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

<jats:title>Summary</jats:title><jats:p>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 (<jats:styled-content style="fixed-case">MAMP</jats:styled-content>s) that, upon recognition by host pattern recognition receptors (<jats:styled-content style="fixed-case">PRR</jats:styled-content>s), trigger immune responses conferring disease resistance. Chito‐oligosaccharides [β‐1,4‐<jats:italic>N</jats:italic>‐acetylglucosamine oligomers, (Glc<jats:styled-content style="fixed-case">NA</jats:styled-content>c)<jats:sub><jats:italic>n</jats:italic></jats:sub>] are the only glycosidic structures from fungal walls that have been well‐demonstrated to function as <jats:styled-content style="fixed-case">MAMP</jats:styled-content>s in plants. Perception of (Glc<jats:styled-content style="fixed-case">NA</jats:styled-content>c)<jats:sub>4–8</jats:sub> by Arabidopsis involves <jats:styled-content style="fixed-case">CERK</jats:styled-content>1, <jats:styled-content style="fixed-case">LYK</jats:styled-content>4 and <jats:styled-content style="fixed-case">LYK</jats:styled-content>5, three of the eight members of the LysM <jats:styled-content style="fixed-case">PRR</jats:styled-content> family. We found that a glucan‐enriched wall fraction from the pathogenic fungus <jats:italic>Plectosphaerella cucumerina</jats:italic> which was devoid of Glc<jats:styled-content style="fixed-case">NA</jats:styled-content>c activated immune responses in Arabidopsis wild‐type plants but not in the <jats:italic>cerk1</jats:italic> mutant. Using this differential response, we identified the non‐branched 1,3‐β‐<jats:sc>d</jats:sc>‐(Glc) hexasaccharide as a major fungal <jats:styled-content style="fixed-case">MAMP</jats:styled-content>. Recognition of 1,3‐β‐<jats:sc>d</jats:sc>‐(Glc)<jats:sub>6</jats:sub> was impaired in <jats:italic>cerk1</jats:italic> but not in mutants defective in either each of the LysM <jats:styled-content style="fixed-case">PRR</jats:styled-content> family members or in the <jats:styled-content style="fixed-case">PRR</jats:styled-content>‐co‐receptor <jats:styled-content style="fixed-case">BAK</jats:styled-content>1. Transcriptomic analyses of Arabidopsis plants treated with 1,3‐β‐<jats:sc>d</jats:sc>‐(Glc)<jats:sub>6</jats:sub> further demonstrated that this fungal <jats:styled-content style="fixed-case">MAMP</jats:styled-content> triggers the expression of immunity‐associated genes. <jats:italic>In silico</jats:italic> docking analyses with molecular mechanics and solvation energy calculations corroborated that <jats:styled-content style="fixed-case">CERK</jats:styled-content>1 can bind 1,3‐β‐<jats:sc>d</jats:sc>‐(Glc)<jats:sub>6</jats:sub> at effective concentrations similar to those of (Glc<jats:styled-content style="fixed-case">NA</jats:styled-content>c)<jats:sub>4</jats:sub>. These data support that plants, like animals, have selected as <jats:styled-content style="fixed-case">MAMP</jats:styled-content>s the linear 1,3‐β‐<jats:sc>d</jats:sc>‐glucans present in the walls of fungi and oomycetes. Our data also suggest that <jats:styled-content style="fixed-case">CERK</jats:styled-content>1 functions as an immune co‐receptor for linear 1,3‐β‐<jats:sc>d</jats:sc>‐glucans in a similar way to its proposed function in the recognition of fungal chito‐oligosaccharides and bacterial peptidoglycan <jats:styled-content style="fixed-case">MAMP</jats:styled-content>s.</jats:p>