Plant Volatiles: Recent Advances and Future Perspectives

Volatile compounds act as a language that plants use for their communication and interaction with the surrounding environment. To date, a total of 1700 volatile compounds have been isolated from more than 90 plant families. These volatiles, released from leaves, flowers, and fruits into the atmosphere and from roots into the soil, defend plants against herbivores and pathogens or provide a reproductive advantage by attracting pollinators and seed dispersers. Plant volatiles constitute about 1% of plant secondary metabolites and are mainly represented by terpenoids, phenylpropanoids/benzenoids, fatty acid derivatives, and amino acid derivatives. In this review we focus on the functions of plant volatiles, their biosynthesis and regulation, and the metabolic engineering of the volatile spectrum, which results in plant defense improvement and changes of scent and aroma properties of flowers and fruits.     

Role of Ethylene in the Biosynthetic Pathways of Aroma Volatiles in Ripening Fruit

During the past decade, fruit aroma biosynthetic pathways were established in some climacteric fruits, such as tomato, apple, and melon. Inhibition of ethylene biosynthesis or its action in these fruits can reduce the production of fruit volatiles. Furthermore, ethylene partially regulates expression of a few important enzyme genes in fruit volatile biosynthetic pathways. The aim of this review is to bring together recent advances for understanding the regulatory role of ethylene in the biosynthesis of aroma volatiles in some fruits.     

Metabolic engineering of plant volatiles

Metabolic engineering of the volatile spectrum offers enormous potential for plant improvement because of the great contribution of volatile secondary metabolites to reproduction, defense and food quality. Recent advances in the identification of the genes and enzymes responsible for the biosynthesis of volatile compounds have made this metabolic engineering highly feasible. Notable successes have been reported in enhancing plant defenses and improving scent and aroma quality of flowers and fruits. These studies have also revealed challenges and limitations which will be likely surmounted as our understanding of plant volatile network improves.     

Effect of suppression of ethylene biosynthesis on flavor products in tomato fruits

To elucidate the role of ethylene in the production of flavor compounds by tomato fruits, wild-type tomato (Lycopersicon esculentum L., cv. Lichun) and its transgenic antisense LeACS2 line with suppressed ethylene biosynthesis were used. The metabolism of individual sugars was ethylene-independent. However, citric acid and malic acid were under ethylene regulation. The content of these acids was higher in transgenic tomato fruits and returned to normal level after transgenic fruits were treated with ethylene. Because most of amino acids, which are important precursors of volatiles, were shown to be correlated with ethylene, we surmise that amino acid-related aroma volatiles were also affected by ethylene. Headspace analysis of volatiles showed a significant accumulation of aldehydes in wild-type tomato fruits during fruit ripening and showed a dramatic decrease in most aroma volatiles in transgenic tomato fruits as compared with wild-type fruits. The production of hexanal, hexanol, trans-2-heptenal, cis-3-hexanol, and carotenoid-related volatiles, except β-damascenone and β-ionone, was inhibited by suppression of ethylene biosynthesis. No remarkable differences were observed in the concentrations of cis-3-hexenal and trans-2-hexenal between transgenic and wild-type tomato fruits, indicating these two volatiles to be independent of ethylene. Thus, there are various regulation patterns of flavor profiles in tomato fruits by ethylene. Published in Russian in Fiziologiya Rastenii, 2007, Vol. 54, No. 1, pp. 92–101. The text was submitted by the authors in English. Both authors equally contributed to this work.     

Terpene down-regulation in orange reveals the role of fruit aromas in mediating interactions with insect herbivores and pathogens

Plants use volatile terpene compounds as odor cues for communicating with the environment. Fleshy fruits are particularly rich in volatiles that deter herbivores and attract seed dispersal agents. We have investigated how terpenes in citrus fruit peels affect the interaction between the plant, insects, and microorganisms. Because limonene represents up to 97% of the total volatiles in orange (Citrus sinensis) fruit peel, we chose to down-regulate the expression of a limonene synthase gene in orange plants by introducing an antisense construct of this gene. Transgenic fruits showed reduced accumulation of limonene in the peel. When these fruits were challenged with either the fungus Penicillium digitatum or with the bacterium Xanthomonas citri subsp. citri, they showed marked resistance against these pathogens that were unable to infect the peel tissues. Moreover, males of the citrus pest medfly (Ceratitis capitata) were less attracted to low limonene-expressing fruits than to control fruits. These results indicate that limonene accumulation in the peel of citrus fruit appears to be involved in the successful trophic interaction between fruits, insects, and microorganisms. Terpene down-regulation might be a strategy to generate broad-spectrum resistance against pests and pathogens in fleshy fruits from economically important crops. In addition, terpene engineering may be important for studying the basic ecological interactions between fruits, herbivores, and pathogens.     

植物挥发物代谢工程在改良香气品质和植物防御中的应用

挥发物次生代谢在植物繁殖、植物防御和改良食物品质方面发挥着重要作用。近年来,随着参与挥发物生物合成的基因和酶类的鉴定以及代谢途径和调控机理等研究的不断发展和深入,挥发物代谢工程已经具备较高的可行性。应用代谢工程改良花、果实的香气品质以及提高植物防御能力的研究成效显著。主要介绍了这些方面的最新进展,同时也讨论了植物挥发物代谢工程应用存在的问题和挑战以及研究思路。     

Biosynthesis of plant-derived flavor compounds

Plants have the capacity to synthesize, accumulate and emit volatiles that may act as aroma and flavor molecules due to interactions with human receptors. These low-molecular-weight substances derived from the fatty acid, amino acid and carbohydrate pools constitute a heterogenous group of molecules with saturated and unsaturated, straight-chain, branched-chain and cyclic structures bearing various functional groups (e.g. alcohols, aldehydes, ketones, esters and ethers) and also nitrogen and sulfur. They are commercially important for the food, pharmaceutical, agricultural and chemical industries as flavorants, drugs, pesticides and industrial feedstocks. Due to the low abundance of the volatiles in their plant sources, many of the natural products had been replaced by their synthetic analogues by the end of the last century. However, the foreseeable shortage of the crude oil that is the source for many of the artifical flavors and fragrances has prompted recent interest in understanding the formation of these compounds and engineering their biosynthesis. Although many of the volatile constituents of flavors and aromas have been identified, many of the enzymes and genes involved in their biosynthesis are still not known. However, modification of flavor by genetic engineering is dependent on the knowledge and availability of genes that encode enzymes of key reactions that influence or divert the biosynthetic pathways of plant-derived volatiles. Major progress has resulted from the use of molecular and biochemical techniques, and a large number of genes encoding enzymes of volatile biosynthesis have recently been reported.     

利用类萜代谢工程改良作物风味

类萜是从植物中分离出的一类类异戊二烯物质。其中挥发性萜类除了在吸引授粉媒、异株克生和植物防御中起到一定的生态作用外,还影响到水果、蔬菜和其他作物的香味形成。对类萜生物合成及其代谢工程的最新研究进展进行了综述,探讨了代谢过程中的关键酶基因,尤其是类萜合成酶(TPSs)基因的表达特性以及操纵类萜生物合成途径提高产量的几种可能的策略。随着更多相关基因的分离,利用代谢工程人工改良作物风味将指日可待。     

A non-targeted approach unravels the volatile network in peach fruit

Volatile compounds represent an important part of the plant metabolome and are of particular agronomic and biological interest due to their contribution to fruit aroma and flavor and therefore to fruit quality. By using a non-targeted approach based on HS-SPME-GC-MS, the volatile-compound complement of peach fruit was described. A total of 110 volatile compounds (including alcohols, ketones, aldehydes, esters, lactones, carboxylic acids, phenolics and terpenoids) were identified and quantified in peach fruit samples from different genetic backgrounds, locations, maturity stages and physiological responses. By using a combination of hierarchical cluster analysis and metabolomic correlation network analysis we found that previously known peach fruit volatiles are clustered according to their chemical nature or known biosynthetic pathways. Moreover, novel volatiles that had not yet been described in peach were identified and assigned to co-regulated groups. In addition, our analyses showed that most of the co-regulated groups showed good intergroup correlations that are therefore consistent with the existence of a higher level of regulation orchestrating volatile production under different conditions and/or developmental stages. In addition, this volatile network of interactions provides the ground information for future biochemical studies as well as a useful route map for breeding or biotechnological purposes.     

NON-SMOKY GLYCOSYLTRANSFERASE1 Prevents the Release of Smoky Aroma from Tomato Fruit

Phenylpropanoid volatiles are responsible for the key tomato fruit (Solanum lycopersicum) aroma attribute termed “smoky.” Release of these volatiles from their glycosylated precursors, rather than their biosynthesis, is the major determinant of smoky aroma in cultivated tomato. Using a combinatorial omics approach, we identified the NON-SMOKY GLYCOSYLTRANSFERASE1 (NSGT1) gene. Expression of NSGT1 is induced during fruit ripening, and the encoded enzyme converts the cleavable diglycosides of the smoky-related phenylpropanoid volatiles into noncleavable triglycosides, thereby preventing their deglycosylation and release from tomato fruit upon tissue disruption. In an nsgt1/nsgt1 background, further glycosylation of phenylpropanoid volatile diglycosides does not occur, thereby enabling their cleavage and the release of corresponding volatiles. Using reverse genetics approaches, the NSGT1-mediated glycosylation was shown to be the molecular mechanism underlying the major quantitative trait locus for smoky aroma. Sensory trials with transgenic fruits, in which the inactive nsgt1 was complemented with the functional NSGT1, showed a significant and perceivable reduction in smoky aroma. NSGT1 may be used in a precision breeding strategy toward development of tomato fruits with distinct flavor phenotypes.     

A Role for Differential Glycoconjugation in the Emission of Phenylpropanoid Volatiles from Tomato Fruit Discovered Using a Metabolic Data Fusion Approach

A role for differential glycoconjugation in the emission of phenylpropanoid volatiles from ripening tomato fruit (Solanum lycopersicum) upon fruit tissue disruption has been discovered in this study. Application of a multiinstrumental analytical platform for metabolic profiling of fruits from a diverse collection of tomato cultivars revealed that emission of three discriminatory phenylpropanoid volatiles, namely methyl salicylate, guaiacol, and eugenol, took place upon disruption of fruit tissue through cleavage of the corresponding glycoconjugates, identified putatively as hexose-pentosides. However, in certain genotypes, phenylpropanoid volatile emission was arrested due to the corresponding hexose-pentoside precursors having been converted into glycoconjugate species of a higher complexity: dihexose-pentosides and malonyl-dihexose-pentosides. This glycoside conversion was established to occur in tomato fruit during the later phases of fruit ripening and has consequently led to the inability of red fruits of these genotypes to emit key phenylpropanoid volatiles upon fruit tissue disruption. This principle of volatile emission regulation can pave the way to new strategies for controlling tomato fruit flavor and taste.More than 7,000 metabolites, including volatiles, have already been identified in plant-based foods and beverages (Goff and Klee, 2006). Volatile organic compounds constitute a significant part of the plant metabolome, and the number of individual volatiles already described for various plants is approaching 2,000 (Dudareva et al., 2006). Significant progress has been made on the functional characterization of these plant volatiles over the past decades. For example, volatiles have been shown to play an important role in the interaction between plants and their environment. They are involved in the defense of plants against pathogens, where they serve as airborne signaling molecules to induce a defense response in other plant parts or neighboring plants (Shulaev et al., 1997). They also act as direct repellents of herbivorous pests or as attractants of the predators of these pests as part of the “cry for help” response (Dudareva et al., 2004; Kappers et al., 2005; Baldwin et al., 2006). In addition, flower volatiles are important for the attraction of pollinators (Dudareva et al., 2004), while fruit volatiles may have a role in attracting seed dispersers (Goff and Klee, 2006; Schwab et al., 2008). Besides their physiological and ecological functions, plant volatiles are also important determinants of consumer quality traits in flowers, fruits, and vegetables as well as the processed products derived from them.Tomato (Solanum lycopersicum) is one of the most important vegetable crops worldwide, and its fresh fruits and processed products are consumed and appreciated in every society. Volatiles are considered as major determinants of tomato fruit flavor (Buttery et al., 1987; Buttery and Ling, 1993; Baldwin et al., 1998, 2000; Tandon et al., 2000; Krumbein et al., 2004; Ruiz et al., 2005; Tieman et al., 2006; Kovács et al., 2009; Zanor et al., 2009). Several hundred tomato fruit volatile compounds have been described in the literature (Petro-Turza, 1987), but only a small part of this diversity is believed to have an impact on tomato fruit organoleptic properties (Buttery and Ling, 1993; Baldwin et al., 2000). We have previously screened red-ripe fruits for variation in their volatile metabolome using a collection of 94 tomato cultivars representing the current diversity within commercial germplasm (Tikunov et al., 2005). In that study, three phenylpropanoid (PhP) volatiles, methyl salicylate (MeSA), guaiacol, and eugenol, were found to be discriminatory within this germplasm collection and roughly divided the cultivars into two groups. Fruits from one group had the capacity to emit significant amounts of these three PhP volatiles upon fruit tissue disruption (blending), while fruits from the other group emitted none or hardly any.The considered relevance of these findings relates to their potential importance in consumer perception of fruit taste differences. It has been proposed previously that PhP volatiles likely have an impact on tomato fruit aroma. MeSA, the methyl ester of salicylic acid, is a potent odor component of wintergreen (Gaultheria procumbens). MeSA content has been shown to be negatively correlated with typical tomato flavor (Krumbein and Auerswald, 1998). Guaiacol is also a well-known flavoring compound and has been associated with a so-called “pharmaceutical” aroma in tomato fruits (Causse et al., 2002). Likewise, eugenol is a well-known odorant that gives the distinctive, pungent flavor to cloves (Syzygium aromaticum) and significantly contributes to the aroma of cinnamon (Cinnamomum verum). Although a potential physiological role for these PhP volatiles in tomato fruits remains unclear, they have been implicated to have a signaling and/or defense function (Shulaev et al., 1997; Koeduka et al., 2006; Sasso et al., 2007). Therefore, there is clear potential for the release of these compounds to influence, either negatively or positively, tomato flavor.In plants, PhP volatiles are primarily derived from Phe (Dudareva and Pichersky, 2000). Cinnamic acid, directly derived from Phe by a deamination catalyzed by Phe ammonia lyase, can either be β-oxidatively or nonoxidatively converted into benzoic acid. This can be further hydroxylated into salicylic acid by benzoic acid 2-hydroxylase. Recently, genetic studies in Arabidopsis (Arabidopsis thaliana) revealed the existence of an alternative pathway for the production of salicylic acid from isochorismate, thus bypassing Phe and its derivatives (Wildermuth et al., 2001). Salicylic acid is the immediate precursor of the volatile MeSA, through the action of salicylic acid methyl transferase (Boatright et al., 2004). Cinnamic acid can also be converted to other phenolic acids: p-coumaric acid, caffeic acid, and ferulic acid. Ferulic acid can be converted into coniferyl alcohol and further to eugenol (Gang, 2005). The biochemical origin of guaiacol in plants is not completely known. However, its chemical structure clearly points to the same PhP origin, as has already been demonstrated for bacteria (Chang and Kang, 2004).Glycosylation is a common means to conjugate plant secondary metabolites, in order to facilitate their transport and storage and to reduce their reactivity by blocking reactive hydroxyl groups. In tomato fruit, many volatile compounds, including PhP volatiles, are bound as glycosides, thus representing an aroma reserve (Buttery et al., 1990; Marlatt et al., 1992; Ortiz-Serrano and Gil, 2007). Such glycosidically bound volatiles can be liberated when cell compartmentation is destroyed, as happens on consumption of fresh fruits or industrial processing or as may happen during late ripening stages. As a consequence of this disruption, the contents of different cell compartments can mix and stored volatile glycosides become exposed to endogenous or exogenous cleavage enzymes, such as glycosyl hydrolases (glycosidases), which leads to glycoside cleavage and volatile emission. Thus, understanding the biochemical processes leading to the formation and/or cleavage of volatile glycoconjugates may provide tools to exploit more efficiently the aroma reserve present in tomato fruit in order to improve tomato fruit flavor.In order to gain greater insight into the volatile compound variation in tomato fruit, we previously analyzed the volatile metabolites using solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) in a broad screening of 94 contrasting tomato genotypes representing the variation present in the germplasm of commercial tomato varieties (Tikunov et al., 2005; Van Berloo et al., 2008). This nontargeted metabolomics approach enabled the detection and putative identification of 322 volatiles. Subsequent multivariate analysis revealed (1) differences between tomato types (cherry versus round tomatoes) driven by the accumulation of phenolic-derived volatiles such as phenylethanol and phenylacetaldehyde, and (2) that the PhP-derived volatiles MeSA, guaiacol, and eugenol roughly split the set of genotypes into two distinct groups, independent of tomato fruit (pheno)type, where fruits of one of these groups emitted considerable amounts of these three PhP volatiles but fruits of the other emitted little or none.In this paper, we describe the investigation into the biochemical basis underlying this difference in capacity to emit PhP volatiles. A multiinstrumental metabolomics platform was used to profile fruits of the same broad tomato germplasm collection for both volatile and nonvolatile metabolites. Metabolic data fusion of both liquid chromatography (LC)-MS and GC-MS data sets, followed by multivariate analyses, suggested a principle for the regulation of PhP volatile emission in tomato fruit through differential volatile-sugar conjugation patterns. Subsequent series of quantitative biochemical experiments proved an important role of this process in regulating the emission of PhP volatiles from tomato fruit.     

Functional characterization of enzymes forming volatile esters from strawberry and banana

Volatile esters are flavor components of the majority of fruits. The last step in their biosynthesis is catalyzed by alcohol acyltransferases (AATs), which link alcohols to acyl moieties. Full-length cDNAs putatively encoding AATs were isolated from fruit of wild strawberry (Fragaria vesca) and banana (Musa sapientum) and compared to the previously isolated SAAT gene from the cultivated strawberry (Fragaria x ananassa). The potential role of these enzymes in fruit flavor formation was assessed. To this end, recombinant enzymes were produced in Escherichia coli, and their activities were analyzed for a variety of alcohol and acyl-CoA substrates. When the results of these activity assays were compared to a phylogenetic analysis of the various members of the acyltransferase family, it was clear that substrate preference could not be predicted on the basis of sequence similarity. In addition, the substrate preference of recombinant enzymes was not necessarily reflected in the representation of esters in the corresponding fruit volatile profiles. This suggests that the specific profile of a given fruit species is to a significant extent determined by the supply of precursors. To study the in planta activity of an alcohol acyltransferase and to assess the potential for metabolic engineering of ester production, we generated transgenic petunia (Petunia hybrida) plants overexpressing the SAAT gene. While the expression of SAAT and the activity of the corresponding enzyme were readily detected in transgenic plants, the volatile profile was found to be unaltered. Feeding of isoamyl alcohol to explants of transgenic lines resulted in the emission of the corresponding acetyl ester. This confirmed that the availability of alcohol substrates is an important parameter to consider when engineering volatile ester formation in plants.     

Lycopene accumulation affects the biosynthesis of some carotenoid-related volatiles independent of ethylene in tomato

For elucidating the regulatory mechanism of ethylene on carotenoid-related volatiles (open chain) compounds and the relationship between lycopene and carotenoid-related volatiles,transgenic tomato fruits in which ACC synthase was suppressed were used.The transgenic tomato fruit showed a significant reduction of lycopene and aroma volatiles with low ethylene production.6-methyl-5-hepten-2-one,6-methyl-5-hepten-2-ol and geranylacetone,which were suspected to be lycopene degradation products,were lower than those in wild type tomato fruits.In order to identify whether lycopene accumulation effects the biosynthesis of some carotenoid-related volatiles independent of ethylene in tomato or not,the capability of both wild type and transgenic tomato fruits discs to convert lycopene into carotenoid-related volatiles was evaluated.The data showed that external lycopene could convert into 6-methyl-5-hepten-2-one and 6-methyl-5-hepten-2-ol in vivo,Indicating that the strong inhibition of ethylene production had no effect on enzymes in the biosynthesis pathway of some carotenoid-related volatiles.Therefore,in ACS-suppression transgenic tomato fruits,the low levels of 6-methyl-5-hepten-2-one,6-methyl-5-hepten-2-ol was due to decreased lycopene accumulation,not ethylene production.Ethylene only affected the accumulation of lycopene,and then indirectly influenceed the level of lycopene-related volatiles.     

Identification of a specific isoform of tomato lipoxygenase (TomloxC) involved in the generation of fatty acid-derived flavor compounds

There are at least five lipoxygenases (TomloxA, TomloxB, TomloxC, TomloxD, and TomloxE) present in tomato (Lycopersicon esculentum Mill.) fruit, but their role in generation of fruit flavor volatiles has been unclear. To assess the physiological role of TomloxC in the generation of volatile C6 aldehyde and alcohol flavor compounds, we produced transgenic tomato plants with greatly reduced TomloxC using sense and antisense constructs under control of the cauliflower mosaic virus 35S promoter. The expression level of the TomloxC mRNA in some transgenic plants was selectively reduced by gene silencing or antisense inhibition to between 1% and 5% of the wild-type controls, but the expression levels of mRNAs for the four other isoforms were unaffected. The specific depletion of TomloxC in transgenic tomatoes led to a marked reduction in the levels of known flavor volatiles, including hexanal, hexenal, and hexenol, to as little as 1.5% of those of wild-type controls following maceration of ripening fruit. Addition of linoleic or linolenic acid to fruit homogenates significantly increased the levels of flavor volatiles, but the increase with the TomloxC-depleted transgenic fruit extracts was much lower than with the wild-type control. Confocal imaging of tobacco (Nicotiana tabacum) leaf cells expressing a TomloxC-GFP fusion confirmed a chloroplast localization of the protein. Together, these results suggest that TomloxC is a chloroplast-targeted lipoxygenase isoform that can use both linoleic and linolenic acids as substrates to generate volatile C6 flavor compounds. The roles of the other lipoxygenase isoforms are discussed.     

A putative PhODO1 homologous MYB transcription factor gene, MdMYBB, is not involved in the regulation of aroma volatile biosynthesis in apple

To get deeper insight on the molecular mechanism underlying production of volatile compounds in apple (Malus domestica Borkh.), we performed the isolation and expression analysis of one R2R3-type MYB gene named MdMYBB. The amino acid sequence and the structural features of MdMYBB highly resembled those of PhODO1, which is a key regulator for floral scent biosynthesis in petunia. The expression of MdMYBB was repressed concomitantly with the inhibition of ethylene production, which regulates the volatile synthesis in apple. However, MdMYBB expression was not detected in the flesh from nearly ripened apple fruits, although the detection of exogenous volatiles had actually occurred in the same portion. In addition, overexpression of MdMYBB did not cause any induction of the volatile compounds in the transgenic tobacco lines. Thus, the features of MdMYBB were not in accordance with the aroma volatile emission, unlike the case of PhODO1, suggesting that MdMYBB may not be involved in the regulation of the biosynthesis for apple aroma volatiles. On the basis of the specific expression patterns, we discussed possible physiological roles of MdMYBB in apple.     

挥发性香气物质和乙烯生产在"湖景蜜露"桃果实发育过程中的变化

以"湖景蜜露"水蜜桃(Prunus persica L.)为试材,检测了果实从未成熟到成熟发育过程中乙烯生成、呼吸速率及挥发性香气性物质的变化;同时对果实大小、果皮色泽、果肉硬度、可溶性固形物、可滴定酸进行了测定;对与果实乙烯产生密切相关的1-氨基环丙烷-1-羧酸(ACC)含量、ACC合成酶活性、ACC氧化酶活性也进行了测定.结果表明,随果实成熟度的增加,果实大小、果皮L*值、可溶性固形物含量增加,而果实硬度、果皮h°值、可滴定酸含量减少.在未成熟的果实中,C6的醛类(反式-2-己烯醛)和醇类(顺式-3-己烯醇)是主要的成分;乙烯生成量很低;呼吸速率较高.到跃变阶段C6～C12的内酯类物质明显增加,尤其是γ和δ-内酯类成为果实主要的香气挥发性物质.推测果实乙烯、呼吸作用等基本的生理变化可能调节着内酯类物质的生成.在乙烯跃变上升时果肉中ACC氧化酶的活性下降,ACC含量和ACC合成酶活力的变化与乙烯生成量变化的趋势一致.根据以上结果可以认为桃果实主要的香气挥发性物质的形成与乙烯、呼吸跃变的开始密切相关.香气物质形成速率动态变化可能是桃果实发育过程中成熟度的另一个生理学指标.     

Costs of induced volatile production in maize

Herbivore‐induced plant volatiles have been shown to serve as indirect defence signals that attract natural enemies of herbivores. Parasitoids and predators exploit these plant‐provided cues to locate their victims and several herbivores are repelled by the volatiles. Recently, benefits, in terms of plant fitness, from the action of the parasitoids were shown for a few systems. However, the cost of production of herbivore‐induced volatiles for the plant remains unknown. Here, we estimate the fitness cost of the production of induced volatiles in maize, Zea mays. Plants were treated with regurgitant of Spodoptera littoralis or with the elicitor volicitin and we measured dry weight of plant parts at specific times after treatments. After a two‐week treatment period, the dry‐weight of leaves of induced plants was lower than that of un‐induced plants, suggesting a metabolic cost for induced defence. However, maize plants seem to compensate for this loss during subsequent growth, since seed production at maturity was not different for unharmed plants and plants treated with caterpillar regurgitant. For volicitin treated plants a small but significant reduction in seed production was found. It is likely that the treatments also induced the production of other defence compounds, which will contribute to the cost. Yet, a comparison of six maize inbred lines with distinct differences in volatile emissions showed a strong correlation between the intensity of induced emissions and reduction in plant performance. An analysis of the terpenoids that accumulated in the leaves of the inbred lines revealed non‐volatilised compounds are constitutively present in maize and only the volatilised compounds are induced. Interestingly, the lines that released the largest amounts of induced volatiles also contained more of the non‐volatile terpenoids. Based on these results and results from a previous study on the benefits of attracting parasitoids, we conclude that costs of induced volatile production in plants are counterbalanced by the benefits as long as natural enemies of the herbivores are present in the environment.     

Identification of intermediates involved in the biosynthetic pathway of 3-mercaptohexan-1-ol conjugates in yellow passion fruit (Passiflora edulis f. flavicarpa)

Yellow passion fruit is one of the most well-known tropical fruits and much of its success comes from its typical aroma. Key compounds in explaining yellow passion fruit scent are volatile thiols. These molecules are reported to be present in several fruits and originate from non-volatile precursors. Such free thiols are particularly appreciated in white wines and considerable efforts have been made to try to maximise their production and understand their biosynthesis.Two main precursors have been identified so far: S-glutathionylated and S-cysteinylated precursors, the latter originating in the breaking down of the glycyl and glutamyl moieties of the former. Improving knowledge about this pathway is currently one of the main challenges in the field of aroma chemistry.Only S-cysteinylated precursors have been reported in the literature for yellow passion fruit, thus much of the biochemical pathway remains unknown.In this paper a combination of organic synthesis, MS and NMR experiments was developed in order to investigate this pathway in yellow passion fruit. The three missing stages leading to the S-cysteinylated precursor were clearly identified. Both intermediate species between S-glutathionyl and S-cysteinyl 3-mercaptohexan-1-ol were found, suggesting that the plant is capable of activating both metabolic routes.The information gained would appear to be crucial for study of this important pathway and for potentially extending this knowledge to other plants, in particular the grapevine.