Inhibition of phosphatidylcholine and chitin biosynthesis in Pyricularia oryzae, Botrytis fabae and Fusarium graminearum by edifenphos.
Colony growth of the fungi Pyricularia oryzae, Botrytis fabae and Fusarium graminearum was reduced by 50% (ED50) by edifenphos concentrations of 7, 25 and 190 microM respectively; the phosphatidylcholine (PC) content of biomass of P. oryzae, B. fabae, and F. graminearum harvested from fungicide-containing-cultures was reduced by 50% by 6, 95 and 350 microM-edifenphos respectively. By contrast, the activities of membrane-bound chitin synthase preparations isolated from the three fungi were approximately equally sensitive to edifenphos. A direct relationship was observed between PC contents of biomass grown in the presence of ediphenphos and in vivo rates of chitin synthesis (biomass incubated with [3H]GlcNAc in the absence of fungicide). Membrane-bound chitin synthase preparations from P. oryzae grown in medium containing 3 or 6 microM-edifenphos had, at the same fungicide concentration, a lower rate of in vivo chitin synthesis than preparations isolated from biomass grown in the absence of edifenphos. Membrane-bound chitin synthase preparations from P. oryzae grown in the presence and absence of 6 microM-edifenphos had the same Km values for the substrate (UDP-[14C]GlcNAc) but different Vmax values. The results suggest that chitin synthesis is inhibited directly by non-competitive inhibition of chitin synthase activity, and indirectly following inhibition of PC biosynthesis. P. oryzae is very sensitive to edifenphos because inhibition of PC biosynthesis occurs at very low fungicide concentrations, and therefore in this fungus inhibition of PC biosynthesis probably represents the primary mode of action of the fungicide.