Complementation of Ustilago maydis MAPK Mutants by a Wheat Leaf Rust, Puccinia triticina Homolog: Potential for Functional Analyses of Rust Genes

From a large expressed sequence tag (EST) database representing several developmental stages of Puccinia triticina, we discovered a mitogen-activated protein kinase (MAPK) with homology to kinases with known pathogenic functions in other fungi. This PtMAPK1 is similar to the Ustilago maydis MAPK, Ubc3/Kpp2, but has a longer N-terminal extension of 43 amino acids (aa) with identities to U. maydis Kpp6, a homolog of Ubc3/Kpp2 with a 170-aa N-terminal extension. Ubc3/Kpp2 is involved in mating and subsequent pathogenic development, whereas Kpp6 functions during invasive growth in corn tissue. PtMAPK1, expressed from a Ustilago sp.-specific promoter, was able to complement a ubc3/kpp2 deletion mutant and restore mating. It also substantially increased virulence on corn, measured as tumor formation, of a kpp6 deletion mutant. Moreover, this construct restored to near-full pathogenicity a ubc3/kpp2 kpp6 nonpathogenic double deletion mutant. Complementation of the ubc3/kpp2 mutant with the complete PtMAPK gene and verification of expression by reverse-transcription polymerase chain reaction indicated that the rust promoter is recognized in U. maydis. Phylogenetically, these basidiomycete plant pathogens are related, which was reflected in comparison of P. triticina ESTs to U. maydis gene sequences. The U. maydis heterologous expression system allows functional analysis of rust genes, currently frustrated by the lack of efficient transformation and selection procedures.