receptor-like MLO protein and the RAC/ROP family G-protein RACB modulate actin reorganization in barley attacked by the biotrophic powdery mildew fungus Blumeria graminis f.sp. hordei

• Published in: The Plant journal : for cell and molecular biology 2005-01, Vol.41 (2), p.291-303
• Main Authors: Opalski, K.S, Schultheiss, H, Kogel, K.H, Huckelhoven, R
• Format: Article
• Language: English
• Subjects: actin; Actin Cytoskeleton - metabolism; Actin Cytoskeleton - microbiology; Actins - metabolism; Barley; Biological and medical sciences; Blumeria graminis; epidermis (plant); Erysiphe graminis f. sp. hordei; Fundamental and applied biological sciences. Psychology; Fungal plant pathogens; Fungi; Fungi - pathogenicity; G-proteins; Hordeum - genetics; Hordeum - metabolism; Hordeum - microbiology; Hordeum vulgare; microfilaments; MLO; Pathogens; Pathology, epidemiology, host-fungus relationships. Damages, economic importance; penetration resistance; Phytopathology. Animal pests. Plant and forest protection; Plant Epidermis - microbiology; plant pathogenic fungi; Plant populations; plant proteins; Plant Proteins - genetics; Plant Proteins - metabolism; polarity; powdery mildew; Proteins; rac GTP-Binding Proteins - metabolism; receptor-like protein; small RAC/ROP G‐protein; susceptibility
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/j.1365-313x.2004.02292.x

Cytoskeleton remodelling is a crucial process in determining the polarity of dividing and growing plant cells, as well as during interactions with the environment. Nothing is currently known about the proteins, which regulate actin remodelling during interactions with invading pathogens. The biotrophic powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh) invades susceptible barley (Hordeum vulgare L.) by penetrating epidermal cells, which remain intact during fungal development. In contrast, resistant host plants prevent infection by inhibiting penetration through apoplastic mechanisms, which require focusing defence reactions on the site of attack. We stained actin filaments in a susceptible Mlo-genotype and a near-isogenic race-non-specifically resistant barley mlo5-mutant genotype using fluorescence-labelled phalloidin after chemical fixation. This revealed that the actin cytoskeleton is differentially reorganized in susceptible and resistant hosts challenged by Bgh. Actin filaments were polarized towards the sites of attempted penetration in the resistant host, whereas when susceptible hosts were penetrated, a more subtle reorganization took place around fungal haustoria. Strong actin filament focusing towards sites of fungal attack was closely associated with successful prevention of penetration. Actin focusing was less frequent and seemingly delayed in susceptible wild-type barley expressing the susceptibility factor MLO. Additionally, single cell overexpression of a constitutively activated RAC/ROP G-protein, CA RACB, another potential host susceptibility factor and hypothetical actin cytoskeleton regulator, partly inhibited actin reorganization when under attack from Bgh, whereas knockdown of RACB promoted actin focusing. We conclude that RACB and, potentially, MLO are host proteins involved in the modulation of actin reorganization and cell polarity in the interaction of barley with Bgh.