Activation of latent nucleolus organizers induced by experimental polyploidization in cells of hexaploid wheat Triticum aestivum L.

The effect of prolonged colchicine-induced polyploidization on activation of latent nucleolus-organizing regions (NOR) of chromosomes was studied in diploid meristematic cells and polyploid root cells of Triticum aestivum L. It has been shown that control diploid and tetraploid cells have maximal number of nucleoli equal to four, which corresponds to the number of nucleolar chromosomes (NC) with active (visualized by staining with AgNO3) NOR (two pairs of homologous chromosomes 1B and 6B). Treatment of wheat seedlings with colchicine for 30 h results in following changes in polyploid cells: (1) impregnation of NOR with silver is observed on homologues of either chromosomes 1A or 5D in all tetraploid metaphase plates (4n, 2x, 4c), which is indicative of the NOR activation on this chromosome in pre-mitotic polyploid interphase; (2) In tetraploid metaphase, NOR in all four homologues of activated chromosomes or in only two of them may be stained; (3) maximal number of nucleoli in tetraploid nuclei is increased till 12, which confirms activation of transcription of additional rRNA gene clusters in polyploids; (4) activation of the rRNA gene expression is induced by the cell polyploidization rather than by colchicine, since in the colchicine-treated diploid cells both maximal number of nucleoli and the number of metaphase chromosomes with active NOR is not changed as compared with control. The obtained data allow us to suggest that structural "separation" of NC in polyploid nuclei stimulates activation of latent NOR.

Dynamics of structural and functional association of nucleolar chromosomes in cells of the hexaploid wheat Triticum aestivum L. at different stages of the cell cycle and during genome polyploidization.

Quantitative analysis of interphase association of the nucleolar chromosomes at different stages of the cell cycle and during genome polyploidization was carried out. Cells of various tissues of hexaploid wheat Triticum aestivum L. (Moskovskaya-35) were used, including diploid root meristematic cells, endopolyploid root cells, triploid endosperm cells and antipodal cells with polytene chromosomes. Interphase nucleoli impregnated with silver or stained with autoimmune antibodies to 53 kDa nucleolar protein served as markers of the nucleolar chromosome association. The following data were obtained: (1) silver-staining revealed two pairs of homologous chromosomes 1B and 6B with active nucleolus-organizing regions in the root meristematic cells; (2) maximal number of nucleoli in diploid meristematic cells reaches four, which corresponds to the number of chromosomes with active organizers; (3) analysis of cells at different stages of the cell cycle has shown that the tendency to the nucleoli association is observed as soon as cells pass individual stages of the cycle; (4) after DNA and chromosome reduplication, the nucleolus-organizing regions in sister chromatids function as a common structure-functional complex; (5) in endopolyploid root cells and antipodal cells with polytene chromosomes, the number of nucleoli does not correlate with ploidy level, and an additional nucleolus revealed in some cells is the result of activation of the latent organizer in one of the nucleolar chromosomes; (6) in the triploid endosperm nucleologenesis, the stage of prenucleolar bodies is missing. Our data suggest that "fusion" of nucleoli and reduction of their number due to the "satellite" association of the nucleolar chromosomes are two independent processes regulated by different mechanisms.