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Plant J. 2016 Aug;87(4):355-75. doi: 10.1111/tpj.13205. Epub 2016 Aug 18.

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.

Author information

1
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.
2
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.
3
Martin-Luther-Universität Halle-Wittenberg, Interdisziplinäres Zentrum für Nutzpflanzenforschung, Betty-Heimann-Str. 3., D-06120, Halle/Saale, Germany. holger.deising@landw.uni-halle.de.

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.

KEYWORDS:

Colletotrichum graminicola; KRE genes; PAMP; PR proteins; RNA interference; Zea mays; biotrophy; defense responses; necrotrophy; β-1,6-glucan

PMID:
27144995
DOI:
10.1111/tpj.13205
[Indexed for MEDLINE]
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