| heal.abstract |
Mutants of Ustilago maydis with low resistance to tridemorph isolated in a mutation frequency of 7 x 10-6 after UV-irradiation and selection on media containing 25 μg ml-1 tridemorph. Genetic analysis with nine such mutant isolates resulted in the identification of two unlinked chromosomal loci, U/tdm-1 and U/tdm-2. The U/tdm mutations are responsible for low resistance levels to tridemorph (resistance factor, Rf, of 3 or 5 based on effective concentration causing a 50% reduction in the growth rate (EC50) or minimal inhibitory concentration (MIC) values, respectively) and low to moderate level of resistance to fenpropimorph (Rf 10 or 16 based on MIC or EC50, respectively) and fenpropidin (Rf 5 or 11 based on MIC or EC50, respectively). Haploid strains carrying both U/tdm mutations exhibit higher levels of resistance to the above fungicides, indicating interallelic interaction between nonallelic genes. Cross between mutants carrying the U/tdm-genes with compatible isolates carrying the U/fpm-1 or U/fpm-2 mutations, which were found in previous work to carry fenpropimorph resistance, yielded in all cases a large number of recombinants with wild-type sensitivity, indicating that the mutant genes involved were not allelic. Cross-resistance studies with the inhibitors of C-14 demethylase showed that the U/tdm-mutations were responsible for increased sensitivity to the triazoles triadimefon, triadimenol, propiconazole and flusilazole, and to the pyridine pyrifenox. Study of gene effect on the fitness of U. maydis showed that U/tdm-mutations appeared to be pleiotropic, having more or less adverse effects on growth rate in liquid culture and pathogenicity on young corn plants. |
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