related enzymes in tobacco

GDP-D-mannose pyrophosphorylase (GMPase) and L-galactono-1, 4-lactone dehydrogenase (GalLDH) are key enzymes in L-ascorbic acid (AsA) biosynthesis of plants, and a full-length cDNA for GMPase was isolated from tobacco using PCR. Additionally, expression of GMPase, GalLDH and other AsA-related enzymes was examined in tobacco tissues and cultured BY-2 cells, and the relationship between their expression patterns and AsA content is discussed. It was found that the expression of GalLDH and GMPase mRNAs was markedly suppressed by loading AsA, suggesting that AsA concentration in the cells may regulate AsA biosynthesis. Moreover, the expression of GMPase and GalLDH mRNAs in tobacco leaf also suggested that AsA biosynthesis may be induced by light.     

Metabolic engineering in food crops to enhance ascorbic acid production: crop biofortification perspectives for human health

Ascorbic acid (AsA) also known as vitamin C is considered as an essential micronutrient in the diet of humans. The human body is unable to synthesize AsA, thus solely dependent on exogenous sources to accomplish the nutritional requirement. AsA plays a crucial role in different physiological aspects of human health like bone formation, iron absorption, maintenance and development of connective tissues, conversion of cholesterol to bile acid and production of serotonin. It carries antioxidant properties and is involved in curing various clinical disorders such as scurvy, viral infection, neurodegenerative diseases, cardiovascular diseases, anemia, and diabetes. It also plays a significant role in COVID-19 prevention and recovery by improving the oxygen index and enhancing the production of natural killer cells and T-lymphocytes. In plants, AsA plays important role in floral induction, seed germination, senescence, ROS regulation and photosynthesis. AsA is an essential counterpart of the antioxidant system and helps to defend the plants against abiotic and biotic stresses. Surprisingly, the deficiencies of AsA are spreading in both developed and developing countries. The amount of AsA in the major food crops such as wheat, rice, maize, and other raw natural plant foods is inadequate to fulfill its dietary requirements. Hence, the biofortification of AsA in staple crops would be feasible and cost-effective means of delivering AsA to populations that may have limited access to diverse diets and other interventions. In this review, we endeavor to provide information on the role of AsA in plants and human health, and also perused various biotechnological and agronomical approaches for elevating AsA content in food crops.