Elucidation of ( Z)-3-Hexenyl-β-glucopyranoside Enhancement Mechanism under Stresses from the Oolong Tea Manufacturing Process

Transcriptomic Insights into the Enhanced Aroma of Guangdong Oolong Dry Tea (Camellia sinensis cv. Yashixiang Dancong) in Winter

Winter dry tea (WDT) exhibits a more intense and lasting aroma compared to dry tea from other seasons; however, this conclusion is solely based on sensory outcomes and lacks corroborative theoretical evidence. Our study aimed to analyze the aroma compounds in WDT and investigate the causes behind the formation of WDT's aroma by analyzing the volatile organic compounds (VOCs) in WDT, spring dry tea (SDT), winter fresh leaves (WFLs) and spring fresh leaves (SFLs) by gas chromatography-mass spectrometry (GC-MS), complemented by an analysis of gene expression pertinent to WFLs and SFLs by using transcriptomic analysis. The results revealed a significant increase in total VOCs in WDT compared to SDT, with WDT exhibiting distinct woody aromas as indicated by a higher α-muurolene content. In WFL, the contents of aldehydes and ketones were richer than those in SFL. Notably, the study found that UDP-glycosyltransferase genes in WFLs were significantly up-regulated, potentially promoting the synthesis of terpene glycosides. These terpene glycosides can release terpene aroma compounds during processing, contributing significantly to the intense and lasting aroma of WDT. Overall, this research provides valuable insights into the mechanism behind aroma formation in Guangdong oolong tea harvested during winter.

Determination of Tea Aroma Precursor Glycosides: An Efficient Approach via Liquid Chromatography-Tandem Mass Spectrometry

Tea aroma components are often stored as glycosidically bound forms in the tea plant (Camellia sinensis). However, the determination of these glycosides in tea samples is far from optimal. In the present study, we developed a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method for simultaneous quantification of eight primary aroma glycosides within 10 min. After systematic optimization of multiple reaction monitoring (MRM) parameters, the proposed method was highly sensitive and accurate. Optimization of the method permitted the efficient extraction of aroma glycosides. The developed method was applied to analyze the contents of aroma glycosides in different organs of tea plants, including the bud, leaves, and stem. Contents of aroma glycosides in the harvested 'Shaancha 1' ranged from 36.1 to 40454.4 μg kg^-1. Geranyl glucoside and primeveroside mainly accumulated in young leaves, while other glycosides mainly accumulated in mature leaves. The findings document a rapid, reliable, and efficient analysis method. This method will be helpful in elucidating the biosynthesis and biotransformation mechanism of tea aroma glycosides and in promoting the development of the tea industry using advanced technological control approaches during the cultivation of tea plants and tea manufacture.

Aroma characteristics of Wuyi rock tea prepared from 16 different tea plant varieties

Wuyi rock tea (WRT) is famous for its long history and unique characteristic of floral, fruity and nutty flavors. This study investigated the aroma characteristics of WRTs prepared from 16 different oolong tea plant varieties. The sensory evaluation results showed that all WRTs had an 'Yan flavor' taste, and the odor was strong and lasting. Roasted, floral and fruity odors were the prime aroma profiles for WRTs. Furthermore, a total of 368 volatile compounds were detected using HS-SPME-GC-MS and analyzed with OPLS-DA and HCA methods. The volatile compounds heterocyclic compounds, esters, hydrocarbons, terpenoids and ketones were the major aromatic components of the WRTs. Specifically, the volatile profiles among newly selected cultivars were comparatively analyzed, and 205 differential volatile compounds were found with variable importance in the projection (VIP) values above 1.0. These results indicated that the aroma profiles of WRTs were mainly dependent on the cultivar specificities of volatile compounds.

Dynamic Changes in Volatile Compounds of Shaken Black Tea during Its Manufacture by GC × GC-TOFMS and Multivariate Data Analysis

Changes in key odorants of shaken black tea (SBT) during its manufacture were determined using headspace solid-phase microextraction (HS-SPME) combined with comprehensive two-dimensional gas chromatography−time-of-flight mass spectrometry (GC × GC−TOFMS) and multivariate data analysis. A total of 241 volatiles was identified, comprising 49 aldehydes, 40 esters, 29 alcohols, 34 ketones, 30 aromatics, 24 alkenes, 17 alkanes, 13 furans, and 5 other compounds. A total of 27 volatiles had average relative odor activity values (rOAVs) greater than 1, among which (E)-β-ionone, (E,Z)-2,6-nonadienal, and 1-octen-3-one exhibited the highest values. According to the criteria of variable importance in projection (VIP) > 1, p < 0.05, and |log2FC| > 1, 61 discriminatory volatile compounds were screened out, of which 26 substances were shared in the shaking stage (FL vs. S1, S1 vs. S2, S2 vs. S3). The results of the orthogonal partial least squares discriminate analysis (OPLS-DA) differentiated the influence of shaking, fermentation, and drying processes on the formation of volatile compounds in SBT. In particular, (Z)-3-hexenol, (Z)-hexanoic acid, 3-hexenyl ester, (E)-β-farnesene, and indole mainly formed in the shaking stage, which promoted the formation of the floral and fruity flavor of black tea. This study enriches the basic theory of black tea flavor quality and provide the theoretical basis for the further development of aroma quality control.

Herbivore-Induced (Z)-3-Hexen-1-ol is an Airborne Signal That Promotes Direct and Indirect Defenses in Tea (Camellia sinensis) under Light

Tea (Camellia sinensis) is the most popular nonalcoholic beverage worldwide. During cultivation, tea plants are susceptible to herbivores and pathogens, which can seriously affect tea yield and quality. A previous report showed that (Z)-3-hexenol is a potentially efficient defensive substance. However, the molecular mechanism mediating (Z)-3-hexenol signaling in tea plants and the resulting effects on plant defenses remain uncharacterized. To clarify the signaling mechanisms in which (Z)-3-hexenol and light are involved, the gene transcription and metabolite levels were assessed, respectively. This study demonstrated that tea plants rapidly and continuously release (Z)-3-hexen-1-ol in response to an insect infestation. (Z)-3-Hexen-1-ol absorbed by adjacent healthy plants would be converted into three insect defensive compounds: (Z)-3-hexenyl-glucoside, (Z)-3-hexenyl-primeveroside, and (Z)-3-hexenyl-vicianoside identified with laboratory-synthesized standards. Moreover, (Z)-3-hexen-1-ol also activates the synthesis of jasmonic acid to enhance the insect resistance of tea plants. Additionally, a continuous light treatment induces the accumulation of (Z)-3-hexenyl-glycosides. Hence, (Z)-3-hexenol serves as a light-regulated signaling molecule that activates the systemic defenses of adjacent plants. Our study reveals the molecular mechanisms by which biotic and abiotic factors synergistically regulate the signaling functions of herbivore-induced plant volatiles in plants, providing valuable information for future comprehensive analyses of the systemic defense mechanisms in plants.

The appearance of volatile aromas in Tieguanyin tea with different elevations

Chinese people have consistentlypreferred high mountain tea because specific flavors are memorable for them, and also, people have traditionally considered this type of tea to be good for health. Tieguanyin is one of the famous traditional Chinese tea that has ever-changing aromas. To illustrate the various characteristics in volatile fragment compounds from Tieguanyin tea, fresh tea leaves collected from different elevations (450, 650, and 900 m) were detected using GC-MS by solid-liquid extraction. The results showed that volatile aromatic compounds, such as benzyl alcohol, phenyl ethanol, and acetophenone, were the most abundant in tea leaves located with high elevation. Meanwhile, 1-hexanol, 1-nananol, and nanoic acid, as a type of aliphatic aroma, were more prevalent in low-elevation tea orchards. Catechols and alkaloids are largely cumulated in low- and high-elevation tea leaves, respectively. Our findings also showed that elemene was widely consisted of high-elevation tea metabolites. It provided practicality for the preparation of tea manufacturing in major Tieguanyin tea-producing regions.

Integrated proteomic and metabolomic analyses reveal the importance of aroma precursor accumulation and storage in methyl jasmonate-primed tea leaves

In response to preharvest priming with exogenous methyl jasmonate (MeJA), tea plants adjust their physiological behavior at the molecular level. The whole-organism reconfiguration of aroma formation from the precursor to storage is poorly understood. In this study, we performed iTRAQ proteomic analysis and identified 337, 246, and 413 differentially expressed proteins in tea leaves primed with MeJA for 12 h, 24 h, and 48 h, respectively. Furthermore, a total of 266 nonvolatile and 100 volatile differential metabolites were identified by utilizing MS-based metabolomics. A novel approach that incorporated the integration of extended self-organizing map-based dimensionality was applied. The vivid time-scale changes tracing physiological responses in MeJA-primed tea leaves are marked in these maps. Jasmonates responded quickly to the activation of the jasmonic acid pathway in tea leaves, while hydroxyl and glycosyl jasmonates were biosynthesized simultaneously on a massive scale to compensate for the exhausted defense. The levels of α-linolenic acid, geranyl diphosphate, farnesyl diphosphate, geranylgeranyl diphosphate, and phenylalanine, which are crucial aroma precursors, were found to be significantly changed in MeJA-primed tea leaves. Green leaf volatiles, volatile terpenoids, and volatile phenylpropanoids/benzenoids were spontaneously biosynthesized from responding precursors and subsequently converted to their corresponding glycosidic forms, which can be stably stored in tea leaves. This study elucidated the physiological response of tea leaves primed with exogenous methyl jasmonate and revealed the molecular basis of source and sink changes on tea aroma biosynthesis and catabolism in response to exogenous stimuli. The results significantly enhance our comprehensive understanding of tea plant responses to exogenous treatment and will lead to the development of promising biotechnologies to improve fresh tea leaf quality.

Analysis of aroma-active volatiles in an SDE extract of white tea

White tea is a famous Chinese tea that is cooked at boiling point before drinking. The simultaneous distillation-extraction (SDE) was used to collect volatile compounds during tea cooking. The SDE extract was dominated with green, floral, roasted and woody notes, and weak sweet note. There were 32 volatile compounds identified via gas chromatography-mass spectrometry analysis, and 19 of them had strong fragrance based on the gas chromatography-olfactometry analyzed results. Hexanal, 2-hexenal, cis-3-hexen-1-ol, and camphene were the main contributors to the green note. The floral note was mainly contributed by 2-hexanone, benzeneacetaldehyde, trans-linalool oxide, and linalool, and the sweet note was induced by trans-β-damascenone. The roasted note was mainly contributed by 2-pentyl-furan. The woody note was mainly contributed by trans-α-ionone and trans-β-ionone. Four putative reaction pathways, including amino acid degradation, carotene degradation, Maillard reaction, and glycosides hydrolysis, were figured out to explain the generation of aromatic-active volatiles at high temperatures. This study added our knowledge on tea aroma under cooking as well as other thermal treatments.

How does tea (Camellia sinensis) produce specialized metabolites which determine its unique quality and function: a review

Tea (Camellia sinensis) is both a plant and a foodstuff. Many bioactive compounds, which are present in the final tea product and related to its quality or functional properties, are produced during the tea manufacturing process. However, the characteristic secondary metabolites, which give tea its unique qualities and are beneficial to human health, are produced mainly in the leaves during the process of plant growth. Therefore, it is important to understand how tea leaves produce these specialized metabolites. In this review, we first compare the common metabolites and specialized metabolites in tea, coffee, cocoa, and grape and discuss the occurrence of characteristic secondary metabolites in tea. Progress in research into the formation of these characteristic secondary metabolites in tea is summarized, including establishing a biological database and genetic transformation system, and the biosynthesis of characteristic secondary metabolites. Finally, speculation on future research into the characteristic secondary metabolites of tea is provided from the viewpoints of the origin, resources, cultivation, and processing of tea. This review provides an important reference for future research on the specialized metabolites of tea in terms of its characteristics.

Architecture and Dynamics of the Wounding-Induced Gene Regulatory Network During the Oolong Tea Manufacturing Process (Camellia sinensis)

Understanding extensive transcriptional reprogramming events mediated by wounding during the oolong tea manufacturing process is essential for improving oolong tea quality. To improve our comprehension of the architecture of the wounding-induced gene regulatory network, we systematically analyzed the high-resolution transcriptomic and metabolomic data from wounding-treated (after turnover stage) tea leaves at 11 time points over a 220-min period. The results indicated that wounding activates a burst of transcriptional activity within 10 min and that the temporal expression patterns over time could be partitioned into 18 specific clusters with distinct biological processes. The transcription factor (TF) activity linked to the TF binding motif participated in specific biological processes within different clusters. A chronological model of the wounding-induced gene regulatory network provides insight into the dynamic transcriptional regulation event after wounding treatment (the turnover stage). Time series data of wounding-induced volatiles reveal the scientific significance of resting for a while after wounding treatment during the actual manufacturing process of oolong tea. Integrating information-rich expression data with information on volatiles allowed us to identify many high-confidence TFs participating in aroma formation regulation after wounding treatment by using weighted gene co-expression network analysis (WGCNA). Collectively, our research revealed the complexity of the wounding-induced gene regulatory network and described wounding-mediated dynamic transcriptional reprogramming events, serving as a valuable theoretical basis for the quality formation of oolong tea during the post-harvest manufacturing process.

Uncovering reasons for differential accumulation of linalool in tea cultivars with different leaf area

It is generally proposed that tea cultivars with larger leaves contain more linalool, an important tea aroma contributor, than ones with smaller leaves. The objective of this study was to confirm the trait and explore the involved reason. Investigation on ten tea cultivars with different leaf areas demonstrated a significant positive correlation between linalool content and leaf area (R² = 0.739, p = 0.010). Analysis of metabolite and gene expression level showed that the transform ability of linalool into linalool oxides was the key factor. Feeding experiments that supplied tea leaves of different leaf areas with [²H₃]linalool under different light conditions revealed that the larger tea leaves receive more light and are less capable of transformation of linalool to linalool oxides, thus leading to linalool accumulation. This information will advance understanding of the variation of linalool content in tea varieties and will provide assistance in breeding and screening of high-linalool tea cultivars.

Hop (Humulus lupulus L.) terroir has large effect on a glycosylated green leaf volatile but not on other aroma glycosides

Genetics and environment both influence the content of hop (Humulus lupulus L.) aroma compounds. The effects of these two factors on aroma glycosides, which can change the aroma profile of beer over time, were examined in a preliminary study. Twenty-three hop cultivars were grown in the northwestern United States in two locations with distinct terroirs. UPLC-MS/MS analysis of hop cone extracts revealed that growing location had a large effect on hexyl glucoside levels but only a negligible effect on levels of linalyl, raspberry ketone, and 2-phenylethyl glucoside, which were mostly affected by genetic differences. The large terroir effect on hexyl glucoside, which releases a green leaf volatile with a grassy aroma when hydrolyzed, but not on the other aroma glucosides, which have more desirable aromas when hydrolyzed, could have an impact on beer aroma profiles.

Integrative Transcriptomic and Metabolic Analyses Provide Insights into the Role of Trichomes in Tea Plant (Camellia Sinensis)

Trichomes, which develop from epidermal cells, are regarded as one of the key features that are involved in the evaluation of tea quality and tea germplasm resources. The metabolites from trichomes have been well characterized in tea products. However, little is known regarding the metabolites in fresh tea trichomes and the molecular differences in trichomes and tea leaves per se. In this study, we developed a method to collect trichomes from tea plant tender shoots, and their main secondary metabolites, including catechins, caffeine, amino acids, and aroma compounds, were determined. We found that the majority of these compounds were significantly less abundant in trichomes than in tea leaves. RNA-Seq was used to investigate the differences in the molecular regulatory mechanism between trichomes and leaves to gain further insight into the differences in trichomes and tea leaves. In total, 52.96 Gb of clean data were generated, and 6560 differentially expressed genes (DEGs), including 4471 upregulated and 2089 downregulated genes, were identified in the trichomes vs. leaves comparison. Notably, the structural genes of the major metabolite biosynthesis pathways, transcription factors, and other key DEGs were identified and comparatively analyzed between trichomes and leaves, while trichome-specific genes were also identified. Our results provide new insights into the differences between tea trichomes and leaves at the metabolic and transcriptomic levels, and open up new doors to further recognize and re-evaluate the role of trichomes in tea quality formation and tea plant growth and development.

Alternative Splicing of Key Genes in LOX Pathway Involves Biosynthesis of Volatile Fatty Acid Derivatives in Tea Plant (Camellia sinensis)

Volatile fatty acid derivatives (VFADs) produced in tea plants (Camellia sinensis) not only have been shown to function as defense compounds but also impart a "fresh green" odor to green tea products; however, little is known about alternative splicing (AS) of genes in regulating the production of VFADs in plants. In this study, the contents of VFADs and corresponding transcriptome profiles were obtained in five different months (April, June, August, September, and October). Correlation analysis identified seven unique transcripts of enzyme-coding genes (CsLOX2, CsLOX4, CsADH4, CsADH8, and CsADH10), which are responsible for regulating VFAD biosynthesis; four AS transcripts of these genes (CsLOX2, CsLOX4, CsADH4, and CsADH8) were validated by RT-PCR. By employing the gene-specific antisense oligodeoxynucleotide-mediated reduction method, we found the expression levels of alternatively spliced transcripts of CsLOX4-iso1, CsLOX4-iso2, and CsADH4-iso3 were lower, and the contents of cis-3-hexenol were correspondingly reduced in the leaves of tea plant; this result suggested that the AS play important roles in regulating biosynthesis of VFADs in C. sinensis. Our results provide new insights into the important contribution of AS events in regulating the VFAD biosynthesis in tea plant.

Increasing Temperature Changes Flux into Multiple Biosynthetic Pathways for 2-Phenylethanol in Model Systems of Tea (Camellia sinensis) and Other Plants

2-Phenylethanol (2PE) is a representative aromatic aroma compound in tea (Camellia sinensis) leaves. However, its formation in tea remains unexplored. In our study, feeding experiments of [²H₈]L-phenylalanine (Phe), [²H₅]phenylpyruvic acid (PPA), or (E/Z)-phenylacetaldoxime (PAOx) showed that three biosynthesis pathways for 2PE derived from L-Phe occurred in tea leaves, namely, pathway I (via phenylacetaldehyde (PAld)), pathway II (via PPA and PAld), and pathway III (via (E/Z)-PAOx and PAld). Furthermore, increasing temperature resulted in increased flux into the pathway for 2PE from L-Phe via PPA and PAld. In addition, tomato fruits and petunia flowers also contained the 2PE biosynthetic pathway from L-Phe via PPA and PAld and increasing temperatures led to increased flux into this pathway, suggesting that such a phenomenon might be common among most plants containing 2PE. This represents a characteristic example of changes in flux into the biosynthesis pathways of volatile compounds in plants in response to stresses.