Structure-dependent modulation of a pathogen response in plants by synthetic O-antigen polysaccharides

Many phytopathogenic bacteria display lipopolysaccharides (LPS) with the O-chain repeating unit [α-L-Rha-(1→3)-α-L-Rha-(1→3)-α-L-Rha-(1→2)]n. This trisaccharide unit was synthesized and oligomerized to obtain hexa- and nonasaccharides. The deprotected rhamnans were effective in suppressing the hypersensitive response (HR) and in inducing PR-1 gene expression in Arabidopsis thaliana. Conformational analysis of the oligorhamnans by NMR spectroscopy and molecular dynamics calculations revealed that a coiled structure develops with increasing chain length of the oligosaccharide. This is associated with increasing efficacy in HR suppression and PR-1 gene expression. We therefore infer that the coiled structure of phytopathogenic bacteria is a plant-recognizable pathogen-associated molecular pattern (PAMP).