Carboxin-resistant mutant of Ustilago maydis is impaired in its pathogenicity for Zea mays

We analyzed the pathogenicity of chitin synthetase (chs) disruptants of Ust ilago maydis obtained with the carboxin-resistant or the hygromycin-resista nt cassettes. We found that only chitin synthetase (chs) mutants obtained b y gene disruption with:the carboxin resistance cassette lost their virulenc e to maize (Zea mays) seedlings. Carboxin is a systemic fungicide that inhi bits respiration by preventing the oxidation of succinate. We demonstrated that carboxin-resistant transformants were affected in the levels of succin ate dehydrogenase and respiratory activities when compared with hygromycin- resistant disruptants. We propose that loss of virulence in the carboxin-re sistant transformants is owing to loss of respiratory fitness, which probab ly represents an important component of virulence in this fungus.