# VOCs Are Relevant Biomarkers of Elicitor-Induced Defences in Grapevine
## 1 Introduction  
Grapevine (_Vitis vinifera_ L.) is an economically important crop highly susceptible to fungal diseases such as downy mildew and powdery mildew, caused by theomycete _Plasmopara viticola_ (_Pv_) and the ascomycete _Erysiphe reactor_, respectively. Controlling these diseases mainly relies on the application of chemical fungicides. However, the extensive use of these pesticides raises environmental and health issues. Elicitor-induced resistance (IR) of plants against diseases is an attractive strategy for reducing the use of chemical fungicides. Elicitors trigger plant defence reactions, resulting in antimicrobial production or/and cell-wall reinforcement that stop or limit pathogen infection [1]. Elicitors are compounds of various biochemical classes, including lipids, proteins, and carbohydrates. Some of them are microbe- or damage-associated molecular patterns (MAMPs and DAMPs),recognised by specific plant cell receptors [2; 3]. Other elicitors are phytohormones e.g., jasmonic acid and salicylic acid and some of their chemical functional analogues [4; 5].  
Successful applications of elicitors in field conditions have been reported [6; 7]. Some are commercialised in France to protect grapevine against downy mildew, powdery mildew, and grey mould. For example, elicitors composed of chito-oligosaccharides (COS-OGA), disodium phosphonate or cerevisiae (yeast extract) are commercialised on the French market. Although they are increasingly used, they are not commonly used yet, mainly due to a variable level of efficiency. The plant responsiveness to elictor application can be influenced by a number of factors such as the plant developmental stage, genotype, abiotic stress, or nutrition [8; 9]. Therefore, further investigations are required to optimise this strategy in field conditions. In this context, relevant, easy-to-analyse biomarkers are needed to monitor the plant defence responses induced by elictor application, so as to determine the conditions for optimal efficiency of such treatments and in turn their positioning in integrated crop protection strategies. Plant defence compounds are logical candidate markers. Stilbene phytoalexins have been the most studied defence response of grapevine so far [10]. They are antimicrobial compounds inducible by various elicitors such as chitosan [11], methyl jasmonate [4], lamininin [12] or sulphated lamininin [13], and are associated to induced resistance against pathogens [10]. However, their analysis requires plant organ sampling and extraction steps, and is therefore tricky to implement in field studies. Besides stilbenes, volatile organic compounds (VOCs) are secondary metabolites that play important roles in the adaptation of the plant to its environment, e.g., pollination or resistance to abiotic and biotic stresses [14]. Regarding plant-pathogen interactions, VOCs can act indirectly as airborne signals that prime defence reactions against pathogens in neighbouring plants [15; 16; 17], or directly as antimicrobial compounds [18]. A limited number of works have reported that elicitors can trigger VOC emission in plants [16; 19; 20; 21]. Sulphated laminarin induces VOC emission--mainly mono- and sesquiterpen--in grapevine [21].  
An advantage of VOC analysis is that VOC collection does not require organ sampling, and there is no extraction step. Thus, VOCs may represent valuable, relatively easy to assess markers of grapevine response to elicitors. In order to verify this hypothesis and pave the way for future experiments in field conditions, we analysed and compared VOC emission by grapevine leaves treated by different elicitors, with a special focus on three commercial ones: Bastid(r) (COS-OGA), Redeli(r) (disodium phosphonate), and Romeo(r) (cerevisiae).