Perspective Cite This: J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Smell of Stress: Identification of Induced Biochemical Pathways Affecting the Volatile Composition and Flavor Quality of Crops Matthias Wüst* Chair of Bioanalytics/Food Chemistry, Department of Nutritional and Food Sciences, University of Bonn Endenicher Allee 11-13, D-53115 Bonn, Germany ABSTRACT: Recent research has shown that the biosynthesis of several key odorants is controlled by genes whose expression is altered or even induced by biotic or abiotic stress. These new findings provide a roadmap for improvement of flavor quality by the application of moderate, well-controlled stress. This strategy aims at reducing the flavor deficiencies in modern commercial varieties as a “green” alternative to genetic engineering. The workflow for a successful implementation of this approach, from the identification of key odorants by molecular science techniques to the investigation of mechanisms controlling their biosynthesis, is complex and calls for interdisciplinary research. KEYWORDS: odorant, stress, flavor quality, odor code, aroma, taste
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pathway is known down to the enzyme level, a redirection of the metabolic flux by genetic engineering is nowadays feasible. An illustrative example is the production of transgenic tomato fruits with an increased monoterpene synthesis as a result of a redirection of the metabolic flux toward geranyl diphosphate in plastids.7 A general scheme that depicts the different stages of a workflow that aims at altering the aroma of fruits or vegetables is shown in the right part of Figure 1. While this strategy is straightforward, the consumer acceptance of genetically modified fruits and vegetables is generally low, especially in countries of western Europe.8 An alternative strategy is based on the recent findings that many of these key odorants have various direct and indirect defense roles against abiotic stresses (such as temperature, water stress, ozone, salt stress, and heavy
eing sessile organisms, plants have to develop strategies to communicate with their environment and, therefore, produce thousands of structurally diverse volatile signal compounds to attract pollinating insects and seed-dispersing animals and to mediate interactions with other plants. These small chemical compounds represent the evolutionary oldest form of communication, and Mithöfer and Boland have recently concluded that plants simply “speak chemistry” and that we need to decipher and learn this chemical language.1 Indeed, during evolution, humans have acquired the ability to perceive and “understand” these compounds as a specific ripe fruit or vegetable aroma and have developed preferences for certain plants that are now produced as crops. During the last 2 decades, numerous aroma-active compounds have been successfully identified by the so-called “sensomics” approach, comprising the bioactivity-guided discovery of key odorants by gas chromatography (GC)−olfactometry, their accurate quantitation by stable isotope dilution assays, followed by aroma reconstitution and omission experiments. A recently performed meta-analysis has now revealed the fascinating fact that characteristic ratios of only about 3−40 genuine key odorants for each food from a group of about 230 out of ca. 10 000 food volatiles is sufficient to decipher nature’s odor code.2 Thus, the foodborne stimulus space has co-evolved with and roughly matches our ca. 400 olfactory receptors as best natural agonists. This knowledge of the odor code of crops holds promise for major improvements in future precision molecular breeding, which in the past have been targeted primarily toward field performance, yield, and storage characteristics, while ignoring quality traits, such as aroma and taste.3 Thus, modern commercial varieties contain often significantly lower amounts of many of these important odorants than older varieties.4 Whole genome sequencing and genome-wide association studies can identify genetic loci that affect most of the target aroma compounds. However, these studies need to be supplemented by the elucidation of the formation pathways of key odorants by labeled precursors, which are often multi-step reactions that comprise enzymatic but also non-enzymatic reactions.5,6 Once, the formation © XXXX American Chemical Society
Figure 1. General scheme that depicts the different stages of a workflow that aims at altering the aroma of fruits or vegetables: (left part) genetic engineering approach and (right part) alternative “green” approach using controlled stress. Received: January 28, 2018 Accepted: March 24, 2018 Published: March 24, 2018 A
DOI: 10.1021/acs.jafc.8b00522 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry
Figure 2. Metabolic pathways leading to the biosynthesis of plant-derived odorants. Metabolites in blue ovals have already been enhanced by the application of controlled stress and/or the exogenous application of methyl jasmonate. MEP, methylerythritol phosphate; LOX, lipoxygenase; MVA, mevalonic acid; and CoA, coenzyme A.
metals) and biotic stresses, such as herbivores and pathogens.9 The biosynthesis of these compounds is thus controlled by genes whose expression is altered or even induced by biotic or abiotic stress as opposed to their biosynthesis by constitutively expressed genes. These new findings provide a roadmap for improvement of flavor quality by the application of moderate, well-controlled stress. This strategy aims at reducing the flavor deficiencies in modern commercial varieties as a “green” alternative to genetic engineering (left part of Figure 1). The concept of the application of controlled stress is not new and has been proposed for the production of phytoalexin-enriched functional foods, which could benefit the consumer by providing “health-enhanced” food choices.10 Indeed, it could be shown recently that hot air treatment enhanced phenylpropanoid metabolism in cherry tomato fruit, as evidenced by elevated levels of phenolics and flavonoids.11 Surprisingly, the application of controlled stress to influence the aroma of crops is not new as well and has evolved in China hundreds of years before, leading to the complex manufacturing process of famous traditional Chinese teas, such as oolong. We are just now at the beginning to understand on a molecular level how specific production steps that cause abiotic stress selectively enhance the expression of aroma-related genes. It could be shown that accumulation of the potent odorant indole in oolong tea was due to the activation of CsTSB2 (catalyzing the transformation from indole-3-glycerol phosphate to indole) by continuous wounding stress caused by mechanical damage from the turnover process. Stable isotope labeling indicated that tea leaf cell disruption from the rolling process of black tea did not lead to the conversion of indole but terminated the synthesis of
indole.12 The same group could show that the combination of low-temperature stress and mechanical damage had a synergistic effect on (E)-nerolidol formation by activating the nerolidol synthase CsNES gene expression resulting in (E)nerolidol accumulation, which is a potent odorant in tea as well.13 Also, biotic stress by herbivores at preharvest stages can decisively influence the aroma quality of tea. The tea green leafhopper responsible for inducing the production of characteristic volatiles in the oolong tea Oriental Beauty was identified as Empoasca (Matsumurasca) onukii Matsuda. This attack significantly induced the emission of linalool from tea leaves as a result of the upregulation of the linalool synthases (CsLIS1 and CsLIS2) and caused the unique aroma.14 These tea-related findings illustrate nicely how the use of stress responses of plant secondary metabolism may lead to the improvement of the aroma of agricultural products. Recent research has shown that numerous stress responses can be triggered by the application of elicitors without the need of stressor application. The plant hormone methyl jasmonate (MeJA) acts as an efficient elicitor of volatile secondary metabolite production through an extensive transcriptional reprogramming of plant metabolites. Indeed, exogenous application of MeJA could induce higher levels of aroma volatiles in tea leaves, especially the potent odorants geraniol and linalool and its oxides. It is noteworthy that, upon leaf wounding, the de novo synthesized jasmonolyl isoleucine acts as a jasmonate transport derivative and mobile signal in tomato plants, which induces a systemic wound response.15 Thus, also fruits may be susceptible to transcriptional reprogramming by this mobile signal, and there is now solid experimental evidence B
DOI: 10.1021/acs.jafc.8b00522 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry
(8) Ishii, T.; Araki, M. Consumer acceptance of food crops developed by genome editing. Plant Cell Rep. 2016, 35 (7), 1507−1518. (9) Rani, K.; Arya, S. S.; Devi, S.; Kaur, V. Plant Volatiles and Defense. In Volatiles and Food Security; Choudhary, D. K., Sharma, A. K., Agarwa, P., Varma, A., Tuteja, N., Eds.; Springer: Singapore, 2017; pp 113−134, DOI: 10.1007/978-981-10-5553-9_7. (10) Boue, S. M.; Cleveland, T. E.; Carter-Wientjes, C.; Shih, B. Y.; Bhatnagar, D.; McLachlan, J. M.; Burow, M. E. Phytoalexin-Enriched Functional Foods. J. Agric. Food Chem. 2009, 57 (7), 2614−2622. (11) Wei, Y.; Zhou, D.; Peng, J.; Pan, L.; Tu, K. Hot Air Treatment Induces Disease Resistance through Activating the Phenylpropanoid Metabolism in Cherry Tomato Fruit. J. Agric. Food Chem. 2017, 65 (36), 8003−8010. (12) Zeng, L.; Zhou, Y.; Gui, J.; Fu, X.; Mei, X.; Zhen, Y.; Ye, T.; Du, B.; Dong, F.; Watanabe, N.; Yang, Z. Formation of Volatile Tea Constituent Indole during the Oolong Tea Manufacturing Process. J. Agric. Food Chem. 2016, 64 (24), 5011−5019. (13) Zhou, Y.; Zeng, L.; Liu, X.; Gui, J.; Mei, X.; Fu, X.; Dong, F.; Tang, J.; Zhang, L.; Yang, Z. Formation of (E)-nerolidol in tea (Camellia sinensis) leaves exposed to multiple stresses during tea manufacturing. Food Chem. 2017, 231, 78−86. (14) Mei, X.; Liu, X.; Zhou, Y.; Wang, X.; Zeng, L.; Fu, X.; Li, J.; Tang, J.; Dong, F.; Yang, Z. Formation and emission of linalool in tea (Camellia sinensis) leaves infested by tea green leafhopper (Empoasca (Matsumurasca) onukii Matsuda). Food Chem. 2017, 237, 356−363. (15) Matsuura, H.; Takeishi, S.; Kiatoka, N.; Sato, C.; Sueda, K.; Masuta, C.; Nabeta, K. Transportation of de novo synthesized jasmonoyl isoleucine in tomato. Phytochemistry 2012, 83, 25−33. (16) Chang, L. L.; Zhang, Y. T.; Wang, G. X.; Dong, J.; Zhong, C. F.; Wang, L. N.; Li, T. H. The effects of exogenous methyl jasmonate on FaNES1 gene expression and the biosynthesis of volatile terpenes in strawberry (Fragaria × ananassa Duch.) fruit. J. Hortic. Sci. Biotechnol. 2013, 88 (4), 393−398. (17) Kondo, S.; Setha, S.; Rudell, D. R.; Buchanan, D. A.; Mattheis, J. P. Aroma volatile biosynthesis in apples affected by 1-MCP and methyl jasmonate. Postharvest Biol. Technol. 2005, 36 (1), 61−68. (18) May, B.; Wüst, M. Induction of de Novo Mono- and Sesquiterpene Biosynthesis by Methyl Jasmonate in Grape Berry Exocarp. In Advances in Wine Research; ACS Symposium Series. ACS Symp. Ser. 2015, 1203, 191−201. (19) Ruiz-García, Y.; López-Roca, J. M.; Bautista-Ortín, A. B.; GilMuñ o z, R.; Gó m ez-Plaza, E. Effect of Combined Use of Benzothiadiazole and Methyl Jasmonate on Volatile Compounds of Monastrell Wine. Am. J. Enol. Vitic. 2014, 65 (2), 238−243. (20) Vitalini, S.; Ruggiero, A.; Rapparini, F.; Neri, L.; Tonni, M.; Iriti, M. The application of chitosan and benzothiadiazole in vineyard (Vitis vinifera L. cv Groppello Gentile) changes the aromatic profile and sensory attributes of wine. Food Chem. 2014, 162, 192−205. (21) D’Onofrio, C.; Matarese, F.; Cuzzola, A. Effect of methyl jasmonate on the aroma of Sangiovese grapes and wines. Food Chem. 2018, 242, 352−361.
that exogenous MeJA can indeed enhance the production of aroma compounds in fruits, such as strawberry,16 apple,17 and grape.18 Most notably, the in vineyard application of the elicitors chitosan, benzothiadiazole, and MeJA changed the aromatic profile and sensory attributes of grapes and wines.19,20 In a recent landmark publication, it was demonstrated that MeJA application led to a delay in grape technological maturity and a significant increase in the concentration of monoterpenes.21 An analysis of the expression of terpenoid biosynthesis genes [terpene synthases (TPS)] confirmed that the MeJA application activated the related biosynthetic pathway. Also, the wines produced by microvinification of Sangiovese treated and untreated grapes showed a rise in the aroma concentration as in berries, with an important impact on longevity and sensorial characters of wines. This study impressively demonstrates a successful implementation of the aforementioned “green” approach that constitutes an alternative to the genetic engineering strategies. Figure 2 illustrates the metabolic pathways leading to the biosynthesis of plant-derived odorants and shows the complexity that still needs to be fully investigated with respect to effects of biotic and abiotic stress. This calls for more cooperation between plant scientists and food chemists when it comes to the improvement of important quality traits, such as aroma and taste. Back to the bench!
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +49-228-732361. Fax: +49-228-733499. E-mail:
[email protected]. ORCID
Matthias Wüst: 0000-0001-6808-5555 Notes
The author declares no competing financial interest.
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REFERENCES
(1) Mithöfer, A.; Boland, W. Do you speak chemistry?: Small chemical compounds represent the evolutionary oldest form of communication between organisms. EMBO Rep. 2016, 17, 626. (2) Dunkel, A.; Steinhaus, M.; Kotthoff, M.; Nowak, B.; Krautwurst, D.; Schieberle, P.; Hofmann, T. Nature’s Chemical Signatures in Human Olfaction: A Foodborne Perspective for Future Biotechnology. Angew. Chem., Int. Ed. 2014, 53 (28), 7124−7143. (3) Hofmann, T.; Krautwurst, D.; Schieberle, P. Current Status and Future Perspectives in Flavor Research: Highlights of the 11th Wartburg Symposium on Flavor Chemistry & Biology. J. Agric. Food Chem. 2018, 66 (10), 2197−2203. (4) Tieman, D.; Zhu, G.; Resende, M. F. R.; Lin, T.; Nguyen, C.; Bies, D.; Rambla, J. L.; Beltran, K. S. O.; Taylor, M.; Zhang, B.; Ikeda, H.; Liu, Z.; Fisher, J.; Zemach, I.; Monforte, A.; Zamir, D.; Granell, A.; Kirst, M.; Huang, S.; Klee, H. A chemical genetic roadmap to improved tomato flavor. Science 2017, 355 (6323), 391−394. (5) Schwab, W.; Wüst, M. Understanding the Constitutive and Induced Biosynthesis of Mono- and Sesquiterpenes in Grapes (Vitis vinifera): A Key to Unlocking the Biochemical Secrets of Unique Grape Aroma Profiles. J. Agric. Food Chem. 2015, 63 (49), 10591− 10603. (6) Wüst, M. Biosynthesis of Plant-Derived Odorants. In Springer Handbook of Odor; Buettner, A., Ed.; Springer: Cham, Switzerland, 2017; pp 9−10, DOI: 10.1007/978-3-319-26932-0_2. (7) Gutensohn, M.; Orlova, I.; Nguyen, T. T. H.; DavidovichRikanati, R.; Ferruzzi, M. G.; Sitrit, Y.; Lewinsohn, E.; Pichersky, E.; Dudareva, N. Cytosolic monoterpene biosynthesis is supported by plastid-generated geranyl diphosphate substrate in transgenic tomato fruits. Plant J. 2013, 75 (3), 351−363. C
DOI: 10.1021/acs.jafc.8b00522 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
