植物尼克酸代谢分子机制及对NAD生物合成的作用研究

NAD (Nicotinamide adenine dinucleotide) is an essential cofactor for cellular redox reactions and energy metabolism in all living organisms. Recent studies revealed that the chemicals from NAD degradation catalyzed by various enzymes, along with NAD itself, play key roles in many important biological processes. Thus the level of NAD in plant cell is tightly regulated by a variety of mechanisms, including its de novo biosynthesis using Asp as precusor and salvage pathway started with nicotinamide and nicotinate. In our previous report, we found that nicotinate, a key intermediate in the NAD salvage pathway, could be modified by methylation and glycosylation except be directed into NAD biosynthesis by NaPRT (nicotinate phosphoribosyltransferase). However, the underlying molecular mechanisms and the effects of these modifications of NA on the NAD biosynthesis are poorly understood. In this project, a combined approach, including molecular biology, analytical biochemistry，enzymology, genetics and bioinformatics, will be applied to discover the genes responsible for the NA modification and their physiological functions in NAD biosynthesis in Arabidopsis. This study will lay a foundation for a complete understanding of NAD biosynthesis network, and also provide breeding and transgenic approaches to improving valuable traits in agriculturally important crops, such as rice, by fine tuning the NAD homeostasis in planta.

NAD除了作为生物体内氧化还原反应电子传递载体与生物能量的提供者，还会被酶促降解，降解产物参与多种重要的生理过程，其作用不可或缺。NAD在生物体内通过从头（de novo pathway）和补偿（salvage pathway）两条途径进行生物合成，以维持NAD在生物体内的动态平衡。我们的前期研究表明，NAD补偿途径中的关键代谢物尼克酸在植物体内不仅可以在尼克酸磷酸核糖转移酶（NaPRT）催化下进入NAD补偿合成途径，还会被甲基化、糖基化等修饰，但这些修饰的分子机理、生理功能及对NAD生物合成的影响尚不清楚。本项目以拟南芥为材料，从代谢角度入手，结合生化试验、分子生物学和遗传学方法探索尼克酸代谢的关键酶作用的分子机制及对NAD生物合成的影响。这一研究有助于进一步阐明植物NAD合成途径的代谢及调控机制，具有重要的科学意义；同时研究成果也可以用于水稻等多种重要作物，具有很好的应用前景。

通过本项目（2013.01-2016.12）的实施，我们在代谢水平和分子水平上详细解析植物体系中（以拟南芥为主要研究材料）尼克酸（Nicotinate，NA）代谢的分子机制，功能鉴定了拟南芥NA氮位和羧酸位的糖基转移酶和甲基转移酶和NaPRTase。利用相关的转基因植物（包括基因过表达和基因沉默）并阐明NA代谢修饰与NAD补救生物合成途径的关系，为探索NAD代谢的调控奠定坚实的理论基础并提供可能的分子手段。发表高水平论文一篇（Plant Cell, IF > 9.0），两篇文章在整理投稿过程中，获得专利1项，培养博士生各三名，完成项目的各项既定目标。整个的项目经费大体按照年度预算计划进行。