Protein kinase C is required for the pathogenicity of Setosphaeria turcica on maize

Northern corn leaf blight caused by Setosphaeria turcica is one of the most important fungal diseases of maize. We previously cloned and analyzed the gene encoding protein kinase C (PKC) in S. turcica, as this Ca2+ signaling component is required for growth and pathogenicity of several other plant pathogenic fungi. Here, we assessed the effect of PKC knockout on S. turcica pathogenicity using four quinic-acid-inducible PKC antisense mutants. The conditional mutants enabled us to study the role of PKC in growth, germination, appressorium development, and pathogenicity despite the fact that constitutive PKC knockout is typically lethal in filamentous fungi. The growth rates of the mutants were the same as that of the wild type (WT) on PDA medium but were much lower on PDA medium that contained the inducer 15 mM quinic acid. Conidial germination and appressorium development were almost completely abolished in the induced mutants, and penetration experiments showed that appressoria from the WT could penetrate cellophane, whereas those of the induced mutants could not. Inoculation experiments showed that the mutant strains failed to produce visible lesions on unwounded maize leaves, although they were able to infect wounded leaves, suggesting that they were impaired in the process of host penetration. Characterization of PKC antisense mutants thus highlighted multiple roles of PKC in S. turcica pathogenicity, laying a foundation for understanding and exploiting a key signal transduction pathway in plant pathogenic fungi.