In nature, plants are constantly exposed to potentially pathogenic microbes such as bacteria, fungi, oomycetes or viruses. However, plants have developed effective immune systems triggering various defence reactions against invading pathogens upon the perception of pathogen-associated molecular patterns (PAMPs; Dodds and Rathjen, 2010). The recognition of these conserved microbial signatures is ensured by Pattern Recognition Receptors (PRRs) which also detect plant endogenous molecules released during pathogen invasion, called damage-associated molecular patterns (DAMPs; Boller and Felix, 2009). Chitin, a fungal cell wall component, is a well-known PAMP that triggers defence responses in many mammal and plant species. The aim of the study was to determine the effects of chito-oligosaccharides on grapevine’s immunity and identify the receptor(s) involved in the perception of chito-oligosaccharides in grapevine. Material and methods Grapevine cells (Vitis vinifera cv Gamay) were cultivated as described in Gauthier et al. (2014). Arabidopsis thaliana plants from wild-type (WT) Columbia (Col-0), mutant and transgenic lines were grown in vitro for two weeks in controlled conditions for defence responses or in jiffy peat pellets in a controlled growth chamber for four weeks for protection assays. Grapevine cells or Arabidopsis plants were treated with water, chitin, chitosan (Elicityl, 0.1 g/l for cells and 1 g/l for plants) or flagelline (10 μM) taken as a positive control. ROS production and cytosolic Ca2+ variations ([Ca2+]cyt) in grapevine cells were performed according to Dubreuil-Maurizi et al. (2010) after elicitor treatments, by measuring the chemiluminescence of luminol for H2O2 production and using apoaequorin expressing cells to detect variations of [Ca2+]cyt. Protein extraction, SDS-PAGE and western blotting for MAPK phosphorylation analysis were carried out as previously described (Trdà et al., 2014). RNA extraction and quantitative real-time PCR were performed using primers for the Highlights • Two Pattern Recognition Receptors (PRRs) VvLYK1-1 and VvLYK1-2 participate in the signaling of chito-oligosaccharides in grapevine • VvLYK1-1 is involved in powdery mildew resistance amplification of defence marker genes (CHIT4C, STS1-2, PAL, RBOHD, and FRK1). Two days after elicitor treatment, Botrytis cinerea and Plasmopara viticola infections were performed on grapevine plants. For protection assays to Erysiphe necator, leaves were infected, maintained on agar medium in the incubator and then sampled at 0, 4, 8, 12 and 24 hours post inoculation. Results and discussion In grapevine cells, chitin treatment induced a rapid and transient increase in free [Ca2+]cyt that peaked at 2 min but not chitosan, even if the basal level remained higher during the whole experiment. Both chito-oligosaccharides did not trigger any H2O2 production contrary to the flagelline epitope flg-22. Chitin and chitosan treatment induced the phosphorylation of two MAPKs with relative molecular masses of 45 and 49 kDa in grapevine cells but chitosan activated the phosphorylation of these two MAPKs longer than the chitin treatment. The expression of defence marker genes activated by different elicitors was then followed by qPCR. Among them, both chito-oligosaccharides induced the expression of four grapevine defence genes encoding an acidic chitinase (CHIT4C), a stilbene synthase (STS1-2), a phenylalanine ammonia lyase (PAL) and a respiratory burst oxidase homolog D (RBOHD), 1 hour post-treatment (hpt). The efficacy of chitin- and chitosan-induced immunity was investigated in Vitis vinifera leaf discs infected by the necrotrophic fungus B. cinerea or with the biotrophic oomycete P. viticola, the causal agents of gray mold and downy mildew, respectively. If chitin pretreatment induced a low but significant resistance against these pathogens, chitosan reduced very significantly the B. cinerea lesion diameter and the P. viticola sporulation. Taken toghether, these results demonstrate that grapevine perceives chitin and chitosan suggesting that at least one PRR for chito-oligosaccharides perception exists. To identify the receptor of chito-oligosaccharides in grapevine, the grapevine family of LysM receptor like kinases was characterised and three proteins, respectively named VvLYK1-1, VvLYK1-2 and VvLYK1-3, showed a close relation to Arabidopsis CERK1/LYK1 (Chitin-Elicitor Receptor Kinase 1) and the rice ortholog CERK1. By functional complementation of the Arabidopsis cerk1/lyk1 mutant, impaired in chitin perception and signalling, we demonstrated that VvLYK1-1 and VvLYK1-2 are involved in the signalling of chito-oligosaccharides in Vitis vinifera. Moreover, VvLYK1-1 plays a key role in basal resistance against the grapevine powdery mildew causal agent E. necator.