After surface treatment of plane tree leaves, CP induces defence-related responses and programmed cell death. In addition, CP has been reported to induce the production of phenolic compounds in various plants. The mechanism by which CP elicits host and non-host plants after foliar treatment is still poorly understood, because the protein is unable to penetrate the leaf cuticle and a receptor has not been identified yet. The data present in the literature about other CPPs do not help to unravel the question because these proteins are usually injected into the tissues by infiltration. Also the signalling pathways activated in the plants by CP or CPPs are unknown, although an involvement of the plant hormone salicylic acid has been recently reported in tobacco after treatment with BcSpl1 from Botrytis cinerea. In the present study we aimed to clarify the questions concerning the resistance-inducing mechanism triggered by CP in the model plant Arabidopsis. In particular, we investigated the role played by the stomata, the activation of the signalling pathways and the biosynthesis of the phytoalexin camalexin following foliar treatment with CP. ROS are central players in the complex signalling network of cells and it is well known that MAMPs cause an oxidative burst upon their recognition by the plant. An initial burst of ROS production can trigger a cascade of cell-to-cell communication events that propagate the signal over long distances like a wave. In the present study, we analysed the production of H2O2 after treatment with CP at the level of stomata. We treated Arabidopsis leaves on the external epidermal surface for two reasons: the external application mimics the first natural contact between a MAMP and the plant tissue, and above all it can be representative of a possible use of CP in plant protection. Hypothesizing a role for stomata in the perception mechanism of CP, we assumed that H2O2 had to be produced first by the stomata. Accordingly, we designed the experiment to monitor the H2O2 evolution on the epidermis and we started the analysis almost instantaneously. To our knowledge, there are no other data in the literature that show the initial production of H2O2 at the level of stomata and the subsequent spreading on the epidermis after surface treatment with microbial-derived elicitors. Among the CPPs, the H2O2 production in Arabidopsis leaves has been reported for MgSM1 and BcSpl1. However, MgSM1 had been assessed 24 h after the ectopic expression in transgenic plants, whereas BcSpl1 had been assessed 4 h after the infiltration. Our results suggest that CP LY294002 elicited defence responses by entering through the stomata, and perhaps its perception occurred at the level of the inner surface of the guard cells. This action model was suggested by some clear observations: the H2O2 production after the treatment with CP was initiated by guard cells and only subsequently spread from the stomata to the neighbouring epidermal cells ; this occurred even when the peels were treated on the underside, which is devoid of cuticle; the treatment of the lower leaf surface, which normally presents a higher stomatal density, resulted in a higher product.