Terbutol Affects Spindle Microtubule Organizing Centres

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Abstract
Light, immunofluorescence, and electron microscopy were utilized to investigate the effects of the herbicide terbutol (2,6-di-tert-butyl-p-tolyl methylcarbamate) on onion root tips. So-called ‘star anaphases,’ chromosomes drawn in at their centromeres at both poles, resulting in a starburst of chromosomes were the predominant form of mitotic abnormality noted in root tip squashes of the terbutol-treated roots. Immunofluorescence microscopy using antitubulin sera reveals a cluster of microtubules radiating from the centre of the chromosome mass at each of the poles. Nuclear envelopes apparently reform around the radially-arranged chromosome masses, resulting in extensively lobed ‘star telophase’ nuclei. Branched and curved phragmoplast arrays are observed, due to interference by the lobes of the star telophase nucleus. These abnormal phragmoplasts result in incomplete and/or abnormally-oriented cell walls. Star anaphase figures are observed after 2 h of herbicide treatment, indicating that this terbutolinduced chromosomal abnormality is a primary effect of the herbicide. Tradescantia stamen hairs were treated with terbutol and mitosis was monitored in vivo by Nomarski differential interference microscopy; these treated stamen hairs produce star anaphase figures as a primary effect of the herbicide. This series of abnormalities has not been observed with any other herbicide, indicating that terbutol has a unique mechanism of action, perhaps interacting with spindle microtubule organizing centres.

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