Immediate disruption of spindle poles and induction of additional microtubule-organizing centres by a phenylcarbamate, during plant mitosis

The herbicide carbetamide [(R)-1-(ethylcarbamoyl) ethylphenylcarbamate], in the 0.4 to 0.8 mM range, efficiently induced multipolar mitoses in Allium cepa L. The frequency of multipolar anaphases rose earlier and reached high er values when both concentration and time of treatment increased, up to a maximum of 90% after 1 h of treatment. To identify the physiological target , the kinetics of induction of multipolar mitoses were followed during reco very from very short treatments (5, 10, and 15 min). Tubulin immunodetectio n showed that phenylcarbamate immediately disrupts the cohesion between the different bundles of microtubule minus ends which converge at the pole. Th e spindle was rendered multipolar about three times more efficiently in met aphase than in anaphase. The observations do not support any effect of the herbicide on the tubulin polymerization-depolymerization cycle, and suggest that the minus ends of the microtubules remained stabilized in carbetamide . Thus, the density of kinetochore microtubules and their lengths were unmo dified in the individual chromosomes which became detached from both spindl e poles in response to the herbicide. Extra microtubule-organizing centres for the assembly of both preprophase band and phragmoplast (the tubulin arr ays which characterize the microtubular cycle responsible for cytokinesis i n plant cells) were also rapidly induced.