Anticytoskeletal Herbicides

Summary

Approximately one quarter of all of the marketed herbicides are classified into the mitotic disrupter herbicide group, including the widely used dinitroaniline and carbamate herbicides. Most of the herbicides in this group are used commercially to control grasses and other small-seeded weed species in larger-seeded dicot crop species. Gross morphology of the seedlings after treatment with mitotic disrupter herbicides is distinctly club-shabed or swollen, compared to the uniformly tapered roots found in untreated controls, and is similar to those effects noted for the classical microtubule disrupter colchicine. Microscopic examination of herbicide-treated roots reveals a concentration-dependent loss of microtubules, with phragmoplast and spindle arrays being affected at lowest concentration, and cortical and kinetochore microtubules being the least affected. The loss of these microtubule arrays results in the production of irregular cell walls, C-metaphase figures and lobed nuclei. Loss of the cortical microtubule array results in isodiametric growth, which leads to root clubbing in the zone of root elongation. Limited biochemical analysis indicates that the herbicides oryzalin and pronamide (propyzamide) bind directly to tubulin and that the carbamates and phosphoric amides can inhibit polymerization in vitro, indirectly confirming this mode of action for these herbicides as well. A possible non-tubulin target has been suggested for the herbicide dithiopyr, although the effects induced are identical to other members of this group. Data from both resistant mutants and molecular modelling indicate that the dinitroaniline and phosphoric amides bind to a similar site, possibly on the alpha-tubulin molecule. Microtubule disrupter herbicides, because of their high selectivity, have great potential in screens for mutants with altered herbicide sensitivity that could be of tremendous agronomic importance.