Action of bleomycin on proliferating plant cells

Bleomycin (10-(6) M) has been tested in Allium cepa L. meristems which are formed by a proliferating cell population growing under steady state conditions. Chromosome breaks were apparently induced by the antibiotic in cells in G2 period since anaphases with chromatid breaks were formed at a time shorter than G2 + prophase duration. Stimulation of entrance of G2 cells into mitosis is suggested both by an increase in the frequency of early prophases and by the study of waves of prophases in a synchronous subpopulation labelled by caffeine. Progression of other mitotic phases was unaffected. Nucleologenesis rate was increased by the antibiotic in a fashion resembling protein synthesis inhibitors. Protein synthesis is inhibited by 10-(6) M bleomycin to the same extent as 4 X 10(-6) M anisomycin. Both facts suggest that bleomycin has a direct inhibitory effect on protein synthesis in meristems. Given the nucleologenesis sensitivity to nucleolar RNA inhibition it is suggested that the antibiotic activity on nucleolar transcription is mediated through DNA.