Caterpillar Herbivory and Salivary Enzymes Decrease Transcript Levels of Medicago truncatula genes Encoding Early Enzymes in Terpenoid Biosynthesis

Abstract

In response to caterpillar herbivory, alfalfa and related plant species defend themselves through the induction of saponin and volatile terpenoid biosynthesis. Both these types of defensive compounds are derived from the metabolic intermediate, isopentenyl diphosphate (IPP). In plants, two distinct biosynthetic pathways can generate IPP; the cytosolic mevalonate pathway and the plastid-associated 2C-methyl erythritol 4-phosphate (MEP) pathway. In Medicago truncatula, transcript levels of key regulatory genes active in the early steps of these biosynthetic pathways were measured in response to larval herbivory by the beet army worm, Spodoptera exigua. Transcripts encoding enzymes at early steps of both terpenoid pathways were lower in caterpillar-damaged leaves. Higher degrees of herbivore damage accentuated the decrease in transcript levels; however, transcript amounts were not affected by insect larval stage. Insect larvae, manipulated to reduce labial gland salivary secretions, were used to examine the role of the salivary elicitors in modulating gene expression. Results suggest that an insect salivary factor, possibly glucose oxidase (GOX), may be involved in reduction of transcript levels following herbivory. Addition of GOX or hydrogen peroxide to mechanically wounded leaves confirm these findings. In comparison, transcript levels of a gene encoding a putative terpene synthase are induced in mechanically- or insect-damaged leaves. These data show that insect salivary factors can act to suppress transcript levels of genes involved in plant defense pathways. Findings also suggest that in response to stress such as insect herbivory, regulation occurs at the early steps of the MEP pathway.

The Role of Insect-Derived Cues in Eliciting Indirect Plant Defenses in Tobacco,Nicotiana tabacum

“…These results are consistent with the recent finding that factors associated with caterpillar saliva suppress the transcription of genes encoding early enzymes involved in the production of terpenoids. 17 Notably, several of the compounds released only in response to ablated caterpillars have previously been shown to be released by tobacco during the dark phase of the photoperiod in response to feeding by intact H. virescens and to be repellent to female H. virescens moths. 32 A major difference in plant responses to feeding by ablated and intact caterpillars was the release of significantly greater amounts of nicotine in response to feeding by ablated caterpillars ( Fig.…”

“…3,5,8,9 Several insect-derived cues involved in the mediation of plant responses to herbivore attack have been isolated and identified from both insect regurgitant 1,[10][11][12][13][14][15] and saliva. 16,17 Regurgitant and saliva are distinct secretions differing in origin and composition. 18 Regurgitant arises from the fore-and midgut and can be collected from caterpillars by gently squeezing them with forceps.…”

“…16 Glucose oxidase also suppresses transcription of genes involved in terpenoid biosynthesis in alfalfa, Medicago truncatula. 17 In addition to suppressing plant defenses, saliva has been shown to induce the production of some plant volatiles. 28 In maize, application of salivary gland extracts of the western tarnished plant bug, Lygus hesperus, induces the release of the same blend of volatiles as that released in response to L. hesperus feeding.…”

Molecular Interactions between the Specialist Herbivore Manduca sexta (Lepidoptera, Sphigidae) and Its Natural Host Nicotiana attenuata. VIII. An Unbiased GCxGC-ToFMS Analysis of the Plant's Elicited Volatile Emissions

“…GOX enzymatic activity also decreased the emission of nicotine into the head space as well as other commonly emitted OS-induced volatiles (Delphia et al, 2007). GOX represses the transcription of genes involved in terpenoid biosynthesis in alfalfa (Medicago truncatula; Bede et al, 2006). C. Diezel and I.T.…”

Salivary signals of European corn borer induce indirect defenses in tomato

“…Caterpillar saliva not only modulates the direct defenses in plants, but also alters the production of volatile, indirect defenses, and/or the genes encoding for their synthesis. [8][9][10] To determine if ECB saliva affects indirect gene expression in tomato, the spinneret, the structure that secretes saliva, was cauterized/ablated to prevent salivation. 7 This allowed us to compare the gene expression in tomato that were infested with ablated (no saliva) or unablated (saliva secreted) ECB larvae for 24 h. Using qRT-PCR we monitored the expression of TERPENE SYNTHASE5 (TPS5) and HYDROPEROXIDE LYASE (HPL), genes which regulate biosynthesis of terpene and green leafy volatiles (GLVs) in tomato, after ECB infestation.…”