# VOCs Are Relevant Biomarkers of Elicitor-Induced Defences in Grapevine
## 2 Results and Discussion
### Comparison of VOC Emission Elicited by Other Elicitors
The time course of VOC emission was also variable. Elicitor-induced VOC release has been reported in a limited number of studies [16; 19; 20], but to our knowledge a comparative study like ours has never been performed. Moreover, the capacity of a yeast extract and of a phosphonate to enhance grapevine VOC emission is demonstrated for the first time. In order to complete this study, VOCs were analysed in response to chitosan and an oligogalacturonide (i.e., COS-OGA, made up of such compounds), as well as Bion(r). Chitosan and oligogalacturonides are known elicitors of grapevine defence reactions [11; 32; 33; 34]. Their activity depends on their degree of polymerisation and degree of deacetylation for chitosan [11; 32; 35]. Bion(r) is also a well-known elicitor of defence reactions whose active ingredient is acibenzolar-S-methyl or benzothiadiazole (BTH), a hormone-like analogue of salicylic acid. BTH has been reported as an elicitor of grapevine defence reactions [29]. These three elicitors also induced VOC emission with different profiles (Table S1). However, a lower number of VOCs was detected following chitosan treatment, and to a lesser extent following Bion(r) treatment. Few reports have addressed the effect of chitosan on VOC emission by plants. It can induce the release of various VOCs, including mono- and sesquiterpenes in tomato [19] and in rice [20]. Bion(r) was previously shown to induce VOC emission in bean, and this property was associated with enhanced resistance to a bacterial pathogen [36]. Four VOCs have been recurrently induced by the three elicitors, namely \(\alpha\)-farnesene, \(\beta\)-caryophyllene, 2-oxo-4-phenyl-6-(4-chlorophenylenyl)-1,2-dihydropyrimidine, and toluene. The presence of 2-oxo-4-phenyl-6-(4-chlorophenylenyl)-1,2-dihydropyrimidine, induced by an elicitor prepared in water (no formulation), rules out the hypothesis of its presence in commercial mixtures. The molecule oxime-, methoxy-phenyl- was detected in the water blank experiment in the absence of plants (data not shown), suggesting that it was present in the greenhouse ambient air. This VOC is listed among the molecules increased in fermentation processes of fish and wine [37; 38] or found naturally in bamboo shoots [39], _Pseudomonas aeruginosa_[40], and myxobacteria [41]. As for 1,3-benzenedicarboxylic acid, 5-(dimethylamino)-, its presence is hardly reported. VOC emission is therefore consistently induced by elicitors, with different blends despite a relevant core of molecules including \(\alpha\)-farnesene. It is now well established that VOCs belong to the defence repertoire of plants against pathogens, especially pests such as caterpillars. In Arabidopsis, \(\beta\)-caryophyllene and GLVs also directly inhibit bacterial growth [42]. The release of GLV 4-hexen-ol-acetate was triggered by insect infestation on maple [27]. Together with oxime-, methoxyphenyl- production, 4-hexen-ol-acetate production can be induced by aphid attacks on apple [26]. VOCs can prime defence reac  
Figure 3: Venn diagram of the VOCs induced by commercial elicitors. Distribution of 43 VOCs detected with FC > 1.5 at least once at two time points or at least in two replicates for one time point (Table 2), following Bastid®, Redeli® or Romeo® treatment. The 12 commonly induced VOCs are listed in the table on the right. tions like airborne signals [15; 16; 17] or act directly on targets thanks to their antimicrobial properties [42]. In grapevine, increased VOC emission has been reported in the course of interactions with insects. For instance, the feeding of the Japanese beetle on leaves of a _Vitis labrusca cv_. resulted in higher abundance of the GLV MeSA and terpen, including \(\beta\)-ocimene and \(\alpha\)-farnesene [43]. Moreover, the two latter VOCs were the only ones significantly increased following feeding and oviposition of a xylem-feeding sharpshooter on Chardonnay cultivar [44]. Three recent reports also support the role of VOCs in grapevine defence against _P. viticola_.