However, little is known about the role of signal transduction pathways in Pst and other rust fungi due to their obligate nature and the lack of an efficient and reliable transformation system. When expressed in Ustilago maydis, the PtMAPK1 MAPK gene of Puccinia triticina was able to partially complement the kpp2 kpp6 mutant for mating, virulence, and pathogenicity. In a previous study, a MAPK gene, designated PsMAPK1, was identified in ESTs generated from a Pst cDNA library. Here we examined the expression profiles of PsMAPK1 and its activities in two ascomycetous pathogens. PsMAPK1 could partially complement the F. graminearum map1 and M. oryzae pmk1 mutants. Results from these studies indicate that F. graminearum or M. oryzae can be used as a heterologous expression system for functional studies with Pst genes and PsMAPK1 may play an important role in regulating penetration and infectious growth of the wheat stripe rust fungus. In this study we described the isolation and characterization of the first MAPK gene from the wheat stripe rust fungus Pst. Single nucleotide polymorphism markers are important tools for various studies, such as recombination, chromosomal dynamics, genome rearrangement, and LY294002 genetic relatedness between individuals. SNPs can be located in the coding or intergenic regions. In coding regions, SNPs may result changes in protein structures and functions. In this study, we identified SNPs in the PsMAPK1 coding region among different field isolates of Pst. In the human genome, SNPs are estimated to occur once every 1 kb. In Candida albicans, the average SNP frequency set was 1 SNP per 83 bp. Between the PsMAPK1 sequences from Pst isolates CYR29 and CYR32, the SNP frequency was about 1 SNP per 50 bp, indicating that significant genetic variations exist among different isolates of Pst. In a previous study, a higher genetic recombination rate was observed in strains originating from the Tianshui county, suggesting the existence of sexual reproduction in Pst. A recent study also revealed that Berberis can serve as the alternate host for Pst. Therefore, we speculate that sexual recombination may account for the high gene variations in Pst. In addition, the SNPs identified from Pst reference isolates can be used as useful molecular markers to distinguish different races in the field. In a number of plant pathogenic fungi, Fus3/Kss1 orthologs have been shown to regulate various plant infection processes, such as appressorium formation in M. oryzae, Colletotrichum lagenarium, and Cochliobolus heterostrophus. In the barley powdery mildew fungus Blumeria graminis, a MAPK gene also has been implicated in the regulation of appressorium development by complementation assays. In this study, we found that PsMAPK1 could partially complement the pmk1 mutant in appressorium formation and plant infection.