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 Fusa rium graminearum was reduced by 50% (ED50) by edifenphos concentration s of 7, 25 and 190 muM respectively, the phosphatidylcholine (PC) cont ent of biomass of P. oryzae, B. fabae, and F. graminearum harvested fr om fungicide-containing-cultures was reduced by 50% by 6, 95 and 350 m uM-edifenphos respectively. By contrast, the activities of membrane-bo und chitin synthase preparations isolated from the three fungi were ap proximately equally sensitive to edifenphos. A direct relationship was observed between PC contents of biomass grown in the presence of edip henphos and in vivo rates of chitin synthesis (biomass incubated with [H-3]GlcNAc in the absence of fungicide). Membrane-bound chitin syntha se preparations from P. oryzae grown in medium containing 3 or 6 muM-e difenphos had, at the same fungicide concentration, a lower rate of in vivo chitin synthesis than preparations isolated from biomass grown i n the absence of edifenphos. Membrane-bound chitin synthase preparatio ns from P. oryzae grown in the presence and absence of 6 mum-edifenpho s had the same K(m) values for the substrate (UDP-[C-14]GlcNAc) but di fferent V(max) values. The results suggest that chitin synthesis is in hibited directly by non-competitive inhibition of chitin synthase acti vity, and indirectly following inhibition of PC biosynthesis. P. oryza e is very sensitive to edifenphos because inhibition of PC biosynthesi s occurs at very low fungicide concentrations, and therefore in this f ungus inhibition of PC biosynthesis probably represents the primary mo de of action of the fungicide.