# Salicylic Acid Modulates Volatile Organic Compound Profiles During CEVd Infection in Tomato Plants
## 4 Discussion
This balanced metabolic profile suggests that wild-type plants activate both SA-dependent and SA-independent pathways, enabling a defence mechanism that minimizes disruption to overall metabolism. The ability to engage multiple pathways, while maintaining metabolic stability, likely underpins their natural CEVd resistance and supports the evidence on SA-JA crosstalk in balancing responses against diverse pathogens [48]. This classic crosstalk between SA and JA was confirmed in _RNAi_55H_ plants, which showed not only enhanced SA production but also compromised JA signalling, displaying susceptibility to _Botrytis cinerea_[9]. Recently, a novel metabolic crosstalk between SA and HMTPs has also been described, due to competition between the MEP pathway involved in the HMIP production and SA biosynthesis [30]. These findings highlight the intricate interplay between hormonal signalling and metabolic pathways, underscoring the complexity of plant defence mechanisms against diverse pathogens. The specific comparison of VOC profiles from _NahG_ vs. _RNAi_55H_ genotypes (Figure 5) revealed that _NahG_ plants were characterized by a predominance of monoterpenoids, emerging as potential indicators of plant disease. Terpenes are well documented for their antimicrobial properties and their roles in activating both localized and systemic plant defence responses [20]. Notably, \(\beta\)-myrccene has been described as an antifungal agent against _Fusarium_[49] and an activator of defence responses in rice [50]. Moreover, in tomato plants, monoterpenoids have been described to activate the defence response by inducing stomatal closure and pathogenesis-related protein 1 (PR1) expression [30]. Additionally, our findings align with the role of herbivore-induced volatiles (HIPVs), such as (\(E\))-\(\beta\)-cimene, in enhancing inter-plant defences against pathogens and herbivores [51]. On the other hand, _RNAi_55H_ plants exhibited higher levels of benzenoids, uncovering the MeSA differential emission, and aldehyde derivatives of fatty acids, including (E)-3-hexenal, (E)-2-hexenal, and hexanal. These aldehydes belong to the green leaf volatile (GLV) family, which has been described to have an important role in the activation of plant immunity [28; 29]. As for MeSA, it is also involved in the plant defence response against pathogens and herbivores, playing a crucial role in plant-to-plant communication [16; 51]. Future research could focus on performing targeted qRT-PCR analyses of genes participating in the biosynthesis of these key VOCs, adjusting sampling times to capture dynamic changes in their expression during infection. Such studies would enhance our understanding of VOC production under stress and support strategies for improving plant resilience through targeted metabolic engineering. This study demonstrates that CEVd infection triggers genotype-dependent metabolic changes in VOC production, which is largely influenced by SA accumulation levels. SA-deficient plants exhibited broader metabolic shifts, while SA-accumulating plants displayed more targeted and efficient defence responses. The identification of key VOCs as markers of pathogen tolerance could offer valuable insights for breeding programmes and targeted metabolic engineering, enabling the development of more resilient crops with enhanced defence mechanisms. Specifically, we propose integrating CRISPR/Cas9 or RNA interference to modulate SA levels or regulate VOC biosynthesis pathways. These strategies could be used to optimize the production of these key defence-related VOCs in plants. Our findings contribute to a deeper understanding of plant-pathogen interactions and pave the way for the development of disease-resistant crop genotypes through metabolic and genetic optimization. By leveraging genotype-specific VOC signatures, future strategies can integrate metabolic traits into sustainable disease management and crop improvement efforts.