Gibberellin Homeostasis and Biosynthesis in Relation to Shoot Growth in Hybrid Aspen

Israelsson, M. 2004. Gibberellin biosynthesis and homeostasis in relation to shoot growth in hybrid aspen. Doctoral thesis. Silvestria 307. ISSN 1401-6230, ISBN 91-576-6541-9 The plant growth substance gibberellin (GA) is a key regulator of shoot growth in plants. In the work underlying this thesis I explored the biosynthesis and homeostasis of GA in the model tree hybrid aspen (Populus tremula x tremuloides). In addition, the effect of bioactive GA on growth and development was examined. Two genes encoding key enzymes in GA biosynthesis, GA 20-oxidase (GA20ox) and GA 3-oxidase (GA3ox), were individually over-expressed by standard transgenic techniques. The phenotype of GA20ox overexpressers (OEs) was associated with up to 20-fold increases in bioactive GA, leading to highly increased stem elongation and secondary growth. In contrast, ectopic expression of GA3ox did not dramatically alter growth and development. Thus it was concluded that the activity of GA20ox, relative to GA3ox, acts as a limiting step during GA biosynthesis in the control of shoot growth in hybrid aspen. Furthermore, several line