Genes constitute only a small portion of the total genome and precisely controlling their expression represents a substantial problem for their regulation. Furthermore, non-coding DNA, that contains introns repetitive elements and active transposable elements, demands effective mechanisms to silence it long-term. Cell differentiation and development are controlled through temporal and spatial activation and silencing of specific genes. These patterns of gene expression must remain stable for many cell generations and last or change when inductive developmental signals have disappeared or new ones induce new programmes.

What turns genes on and off? Among others, gene regulation is controlled by epigenetic mechanisms, defined as any gene-regulating activity that does not also involve changes in the DNA code and is capable of persisting. It has become apparent that epigenetic control of transcription is mediated through specific states of the chromatin structure. Associations of specific chromosomal proteins, posttranslational histone modifications and DNA methylation are some of the epigenetic mechanisms that are involved in controlling chromatin states. DNA methylation research can be approached from several standpoints, since there is a wide range of techniques available to study the occurrence and localisation of methyldeoxycytosine in the genome. Several studies dealing with DNA methylation in relation to tree development, microproprogation and somaclonal variation will be presented, with the final aim of demonstrating that DNA methylation levels are hallmarks for growing seasonal periods and are related to open windows of competence in plants.