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

An improved genome assembly of Chrysanthemum nankingense reveals expansion and functional diversification of terpene synthase gene family

BACKGROUND

Terpenes are important components of plant aromas, and terpene synthases (TPSs) are the key enzymes driving terpene diversification. In this study, we characterized the volatile terpenes in five different Chrysanthemum nankingense tissues. In addition, genome-wide identification and expression analysis of TPS genes was conducted utilizing an improved chromosome-scale genome assembly and tissue-specific transcriptomes. The biochemical functions of three representative TPSs were also investigated.

RESULTS

We identified tissue-specific volatile organic compound (VOC) and volatile terpene profiles. The improved Chrysanthemum nankingense genome assembly was high-quality, including a larger assembled size (3.26 Gb) and a better contig N50 length (3.18 Mb) compared to the old version. A total of 140 CnTPS genes were identified, with the majority representing the TPS-a and TPS-b subfamilies. The chromosomal distribution of these TPS genes was uneven, and 26 genes were included in biosynthetic gene clusters. Closely-related Chrysanthemum taxa were also found to contain diverse TPS genes, and the expression profiles of most CnTPSs were tissue-specific. The three investigated CnTPS enzymes exhibited versatile activities, suggesting multifunctionality.

CONCLUSIONS

We systematically characterized the structure and diversity of TPS genes across the Chrysanthemum nankingense genome, as well as the potential biochemical functions of representative genes. Our results provide a basis for future studies of terpene biosynthesis in chrysanthemums, as well as for the breeding of improved chrysanthemum varieties.

Comparative transcriptome and metabolite profiling reveal diverse pattern of CYP-TS gene expression during corosolic acid biosynthesis in Lagerstroemia speciosa (L.) Pers

KEY MESSAGE

Extensive leaf transcriptome profiling and differential gene expression analysis of field grown and elicited shoot cultures of L. speciosa suggest that differential synthesis of CRA is mediated primarily by CYP and TS genes, showing functional diversity. Lagerstroemia speciosa L. is a tree species with medicinal and horticultural attributes. The pentacyclic triterpene, Corosolic acid (CRA) obtained from this species is widely used for the management of diabetes mellitus in traditional medicine. The high mercantile value of the compound and limited availability of innate resources entail exploration of alternative sources for CRA production. Metabolic pathway engineering for enhanced bioproduction of plant secondary metabolites is an attractive proposition for which, candidate genes in the pathway need to be identified and characterized. Therefore, in the present investigation, we focused on the identification of cytochrome P450 (CYP450) and oxidosqualene cyclases (OSC) genes and their differential expression during biosynthesis of CRA. The pattern of differential expression of these genes in the shoot cultures of L. speciosa, elicited with different epigenetic modifiers (azacytidine (AzaC), sodium butyrate (NaBu) and anacardic acid (AA)), was studied in comparison with field grown plant. Further, in vitro cultures with varying (low to high) concentrations of CRA were systematically assessed for the expression of CYP-TS and associated genes involved in CRA biosynthesis by transcriptome sequencing. The sequenced samples were de novo assembled into 180,290 transcripts of which, 92,983 transcripts were further annotated by UniProt. The results are collectively given in co-occurrence heat maps to identify the differentially expressed genes. The combined transcript and metabolite profiles along with RT-qPCR analysis resulted in the identification of CYP-TS genes with high sequence variation. Further, instances of concordant/discordant relation between CRA biosynthesis and CYP-TS gene expression were observed, indicating functional diversity in genes.

A geraniol synthase regulates plant defense via alternative splicing in tea plants

Geraniol is an important contributor to the pleasant floral scent of tea products and one of the most abundant aroma compounds in tea plants; however, its biosynthesis and physiological function in response to stress in tea plants remain unclear. The proteins encoded by the full-length terpene synthase (CsTPS1) and its alternative splicing isoform (CsTPS1-AS) could catalyze the formation of geraniol when GPP was used as a substrate in vitro, whereas the expression of CsTPS1-AS was only significantly induced by Colletotrichum gloeosporioides and Neopestalotiopsis sp. infection. Silencing of CsTPS1 and CsTPS1-AS resulted in a significant decrease of geraniol content in tea plants. The geraniol content and disease resistance of tea plants were compared when CsTPS1 and CsTPS1-AS were silenced. Down-regulation of the expression of CsTPS1-AS reduced the accumulation of geraniol, and the silenced tea plants exhibited greater susceptibility to pathogen infection than control plants. However, there was no significant difference observed in the geraniol content and pathogen resistance between CsTPS1-silenced plants and control plants in the tea plants infected with two pathogens. Further analysis showed that silencing of CsTPS1-AS led to a decrease in the expression of the defense-related genes PR1 and PR2 and SA pathway-related genes in tea plants, which increased the susceptibility of tea plants to pathogens infections. Both in vitro and in vivo results indicated that CsTPS1 is involved in the regulation of geraniol formation and plant defense via alternative splicing in tea plants. The results of this study provide new insights into geraniol biosynthesis and highlight the role of monoterpene synthases in modulating plant disease resistance via alternative splicing.

Identification of genes associated with abiotic stress tolerance in sweetpotato using weighted gene co-expression network analysis

Sweetpotato, Ipomoea batatas (L.), a key food security crop, is negatively impacted by heat, drought, and salinity stress. The orange-fleshed sweetpotato cultivar "Beauregard" was exposed to heat, salt, and drought treatments for 24 and 48 h to identify genes responding to each stress condition in leaves. Analysis revealed both common (35 up regulated, 259 down regulated genes in the three stress conditions) and unique sets of up regulated (1337 genes by drought, 516 genes by heat, and 97 genes by salt stress) and down regulated (2445 genes by drought, 678 genes by heat, and 204 genes by salt stress) differentially expressed genes (DEGs) suggesting common, yet stress-specific transcriptional responses to these three abiotic stressors. Gene Ontology analysis of down regulated DEGs common to both heat and salt stress revealed enrichment of terms associated with "cell population proliferation" suggestive of an impact on the cell cycle by the two stress conditions. To identify shared and unique gene co-expression networks under multiple abiotic stress conditions, weighted gene co-expression network analysis was performed using gene expression profiles from heat, salt, and drought stress treated 'Beauregard' leaves yielding 18 co-expression modules. One module was enriched for "response to water deprivation," "response to abscisic acid," and "nitrate transport" indicating synergetic crosstalk between nitrogen, water, and phytohormones with genes encoding osmotin, cell expansion, and cell wall modification proteins present as key hub genes in this drought-associated module. This research lays the groundwork for exploring to a further degree, mechanisms for abiotic stress tolerance in sweetpotato.

Comparative Analysis and Identification of Terpene Synthase Genes in Convallaria keiskei Leaf, Flower and Root Using RNA-Sequencing Profiling

The 'Lilly of the Valley' species, Convallaria, is renowned for its fragrant white flowers and distinctive fresh and green floral scent, attributed to a rich composition of volatile organic compounds (VOCs). However, the molecular mechanisms underlying the biosynthesis of this floral scent remain poorly understood due to a lack of transcriptomic data. In this study, we conducted the first comparative transcriptome analysis of C. keiskei, encompassing the leaf, flower, and root tissues. Our aim was to investigate the terpene synthase (TPS) genes and differential gene expression (DEG) patterns associated with essential oil biosynthesis. Through de novo assembly, we generated a substantial number of unigenes, with the highest count in the root (146,550), followed by the flower (116,434) and the leaf (72,044). Among the identified unigenes, we focused on fifteen putative ckTPS genes, which are involved in the synthesis of mono- and sesquiterpenes, the key aromatic compounds responsible for the essential oil biosynthesis in C. keiskei. The expression of these genes was validated using quantitative PCR analysis. Both DEG and qPCR analyses revealed the presence of ckTPS genes in the flower transcriptome, responsible for the synthesis of various compounds such as geraniol, germacrene, kaurene, linalool, nerolidol, trans-ocimene and valencene. The leaf transcriptome exhibited genes related to the biosynthesis of kaurene and trans-ocimene. In the root, the identified unigenes were associated with synthesizing kaurene, trans-ocimene and valencene. Both analyses indicated that the genes involved in mono- and sesquiterpene biosynthesis are more highly expressed in the flower compared to the leaf and root. This comprehensive study provides valuable resources for future investigations aiming to unravel the essential oil-biosynthesis-related genes in the Convallaria genus.

Genome-wide identification and expression analysis of terpene synthases in Gossypium species in response to gossypol biosynthesis

Cottonseed is an invaluable resource, providing protein, oil, and abundant minerals that significantly contribute to the well-being and nutritional needs of both humans and livestock. However, cottonseed also contains a toxic substance called gossypol, a secondary metabolite in Gossypium species that plays an important role in cotton plant development and self-protection. Herein, genome-wide analysis and characterization of the terpene synthase (TPS) gene family identified 304 TPS genes in Gossypium. Bioinformatics analysis revealed that the gene family was grouped into six subgroups TPS-a, TPS-b, TPS-c, TPS-e, TPS-f, and TPS-g. Whole-genome, segmental, and tandem duplication contributed to the evolution of TPS genes. According to the analysis of selection pressure, it was predicted that TPS genes experience predominantly negative selection, with positive selection occurring subsequently. RT-qPCR analysis in TM-1 and CRI-12 lines revealed GhTPS48 gene as the candidate gene for silencing experiments. To summarize, comprehensive genome-wide studies, RT-qPCR, and gene silencing experiments have collectively demonstrated the involvement of the TPS gene family in the biosynthesis of gossypol in cotton.

Anti-Inflammatory and Anti-Quorum Sensing Effect of Camellia sinensis Callus Lysate for Treatment of Acne

Cutibacterium acnes (C. acnes) is involved in the pathogenesis of acne by inducing inflammation and biofilm formation, along with other virulence factors. A Camellia sinensis (C. sinensis) callus lysate is proposed to reduce these effects. The aim of the present work is to study the anti-inflammatory properties of a callus extract from C. sinensis on C. acnes-stimulated human keratinocytes and the quorum-quenching activities. Keratinocytes were stimulated with thermo-inactivated pathogenic C. acnes and were treated with the herbal lysate (0.25% w/w) to evaluate its anti-inflammatory effect. C. acnes biofilm was developed in vitro and treated with 2.5 and 5% w/w of the lysate to evaluate quorum sensing and the lipase activity. The results showed that the lysate was able to reduce the production of interleukin-6 (IL-6), interleukin-8 (IL-8), tumor necrosis factor-α (TNF-α), and C-X-C motif chemokine ligand 1 (CXCL1), and decrease the nuclear translocation of nuclear factor kappa light chain enhancer of activated B cells (NF-κB). The lysate did not show bactericidal activity but showed diminished biofilm formation, the lipase activity, and the production of autoinducer 2 (AI-2), a member of a family of signaling molecules used in quorum sensing. Therefore, the proposed callus lysate could have the potential to reduce acne-related symptoms without the eradication of C. acnes, which is part of the natural skin microbiome.

Genome-wide identification of terpenoid synthase family genes in Gossypium hirsutum and functional dissection of its subfamily cadinene synthase A in gossypol synthesis

Plant terpenoid synthase (TPS) family genes participate in metabolite synthesis, hormones, gossypol, etc. Here, we genome-widely identified TPS family genes in 12 land plant species. Four hundred and thirty TPS-related genes were divided into seven subfamilies. The TPS-c in Bryophytes was suggested to be the earliest subfamily, followed by the TPS-e/f and TPS-h presence in ferns. TPS-a, the largest number of genes, was derived from monocotyledonous and dicotyledonous plants. Collinearity analysis showed that 38 out of the 76 TPS genes in G. hirsutum were collinear within G. arboreum and G. raimondii. Twenty-one GhTPS-a genes belong to the cadinene synthase (GhCDN) subfamily and were divided into five groups, A, B, C, D, and E. The special cis-elements in the promoters of 12 GhCDN-A genes suggested that the JA and ethylene signaling pathways may be involved in their expression regulation. When 12 GhCDN-A genes were simultaneously silenced through virus-induced gene silencing, the glandular color of GhCDN-A-silenced plants was lighter than that of the control, supported by a gossypol content decrease based on HPLC testing, suggesting that GhCDN-A subgroup genes participate in gossypol synthesis. According to RNA-seq analysis, gossypol synthesis-related genes and disease-resistant genes in the glandular variety exhibited upregulated expression compared to the glandless variety, whereas hormone signaling-related genes were downregulated. All in all, these results revealed plant TPS gene evolution rules and dissected the TPS subfamily, GhCDN-A, function in gossypol synthesis in cotton.

Biosynthesis and the Transcriptional Regulation of Terpenoids in Tea Plants (Camellia sinensis)

Terpenes, especially volatile terpenes, are important components of tea aroma due to their unique scents. They are also widely used in the cosmetic and medical industries. In addition, terpene emission can be induced by herbivory, wounding, light, low temperature, and other stress conditions, leading to plant defense responses and plant-plant interactions. The transcriptional levels of important core genes (including HMGR, DXS, and TPS) involved in terpenoid biosynthesis are up- or downregulated by the MYB, MYC, NAC, ERF, WRKY, and bHLH transcription factors. These regulators can bind to corresponding cis-elements in the promoter regions of the corresponding genes, and some of them interact with other transcription factors to form a complex. Recently, several key terpene synthesis genes and important transcription factors involved in terpene biosynthesis have been isolated and functionally identified from tea plants. In this work, we focus on the research progress on the transcriptional regulation of terpenes in tea plants (Camellia sinensis) and thoroughly detail the biosynthesis of terpene compounds, the terpene biosynthesis-related genes, the transcription factors involved in terpene biosynthesis, and their importance. Furthermore, we review the potential strategies used in studying the specific transcriptional regulation functions of candidate transcription factors that have been discriminated to date.

Ethylene and Jasmonates Signaling Network Mediating Secondary Metabolites under Abiotic Stress

Plants are sessile organisms that face environmental threats throughout their life cycle, but increasing global warming poses an even more existential threat. Despite these unfavorable circumstances, plants try to adapt by developing a variety of strategies coordinated by plant hormones, resulting in a stress-specific phenotype. In this context, ethylene and jasmonates (JAs) present a fascinating case of synergism and antagonism. Here, Ethylene Insensitive 3/Ethylene Insensitive-Like Protein1 (EIN3/EIL1) and Jasmonate-Zim Domain (JAZs)-MYC2 of the ethylene and JAs signaling pathways, respectively, appear to act as nodes connecting multiple networks to regulate stress responses, including secondary metabolites. Secondary metabolites are multifunctional organic compounds that play crucial roles in stress acclimation of plants. Plants that exhibit high plasticity in their secondary metabolism, which allows them to generate near-infinite chemical diversity through structural and chemical modifications, are likely to have a selective and adaptive advantage, especially in the face of climate change challenges. In contrast, domestication of crop plants has resulted in change or even loss in diversity of phytochemicals, making them significantly more vulnerable to environmental stresses over time. For this reason, there is a need to advance our understanding of the underlying mechanisms by which plant hormones and secondary metabolites respond to abiotic stress. This knowledge may help to improve the adaptability and resilience of plants to changing climatic conditions without compromising yield and productivity. Our aim in this review was to provide a detailed overview of abiotic stress responses mediated by ethylene and JAs and their impact on secondary metabolites.

Genome-Wide Identification and Expression Analysis of TPS Gene Family in Liriodendron chinense

Terpenoids play a key role in plant growth and development, supporting resistance regulation and terpene synthase (TPS), which is the last link in the synthesis process of terpenoids. Liriodendron chinense, commonly called the Chinese tulip tree, is a rare and endangered tree species of the family Magnoliaceae. However, the genome-wide identification of the TPS gene family and its transcriptional responses to development and abiotic stress are still unclear. In the present study, we identified a total of 58 TPS genes throughout the L. chinense genome. A phylogenetic tree analysis showed that they were clustered into five subfamilies and unevenly distributed across six chromosomes. A cis-acting element analysis indicated that LcTPSs were assumed to be highly responsive to stress hormones, such as methyl jasmonate (MeJA) and abscisic acid (ABA). Consistent with this, transcriptome data showed that most LcTPS genes responded to abiotic stress, such as cold, drought, and hot stress, at the transcriptional level. Further analysis showed that LcTPS genes were expressed in a tissue-dependent manner, especially in buds, leaves, and bark. Quantitative reverse transcription PCR (qRT-PCR) analysis confirmed that LcTPS expression was significantly higher in mature leaves compared to young leaves. These results provide a reference for understanding the function and role of the TPS family, laying a foundation for further study of the regulation of TPS in terpenoid biosynthesis in L. chinense.

The genome of Magnolia hypoleuca provides a new insight into cold tolerance and the evolutionary position of magnoliids

Magnolia hypoleuca Sieb. & Zucc, a member of the Magnoliaceae of magnoliids, is one of the most economically valuable, phylogenetic and ornamental tree species in Eastern China. Here, the 1.64 Gb chromosome-level assembly covers 96.64% of the genome which is anchored to 19 chromosomes, with a contig N50 value of 1.71 Mb and 33,873 protein-coding genes was predicted. Phylogenetic analyses between M. hypoleuca and other 10 representative angiosperms suggested that magnoliids were placed as a sister group to the eudicots, rather than sister to monocots or both monocots and eudicots. In addition, the relative timing of the whole-genome duplication (WGD) events about 115.32 Mya for magnoliid plants. M. hypoleuca was found to have a common ancestor with M. officinalis approximately 23.4 MYA, and the climate change of OMT (Oligocene-Miocene transition) is the main reason for the divergence of M. hypoleuca and M. officinalis, which was along with the division of Japanese islands. Moreover, the TPS gene expansion observed in M. hypoleuca might contribute to the enhancement of flower fragrance. Tandem and proximal duplicates of younger age that have been preserved have experienced more rapid sequence divergence and a more clustered distribution on chromosomes contributing to fragrance accumulation, especially phenylpropanoid, monoterpenes and sesquiterpenes and cold tolerance. The stronger selective pressure drived the evolution of tandem and proximal duplicates toward plant self-defense and adaptation. The reference M. hypoleuca genome will provide insights into the evolutionary process of M. hypoleuca and the relationships between the magnoliids with monocots and eudicots, and enable us to delve into the fragrance and cold tolerance produced by M. hypoleuca and provide more robust and deep insight of how the Magnoliales evolved and diversified.

Genome-wide analysis of terpene synthase gene family to explore candidate genes related to disease resistance in Prunus persica

In plants, a family of terpene synthases (TPSs) is responsible for the biosynthesis of terpenes and contributes to species-specific diversity of volatile organic compounds, which play essential roles in fitness of plants. However, little is known about the TPS gene family in peach and/or nectarine (Prunus persica L.). In this study, we identified 40 PpTPS genes in peach genome v2.0. Although these PpTPSs could be clustered into five classes, they distribute in several gene clusters of three chromosomes, share conserved exon-intron organizations, and code similar protein motifs. Thirty-five PpTPSs, especially PpTPS2, PpTPS23, PpTPS17, PpTPS18, and PpTPS19, altered their transcript levels after inoculation with Botryosphaeria dothidea, a cause of peach gummosis, compared to the mock treatments, which might further affect the contents of 133 terpenoids at 48 hours and/or 84 hours post inoculations in the current-year shoots of 'Huyou018', a highly susceptible nectarine cultivar. Moreover, about fifteen PpTPSs, such as PpTPS1, PpTPS2, PpTPS3, and PpTPS5, showed distinct expression patterns during fruit development and ripening in two peach cultivars, yellow-fleshed 'Jinchun' and white-fleshed 'Hikawa Hakuho'. Among them, the transcription level of chloroplast-localized PpTPS3 was obviously related to the content of linalool in fruit pulps. In addition, elevated concentrations (0.1 g/L to 1.0 g/L) of linalool showed antifungal activities in PDA medium. These results improve our understanding of peach PpTPS genes and their potential roles in defense responses against pathogens.

Molecular basis of genetic plasticity to varying environmental conditions on growing rice by dry/direct-sowing and exposure to drought stress: Insights for DSR varietal development

Rice requires plenty of water for its cultivation by transplanting. This poses several challenges to its cultivation due to erratic rainfall resulting in drought, flood, and other abiotic stresses of varying intensity. Dry/direct-sown rice (DSR) has emerged as a water-saving/climate-smart alternative to transplanted rice (TPR). The performance of a rice cultivar on growing by different methods of planting under varying environmental conditions varies considerably. However, the molecular basis of the observed phenotypic plasticity of rice to varying environmental conditions is still elusive. Resilience to various environmental fluctuations is important to ensure sustainable rice production in the present era of global climate change. Our observations on exclusively up-regulated genes in leaf of Nagina 22 (N 22) grown by dry/direct-sowing and subjected to drought stress at panicle initiation stage (compared to that in leaf of IR 64), and another set of genes exclusively down-regulated in leaf of N 22 (compared to that in leaf of IR 64) indicate important roles of leaf in stress resilience. A large number of genes down-regulated exclusively in root of N 22 on dry/direct-sowing subjected to drought stress indicates a major contribution of roots in stress tolerance. The genes for redox-homeostasis, transcription factors, stress signaling, carbohydrate metabolism, and epigenetic modifications play important roles in making N 22 better adapted to DSR conditions. More importantly, the involvement of genes in rendering genetic plasticity to N 22 under changing environmental conditions was confirmed by reversal of the method of planting. To the best of our knowledge, this is the first report on decoding the molecular basis of genetic plasticity of rice grown by two different methods of planting subjected to drought stress at the reproductive stage of plant growth. This might help in DSR varietal development program to enhance water-productivity, conserve natural resources, and minimize the emission of greenhouse gases, thus achieving the objectives of negative-emission agriculture.

Integrating metabolite and transcriptome analysis revealed the different mechanisms of characteristic compound biosynthesis and transcriptional regulation in tea flowers

The flowers of tea plants (Camellia sinensis), as well as tea leaves, contain abundant secondary metabolites and are big potential resources for the extraction of bioactive compounds or preparation of functional foods. However, little is known about the biosynthesis and transcriptional regulation mechanisms of those metabolites in tea flowers, such as terpenoid, flavonol, catechins, caffeine, and theanine. This study finely integrated target and nontarget metabolism analyses to explore the metabolic feature of developing tea flowers. Tea flowers accumulated more abundant terpenoid compounds than young leaves. The transcriptome data of developing flowers and leaves showed that a higher expression level of later genes of terpenoid biosynthesis pathway, such as Terpene synthases gene family, in tea flowers was the candidate reason of the more abundant terpenoid compounds than in tea leaves. Differently, even though flavonol and catechin profiling between tea flowers and leaves was similar, the gene family members of flavonoid biosynthesis were selectively expressed by tea flowers and tea leaves. Transcriptome and phylogenetic analyses indicated that the regulatory mechanism of flavonol biosynthesis was perhaps different between tea flowers and leaves. However, the regulatory mechanism of catechin biosynthesis was perhaps similar between tea flowers and leaves. This study not only provides a global vision of metabolism and transcriptome in tea flowers but also uncovered the different mechanisms of biosynthesis and transcriptional regulation of those important compounds.

A monoterpene synthase gene cluster of tea plant (Camellia sinensis) potentially involved in constitutive and herbivore-induced terpene formation

Monoterpenes and sesquiterpenes are the most abundant volatiles in tea plants and have dual functions in aroma quality formation and defense responses in tea plants. Terpene synthases (TPS) are the key enzymes for the synthesis of terpenes in plants; however, the functions of most of them in tea plants are still unknown. In this study, six putative terpene biosynthesis gene clusters were identified from the tea plant genome. Then we cloned three new TPS-b subfamily genes, CsTPS08, CsTPS10 and CsTPS58. In vitro enzyme assays showed that CsTPS08 and CsTPS58 are two multiple-product terpene synthases, with the former synthesizing linalool as the main product, and β-myrcene, α-phellandrene, α-terpinolene, D-limonene, cis-β-ocimene, trans-β-ocimene and (4E,6Z)-allo-ocimene as minor products are also detected, while the latter catalyzing the formation of α-pinene and D-limonene using GPP as the substrate. No product of CsTPS10 was detected in the prokaryotic expression system, but geraniol production was detected when transiently expressed in tobacco leaves. CsTPS08 and CsTPS10 are two functional members of a monoterpene synthase gene cluster, which were significantly induced during both Ectropis oblique feeding and fresh leaf spreading treatments, suggesting that they have dual functions involved in tea plant pest defense and tea aroma quality regulation. In addition, the differences in their expression levels in different tea plant cultivars provide a possibility for the subsequent screening of tea plant resources with a specific aroma flavor. Our results deepen the understanding of terpenoid synthesis in tea plants.

Regulation of biosynthesis of the main flavor-contributing metabolites in tea plant (Camellia sinensis): A review

In the process of adapting to the environment, tea plants (Camellia sinensis) endow tea with unique flavor and health functions, which should be attributed to secondary metabolites, including catechins, L-theanine, caffeine and terpene volatiles. Since the content of these flavor-contributing metabolites are mainly determined by the growth of tea plant, it is very important to understand their alteration and regulation mechanisms. In the present work, we first summarize the distribution, change characteristics of the main flavor-contributing metabolites in different cultivars, organs and under environmental stresses of tea plant. Subsequently, we discuss the regulating mechanisms involved in the biosynthesis of these metabolites based on the existing evidence. Finally, we propose the remarks and perspectives on the future study relating flavor-contributing metabolites. This review would contribute to the acceleration of research on the characteristic secondary metabolites and the breeding programs in tea plants.

Cytosolic Nudix Hydrolase 1 Is Involved in Geranyl β-Primeveroside Production in Tea

Geraniol is a potent tea odorant and exists mainly as geranyl glycoside in Camellia sinensis. Understanding the mechanisms of geraniol biosynthesis at molecular levels in tea plants is of great importance for practical improvement of tea aroma. In this study, geraniol and its glycosides from tea plants were examined using liquid chromatography coupled with mass spectrometry. Two candidate geraniol synthase (GES) genes (CsTPS) and two Nudix hydrolase genes (CsNUDX1-cyto and CsNUDX1-chlo) from the tea genome were functionally investigated through gene transcription manipulation and gene chemical product analyses. Our data showed that in tea leaves, levels of geranyl β-primeveroside were dramatically higher than those of geranyl β-glucoside, while free geraniol was undetectable in this study. A tempo-spatial variation of geranyl β-primeveroside abundance in tea plants existed, with high levels in young and green tissues and low levels in mature or non-green tissues. Cytosolic CsNUDX1-cyto showed higher hydrolysis activity of geranyl-pyrophosphate to geranyl-monophosphate (GP) in vitro than did chloroplastidial CsNUDX1-chlo. A transgenic study revealed that expression of CsNUDX1-cyto resulted in significantly more geranyl β-primeveroside in transgenic Nicotiana benthamiana compared with non-transgenic wild-type, whereas expression of CsNUDX1-chlo had no effect. An antisense oligo-deoxynucleotide study confirmed that suppression of CsNUDX1-cyto transcription in tea shoots led to a significant decrease in geranyl β-primeveroside abundance. Additionally, CsNUDX1-cyto transcript levels and geranyl β-primeveroside abundances shared the same tempo-spatial patterns in different organs in the tea cultivar "Shucha Zao," indicating that CsNUDX1-cyto is important for geranyl β-primeveroside formation in tea plants. Results also suggested that neither of the two candidate GES genes in tea plants did not function as GES in transgenic N. benthamiana. All our data indicated that CsNUDX1-cyto is involved in geranyl β-primeveroside production in tea plants. Our speculation about possible conversion from the chemical product of CsNUDX1-cyto to geranyl β-primeveroside in plants was also discussed.

Genomic Analysis Based on Chromosome-Level Genome Assembly Reveals an Expansion of Terpene Biosynthesis of Azadirachta indica

Azadirachta indica (neem), an evergreen tree of the Meliaceae family, is a source of the potent biopesticide azadirachtin. The lack of a chromosome-level assembly impedes an in-depth understanding of its genome architecture and the comparative genomic analysis of A. indica. Here, a high-quality genome assembly of A. indica was constructed using a combination of data from Illumina, PacBio, and Hi-C technology, which is the first chromosome-scale genome assembly of A. indica. Based on the length of our assembly, the genome size of A. indica is estimated to be 281 Mb anchored to 14 chromosomes (contig N50 = 6 Mb and scaffold N50 = 19 Mb). The genome assembly contained 115 Mb repetitive elements and 25,767 protein-coding genes. Evolutional analysis revealed that A. indica didn't experience any whole-genome duplication (WGD) event after the core eudicot γ event, but some genes and genome segment might likely experienced recent duplications. The secondary metabolite clusters, TPS genes, and CYP genes were also identified. Comparative genomic analysis revealed that most of the A. indica-specific TPS genes and CYP genes were located on the terpene-related clusters on chromosome 13. It is suggested that chromosome 13 may play an important role in the specific terpene biosynthesis of A. indica. The gene duplication events may be responsible for the terpene biosynthesis expansion in A. indica. The genomic dataset and genomic analysis created for A. indica will shed light on terpene biosynthesis in A. indica and facilitate comparative genomic research of the family Meliaceae.

Tissue-Specific Expression of the Terpene Synthase Family Genes in Rosa chinensis and Effect of Abiotic Stress Conditions

Rose (Rosa chinensis) is one of the most famous ornamental plants worldwide, with a variety of colors and fragrances. Terpene synthases (TPSs) play critical roles in the biosynthesis of terpenes. In this work, we report a comprehensive study on the genome-wide identification and characterization of the TPS family in R. chinensis. We identified 49 TPS genes in the R. chinensis genome, and they were grouped into five subfamilies (TPS-a, TPS-b, TPS-c, TPS-g and TPS-e/f). Phylogenetics, gene structure and conserved motif analyses indicated that the RcTPS genes possessed relatively conserved gene structures and the RcTPS proteins contained relatively conserved motifs. Multiple putative cis-acting elements involved in the stress response were identified in the promoter region of RcTPS genes, suggesting that some could be regulated by stress. The expression profile of RcTPS genes showed that they were predominantly expressed in the petals of open flowers, pistils, leaves and roots. Under osmotic and heat stresses, the expression of most RcTPS genes was upregulated. These data provide a useful foundation for deciphering the functional roles of RcTPS genes during plant growth as well as addressing the link between terpene biosynthesis and abiotic stress responses in roses.

Diversity and Functional Evolution of Terpene Synthases in Rosaceae

Terpenes are organic compounds and play important roles in plant development and stress response. Terpene synthases (TPSs) are the key enzymes for the biosynthesis of terpenes. For Rosaceae species, terpene composition represents a critical quality attribute, but limited information is available regarding the evolution and expansion occurring in the terpene synthases gene family. Here, we selected eight Rosaceae species with sequenced and annotated genomes for the identification of TPSs, including three Prunoideae, three Maloideae, and two Rosoideae species. Our data showed that the TPS gene family in the Rosaceae species displayed a diversity of family numbers and functions among different subfamilies. Lineage and species-specific expansion of the TPSs accompanied by frequent domain loss was widely observed within different TPS clades, which might have contributed to speciation or environmental adaptation in Rosaceae. In contrast to Maloideae and Rosoideae species, Prunoideae species owned less TPSs, with the evolution of Prunoideae species, TPSs were expanded in modern peach. Both tandem and segmental duplication significantly contributed to TPSs expansion. Ka/Ks calculations revealed that TPSs genes mainly evolved under purifying selection except for several pairs, where the divergent time indicated TPS-e clade was diverged relatively anciently. Gene function classification of TPSs further demonstrated the function diversity among clades and species. Moreover, based on already published RNA-Seq data from NCBI, the expression of most TPSs in Malus domestica, Prunus persica, and Fragaria vesca displayed tissue specificity and distinct expression patterns either in tissues or expression abundance between species and TPS clades. Certain putative TPS-like proteins lacking both domains were detected to be highly expressed, indicating the underlying functional or regulatory potentials. The result provided insight into the TPS family evolution and genetic information that would help to improve Rosaceae species quality.

Full-length transcriptome analysis of multiple organs and identification of adaptive genes and pathways in Mikania micrantha

Mikania micrantha is a notorious invasive weed that has caused huge economic loss and negative ecological consequences in invaded areas. This species can adapt well to invasive environments with various stress factors. The identification of gene families and functional pathways related to environmental adaptability is lack in M. micrantha at the multi-organ full-length transcriptome level. In this study, we sequenced the transcriptomes of five M. micrantha organs using PacBio single-molecule real-time sequencing and Illumina RNA sequencing technologies. Based on the transcriptome data, full-length transcripts were captured and gene expression patterns among the five organs were analyzed. KEGG enrichment analysis of genes with higher expression indicated their special roles in environmental stress response and adversity adaptation in the various five organs. The gene families and pathways related to biotic and abiotic factors, including terpene synthases, glutathione S-transferases, antioxidant defense system, and terpenoid biosynthesis pathway, were characterized. The expression levels of most differentially expressed genes in the antioxidant defense system and terpenoid biosynthesis pathway were higher in root, stem, and leaf than in the other two organs, suggesting that root, stem, and leaf have strong ability to respond to adverse stresses and form the important organs of terpenoid synthesis and accumulation. Additionally, a large number of transcription factors and alternative splicing events were predicted. This study provides a comprehensive transcriptome resource for M. micrantha, and our findings facilitate further research on the adaptive evolution and functional genomics of this species.

Comparative Transcriptome and Phytochemical Analysis Provides Insight into Triterpene Saponin Biosynthesis in Seeds and Flowers of the Tea Plant (Camellia sinensis)

Triterpene saponins exhibit various biological and pharmacological activities. However, the knowledge on saponin biosynthesis in tea plants (Camellia sinensis L.) is still limited. In this work, tea flower and seed samples at different developmental stages and leaves were collected and analyzed with UPLC-PDA-MS and RNA sequencing for saponin determination and transcriptome comparison. The saponin content reached around 19% in the freshly mature seeds and 7% in the green flower buds, and decreased with the fruit ripeness and flower blooming. Almost no saponins were detected in leaf samples. PCA and KEGG analysis suggested that the gene expression pattern and secondary metabolism in TF1 and TS2 vs. leaf samples were significantly different. Weighted gene coexpression network analysis (WGCNA) uncovered two modules related to saponin content. The mevalonate (MVA) instead of 2-C-methyl-d-erythritol-4-phospate (MEP) pathway was responsible for saponin accumulation in tea plants, and 3-hydroxy-3-methylglutaryl-CoA synthase (HMGS), diphosphomevalonate decarboxylase (MVD) and isopentenyl diphosphate isomerase (IDI) may be the key enzymes involved in saponin biosynthesis in tea seeds and flowers. Moreover, ten transcription factors (TFs) were predicted to regulate saponin biosynthesis in the tea plant. Taken together, our study provides a global insight into the saponin biosynthesis and accumulation in the tea plant.

Molecular and Metabolic Changes under Environmental Stresses: The Biosynthesis of Quality Components in Preharvest Tea Shoots

Severe environments impose various abiotic stresses on tea plants. Although much is known about the physiological and biochemical responses of tea (Camellia sinensis L.) shoots under environmental stresses, little is known about how these stresses impact the biosynthesis of quality components. This review summarizes and analyzes the changes in molecular and quality components in tea shoots subjected to major environmental stresses during the past 20 years, including light (shade, blue light, green light, and UV-B), drought, high/low temperature, CO2, and salinity. These studies reveal that carbon and nitrogen metabolism is critical to the downstream biosynthesis of quality components. Based on the molecular responses of tea plants to stresses, a series of artificial methods have been suggested to treat the pre-harvest tea plants that are exposed to inhospitable environments to improve the quality components in shoots. Furthermore, many pleiotropic genes that are up- or down-regulated under both single and concurrent stresses were analyzed as the most effective genes for regulating multi-resistance and quality components. These findings deepen our understanding of how environmental stresses affect the quality components of tea, providing novel insights into strategies for balancing plant resistance, growth, and quality components in field-based cultivation and for breeding plants using pleiotropic genes.

The reference genome of camellia chekiangoleosa provides insights into camellia evolution and tea oil biosynthesis

Camellia oil extracted from Camellia seeds is rich in unsaturated fatty acids (UFAs) and secondary metabolites beneficial to human health. However, no oil-tea tree genome has yet been published, which is a major obstacle to investigating the heredity improvement of oil-tea trees. Here, using both Illumina and PicBio sequencing technologies, we present the first chromosome-level genome sequence of the oil-tea tree species Camellia chekiangoleosa Hu. (CCH). The assembled genome consists of 15 pseudochromosomes with a genome size of 2.73 Gb and a scaffold N50 of 185.30 Mb. At least 2.16 Gb of the genome assembly consists of repetitive sequences, and the rest involves a high-confidence set of 64 608 protein-coding gene models. Comparative genomic analysis revealed that the CCH genome underwent a whole-genome duplication (WGD) event shared across the Camellia genus at ~57.48 MYA and a γ-WGT event shared across all core eudicot plants at ~120 MYA. Gene family clustering revealed that the genes involved in terpenoid biosynthesis have undergone rapid expansion. Furthermore, we determined the expression patterns of oleic acid accumulation- and terpenoid biosynthesis-associated genes in six tissues. We found that these genes tend to be highly expressed in leaves, pericarp tissues, roots, and seeds. The first chromosome-level genome of oil-tea trees will provide valuable resources for determining Camellia evolution and utilizing the germplasm of this taxon.

Genome-Wide Identification and Expression Analysis of Terpene Synthase Genes in Cymbidium faberi

Terpene synthases (TPSs) are essential for forming terpenes, which play numerous functional roles in attracting pollinators, defending plants, and moderating the interaction between plants. TPSs have been reported in some orchids, but genome-wide identification of terpenes in Cymbidium faberi is still lacking. In this study, 32 putative TPS genes were classified in C. faberi and divided into three subfamilies (TPS-a, TPS-b, and TPS-e/f). Motif and gene structure analysis revealed that most CfTPS genes had the conserved aspartate-rich DDxxD motif. TPS genes in the TPS-a and TPS-b subfamilies had variations in the RRX₈W motif. Most cis-elements of CfTPS genes were found in the phytohormone responsiveness category, and MYC contained most of the numbers associated with MeJA responsiveness. The Ka/Ks ratios of 12/13 CfTPS gene pairs were less than one, indicated that most CfTPS genes have undergone negative selection. The tissue-specific expression patterns showed that 28 genes were expressed in at least one tissue in C. faberi, and TPS genes were most highly expressed in flowers, followed by leaves and pseudobulbs. In addition, four CfTPS genes were selected for the real-time reverse transcription quantitative PCR (RT-qPCR) experiment. The results revealed that CfTPS12, CfTPS18, CfTPS23, and CfTPS28 were mainly expressed in the full flowering stage. CfTPS18 could convert GPP to β-myrcene, geraniol, and α-pinene in vitro. These findings of CfTPS genes of C. faberi may provide valuable information for further studies on TPSs in orchids.

RNASeq analysis of drought-stressed guayule reveals the role of gene transcription for modulating rubber, resin, and carbohydrate synthesis

The drought-adapted shrub guayule (Parthenium argentatum) produces rubber, a natural product of major commercial importance, and two co-products with potential industrial use: terpene resin and the carbohydrate fructan. The rubber content of guayule plants subjected to water stress is higher compared to that of well-irrigated plants, a fact consistently reported in guayule field evaluations. To better understand how drought influences rubber biosynthesis at the molecular level, a comprehensive transcriptome database was built from drought-stressed guayule stem tissues using de novo RNA-seq and genome-guided assembly, followed by annotation and expression analysis. Despite having higher rubber content, most rubber biosynthesis related genes were down-regulated in drought-stressed guayule, compared to well-irrigated plants, suggesting post-transcriptional effects may regulate drought-induced rubber accumulation. On the other hand, terpene resin biosynthesis genes were unevenly affected by water stress, implying unique environmental influences over transcriptional control of different terpene compounds or classes. Finally, drought induced expression of fructan catabolism genes in guayule and significantly suppressed these fructan biosynthesis genes. It appears then, that in guayule cultivation, irrigation levels might be calibrated in such a regime to enable tunable accumulation of rubber, resin and fructan.

Combined Transcriptome and Proteome Analysis to Elucidate Salt Tolerance Strategies of the Halophyte Panicum antidotale Retz

Panicum antidotale, a C4 monocot, has the potential to reclaim saline and drylands and to be utilized as fodder and forage. Its adaptability to survive saline stress has been proven with eco-physiological and biochemical studies. However, little is known about its molecular mechanisms of salt tolerance. In this study, an integrated transcriptome and proteome analysis approach, based on RNA sequencing and liquid chromatography tandem mass spectrometry (LC-MS/MS), was used to identify the said mechanisms. Plants were treated with control (0 mM), low (100 mM), and high (300 mM) sodium chloride (NaCl) treatments to distinguish beneficial and toxic pathways influencing plant biomass. The results indicated differential expression of 3,179 (1,126 upregulated/2,053 downregulated) and 2,172 (898 upregulated/1,274 downregulated) genes (DEGs), and 514 (269 upregulated/245 downregulated) and 836 (494 upregulated/392 downregulated) proteins (DEPs) at 100 and 300 mM NaCl, respectively. Among these, most upregulated genes and proteins were involved in salt resistance strategies such as proline biosynthesis, the antioxidant defense system, ion homeostasis, and sugar accumulation at low salinity levels. On the other hand, the expression of several genes and proteins involved in the respiratory process were downregulated, indicating the inability of plants to meet their energy demands at high salinity levels. Moreover, the impairments in photosynthesis were also evident with the reduced expression of genes regulating the structure of photosystems and increased expression of abscisic acid (ABA) mediated pathways which limits stomatal gas exchange. Similarly, the disturbance in fatty acid metabolism and activation of essential ion transport blockers damaged the integrity of the cell membrane, which was also evident with enhanced malondialdehyde (MDA). Overall, the analysis of pathways revealed that the plant optimal performance at low salinity was related to enhanced metabolism, antioxidative defense, cell growth, and signaling pathways, whereas high salinity inhibited biomass accumulation by altered expression of numerous genes involved in carbon metabolism, signaling, transcription, and translation. The data provided the first global analysis of the mechanisms imparting salt stress tolerance of any halophyte at transcriptome and proteome levels.

Water deficit stress responses of monoterpenes and sesquiterpenes in different Iranian cultivars of basil

Ocimum basilicum, a popular aromatic plant, contains aromatic terpenes of terpenoids with in vivo and in vitro verified cytotoxicity. Considering the characteristics and potential of its utilization, it would be attractive to reveal its regulation and biosynthesis, originally at the molecular level under water deficit stress. For this aim, for the first time, the gene encoding the enzyme involved in the end step of the MEP biosynthetic pathways (HDR) was cloned, and the accumulation ratio of linalool, germacrene D and γ-cadinene compounds as well as the expression trait of four critical genes (i.e., HDR, LinS, GerS, and GadS) was assessed under water deficit stress in three Iranian cultivars of basil. The highest value of linalool and γ-cadinene were detected for Cultivar 1 (Cult. 1) under mild stress (W1; 52.6 and 21.1%), while insignificant amounts were obtained for Cultivar 3 (Cult. 3). The germacrene D level of Cultivar 2 (Cult. 2) increased under severe and moderate water stresses as compared with mild water deficit stress. Apart from some expectation, all the studied genes demonstrated divergent transcription ratios under water deficit stress. Principal component analyses (PCA) showed that the relative water content (RWC) and HDR gene expression correlated significantly with essential oil components and gene expression in Cult. 1 and 2, which could represent an elevated demand for corresponding metabolites in the plant tissues. The present work elaborates on the regulation of the mentioned genes, and the results indicate that the production of terpenoids might be a drought stress-dependent and cultivar-dependent procedure.

Pan-transcriptome identifying master genes and regulation network in response to drought and salt stresses in Alfalfa (Medicago sativa L.)

Alfalfa is an important legume forage grown worldwide and its productivity is affected by environmental stresses such as drought and high salinity. In this work, three alfalfa germplasms with contrasting tolerances to drought and high salinity were used for unraveling the transcriptomic responses to drought and salt stresses. Twenty-one different RNA samples from different germplasm, stress conditions or tissue sources (leaf, stem and root) were extracted and sequenced using the PacBio (Iso-Seq) and the Illumina platforms to obtain full-length transcriptomic profiles. A total of 1,124,275 and 91,378 unique isoforms and genes were obtained, respectively. Comparative analysis of transcriptomes identified differentially expressed genes and isoforms as well as transcriptional and post-transcriptional modifications such as alternative splicing events, fusion genes and nonsense-mediated mRNA decay events and non-coding RNA such as circRNA and lncRNA. This is the first time to identify the diversity of circRNA and lncRNA in response to drought and high salinity in alfalfa. The analysis of weighted gene co-expression network allowed to identify master genes and isoforms that may play important roles on drought and salt stress tolerance in alfalfa. This work provides insight for understanding the mechanisms by which drought and salt stresses affect alfalfa growth at the whole genome level.

Genome-wide detection and classification of terpene synthase genes in Aquilaria agallochum

Agarwood, one of the precious woods in the globe, is produced by Aquilaria plant species during an upshot of wounding and infection. Produced as a defence response, the dark, fragrant resin gets secreted in the plant's duramen, which is impregnated with fragrant molecules with the due course. Agarwood has gained worldwide popularity due to its high aromatic oil, fragrance, and pharmaceutical value, which makes it highly solicited by numerous industries. Predominant chemical constituents of agarwood, sesquiterpenoids, and 2-(2-phenylethyl) chromones have been scrutinized to comprehend the scientific nature of the fragrant wood and develop novel products. However, the genes involved in the biosynthesis of these aromatic compounds are still not comprehensively studied in Aquilaria. In this study, publicly available genomic and transcriptomics data of Aquilaria agallochum were integrated to identify putative functional terpene synthase genes (TPSs). The in silico study enabled us to identify ninety-six TPSs, of which thirty-nine full-length genes were systematically classified into TPS-a, TPS-b, TPS-c, TPS-e, TPS-f, and TPS-g subfamilies based on their gene structure, conserve motif, and phylogenetic comparison with TPSs from other plant species. Analysis of the cis-regulatory elements present upstream of AaTPSs revealed their association with hormone, stress and light responses. In silico expression studies detected their up-regulation in stress induced tissue. This study provides a basic understanding of terpene synthase gene repertoire in Aquilaria agallochum and unlatches opportunities for the biochemical characterization and biotechnological exploration of these genes.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01040-z.

Genome-wide identification and expression analysis of terpene synthase gene family in Aquilaria sinensis

Agarwood is the resinous portion of Aquilaria trees, and has been widely used as medicine and incense. Sesquiterpenes are the main chemical characteristic constituents of agarwood. Terpene synthase (TPS) is a critical enzyme responsible for biosynthesis of sesquiterpene compounds. However, limited information is available on genome-wide identification and characterization of the TPS family in Aquilaria trees. In this study, TPS gene family was identified and characterized in Aquilaria sinensis by bioinformatics methods. The expression of those genes was analyzed by RNA-seq and quantitative real-time PCR. Transcription factors regulating TPS gene expression were identified by yeast one-hybrid and dual-luciferase assay. In total, 26 AsTPS genes (AsTPS1-AsTPS26) were identified, which were classified into five subgroups. Many putative cis-elements putatively involved in stresses and phytohormones (especially jasmonic acid) were identified in the promoter regions of AsTPSs, suggesting that AsTPSs genes may be regulated by stresses and jasmonic acid. Expression analysis revealed seven TPS genes encoding sesquiterpene synthetases were induced by wounding and methyl jasmonic acid (MeJA), which may be related to sesquiterpene biosynthesis. By yeast one-hybrid screening, a ERF transcription factor AsERF1 was identified to interact with the AsTPS1 promoter. Subcellular localization analysis indicated AsERF1 was a nucleus-localized protein. Transient transfection of AsERF1 in leaves of Nicotiana benthamiana significantly enhanced the promoter activation of AsTPS1, suggesting AsERF1 may participate in sesquiterpene biosynthesis by regulating AsTPS1 expression. These data generated in this study provide a foundation for future studies on functional roles and regulation mechanisms of AsTPS in sesquiterpene biosynthesis and agarwood formation.

Genetic basis of high aroma and stress tolerance in the oolong tea cultivar genome

Tea plants (Camellia sinensis) are commercially cultivated in >60 countries, and their fresh leaves are processed into tea, which is the most widely consumed beverage in the world. Although several chromosome-level tea plant genomes have been published, they collapsed the two haplotypes and ignored a large number of allelic variations that may underlie important biological functions in this species. Here, we present a phased chromosome-scale assembly for an elite oolong tea cultivar, "Huangdan", that is well known for its high levels of aroma. Based on the two sets of haplotype genome data, we identified numerous genetic variations and a substantial proportion of allelic imbalance related to important traits, including aroma- and stress-related alleles. Comparative genomics revealed extensive structural variations as well as expansion of some gene families, such as terpene synthases (TPSs), that likely contribute to the high-aroma characteristics of the backbone parent, underlying the molecular basis for the biosynthesis of aroma-related chemicals in oolong tea. Our results uncovered the genetic basis of special features of this oolong tea cultivar, providing fundamental genomic resources to study evolution and domestication for the economically important tea crop.

Biochemical Characterization and Function of Eight Microbial Type Terpene Synthases from Lycophyte Selaginella moellendorffii

Selaginella moellendorffii is a lycophyte, a member of an ancient vascular plant lineage. Two distinct types of terpene synthase (TPS) genes were identified from this species, including S. moellendorffii TPS genes (SmTPSs) and S. moellendorffii microbial TPS-like genes (SmMTPSLs). The goal of this study was to investigate the biochemical functions of SmMTPSLs. Here, eight full-length SmMTPSL genes (SmMTPSL5, -15, -19, -23, -33, -37, -46, and -47) were functionally characterized from S. moellendorffii. Escherichia coli-expressed recombinant SmMTPSLs were tested for monoterpenes synthase and sesquiterpenes synthase activities. These enzymatic products were typical monoterpenes and sesquiterpenes that have been previous shown to be generated by typical plant TPSs when provided with geranyl diphosphate (GPP) and farnesyl diphosphate (FPP) as the substrates. Meanwhile, SmMTPSL23, -33, and -37 were up-regulated when induced by alamethicin (ALA) and methyl jasmonate (MeJA), suggesting a role for these genes in plants response to abiotic stresses. Furthermore, this study pointed out that the terpenoids products of SmMTPSL23, -33, and -37 have an antibacterial effect on Pseudomonas syringae pv. tomato DC3000 and Staphylococcus aureus. Taken together, these results provide more information about the catalytic and biochemical function of SmMTPSLs in S. moellendorffii plants.

Enhancing Salt Tolerance of Plants: From Metabolic Reprogramming to Exogenous Chemical Treatments and Molecular Approaches

Plants grow on soils that not only provide support for root anchorage but also act as a reservoir of water and nutrients important for plant growth and development. However, environmental factors, such as high salinity, hinder the uptake of nutrients and water from the soil and reduce the quality and productivity of plants. Under high salinity, plants attempt to maintain cellular homeostasis through the production of numerous stress-associated endogenous metabolites that can help mitigate the stress. Both primary and secondary metabolites can significantly contribute to survival and the maintenance of growth and development of plants on saline soils. Existing studies have suggested that seed/plant-priming with exogenous metabolites is a promising approach to increase crop tolerance to salt stress without manipulation of the genome. Recent advancements have also been made in genetic engineering of various metabolic genes involved in regulation of plant responses and protection of the cells during salinity, which have therefore resulted in many more basic and applied studies in both model and crop plants. In this review, we discuss the recent findings of metabolic reprogramming, exogenous treatments with metabolites and genetic engineering of metabolic genes for the improvement of plant salt tolerance.

Isolation and functional characterization of four microbial type terpene synthases from ferns

Typical plant terpene synthases (TPSs) are responsible for the production of terpenes, a major class of plant secondary metabolites. However, various nonseed plants also harbor genes encoding microbial terpene synthase-like (MTPSL) enzymes. Here, a scan of 31 ferns transcriptomes revealed 40 sequences putatively encoding MTPSLs. Two groups of sequences were recognized based on the key conserved motifs. Four representative genes were isolated from each of the four species Adiantum capillus-veneris, Cyclosorus parasiticus, Drynaria bonii and Microlepia platyphylla. Following their heterologous expression in E. coli, the recombinant proteins were tested for monoterpene synthase and sesquiterpene synthase activity. These enzymatic products were typical monoterpenes and sesquiterpenes that have been previous shown to be generated by classical plant TPSs when provided with GPP and FPP as substrates. Subcellular localization experiments in the leaf epidermis of Nicotiana benthamiana and onion (Allium cepa) inner epidermal cells indicated that AcMTPSL1 and DbMTPSL were deposited in both the cytoplasm and nucleus, whereas CpMTPSL1 and MpMTPSL were localized in the cytoplasm, chloroplasts and nucleus. AcMTPSL1 was up-regulated in plants exposed to methyl jasmonate treatment, suggesting a role for this gene in host defense. This study provides more information about the catalytic function of MTPSLs in nonseed plants and for the first time, the subcellular localization of MTPSLs was experimentally characterized.

Functional Characterization of a Dendrobium officinale Geraniol Synthase DoGES1 Involved in Floral Scent Formation

Floral scent is a key ornamental trait that determines the quality and commercial value of orchids. Geraniol, an important volatile monoterpene in orchids that attracts pollinators, is also involved in responses to stresses but the geraniol synthase (GES) responsible for its synthesis in the medicinal orchid Dendrobium officinale has not yet been identified. In this study, three potential geraniol synthases were mined from the D. officinale genome. DoGES1, which was localized in chloroplasts, was characterized as a geraniol synthase. DoGES1 was highly expressed in flowers, especially in petals. DoGES1 transcript levels were high in the budding stage of D. officinale flowers at 11:00 a.m. DoGES1 catalyzed geraniol in vitro, and transient expression of DoGES1 in Nicotiana benthamiana leaves resulted in the accumulation of geraniol in vivo. These findings on DoGES1 advance our understanding of geraniol biosynthesis in orchids, and lay the basis for genetic modification of floral scent in D. officinale or in other ornamental orchids.

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.

Genome-Wide Identification and Expression Profile of TPS Gene Family in Dendrobium officinale and the Role of DoTPS10 in Linalool Biosynthesis

Terpene synthase (TPS) is a critical enzyme responsible for the biosynthesis of terpenes, which possess diverse roles in plant growth and development. Although many terpenes have been reported in orchids, limited information is available regarding the genome-wide identification and characterization of the TPS family in the orchid, Dendrobium officinale. By integrating the D. officinale genome and transcriptional data, 34 TPS genes were found in D. officinale. These were divided into four subfamilies (TPS-a, TPS-b, TPS-c, and TPS-e/f). Distinct tempospatial expression profiles of DoTPS genes were observed in 10 organs of D. officinale. Most DoTPS genes were predominantly expressed in flowers, followed by roots and stems. Expression of the majority of DoTPS genes was enhanced following exposure to cold and osmotic stresses. Recombinant DoTPS10 protein, located in chloroplasts, uniquely converted geranyl diphosphate to linalool in vitro. The DoTPS10 gene, which resulted in linalool formation, was highly expressed during all flower developmental stages. Methyl jasmonate significantly up-regulated DoTPS10 expression and linalool accumulation. These results simultaneously provide valuable insight into understanding the roles of the TPS family and lay a basis for further studies on the regulation of terpenoid biosynthesis by DoTPS in D. officinale.