Regulation of Cytokinin Metabolic Genes

细胞分裂素是一种调节细胞分裂和分化的植物激素。最普遍的自然产生的细胞分裂素是玉米素，一种具有高生物活性的化合物。在植物组织中发现了各种玉米素代谢物，其中一些是活性的，另一些不是；因此，了解玉米素代谢是如何调节的是很重要的。该项目的长期目标是分离出发挥这种控制作用的遗传因素。通过对豆类种子玉米素代谢的研究，鉴定出四种玉米素专一性酶，即玉米素O-葡萄糖基转移酶、O-木糖基转移酶、还原酶和顺反异构酶。基因克隆是从O-糖基转移酶开始的，现在已经在豆类和大豆中鉴定出一个基因家族。这些基因的序列非常相似，但编码的酶对不同的细胞分裂素和糖类供体具有不同的亲和力。由于O-糖基化玉米素广泛存在于植物组织中，作为细胞分裂素的可逆储存形式(通过水解)和作为抵抗酶降解的代谢物，这些基因的活性可能会对活性细胞分裂素的水平产生重大影响。因此，这些基因是进一步分子和生理研究的重要靶点。这项拟议的工作将寻求分离该基因家族的其他成员，并确定它们的表达。细胞分裂素O-糖基化对植物生长的影响将通过产生含有选定克隆基因成员的转基因植株来研究。此外，识别特定底物的关键酶上的氨基酸序列将通过构建杂交基因来确定。这项拟议的工作将定义一组控制植物中普遍存在的新陈代谢过程的细胞分裂素特定基因的特性和影响。这些基因是转基因和作物改良的潜在靶点。

Cytokinins are plant hormones mediating cell division and differentiation. The most prevalent naturally occurring cytokinin is zeatin, a compound with high biological activity. Various zeatin metabolites are found in plant tissues, some of which are active, others not; therefore, it is important to understand how zeatin metabolism is regulated. The long-term goal of the project is to isolate genetic elements exerting such controls. Studies of zeatin metabolism in seeds of beans led to the identification of four zeatin-specific enzymes, zeatin O-glucosyltransferase, O-xylosyltransferase, reductase and cis-trans isomerase. Gene cloning was initiated with the O-glycosyltransferases, and a family of genes has now been identified in beans and soybean. The sequences of the genes are very similar but encode enzymes with distinct affinity to different cytokinins and sugar donors. Since O-glycosylated zeatin occurs widely in plant tissues and functions as a reversible storage form of cytokinin (via hydrolysis) and as metabolites resistant to enzymatic breakdown, the activity of these genes is likely to have significant impact on the level of active cytokinins. Therefore, these genes are important targets for further molecular and physiological studies. The proposed work will seek to isolate other members of the gene family and determine their expression. The influence of cytokinin O-glycosylation on plant growth will be investigated by generating transgenic plants harboring selected members of the cloned genes. In addition, amino acid sequences on the enzymes critical to recognizing specific substrates will be determined by constructing hybrid genes. The proposed work will define the properties and the effects of a set of cytokinin-specific genes governing a metabolic process ubiquitous in plants. These genes are potential targets for genetic modification and crop improvement.