Attenuation of PAMP-triggered immunity in maize requires down-regulation of the key [beta]-1,6-glucan synthesis genes KRE5 and KRE6 in biotrophic hyphae of Colletotrichum graminicola

• Published in: The Plant journal : for cell and molecular biology Vol. 87; no. 4; p. 355
• Main Authors: Oliveira-Garcia, Ely, Deising, Holger B
• Format: Journal Article
• Language: English
• Published: Oxford Blackwell Publishing Ltd 01.08.2016
• Subjects: Corn; Crop diseases; Genes; Plant pathology; Plant resistance
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/tpj.13205

Summary 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 [beta]-glucans are essential for infection structure rigidity and pathogenicity, but at the same time represent PAMPs. Kre5 and Kre6 are key enzymes in [beta]-1,6-glucan synthesis and formation of branch points of the [beta]-glucan network. In spite of the importance of branched [beta]-glucan for hyphal rigidity and plant-fungus interactions, neither the role of KRE5 and KRE6 in pathogenesis nor mechanisms allowing circumventing branched [beta]-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 [beta]-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 [beta]-1,6-glucan-binding protein showed expression of these genes and exposure of [beta]-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 H2O2 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 [beta]-1,3-[beta]-1,6-glucan in biotrophic hyphae is required for attenuation of plant immune responses. Significance Statement Pathogen-associated molecular patterns (PAMPs) activate broad-spectrum defense responses in plants. One way that pathogens attenuate this activation is to down-regulate PAMP synthesis at critical stages of pathogenesis. Here we show that down-regulation of key genes of [beta]-1,6-glucan synthesis allows the pathogen to escape defense initiation and thus establish a compatible parasitic interaction.