A Yeast STE11 Homologue CoMEKK1 Is Essential for Pathogenesis-Related Morphogenesis in Colletotrichum orbiculare

Several signal transduction pathways, including mitogen-activated protein kinase (MAPK) pathways, are Involved in appressorium development in Colletotrichum orbtculare, the causal agent of cucumber anthracnose disease In this study, CoMEKK1, a yeast MAPK kinases (MAPKK) kinase STE11 homolog, was identified as a disrupted gene in an Agrobacterium tumefaciens-mediated transformation mutant The phenotype of comekk1 disruptant was similar to that of cmk1, a Saccharomyces cerevistae Fus3/Kss1 MAPK homolog mutant Moreover, comekk1 and cmk1 mutants were sensitive to high osmotic and salinity stresses, indicating that Comekk1p/Cmk1p signal transduction is involved in stress tolerance The transformants of the wild type and the comekk1 mutant expressing a constitutively active form of the CoMEKK1 showed slower hyphal growth and abnormal appressorium formation, whereas those of the cmk1 disruptant did not A Cmk1p-green fluorescent protein (GFP) intracellular localization experiment indicated that nuclear localization of the Cmk1p-GFP fusion protein induced by salt stress was diminished in comekk1 mutants These results indicate that Comekk1p functions upstream of Cmk1p