Caterpillar with intact spinneret induces oxidative response in plants
The level of total, reduced, and oxidized forms of ascorbate and glutathione was measured in M. truncatula leaves at 15, 30 and 45 minutes after herbivory by caterpillars with intact or cauterized caterpillars in wild type, jemolong A17. There was no significant increase in the total ascorbate and total glutathione in both genotypes until 45 minutes of herbivory. However, although both caterpillar feedings showed no significant changes in oxidized as well as reduced form of ascorbate, caterpillar with intact spinneret significantly increased the ratio of oxidized to reduced form of ascorbate at 45 minute. As caterpillar without labial salivary secretion caused no significant difference in the ratio of oxidized to reduced ascorbate, it indicates that the effectors in labial saliva creates oxidative environment in the wounded tissues.
Similar to reduced ascorbate, the level of reduced glutathione was not changed due to herbivory with or without labial salivary secretion at any time point. However, normal caterpillar accumulated significantly higher level of oxidized glutathione (GSSG) than control and cauterized caterpillar after 15 and 30 minutes of herbivory. At 45 minute, the level of GSSG was reduced to constitutive level. Higher level of GSSG due normal caterpillar herbivory and thus the increased ratio of oxidized to reduced glutathione confirmed the labial salivary-specific induction of oxidative stress in caterpillar fed plants. Together with this and a trend of decreasing concentration of reduced ascorbate (although not significant) after 15 min of herbivory by normal caterpillar reveals the activation of ascorbate-glutathione cycle in wounded tissues after normal caterpillar herbivory.