Double chitin synthetase mutants from the corn smut fungus Ustilago maydis

Using genetic crosses between single chs mutants of Ustilago maydis inocula ted into maize (Zea mays) seedlings, two classes of double mutants affected in genes coding for chitin synthetases were isolated: chs3/chs4, and chs4/ chs5. Analysis of the mutants showed almost no change in their phenotype co mpared with wild-type strains. Growth rate, effect of stress conditions, di morphic transition and mating were not affected. The only salient differenc es were increased sensitivity to osmotics at acid pH, and decrease in chiti n synthetase activity, especially when measured with CO2+, and in chitin co ntent. Most significant was a decrease in virulence, although this appeared to be due a factor unrelated to CHS genes. These data can be taken as furt her evidence that multigenic control of chitin synthetase in fungi operates as a safety mechanism to guarantee fungal viability in changing and hostil e environmental conditions.