Analysis of Gibberellin Signal Transduction Pathway in Arabidopsis

生物活性赤霉素（giberellins, GAs）是高等植物中重要的植物生长激素，具有促进种子萌发、叶片扩张、茎伸长和花发育的作用。在60年代和70年代“绿色革命”期间开发的小麦和水稻矮秆品种显著提高了粮食产量。近年来的研究表明，这些品种在赤霉素生产或赤霉素响应方面发生了修饰，说明赤霉素在调控农业相关植物发育中的重要作用。GA信号通路受到抑制蛋白的抑制。生长和发育是通过GA信号使这些抑制蛋白失活而发生的。在模式植物拟南芥中已经发现了两种这样的抑制因子RGA和GAI。在水稻、小麦、玉米、大麦和葡萄等作物中也分离到了功能保守的类似蛋白（RGA/GAI同源物）。事实上，矮小麦品种（绿色革命的一部分）携带小麦RGA/GAI同源物（Rht）的特定突变。因此，研究拟南芥RGA/GAI蛋白及其信号转导对提高作物品质具有重要意义。RGA和GAI的氨基酸序列同源性超过80%，在抑制拟南芥GA信号传导中具有部分冗余功能。RGA和GAI的主要作用是抑制ga刺激的茎生长、叶片扩张和花诱导。然而，它们不控制种子发芽和花的发育，这表明必须有额外的抑制因子调节这些过程。通过DNA序列比较，鉴定出拟南芥候选基因RGL1、RGL2和RGL3（用于RGA-LIKE）。为了确定哪个RGL基因在调节花发育中起主要作用，将对这些基因缺陷突变体的表型进行表征。GA通过引起RGA蛋白的降解来抑制其信号通路。我们将研究RGL基因的表达模式和RGL蛋白的稳定性，以阐明RGL调节GA信号的分子机制。RGA/GAI蛋白被认为可以抑制GA反应通路中下游基因的表达。为了确定RGA和GAI的靶基因，新的基因芯片技术将被用于研究GA处理后整个拟南芥基因组表达谱的变化，以及RGA和/或GAI基因的突变。GA应答基因和RGA/GAI靶点的鉴定将有助于解析GA信号转导通路。由于RGA/GAI的同源物在植物中高度保守，因此从该项目中获得的知识也将有助于未来操纵作物的策略。

Bioactive gibberellins (GAs) are important plant growth hormones that promote seed germination, leaf expansion, stem elongation and flower development in higher plants. The dwarf cultivars of wheat and rice developed during the 'Green Revolution' in the 60s and 70s remarkably increased grain yields. Recent studies revealed that these cultivars are modified in their GA production or GA response, illustrating the important role of GA in regulating plant development relevant to agriculture. The GA signaling pathway is inhibited by repressor proteins. Growth and development occur through inactivation of these repressor proteins by the GA signal. Two such repressors, RGA and GAI, have been identified in the model plant Arabidopsis. Similar proteins (homologs of RGA/GAI) with conserved function have also been isolated in crops, including rice, wheat, corn, barley and grape. In fact, the dwarf wheat cultivars (part of the Green Revolution) carry specific mutations in the RGA/GAI homolog in wheat (Rht). Therefore, studies on RGA/GAI proteins and GA signaling in Arabidopsis may have a broad impact in improving the quality of agricultural crops. The amino acid sequences of RGA and GAI are over 80% identical, and they have partially redundant function in repressing GA signaling in Arabidopsis. The major role of RGA and GAI is to inhibit GA-stimulated stem growth, leaf expansion and floral induction. However, they do not control seed germination and flower development, suggesting that additional repressors must modulate these processes. Three candidate Arabidopsis genes RGL1, RGL2 and RGL3 (for RGA-LIKE) were identified by DNA sequence comparison. To determine which RGL gene(s) play a major role in modulating flower development, the phenotypes of mutants that are defective in these genes will be characterized. GA de-represses its signaling pathway by causing degradation of the RGA protein. Expression patterns of RGL genes and the RGL protein stability in response to GA will be examined to elucidate the molecular mechanisms by which RGLs modulates GA signaling. RGA/GAI proteins are thought to repress expression of downstream genes in the GA response pathway. To identify the target genes of RGA and GAI, the new GeneChip technique will be employed to survey alterations in expression profiles of the whole Arabidopsis genome after GA treatment and by mutations in RGA and/or GAI genes. Identification of GA-response genes and RGA/GAI targets will help to dissect GA signal transduction pathway. Because the homologs of RGA/GAI are highly conserved in plants, the knowledge gained from this project will also facilitate strategies in future manipulation of crops.