Metabolic profiling of tobacco leaves at different growth stages or different stalk positions by gas chromatography–mass spectrometry

Application of PLSR in correlating sensory and chemical properties of middle flue-cured tobacco leaves with honey-sweet and burnt flavour

Among the eight types of aroma and flavour characteristics of Chinese flue-cured tobacco (FCT), tobacco grown in Shandong is classified as having a honey-sweet and burnt aroma. To elucidate the key chemical components that determine the characteristics of the honey-sweet and burnt aroma styles of Shandong FCT, we qualitatively and quantitatively evaluated the smoke flavour quality and five categories of flavour-related chemical components (routine components, water-soluble sugars, free amino acids, Amadori compounds and key aroma-active compounds) in Shandong middle FCT leaves using sensory analysis and modern instrumental analysis techniques. The association between the chemical components and sensory quality was analysed. Our results showed that the total sugars, reducing sugars (fructose, glucose, and psicose), total sugar-nicotine ratio, proline-total amino acid ratio, sulphur-containing amino acid-total amino acid ratio and fructosyl-proline (Fru-Pro) were high in premium FCT leaves. The aroma-active compounds associated with the honey-sweet burnt flavour style of the Shandong Middle FCT included sweet-scented 2,3-pentanedione, 2,3-butanedione, butyrolactone, 2-furanmethanol, roasted-like 2-pentylfura, and green-like 1-penten-3-one. Partial least squares regression (PLSR) analysis revealed that 29 aroma precursors were positively correlated with the sensory quality characteristics of Shandong FCT. The results of our study can provide guidance for the targeted improvement and precise regulation of the flavour-style characteristics of Shandong FCT.

Gas Chromatography-Mass Spectrometry (GC-MS) Metabolite Profiling of Citrus limon (L.) Osbeck Juice Extract Evaluated for its Antimicrobial Activity Against Streptococcus mutans

Purpose The present study aimed to determine the antimicrobial nature of Citrus limon juice extract against Streptococcus mutans and to identify its metabolic profile by gas chromatography-mass spectrometry (GC-MS) technique. Materials and methods The cariogenic bacteria S. mutans were procured from Microbial Type Culture Collection and Gene Bank (MTCC), Chandigarh, India, and revived on brain heart infusion (BHI) agar. The C. limon (L.) Osbeck fruits were authenticated from the University of Trans-Disciplinary Health Sciences and Technology (TDU), Bengaluru, India. The antibacterial property of lyophilized lemon juice extract (LJE) dissolved in methanol was evaluated against S. mutans (MTCC 497) by the agar well diffusion assay. GC-MS technique was employed to identify the volatile metabolite profile of the methanolic LJE sample. The metabolite masses of the respective compounds were identified using the National Institute of Standards and Technology (NIST) library. Results The methanolic LJE sample concentration from 5 to 25 mg/ml did not demonstrate antimicrobial activity, while 30 to 100 mg/ml displayed antibacterial activity against S. mutans. Chlorohexidine (100 µg/ml) was used as the positive control, while methanol was used as the negative control. Significant antimicrobial metabolites were detected in the methanolic LJE sample by GC-MS analysis. Maleic anhydride, succinic anhydride, 6-Oxa-bicyclo[3.1.0] hexan-3-one, and 3-methyl-2,5-Furandione were the key metabolites identified in the methanolic LJE sample.  Conclusion The present study reports that C. limon is a potential candidate for the pharmaceutical industry as it possesses bioactive compounds demonstrating therapeutic properties. Further investigations are warranted to determine the individual and synergetic effects of identified metabolites in the methanolic LJE sample for its antimicrobial property. Special attention needs to be focussed on translational research for the development of anti-caries products from C. limon.

Metabolomics and proteomics revealed the synthesis difference of aroma precursors in tobacco leaves at various growth stages

Tobacco has a high economic value as the largest cash crop worldwide. The quality of flue-cured tobacco is closely related to the overall status of compounds in fresh tobacco leaves, and the aroma precursor plays a key role in the aroma quality of flue-cured tobacco. The untargeted metabolomics and label-free quantitative proteomics analysis of tobacco leaves in three growth stages (root stretching, prosperous growth, and maturation) retrieved 243 metabolites and 4313 proteins (944 differentially expressed proteins), which showed that carbohydrate, amino acid, and fatty acid metabolism varies among the three growth stages. Also, the most of amino acids, organic acids, fatty acids, and polyphenols reduced in the vegetative growth stage, while increased in the reproductive growth stage. On the other hand, alkaloids such as nicotine, nornicotine, and anatabine increased continuously in tobacco leaves during the three growth stages. This study helps us understand the growth and development characteristics of Yun87 flue-cured tobacco in the field before harvest, and it provides a certain omics basis for the industrial crop flue-cured tobacco.

Simultaneous quantification of nicotine salts in e-liquids by LC-MS/MS and GC-MS

Nicotine salts, formed by nicotine and organic acids, are commonly added to electronic cigarette liquids for their ability to provide desirable sensory effects. Analytical strategies have been developed to detect the types of organic acids and nicotine levels, but methods for directly measuring nicotine salts are still desirable. Herein, a novel approach is presented for the simultaneous quantification of non-volatile and volatile nicotine salts via liquid chromatography/tandem mass spectroscopy (LC-MS/MS) and gas chromatography/mass spectroscopy (GC-MS). This approach was validated with recovery experiments, which yielded recovery values between 92.0% and 110.8%. This method is the first technique for quantifying multiple nicotine salts that could be present in commercial e-liquids. Without using derivatization steps, different nicotine salts could be detected rapidly and conveniently. This new method was demonstrated with 10 e-cigarette liquid samples, providing satisfactory outcomes. It could be used to study organic acids and protonated nicotine in e-liquids and the release behaviour of nicotine salts in electronic cigarettes.

Dose differentiation in elevated UV-B manifests variable response of carbon-nitrogen content with changes in secondary metabolites of Curcuma caesia Roxb

Despite acting as environmental stress, UV-B also plays a regulatory role in the plant's growth and secondary metabolism. UV-B-induced changes show variations between and among the species. The present study mainly focuses on variations in carbon and nitrogen contents and their relation with the phytochemical constituents of Curcuma caesia exposed to two different doses of UV-B (ambient ± elevated UV-B for 1 h (2.4 kJ m^-2 day^-1) and 2 h (4.8 kJ m^-2 day^-1)) under natural field conditions. Results showed that increasing the dose of eUV-B leads to high tuber biomass and reduced rhizome biomass (the medicinally important part). Increased expression of compounds at the initial rhizome formation stage might be due to the increased carbon content, whereas no such trend was found at the final growth or rhizome maturation stage. After final harvesting, carbon content was reduced, with an increase of nitrogen content which might be responsible for enhanced production of major components of essential oil (D-camphor and 1,8-cineole) in 2 h of UV-B exposure followed by 1 h. The phytochemical analysis at the final stage showed induction of compounds (15 and 10 in 1 h and 2 h, respectively) after UV-B exposure which was not detected in controls. The present study suggests that the change in carbon-nitrogen played an important role in the fraction of compounds at different stages, and a lower dose of UV-B (1 h) favoured the increased production of essential oil; however, 2 h dose is important for the enhanced production of major active compounds of essential oil.

Proteomic and Metabolomic Revealed Differences in the Distribution and Synthesis Mechanism of Aroma Precursors in Yunyan 87 Tobacco Leaf, Stem, and Root at the Seedling Stage

Tobacco, as an important cash crop and model plant, has been the subject of various types of research. The quality of flue-cured tobacco products depends on the compound collection of tobacco leaves, including pigments, carbohydrates, amino acids, polyphenols, and alkaloid aroma precursors. The present study investigates tobacco seedling organs (leaf, stem, and root) with the assistance of label-free proteomic technology and untargeted metabonomic technology. We analyzed 4992 proteins and 298 metabolites obtained in the leaf, stem, and root groups and found that there were significant differences in both primary and secondary metabolism processes involved in aroma precursor biosynthesis, such as carbohydrate metabolism, energy metabolism, and amino acid biosynthesis, and phenylpropanoid, flavonoid, and alkaloid biosynthesis. The findings showed that the contents of alkaloid metabolites such as nornicotine, anatabine, anatalline, and myosmine were significantly higher in tobacco roots than in leaves and stems at the seedling stage.

Application of Chromatographic Technology to Determine Aromatic Substances in Tobacco during Natural Fermentation: A Review

Flavor is an important index to evaluate the sensory quality of tobacco. The process of fermentation is a key step in the production of aromatic substances in tobacco leaves and an important factor in improving their quality. Worldwide, reams of research show that chromatographic technology plays an irreplaceable role in the tobacco aromatic chemistry. Nevertheless, the degradation mechanism of latent aromatic compounds and the formation mechanism of characteristic aromatic substances have not been fully and systematically elucidated. In this study, the latest progress of basic methods, techniques, and research results of the separation, analysis, and identification of aromatic substances in fermented tobacco leaves were reviewed, and the next research and application directions were prospected. It is expected to provide theoretical reference for the study of molecular mechanism of tobacco flavor, reveal the degradation mechanism of potential aroma compounds, and help improve the quality of tobacco.

Correlation between Irradiation Treatment and Metabolite Changes in Bactrocera dorsalis (Diptera: Tephritidae) Larvae Using Solid-Phase Microextraction (SPME) Coupled with Gas Chromatography-Mass Spectrometry (GC-MS)

The metabolites produced by the larvae of Bactrocera dorsalis (Diptera: Tephritidae) exposed to different doses of irradiation were analyzed using solid phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS), and a metabonomic analysis method of irradiated insects based on GC-MS was established. The analysis revealed 67 peaks, of which 23 peaks were identified. The metabolites produced by larvae treated with different irradiation doses were compared by multivariate statistical analysis, and eight differential metabolites were selected. Irradiation seriously influenced the fatty acid metabolic pathway in larvae. Using the R platform combined with the method of multivariate statistical analysis, changes to metabolite production under four irradiation doses given to B. dorsalis larvae were described. Differential metabolites of B. dorsalis larvae carried chemical signatures that indicated irradiation dose, and this method is expected to provide a reference for the detection of irradiated insects.

Rapid quantification of alkaloids, sugar and yield of tobacco (Nicotiana tabacum L.) varieties by using Vis-NIR-SWIR spectroradiometry

Tobacco genetic improvement programs, as well as the tobacco industry, require techniques that allow the estimation of its attributes in a fast and cheap way. The use of remote sensing through visible, near infrared and short-wave spectroscopy (Vis-NIR-SWIR) has been studied aiming to meet such demand. Thus, the aim of this work was to evaluate the use of Vis-NIR-SWIR spectroradiometer as a rapid tool to estimate alkaloids, sugars and yield of tobacco varieties. For that purpose, a study was carried out in a greenhouse with plants grown in pots (18 dm-3) containing nutrient solutions. The experimental design was completely randomized, with 30 treatments (tobacco varieties) and 10 repetitions. Tobacco leaf reflectance was collected at 13, 34 and 68 days after transplantation (DAT) with a plant-probe device connected to the spectroradiometer by an optical fiber. Subsequently, leaf analysis of alkaloids, sugars and yield were performed, and such attributes were estimated by using the Partial Least Squares Regression (PLSR), combined with the following pre-processing (PP) techniques: multiplicative scatter correction (MSC), Savitzky-Golay (SG) and standard normal variate (SNV). The results showed presence of typical inflections of chemical and structural components of the plants, which allowed obtaining PLSR models with R2p and RPDp superior to 0.71 and 2.27, respectively, for all PP techniques and attributes evaluated. The most important wavelengths were well distributed within the three operating ranges of the spectroradiometer (Vis-NIR-SWIR). Thus, the methodology proposed by this research was able to simultaneously determine all the three attributes (alkaloids, sugars and yield) with excellent predictive capacity. This is a promising result for genetic improvement and processing of tobacco (as well as other crops), since it is necessary to evaluate a large number of samples within a short period and at a low cost.

The chemotype core collection of genus Nicotiana

Sustainable production of chemicals and improving these biosources by engineering metabolic pathways to create efficient plant-based biofactories relies on the knowledge of available chemical/biosynthetic diversity present in the plant. Nicotiana species are well known for their amenability towards transformation and other new plant breeding techniques. The genus Nicotiana is primarily known through Nicotiana tabacum L., the source of tobacco leaves and all respective tobacco products. Due to the prevalence of the latter, N. tabacum and related Nicotiana species are one of the most extensively studied plants. The majority of studies focused solely on N. tabacum or other individual species for chemotyping. The present study analysed a diversity panel including 17 Nicotiana species and six accessions of Nicotiana benthamiana and created a data set that effectively represents the chemotype core collection of the genus Nicotiana. The utilisation of several analytical platforms and previously published libraries/databases enabled the identification and measurement of over 360 metabolites of a wide range of chemical classes as well as thousands of unknowns with dedicated spectral and chromatographic properties.

Geographic and position-based variations in phyllospheric bacterial communities present on flue-cured tobacco

Although tobacco leaves (TLs) contain abundant bacteria, how the geography and leaf position of TLs affect these bacteria is unclear. Here, TLs at different positions from Henan (HN, strong flavor style) and Yunnan (YN, fresh flavor style) provinces were collected, and the bacteria were characterized by Illumina sequencing at harvest and 1 year of storage. Bacterial communities were very different between TLs originating from different geographical areas and positions, and beta diversity analysis showed that leaf position was the most important factor for phyllospheric bacterial communities, followed by geographical area and storage time. At the genus level, Subdoligranulum, Thermus, and Acinetobacter were obviously more abundant in HN than in YN, while Blautia and Ruminococcus were significantly more abundant in YN. These differences in bacterial communities decreased after 1 year of storage, indicating that the microbiota tends to become similar during tobacco processing. Storage time also affected the phyllospheric bacteria of TLs, as the bacterial communities shifted significantly on both HN and YN TLs after 1 year of storage. Significant differences in the predicted genes were also observed between the different geographic locations and leaf positions. Potential human pathogens, including Acinetobacter, Methylobacterium, and Escherichia-Shigella, were greatly different between TLs originating from different areas and positions. These data suggested that geographic variations and positions were associated with phyllospheric bacterial communities on TLs, which may be related to not only the flavor style and quality of TLs but also the potential health risks to humans. KEY POINTS: • Tobacco leaf position and tobacco growth location affected bacterial communities. • Microbial communities of TLs shifted significantly after one year of storage. • Potential human pathogens differed at different leaf positions and growth locations.

Metabolic changes in leaves of N. tabacum and N. benthamiana during plant development

Dwindling fossil fuel reserves and poor environmental credentials of chemical synthesis means, new renewable sources for the production and manufacture of valuable chemicals and pharmaceuticals are required. Presently, tobacco is an underutilised non-food crop with the potential to act as a biofactory. In this study, metabolite profiling across vegetative development has been carried out to provide a quantitative baseline of metabolites, their formation and interaction. Two tobacco platforms have been used, Nicotiana benthamiana and Nicotiana tabacum. Our data generated has provided the quantitative and qualitative baseline levels for exploitable pathways and metabolites, across two complementary Nicotiana species. N. benthamiana is the chassis of choice for transient expression. The metabolite data obtained for N. benthamiana highlighted that before flower emergence, the increased central carbon metabolism and high amino acid levels are available for the biosynthesis of endogenous or heterologous metabolites. In the future, engineering pathways or biocatalysts into N. benthamiana could add value to the process presently used to produce low volume, high cost pharmaceuticals. Similar outputs were obtained for N. tabacum, which has the advantage of providing a large biomass and hence, high product yield. These data provide an insight into the metabolite pools available in tobacco for future exploitation by emerging New Plant Breeding Techniques.

Metabolic Profiling of Resistant and Susceptible Tobaccos Response Incited by Ralstonia pseudosolanacearum Causing Bacterial Wilt

The causal agent of bacterial wilt, Ralstonia pseudosolanacearum, can cause significant economic losses during tobacco production. Metabolic analyses are a useful tool for the comprehensive identification of plant defense response metabolites. In this study, a gas chromatography-mass spectrometry (GC-MS) approach was used to identify metabolites differences in tobacco xylem sap in response to R. pseudosolanacearum CQPS-1 in two tobacco cultivars: Yunyan87 (susceptible to R. pseudosolanacearum) and K326 (quantitatively resistant). Metabolite profiling 7 days post inoculation with R. pseudosolanacearum identified 88 known compounds, 42 of them enriched and 6 depleted in the susceptible cultivar Yunyan87, while almost no changes occurred in quantitatively resistant cultivar K326. Putrescine was the most enriched compound (12-fold) in infected susceptible tobacco xylem, followed by methyl-alpha-d-glucopyranoside (9-fold) and arabinitol (6-fold). Other sugars, amino acids, and organic acids were also enriched upon infection. Collectively, these metabolites can promote R. pseudosolanacearum growth, as shown by the increased growth of bacterial cultures supplemented with xylem sap from infected tobacco plants. Comparison with previous metabolic data showed that beta-alanine, phenylalanine, and leucine were enriched during bacterial wilt in both tobacco and tomato xylem.

A Widely Metabolomic Analysis Revealed Metabolic Alterations of Epimedium Pubescens Leaves at Different Growth Stages

Epimedium folium is the major medicinally-used organ of Epimedium species and its metabolic changes during the leaf growth have not been studied at the metabolomic level. E. pubescens is one of five recorded species in the Pharmacopoeia of the People’s Republic of China and widely grows in China. A UPLC-ESI-MS/MS-based targeted metabolomic analysis was implemented to explore the metabolite composition in E. pubescens leaves under the cultivation condition and further to investigate their temporal variations among four representative growth stages. A total of 403 metabolites, including 32 hitherto known in Epimedium species, were identified in E. pubescens leaf, of which 302 metabolites showed the growth/development-dependent alterations. Flavonoid-type compounds were the major composition of the metabolites identified in this study. Most flavonoids, together with tannin-type and lignans and coumarin-type compounds, were up-regulated with E. pubescens leaf growth and maturation after the full flowering stage. Our results not only greatly enriched the existing Epimedium phytochemical composition database and also, for the first time, provided the metabolomics-wide information on metabolic changes during E. pubescens leaf growth and development, which would facilitate in the choice of an optimum harvest time to balance a higher biomass yield of Epimedium folium with its better medicinal quality.

Non-target Effects of Hyperthermostable α-Amylase Transgenic Nicotiana tabacum in the Laboratory and the Field

Thermostable α-amylases are important enzymes used in many industrial processes. The expression of recombinant Pyrococcus furiosus α-amylase (PFA) in Nicotiana tabacum has led to the accumulation of high levels of recombinant protein in transgenic plants. The initial steps to registering the transgenic tobacco at a commercial production scale and growing it in the field requires a risk assessment of potential non-target effects. The objective of this study was to assess the effect of feeding on transgenic tobacco with 2 indigenous insect species commonly associated with wild and commercial tobacco involving plants grown and evaluated under laboratory and field conditions. The highest levels of PFA ranged from 1.3 to 2.7 g/kg leaf fresh weight produced in the field-grown cultivars Con Havana and Little Crittenden, respectively. These two cultivars also had the highest nicotine (ranging from 4.6 to 10.9 mg/g), but there was little to no negative effect for either tobacco hornworm Manduca sexta L. or aphid Myzus nicotianae (Blackman). Both laboratory and field trials determined no short term (5 days) decrease in the survival or fecundity of the tobacco aphid after feeding on PFA transgenic tobacco compared to non-transgenic plants. In the field, tobacco hornworm larvae showed no differences in survival, final larval weights or development time to adult stage between transgenic lines of four cultivars and their corresponding wild type controls. Laboratory studies confirmed the field trial results indicating the low risk association of PFA expressed in tobacco leaves with tobacco hornworms and aphids that would feed on the transgenic plants.