Tomato plants damaged by S. exigua caterpillars with intact ventral eversible gland (VEGI) expressed significantly higher amounts of defense-related enzymes and genes, and headspace VOCs than plants damaged by caterpillars with ablated VEG (VEGA). These results suggest that secretions from the VEG of S. exigua caterpillars contain elicitors of late defense signaling in tomato. To our knowledge, this is the first report on the role of caterpillar VEG secretions as an elicitor of late defense signaling in plants. A previous study by Zebelo and Maffei  showed that VEG secretions of Spodoptera littoralis caterpillars induced early defense signaling in Arabidopsis thaliana.
The three defense-related enzymes, peroxidase (POD), polyphenol oxidase (PPO), and lipoxigenase (LOX), were expressed in significantly higher amounts in plants damaged by VEGI caterpillars or mechanically injured plants treated with oral secretion (OS) from VEGI caterpillars (MI + OSVEGI) than in plants damaged by VEGA caterpillars, mechanically injured (MI) plants, mechanically injured plants treated with oral secretion (OS) from VEGA caterpillars (MI + OSVEGA), or untreated (control) plants. All three enzymes are components of the octadecanoid signal transduction pathway, which regulates the production of the phytohormone, jasmonic acid (JA) [39–41]. Peroxidases (PODs) are a group of plant defense-related enzymes, which limit plant nutritional quality to insect herbivores through quinone and reactive oxygen species generation with subsequent inhibition of insect digestion of plant material [41, 42]. Over-expression of PODs can enhance plant resistance to insects  and limit plant nutritional quality to insect herbivores . Suzuki et al.  reported that herbivory by caterpillars and high POD activity resulted in the up-regulation of several tomato genes including genes encoding proteinase inhibitors.
Polyphenol oxidase (PPO) is an inducible enzyme that is found throughout the plant kingdom and known to have defensive role against herbivores [40, 44] and pathogens [45, 46]. Production of PPO is induced by mechanical injury, methyl jasmonate (MeJa) and herbivory . Similar to our results, Chung et al.  reported high PPO levels in tomato plants wounded mechanically and treated with oral secretions (OS) from Colorado potato beetle, Leptinotarsa decemlineata, suggesting that insect OS contain elicitors of PPO activity . Lipoxygenases (LOXs) are another group of anti-oxidative enzymes involved in plant defense against herbivory and pathogens through the octadecanoid pathway . One of the most important functions of LOX in plant defense is the oxidation of linolenic acid in the JA signaling pathway . Allene oxide synthase (AOS) catalyzes the first step of the LOX pathway that leads to JA biosynthesis . In the present study, we observed an early induction of LOX-specific activity within 24 h of feeding by caterpillars with intact VEG (VEGI). Likewise, the transcript levels of LOX and AOS genes were higher in plants damaged by VEGI caterpillars compared to plants damaged by caterpillars with ablated VEG (VEGA). These results are consistent with previous studies which demonstrate that caterpillar feeding up-regulates the expression of LOX genes in tomato .
Our results also showed increased emission of VOCs in tomato plants damaged by VEGI caterpillars compared to plants damaged by VEGA caterpillars or mechanically injured plants. Among the common VOCs induced by herbivory are those that are LOX-derived, such as green leaf volatiles (GLVs), terpenoids and methyl salicylate . Numerous plants emit GLVs and other VOCs as an indirect defense strategy against herbivory, as these volatiles can attract predacious and parasitic natural enemies of herbivores [52–54]. In this study, GLVs and certain monoterpenes were emitted in higher amounts by plants damaged by VEGI caterpillars, suggesting the involvement of the VEG in the induction of plant VOCs. For instance, most GLVs including (E)-2-hexenal, (Z)-3-hexenal, (Z)-2-hexenol and (Z)-3-hexenyl acetate were detected in higher amounts in plants damaged by VEGI caterpillars compared to plants damaged by VEGA caterpillars. Interestingly, many of these GLVs are used as host location cues by caterpillar parasitoids , suggesting that VEG secretions may impact tritrophic interactions. GLVs have also been reported to play a role in plant-plant interactions .
Another highly diverse group of plant compounds are the terpenoids, which are synthesized by a group of enzymes called terpene synthases (TPS) to produce mono-, sesqui- and diterpenes . Terpenes are more costly to synthesize per gram than most other primary and secondary plant compounds . Studies have shown that a single mechanical plant tissue injury event may not elicit induced defense related volatile organic compounds (VOCs) [58, 59]. However, application of OS to mechanically wounded site could elicit the release of inducible volatile compounds and thereby mimic herbivory [58, 59]. In the present study, we observed significantly higher emission of the monoterpenes, β-linalool and γ-terpinene, in plants damaged by VEGI caterpillars compared to those damaged by VEGA caterpillars. However, there were no significant differences recorded among the treatments in the emission of other monoterpenes such as α-pinene, β-phellendrene, β-pinene and β-ocimene, suggesting that not all VOCs are inducible by VEG secretions.
Like monoterpenes, sesquiterpenes are phytoalexins which play a pivotal role in direct and indirect defenses against herbivores . In the present study, several sesquiterpenes (i.e. (E)-β-caryophyllene, α-humulene and β-elemene), some fatty acids (3- octanol and nonanal) and an organic ester (methyl salicylate) were emitted in significantly higher amounts by plants damaged by VEGI caterpillars compared to plants damaged by VEGA caterpillars.
Results from gene expression studies showed that most terpene synthase genes (i.e. TPS7 which encodes the monoterpene, β-Ocimene and TPS12 which encodes the sesquiterpenes, (E)-β-caryophyllene and α-humulene) and the genes involved in the biosynthesis of GLVs and jasmonic acid (i.e. LOX and AOS) were up-regulated in plants damaged by VEGI caterpillars as well as in mechanically injured plants treated with oral secretion from VEGI caterpillars (MI + OSVEGI). However, the transcript levels of these genes were not up-regulated in plants damaged by VEGA caterpillars, mechanically injured plants treated with oral secretion from VEGA caterpillars (MI + OSVEGA), or mechanically injured (MI) plants. These results suggest that an intact VEG in S. exigua caterpillars is crucial for eliciting late defense signaling via the expression of defense-related genes. These findings are in agreement with those of Bricchi et al. which showed that mechanical injury alone failed to increase the transcript levels of terpene synthase and JA biosynthesis genes in Arabidobsis thaliana, but mechanical injury treated with Spodoptera littoralis oral secretion activated the genes.
In a recent review of the role of caterpillar secretions on induced plant defenses, Felton  suggested that the VEG may play an important role in secretion during feeding by caterpillars in the family Noctuidae. The structure and proximity of the VEG to the caterpillar mouthparts lend credence to this proposal. When a caterpillar feeds on a plant material the VEG is distended from its eversible position on the ventral surface of the caterpillar thorax and reaches the injured plant surface [32, 61]. Furthermore, because the tip of the everted VEG can reach the mandibles during feeding , the VEG secretions are usually deposited onto the food substrate with the OS . Our results confirm that the VEG secretions, which are deposited along with oral secretions or regurgitate onto plants during caterpillar feeding can induce late defense signaling in tomato. Further studies are needed to identify the bioactive components of the VEG secretions that trigger plant defense signaling.