The Ventral Eversible Gland (VEG) of Spodoptera littoralis triggers early responses to herbivory in Arabidopsis thaliana

The ventral eversible gland (VEG) in Lepidopteran larvae was first reported by De Geer in 1745. Secretions from VEG have been associated with defense against predators and the production of antiaggregation pheromones; however, the role of the VEG in arthropod–plant interactions is still unclear. Here, we show that the ablation of Spodoptera littoralis larvae VEG affects early Arabidopsis thaliana responses to herbivory and insect’s oral secretions (OS). We measured the plasma transmembrane potential (Vm) variation in Arabidopsis mesophyll palisade cells upon feeding by untreated (N) and VEG-ablated (VEGA) S. littoralis larvae. OS from both N and VEGA were collected from larvae feeding on either artificial diet (ADOS) or Arabidopsis green leaves (GLOS) and tested for their ability to affect Vm on intact Arabidopsis leaves. Calcium and hydrogen peroxide (H2O2) signaling were also evaluated by confocal laser scanning microscopy by using the fluorescent probes calcium orange and Amplex red, respectively, upon herbivory by N and VEGA, and after application of either ADOS or GLOS from both N and VEGA to Arabidopsis leaves. Ablation of VEG prompted a significant reduction of the Vm depolarization and significantly reduced both cytosolic calcium concentration ([Ca2+]cyt) and H2O2 burst. OS extracted from VEGA larvae showed the same pattern, suggesting that a functional VEG is required for the synthesis of VEG secretions able to induce early responses in the fed plant tissues. These results suggest that VEG might contain elicitors able to trigger early responses (Vm depolarization, [Ca2+]cyt influx and H2O2 burst) of Arabidopsis to S. littoralis herbivory.