Chromatin Organisation during Arabidopsis root development

3 OUTLINE The different cell types of a multicellular organism express different sets of genes. This is one of the oldest statements of developmental genetics. However, how different patterns of gene expression are established in genetically identical cells and maintained during subsequent cell division is still an active topic of research. Especially studies on the involvement of epigenetic modifications in development are a rapidly expending area. The eukaryotic nucleus stores a great amount of information in an extremely compact way. This high level of compaction of the chromatin raises challenges for processes such as transcription. To access the chromatin, 'chromatin modifiers' are essential to 'open' or 'close' the chromatin and in this way control gene expression and establish epigenetic marks that can be inherited. Chromatin modifiers can modify the DNA itself like DNA methyltransferases, which methylate cytosine; but also the histones can be modified by for example histone acetyltransferases or histones methyltransferases. Further, the nucleosomes can be repositioned by for example ATP-dependent chromatin remodelling enzymes, loosing up' or 'tightening up' the chromatin structure. In chapter 1, I will introduce the organisation of the chromatin in relation to development of a multicellular organism. An overview will be provided on what is known about cytosine methylation; about Heterochromatin Protein 1, one of the first chromatin modifier studied in animals and its homolog LHP1 in plants; and about plant histone lysine 9 methyltransferases, which interact with HP1 in animals. We used as model system the root of Arabidopsis thaliana. Arabidopsis has a small genome size of 150 Mbp, and a simple organisation of euchromatin and heterochromatin. The root of Arabidopsis has a simple organisation; furthermore this thin organ (150 µm in thickness) allows microscopic studies on nuclei in an intact organ. In Chapter 2, we first studied whether the structure of the chromatin changes during differentiation of root cells. Level of DNA methylation was used as a 4 characteristic epigenetic mark of the heterochromatin and this was studied and quantified in a developing root. We showed that the Quiescent Center cells and surrounding stem cells have the highest level of methylation. Further, the level of DNA methylation decreases in the division zone of the root and increases again when cells differentiate. As a second criterion to study changes of chromatin during development, Like Heterochromatin Protein 1 was chosen and analyzed in chapter 3. Its homolog in animals, HP1 was first identified as a …