Direct proof of ingested food regurgitation by Spodoptera littoralis caterpillars during feeding on Arabidopsis.
ABSTRACT Oral secretions of herbivorous lepidopteran larvae contain a mixture of saliva and regurgitant from the insect gut. Different compounds from the oral secretions can be recognized by the host plants and, thus, represent elicitors that induce plant defenses against feeding herbivores. Exogenously applied oral secretions can initiate the biosynthesis of jasmonates, phytohormones involved in the regulation of plant defense. However, it is not known (a) whether or not non-manipulated insects indeed release oral secretions including gut-derived compounds into a leaf wound during the natural feeding process, or (b) whether they adjust the release of gut components to the state of plant defense. We addressed these questions by using Arabidopsis thaliana as host plant and larvae of the generalist herbivorous insect Spodoptera littoralis. We investigated the conversion of the plant-derived jasmonate precursor, cis-12-oxophytodienoic acid (cis-OPDA), to iso-OPDA by the larvae. This enzymatic reaction is mediated by a specific glutathione-S-transferase in the insect gut, but not in the plant. Any presence of iso-OPDA in plant tissue, thus, indicated that gut content had been regurgitated into the plant wound. Our study demonstrates that the plant is the only source for the substrate cis-OPDA by using aos (allene oxide synthase) mutants that are unable to synthesize OPDA. The fact that iso-OPDA accumulated over time on feeding-damaged leaves shows that the feeding larvae are constantly regurgitating on leaves. Although the larvae provided the signaling compounds that were recognized by the plant and elicited defense reactions, the larval regurgitation behavior did not depend on whether they fed on a defensive wild type plant or on a non defensive coi1-16 plant. This suggests that S. littoralis larvae do not adjust regurgitation to the state of plant defense.
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ABSTRACT: Volicitin (N-[17-hydroxylinolenoyl]-L glutamine) present in the regurgitant of beet armyworm (Spodoptera exigua) activates the emissions of volatile organic compounds (VOCs) when in contact with damaged corn (Zea mays L.) leaves. VOC emission in turn serves as a signaling defense for the plant by attracting female parasitic wasps that prey on herbivore larvae. Chemical tracking of volicitin within plants has yet to be reported. Here we present biochemical data that beet armyworm regurgitant serves as a vector for the introduction of volicitin to the site of leaf damage under natural feeding conditions. Corn seedlings were 14CO2-labeled in situ, and beet armyworm larvae were allowed to feed on the labeled leaves. Herbivore oral secretions collected from late-third-instar larvae contained approximately 120 pmol volicitin (0.05 nCi pmol(-1)) per larva. When radiochemically labeled larvae were placed on unlabeled leaves, the amount of volicitin introduced to the damaged site was approximately 5.0 nCi (calc. 100 pmol/larvae). The mobility of volicitin in leaves was examined by allowing radiolabeled beet armyworms to feed on unlabeled plants. In such tracking experiments, radioactivity was not detected in the upper leaves; however, the exogenous application of 5 nCi of [U-14C]sucrose to the lower leaf did result in subsequent radioactivity being detected in the upper portion of the plant. The detection of labeled sucrose with the same radioactivity as that of administered volicitin indicated that volicitin was not readily transported to undamaged leaves and that volicitin may not directly serve as a mobile messenger in triggering the emissions of VOCs systemically.Planta 05/2004; 218(6):999-1007. · 3.35 Impact Factor
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ABSTRACT: Plants can perceive a wide range of biotic attackers and respond with targeted induced defenses. Specificity in plant non-self-recognition occurs either directly by perception of pest-derived elicitors or indirectly through resistance protein recognition of host targets that are inappropriately proteolyzed. Indirect plant perception can occur during interactions with pathogens, yet evidence for analogous events mediating the detection of insect herbivores remains elusive. Here we report indirect perception of herbivory in cowpea (Vigna unguiculata) plants attacked by fall armyworm (Spodoptera frugiperda) larvae. We isolated and identified a disulfide-bridged peptide (+ICDINGVCVDA-), termed inceptin, from S. frugiperda larval oral secretions that promotes cowpea ethylene production at 1 fmol leaf(-1) and triggers increases in the defense-related phytohormones salicylic acid and jasmonic acid. Inceptins are proteolytic fragments of chloroplastic ATP synthase gamma-subunit regulatory regions that mediate plant perception of herbivory through the induction of volatile, phenylpropanoid, and protease inhibitor defenses. Only S. frugiperda larvae that previously ingested chloroplastic ATP synthase gamma-subunit proteins and produced inceptins significantly induced cowpea defenses after herbivory. Digestive fragments of an ancient and essential plant enzyme, inceptin functions as a potent indirect signal initiating specific plant responses to insect attack.Proceedings of the National Academy of Sciences 07/2006; 103(23):8894-9. · 9.74 Impact Factor
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ABSTRACT: Oxylipins play important roles in stress signaling in plants. The compound 12-oxophytodienoic acid (cis-OPDA) is an early biosynthetic precursor of jasmonic acid (JA), the key phytohormone orchestrating the plant anti-herbivore defense. When consumed by feeding Lepidopteran larvae, plant-derived cis-OPDA suffers rapid isomerization to iso-OPDA in the midgut and is excreted in the frass. Unlike OPDA epimerization (yielding trans-OPDA), the formation of iso-OPDA is enzyme-dependent, and is catalyzed by an inducible glutathione transferase (GSTs) from the larval gut. Purified GST fractions from the gut of Egyptian cotton leafworm (Spodoptera littoralis) and cotton bollworm (Helicoverpa armigera) both exhibited strong OPDA isomerization activity, most likely via transient formation of a glutathione-OPDA conjugate. Out of 16 cytosolic GST proteins cloned from the gut of cotton bollworm larvae and expressed in E. coli, only one catalyzed the OPDA isomerization. The biological function of the double bond shift might be seen in an inactivation of cis-OPDA, similar to the inactivation of prostaglandin A1 to prostaglandin B1 in mammalian tissue. The enzymatic isomerization is particularly widespread among generalist herbivores that have to cope with various amounts of cis-OPDA in their spectrum of host plants.Proceedings of the National Academy of Sciences 09/2009; 106(38):16304-9. · 9.74 Impact Factor