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We thank Dr D. Scheel (Leibniz Institute of Plant Biochemistry, Germany), Dr C. Zipfel (Sainsbury Laboratory, UK), Dr T. Nurnberger and Dr Andrea Gust (University of Tubingen, Germany), Dr M. Knight (Durham University, UK) and Dr J. E. Parker (Max Planck Institute for Plant Breeding Research, Cologne) for providing biological materials. We thank the Megazyme Technical Support Team and Dr Benoit Darblade (Elicytil, France) for providing quality control analyses of the MAMPs/DAMPs used in this article. The MALDI-TOF MS analyses were performed in the Proteomics Unit of Complutense University of Madrid that belongs to ProteoRed (PRB2-ISCIII supported by grant PT13/0001). This work was supported by Spanish Ministry of Economy and Competitiveness (MINECO) grants BIO2015-64077-R and BIO2012-32910 to AM and BIO2013-41403-R to LFP. HM was funded by an IEF grant (Sign-WALLINg-624721) from the European Union and LB was recipient of a FPI fellowship (BES-2013-065010) from MINECO.

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Non-branched-1,3-glucan oligosaccharides trigger immune responses in Arabidopsis

Publicat a:Plant Journal. 93 (1): 34-49 - 2018-01-01 93(1), DOI: 10.1111/tpj.13755

Autors: Melida, Hugo; Sopena-Torres, Sara; Bacete, Laura; Garrido-Arandia, Maria; Jorda, Lucia; Lopez, Gemma; Munoz-Barrios, Antonio; Pacios, Luis F.; Molina, Antonio;

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Fungal cell walls, which are essential for environmental adaptation and host colonization by the fungus, have been evolutionarily selected by plants and animals as a source of microbe-associated molecular patterns (MAMPs) that, upon recognition by host pattern recognition receptors (PRRs), trigger immune responses conferring disease resistance. Chito-oligosaccharides [-1,4-N-acetylglucosamine oligomers, (GlcNAc)(n)] are the only glycosidic structures from fungal walls that have been well-demonstrated to function as MAMPs in plants. Perception of (GlcNAc)(4-8) by Arabidopsis involves CERK1, LYK4 and LYK5, three of the eight members of the LysM PRR family. We found that aglucan-enriched wall fraction from the pathogenic fungus Plectosphaerella cucumerina which was devoid of GlcNAc activated immune responses in Arabidopsis wild-type plants but not in the cerk1 mutant. Using this differential response, we identified the non-branched 1,3--d-(Glc) hexasaccharide as a major fungal MAMP. Recognition of 1,3--d-(Glc)(6) was impaired in cerk1 but not in mutants defective in either each of the LysM PRR family members or in the PRR-co-receptor BAK1. Transcriptomic analyses of Arabidopsis plants treated with 1,3--d-(Glc)(6) further demonstrated that this fungal MAMP triggers the expression of immunity-associated genes. In silico docking analyses with molecular mechanics and solvation energy calculations corroborated that CERK1 can bind 1,3--d-(Glc)(6) at effective concentrations similar to those of (GlcNAc)(4). These data support that plants, like animals, have selected as MAMPs the linear 1,3--d-glucans present in the walls of fungi and oomycetes. Our data also suggest that CERK1 functions as an immune co-receptor for linear 1,3--d-glucans in a similar way to its proposed function in the recognition of fungal chito-oligosaccharides and bacterial peptidoglycan MAMPs. Significance statement Despite pioneering work and recently regained momentum, the plant-specific knowledge about the mechanisms of -glucan perception and its role in activation of plant defence clearly lags behind the animal field. This article demonstrates that Arabidopsis can perceive non-branched 1,3--glucans of various degrees of polymerization and that this recognition is at least partially mediated by CERK1, but it does not involve BAK1, the co-receptor of other PRRs.

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