title: Barley disease susceptibility factor RACB acts in epidermal cell polarity and positioning of the nucleus.
abstract: RHO GTPases are regulators of cell polarity and immunity in eukaryotes. In plants, RHO-like RAC/ROP GTPases are regulators of cell shaping, hormone responses, and responses to microbial pathogens. The barley (Hordeum vulgare L.) RAC/ROP protein RACB is required for full susceptibility to penetration by Blumeria graminis f.sp. hordei (Bgh), the barley powdery mildew fungus. Disease susceptibility factors often control host immune responses. Here we show that RACB does not interfere with early microbe-associated molecular pattern-triggered immune responses such as the oxidative burst or activation of mitogen-activated protein kinases. RACB also supports rather than restricts expression of defence-related genes in barley. Instead, silencing of RACB expression by RNAi leads to defects in cell polarity. In particular, initiation and maintenance of root hair growth and development of stomatal subsidiary cells by asymmetric cell division is affected by silencing expression of RACB. Nucleus migration is a common factor of developmental cell polarity and cell-autonomous interaction with Bgh RACB is required for positioning of the nucleus near the site of attack from Bgh We therefore suggest that Bgh profits from RACB's function in cell polarity rather than from immunity-regulating functions of RACB.
authors.affiliation: Phytopathology, Technische Universität München, D-85354 Freising-Weihenstephan, Germany.
authors.forename: Björn
authors.initials: B
authors.lastname: Scheler
authors.affiliation: Phytopathology, Technische Universität München, D-85354 Freising-Weihenstephan, Germany.
authors.forename: Vera
authors.initials: V
authors.lastname: Schnepf
authors.affiliation: Phytopathology, Technische Universität München, D-85354 Freising-Weihenstephan, Germany.
authors.forename: Carolina
authors.initials: C
authors.lastname: Galgenmüller
authors.affiliation: Phytopathology, Technische Universität München, D-85354 Freising-Weihenstephan, Germany.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Ranf
authors.affiliation: Phytopathology, Technische Universität München, D-85354 Freising-Weihenstephan, Germany hueckelhoven@wzw.tum.de.
authors.forename: Ralph
authors.initials: R
authors.lastname: Hückelhoven
mesh_terms: D001203:Ascomycota / Q000502:physiology*; D004198:Disease Susceptibility / Q000276:immunology / Q000382:microbiology; D018506:Gene Expression Regulation, Plant / Q000276:immunology*; D001467:Hordeum / Q000235:genetics* / Q000276:immunology* / Q000382:microbiology; D010935:Plant Diseases / Q000276:immunology* / Q000382:microbiology; D057865:Plant Immunity / Q000235:genetics*; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism
chemical_list: D010940:Plant Proteins
keywords: Blumeria graminis; MAP kinase; ROP GTPase.; disease susceptibility; epidermis; nucleus; oxidative burst; polarity