How the Pathogenic Fungus Alternaria alternata Copes with Stress via the Response Regulators SSK1 and SHO1

• Published in: PloS one 2016-02, Vol.11 (2), p.e0149153-e0149153
• Main Authors: Yu, Pei-Ling, Chen, Li-Hung, Chung, Kuang-Ren
• Format: Article
• Language: English
• Subjects: Alternaria - genetics; Alternaria - metabolism; Alternaria - physiology; Alternaria alternata; Amino acids; Antifungal Agents - chemistry; Baking yeast; Biology and Life Sciences; Brown spot; Cell differentiation; Citrus; Citrus fruits; Clonal deletion; Conidia; Cultivars; Dicarboximide; Diseases and pests; Drug resistance; Environmental effects; Fungal Proteins - metabolism; Fungi; Fungicides; Fungicides, Industrial - pharmacology; Gene Deletion; Gene expression; Gene Expression Regulation, Fungal; Genetic aspects; Germ tubes; Health aspects; Histidine; Histidine kinase; Hog1 protein; Homology; Hydrogen peroxide; Hydrogen Peroxide - chemistry; Infections; Kinases; MAP kinase; MAP Kinase Signaling System; Membrane Proteins - metabolism; Membrane Proteins - physiology; Mitogen-Activated Protein Kinases - metabolism; Multidrug resistance; Mutants; Mutation; Natural resources; Oligonucleotides - genetics; Oryza; Osmolarity; Osmosis; Osmotic stress; Oxidants; Oxidative stress; Oxidizing agents; Pathogenesis; Pesticides; Phenotype; Physical Sciences; Physiological aspects; Phytopathogenic fungi; Plant pathology; Polypeptides; Proteins; Reactive oxygen species; Reactive Oxygen Species - metabolism; Regulators; Saccharomyces cerevisiae; Saccharomyces cerevisiae Proteins - metabolism; Saccharomyces cerevisiae Proteins - physiology; Salts; Sensitivity; Signal Transduction; Stress (Physiology); Stress response; Stress, Physiological; Stresses; Sugar; Tubes; Virulence; Yeast
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0149153

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.