The spatial localization of 18 S rRNA genes, in relation to the descent of the cells, in the root cortex of Petunia hybrida.

The 3-D localization of transcription inactive 18 S rRNA genes was studied in interphase nuclei of Petunia hybrida root tip cells. To enable a cell type (i.e. cortex)-specific study in which also the orientation and descent of the cells could be taken into account, a method was developed to preserve the spatial organization of the root meristem. The ribosomal genes were detected by fluorescence in situ hybridization using a biotinylated cDNA probe. 3-D images of 81 nuclei, obtained by confocal scanning laser microscopy, were processed with newly developed computer software. 3-D nucleolar and nuclear dimensions, and the localization of the FISH-spots, were recorded interactively. We compared the absolute and relative position of the genes within and between files of cells of the cortex region of several roots, taking into account the genealogical relationship of the cells. Statistical analysis showed that both the relative and absolute positions of the inactive genes were random, also in more closely related cells within a file of cells. A 'relict telophase orientation' of the genes (i.e. the position of the genes in the daughter cells are mirror images of each other) could only be observed in the G0/1 phase of 'true' daughter cells; the orientation was not preserved throughout the next cell cycle.