Chromocenter association in plant cell nuclei: determinants, functional significance, and evolutionary implications

Owing to close associations of different chromosomes at their heterochromatic regions, nuclear repatternings consisting of changes in the number and size of the chromocenters may occur with tissue differentiation in plant species. By studying these changes in the nuclear structure in a number of Magnoliopsida with chromocentric nuclei, we found: (i) that chromocenter association is affected by the degree of partitioning of the genome, since the extent of the phenomenon correlates positively with the number of chromosomes in the complement, the level of ploidy, and the number of chromocenters in meristematic nuclei, whereas the correlation with the mean DNA content per chromosome is negative; (ii) that this kind of nuclear repatterning plays a role in or is a marker of events that have a part in the control of nuclear activity, since a negative correlation was found to exist, as a rule, between the extent of chromocenter association and that of RNA synthesis in the nucleus; and (iii) that this large-scale control mechanism of gene expression is exploited in cell differentiation from its early stages. Indeed, chromocenter association takes place at the base of the meristems and remains unchanged during further tissue development. In certain species, this phenomenon has not been observed; its presence or absence may represent a character of entire families, and may be linked to the position of a species on an evolutionary scale.