Are chitin synthases targets for antimicrobial compounds and sources of MAMPs in oomycetes?

National audience; Chitin is a crystalline N-Acetyl-Glucosamine (GlcNAc) polymer that is essential to cell wall function in Fungi, and chitooligosaccharides derived from it are Microbe-Associated Molecular Patterns recognized by LysM-containing receptors. Whereas GlcNAc is usually a minor component in the cell wall of most oomycetes, large scale sequencing shows the presence of chitin synthase (CHS) genes in their genomes. We are interested in determining the biological role of oomycete, and their involvement in the generation of signals perceived by the plant cell. Our microbial models are Aphanomyces euteiches, a legume root parasite, and a Phytophthora parasitica strain pathogenic to tobacco. Whereas chitin has never been detected in P. parasitica, our data suggest that its CHS gene(s) play(s) an essential role. In A. euteiches, we recently showed that amorphous GlcNAc polymers (chitosaccharides) are involved in cell wall function (Badreddine et al 2008). We are now engaged in characterizing these chitosaccharides in order to determine their links to the other cell wall polymers, and to understand how the host plant Medicago truncatula distinguishes chitosaccharide-derived fragments from other microbial signals such as symbiotic Nod factors. Our approaches include 13C-NMR studies of the cell wall polysaccharides, purification of the chitosaccharides after sequential chemical and enzymatic hydrolyses, elicitation bioassays involving the measurement of reactive oxygen species and expression of defense-related genes, and genetic studies targeting candidate genes of the LysM-containing putative receptors family. Last data obtained on both the microbe and plant sides will be presented.