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