Mode of Action of Toiclofos-methyl in Ustilago maydis

Abstract

When tolclofos-methyl was added to the liquid culture of Ustilago maydis at 4μg/ml, sporidial multiplication was immediately prevented. The treated sporidia did not form buds of daughter cells and became enlarged. After a 2-hr incubation period, the sporidial cells initiated a division by forming cross walls, so that the sporidia became multicellular. This indicates that the sporidia changed the manner of cell division in the presence of the fungicide. Within the succeeding several hours, the sporidial cells ceased to divide and bursting of the cells began to occur. Most of the cells lost their cytoplasm by after 24hr. On the other hand, increase of dry weight of the sporidia and RNA synthesis was not immediately affected by tolclofos-methyl. DNA and protein synthesis, however, were significantly reduced during an early incubation period. This was suggested to be a reflection of the inhibition of cell division. Endogenous respiration and glucose oxidation were not inhibited by tolclofos-methyl at 4μg/ml. Although succinate oxidation was moderately suppressed by the fungicide at the same concentration, this does not fully explain its toxicity. The present experiments therefore suggested that tolclofos-methyl primarily affected cytokinesis and particularly inhibited bud formation in U. maydis.