# Salicylic Acid Modulates Volatile Organic Compound Profiles During CEVd Infection in Tomato Plants
## 3 Results
### Salicylic Acid Accumulation Modulate Volatile Profile in CEVd-Infected Tomato Plants
Alternatively, this response might reflect a general stress-induced increase in VOC production to mitigate pathogen impact or signal neighbouring plants. Further analysis is needed to determine whether these compounds play a direct role in the defence  
In turn, the monoterepenoid \(\alpha\)-terpinene-4-ol, along with the monoterepene D-limonene, the benzenoid guaiacol, and the fatty aldehyde octanal, were specifically associated with the hypersusceptible _NahG_-infected plants. These VOCs are shared components of the so-called \"aroma of death\", suggesting that their production may be associated with symptom development and could therefore be used as biomarkers for vivid infections. Particularly, the monoterepene D-limonene was also found to be over-accumulated in infected wild-type tomato plants, whilst terpinene-4-ol was statistically over-accumulated in _RNAi_55H_-infected plants when compared with the corresponding non-infected control plants (Figure 4B). Finally, the linear monoterepene \(\beta\)-myrcene and the phenolic methyl salicate could be proposed as key components of the \"aroma of tolerance\" against CEVd, as they are significantly induced exclusively in _RNAi_55H_ plants but not in _NahG_ plants (Figure 4C). This highlights their potential role as specific markers of salicylic acid-associated resistance pathways in response to CEVd infection. To further investigate the volatiles associated with both the \"tolerance\" and \"death\" aromas, a PCA was performed with data corresponding to CEVd-infected _NahG_ plants and CEVd-infected _RNAi_55H_ plants. To reduce potential basal effects introduced by transgenesis, the data were normalized as outlined in the Section 2. As Figure 5A shows, PCA clearly separated the two genotypes upon CEVd infection. Notably, analysis of the loading plot identified the VOCs that contributed most to the observed variance. In _RNAi_55H_ plants, which exhibit tolerance to CEVd infection, specific monoterepenes and fatty aldehyde derivatives were highly representative. These include terpinene-4-ol, linalool, \(\alpha\)-terpineol, the benzenoid methyl salicate (MeSA), and the fatty aldehydes (\(E\))-3-hexenal, (\(E\))-2-hexenal, and hexanal. Conversely, VOCs associated with the \"aroma of death\" predominantly included monoterepends and monoterepes, such as \(\alpha\)-phellandran-8-ol, (\(Z\))-caren-2-ol, \(\Upsilon\)-terpinene, o-cymene, (\(E\),\(E\))-cosmene, D-limonene, and sabinene. These findings suggest that two distinct metabolic pathways are involved in CEVd pathogenesis, leading to either tolerance or susceptibility to vivid infection. Figure 4: Relative accumulation levels of selected VOCs in tomato plants of different genotypes (WT, RNAi_55H, and _NahG_) after 21 days of CEVd infection. (**A**) Relative quantification of nonanal, \(\alpha\)-pinene, 3-caren, and \(\alpha\)-terpinene, characteristic of both the aroma of death and resistance; (**B**) relative quantification of linalool oxide, octanal, guaiacol, and D-limonene, specific to the scent of death; and (**C**) relative quantification of \(\beta\)-myrcene and methyl salicate, specific to the aroma of resistance. The accumulation levels were quantified based on the peak ion area corresponding to the VOCs from plants inoculated without the pathogen (mock) and inoculated with the CEVd pathogen. The letters show the grouping information using [PERSON]’s range test (one-way ANOVA method) with a significance level of 5% (\(p\)-value \(<\) 0.05).