Study of mitosis in root-tip cells of Triticum turgidum treated with the DNA-intercalating agent ethidium bromide

This work examines mitosis in root-tip cells of Triticum turgidum treated with the RNA synthesis inhibitor ethidium bromide, using tubulin immunolabeling and electron microscopy. The following aberrations were observed in ethidium bromide-affected cells: (1) incomplete chromatin condensation and nuclear-envelope breakdown; (2) delay of preprophase microtubule band maturation; (3) preprophase microtubule band assembly in cells displaying an interphase appearance of the nucleus; (4) prevention of the prophase spindle formation, caused by inhibition of perinuclear microtubule (Mt) formation and/or inability of the perinuclear Mts to assume bipolarity; (5) organization of an atypical metaphase spindle which is unable to arrange the chromosomes on the equatorial plane; (6) formation of an atypical 'perinuclear metaphase spindle' in cells in which nuclear-envelope breakdown has been almost completely inhibited; (7) inhibition of the anaphase spindle formation as well as of anaphase chromosome movement; (8) disorganization of the atypical mitotic spindle during transition from mitosis to cytokinesis. The observations favor the following hypotheses. Nucleation of prophase spindle Mts is related to the mechanism that causes nuclear-envelope breakdown. The mitotic poles lack Mt-nucleating and -organizing properties, and their function does not account for prophase and metaphase spindle assembly. The organization of the prophase spindle is not a prerequisite for the formation of the metaphase spindle; the metaphase spindle seems to be formed de novo by Mts nucleated on the nuclear envelope and/or in the immediate vicinity of chromosomes.