Dynamics of functional heterochromatic domains in the plant interphase nucleus.

Biologists have been fascinated for more than 2 centuries about how the nucleus in eukaryotes is organised. Certain of the component parts are well known, but the overall picture is blurred and often confusing. Small genome species have chromosomes in their interphase nuclei disposed in diffuse chromosome territories, without any Rabl arrangement, while in large genomes the chromosomes run string-like through the nucleus with a Rabl orientation following through the cell cycle. What happens in genomes of intermediate size is either a bit of both, depending on the tissue being studied, or still remains to be determined. The centromeres are the most dynamic and least well understood part of the nucleus, subject to rapid evolutionary change and with an epigenetic mark based on a special form of histone CENH3. Nonetheless, the centromere epigenetic mark has been inherited for millions of years by a process that is a complete mystery. Centromeres are involved with the dynamic interactions between chromosomes and other parts of the nuclear environment, such as the nuclear matrix and inner nuclear membrane, and they also engage with the spindle when the order within the nucleus changes during its division. The nucleolus organizer regions have likewise posed tantalising problems about their massive amplification of rDNA sequences, and how they are regulated and expressed. Some of these issues are now becoming clearer with advances in the science and the ongoing development of new molecular tools. These developments are discussed in this contribution, with particular reference to the centromere and the nucleolus organizer.