Genetic changes involved in the juvenile-to-adult transition in the shoot apex of Olea europaea L. occur years before the first flowering

Plant development is a complex process scarcely studied in woody plants. For a study of the genetic control of the juvenile-to-adult transition in Olea europaea L. and for a determination of the timing of the genetic changes, five seedlings were studied by transcriptomic analysis during the developmental process. Microarray analysis over 33 months of growth showed a general trend toward gene repression. There were 1,034 unigenes at a fourfold change, which decreased over time, and 381 that were induced. The expression profile of putative homologs to Arabidopsis thaliana transition genes as squamosa promoter binding protein-like, short vegetative phase or APETALA 2-like transcription-factor families, and the microarray analysis showed two main patterns of changes over development. Thus, at eightfold change, 49 unigenes were repressed and five induced in a quite constant slope over time, while another group underwent notable changes near a genetic transition point. In fact, 22 unigenes of the latter group had sharp repression and seven a strong induction close to the transition time between 9 and 15 months. Notably, a seedling deficiency in the juvenile-to-adult transition failed to induce the genes that are usually upregulated in the transition of normal seedlings. The genetic changes that control the juvenile-to-adult transition appear to occur when the juvenile olive tree reaches around 45 nodes in size. Finally, at least some of the main components of the Arabidopsis pathway for the genetic control of the development phase transitions were found in olive.