甜菜碱合成途径及其在基因工程育种中的应用

在不良的环境胁迫条件下许多生物体内大量积累甜菜碱,甜菜碱是一种小分子渗透调节物质,它可以调节细胞渗透势,保护蛋白质结构和代谢酶类。由于甜菜碱的合成途径相对简单,通过基因工程技术将甜菜碱生物合成途径导入到不能自身合成渗透调节物并对胁迫条件敏感的重要农作物中,赋予其合成甜菜碱的能力,最终提高其抗性,已经成为当今抗逆育种的重要手段之一,并取得了一定的成绩。不同生物体内甜菜碱的合成途径、催化酶类、反应底物有所不同。在高等植物体、大肠杆菌和土壤细菌中,甜菜碱以胆碱为直接底物,经一或两步氧化而成。其底物胆碱来自于生物体内丝氨酸衍生物的甲基化。反应涉及的相关酶类和基因已经获得,该途径的基因工程研究也在一定程度上提高了转化株的抗性,但底物限制问题没有得到解决。嗜盐隐杆藻中存在另一条完全不同的途径,以甘氨酸为底物由2个不同的甲基转移酶催化完成几个连续的甲基化反应生成甜菜碱,该途径不存在底物限制。本文全面地介绍了不同生物体中甜菜碱合成途径、相关酶类基因及各途径在植物抗逆育种中的应用现状的同时总结了个别途径应用中存在的问题,并提出了可行的解决方案。

Biosynthetic Pathway of Glycine Betaine and its Application in Genetic Engineering

Many kinds of organism accumulated a large amount of Gly Betaine(GB) in extreme environments, which is a kind of low-molecular weight osmotica. It can regulate cytoplasmic osmotic pressure in stressed cells and stabilize protein and metabolic enzyme. Nowadays utilizing genetic engineering technology to introduce the biosynthesis pathway of GB into important stress-sensitive crops which are incapable of synthesizing compatible solute is one of the important strategies in resistance breeding and many improvements have had good results because the biosynthetic pathway of GB is simple. The biosynthetic pathways, enzymes involved in catalyzed reactions and substrates are different in different organism. GB is synthesized by one or two-step oxidation from choline in plants, Escherichia coli and Arthrobacter globiformis, and choline was the product of the methylation of the derivative of serine in these reactions. Various genes and enzymes involved have been gained and the genetic engineering of the biosynthesis of betaine from choline has been exploited to a greater or lesser extent in efforts to increase the ability of transgenic plants to tolerate environmental stress. But the problem of the limition of the substrate has not been solved. There is a completely different biosynthentic pathway in a halotolerant cyanobacterium, Aphanothece halophytica. In this pathways, betaine was synthesized from glycine by a series of methylation reactions and catalyzed by two N-methyltransferase enzymes. This paper reviews the biosynthesis pathways, genes of enzymes involved and the current application situation in resistance breeding, limiting factors in application are summerized and the solution to these problems is put forward at the same time.