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

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 [β-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 a glucan-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-β-d-(Glc) hexasaccharide as a major fungal MAMP. Recognition of 1,3-β-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-β-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-β-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-β-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-β-d-glucans in a similar way to its proposed function in the recognition of fungal chito-oligosaccharides and bacterial peptidoglycan MAMPs.