Abstract
BackgroundPlant induced defense against herbivory are generally associated with metabolic costs that result in the allocation of photosynthates from growth and reproduction to the synthesis of defense compounds. Therefore, it is essential that plants are capable of sensing and differentiating mechanical injury from herbivore injury. Studies have shown that oral secretions (OS) from caterpillars contain elicitors of induced plant responses. However, studies that shows whether these elicitors originated from salivary glands or from other organs associated with feeding, such as the ventral eversible gland (VEG) are limited. Here, we tested the hypothesis that the secretions from the VEG gland of Spodoptera exigua caterpillars contain elicitors that induce plant defenses by regulating the expression of genes involved in the biosynthesis of volatile organic compounds (VOCs) and other defense-related genes. To test this hypothesis, we quantified and compared the activity of defense-related enzymes, transcript levels of defense-related genes and VOC emission in tomato plants damaged by S. exigua caterpillars with the VEG intact (VEGI) versus plants damaged by caterpillars with the VEG ablated (VEGA).ResultsThe quantified defense-related enzymes (i.e. peroxidase, polyphenol oxidase, and lipoxigenase) were expressed in significantly higher amounts in plants damaged by VEGI caterpillars than in plants damaged by VEGA caterpillars. Similarly, the genes that encode for the key enzymes involved in the biosynthesis of jasmonic acid and terpene synthase genes that regulate production of terpene VOCs, were up-regulated in plants damaged by VEGI caterpillars. Moreover, the OS of VEGA caterpillars were less active in inducing the expression of defense genes in tomato plants. Increased emissions of VOCs were detected in the headspace of plants damaged by VEGI caterpillars compared to plants damaged by VEGA caterpillars.ConclusionThese results suggest that the VEG of S. exigua caterpillars contains elicitors of late plant defense signaling in tomato which trigger defense-related enzymatic activity, regulate expression of defense-related genes, and induce emission of plant VOCs. These signaling cascades may have important ramifications for plant-insect and tritrophic interactions.
Highlights
Plant induced defense against herbivory are generally associated with metabolic costs that result in the allocation of photosynthates from growth and reproduction to the synthesis of defense compounds
ventral eversible gland (VEG) secretions activate defense-related enzymes in tomato The selected defense-related enzymes, peroxidase (POD), polyphenol oxidase (PPO), and lipoxygenase (LOX), were expressed in significantly higher amounts in plants damaged by VEG intact (VEGI) caterpillars and MI + OSVEGI than in plants damaged by VEG ablated (VEGA) caterpillars, mechanically injured (MI) plants, MI + OSVEGA plants, and untreated plants
Activity of PPO 48 h after treatment was 8.2, 9.1, 8.8 and 8.5% higher in plants damaged by VEGI caterpillars than in plants damaged by VEGA caterpillars, MI, MI + OSVEGA, or undamaged plants, respectively (Figure 1B)
Summary
Plant induced defense against herbivory are generally associated with metabolic costs that result in the allocation of photosynthates from growth and reproduction to the synthesis of defense compounds. Plant defense signaling cascades induced by insects begin with plant recognition of insect-derived elicitors followed by plasma trans-membrane potential (Vm) depolarization [6,7,8], the rise in cytosolic calcium ions [9] and a burst of reactive oxygen species (ROS), including hydrogen peroxide (H2O2) and nitric oxide (NO) [2,10,11] These cascades lead to a rise in production of the phytohormone, jasmonic acid (JA) and salicylic acid (SA) [3,12] that regulate the transcript level of defense-related genes [3,13], and end with metabolic changes including release of volatile organic compounds (VOCs) [1,3,13,14,15] and production of toxic compounds in the plants [16,17]. Despite the discovery of several elicitors, studies that show whether these elicitors originated from salivary glands or from other organs associated with feeding, such as the ventral eversible gland (VEG) are limited
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