Mechanism of action and selectivity of a novel fungicide, pyribencarb

Pyribencarb is a novel benzylcarbamate-type fungicide, which is active against a wide range of plant pathogenic fungi. In this paper, the inhibitory effects of this fungicide on the electron transport system of fungi, plants, rat and carp were examined to elucidate its mode of action and selectivity. Pyribencarb potently inhibited succinate-cytochrome c reductase (SCR) activities of Botrytis cinerea (cucumber gray mold), Corynespora cassiicola (leaf spot) and decylubiquinol-cytochrome c reductase (UCR) activity of B. cinerea. Pyribencarb inhibited the UCR of B. cinerea in an uncompetitive manner with respect to decylubiquinol, which was the same as strobilurin fungicides, and the substrate-dependent inhibition constant was found from calculation to be 13 nM. These results suggested that the target site of pyribencarb is cytochrome b of complex III in the electron transport system of the respiratory chain. On the other hand, the inhibitory potency of pyribencarb on SCR activities of plants, rats and carp was relatively weak compared with that of strobilurin fungicides, indicating that pyribencarb is a Qo inhibitor of cytochrome b, whose properties are superior to well-known Qo inhibitor fungicides in terms of target. The binding site of pyribencarb on cytochrome b was assumed to be a little different from that of strobilurin fungicides, because pyribencarb inhibited SCRs of strobilurin fungicide-resistant strains of B. cinerea and C. cassiicola with relatively low concentrations. The binding site was also discussed through comparison of amino acid sequences of plants, rats, carp, yeast and fungi, including B. cinerea, which was elucidated in this paper.