The abnormal cell plates formed after microtubule disrupter herbicide treatment are enriched in callose

Mitotic disrupter herbicides are best known for their macroscopic effect on root tip swelling and their microscopic effect on the progression of chromosomes through mitosis. However, irregularities with the phragmoplast microtubules and cell plate formation occur at lower herbicide concentrations than these more familiar effects. Instead of the relatively straight cell plates found in control tissue, cell plates after mitotic disrupter treatment are often branched and grow irregularly throughout the cytoplasm. Sometimes these abnormal plates adhere to one wall and in most cases do not effectively divide the potential daughter cytoplasms. To determine the chemical composition of these abnormal cell plates, thin sections of treated onion root tips were probed with a battery of antibodies and cytochemical probes. Abnormal cell plates are greatly enriched in callose compared to control cell plates and accumulate very low levels of cellulose. The development of these wildly undulated and excessively branched or heavily thickened cell plates indicates the importance of microtubules in forming a proper cell plate and perhaps the necessity of stable microtubule arrays for the addition of cellulose to these structures. Because the abnormal plates occur at herbicide concentrations below that required for induction of mitotic arrest or root tip swelling, this effect may be the primary phytotoxic effect of these herbicides.