Isoprene is the most abundant biogenic volatile hydrocarbon compound naturally emitted by plants and plays a major role in atmospheric chemistry. It has been proposed that isoprene synthases (IspS) may readily evolve from other terpene synthases, but this hypothesis has not been experimentally investigated. We isolated and functionally validated in Arabidopsis the first isoprene synthase gene, AdoIspS, from a monocotyledonous species (Arundo donax L., Poaceae). Phylogenetic reconstruction indicates that AdoIspS and dicots isoprene synthases most likely originated by parallel evolution from TPS-b monoterpene synthases. Site-directed mutagenesis demonstrated invivo the functional and evolutionary relevance of the residues considered diagnostic for IspS function. One of these positions was identified by saturating mutagenesis as a major determinant of substrate specificity in AdoIspS able to cause invivo a dramatic change in total volatile emission from hemi- to monoterpenes and supporting evolution of isoprene synthases from ocimene synthases. The mechanism responsible for IspS neofunctionalization by active site size modulation by a single amino acid mutation demonstrated in this study might be general, as the very same amino acidic position is implicated in the parallel evolution of different short-chain terpene synthases from both angiosperms and gymnosperms. Based on these results, we present a model reconciling in a unified conceptual framework the apparently contrasting patterns previously observed for isoprene synthase evolution in plants. These results indicate that parallel evolution may be driven by relatively simple biophysical constraints, and illustrate the intimate molecular evolutionary links between the structural and functional bases of traits with global relevance.

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Botrytis cinerea, the main pathogen of strawberry, has the ability to remain quiescent in unripe tissue and develop disease symptoms in ripe fruit. As strawberry ripening is characterized by an increase of aroma compounds, the role of volatile emission in the development of infection was investigated. Thirty-five strawberry volatile organic compounds (VOCs) were tested on B. cinerea in vitro and volatile emission was analysed in strawberry harvested at four ripening stages by headspace solid-phase microextraction/gas chromatography-mass spectrometry and proton transfer reactiontime of flight-mass spectrometry. The coupling of such data sets made it possible to conclude that key strawberry aroma compounds stimulate B. cinerea conidial germination and some typical wound-volatiles stimulate pathogen conidial germination or mycelial growth. This study is the first report of fungal stimulation by some VOCs naturally occurring in strawberry: the esters ethyl butanoate, cis-3-hexenyl acetate, trans-2-hexenyl acetate, methyl butanoate and hexyl butanoate, the furanones furaneol and mesifurane, and the alcohol trans-2-hexenol. The results of this work provide advances in understanding the functional role of fruit VOCs and suggest, for the first time, that fruit VOCs may influence the development of B. cinerea from the latent phase and that they could favour the invasive growth of B. cinerea after wounding. In particular, ethyl butanoate and furaneol could signal strawberry ripening, and the green leaf volatiles trans-2-hexenol, trans-2-hexenyl acetate and cis-3-hexenyl acetate could signal the presence of damaged tissues that are easier sites for penetration by B. cinerea.

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The grapevine (Vitis vinifera) is one of the most widely cultivated fruit crops globally, and one of its most important diseases in terms of economic losses is downy mildew, caused by Plasmopara viticola. Several wild Vitis species have been found to be resistant to this pathogen and have been used in breeding programs to introduce resistance traits to susceptible cultivars. Plant defense is based on different mechanisms, and volatile organic compounds (VOCs) play a major role in the response to insects and pathogens. Although grapevine resistance mechanisms and the production of secondary metabolites have been widely characterized in resistant genotypes, the emission of VOCs has not yet been investigated following P. viticola inoculation. A Proton Transfer Reaction-Time of Flight-Mass Spectrometer (PTR-ToF-MS) was used to analyze the VOCs emitted by in vitro-grown plants of grapevine genotypes with different levels of resistance. Downy mildew inoculation significantly increased the emission of monoterpenes and sesquiterpenes by the resistant SO4 and Kober 5BB genotypes, but not by the susceptible V. vinifera Pinot noir. Volatile terpenes were implicated in plant defense responses against pathogens, suggesting that they could play a major role in the resistance against downy mildew by direct toxicity or by inducing grapevine resistance. The grapevine genotypes differed in terms of the VOC emission pattern of both inoculated and uninoculated plants, indicating that PTR-ToF-MS could be used to screen hybrids with different levels of downy mildew resistance. Copyright © 2015 John Wiley & Sons, Ltd.

Fruit and vegetables are regularly stored by consumers in the refrigerator at temperatures that may be well below the recommended storage temperatures. Apart from causing visible symptoms such as watery, sunken areas on the skin, chilling may also induce changes in fruit textural properties and flavour. The aim of this research was to investigate the effect of low temperature storage on tomato flavour and off-flavour production. To more closely mimic the real-consumer aroma perception while eating, in addition to the standard solid-phase microextraction gas chromatography–mass spectrometry (SPME/GC-MS) analysis, volatiles were also measured using a chewing device connected to a proton-transfer reaction–mass spectrometer (PTR-MS). Aroma volatiles were assessed in red ripe tomatoes of the cvs Cappricia RZ (round truss) and Amoroso RZ (cocktail truss) stored at refrigerator temperature (4 °C) and at higher temperatures (16 and 22 °C) for 20 days. The changes in aroma production were also monitored when the fruit was brought from room to refrigerator temperature and vice versa. After bringing the fruit from room to refrigerator temperature, the abundance of most volatiles was greatly reduced within 3 to 5 h, closely following the decrease in fruit temperature. When temperature was restored to room temperature following varying times of cold storage, the abundance of most volatiles increased again, but generally not to the original levels. Overall, the effects of low temperature storage on the decrease in volatile abundance were more pronounced in cv Cappricia RZ than in cv Amoroso RZ. On the contrary, the production of off flavours following prolonged cold storage was more pronounced in cv Amoroso RZ than in cv Cappricia RZ. Apart from changes in the overall abundance of the volatiles, marked changes in the volatile profile were observed in fruit stored for longer times in the cold and this may at least in part explain the negative effect of cold storage on overall tomato flavour.

Abstract. Volatile organic compounds (VOCs) emitted by forests strongly affect the chemical composition of the atmosphere. While the emission of isoprenoids has been largely characterized, forests also exchange many oxygenated VOCs (oVOCs), including methanol, acetone, methyl ethyl ketone (MEK), and acetaldehyde, which are less well understood. We monitored total branch-level exchange of VOCs of a strong isoprene emitter (Quercus rubra L.) in a mixed forest in New England, where canopy-level fluxes of VOCs had been previously measured. We report daily exchange of several oVOCs and investigated unknown sources and sinks, finding several novel insights. In particular, we found that emission of MEK is linked to uptake of methyl vinyl ketone (MVK), a product of isoprene oxidation. The link was confirmed by corollary experiments proving in vivo detoxification of MVK, which is harmful to plants. Comparison of MEK, MVK, and isoprene fluxes provided an indirect indication of within-plant isoprene oxidation. Furthermore, besides confirming bidirectional exchange of acetaldehyde, we also report for the first time direct evidence of benzaldehyde bidirectional exchange in forest plants. Net emission or deposition of benzaldehyde was found in different periods of measurements, indicating an unknown foliar sink that may influence atmospheric concentrations. Other VOCs, including methanol, acetone, and monoterpenes, showed clear daily emission trends but no deposition. Measured VOC emission and deposition rates were generally consistent with their ecosystem-scale flux measurements at a nearby site.

<p><strong>Abstract.</strong> Volatile organic compounds (VOCs) emitted by forests strongly affect the chemical composition of the atmosphere. While the emission of isoprenoids has been largely characterized, forests also exchange many oxygenated VOCs (oVOCs), including methanol, acetone, methyl ethyl ketone (MEK) and acetaldehyde, which are less well understood. We monitored total branch-level exchange of VOCs of a strong isoprene emitter (<i>Quercus rubra</i> L.) in a mixed forest in New England, where canopy-level fluxes of VOCs had been previously measured. We report daily exchange of several oVOCs and investigated unknown sources and sinks, finding several novel insights. In particular, we found that emission of MEK is linked to uptake of methyl vinyl ketone (MVK), a product of isoprene oxidation. The link was confirmed by corollary experiments proving in vivo detoxification of MVK, which is harmful to plants. Comparison of MEK, MVK and isoprene fluxes provided an indirect indication of within-plant isoprene oxidation. Furthermore, besides confirming bidirectional exchange of acetaldehyde, we also report for the first time direct evidence of benzaldehyde bidirectional exchange in forest plants. Net emission or deposition of benzaldehyde was found in different periods of measurements, indicating an unknown foliar sink that may influence atmospheric concentrations. Other VOCs, including methanol, acetone and monoterpenes, showed clear daily emission trends, but no deposition. Measured VOC emission and deposition rates were generally consistent with their ecosystem-scale flux measurements at a nearby site.</p>