Synthesis of L-ascorbic acid in the phloem.
Synthesis of L-ascorbic acid in the phloem.
复制标题
DOI：
10.1186/1471-2229-3-7
复制
发表时间：
2003-11-24
影响因子：
5.3
通讯作者：
Viola R
中科院分区：
文献类型：
--
作者：
Hancock RD;McRae D;Haupt S;Viola R
尽管植物是人类饮食中维生素 C 的主要来源，但我们对植物如何合成 L-抗坏血酸 (AsA) 以及不同植物组织中其浓度的调节因素仍然了解有限。特别是，不同植物的储存器官中维生素 C 含量的巨大差异仍然无法解释。植物贮藏器官中AsA的可能来源包括原位合成和通过韧皮部在其他组织中合成的AsA的长距离运输。在本文中，我们研究了第三种可能性，即韧皮部内的合成。我们使用蚜虫花管切除术和组织化学方法提供了多种作物的韧皮部汁液中存在 AsA 的证据。在来自南瓜果实的富含韧皮部的维管渗出物中检测到了主要 AsA 生物合成途径的几乎所有酶的活性，并且在与一系列前体（D-葡萄糖、D-甘露糖、L-半乳糖和 L-半乳糖-1,4-内酯）一起孵育的芹菜叶柄中分离的韧皮部链中证明了 AsA 生物合成。在本塞姆氏烟草叶盘中观察到韧皮部摄取 D-[U-14C]甘露糖和 L-[1-14C]半乳糖（AsA 生物合成途径的中间体）以及 L-[1-14C]AsA 和 L-[1-14C]DHA。我们提出了新的发现，即活跃的 AsA 生物合成发生在韧皮部。现在必须在涉及全植物 AsA 分布的机制的背景下考虑这个过程。这项工作应该激发旨在阐明韧皮部 AsA 生物合成的体内底物及其对植物贮藏器官中 AsA 积累的贡献的研究。
Although plants are the main source of vitamin C in the human diet, we still have a limited understanding of how plants synthesise L-ascorbic acid (AsA) and what regulates its concentration in different plant tissues. In particular, the enormous variability in the vitamin C content of storage organs from different plants remains unexplained. Possible sources of AsA in plant storage organs include in situ synthesis and long-distance transport of AsA synthesised in other tissues via the phloem. In this paper we examine a third possibility, that of synthesis within the phloem. We provide evidence for the presence of AsA in the phloem sap of a wide range of crop species using aphid stylectomy and histochemical approaches. The activity of almost all the enzymes of the primary AsA biosynthetic pathway were detected in phloem-rich vascular exudates from Cucurbita pepo fruits and AsA biosynthesis was demonstrated in isolated phloem strands from Apium graveolens petioles incubated with a range of precursors (D-glucose, D-mannose, L-galactose and L-galactono-1,4-lactone). Phloem uptake of D-[U-14C]mannose and L-[1-14C]galactose (intermediates of the AsA biosynthetic pathway) as well as L-[1-14C]AsA and L-[1-14C]DHA, was observed in Nicotiana benthamiana leaf discs. We present the novel finding that active AsA biosynthesis occurs in the phloem. This process must now be considered in the context of mechanisms implicated in whole plant AsA distribution. This work should provoke studies aimed at elucidation of the in vivo substrates for phloem AsA biosynthesis and its contribution to AsA accumulation in plant storage organs.