[Heterochromatin, a plastic component in the nucleus of Arabidopsis thaliana cells].

The cytogenetic observation of the nucleus of Arabidopsis thaliana, a plant member of the brassicaceae family, reveals a simple organization of the nuclear content. Indeed, the nuclear volume is occupied by two distinct and easily distinguishable forms of chromatin: a large fraction of relatively decondensed and transcriptionnally active euchromatin surrounds about ten conspicuous regions, the chromocenters, which contain most repeated and highly condensed heterochromatic sequences. Remarkably, during the development of A. thaliana or when the plant is exposed to certain environmental variations, dramatic changes in the appearance, the size or the presence of the chromocenters occur. A number of cytogenetic studies have not only characterized the genomic sequences accommodated in the chromocenters, but have also established the dynamics of their assembly and disruption. Moreover, various endogenous and exogenous factors involved in the presence and the size of chromocenters were recently identified. Taken together, these studies carried out in A. thaliana suggest that heterochromatin is a truly "malleable" fraction of the genome whose dynamic organization is not controlled only by epigenetic marks and whose importance in nuclear function goes beyond merely grouping together non-coding genomic sequences.