# Salicylic Acid Modulates Volatile Organic Compound Profiles During CEVd Infection in Tomato Plants
## 1 Introduction  
Plants are exposed to a variety of stresses that can significantly impact their growth, development, and overall fitness. These stresses are classified as abiotic, caused by environmental factors such as drought, extreme temperature, salinity, and shading [1], or biotic, resulting from pathogen and herbivore attacks [2]. In response to these challenges, plants have evolved complex and highly regulated defence mechanisms. In this context, phytohormones serve as critical signalling molecules in the plant's response to various stresses. Salicylic acid (SA) plays a central role in plant defence against biotrophic and hemibiotrophic pathogens [2]. The generation of transgenic plants overexpressing the bacterial _NahG_ gene, which prevents SA accumulation by convertingit to catechol, was fundamental in elucidating the pivotal role of SA in plant defence. These _NaH_G plants were unable to establish effective defence mechanisms [3; 4]. Additionally, the exogenous application of SA has been shown to induce the accumulation of pathogenesis-related (PR) proteins, which confer protective effects in infected plants [5], further corroborating the essential role of SA in plant defence. SA accumulates in varying quantities depending on the specific plant-pathogen system and can also be converted into some derivates like gentisic acid (GA, 2,5-dihydroxybenzoic acid), a 5-hydroxylated derivate, that dramatically accumulates upon _Citrus Exocortis Viroid_ (CEVd) infection, inducing specific defence proteins not triggered by SA, suggesting distinct roles of SA and GA in plant defence responses [6]. The hydroxylation of SA to produce GA is catalyzed by the SSH (salicylate 5-hydroxylase) enzyme in both _Arabidopsis thaliana_[7; 8] and tomato plants [9]. Notably, _RNAi_SSH_-silenced tomato plants, which accumulate higher levels of SA, display a reduced size, early senescence, and hypersusceptibility to the heterotrophic fungus _Botrytis cinerea_, while exhibiting resistance to biotrophic pathogens such as _Pseudomonas syringae_ or CEVd [9]. Viroids are the smallest known plant pathogens, consisting of a small circular single-strand RNA molecule (250-400 nucleotides) with a highly stable secondary structure and lacking any envelope or protein capsid [10; 11]. Unable to encode any proteins, they parasitize the transcriptional machinery of the host cell, depending on their RNA polymerases and processing enzymes for replication [12]. Among these pathogens, CEVd is notable for infecting various agronomic plant species, including tomato (_Solanum lycopersicum_). Infected plants exhibit symptoms such as bark scaling and stunted growth. CEVd is one of the most extensively studied viroids due to its significant impact on agricultural production [11; 13; 14]. The role of SA in plant-vivoid interactions is well established. For instance, _NaH_G tomato plants, which are impaired in SA accumulation, exhibit enhanced susceptibility to CEVd [15], while transgenic _RNAi_SSH_ lines with increased SA levels demonstrate greater resistance to this pathogen [9]. Moreover, treatments with BTH, a SA functional analogue, have been described to reverse the enhanced susceptibility of NahG plants to CEVd [15]. Beyond SA, plants produce volatile organic compounds (VOCs) in response to biotic stresses. These VOCs play a dual role since they activate defence responses both in the plant's own distal tissues and in neighbouring plants, thereby mitigating potential infections [16; 17; 18; 19]. VOCs include a wide range of chemical structures including terpenoids, phenylpropanoids-benzenoids, and derivatives of fatty and amino acids [20; 21; 22]. These compounds are characterized by their low molecular weight, high vapour pressure, and capacity to cross membranes and be released into the atmosphere [23]. In this context, VOCs have emerged as key players in plant defence, functioning as direct antimicrobial agents, signalling molecules, or attractants for beneficial organisms that aid in controlling pathogen spread [24; 25].