Epigenetic regulation controls a broad range of heritable, yet reversible, changes in gene expression. It generates an additional level of genetically transmitted information and gene expression diversity in many eukaryotes. It is involved in the defence against intruding DNA and RNA molecules, in genome stabilisation and in the regulation of development and morphology. Our group is interested in the epigenetic consequences of polyploidy and of abiotic stress. We study both aspects in Arabidopsis thaliana, based on well-established genetic and transgenic resources. Epialleles differing in expression of a selectable transgene were found in tetraploid lines. These epialleles, stable over many generations of selfing, show an unexpected interaction when combined in tetraploid plants, leading to trans-silencing and DNA hypermethylation. We investigate structural and chromatin features of the epialleles in wild-type and mutant backgrounds, we analyse the requirements for their interaction, the role of meiosis and try to discover the trans-acting factors that are required for maintenance of the silent epiallele. With genetic tools, we further explore the role of epigenetic factors in stress memory.