Characterization of chlorate-resistant sectors from isolates of Fusarium moniliforme and F. proliferatum

Abstract

Fusarium moniliforme and Fusarium proliferatum produce spontaneous chlorate-resistant sectors on chlorate-containing media. Most of these sectors are nitrate non-utilizing mutants ( nit mutants). We recovered nit mutants as spontaneous chlorate-resistant sectors from 18 isolates of F. moniliforme, and 16 isolates of F. proliferatum of Italian origin. Three media, each containing two different chlorate concentrations, were used. The nit mutants were characterized by their colony morphology on different nitrogen sources. In some cases nitrate reductase activity was evaluated by testing for nitrite excretion. We found three classes of mutants ( nit1, nit3, NitM) corresponding to those assigned in previous studies to mutants of Fusarium species, and, with a low frequency, two additional unexpected classes. One of these, designated Nit8, presumably reflected a mutation at a nitrite reductase structural locus as mutants were positive in the nitrite excretion test. Mutants that grew prototrophically only when reduced sulphur sources were added to the medium were assigned to the other class, and considered as sulphate non-utilizing mutants ( sul). The majority of sul mutants responded to thiosulphate supplementation but two required cysteine or methionine. Complementation was also observed when some of these mutants where co-cultured on MM. The pattern of positive reaction obtained suggested that our sul mutants could result from mutations of at least three loci associated with sulphate metabolism. The sul mutants were recovered from F. moniliforme isolates only. Sectoring on chlorate-containing media provides a method for the isolation of Nit8 mutants from F. moniliforme and F. proliferatum, and of sul mutants from F. moniliforme.