# Signaling in the Tomato Immunity against _Fusarium oxysporum_
## 2 Results
### Fungal Infection Induces the Specific Expression of Genes Involved in VOC Biosynthesis  
To relate the differential volatile production with the transcriptional activation, the expression levels of several key genes involved in VOC biosynthesis were analyzed by RT-qPCR (Figure 5). To study the genes involved in aldehydes, esters, and alcohols of 9-, 6-, and 5-carbons, and jasmonate biosynthesis, the possible activation pathway of fatty acid derivatives was analyzed. In this sense, the lipoxygenases (LOX) are a group of enzymes responsible for converting both linoleic and linolenic acids into 9- and 13-hydroperoxides which are cleaved by hydroperoxidelyases (HPL) to form the corresponding short-chain aldehydes and oxo-acids. Besides, the allene oxide synthase (AOS) produces jasmonate derivatives from 13-hydroperoxide [22]. Six genes which codify for lipoxygenases were described in tomatoes [23]. _TomloxA_, _TomloxB_, and _TomloxE_ codify for 9-LOX, which catalyzes the first step in the synthesis of 9C compounds, while _TomloxC_, _TomloxD_, and _TomloxF_ codify for 13-LOX involved in the formation of oxylipins. Specifically, _TomloxD_ is responsible for JA emission in the wound response, and _TomloxC_ and _TomloxF_ take part in the production of 5-carbon and 6-carbon volatiles, respectively. In addition to genes involved in fatty acid derivatives, _MTS1_ and _SAMT1_ were studied as marker genes for the biosynthesis pathways of terpenids like monoterpenes and sesquiterpenes, and benzenoids, respectively [22]. Log(ratio) of the relative expression levels of these genes between _Fol_-infected and mock-inoculated plants in both MOM and MOV isogenic lines is represented in Figure 5.  
A significant induction of the relative expression levels was observed for _TomloxA_ at 3 dpi (Figure 5B), as well as for _TomloxF_ (Figure 5D) and HPL (Figure 5A) at 7 dpi in  
Figure 5: Logarithm of the ratios of the relative expression levels between _Fol_-infected and mock-inoculated leaves of _TomloxC_ (**A**), _TomloxA_ (**B**), _TomloxD_ (**C**), _TomloxF_ (**D**), _HPL_ (**E**), _AOS_ (**F**), _MTS1_ (**G**), and _SAMT1_ (**H**) in susceptible Momor (MOM; white) and resistant Movione (MOV; gray) tomato plants upon _Fusarium oxysporum_ f. sp. _Icyopersici_ infection at 3 dpi and 7 dpi by RT-qPCR. cDNA expression levels were expressed as the average log(ratio) values of a representative experiment. Values were expressed in relation to the endogenous gene, elongation factor 1 \(\alpha\). Statistical analysis was done by means of a _t_-test or a Mann–Whitney test. Asterisks (*) mean \(p\) < 0.05 (*), \(p\) < 0.01 (**), and \(p\) < 0.001 (***).  
susceptible MOM with respect to resistant MOV plants, correlating with 2-nonenal and 2-hexanone emission in the compatible interaction at any time.  
Moreover, statistically significant differences between the log(ratio) of _TomloxC_ induction were measured at 7 dpi in the resistant MOV with respect to the susceptible MOM (Figure 5A), in agreement with the further over-emission of (_E_)-2-pentenal during ETI establishment at 14 dpi (Figure 4), while no differences were found between both interactions at 3 dpi. Interestingly, the significantly higher log(ratio) of _TomloxD_ (Figure 5C) and _AOS_ (Figure 5F) in MOV at 3 dpi could explain the levels of JA accumulation at 3 dpi (Figure 3B), as well as the overexpression of _JAZ7_ (Figure 2B) in this resistant isogenic line. Additionally, the differential terpene emission in the MOV immune plants was in accordance with the statistically significant higher levels of _MTS1_ analyzed at 3 dpi and 7 dpi (Figure 5G). A repression in both lines and times was observed for _SAMT1_ gene, implicated in SA methylation, the downregulation being significantly lower in MOV than in MOM.