An ultrastructural and radioautographic study of the effect of ethidium bromide on the interphase nucleus of meristematic plant cells.

In the present work the effects of an intermediate dose (20 fig/ml) of ethidium bromide on nuclear structures of meristematic plant cells (Raphanus sativtis) have been studied under both light and electron microscopy. Radioautography was also exploited to assess the action of this drug on the incorporation activity of nucleic acid precursors. After a 30-min treatment with ethidium bromide, no noticeable ultrastructural modifications are detected but the incorporation of both thymidine and uridine is already markedly impaired. As treatment is extended to 1 h, the numerous light lacunae characterizing the fibrillar nucleolar zones in normal cells transform into elongated areas which are often found to extend to the surface of this organelle and are continuous with perinucleolar chromatin masses. After 2 h, many small bodies consisting of granular material appear in the nucleoplasm. The nucleolus gradually loses most of its granular component and, eventually, becomes predominantly fibrillar in texture; in many cells it breaks up into a number of irregular masses. Throughout various stages of the experiment, certain cells show small spherical nucleoli characterized by the presence on their surface of one or two conspicuous reticulate masses which are heavily labelled with thymidine and, consequently, consist mostly of chromatin. In view of the fact that ethidium bromide has been observed in this study to induce a certain degree of condensation of chromatin within the nuclear cavity, it is surmised that this drug also brings about migration of intranucleolar DNA to the surface of this organelle.