Plant-induced defense against herbivores is typically done by rerouting
photosynthates to the synthesis of defense compounds rather than growth and
reproduction. Hence, it is important that plants are able to sense and
differentiate mechanical injury from injury caused by herbivores. Oral
secretions from caterpillars were found to contain elicitors of
plant-induced responses such as production of volatile organic compounds
(VOCs). However, there are limited studies that show whether these elicitors
are from salivary glands or from other organs related to feeding, such as
the ventral eversible gland (VEG).

In this study, secretions from the VEG of beet armyworm (Spodoptora exigua)
caterpillars were examined for the presence of elicitors that could induce
plant defenses. To test this, the activity of defense-related enzymes,
transcript levels of defense-related genes and volatile organic compounds
(VOC) emission in tomato plants damaged by S. exigua caterpillars with the
intact VEG (VEGI) and compared it with tomato plants damaged by caterpillars
without the VEG (VEGA).

Tomato plants damaged by the VEGI caterpillars expressed significantly
higher amounts of defense-related enzymes than plants damaged by VEGA
caterpillars. There was also an up-regulation in genes encoding for jasmonic
acid as well as those genes involved in the production of terpene VOCs in
plants damaged by VEGI caterpillars. This led to increased emissions of VOCs
detected in the headspace of plants damaged by VEGI caterpillars.
Furthermore, the oral secretions of the VEGA caterpillars were less
effective in inducing expression of the defense genes. This verifies that
VEG secretions of beet armyworm caterpillars contain elicitors of plant
defense response in tomato.

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