MITOTIC DISRUPTOR HERBICIDES ACT BY A SINGLE MECHANISM BUT VARY IN EFFICACY

Although there are numerous herbicides that disrupt mitosis as a mecha nism of action, to date not one has compared the effects of these disr upters on a single species and over a range of concentrations. Oat see dlings, treated with a range of concentrations of nine different ''mit otic disrupter herbicides'', were examined by immunofluorescence micro scopy of tubulin in methacrylate sections. All herbicides caused the s ame kinds of microtubule disruption, although the concentrations requi red to cause the effects differed markedly between the herbicides. Eff ects on spindle and phragmoplasts mitotic microtubule arrays were seen at the lowest concentrations and manifested as multipolar spindles an d bifurcated phragmoplasts (which subsequently resulted in abnormal ce ll plate formation). At increasing concentrations, effects on mitotic microtubule arrays manifested as microtubule tufts at kinetochores and reduction of cortical microtubules resulting in arrested prometaphase figures and isodiametric cells. These data indicate that all mitotic disrupter herbicides have a common primary mechanism of action, inhibi tion of microtubule polymerization, and that marginal effects observed in the past were the result of incomplete inhibition and/or different ial sensitivity of the microtubule arrays.