Changes in the transcriptome and metabolome during the initiation of growth cessation in hybrid aspens

At 9 550 years of age, Old Rasmus, a Norway spruce (Picea abies), located in the Dalarna province in Sweden is recognized as the oldest surviving tree in the world. Old Rasmus exemplifies a robust capacity to survive in changing conditions, making trees a unique and fascinating subject for study. For perennial trees located in high latitudes one of the most important survival mechanisms is to be able to predict the coming winter and to halt growth in time. Poplars (Populus sp.) use mainly two environmental signals, temperature and light (day length), to predict the coming winter. However, of these two signals day length predominates. Towards the end of summer when the days shorten the trees recognize a critical day length and thereafter start the processes to halt growth and prepare for the impending winter. This thesis focuses on the early gene expression and metabolic responses exhibited in hybrid aspen (Populus tremula L. x tremuloides) induced by a shorter day length below the critical day length. In our effort to identify and describe these responses we used high throughput technologies such as microarray and GC-MS to generate data. We show that similar responses exists in hybrid aspen as in Arabidopsis, with initial responses in carbohydrate and secondary metabolism pathways and circadian clock associated genes. Our results provide evidence that the hormone Gibberellin (GA) is linked with the photoperiodic regulation pathway by affecting PHYTOCHROME A (PHYA) expression and thereby the levels of transcripts that act downstream of PHYA. We also suggest that in Populus the protein GIGANTEA1 (GI1) might be able to influencing growth cessation process by regulating the expression of FLOWERING LOCUS T2 (FT2) independently of CONSTANS2 (CO2)