Genome-Wide Investigation of Oxidosqualene Cyclase Genes Deciphers the Genetic Basis of Triterpene Biosynthesis in Tea Plants

Triterpenoids from Camellia species comprise a diverse class of bioactive compounds with great therapeutic potential. However, triterpene biosynthesis in tea plants (Camellia sinensis) remains elusive. Here, we identified eight putative 2,3-oxidosqualene cyclase (OSC) genes (CsOSC1-8) from the tea genome and characterized the functions of five through heterologous expression in yeast and tobacco and transient overexpression in tea plants. CsOSC1 was found to be a β-amyrin synthase, whereas CsOSC4, 5, and 6 exhibited multifunctional α-amyrin synthase activity. Molecular docking and site-directed mutagenesis showed that the CsOSC6M259T/W260L double mutant yielded >40% lupeol, while the CsOSC1 W259L single mutant alone was sufficient for lupeol production. The V732F mutation in CsOSC5 altered product formation from friedelin to taraxasterol and ψ-taraxasterol. The L254 M mutation in the cycloartenol synthase CsOSC8 enhanced the catalytic activity. Our findings shed light on the molecular basis governing triterpene diversity in tea plants and offer potential avenues for OSC engineering.

γ-Glutamyl-transpeptidase CsGGT2 functions as light-activated theanine hydrolase in tea plant (Camellia sinensis L.)

Theanine is an important secondary metabolite endowing tea with umami taste and health effects. It is essential to explore the metabolic pathway and regulatory mechanism of theanine to improve tea quality. Here, we demonstrated that the expression patterns of CsGGT2 (γ-glutamyl-transpeptidase), participated in theanine synthesis in vitro in our previous research, are significantly different in the aboveground and underground tissues of tea plants and regulated by light. Light up-regulated the expression of CsHY5, directly binding to the promoter of CsGGT2 and acting as an activator of CsGGT2, with a negative correlation with theanine accumulation. The enzyme activity assays and transient expression in Nicotiana benthamiana showed that CsGGT2, acting as bifunctional protein, synthesize and degrade theanine in vitro and in planta. The results of enzyme kinetics, Surface plasmon resonance (SPR) assays and targeted gene-silencing assays showed that CsGGT2 had a higher substrate affinity of theanine than that of ethylamine, and performed a higher theanine degradation catalytic efficiency. Therefore, light mediates the degradation of theanine in different tissues by regulating the expression of the theanine hydrolase CsGGT2 in tea plants, and these results provide new insights into the degradation of theanine mediated by light in tea plants.

Camellia sinensis small GTPase gene (CsRAC1) involves in response to salt stress, drought stress and ABA signaling pathway

RAC/ROP gene (RACs) is a plant-specific small GTPases. RACs play an irreplaceable role in the tissue dynamics of cytoskeleton, vesicle transport and hormone signal transmission in plants. In the present study, a novel gene from RACs family, CsRAC1, was identified from tea [Camellia sinensis (L.) O. Kuntze]. CsRAC1 contained a 591-bp open reading frame and encoded a putative protein of 197 amino acids. Subcellular localization analysis in leaves of transgenic tobacco and root tips of Arabidopsis thaliana showed that CsRAC1 targeted the nucleus and cell membrane. The expression of CsRAC1 induced by abiotic stresses such as cold, heat, drought, salt and abscisic acid has also been verified by RT-qPCR. Further verification of biological function of CsRAC1 showed that overexpression of CsRAC1 increased the sensitivity of A. thaliana to salt stress, improved the tolerance of mature A. thaliana to drought stress, and enhanced the inhibition of ABA on seed germination of A. thaliana. In addition, the antioxidant system regulated by CsRAC1 mainly worked in mature A. thaliana. The results indicate that CsRAC1 is involved in the response of C. sinensis to salt, drought stress and ABA signaling pathway.

A novel adenylate isopentenyltransferase 5 regulates shoot branching via the ATTTA motif in Camellia sinensis

BACKGROUND

Shoot branching is one of the important agronomic traits affecting yields and quality of tea plant (Camellia sinensis). Cytokinins (CTKs) play critical roles in regulating shoot branching. However, whether and how differently alternative splicing (AS) variant of CTKs-related genes can influence shoot branching of tea plant is still not fully elucidated.

RESULTS

In this study, five AS variants of CTK biosynthetic gene adenylate isopentenyltransferase (CsA-IPT5) with different 3' untranslated region (3' UTR) and 5' UTR from tea plant were cloned and investigated for their regulatory effects. Transient expression assays showed that there were significant negative correlations between CsA-IPT5 protein expression, mRNA expression of CsA-IPT5 AS variants and the number of ATTTA motifs, respectively. Shoot branching processes induced by exogenous 6-BA or pruning were studied, where CsA-IPT5 was demonstrated to regulate protein synthesis of CsA-IPT5, as well as the biosynthesis of trans-zeatin (tZ)- and isopentenyladenine (iP)-CTKs, through transcriptionally changing ratios of its five AS variants in these processes. Furthermore, the 3' UTR AS variant 2 (3AS2) might act as the predominant AS transcript.

CONCLUSIONS

Together, our results indicate that 3AS2 of the CsA-IPT5 gene is potential in regulating shoot branching of tea plant and provides a gene resource for improving the plant-type of woody plants.

Genome-wide identification and expression analysis of SABATH methyltransferases in tea plant (Camellia sinensis): insights into their roles in plant defense responses

SABATH methyltransferases convent plant small-molecule metabolites into volatile methyl esters, which play important roles in many biological processes and defense reactions in plants. In this study, a total of 32 SABATH genes were identified in the Camellia sinensis var. sinensis (CSS) genome, which were renamed CsSABATH1 to CsSABATH32. Genome location annotation suggested that tandem duplication was responsible for the expansion of SABATH genes in tea plant. Multiple sequence alignment and phylogenetic analysis showed that the CsSABATHs could be classified into three groups (I, II and III), which were also supported by gene structures and conserved motifs analysis. Group II contained only two CsSABATH proteins, which were closely related to PtIAMT, AtIAMT and OsIAMT. The group III SABATH genes of tea plant exhibited expansion on the CSS genome compared with Camellia sinensis var. assamica (CSA) genome. Based on RNA-seq data, the CsSABATHs exhibited tissue-specific expression patterns, and the members with high expression in buds and young leaves were also obviously upregulated after MeJA treatment. The expression of many transcription factors was significantly correlated with that of different members of the CsSABATH gene family, suggesting a potential regulatory relationship between them. Quantitative real-time PCR (qPCR) expression analysis showed that CsSABATHs could respond to exogenous JA, SA and MeSA treatments in tea plants. RNA-seq data analysis and qPCR validation suggested that CsSABATH8, 11, 16, 25, 29 and 32 might play a special role in plant defense against insect herbivory. These results provide references for evolutionary studies of the plant SABATH family and the exploration of the potential roles of CsSABATHs in tea plant defense responses.

Identification and characterization of two sesquiterpene synthase genes involved in volatile-mediated defense in tea plant (Camellia sinensis)

Terpenes and their derivatives are vital components of tea aroma. Their constitution and quantity are highly important criteria for the sensory evaluation of teas. Biologically, terpenes are involved in chemical resistance of tea plant against biotic and/or abiotic stresses. The goal of this study is to identify volatile terpenes of tea plants implicated in defense against herbivores and to identify terpene synthase (TPS) genes for their biosynthesis. Upon herbivory by tea geometrid (Ectropis obliqua Prout), tea plants were found to emit two sesquiterpenes, (E, E)-α-farnesene and (E)-nerolidol, which were undetectable in intact tea plants. The induced emission of (E, E)-α-farnesene and (E)-nerolidol suggests that they function in either direct or indirect defense of tea plants against the tea geometrid. Candidate TPS genes were identified from the transcriptomes of tea plants infested by tea geometrids. Two dedicated sesquiterpene synthases, CsAFR and CsNES2, were identified. CsAFR belongs to the TPS-b clade and can catalyze the formation of (E, E)-α-farnesene from (E, E)-FPP. CsNES2 belongs to the TPS-g clade and can synthesize (E)-nerolidol using (E, E)-FPP. The two genes were also both dramatically upregulated by herbivore damage. In summary, we showed that two novel sesquiterpene synthase genes CsAFR and CsNSE2 are inducible by herbivory and responsible for the elevated emission of herbivore-induced (E, E)-α-farnesene and (E)-nerolidol, which are implicated in tea plant defense against herbivores.

Genome-wide identification of the HDAC family proteins and functional characterization of CsHD2C, a HD2-type histone deacetylase gene in tea plant (Camellia sinensis L. O. Kuntze)

The histone deacetylases (HDACs) are involved in growth, development and stress responses in many plants. However, the functions of HDACs in tea plant (Camellia sinensis L. O. Kuntze) and other woody plants remain unclear. Here, 18 CsHDAC genes were identified by genome-wide analysis in tea plant. The phylogenetic analysis demonstrated that the CsHDAC proteins were divided into three subfamilies, namely, the RPD3/HDA1 subfamily (8 members), the SIR2 subfamily (4 members) and the plant specific HD2 subfamily (6 members). The expression patterns showed that most members of CsHDACs family were regulated by different abiotic stress. High correlation was found between the expression of the CsHDACs and the accumulation of theanine, catechin, EGCG and other metabolites in tea plant. Most of the CsHDAC proteins were negative regulators. We further studied a specific gene CsHD2C (NCBI-ID: KY364373) in tea plant, which is the homolog of AtHD2C, encoded a protein of 306 aa. CsHD2C was highly expressed in leaves, young buds and stems. The transcription of CsHD2C was inhibited by ABA, NaCl and low temperature. It was found localized in the nucleus when fused with a YFP reporter gene. Overexpression of CsHD2C can rescue the phenotype related to different abiotic stresses in the mutant of AtHD2C in Arabidopsis. The stress-responsive genes RD29A, RD29B, ABI1 and ABI2 were also investigated to understand the regulating role of CsHD2C under abiotic stresses. We also found that CsHD2C could renew the change of acetylation level for histone H4 and the RNAP-II occupancy accumulation in the promoter of abiotic stress responses gene in the hd2c Arabidopsis mutant. Together, our results suggested that CsHD2C may act as a positive regulator in abiotic stress responses in tea plant.

Isolation and characterization of chloroplastic glutamine synthetase gene (CsGS2) in tea plant Camellia sinensis

Tea plant (Camellia sinensis) is an ammonium preferring plant species. However, little is known about the mechanism underlying this preference. Herein, a chloroplastic glutamine synthetase gene (CsGS2), which is vital for nitrogen assimilation in mesophyll tissue, was isolated from tea cultivar C. sinensis cv. 'Longjing43'. The full length cDNA of CsGS2 was 1622 bp, having a 1299 bp open reading frame encoding a 432-amino acid protein. Homology search and sequence analysis demonstrated that CsGS2 protein carried the basic characteristics of a canonical GS2 domain and shared high identity with GS2s from other plant species. Subcellular localization and immunolocalization of CsGS2 revealed that it is localized in chloroplast. qRT-PCR and Western blot analyses showed that CsGS2 was expressed in a leaf-specific pattern, such that both CsGS2 and its protein were most abundant in mature leaves. Temporal expression patterns of CsGS2 showed minor differences in response to ammonium and nitrate nutrition. The transcript level of CsGS2 was significantly induced in mature leaves during the development of new shoots, whereas darkness inhibited this induction significantly. These results suggested that CsGS2 does not play a role in the differential utilization mechanisms of differing nitrogen forms in tea, and imply a light dependent transcription regulation in mature leaves during the development of new shoots.

Dehydration-Induced Carotenoid Cleavage Dioxygenase 1 Reveals a Novel Route for β-Ionone Formation during Tea (Camellia sinensis) Withering

β-Ionone is a carotenoid-derived flavor and fragrance compound with a complex fruity and woody scent, known for its violet aroma. Due to the low odor threshold, β-ionone dramatically affects the aroma and quality of tea. Previous studies have shown that β-ionone increases during tea withering; however, its formation and regulation during the withering process are far from being understood. As dehydration is the most important stress during the withering of the tea leaves, we isolated a dehydration-induced gene belonging to the subfamily of carotenoid cleavage dioxygenases called carotenoid cleavage dioxygenase 1a (CsCCD1a) from Camellia sinensis and expressed it in Escherichia coli. The recombinant protein could generate volatile β-ionone and pseudoionone from carotenoids. CsCCD1a was induced by dehydration stress, and its expression was related to the β-ionone accumulation during tea withering. Overall, this study elucidated that CsCCD1a catalyzes the formation of β-ionone in C. sinensis and enhanced the understanding of the β-ionone formation under multiple stresses during the processing of tea.

Characterization of l-Theanine Hydrolase in Vitro and Subcellular Distribution of Its Specific Product Ethylamine in Tea (Camellia sinensis)

l-Theanine has a significant role in the taste of tea (Camellia sinensis) infusions. Our previous research indicated that the lower l-theanine metabolism in ethylamine and l-glutamate is a key factor that explains the higher content of l-theanine in albino tea with yellow or white leaves, compared with that of normal tea with green leaves. However, the specific genes encoding l-theanine hydrolase in tea remains unknown. In this study, CsPDX2.1 was cloned together with the homologous Arabidopsis PDX2 gene and the recombinant protein was shown to catalyze l-theanine hydrolysis into ethylamine and l-glutamate in vitro. There were higher CsPDX2.1 transcript levels in leaf tissue and lower transcripts in the types of albino (yellow leaf) teas compared with green controls. The subcellular location of ethylamine in tea leaves was shown to be in the mitochondria and peroxisome using a nonaqueous fractionation method. This study identified the l-theanine hydrolase gene and subcellular distribution of ethylamine in tea leaves, which improves our understanding of the l-theanine metabolism and the mechanism of differential accumulation of l-theanine among tea varieties.

Enzyme Catalytic Efficiencies and Relative Gene Expression Levels of (R)-Linalool Synthase and (S)-Linalool Synthase Determine the Proportion of Linalool Enantiomers in Camellia sinensis var. sinensis

Linalool is abundant in tea leaves and contributes greatly to tea aroma. The two isomers of linalool, (R)-linalool and (S)-linalool, exist in tea leaves. Our study found that (R)-linalool was the minor isomer in nine of Camellia sinensis var. sinensis cultivars. The (R)-linalool synthase of tea plant CsRLIS was identified subsequently. It is a chloroplast-located protein and specifically catalyzes the formation of (R)-linalool in vitro and in vivo. CsRLIS was observed to be a stress-responsive gene and caused the accumulation of internal (R)-linalool during oolong tea manufacture, mechanical wounding, and insect attack. Further study demonstrated that the catalytic efficiency of CsRLIS was much lower than that of (S)-linalool synthase CsSLIS, which might explain the lower (R)-linalool proportion in C. sinensis var. sinensis cultivars. The relative expression levels of CsRLIS and CsSLIS may also affect the (R)-linalool proportions among C. sinensis var. sinensis cultivars. This information will help us understand differential distributions of chiral aroma compounds in tea.

Divergent Evolution of the Diterpene Biosynthesis Pathway in Tea Plants (Camellia sinensis) Caused by Single Amino Acid Variation of ent-Kaurene Synthase

Most plant terpenoids are classified as secondary metabolites. A small portion of them are products of primary metabolism biosynthesized by relatively conserved pathways. Gibberellins (GAs), which are essential for plant growth and development, are diterpenoid phytohormones. (E,E,E)-Geranylgeranyl diphosphate (GGPP) is the precursor for both GAs and other diterpenoids of secondary metabolism. ent-Kaurene biosynthesis from GGPP is a key step of GA formation, which is catalyzed by two sequential and dedicated diterpene synthases (diTPSs): ent-copalyl diphosphate synthase (CPS) and ent-kaurene synthase (KS) of the terpene synthase gene family. Sharing a common evolutionary origin, CPS and KS belong to different TPS subfamilies. Tea plant (Camellia sinensis), the subject of this study, is a leaf-based economic crop. Budbreak mainly manipulated by GAs is a primary factor for targeted tea breeding. The key genes for gibberellin biosynthesis are known; however, they have not yet been characterized in tea plants. Here, we identified and functionally characterized three diterpene biosynthesis-related genes, including one CPS and two highly similar KSs in tea plants. These genes were initially identified through transcriptome sequencing. The functional characterization determined by enzymatic activity assay indicated that CsCPS could catalyze GGPP to form ent-copalyl diphosphate (ent-CPP), which was further used as the substrate by CsKS1 to produce ent-kaurene or by CsKS2 to produce 16α-hydroxy-ent-kaurane with ent-kaurene as a minor product, respectively. We demonstrated that the divergent evolution of diterpene biosynthesis in tea plants resulted from gene duplication of KSs, followed by functional divergence caused by single amino acid variation. This study would provide an insight into the diterpenoid metabolism and GA biosynthesis in tea plants to further understand leaf bud development or insect resistance and to provide a genetic basis for tea plant breeding.

The R2R3-MYB transcription factor CsMYB73 negatively regulates l-Theanine biosynthesis in tea plants (Camellia sinensis L.)

l-Theanine, a non-proteinaceous amino acid abundantly present in tea (Camellia sinensis), contributes to the umami flavor of tea and has beneficial effects on human health. While key l-theanine biosynthetic genes have been well documented, their transcriptional regulation remains poorly understood. In this study, we determined the l-theanine contents in tea leaves of two cultivars at three developmental stages and investigated the expression patterns of the l-theanine biosynthetic genes CsGS1 and CsGS2. Additionally, we identified an R2R3-MYB transcription factor, CsMYB73, belonging to subgroup 22 of the R2R3-MYB family. CsMYB73 expression negatively correlated with l-theanine accumulation during leaf maturation. We found that CsMYB73, as a nuclear protein, binds to the promoter regions of CsGS1 and CsGS2 via MYB recognition sequences and represses the transcription of CsGS1 and CsGS2 in tobacco leaves. Collectively, our results demonstrate that CsMYB73 is a transcriptional repressor involved in l-theanine biosynthesis in tea plants. Our findings might contribute to future tea plant breeding strategies.

The ascorbate peroxidase 1 regulates ascorbic acid metabolism in fresh-cut leaves of tea plant during postharvest storage under light/dark conditions

Postharvest storage conditions affect the ascorbic acid (AsA) levels in fresh-cut leaves of horticultural crops. However, the detailed mechanism of AsA metabolism in the fresh-cut leaves of tea plant (Camellia sinensis) during postharvest storage under light/dark conditions remains unclear. To investigate the AsA mechanism, we treated fresh-cut tea leaves with light/dark during postharvest storage. An ascorbate peroxidase 1 (CsAPX1) protein involved in AsA metabolism was identified by iTRAQ analysis. Gene expression profile of CsAPX1 encoding ascorbate peroxidase (APX) was regulated by light/dark conditions. AsA accumulation and APX activity were suppressed by light/dark conditions. SDS-PAGE analysis showed that the molecular mass of recombinant CsAPX1 protein was about 34.45 kDa. Subcellular localization indicated that CsAPX1 protein was a cytosol ascorbate peroxidase. Overexpression CsAPX1 in Arabidopsis indicated that the decrease of AsA content and APX activity in transgenic lines were less significant than that of WT during postharvest storage under light/dark conditions. These data suggested that CsAPX1 involved in regulating AsA metabolism through effecting on the changes of AsA accumulation and APX activity in fresh-cut tea leaves during postharvest storage under light/dark conditions.

Sesquiterpene glucosylation mediated by glucosyltransferase UGT91Q2 is involved in the modulation of cold stress tolerance in tea plants

Plants produce and emit terpenes, including sesquiterpenes, during growth and development, which serve different functions in plants. The sesquiterpene nerolidol has health-promoting properties and adds a floral scent to plants. However, the glycosylation mechanism of nerolidol and its biological roles in plants remained unknown. Sesquiterpene UDP-glucosyltransferases were selected by using metabolites-genes correlation analysis, and its roles in response to cold stress were studied. We discovered the first plant UGT (UGT91Q2) in tea plant, whose expression is strongly induced by cold stress and which specifically catalyzes the glucosylation of nerolidol. The accumulation of nerolidol glucoside was consistent with the expression level of UGT91Q2 in response to cold stress, as well as in different tea cultivars. The reactive oxygen species (ROS) scavenging capacity of nerolidol glucoside was significantly higher than that of free nerolidol. Down-regulation of UGT91Q2 resulted in reduced accumulation of nerolidol glucoside, ROS scavenging capacity and tea plant cold tolerance. Tea plants absorbed airborne nerolidol and converted it to its glucoside, subsequently enhancing tea plant cold stress tolerance. Nerolidol plays a role in response to cold stress as well as in triggering plant-plant communication in response to cold stress. Our findings reveal previously unidentified roles of volatiles in response to abiotic stress in plants.

Carotenoid Cleavage Dioxygenase 4 Catalyzes the Formation of Carotenoid-Derived Volatile β-Ionone during Tea (Camellia sinensis) Withering

The carotenoid-derived volatile β-ionone plays an important role in the formation of green and black tea flavors due to its low odor threshold, but its formation and the gene(s) involved in its biosynthesis during the tea withering process is(are) still unknown. In this study, we found that the content of β-ionone increased during the tea withering process catalyzed by an unknown enzyme(s). Correlation analysis of expression patterns of Camellia sinensis carotenoid cleavage dioxygenase genes (CsCCDs) and the β-ionone content during the withering period revealed CsCCD4 as the most promising candidate. The full-length CsCCD4 gene was amplified from C. sinensis, and the biochemical function of the recombinant CsCCD4 protein was studied after coexpression in Escherichia coli strains engineered to accumulate β-carotene. The recombinant protein was able to cleave a variety of carotenoids at the 9-10 and 9'-10' double bonds. Volatile β-ionone was detected as the main product by gas and liquid chromatography-mass spectrometry. The accumulation of β-ionone was consistent with the expression levels of CsCCD4 in different tissues and during the withering process. The CsCCD4 expression was induced by low temperature and mechanical damage stress but not by dehydration stress. The results demonstrate that CsCCD4 catalyzes the production of β-ionone in the tea plant and provide insight into its formation mechanism during the withering process.

Analysis of Terpene Synthase Family Genes in Camellia sinensis with an Emphasis on Abiotic Stress Conditions

For a better understanding terpenoid volatile production in Camellia sinensis, global terpenoid synthase gene (TPS) transcription analysis was conducted based on transcriptomic data combined with terpenoid metabolic profiling under different abiotic stress conditions. Totally 80 TPS-like genes were identified. Twenty-three CsTPS genes possessed a complete coding sequence and most likely were functional. The remaining 57 in the currently available database lack essential gene structure or full-length transcripts. Distinct tempo-spatial expression patterns of CsTPS genes were found in tea plants. 17 genes were substantially expressed in all the tested organs with a few exceptions. The other 17 were predominantly expressed in leaves whereas additional eight were primarily expressed in flowers. Under the treatments of cold acclimation, salt and polyethylene glycol, CsTPS67, -69 and -71 were all suppressed and the inhibited expression of many others were found in multiple stress treatments. However, methyl jasmonate resulted in the enhanced expression of the majority of CsTPS genes. These transcription data were largely validated using qPCR. Moreover, volatile terpenoid profiling with leaves, flowers and stress-treated plants revealed a general association between the abundances of mono- and sesqui-terpenoids and some CsTPS genes. These results provide vital information for future studies on CsTPS regulation of terpenoid biosynthesis.

Wound- and pathogen-activated de novo JA synthesis using different ACX isozymes in tea plant (Camellia sinensis)

Acyl-CoA oxidase (ACX; EC 1.3.3.6) plays a vital role in the biosynthesis of jasmonic acid (JA) in plant peroxisomes. We previously identified an herbivore-induced gene CsACX1 in tea plant (Camellia sinensis) and showed CsACX1 was involved in the wound-induced synthesis of jasmonic acid (JA). Here, another ACX gene CsACX3 was isolated from tea plant. CsACX3 was predicted to consist of 684 amino acid residues. CsACX3 can be induced by mechanical wounding, JA application, and infestation by the tea geometrid Ectropis obliqua Prout and the tea green leafhopper Empoasca (Matsumurasca) onukii Matsuda. These expression patterns are consistent with the previously reported expression pattern of CsACX1 under such treatments. Recombinant CsACX3 showed preference for medium-chain acyl-coA oxidase substrates (C₈- to C₁₄-CoA). CsACX3 expression could also be induced by the infection of a pathogen Colletotrichum gloeosporioides (Cgl), and the increased ACX activities in tea plants were correlated with the Cgl-induced CsACX3 expression. Cgl could not induce the expression of CsACX1, which showed preference for C₁₂- to C₁₆-CoA substrates. The constitutive expression of CsACX3 rescued wound-induced JA biosynthesis and enhanced the Cgl-induced JA biosynthesis in Arabidopsis mutant atacx1. However, constitutive expression of CsACX1 could not enhance the Cgl-induced JA biosynthesis in atacx1 plant. These results indicate that CsACX1 and CsACX3 functions overlap and have distinct roles in the wound- and pathogen-activated de novo JA synthesis via enzymatic routes that utilize different ACX isozymes in tea plant.

Short-term inhibition of glutamine synthetase leads to reprogramming of amino acid and lipid metabolism in roots and leaves of tea plant (Camellia sinensis L.)

BACKGROUND

Nitrogen (N) nutrition significantly affected metabolism and accumulation of quality-related compounds in tea plant (Camellia sinensis L.). Little is known about the physiological and molecular mechanisms underlying the effects of short-term repression of N metabolism on tea roots and leaves for a short time.

RESULTS

In this study, we subjected tea plants to a specific inhibitor of glutamine synthetase (GS), methionine sulfoximine (MSX), for a short time (30 min) and investigated the effect of the inhibition of N metabolism on the transcriptome and metabolome of quality-related compounds. Our results showed that GS activities in tea roots and leaves were significantly inhibited upon MSX treatment, and both tissue types showed a sensitive metabolic response to GS inhibition. In tea leaves, the hydrolysis of theanine decreased with the increase in theanine and free ammonium content. The biosynthesis of all other amino acids was repressed, and the content of N-containing lipids declined, suggesting that short-term inhibition of GS reduces the level of N reutilization in tea leaves. Metabolites related to glycolysis and the tricarboxylic acid (TCA) cycle accumulated after GS repression, whereas the content of amino acids such as glycine, serine, isoleucine, threonine, leucine, and valine declined in the MXS treated group. We speculate that the biosynthesis of amino acids is affected by glycolysis and the TCA cycle in a feedback loop.

CONCLUSIONS

Overall, our data suggest that GS repression in tea plant leads to the reprogramming of amino acid and lipid metabolic pathways.

Three Camellia sinensis glutathione S-transferases are involved in the storage of anthocyanins, flavonols, and proanthocyanidins

Biochemical, transgenic, and genetic complementation data demonstrate that three glutathione S-transferases are involved in the storage of anthocyanins, flavonols, and proanthocyanins in plant cells. Flavonoids are compounds in tea (Camellia sinensis) that confer the characteristic astringent taste of tea beverages; these compounds have numerous benefits for human health. In plant cells, flavonoids are synthesized in different locations within the cytoplasm and are then transported and finally stored in vacuoles. To date, the mechanism involved in the intracellular transport of flavonoids in tea has not been well elucidated. In this study, we report the functional characterization of three cDNAs encoding glutathione S-transferases (CsGSTs) of C. sinensis, namely, CsGSTa, CsGSTb, and CsGSTc. The expression profiles of CsGSTa and CsGSTb were positively correlated with the accumulation of flavonols, anthocyanins and proanthocyanins in tea tissues and cultivars. These three recombinant CsGSTs showed a high affinity for flavonols (kaempferol-3-O-glucoside and quercetin-3-O-glucoside) and anthocyanin (cyanidin-3-O-glucoside) in vitro but had no or weak affinity for epicatechin. In vivo, CsGSTa, CsGSTb and CsGSTc fully or partially restored the storage of anthocyanins and proanthocyanidins in transgenic tt19 mutants. Metabolic profiling revealed that the contents of anthocyanins, flavonols, and proanthocyanidins were increased in the transgenic petals of Nicotiana tabacum. Taken together, all data showed that CsGSTa, CsGSTb, and CsGSTc are associated with the storage of anthocyanins, flavonols, and proanthocyanins in C. sinensis cells.

Isolation and functional analysis of squalene synthase gene in tea plant Camellia sinensis

Tea contains high quantities and diverse types of triterpenoids, particularly in the form of saponins. However, little is yet known about the molecular basis of triterpenoid biosynthesis in tea plant. Here we report on isolation and functional analysis of squalene synthase (SQS) gene from tea plant (Camellia sinensis var. sinensis), which controls the biosynthesis of triterpenoids precursor. First, a full-length cDNA of squalene synthase, designated CsSQS, was isolated from tea plant. The protein is highly homologous to SQSs from other plants. Using CsSQS-reporter assays, CsSQS was demonstrated to be endoplasmic reticulum membrane-bound. The coding region of CsSQS excluding transmemberane sequence was expressed in Escherichia coli. Recombinant CsSQS catalyzed the formation of squalene using farnesyl-pyrophosphate (FPP) as substrate with NADPH and Mg2+. In tea plant leaves, CsSQS expression was significantly induced by both herbivore and mechanical damages. Consistent with the stronger induction of CsSQS expression by mechanical damage than herbivory, tea plants injured mechanically released squalene as a volatile compound, which however was not detected from herbivore-damaged tea plants. Furthermore, it was found that the flowers of another tea plant cultivar Camellia sinensis var. assamica contain higher concentrations of squalene than the cultivar sinensis, indicating variations among tea plant varieties. With the identification and molecular characterization of squalene synthase in tea plant, next, we can ask the questions about the roles of squalene as a volatile product as well as a precursor for triterpenoids, which may promote product development from diverse tea materials and mining of excellent tea germplasm resources.

Expression patterns of alpha-amylase and beta-amylase genes provide insights into the molecular mechanisms underlying the responses of tea plants (Camellia sinensis) to stress and postharvest processing treatments

The alpha-amylase and beta-amylase genes have been identified from tea plants, and their bioinformatic characteristics and expression patterns provide a foundation for further studies to elucidate their biological functions. Alpha-amylase (AMY)- and beta-amylase (BAM)-mediated starch degradation plays central roles in carbohydrate metabolism and participates extensively in the regulation of a wide range of biological processes, including growth, development and stress response. However, the AMY and BAM genes in tea plants (Camellia sinensis) are poorly understood, and the biological functions of these genes remain to be elucidated. In this study, three CsAMY and nine CsBAM genes from tea plants were identified based on genomic and transcriptomic database analyses, and the genes were subjected to comprehensive bioinformatic characterization. Phylogenetic analysis showed that the CsAMY proteins could be clustered into three different subfamilies, and nine CsBAM proteins could be classified into four groups. Putative catalytically active proteins were identified based on multiple sequence alignments, and the tertiary structures of these proteins were analyzed. Cis-element analysis indicated that CsAMY and CsBAM were extensively involved in tea plant growth, development and stress response. In addition, the CsAMY and CsBAM genes were differentially expressed in various tissues and were regulated by stress treatments (e.g., ABA, cold, drought and salt stress), and the expression patterns of these genes were associated with the postharvest withering and rotation processes. Taken together, our results will enhance the understanding of the roles of the CsAMY and CsBAM gene families in the growth, development and stress response of tea plants and of the potential functions of these genes in determining tea quality during the postharvest processing of tea leaves.

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

The enzymatic hydrolysis of glycosidically bound volatiles (GBVs) plays an important role in tea aroma formation during the tea manufacturing process. However, during the enzyme-active manufacturing process of oolong tea, most GBVs showed no reduction, while ( Z)-3-hexenyl-β-glucopyranoside significantly enhanced at the turnover stage. This study aimed to determine the reason for this increase in ( Z)-3-hexenyl-β-glucopyranoside. Continuous wounding stress in the turnover stage did not enhance the expression level of glycosyltransferase 1 ( CsGT1), while it induced a significant increase in the ( Z)-3-hexenol content ( p ≤ 0.05). Furthermore, observing the cell structures of tea leaves exposed to continuous wounding and subcellular localizations of CsGTs suggested that the interaction of ( Z)-3-hexenol (substrate) and CsGT1 (enzyme) was available. In conclusion, both continuous wounding and subcellular localizations led to a ( Z)-3-hexenyl-β-glucopyranoside enhancement mechanism during the oolong tea's turnover stage. These results advance our understanding of GBV formation during the tea manufacturing process and their relationship with the stress from the tea manufacturing process. In addition, the information will help us further evaluate contribution of GBVs to enzymatic formation of oolong tea aroma compounds.

A variable loop involved in the substrate selectivity of pinoresinol/lariciresinol reductase from Camellia sinensis

Pinoresinol/lariciresinol reductase (PLR), an NADPH-dependent reductase that catalyzes the sequential reduction of pinoresinol into secoisolariciresinol via Lariciresinol, can lead to the structural and stereochemical diversity of lignans. The relationship between substrate-selective reaction of PLR and sequence homology still remains unclear. In this study, we focused on the contribution of the variable region between PLRs in determining substrate selectivity. Here, two CsPLRs (CsPLR1 and CsPLR2) were identified in the tea plant (Camellia sinensis var. sinensis cv. Shuchazao). In vitro enzymatic assays showed that CsPLR1 could convert (+)- and (-)-pinoresinol into lariciresinol or secoisolariciresinol, whereas CsPLR2 catalyzed (+)-pinoresinol enantioselectively into (-)-secoisolariciresinol. Homology modeling and site-directed mutagenesis were used to examine the role of a variable loop in catalysis and substrate selectivity. The L174I mutant in CsPLR1 lost the capacity to reduce either (+)- or (-)-pinoresinol but retained the ability to catalyze the reduction of (-)-lariciresinol. These findings provide a basis for better understanding of the substrate-selective reaction of PLR.

Identification of Tea Plant Purple Acid Phosphatase Genes and Their Expression Responses to Excess Iron

Purple acid phosphatase (PAP) encoding genes are a multigene family. PAPs require iron (Fe) to exert their functions that are involved in diverse biological roles including Fe homeostasis. However, the possible roles of PAPs in response to excess Fe remain unknown. In this study, we attempted to understand the regulation of PAPs by excess Fe in tea plant (Camellia sinensis). A genome-wide investigation of PAP encoding genes identified 19 CsPAP members based on the conserved motifs. The phylogenetic analysis showed that PAPs could be clustered into four groups, of which group II contained two specific cysteine-containing motifs “GGECGV” and “YERTC”. To explore the expression patterns of CsPAP genes in response to excessive Fe supply, RNA-sequencing (RNA-seq) analyses were performed to compare their transcript abundances between tea plants that are grown under normal and high iron conditions, respectively. 17 members were shown to be transcribed in both roots and leaves. When supplied with a high amount of iron, the expression levels of four genes were significantly changed. Of which, CsPAP15a, CsPAP23 and CsPAP27c were shown as downregulated, while the highly expressed CsPAP10a was upregulated. Moreover, CsPAP23 was found to be alternatively spliced, suggesting its post-transcriptional regulation. The present work implicates that some CsPAP genes could be associated with the responses of tea plants to the iron regime, which may offer a new direction towards a further understanding of iron homeostasis and provide the potential approaches for crop improvement in terms of iron biofortification.

A coupled role for CsMYB75 and CsGSTF1 in anthocyanin hyperaccumulation in purple tea

Cultivars of purple tea (Camellia sinensis) that accumulate anthocyanins in place of catechins are currently attracting global interest in their use as functional health beverages. RNA-seq of normal (LJ43) and purple Zijuan (ZJ) cultivars identified the transcription factor CsMYB75 and phi (F) class glutathione transferase CsGSTF1 as being associated with anthocyanin hyperaccumulation. Both genes mapped as a quantitative trait locus (QTL) to the purple bud leaf color (BLC) trait in F1 populations, with CsMYB75 promoting the expression of CsGSTF1 in transgenic tobacco (Nicotiana tabacum). Although CsMYB75 elevates the biosynthesis of both catechins and anthocyanins, only anthocyanins accumulate in purple tea, indicating selective downstream regulation. As glutathione transferases in other plants are known to act as transporters (ligandins) of flavonoids, directing them for vacuolar deposition, the role of CsGSTF1 in selective anthocyanin accumulation was investigated. In tea, anthocyanins accumulate in multiple vesicles, with the expression of CsGSTF1 correlated with BLC, but not with catechin content, in diverse germplasm. Complementation of the Arabidopsis tt19-8 mutant, which is unable to express the orthologous ligandin AtGSTF12, restored anthocyanin accumulation, but did not rescue the transparent testa phenotype, confirming that CsGSTF1 did not function in catechin accumulation. Consistent with a ligandin function, transient expression of CsGSTF1 in Nicotiana occurred in the nucleus, cytoplasm and membrane. Furthermore, RNA-Seq of the complemented mutants exposed to 2% sucrose as a stress treatment showed unexpected roles for anthocyanin accumulation in affecting the expression of genes involved in redox responses, phosphate homeostasis and the biogenesis of photosynthetic components, as compared with non-complemented plants.

Characterization of a heat responsive UDP: Flavonoid glucosyltransferase gene in tea plant (Camellia sinensis)

Tea plant (Camellia sinensis) accumulates abundant flavonoid glycosides that are the major bioactive ingredients in tea. Biosynthesis of flavonoid glycosides are catalyzed by UDP-glucosyltransferases (UGTs) that are widely present in plants. Among one hundred and seventy-eight UGTs genes that we have previously identified in tea plant, few of them have been functionally characterized. In the present study, we further identified UGT73A17 gene that is responsible for the biosynthesis of a broad range of flavonoid glycosides. Sequence analysis revealed that the deduced UGT73A17 protein showed high identity with 7-O-glycosyltransferases at amino acid level and it was clustered into the clade containing several 7-O-glycosyltransferases from other plant species. Enzymatic assays revealed that the recombinant UGT73A17 protein (rUGT73A17) exhibited activity toward flavonols (kaempferol, quercetin, and myricetin), flavones (apigenin, luteolin, and tricetin), flavanone (naringenin), isoflavones (genistein) and epicatechin gallate, yielding 7-O-glucosides as the major in vitro products. In particular, rUGT73A17 displayed higher activity at high temperatures (eg. 50°C) than at low temperatures, which was consistent with its relatively high expression level at high temperatures. Two amino acid substitutions at I296L and V466A improved the enzymatic activity of rUGT73A17. Our study demonstrated that UGT73A17 is responsible for the biosynthesis of a broad range of flavonoid glucosides, which is also involved in heat response and quality of tea plant.

Four flavonoid glycosyltransferases present in tea overexpressed in model plants Arabidopsis thaliana and Nicotiana tabacum for functional identification

Tea possesses a distinctive flavor profile and can have health benefits owing to the high levels of flavonoids in its leaves. However, the mechanism of the flavonoid glycosylation hasn't been well studied in tea plants, especially glycosylation at the 7-OH site has rarely been reported. In this study, four UGT genes CsUGT73A20, CsUGT75L12, CsUGT78A14 and CsUGT78A15 were isolated from tea leaves and overexpressed in the model plants Arabidopsis thaliana and Nicotiana tabacum for the functional identification of genes in vivo. In order to characterize the CsUGT functions in model plants, flavonoids in seeds of Arabidopsis and the flowers of tobacco were identified first. In CsUGT73A20-overexpressing Arabidopsis and tobacco, the level of certain flavonol glycosides involved in glycosylation reactions at the 3-OH and 7-OH sites increased considerably, but the level of flavan-3-ols decreased. In CsUGT75L12 transgenic Arabidopsis, the level of flavonol glycosides exhibiting glucosyltransferase activity at the 7-OH position increased markedly, but the concentrations of quercetin and kaempferol and flavan-3-ols decreased. In both transgenic Arabidopsis and tobacco, CsUGT78A14 promoted the synthesis of more flavonol glucosides with UDP-glucose as a sugar donor at the 3-OH glycosylation site. In CsUGT78A15 transgenic plants, flavonol galactosides at the 3-OH glycosylation site with UDP-galactose as a sugar donor were increased. In the tea plant, the corresponding flavonoid glycosides such as kaempferol‑3‑O‑β‑d‑glucosides, kaempferol‑3‑O‑β‑d‑galactosides, kaempferol‑7‑O‑β‑d‑glucoside, and luteolin‑7‑O‑β‑d‑glucoside were identified. And it could be possible that they were products of CsUGT78A14, CsUGT78A15, CsUGT73A20 and CsUGT75L12, respectively.

Genome-wide identification and characterization of the CsSnRK2 family in Camellia sinensis

The sucrose nonfermenting 1 (SNF1)-related protein kinase 2 (SnRK2) genes play central roles in plant stress signal transduction. In this study, 8 SnRK2 genes were identified from the tea plant genome database and named CsSnRK2.1-8. Phylogenetic analysis showed that the CsSnRK2 genes were classifiable into three groups, similar to those of Arabidopsis thaliana, Oryza sativa and maize. The coding sequences (CDSs) of all CsSnRK2s were separated by eight introns, and their exon-intron organizations exhibited high similarity to those of other plants. The fluorescence of GFP fused with CsSnRK2.3 was detected in only the cytoplasm, while the rest of the proteins showed GFP signal in both the nucleus and the cytoplasm. The results of the expression patterns of the CsSnRK2 genes showed that CsSnRK2s were differentially induced by salt, polyethylene glycol (PEG) and abscisic acid (ABA) stress. Interestingly, The expression of CsSnRK2.3 was inhibited by ABA, suggesting the complicated roles of CsSnRK2s in the ABA signal transduction pathway. Some CsSnRK2 gene pairs showed significant expression change correlations under stresses, indicating that CsSnRK2s might exhibit synergistic effects of signal regulation in response to various stresses. In summary, this comprehensive analysis will facilitate further studies of the SnRK2 family of Camellia sinensis and provide useful information for the functional validation of CsSnRK2s.

Genome-wide identification and characterization of ALTERNATIVE OXIDASE genes and their response under abiotic stresses in Camellia sinensis (L.) O. Kuntze

Four typical ALTERNATIVE OXIDASE genes have been identified in tea plants, and their sequence features and gene expression profiles have provided useful information for further studies on function and regulation. Alternative oxidase (AOX) is a terminal oxidase located in the respiratory electron transport chain. AOX catalyzes the oxidation of quinol and the reduction of oxygen into water. In this study, a genome-wide search and subsequent DNA cloning were performed to identify and characterize AOX genes in tea plant (Camellia sinensis (L.) O. Kuntze cv. Longjing43). Our results showed that tea plant possesses four AOX genes, i.e., CsAOX1a, CsAOX1d, CsAOX2a and CsAOX2b. Gene structure and protein sequence analyses revealed that all CsAOXs share a four-exon/three-intron structure with highly conserved regions and amino acid residues, which are necessary for AOX secondary structures, catalytic activities and post-translational regulations. All CsAOX were shown to localize in mitochondria using the green fluorescent protein (GFP)-targeting assay. Both CsAOX1a and CsAOX1d were induced by cold, salt and drought stresses, and with different expression patterns in young and mature leaves. Reactive oxygen species (ROS) accumulated strongly after 72 and 96 h cold treatments in both young and mature leaves, while the polyphenol and total catechin decreased significantly only in mature leaves. In comparison to AtAOX1a in Arabidopsis thaliana, CsAOX1a lost almost all of the stress-responsive cis-acting regulatory elements in its promoter region (1500 bp upstream), but possesses a flavonoid biosynthesis-related MBSII cis-acting regulatory element. These results suggest a link between CsAOX1a function and the metabolism of some secondary metabolites in tea plant. Our studies provide a basis for the further elucidation of the biological function and regulation of the AOX pathway in tea plants.

A Novel F3' 5' H Allele with 14 bp Deletion Is Associated with High Catechin Index Trait of Wild Tea Plants and Has Potential Use in Enhancing Tea Quality

Catechins are important chemical components determining the quality of tea. The catechin index (CI, ratio of dihydroxylated catechin (DIC)/trihydroxylated catechin (TRIC)) in the green leaf has a major influence on the amounts of theaflavins in black tea. In this work, the major catechin profiles of wild tea plants originating from Guizhou Province with high CI trait were investigated. We identified a novel flavonoid 3',5' hydroxylase gene ( F3' 5' H) allele with a 14 bp deletion in the upstream regulation region and developed an insertion/deletion (InDel) marker accordingly. The 14 bp deletion in the novel  F3' 5' H allele was associated with low F3' 5' H mRNA expression, thereby resulting in low TRIC content and high CI value. The allelic variant in the novel F3' 5' H allele associated with high CI values and DIC contents was confirmed by the introgression lines derived from a distant cross population. The novel F3' 5' H allele in wild tea plants is a valuable gene resource, which could be applied to breeding improvement on tea quality.

Characterization and Alternative Splicing Profiles of the Lipoxygenase Gene Family in Tea Plant (Camellia sinensis)

Oxylipins, including jasmonic acid (JA) and volatiles, are important for signaling in plants, and these are formed by the lipoxygenase (LOX) enzyme family. There is a large gap in understanding of the underlying molecular basis of their roles in tea plants. Here, we identified 11 CsLOX genes from the tea plant (Camellia sinensis), and characterized their phylogeny, gene structure and protein features into three subclasses. We then examined their enzymatic activities, LOX expression and alternative splicing of transcripts during development and in response to abiotic or biotic stresses in tea plants. In vitro expressed protein assays showed that the CsLOX2, 3 and 9 enzymatically function to produce 9/13-HPOT, 13-HPOT and 9-HPOT, respectively. CsLOX2 and CsLOX9 green fluorescent protein (GFP) fusion proteins localized to chloroplasts and the cytoplasm, respectively. RNA sequencing, quantitative reverse transcription-PCR and Northern blot analysis suggested that CsLOX5, 6 and 9 were predominantly expressed in seeds, flowers and roots, respectively. CsLOX2, 3, 4, 6 and 7 were up-regulated after attack by the insect Ectropis oblique, while CsLOX1 was induced after infection with the pathogen Glomerella cingulata. CsLOX3, 7 and 10 were up-regulated by JA but not ABA or salicylic acid. Long-term cold stress down-regulated CsLOX expression while a short duration of cold induced the expression of CsLOX1, 6 and 7. Alternatively spliced transcripts of six CsLOX genes were dynamically regulated through time and varied in relative abundances under the investigated stresses; we propose a mechanism of competing or compensating regulation between isoforms. This study improves our understanding of evolution of LOXs and regulation of their diverse functions in plants.

Two New Polyphenol Oxidase Genes of Tea Plant (Camellia sinensis) Respond Differentially to the Regurgitant of Tea Geometrid, Ectropis obliqua

Polyphenol oxidases (PPOs) have been reported to play an important role in protecting plants from attacks by herbivores. Though PPO genes in other plants have been extensively studied, research on PPO genes in the tea plant (Camellia sinensis) is lacking. In particular, which members of the PPO gene family elicit the defense response of the tea plant are as yet unknown. Here, two new PPO genes, CsPPO1 and CsPPO2, both of which had high identity with PPOs from other plants, were obtained from tea leaves. The full length of CsPPO1 contained an open reading frame (ORF) of 1740 bp that encoded a protein of 579 amino acids, while CsPPO2 contained an ORF of 1788 bp that encoded a protein of 595 amino acids. The deduced CsPPO1 and CsPPO2 proteins had calculated molecular masses of 64.6 and 65.9 kDa; the isoelectric points were 6.94 and 6.48, respectively. The expression products of recombinant CsPPO1 and CsPPO2 in Escherichia coli were about 91 and 92 kDa, respectively, but the recombinant proteins existed in the form of an inclusion body. Whereas CsPPO1 is highly expressed in stems, CsPPO2 is highly expressed in roots. Further results showed that the expression of CsPPO1 and CsPPO2 was wound- and Ectropis obliqua-induced, and that regurgitant, unlike treatment with wounding plus deionized water, significantly upregulated the transcriptional expression of CsPPO2 but not of CsPPO1. The difference between regurgitant and wounding indicates that CsPPO2 may play a more meaningful defensive role against E. obliqua than CsPPO1. Meanwhile, we found the active component(s) of the regurgitant elicited by the expression of CsPPO may contain small molecules (under 3-kDa molecular weight). These conclusions advance the understanding of the biological function of two new PPO genes and show that one of these, CsPPO2, may be a promising gene for engineering tea plants that are resistant to E. obliqua.

Evolutionary and functional characterization of leucoanthocyanidin reductases from Camellia sinensis

MAIN CONCLUSION

LARs promoted the biosynthesis of catechin monomers and inhibited their polymerization. The accumulation of catechin monomers and polymers was increased by up-regulating the expression of NtLAR and NtANR s in CsMYB5b transgenic tobacco. Tea is rich in polyphenolic compounds, and catechins are the major polyphenols in tea. The biosynthesis of polyphenols is closely related to the expression of the leucoanthocyanidin reductase (LAR) and anthocyanidin reductase (ANR) genes. In this paper, an evolutionary analysis and functional characterization of three CsLARs were performed. The phylogenetic tree showed that plant LARs could be grouped into three, including gymnosperms, monocotyledons and dicotyledons (clusters I and II). The eighth amino acid residue in a conserved LAR-specific motif is changeable due to a transversion (G → T) and transition (G → C) that occur in the corresponding codon. Therefore, plant LARs can be classified as G-type, A-type and S-type LARs due to this variable amino acid residue. Although (2R, 3S)-trans-flavan-3-ols were the products of recombinant CsLARs proteins expressed in Escherichia coli, both (2R, 3S)-trans and (2R, 3R)-cis-flavan-3-ols were detected in tobacco overexpressing CsLARs. However, a butanol/HCl hydrolysis assay indicated that overexpression of the CsLARs caused a decrease in polymerized catechins. A hybridization experiment with CsLARc + AtPAP1 also showed that no polymers other than epicatechin, catechin and glycoside were detected, although the accumulation of anthocyanins was markedly decreased. CsMYB5b promoted the biosynthesis of both flavan-3-ols and proanthocyanidins (PAs). Therefore, LARs promoted the biosynthesis of catechin monomers and inhibited their polymerization. The accumulation of catechin monomers and polymers was increased by up-regulating the expression of the NtLAR and NtANRs in CsMYB5b transgenic tobacco.

Functional Characterization of Salicylic Acid Carboxyl Methyltransferase from Camellia sinensis, Providing the Aroma Compound of Methyl Salicylate during the Withering Process of White Tea

Methyl salicylate (MeSA) is one of the volatile organic compounds (VOCs) that releases floral scent and plays an important role in the sweet flowery aroma of tea. During the withering process for white tea producing, MeSA was generated by salicylic acid carboxyl methyltransferase (SAMT) with salicylic acid (SA), and the specific floral scent was formed. In this study, we first cloned a CsSAMT from tea leaves (GenBank accession no. MG459470) and used Escherichia coli and Saccharomyces cerevisiae to express the recombinant CsSAMT. The enzyme activity in prokaryotic and eukaryotic expression systems was identified, and the protein purification, substrate specificity, pH, and temperature optima were investigated. It was shown that CsSAMT located in the chloroplast, and the gene expression profiles were quite different in tea organs. The obtained results might give a new understanding for tea aroma formation, optimization, and regulation and have great significance for improving the specific quality of white tea.

Functional Analysis of an Uridine Diphosphate Glycosyltransferase Involved in the Biosynthesis of Polyphenolic Glucoside in Tea Plants (Camellia sinensis)

Polyphenols are one of the largest groups of compounds that confer benefits to the health of plants and humans. Flavonol glycosides are a major ingredient of polyphenols in Camellia sinensis. Flavonol-3-O-glycosides are characteristic astringent taste compounds in tea infusion. A polyphenolic glycosyltransferase (CsUGT72AM1) belonging to cluster IIIb was isolated from the tea plant. The full-length cDNA of CsUGT72AM1 is 1416 bp. It encodes 472 amino acids with a calculated molecular mass of 50.92 kDa and an isoelectric point of 5.21. The recombinant CsUGT72AM1 protein was expressed in Escherichia coli and exhibited catalytic activity toward multiple flavonoids and coniferyl aldehyde. The enzyme assay indicated that rCsUGT72AM1 could perform glycosidation of flavonols or coniferyl aldehyde in vitro to form 3-O-glucoside or 4-O-glucoside, respectively. Interestingly, this enzyme also had activities and performed multisite glycosidation toward flavanones. The consistent products were confirmed to be naringenin-7-O-glucoside and -4'-O-glucoside by the nuclear magnetism assay. In addition, in the enzyme assay with cyanidin as the substrate, the results suggested that the glycosylated activity of CsUGT72AM1 was remarkably inhibited by a high concentration of anthocyanins. The above results indicate that CsUGT72AM1 may be involved in the metabolism of flavonol, flavanone, anthocyanin, and lignin.

Six phenylalanine ammonia-lyases from Camellia sinensis: Evolution, expression, and kinetics

Phenylalanine ammonia-lyase (PAL), the branch point enzyme controlling the flow of primary metabolism into second metabolism, converts the L-phenylalanine (L-Phe) to yield cinnamic acid. Based on the sequencing data available from eight transcriptome projects, six PAL genes have been screened out, cloned, and designated as CsPALa-CsPALf. The phylogenetic tree showed that CsPALs were divided into three subgroups, PALa and PALb, PALc and PALd, and PALe and PALf. All six CsPALs exhibited indiscriminate cytosolic locations in epidermis cells and mesophyll cells. Then, the expression profiles of six PAL genes were qualitatively investigated and they displayed tissue-/induced-expression specificity in several tissues or under different exogenous treatments. Furthermore, in vitro enzymatic assays showed that all six recombinant proteins were characterized by the strict substrate specificity toward L-Phe, but no activity toward L-Tyr, and they displayed subtle differences in kinetics and enzymatic properties. These results indicate that CsPALs play both distinct and overlapping roles in plant growth and responses to environmental cues.

Studies on the Biochemical Formation Pathway of the Amino Acid l-Theanine in Tea (Camellia sinensis) and Other Plants

Tea (Camellia sinensis) is the most widely consumed beverage aside from water. The flavor of tea is conferred by certain metabolites, especially l-theanine, in C. sinensis. To determine why more l-theanine accumulates in C. sinensis than in other plants, we compare l-theanine contents between C. sinensis and other plant species (Camellia nitidissima, Camellia japonica, Zea mays, Arabidopsis thaliana, and Solanum lycopersicum) and use a stable isotope labeling approach to elucidate its biosynthetic route. We quantify relevant intermediates and metabolites by mass spectrometry. l-Glutamic acid, a precursor of l-theanine, is present in most plants, while ethylamine, another precursor of l-theanine, specifically accumulates in Camellia species, especially C. sinensis. Most plants contain the enzyme/gene catalyzing the conversion of ethylamine and l-glutamic acid to l-theanine. After supplementation with [²H₅]ethylamine, all the plants produce [²H₅]l-theanine, which suggests that ethylamine availability is the reason for the difference in l-theanine accumulation between C. sinensis and other plants.

Molecular Cloning and Characterization of Galactinol Synthases in Camellia sinensis with Different Responses to Biotic and Abiotic Stressors

Galactinol synthase (GolS) is a key biocatalyst for the synthesis of raffinose family oligosaccharides (RFOs). RFOs accumulation plays a critical role in abiotic stress adaptation, but the relationship between expression of GolS genes and biotic stress adaptation remains unclear. In this study, two CsGolS genes were found to be highly up-regulated in a transcriptome library of Ectropic oblique-attacked Camellia sinensis. Three complete CsGolS genes were then cloned and characterized. Gene transcriptional analyses under biotic and abiotic stress conditions indicated that the CsGolS1 gene was sensitive to water deficit, low temperature, and abscisic acid, while CsGolS2 and CsGolS3 genes were sensitive to pest attack and phytohormones. The gene regulation and RFOs determination results indicated that CsGolS1 was primarily related to abiotic stress and CsGolS2 and CsGolS3 were related to biotic stress. GolS-mediated biotic stress adaptations have not been studied in depth, so further analysis of this new biological function is required.

Functional Characterization of a New Tea (Camellia sinensis) Flavonoid Glycosyltransferase

Tea (Camellia sinensis) is an important commercial crop, in which the high content of flavonoids provides health benefits. A flavonoid glycosyltransferase (CsUGT73A20), belonging to cluster IIIa, was isolated from tea plant. The recombinant CsUGT73A20 in Escherichia coli exhibited a broad substrate tolerance toward multiple flavonoids. Among them, kaempferol was the optimal substrate compared to quercetin, myricetin, naringenin, apigenin, and kaempferide. However, no product was detected when UDP-galactose was used as the sugar donor. The reaction assay indicated that rCsUGT73A20 performed multisite glycosidation toward flavonol compounds, mainly forming 3-O-glucoside and 7-O-glucoside in vitro. The biochemical characterization analysis of CsUGT73A20 showed more K7G product accumulated at pH 8.0, but K3G was the main product at pH 9.0. Kinetic analysis demonstrated that high pH repressed the glycosylation reaction at the 7-OH site in vitro. Besides, the content of five flavonol-glucosides was increased in CsUGT73A20-overexpressing tobaccos (Nicotiana tabacum).

Functional Characterization of a New Tea (Camellia sinensis) Flavonoid Glycosyltransferase

Tea (Camellia sinensis) is an important commercial crop, in which the high content of flavonoids provides health benefits. A flavonoid glycosyltransferase (CsUGT73A20), belonging to cluster IIIa, was isolated from tea plant. The recombinant CsUGT73A20 in Escherichia coli exhibited a broad substrate tolerance toward multiple flavonoids. Among them, kaempferol was the optimal substrate compared to quercetin, myricetin, naringenin, apigenin, and kaempferide. However, no product was detected when UDP-galactose was used as the sugar donor. The reaction assay indicated that rCsUGT73A20 performed multisite glycosidation toward flavonol compounds, mainly forming 3-O-glucoside and 7-O-glucoside in vitro. The biochemical characterization analysis of CsUGT73A20 showed more K7G product accumulated at pH 8.0, but K3G was the main product at pH 9.0. Kinetic analysis demonstrated that high pH repressed the glycosylation reaction at the 7-OH site in vitro. Besides, the content of five flavonol-glucosides was increased in CsUGT73A20-overexpressing tobaccos (Nicotiana tabacum).

[Engineering of a flavonoid 3'-hydroxylase from tea plant (Camellia sinensis) for biosynthesis of B-3',4'-dihydroxylated flavones]

OBJECTIVE

A flavonoid 3'-hydroxylase from tea plant was engineered to synthesize B-3',4'-dihydroxylated flavones such as eriodictyol, dihydroquercetin and quercetin.

METHODS

Four articifical P450 constructs harboring both flavonoid 3'-hydroxylase gene from Camellia sinensis (CsF3'H) and P450 reductase gene from Arabidopsis thaliana (ATR1 or ATR2) were introduced into Escherichia coli strains TOP10, DH5α and BL21, resultantly engineering strains S1 to S12. The plasmid pYES-Dest52-CsF3'H harboring CsF3'H gene was introduced into yeast Saccharomyces cerevisiae WAT11 designated as strain S13. The plasmid pES-HIS-CsF3H::AtFLS 9 AA was constructed through fusing flavanone 3-hydroxylase gene from Camellia sinensis (CsF3H) and flavonol synthase gene from Arabidopsis thaliana (AtFLS). Plasmid pES-URA-CsF3'H and pES-HIS-CsF3H::AtFLS 9 AA were then co-introduced into yeast S. cerevisiae WAT11 designated as strain S14.

RESULTS

Strain S6 generated highest bioconversion efficiency at 25℃ among all E. coli strains during 24 h fernentation. Supplemented with 1000 μmol/L naringenin, dihydrokaempferol and kaempferol, the maximum amounts of eriodictyol, dihydroquercetin and quercetin produced by strain S13 were 734.32 μmol/L, 446.07 μmol/L and 594.64 μmol/L respectively. Supplemented with 5 mmol/L naringenin, the maximum amounts of eriodictyol, kaempferol, quercetin, dihydroquercetin and dihydrokaempferol produced by strain S14 were 1412.16 μmol/L, 490.25 μmol/L, 445.75 μmol/L, 66.75 μmol/L and 73.50 μmol/L during 36-48 h fermentaion respectively.

CONCLUSION

CsF3'H was engineered for biosynthesis of B-3',4'-dihydroxylated flavone.

Functional natural allelic variants of flavonoid 3',5'-hydroxylase gene governing catechin traits in tea plant and its relatives

Functional allelic variants of the flavonoid 3',5'-hydroxylase (F3'5'H) gene provides new information of F3'5'H function of tea plant and its relatives. This insight may serve as the foundation upon which to advance molecular breeding in the tea plant. Catechins are the active components of tea that determine its quality and health attributes. This study established the first integrated genomic strategy for deciphering the genetic basis of catechin traits of tea plant. With the RNA-sequencing analysis of bulked segregants representing the tails of a F1 population segregated for total catechin content, we identified a flavonoid 3',5'-hydroxylase (F3'5'H) gene. F3'5'H had one copy in the genomic DNA of tea plant. Among 202 tea accessions, we identified 120 single nucleotide polymorphisms (SNPs) at F3'5'H locus. Seventeen significant marker-trait associations were identified by association mapping in multiple environments, which were involved in 10 SNP markers, and the traits including the ratio of di/tri-hydroxylated catechins and catechin contents. The associated individual and combination of SNPs explained 4.5-25.2 and 53.0-63.0% phenotypic variations, respectively. In the F1 population (validation population), the catechin trait variation percentages explained by F3'5'H diplotype were 6.9-74.3%. The genotype effects of ten functional SNPs in the F1 population were all consistent with the association population. Furthermore, the function of SNP-711/-655 within F3'5'H was validated by gene expression analysis. Altogether, our work indicated functional SNP allelic variants within F3'5'H governing the ratio of di/tri-hydroxylated catechins and catechin contents. The strong catechin-associated SNPs identified in this study can be used for future marker-assisted selection to improve tea quality.

Isolation and expression features of hexose kinase genes under various abiotic stresses in the tea plant (Camellia sinensis)

Hexokinases (HXKs, EC 2.7.1.1) and fructokinases (FRKs, EC 2.7.1.4) play important roles in carbohydrate metabolism and sugar signaling during the growth and development of plants. However, the HXKs and FRKs in the tea plant (Camellia sinensis) remain largely unknown. In this manuscript, we present the molecular characterization, phylogenetic relationships, conserved domains and expression profiles of four HXK and seven FRK genes of the tea plant. The 11 deduced CsHXK and CsFRK proteins were grouped into six main classes. All of the deduced proteins, except for CsFKR7, possessed putative ATP-binding motifs and a sugar recognition region. These genes exhibited tissue-specific expression patterns, which suggests that they play different roles in the metabolism and development of source and sink tissues in the tea plant. There were variations in CsHXKs and CsFRKs transcript abundance in response to four abiotic stresses: cold, salt, drought and exogenous abscisic acid (ABA). Remarkably, CsHXK3 and CsHXK4 were significantly induced in the leaves and roots under cold conditions, CsHXK1 was apparently up-regulated in the leaves and roots under salt and drought stresses, and CsHXK3 was obviously stimulated in the leaves and roots under short-term treatment with exogenous ABA. These findings demonstrate that CsHXKs play critical roles in response to abiotic stresses in the tea plant. Our research provides a fundamental understanding of the CsHXK and CsFRK genes of the tea plant and important information for the breeding of stress-tolerant tea cultivars.

Identification of the invertase gene family (INVs) in tea plant and their expression analysis under abiotic stress

Fourteen invertase genes were identified in the tea plant, all of which were shown to participate in regulating growth and development, as well as in responding to various abiotic stresses. Invertase (INV) can hydrolyze sucrose into glucose and fructose, which plays a principal role in regulating plant growth and development as well as the plants response to various abiotic and biotic stresses. However, currently, there is a lack of reported information, regarding the roles of INVs in either tea plant development or in the tea plants response to various stresses. In this study, 14 INV genes were identified from the transcriptome data of the tea plant (Camellia sinensis (L.) O. Kuntze), and named CsINV1-5 and CsINV7-15. Based on the results of a Blastx search and phylogenetic analysis, the CsINV genes could be clustered into 6 acid invertase (AI) genes and 8 alkaline/neutral invertase (A/N-Inv) genes. The results of tissue-specific expression analysis showed that the transcripts of all the identified CsINV genes are detectable in various tissues. Under various abiotic stress conditions, the expression patterns of the 14 CsINV genes were diverse in both the leaves and roots, and some of them were shown to be significantly expressed. Overall, we hypothesize that the identified CsINV genes all participate in regulating growth and development in the tea plant, and most likely through different signaling pathways that regulate the carbohydrate allocation and the ratio of hexose and sucrose for improving the resistance of the leaves and the roots of the tea plant to various abiotic stresses.

Association mapping of caffeine content with TCS1 in tea plant and its related species

Caffeine is the most abundant purine alkaloid in majority of tea plant and its related species. This purine alkaloid contributes to the important flavor and health attributes of tea. Tea caffeine synthase 1 (TCS1, EC 2.1.1.159/2.1.1.160) gene plays a crucial role in caffeine biosynthesis. The objective of this study was to investigate the genetic relationship between the TCS1 and caffeine content of tea plant and its related species using association mapping. We identified 87 single-nucleotide polymorphisms (SNPs, π = 0.00447) by resequencing the TCS1 locus of 44 tea accessions. Linkage disequilibrium (LD) analysis showed that LD did not extend over the entire gene (r(2) < 0.1, within 1000 bp). Two cleaved amplified polymorphism sequence (CAPS) markers were developed from sequence variations (SNP4318 and SNP6252). By association mapping, we identified SNP4318 associated with caffeine content in four environments, explaining 4.0%-7.7% of the phenotypic variance. We also validated the significant marker-trait associations in site-directed mutagenesis experiments. Examination of allelic variation and linkage disequilibrium by a candidate-gene-based approach can help to decipher the genetic basis of caffeine biosynthesis. Moreover, the SNP marker identified in this study can potentially be applied for future marker-assisted selection to improve tea quality.

Formation of Volatile Tea Constituent Indole During the Oolong Tea Manufacturing Process

Indole is a characteristic volatile constituent in oolong tea. Our previous study indicated that indole was mostly accumulated at the turn over stage of oolong tea manufacturing process. However, formation of indole in tea leaves remains unknown. In this study, one tryptophan synthase α-subunit (TSA) and three tryptophan synthase β-subunits (TSBs) from tea leaves were isolated, cloned, sequenced, and functionally characterized. Combination of CsTSA and CsTSB2 recombinant protein produced in Escherichia coli exhibited the ability of transformation from indole-3-glycerol phosphate to indole. CsTSB2 was highly expressed during the turn over process of oolong tea. Continuous mechanical damage, simulating the turn over process, significantly enhanced the expression level of CsTSB2 and amount of indole. These suggested that accumulation of indole in oolong tea was due to the activation of CsTSB2 by continuous wounding stress from the turn over process. Black teas contain much less indole, although wounding stress is also involved in the manufacturing 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. Our study provided evidence concerning formation of indole in tea leaves for the first time.

Identification of UDP-glycosyltransferases involved in the biosynthesis of astringent taste compounds in tea (Camellia sinensis)

Galloylated catechins and flavonol 3-O-glycosides are characteristic astringent taste compounds in tea (Camellia sinensis). The mechanism involved in the formation of these metabolites remains unknown in tea plants. In this paper, 178 UGT genes (CsUGTs) were identified inC. sinensis based on an analysis of tea transcriptome data. Phylogenetic analysis revealed that 132 of these genes were clustered into 15 previously established phylogenetic groups (A to M, O and P) and a newly identified group R. Three of the 11 recombinant UGT proteins tested were found to be involved in the in vitro biosynthesis of β-glucogallin and glycosylated flavonols. CsUGT84A22 exhibited catalytic activity toward phenolic acids, in particular gallic acid, to produce β-glucogallin, which is the immediate precursor of galloylated catechin biosynthesis in tea plants. CsUGT78A14 and CsUGT78A15 were found to be responsible for the biosynthesis of flavonol 3-O-glucosides and flavonol 3-O-galactosides, respectively. Site-directed mutagenesis of the Q373H substitution for CsUGT78A14 indicated that the Q (Gln) residue played a catalytically crucial role for flavonoid 3-O-glucosyltransferase activity. The expression profiles of the CsUGT84A22, CsUGT78A14, and CsUGT78A15 genes were correlated with the accumulation patterns of β-glucogallin and the glycosylated flavonols which indicated that these three CsUGT genes were involved in the biosynthesis of astringent compounds inC. sinensis.

Molecular Cloning and Characterization of Hydroperoxide Lyase Gene in the Leaves of Tea Plant (Camellia sinensis)

Hydroperoxide lyase (HPL, E.C. 4.1.2.) is the major enzyme in the biosynthesis of natural volatile aldehydes and alcohols in plants, however, little was known about HPL in tea plants (Camellia sinensis). A unique cDNA fragment was isolated by suppressive subtractive hybridization (SSH) from a tea plant subjected to herbivory by tea geometrid Ectropis obliqua. This full length cDNA acquired by RACE was 1476 bp and encoded 491 amino acids. DNA and protein BLAST searches showed high homology to HPL sequences from other plants. The His-tag expression vector pET-32a(+)/CsHPL was constructed and transferred into Escherichia coli Rosetta (DE3). The expression product of recombinant CsHPL in E. coli was about 60 kDa. The enzyme activity of CsHPL was 0.20 μmol·min(-1)·mg(-1). Quantitative RT-PCR analysis indicated CsHPL was strongly up-regulated in tea plants after Ectropis obliqua attack, suggesting that it may be an important candidate for defense against insects in tea plants.

Cloning and Characterization of a Flavonoid 3'-Hydroxylase Gene from Tea Plant (Camellia sinensis)

Tea leaves contain abundant flavan-3-ols, which include dihydroxylated and trihydroxylated catechins. Flavonoid 3'-hydroxylase (F3'H: EC 1.14.13.21) is one of the enzymes in the establishment of the hydroxylation pattern. A gene encoding F3'H, designated as CsF3'H, was isolated from Camellia sinensis with a homology-based cloning technique and deposited in the GenBank (GenBank ID: KT180309). Bioinformatic analysis revealed that CsF3'H was highly homologous with the characterized F3'Hs from other plant species. Four conserved cytochrome P450-featured motifs and three F3'H-specific conserved motifs were discovered in the protein sequence of CsF3'H. Enzymatic analysis of the heterologously expressed CsF3'H in yeast demonstrated that tea F3'H catalyzed the 3'-hydroxylation of naringenin, dihydrokaempferol and kaempferol. Apparent Km values for these substrates were 17.08, 143.64 and 68.06 μM, and their apparent Vmax values were 0.98, 0.19 and 0.44 pM·min(-1), respectively. Transcription level of CsF3'H in the new shoots, during tea seed germination was measured, along with that of other key genes for flavonoid biosynthesis using real-time PCR technique. The changes in 3',4'-flavan-3-ols, 3',4',5'-flavan-3-ols and flavan-3-ols, were consistent with the expression level of CsF3'H and other related genes in the leaves. In the study of nitrogen supply for the tea plant growth, our results showed the expression level of CsF3'H and all other tested genes increased in response to nitrogen depletion after 12 days of treatment, in agreement with a corresponding increase in 3',4'-catechins, 3',4',5'-catechins and flavan 3-ols content in the leaves. All these results suggest the importance of CsF3'H in the biosynthesis of 3',4'-catechins, 3',4',5'-catechins and flavan 3-ols in tea leaves.