# 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
\(\alpha\)-terpinen-4-ol and \(\alpha\)-phellandran-8-ol; acyclic monoterreponoids including nerol and geraniol; cyclic monoterreponoids such as linalool oxide; bicyclic monoterepenes such as \(\beta\)-pinene and 3-carene; sesquiterpenoids like \(\alpha\)-humulene and valencene; aldehydes derived from fatty acids like octanal; and benzenoids including guaiacol. These results observed in _NahG_ tomato plants suggest a compensatory defence mechanism in response to the absence of salicylic acid (SA). Conversely, transgenic _NahG_ tomato plants exhibited reduced accumulation of the aldehydes (_E_)-2-hexenal and (_E_,_E_)-2.4-heptalienal, malonamic acid, and the monoterreponoid (_Z_)-2-Caren-4-ol in response to CEvd infection (red colours in column 1 of Figure 3). These results appear to indicate that SA is involved in their biosynthesis or induction during CEvd infection. These compounds are known for their antimicrobial and signalling roles [34; 35], and their diminished levels may reflect a weakened defence response in the absence of SA. This highlights the role of SA in regulating key metabolic pathways involved in plant defence against pathogens. On the other hand, the specific overproduction of the monoterepenes \(\beta\)-myrrene and \(\alpha\)-terpinen-4-ol in CEvd-infected _RNAi_SSH_ tomato plants (Figure S1C) may suggest that their biosynthesis is regulated in an SA-dependent manner. The characteristic \"aroma of tolerance\" (light green colours in column 3 of Figure 3) also includes methane monoterreponoids (eucalyptt, terpinolene, and \(\Upsilon\)-terpinene), two acyclic monoterreponoids isomers (nerol and geraniol), a bicyclic monoterreponoid (myrtenol), and a sesquiterpene (\(\alpha\)-humulene). In contrast, the levels of two benzenoids (eugenol and guaiacol), an acyclic monoterreponoid (\(p\)-cymene), and a fatty aldehyde derivative (nonanal) were reduced in CEvd-infected _RNAii_SSH_ plants (column 3 of Figure 3). In response to CEvd infection, _NahG_ plants, which are deficient in SA accumulation, exhibited significantly higher accumulation of several VOCs. In contrast, _RNAi_SSH_ plants, which accumulate higher levels of SA, responded to CEvd infection in a more measured manner, with fewer compounds showing significant fold changes compared to _NahG_ plants. The controlled response observed in _RNAi_SSH_ plants highlights the regulatory role of SA in modulating volatile accumulation and suggests that plants with sufficient SA levels may rely on more targeted or efficient defence pathways, avoiding the overproduction of volatiles observed in _NahG_ plants. Additionally, Supplementary Table S1 provides a comprehensive catalogue of VOCs significantly altered in CEvd-infected tomato plants compared to mock-treated controls across all genotypes. This detailed analysis highlights the range of VOCs associated with the tomato-CEvd interaction, offering valuable insights into genotype-specific and general responses to infection. Volatile Organic Compounds as Potential Markers of Tolerance and Susceptibility in CEvd-Infected Plants  
To validate the metabolomics analysis, the relative quantification of some discriminant volatiles representative of both the aroma of death and tolerance was conducted. Similar trends were observed for the cyclic monoterpenes \(\beta\)-pinene and 3-carene, the monoterreponoid linalool oxide, and the aldehyde fatty acid derivative nonanal which were found to be common to the aroma of death and tolerance (Figure 4A). The relative quantification of the four VOCs shows a consistent pattern of elevated levels in both _NahG_ and _RNAi_SSH_ tomato plants upon CEvd infection. This consistent accumulation across genotypes may indicate that these VOCs are part of a broader, SA-independent metabolic response to CEvd infection. Their production could be driven by alternative signalling pathways, such as those involving ethylene, which has also been implicated in plant responses to vivid infections [32].