The plant nucleus at war and peace: Genome organization in the interphase nucleus

Abstract

The interphase nucleus is highly dynamic, anything but the 'resting stage' of the cell cycle. In terms of genome organisation the diploid nucleus is the most 'peaceful', but contrasting structural arrangements may be found in apparently comparable plants at every physical level, from the conservation or not of Rabl organisation as genome size increases through to the presence or absence of expressed genes within a species. This plasticity may mean that virtually every individual of a species represents a unique combination of gene copy numbers, heterochromatic and mobile element content and interchromosomal associations. Tensions in the nucleus are most apparent in newly created interspecific hybrids and allopolyploids, where two genomes share a common cytoplasm and experience numerous and rapid interactions, including: loss or gain of sequences, transposon activation, epigenetic changes, interaction of regulatory elements, genome drift and modifications to cell cycling. The story of order and chaos in the plant nucleus is thus incomplete and open-ended, since our current knowledge is based on only a small number of model species. We also have to bear in mind recent findings that many diploids, if not all, have a history of having passed through earlier cycles of ploidy events, and still bear the duplications as evidence. New discoveries on genome readjustment in hybrids and allopolyploids have implications for our understanding of genome change in evolution, as well as presenting opportunities for the release of new forms of genetic and epigenetic variation in crop plants.