Non-Branched-1,3-Glucan Oligosaccharides Trigger Immune Responses In Arabidopsis

PLANT JOURNAL(2018)

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摘要
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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关键词
necrotrophic fungi, plant immunity, cell wall, glucan, chitin, CERK, BAK1
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