ReviewAnalytical strategies for in-vivo evaluation of plant volatile emissions - A review
Graphical abstract
Introduction
All plants emit non-organic volatiles (CO2, O2) during photosynthesis and respiration, but most of them also produce many biogenic volatile organic compounds (BVOCs). These volatiles (chiefly terpenoids, fatty acid degradation products, phenylpropanoids and amino acid-derived products) are produced in different plant organs (leaves, flowers, fruits, but also roots). They are stored in specialised secretory structures, such as glandular trichomes and resin ducts, and are spontaneously released by the plants, even if their emission can vary depending on specific stress conditions. These analytes play a fundamental role in the defence mechanisms against herbivores and pathogens, in the attraction of pollinators and seed dispersers, and as signals in plant–plant communication. Besides their role for the plant, BVOCs can also have important ecological relevance (they can act as precursors of tropospheric phytotoxic compounds) and they also present a wide range of biological activities useful to humans, making them a sustainable and under-exploited source of bioactive compounds [1,2].
The plant volatilome [2,3], is made up of more than 1700 volatile compounds isolated from more than 90 plant families, and its study therefore requires a metabolomic approach [4]. In addition, it is important to avoid alterations to the metabolomic profile so that it can provide a snapshot of the biochemical activity of the investigated biosystem. Specific in-vivo analytical strategies are required to prevent: i) the de-novo formation of compounds biosynthesised in response to any damage resulting from plant collection; ii) possible enzyme-mediated metabolite conversion; and iii) chemical degradation of labile metabolites [5]. Sample preparation plays a fundamental role in this respect, and the development of static and dynamic techniques for the headspace collection of volatiles in combination with a suitable analytical platform (mainly gas chromatography–mass spectrometry (GC-MS)) has significantly contributed to the development of dedicated analytical strategies for the in-vivo investigation of the plant volatilome, thus improving the understanding of the biosynthesis and ecology of plant BVOC compounds [1,2].
In this article, we first provide a review of the main sampling and analytical approaches developed in the last twenty years for the in-vivo evaluation of plant volatiles, with emphasis on some challenging, but crucial, features of the analytical procedure, such as quantification. The second part of this article focuses on the impact of in-vivo volatilomic studies on our knowledge of plant behaviour, examining the spontaneous emission of BVOCs in healthy plants and how the plant volatilome can be affected by different abiotic and biotic stresses.
Section snippets
Sampling strategies
Research of proper analytical strategies to investigate the volatile profile of plants is fundamental to the correct characterisation of their metabolism and of their relationship with an ecosystem. A sample preparation procedure is usually necessary as most biological systems are too complex for a direct analysis [6]. For this reason, the sampling step is crucial in the development of an analytical method [7,8].
The main challenge in the evaluation of plant BVOC emissions is their complexity,
Overview of analytical platforms
After sampling, the next fundamental step for the determination of the volatile profile of a plant sample is its analysis with the adoption of an appropriate analytical platform. When BVOCs are retained in a trapping system, two approaches to desorb the analytes can be adopted: i) solvent back-extraction (mainly with carbon disulfide, methylene chloride, hexane or dichloromethane) [20,38,40,44,50,53,54] and ii) thermal treatment [15,[32], [33], [34],37,42,43,55,57,64,70,74,87]. Organic solvents
Applications
BVOCs are spontaneously released by healthy plants, but their amounts can vary or de-novo compounds can be biosynthesised when the living system is subjected to induced stress (such as mechanical or herbivore damage, lack or excess of specific nutrients, light variations or high or low temperatures) [98] (Fig. 4). This section reports a series of applications of the in-vivo evaluation of volatiles on undamaged plants or in presence of specific biotic or abiotic stress; the text is organized on
Conclusions and future trends
The in-vivo evaluation of the volatiles emitted by a plant is a fundamental step towards clarifying its behaviour when it is submitted to stress or to a multitrophic interaction. The correct choice of sampling approach in combination with a compatible analytical platform is essential to obtaining meaningful information about the plant metabolome, but it requires the development of dedicated analytical tools.
From the analytical point of view, the in-vivo collection of plant volatiles has
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work was financially supported by the ‘Ricerca Locale’ (Ex60%2019) project of the University of Turin, Turin (Italy). This article is based upon work from the Sample Preparation Study Group and Network supported by the Division of Analytical Chemistry of the European Chemical Society.
Carlo Bicchi has been Full Professor of Pharmaceutical Biology at the University of Torino since 1990. His main fields of research are development of analytical technologies for profiling and fingerprinting of biologically active secondary (specialised) metabolites in particular characterizing the volatile fraction of vegetable matrices including important food crops (tea, coffee, cocoa, hazelnuts and olive oil). Analytical technologies include: sample preparation techniques; GC, GC-MS,
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Carlo Bicchi has been Full Professor of Pharmaceutical Biology at the University of Torino since 1990. His main fields of research are development of analytical technologies for profiling and fingerprinting of biologically active secondary (specialised) metabolites in particular characterizing the volatile fraction of vegetable matrices including important food crops (tea, coffee, cocoa, hazelnuts and olive oil). Analytical technologies include: sample preparation techniques; GC, GC-MS, Fast-GC, Multidimensional GC, Enantioselective GC, HPLC, HPLC-MS, SFE and SFC, Chemometric elaboration. He has authored or co-authored more than 290 publications in internationally recognized journals and 350 oral presentations.
Cecilia Cagliero is Associate Professor in Pharmaceutical Biology at the Department of Drug Science and Technology of the University of Turin (Italy) where she obtained the PhD in Science and High Technology in 2010. Her research activity, documented by 60 articles (H-index 20), is focused towards the development of advanced approaches for the characterisation of volatile and nonvolatile fractions of plants. In the occasion of the 2016 International Symposium on Capillary Chromatography she received the Leslie Ettre Award and on October 2018, she was included in the “The Top 40 Under 40 Power List” by The Analytical Scientist.
Arianna Marengo is Assistant Research Fellow at the Department of Drug Science and Technology, University of Torino (Italy). She graduated with honour in Cellular and Molecular Biology, at the University of Torino (Italy), in 2013 and she obtained a doctoral degree in Earth and Environmental Sciences and Technologies at the University of Cagliari (Italy), in 2017. Her research is focused on the phytochemical and biomolecular characterisation of plant species of pharmaceutical, cosmetic and healthy interest. She has published 22 articles and she joined different university laboratories in Italy, France and USA, for short research training periods.
Giulia Mastellone graduated with honour in Pharmacy, at University of Torino (Italy) in April 2020 with a thesis carried out during a traineeship of six months at the University of La Laguna (Tenerife, Spain). She is currently a PhD student at the University of Torino (Italy) under the supervision of Cecilia Cagliero. Her research interest includes the development of sustainable and innovative analytical strategies for the characterisation of botanicals. She is author of two articles published in peer-review International Journals.
Patrizia Rubiolo has been Full Professor of Pharmaceutical Biology and since 2018 Head of the Department of Drug Science and Technology of the University of Turin. Her main field of research is devoted to the study of the biologically active specialised metabolites in plants and in their derivatives through the application of innovative methods of sampling and analysis, in particular through the adoption of solvent-free sampling techniques coupled with GC-MS and LC-MS analytical platforms. Research activity is documented by more than 140 publications in international journals with a H-index of 34 and 3876 citations (June2020).
Barbara Sgorbini is Associate Professor of Pharmaceutical Biology at the Department of Drug Science and Technology of the University of Turin. Her main field of research is devoted to the study of the volatile bioactive secondary metabolites in plants and in their derivatives (in particular essential oils), by adopting innovative sample preparation techniques (especially solvent-free sampling techniques) combined with GC-MS analytical platforms. Her research activity is documented by more than 70 publications in International Journals.
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Both authors contributed equally to this work.