title: How the Pathogenic Fungus Alternaria alternata Copes with Stress via the Response Regulators SSK1 and SHO1.
abstract: The tangerine pathotype of Alternaria alternata is a necrotrophic fungal pathogen causing brown spot disease on a number of citrus cultivars. To better understand the dynamics of signal regulation leading to oxidative and osmotic stress response and fungal infection on citrus, phenotypic characterization of the yeast SSK1 response regulator homolog was performed. It was determined that SSK1 responds to diverse environmental stimuli and plays a critical role in fungal pathogenesis. Experiments to determine the phenotypes resulting from the loss of SSK1 reveal that the SSK1 gene product may be fulfilling similar regulatory roles in signaling pathways involving a HOG1 MAP kinase during ROS resistance, osmotic resistance, fungicide sensitivity and fungal virulence. The SSK1 mutants display elevated sensitivity to oxidants, fail to detoxify H2O2 effectively, induce minor necrosis on susceptible citrus leaves, and displays resistance to dicarboximide and phenylpyrrole fungicides. Unlike the SKN7 response regulator, SSK1 and HOG1 confer resistance to salt-induced osmotic stress via an unknown kinase sensor rather than the "two component" histidine kinase HSK1. SSK1 and HOG1 play a moderate role in sugar-induced osmotic stress. We also show that SSK1 mutants are impaired in their ability to produce germ tubes from conidia, indicating a role for the gene product in cell differentiation. SSK1 also is involved in multi-drug resistance. However, deletion of the yeast SHO1 (synthetic high osmolarity) homolog resulted in no noticeable phenotypes. Nonetheless, our results show that A. alternata can sense and react to different types of stress via SSK1, HOG1 and SKN7 in a cooperative manner leading to proper physiological and pathological functions.
authors: Department of Plant Pathology, College of Agriculture and Natural Resources, National Chung-Hsing University, Taichung, Taiwan.
authors: Pei-Ling
authors: PL
authors: Yu
authors: Department of Plant Pathology, College of Agriculture and Natural Resources, National Chung-Hsing University, Taichung, Taiwan.
authors: Li-Hung
authors: LH
authors: Chen
authors: Department of Plant Pathology, College of Agriculture and Natural Resources, National Chung-Hsing University, Taichung, Taiwan.
authors: Kuang-Ren
authors: KR
authors: Chung
mesh_terms: D000528:Alternaria / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D000935:Antifungal Agents / Q000737:chemistry; D005656:Fungal Proteins / Q000378:metabolism*; D005659:Fungicides, Industrial / Q000494:pharmacology; D017353:Gene Deletion; D015966:Gene Expression Regulation, Fungal; D006861:Hydrogen Peroxide / Q000737:chemistry; D020935:MAP Kinase Signaling System; D008565:Membrane Proteins / Q000378:metabolism* / Q000502:physiology; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D009154:Mutation; D009841:Oligonucleotides / Q000235:genetics; D009995:Osmosis; D010641:Phenotype; D017382:Reactive Oxygen Species / Q000378:metabolism; D012441:Saccharomyces cerevisiae; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism* / Q000502:physiology; D015398:Signal Transduction; D013312:Stress, Physiological; D014774:Virulence
chemical_list: D000935:Antifungal Agents; D005656:Fungal Proteins; D005659:Fungicides, Industrial; D008565:Membrane Proteins; D009841:Oligonucleotides; D017382:Reactive Oxygen Species; C459787:SHO1 protein, S cerevisiae; C087199:SSK1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D006861:Hydrogen Peroxide; C088589:HOG1 protein, S cerevisiae; D020928:Mitogen-Activated Protein Kinases