Tracing Hidden Herbivores: Time-Resolved Non-Invasive Analysis of Belowground Volatiles by Proton-Transfer-Reaction Mass Spectrometry (PTR-MS)

Abstract

Root herbivores are notoriously difficult to study, as they feed hidden in the soil. However, root herbivores may be traced by analyzing specific volatile organic compounds (VOCs) that are produced by damaged roots. These VOCs not only support parasitoids in the localization of their host, but also may help scientists study belowground plant-herbivore interactions. Herbivore-induced VOCs are usually analyzed by gas-chromatography mass spectrometry (GC-MS), but with this off-line method, the gases of interest need to be preconcentrated, and destructive sampling is required to assess the level of damage to the roots. In contrast to this, proton-transfer-reaction mass spectrometry (PTR-MS) is a very sensitive on-line, non-invasive method. PTR-MS already has been successfully applied to analyze VOCs produced by aboveground (infested) plant parts. In this review, we provide a brief overview of PTR-MS and illustrate how this technology can be applied to detect specific root-herbivore induced VOCs from Brassica plants. We also specify the advantages and disadvantages of PTR-MS analyses and new technological developments to overcome their limitations.

Root-emitted volatile organic compounds: can they mediate belowground plant-plant interactions?

“…GC-MS and proton transfer reaction-mass spectrometry (PTR-MS) analyses have shown that insect-damaged Brassica roots release mainly highly volatile sulphur compounds (methanethiol, dimethyl sulphide, dimethyl disulphide and dimethyl trisulphide) and glucosinolate breakdown products (thiocyanates and isothiocyanates) in the soil, with a VOC emission pattern depending on the plant species studied Danner et al 2012;Crespo et al 2012;Danner et al 2015). To date, PTR-MS analyses of root-emitted VOCs have been performed only on potted Brassica spp.…”

“…Due to their belowground location, studying root-root interactions mediated by VOCs is challenging and requires innovative tools in order to (1) identify the volatile signals released into the plant rhizosphere Campos-Herrera et al 2013;Hiltpold et al 2013), (2) follow the temporal variations in VOC emission (composition of the VOC blends and individual VOC concentrations) (Danner et al 2012) and (3) conduct bioassays on the roles played by root-emitted VOCs in plant-plant interactions.…”

“…PTR-MS has been shown to be an efficient tool for the real-time analysis of root-emitted VOCs in situ Danner et al 2012;Crespo et al 2012;Danner et al 2015). The identity of the molecules entering the ionization chamber, however, cannot be determined using the mass spectral data acquired with this technique because only the mass of a small number of product ions (particularly cluster ions) can be obtained .…”

“…(Drawing: Ir Carolina Levicek) exploiting the advantages of both analytical techniques. Whereas GC-MS enables biogenic VOCs to be rapidly identified by comparing their recorded mass spectra with those contained in mass spectral databases, PTR-MS enables the temporal dynamics of root-emitted VOCs to be monitored with a high time resolution, particularly for highly volatile and low molecular weight molecules that are not easily trapped using packed adsorbents prior to GC-MS analyses (Danner et al 2012;Crespo et al 2012).…”

Mechanosensitivity below Ground: Touch-Sensitive Smell-Producing Roots in the Shy Plant Mimosa pudica

“…Of particular relevance is the finding that the changing profile of organosulfur exudates that occurs in Brassica spp. roots in response to herbivore attack or a tissue breach can be monitored in real time by proton transfer reaction-mass spectrometry (Crespo et al, 2012;Danner et al, 2012;van Dam et al, 2012;Samudrala et al, 2015). If conventional metabolome analysis sample preparation methods had been used in these cases (e.g.…”