title: Attenuation of PAMP-triggered immunity in maize requires down-regulation of the key β-1,6-glucan synthesis genes KRE5 and KRE6 in biotrophic hyphae of Colletotrichum graminicola.
abstract: In plants, pathogen defense is initiated by recognition of pathogen-associated molecular patterns (PAMPs) via plasma membrane-localized pattern-recognition receptors (PRRs). Fungal structural cell wall polymers such as branched β-glucans are essential for infection structure rigidity and pathogenicity, but at the same time represent PAMPs. Kre5 and Kre6 are key enzymes in β-1,6-glucan synthesis and formation of branch points of the β-glucan network. In spite of the importance of branched β-glucan for hyphal rigidity and plant-fungus interactions, neither the role of KRE5 and KRE6 in pathogenesis nor mechanisms allowing circumventing branched β-glucan-triggered immune responses are known. We functionally characterized KRE5 and KRE6 of the ascomycete Colletotrichum graminicola, a hemibiotroph that infects maize (Zea mays). After appressorial plant invasion, this fungus sequentially differentiates biotrophic and highly destructive necrotrophic hyphae. RNAi-mediated reduction of KRE5 and KRE6 transcript abundance caused appressoria to burst and swelling of necrotrophic hyphae, indicating that β-1,6-glucosidic bonds are essential in these cells. Live cell imaging employing KRE5:mCherry and KRE6:mCherry knock-in strains and probing of infection structures with a YFP-conjugated β-1,6-glucan-binding protein showed expression of these genes and exposure of β-1,6-glucan in conidia, appressoria and necrotrophic, but not in biotrophic hyphae. Overexpression of KRE5 and KRE6 in biotrophic hyphae led to activation of broad-spectrum plant defense responses, including papilla and H2 O2 formation, as well as transcriptional activation of several defense-related genes. Collectively, our results strongly suggest that down-regulation of synthesis and avoidance of exposure of branched β-1,3-β-1,6-glucan in biotrophic hyphae is required for attenuation of plant immune responses.
authors.affiliation: Martin-Luther-Universität Halle-Wittenberg, Naturwissenschaftliche Fakultät III, Institut für Agrar- und Ernährungswissenschaften, Phytopathologie und Pflanzenschutz, Betty-Heimann-Str. 3., D-06120, Halle/Saale, Germany.
authors.forename: Ely
authors.initials: E
authors.lastname: Oliveira-Garcia
authors.affiliation: Martin-Luther-Universität Halle-Wittenberg, Naturwissenschaftliche Fakultät III, Institut für Agrar- und Ernährungswissenschaften, Phytopathologie und Pflanzenschutz, Betty-Heimann-Str. 3., D-06120, Halle/Saale, Germany. holger.deising@landw.uni-halle.de.|Martin-Luther-Universität Halle-Wittenberg, Interdisziplinäres Zentrum für Nutzpflanzenforschung, Betty-Heimann-Str. 3., D-06120, Halle/Saale, Germany. holger.deising@landw.uni-halle.de.
authors.forename: Holger B
authors.initials: HB
authors.lastname: Deising
mesh_terms: D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D020231:Colletotrichum / Q000235:genetics / Q000276:immunology* / Q000502:physiology; D015536:Down-Regulation; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism*; D025301:Hyphae; D037102:Lectins / Q000235:genetics / Q000378:metabolism; D010935:Plant Diseases / Q000276:immunology* / Q000382:microbiology; D034622:RNA Interference; D003313:Zea mays / Q000235:genetics / Q000276:immunology*; D047071:beta-Glucans / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D005656:Fungal Proteins; D037102:Lectins; D047071:beta-Glucans; C037937:glucan-binding proteins; C064197:beta-1,6-glucan
keywords: Colletotrichum graminicola; KRE genes; PAMP; PR proteins; RNA interference; Zea mays; biotrophy; defense responses; necrotrophy; β-1,6-glucan