Terrazol, a systemic fungicide showing high specifity to oomycetes, inhibits the apical growth of hyphae and promotes at lower concentrations the thickening of the cell wall in Mucor mucedo. As revealed by ultrastructural analysis, particularly the fine structure of some membrane systems is influenced. In the first place the inner membrane of the mitochondria is attacked leading to a complete lysis of mitochondria. However, the sensitivities within a given population are different. The plasmalemma enlarges, forms several invaginations, partly redraws from the cell wall, but remains intact. Only after an extensive treatment with relatively high concentrations of terrazol the nuclear envelope shows vesicles between the double membranes. The mechanism of action of terrazol is discussed.
Higher concentrations of terrazol inhibit the growth of a large number of fungi, but oomycetes are most sensitive. At medium concentrations (ED,, = 10 ppm) in Nucor mucedo several alterations of ultrastructure are recognizable even after short incubation periods. Significant observations are vacuolization of the mitochondria1 cristae, invagination of the cytoplasmic membrane, and thickening of the cell wall. Later on, numerous vacuoles in the cytoplasm are visible. Primary effects on the cell membrane (destructive effect), on 0,-consumption, on fermentation, or on nucleic acid synthesis could not be demonstrated. Lipid synthesis under the influence of terrazol showed characteristic changes. The synthesis of triglycerides and sterolesters was inhibited while the synthesis of free fatty acids and phospholipids was stimulated. Addition of lipids, vitamines, amino acids, or nucleic acids did not counteract the effect of terrazol. Further investigations are necessary t o elucidate the primary mechanism of action.
Comparative investigations on the inhibition of chitin synthetase of Mucor rouxii revealed that in contrast to results obtained in vivo, only few fungicides and other compounds inhibit the enzyme in vitro. Besides the well-known effect of polyoxin D, an inhibition was demonstrated for terrazol, tridemorph, hinosan, and formulated preparations of dimilin (PH 60--40, 60--38). The latter preparations, however, showed growth inhibition also with fungal species which do not synthesize chitin and the pure compounds are ineffective. Inhibition by phospholipase C, unsaturated fatty acids, and the reversibility of the inhibition caused by terrazol after addition of procain-hydrochloride demonstrates that phospholipids are essential for the activity of the enzyme, whereas sterols seem to be ineffective. Action of trypsin, PCNB, pentachlorophenol, and some similar compounds results in significantly increased activity, which in the case of trypsin could be due to the hydrolysis of a protein inhibitor. Hinosan inhibits the enzyme indirectly in a still unexplored manner.
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