Carboxin resistance in the haploid, the heterozygous diploid, and the plant-parasitic dicaryotic phase of Ustilago maydis

Abstract

Respiration and growth of a diploid strain of Ustilago maydis, heterozygous at the oxr-1 locus for resistance to carboxin and carboxin analogs, were less sensitive to the fungicide than the corresponding processes in the wild type haploid. Data on the inhibition of succinate-2,6-dichlorophenolindophenol reductase indicate that the mitochondria of the heterozygous diploid probably contain a mixture of carboxin-sensitive and carboxin-resistant succinic dehydrogenase complexes. This hypothesis is supported by the results of temperature aging experiments: When mitochondria of the heterozygous diploid were subjected to incubation at 30°C which destroyed the enzyme activity of mitochondria from the mutant haploid, the remaining succinate-2,6-dichlorophenol-indophenol reductase activity exhibited a carboxin sensitivity similar to that of mitochondria from the wild type haploid. Carboxin gave very good control of corn smut in seedlings artificially inoculated with two wild type compatible strains but not when both strains were mutant for carboxin resistance, which indicates high resistance in the dicaryon. In a heterozygous cross, the dicaryon seemed to exhibit considerable resistance, if sufficient time was allowed for its formation between inoculation and carboxin treatment of soil.