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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

Oliveira‐Garcia, Ely, Deising, Holger B.
The plant journal 2016 v.87 no.4 pp. 355-375
Glomerella graminicola, Zea mays, appressoria, beta-glucans, cell walls, conidia, corn, ecological invasion, enzymes, fungi, gene overexpression, genes, hydrogen peroxide, hyphae, image analysis, immune response, pathogenesis, pathogenicity, pathogens, polymers, receptors, transcriptional activation
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 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 β‐1,3‐β‐1,6‐glucan in biotrophic hyphae is required for attenuation of plant immune responses.