Salicylic Acid Modulates Volatile Organic Compound Profiles During CEVd Infection in Tomato Plants

<b>Background:</b><i>Citrus Exocortis Viroid</i> (CEVd) is a non-coding RNA pathogen capable of infecting a wide range of plant species, despite its lack of protein-coding ability. Viroid infections induce significant alterations in various physiological and biochemical processes, particularly impacting plant metabolism. This study shows the metabolic changes upon viroid infection in tomato plants (<i>Solanum lycopersicum</i> var. 'MoneyMaker') exhibiting altered levels of salicylic acid (SA), a key signal molecule involved in the plant defence against this pathogen. <b>Methods:</b> Transgenic <i>RNAi_S5H</i> lines, which have the salicylic acid 5-hydroxylase gene silenced to promote SA accumulation, and <i>NahG</i> lines, which overexpress a salicylate hydroxylase to degrade SA into catechol and prevent its accumulation, were used to establish different SA levels in plants, resulting in varying degrees of resistance to viroid infection. The analysis was performed by using gas chromatography-mass spectrometry (GC-MS) to explore the role of volatile organic compounds (VOCs) in plant immunity against this pathogen. <b>Results:</b> Our results revealed distinct volatile profiles associated with plant immunity, where <i>RNAi_S5H</i>-resistant plants showed significantly enhanced production of monoterpenoids upon viroid infection. Moreover, viroid-susceptible <i>NahG</i> plants emitted a broad range of VOCs, whilst viroid-tolerant <i>RNAi_S5H</i> plants exhibited less variation in VOC emission. <b>Conclusions:</b> This study demonstrates that SA levels significantly influence metabolic responses and immunity in tomato plants infected by CEVd. The identification of differential emitted VOCs upon CEVd infection could allow the development of biomarkers for disease or strategies for disease control.