# Signaling in the Tomato Immunity against _Fusarium oxysporum_
## 3 Discussion
Besides, emissions of the products of the _LOX_ pathway, such as C6 aldehydes and alcohols and their derivatives, generally known as GLVs (green leaf volatiles) have been described in biotic stresses [47]. Specifically, GLVs are emitted by leaves undergoing direct damage and they are related to infection severity and the degradation of the cell wall in leaves [48]. Esters of GLVs have also been described to be emitted by tomato plants displaying ETI upon _Pseudomonas syringae_ pv. _tomato_ infection [14]. Terpenoids are emitted due to systemic damage associated with the number of and injury in the trichomes of tomato plants [49] and other secondary stresses [48]. Terpenoids like monoterpenes and sesquiterpenes have been studied in relation to insect-plant interactions for their toxicity and repellent properties [43]. By now, a few studies report that root pathogens like _Fol_ induce a foliar emission of this type of compounds in their hosts as we observed in this study. \(\beta\)-caryophyllene was identified as a terpenoid which generated a plant-growth promotion effect in the interaction between lettuce and _Fusarium oxysporum_[50], and 3-carene was also identified as a VOC emitted by healthy tomato roots and infected ones by _Fol_[51]. Benzenoids are known because of their biological activities, and in tomato-_Pseudomonas_ interaction, a higher concentration of phenolic compounds has been observed, provoking a 60% reduction in a subsequent _Fol_ infection [52]. MeSA is a volatile molecule derived from SA which takes part in SAR [53]. This phenolic compound is able to travel across the phloem and in the air as a volatile signal among plants, producing SA to trigger the systemic defense in the host and the neighboring plants [42]. An accumulation of MeSA has also been identified in tomato plants infected with a virulent bacteria [14]. Besides, MeSA has been described to be involved in tomato defense response against _Fol_, since SAMT-silenced tomato are less susceptible to a virulent strain of this root-invading fungus [25]. The over-emission of MeSA observed in the susceptible MOM plants caused by _Fol_ infection (Figure 4) is in accordance with the SA levels analyzed in these plants (Figure 3A), thus confirming that the endogenous presence of this phenolic defensive phytohormone and its methylated form cannot prevent the infection progress. Furthermore, our findings appear to indicate that MeSA levels correlate with SA accumulation, unlike _SAMT1_ induction (Figure 5H), pointing out the importance of substrate content in the emission of this volatile. The new and wide range of compounds identified in immune MOV tomato plants upon _Fol_ infection after ETI establishment leads to new potential chemical strategies for crop protection against the fusarium wilt or other similar diseases. In this sense, exogenous treatments with compounds differentially emitted by tomato MOV plants resisting _Fol_ infection (some terpenoids, benzenoids, and 2-ethylhexanoic acid; detailed in Figure 4 and Table S2) could lead to the activation of plant defense against this fungus, therefore uncovering new resistance inducers. Besides, transgenic plants over-emitting these VOCs by the constitutive activation of their biosynthetic pathways could represent a biotechnological tool for plant resistance.