Epsilon Open Archive

Abstract

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 lines of evidence now suggest that GA4 has a more pivotal role than GA1 in controlling shoot elongation. GA regulates wood formation as transgenic GA-overproducing plants developed 70% more and 8% longer xylem fibers as compared to WT. Curiously, no major alterations in phloem formation were observed. Based on the results of microarray analyses comparing the wood-forming tissues of GA20ox OE and WT, we suggest that GA mainly up-regulates the expression of genes implicated in cell wall formation and extension. Intriguingly, the microarray analysis helped uncover an interesting shift in the composition of lignin previously undetected in the transgenic trees. The distribution of bioactive GA within the cambial region of Populus was mainly restricted to expanding xylem cells. We therefore suggest that the predominant effect of GA occurs during xylem expansion. Finally, the expression of two genes implicated in GA responsiveness correlated very well with the distribution of bioactive GAs in the cambial region.