Manifold changes occur in the course of a cell’s life. However, not all the changes we detect really result from mutations within genes. Some of the changes are a result of environmental conditions and are described as epigenetic changes. Investigations in this field of research focus on introduced foreign genes and the variations of their expression, and on epigenetic mechanisms possibly involved.
Within multicellular organisms different cell types and tissues have different responsibilities and tasks. Although all cells contain identical sets of chromosomes and genes, not all genes are active in all cells at the same time. Many mechanisms exist to turn genes on or off as well as to enhance or down- regulate the production of proteins. We are investigating some of these versatile control mechanisms of gene expression.
First of all, promoters – specific nucleotide sequences in control regions – are responsible for the regulation and variation of gene expression. They are comparable to on-/off-switches, allowing or restricting genes to be transcribed under certain conditions.
As a consequence of the non-uniform packaging of the genome, some genome sections like heterochromatic regions are hardly or not at all accessible for transcription. Modifying enzymes allow tagging genomic regions by adding chemical flags like methyl- or acetyl groups to specific DNA nucleotides and/or packaging proteins (histones). Such chemical tags affect the expression activity of adjacent genes.
Another level of gene regulation is RNA interference (RNAi), where multiple small RNAs (microRNAs and siRNAs) - either naturally occurring or triggered, e.g., during virus infections - dedicate specific gene transcripts to rapid decay. This type of regulation can be compared to a dimmer.
The majority of the expression patterns of the different cell types are programmed during ontogeny of the organism. However, the developmental stages are not completely fixed. They respond to the environment. Thus, cells can adapt to their conditions of living at least to a certain extent. This guarantees a quicker response than selection of progeny solely on the basis of mutations at the nucleotide level. The expression patterns modified by environmental influences can be transferred to progenitor cells. In plants such ‘experienced’ cells not only give rise to the cell types of the leaves and the roots but also to cells building the male and female flower parts. In this way, environmental experiences can be inherited through generations of progeny. The inheritance of acquired gene expression programs is called epigenetics.
Also, transgenes which are introduced into the plant genome via genetic engineering are subject to epigenetic modifications. The degree of silencing or even shut down of expression of a transgene is assumed to depend on epigenetic factors to a large extent.