Abstract
Studies on aboveground (AG) plant organs have shown that volatile organic compound (VOC) emissions differ between simultaneous attack by herbivores and single herbivore attack. There is growing evidence that interactive effects of simultaneous herbivory also occur across the root-shoot interface. In our study, Brassica rapa roots were infested with root fly larvae (Delia radicum) and the shoots infested with Pieris brassicae, either singly or simultaneously, to study these root-shoot interactions. As an analytical platform, we used Proton Transfer Reaction Mass Spectrometry (PTR-MS) to investigate VOCs over a 3 day time period. Our set-up allowed us to monitor root and shoot emissions concurrently on the same plant. Focus was placed on the sulfur-containing compounds; methanethiol, dimethylsulfide (DMS), and dimethyldisulfide (DMDS), because these compounds previously have been shown to be biologically active in the interactions of Brassica plants, herbivores, parasitoids, and predators, yet have received relatively little attention. The shoots of plants simultaneously infested with AG and belowground (BG) herbivores emitted higher levels of sulfur-containing compounds than plants with a single herbivore species present. In contrast, the emission of sulfur VOCs from the plant roots increased as a consequence of root herbivory, independent of the presence of an AG herbivore. The onset of root emissions was more rapid after damage than the onset of shoot emissions. The shoots of double infested plants also emitted higher levels of methanol. Thus, interactive effects of root and shoot herbivores exhibit more strongly in the VOC emissions from the shoots than from the roots, implying the involvement of specific signaling interactions.Electronic supplementary materialThe online version of this article (doi:10.1007/s10886-015-0601-y) contains supplementary material, which is available to authorized users.
Highlights
Upon herbivore attack, plants employ an arsenal of defense responses, including the production of volatile organic compounds (VOCs)
The present study examines how feeding by P. brassicae on the shoot or root feeding by D. radicum, alone and in combination, influences the emission of sulfur-containing VOCs emitted from wild Brassica rapa plants
We confirmed the positive identification of sulfur compounds monitored on-line with Proton Transfer Reaction Mass Spectrometry (PTR-MS) with GC/MS analysis of the same compounds collected from the plant headspace on Aboveground VOCs Plants infested with both herbivores emitted methanethiol from their shoots in higher quantities than in any other treatment (Fig. 2a, P
Summary
Plants employ an arsenal of defense responses, including the production of volatile organic compounds (VOCs). These herbivore-induced plant volatiles have long been recognized as an indirect defense mechanism against herbivores by attracting natural enemies of the herbivores (Price et al 1980). Most studies on induced indirect defenses, either above or below ground, have investigated natural enemy responses and VOCs of plants damaged by a single herbivore species. AG and BG herbivores and their natural enemies form a complex network of interacting species connected via systemically induced plant responses (Bezemer and van Dam 2005; Wardle et al 2004)
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