title: Independent signaling pathways regulate cellular turgor during hyperosmotic stress and appressorium-mediated plant infection by Magnaporthe grisea.
abstract: The phytopathogenic fungus Magnaporthe grisea elaborates a specialized infection cell called an appressorium with which it mechanically ruptures the plant cuticle. To generate mechanical force, appressoria produce enormous hydrostatic turgor by accumulating molar concentrations of glycerol. To investigate the genetic control of cellular turgor, we analyzed the response of M. grisea to hyperosmotic stress. During acute and chronic hyperosmotic stress adaptation, M. grisea accumulates arabitol as its major compatible solute in addition to smaller quantities of glycerol. A mitogen-activated protein kinase-encoding gene OSM1 was isolated from M. grisea and shown to encode a functional homolog of HIGH-OSMOLARITY GLYCEROL1 (HOG1), which encodes a mitogen-activated protein kinase that regulates cellular turgor in yeast. A null mutation of OSM1 was generated in M. grisea by targeted gene replacement, and the resulting mutants were sensitive to osmotic stress and showed morphological defects when grown under hyperosmotic conditions. M. grisea deltaosm1 mutants showed a dramatically reduced ability to accumulate arabitol in the mycelium. Surprisingly, glycerol accumulation and turgor generation in appressoria were unaltered by the Deltaosm1 null mutation, and the mutants were fully pathogenic. This result indicates that independent signal transduction pathways regulate cellular turgor during hyperosmotic stress and appressorium-mediated plant infection. Consistent with this, exposure of M. grisea appressoria to external hyperosmotic stress induced OSM1-dependent production of arabitol.
authors.affiliation: School of Biological Sciences, University of Exeter, Washington Singer Laboratories, Exeter EX4 4QG, United Kingdom.
authors.forename: K P
authors.initials: KP
authors.lastname: Dixon
authors.forename: J R
authors.initials: JR
authors.lastname: Xu
authors.forename: N
authors.initials: N
authors.lastname: Smirnoff
authors.forename: N J
authors.initials: NJ
authors.lastname: Talbot
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D017931:DNA Primers; D005656:Fungal Proteins / Q000235:genetics*; D020082:Magnaporthe / Q000235:genetics / Q000378:metabolism* / Q000472:pathogenicity; D020928:Mitogen-Activated Protein Kinases / Q000235:genetics; D008969:Molecular Sequence Data; D009154:Mutation; D009997:Osmotic Pressure; D010944:Plants / Q000382:microbiology*; D029701:Saccharomyces cerevisiae Proteins; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D013385:Succinate Dehydrogenase
chemical_list: D017931:DNA Primers; D005656:Fungal Proteins; C063916:OSM1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D013385:Succinate Dehydrogenase; D020928:Mitogen-Activated Protein Kinases