title: Septation of infectious hyphae is critical for appressoria formation and virulence in the smut fungus Ustilago maydis.
abstract: Differentiation of hyphae into specialized infection structures, known as appressoria, is a common feature of plant pathogenic fungi that penetrate the plant cuticle. Appressorium formation in U. maydis is triggered by environmental signals but the molecular mechanism of this hyphal differentiation is largely unknown. Infectious hyphae grow on the leaf surface by inserting regularly spaced retraction septa at the distal end of the tip cell leaving empty sections of collapsed hyphae behind. Here we show that formation of retraction septa is critical for appressorium formation and virulence in U. maydis. We demonstrate that the diaphanous-related formin Drf1 is necessary for actomyosin ring formation during septation of infectious hyphae. Drf1 acts as an effector of a Cdc42 GTPase signaling module, which also consists of the Cdc42-specific guanine nucleotide exchange factor Don1 and the Ste20-like kinase Don3. Deletion of drf1, don1 or don3 abolished formation of retraction septa resulting in reduced virulence. Appressorium formation in these mutants was not completely blocked but infection structures were found only at the tip of short filaments indicating that retraction septa are necessary for appressorium formation in extended infectious hyphae. In addition, appressoria of drf1 mutants penetrated the plant tissue less frequently.
authors.affiliation: Department of Biology, Philipps-University Marburg, Marburg, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Freitag
authors.forename: Daniel
authors.initials: D
authors.lastname: Lanver
authors.forename: Christian
authors.initials: C
authors.lastname: Böhmer
authors.forename: Kay Oliver
authors.initials: KO
authors.lastname: Schink
authors.forename: Michael
authors.initials: M
authors.lastname: Bölker
authors.forename: Björn
authors.initials: B
authors.lastname: Sandrock
mesh_terms: D000205:Actomyosin / Q000096:biosynthesis; D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism; D055786:Gene Knockout Techniques; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism; D025301:Hyphae / Q000166:cytology / Q000235:genetics / Q000502:physiology; D010935:Plant Diseases / Q000382:microbiology*; D018515:Plant Leaves / Q000382:microbiology*; D016133:Polymerase Chain Reaction; D017384:Sequence Deletion; D015398:Signal Transduction; D014588:Ustilago / Q000235:genetics / Q000378:metabolism / Q000472:pathogenicity*; D003313:Zea mays / Q000382:microbiology*; D020764:cdc42 GTP-Binding Protein / Q000235:genetics / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D005656:Fungal Proteins; D020662:Guanine Nucleotide Exchange Factors; D000205:Actomyosin; D020764:cdc42 GTP-Binding Protein