INCOMPATIBILITY CONDITIONED BY THE MLA GENE IN POWDERY MILDEW OF BARLEY - TIMING OF THE EFFECT OF CORDYCEPIN ON HYPERSENSITIVE CELL-DEATH

The effect of cordycepin, an inhibitor of mRNA synthesis, on the hyper sensitive host cell death response (HR), was tested with epidermal tis sues from barley coleoptiles containing the Mla gene for resistance, i noculated with incompatible race 3 of Erysiphe graminis f.sp. hordei. In untreated control tissues, HR occurred 18-30 h after inoculation in 61-91% of cells with haustoria. The percentage of HR was strongly red uced by cordycepin al 10-31 mu m but only if applied 4 h after inocula tion or before; not if applied 8 h or later. Effective cordycepin trea tments had no inhibitory effect on the development of germ tubes or ap pressoria and usually increased the development of haustoria and hypha e. The results indicate that mRNAs required for the HR are synthesized in the host within the first 0-8 h after inoculation and that mRNX sy nthesis is not required thereafter. The timing of protein synthesis re quired for the HR in the host could not be determined because the inhi bitors of protein synthesis used, blasticidin S and cycloheximide, inh ibited parasite development al concentrations required to inhibit the HR.