The MAP Kinase Kinase Gene Regulates Multiple Aspects of Pathogenesis.

Mitogen-activated protein kinase (MAPK) cascades in fungi are ubiquitously conserved signaling pathways that regulate stress responses, vegetative growth, pathogenicity, and many other developmental processes. Previously, we reported that the gene, which encodes a mitogen-activated protein kinase kinase (MAPKK) in , plays a central role in pathogenicity against host cabbage plants. In this research, we further characterized the role of in the pathogenicity of this fungus using mutants. Disruption of the gene of reduced accumulation of metabolites toxic to the host plant in liquid culture media. The mutants could not efficiently detoxify cruciferous phytoalexin brassinin, possibly due to reduced expression of the brassinin hydrolase gene involved in detoxifying brassinin. Disruption of the gene also severely impaired fungal detoxification of reactive oxygen species. gene disruption reduced the enzymatic activity of cell wall-degrading enzymes, including cellulase, β-glucosidase, pectin methylesterase, polymethyl-galacturonase, and polygalacturonic acid transeliminase, during host plant infection. Altogether, the data strongly suggest the MAPKK gene plays a pivotal role in during host infection by regulating multiple steps, and thus increasing pathogenicity and inhibiting host defenses.