Search

The juvenile-to-adult transition is a complex and poorly understood process in plant development required to reach reproductive competence. For woody plants, knowledge of this transition is even scantier and no genes have been definitively identified as involved in this transition. To search for genes involved in the juvenile-to-adult transition in olive, we constructed juvenile and adult subtractive cDNA gene libraries and identified genes that were differentially expressed in the juvenile and adult phases. In the analysis of theses libraries, we found 13 differentially expressed genes. One of these genes designated as juvenile to adult transition (JAT) was of special interest because it was highly expressed at the mRNA level in the early developmental phases but repressed in the adult phase. The analysis of mutant trees altered in the juvenile-to-adult transition, as well as a segregating progeny of 31 trees from a “Picual” x “Jabaluna” cross, support the contention that its activity might be required for a non-delayed transition. The study of an Arabidopsis thaliana JAT mutant strain confirmed this hypothesis as it showed a delayed flowering phenotype. JAT is expressed in different parts of the plant, showing an unexpectedly high level of mRNA in the roots. However, the JAT expression level is not determined by the distance to the roots, but rather depends on the developmental stage of the branch meristems. JAT is a widely represented gene in plants that appears to be involved in the control of the juvenile-to-adult transition in olive.

In woody plants, the juvenile-to-adult transition occurs when the apical and lateral branches reach a sufficient distance from the roots. In olive trees, there is a minimum distance required between the root and the meristem for flowering to occur. However, the way in which the roots may influence the apical meristem to develop juvenile or adult branches is still unknown; despite that, it is of great relevance for breeders. In addition, as in most fruit trees, olive cultivars are propagated by cuttings, while genetic breeding is performed by seedlings. The roots of both types of plants, cuttings, and seedlings, originate from different tissues, giving rise to differences between the two types of roots. For these reasons, we have studied the gene expression differences between the two types of roots in olive plants by microarray hybridization and found 253 genes differentially expressed, 126 over-expressed in roots of adult cuttings, and 127 in roots of juvenile seedlings. The genes over-expressed in cutting roots showed a similar distribution of genes in metabolic and biosynthetic processes, but juvenile seedling roots showed 20 % of over-expressed genes associated with lipid biosynthesis and metabolism processes, while these processes were absent in the over-expressed genes of adult cutting roots and in general, seedling over-expressed root genes are more diverse and evenly distributed than in cutting roots. In addition, eight root-specific genes have been identified.

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.