DMI-RESISTANCE IN USTILAGO-MAYDIS .2. EFFECT OF TRIADIMEFON ON REGENERATING PROTOPLASTS AND ANALYSIS OF FUNGICIDE UPTAKE

Spontaneous triadimefon-resistant mutants of Ustilago maydis were investigated with regard to the mechanism of resistance. At first, the influence of triadimefon on regenerating protoplasts from sensitive and resistant sporidia was studied. The permeability of the plasma membrane of protoplasts from sensitive and resistant strains was affected by the triazole derivative to the same extent. Resistance was provable only in intact sporidia, that is, before protoplast formation and after complete cell wall regeneration but not in the protoplast stage. Following protoplast formation of two sensitive and two resistant strains, it could be demonstrated that the enzymatic cell wall lysis of the resistant sporidia required more time than that of the sensitive ones. Gas chromatographic analyses of fungicide content in a sensitive and a resistant strain showed triadimefon accumulation to be dependent on concentration and nearly constant overtime. However, the amount of fungicide taken up by the resistant strain was 2-4 times as high as that of the sensitive one. In the stationary phase the triadimefon content of the sensitive sporidia was increased 2.5-fold compared with the log phase, whereas that of the resistant strain remained unchanged. No metabolism of triadimefon to triadimenol could be detected in the sensitive or the resistant strain. It is concluded that the cell wall region possibly plays an important role in triadimefon resistance. © 1993 Academic Press. All rights reserved.