Effects of the herbicide dithiopyr on cell division in wheat root tips

Abstract

The effects of dithiopyr [ S,S-dimethyl 2-(difluoromethyl)-4-(2-methylpropyl)-6-(trifluoro-methyl)-3,5-pyridinedicarbothiate], the active ingredient in the Monsanto herbicide Dimension, were examined on wheat root tips. Dithiopyr caused a cessation of root elongation and swelling of the root tip. The mitotic index of root tips increased from 2.5 to 12% with mitotic cells arresting in late prometaphase within a 6-hr treatment period. These data suggested that dithiopyr did not alter processes regulating the entry of cells into mitosis. Micrographs of treated root tips confirmed the accumulation of cells in prometaphase. The spindle microtubules, common to cells in division in untreated root tips, were not present in dithiopyr-treated root tips. However, microtubule fragments, cortical microtubules, and spindle organizing centers could still be found. Cells in the zone of division became increasingly less isodiametric after 1 to 5 hr treatments, the result of abnormal cell wall orientation and deposition, whereas cells in the zone of elongation became more isodiametric. The application of dithiopyr elicited morphological changes similar to phytotoxic reactions to dinitroaniline herbicides and pronamide. Dithiopyr, however, binds a 68-kDa protein, thus removing from consideration a tubulin-binding protein. The mode of action of dithiopyr may be to alter microtubule polymerization and stability by interacting with microtubule associated proteins and/or microtubule organizing centers rather than interaction directly with tubulin.