Epigenetic phenomena: Epigenetic phenomena: Paramutation and gene silencing in plants

Gene silencing is a widespread phenomenon in plants, and can affect endogenous genes as well as transgenes integrated into the genome [1,2]. Gene silencing can be induced by interactions between related genes, and one of the best-known examples is named paramutation. Paramutation is the directed heritable alteration of one allele following exposure to another allele in a heterozygote [3]. Examples are known in many plant species, but only in snapdragon and maize have molecular structures for endogenous paramutant alleles been determined [1,2]. Over the past year, the structure, modification and genetic properties of paramutant allelic series at three loci in maize have been determined in detail [4–9]. In addition, an example of endogenous gene silencing in Arabidopsis resembles some aspects of paramutation, although in this case non-allelic interactions are involved [10]. These examples differ from one another in their genetic properties, but it is conceivable that some underlying feature of the plant genome may tie them together.