Steviol glycosides targeted analysis in leaves of Stevia rebaudiana (Bertoni) from plants cultivated under chilling stress conditions

Improvement and regulation of steviol glycoside biosynthesis in Stevia rebaudiana Bertoni

Stevia rebaudiana Bertoni is a natural sweetener plant that is progressively used not only for its sweetening properties but also for its medicinal properties. The plant contains steviol glycoside (SG) which is reported to be up to 300 times sweeter than sucrose. The plant is said to have no side effects on human health and has been approved by FDA. On the basis of previous studies and available databases, this review discusses the extensive understanding of the different approaches for enhancements of SG in S. rebaudiana. To improve the SG biosynthesis, application of different stress, elicitors, induction of polyploidy, cell culture, genetic engineering, and transcriptomic approaches have been addressed. A brief discussion about the cloning and characterization of important genes of the metabolic pathway of SG biosynthesis is also discussed along with various metabolic engineering pathways viz. methylerythritol 4- phosphate (MEP) and mevalonate (MVA) pathways. This review paper also discusses the different aspects as well as the effects of various nanoparticles on S. rebaudiana growth and development, as well as SG biosynthesis.

Improved drought tolerance of EMS mutagenized Alfalfa (Medicago sativa L.) mutants by in vitro screening at germination stage

The objectives of this study were to determine drought tolerant novel mutant of alfalfa (Medicago sativa L.) genotypes by screening EMS mutagenized 340675 M₃ seeds at germination stages in the presence of osmotic stress of 35% PEG₆₀₀₀. Root growth assay provided several drought tolerant candidate mutants. Of those, 4 mutants were further evaluated at water deficit conditions applied for 24 days after the first cutting at flowering bud stage. The results revealed that mutants determined as drought tolerant at germination stage were also tolerant to water deficit conditions. Protein content and superoxide dismutase values were found to be higher in all mutants than controls. Ascorbate peroxides, glutton reductase and lipid peroxidase values varied based on the mutant genotype and duration of drought stress. Drought stress significantly changed transcriptional levels of MtP5CS, MtDehyd, MseIF-2, MtRD2 and MsNAC genes. These results indicated that in vitro screening of alfalfa mutant seeds for osmatic tolerance at germination and early seedling growth stages was successfully able to determine the drought tolerant alfalfa mutants which were also tolerant to water deficit conditions after the first cutting at flowering bud stage.

Biotechnological interventions of in vitro propagation and production of valuable secondary metabolites in Stevia rebaudiana

Plant cell and tissue culture makes provision of a sustainable and nature-friendly strategy for the production of secondary metabolites, and modern progress in gene editing and genome engineering provides novel possibilities to improve both the qualitative and quantitative aspects of such phytochemicals. The ever-expanding quest for plant-based medicine to treat diabetes facilitates large-scale cultivation of Stevia rebaudiana to enhance the yield of its much-coveted low-calorie sweetener glycosides. The potential to process stevia as a "natural" product should enhance the acceptance of steviosides as a natural calorie-free sweetener especially suitable for use in diabetic and weight control drinks and foods. Besides sweetener agents, S. rebaudiana is a potent source of many antioxidant compounds and is used to cure immunodeficiencies, neurologic disorders, inflammation, diabetes mellitus, Parkinson's disease, and Alzheimer's disease. This comprehensive review presents the research outcomes of the many biotechnological interventions implicated to upscale the yield of steviol glycosides and its derivatives in in vitro cell, callus, tissue, and organ cultures with notes on the use of bioreactor and genetic engineering in relation to the production of these valuable compounds in S. rebaudiana. KEY POINTS: • Critical and updated assessment on sustainable production of steviol glycosides from Stevia rebaudiana. • In vitro propagation of S. rebaudiana and elicitation of steviol glycosides production. • Genetic fidelity and diversity assessment of S. rebaudiana using molecular markers.

Influence of calcium and magnesium elimination on plant biomass and secondary metabolites of Stevia rebaudiana Bertoni

This study reports the increment in the secondary metabolites in Stevia rebaudiana plant after exposure to the elimination of Ca and Mg from Murashige and Skoog culture medium. The effect of nutrient stress on regenerants of S. rebaudiana is measured, which reveals significantly enhanced growth parameters, steviol glycosides (SGs) content, and nonenzymatic antioxidants; total phenolic content, total flavonoid content, total antioxidant capacity, total reducing power, and DPPH-free radical scavenging activity as compared with the control treatment. However, significantly highest amounts are obtained in a medium with only Ca deficiency. The amount of rebaudioside A (Reb A) and stevioside (ST) obtained in the case of Ca-deficient medium is 4.08 and 0.69%, respectively. It is followed by the results obtained from both Ca- and Mg-deprived medium [Reb A (3.23%) and ST (0.52%)] and the lowest values are obtained from medium lacking Mg only [Reb A (2.60%) and ST (0.40%)]. The most probable adaptation mechanism might be the production of reactive oxygen species by nutrients' stress, which results in secondary metabolites production as defensive moieties to overcome stress situation. This effective protocol needs to be refined to apply on an industrial scale in bioreactors for increasing quantities of commercially important pharmaceutical compounds.

Modelling of multilinear gradient retention time of bio-sweetener rebaudioside A in HPLC analysis

Artificial neural network (ANN), as one of the artificial intelligence methods, has been widely using in HPLC studies for modelling purposes. Stevia rebaudiana is an important industrial plant due to its bio-sweetener molecule, rebaudioside-a, in its leaves. Although rebaudioside-a is up to 300-fold sweeter than sucrose, its calorie is almost zero. In this study, HPLC optimization of rebaudioside-a was studied and the optimization data based on multilinear gradient retention times were modelled by ANN. The input parameters were selected as concentrations, column temperatures, initial acetonitrile percentage for the first step of gradient elution, initial acetonitrile percentage for the second step of gradient elution, slope of acetonitrile, wavelengths, flow rates. The retention time was the output. Also, dried S. rebaudiana leaves were extracted and the concentrations were evaluated by HPLC. According to the ANN results, the most effective parameters on the prediction of non-linear gradient retention time for rebaudioside-a were found as flow rate and initial acetonitrile percentage for the second step of gradient. The best back propagation was selected as Levenberg-Marquardt algorithm. The highest rebaudioside-a level was found as 96.53 ± 6.36 μg mL^-1. ANN modelling methods can be used in preparative HPLC applications to estimate the retention time of steviol glycosides.

LC-ESI/LTQ-Orbitrap-MS Based Metabolomics in Evaluation of Bitter Taste of Arbutus unedo Honey

Strawberry tree honey is a high-value honey from the Mediterranean area and it is characterised by a typical bitter taste. To possibly identify the secondary metabolites responsible for the bitter taste, the honey was fractionated on a C18 column and the individual fractions were subjected to sensory analysis and then analysed by liquid chromatography coupled with high-resolution tandem mass spectrometry in negative ion mode, using a mass spectrometer with an electrospray source coupled to a hybrid high resolution mass analyser (LC-ESI/LTQ-Orbitrap-MS). A chemometric model obtained by preliminary principal component analysis (PCA) of LC-ESI/LTQ-Orbitrap-MS data allowed the identification of the fractions that caused the perception of bitterness. Subsequently, a partial least squares (PLS) regression model was built. The studies carried out with multivariate analysis showed that unedone (2-(1,2-dihydroxypropyl)-4,4,8-trimethyl-1-oxaspiro [2.5] oct-7-en-6-one) can be considered responsible for the bitter taste of strawberry tree honey. Confirmation of the bitter taste of unedone was obtained by sensory evaluation of a pure standard, allowing it to be added to the list of natural compounds responsible for giving the sensation of bitterness to humans.

Greenhouse evaluation of branching, leaf yield and biochemical compositions of Stevia rebaudiana Bertoni to decapitation and foliar application of abscisic acid and fluridone

Stevia is a herbaceous plant containing non-sugar sweeteners that could be regarded as a successor to sugar for diabetics. However, there are some problems with the cultivation of stevia, especially in the greenhouse, including unbranching and low sweetening agents. To overcome this issue, an experiment was designed to increase not only the branching and leaf production but also the sweetness. Therefore, a novel strategy using abscisic acid (0, 50, 100, 150 μM), its inhibitor, i.e. fluridone (0, 50, 100, 150 μM) and decapitation of plant apical meristems was applied. Results showed that when stevia was decapitated, dormant buds responded to the application of abscisic acid and fluridone. Under these conditions, axillary buds were developed to branches. As well leaf, total dry weight, soluble sugars and steviol glycosides (SGs) were significantly increased. In addition, the interaction of abscisic acid (50 and 100 μM) and fluridone (50 μM) had the highest positive effects on plant growth and steviol glycosides production rather than their sole applications. Results also indicated that decapitation removed a terminal dominance over a limited period of time and the terminal dominance was re-established with the growth of the terminal branches, whereas the influence of 50 µM fluridone on stevia was long term and the number of shoots was greater. Since stevia is a costly sweetener, the results of this study could be used in greenhouses, where the cultivation of stevia seems to be reasonable in terms of economic aspects.

Membrane separation processes for the extraction and purification of steviol glycosides: an overview

Steviol glycosides (SGs), as natural sweeteners from Stevia rebaudiana, are currently employed for replacing sugar and its derivatives in several food products and formulations. Such compounds play an essential role in human health. Their usage provides a positive effect on preventing diseases related to sugar consumption, including diabetes mellitus, cancer, and lipid metabolism disorders. The traditional extraction of SGs is performed by means of solvent extraction, which limits their application since the removal of residual solvents is a challenging task requiring further downstream purification steps. In addition, the presence of residual solvents negatively affects the quality of such compounds. Today, food technicians are looking for innovative and improved techniques for the extraction, recovery and purification of SGs. Membrane-based technologies, including microfiltration, ultrafiltration, and nanofiltration, have long been proven to be a valid alternative for efficient extraction and purification of several high added-value molecules from natural sources. Such processes and their possible coupling in integrated membrane systems have been successfully involved in recovery protocols of several compounds, such as metabolites, polyphenols, anthocyanins, natural pigments, proteins, from different sources (e.g., agro-food wastes, plant extracts, fruits, fermentation broths, among others). Herein, we aim to review the current progresses and developments about the extraction of SGs with membrane operations. Our attention has been paid to the latest insights in the field. Furthermore, key process parameters influencing the extraction and purification of SGs are also discussed in detail.

Stevia residue as new precursor of CO2-activated carbon: Optimization of preparation condition and adsorption study of triclosan

The present work reports the preparation of CO₂-activated carbon (AC) using Stevia rebaudiana (Bertoni) residue as a new carbon precursor. The experimental parameters were optimized via chemometrics tools to obtain an AC with high BET surface area (S_BET). The found optimum condition was: activation temperature of 900 °C, CO₂ flow of 165 cm³ g^-1 and activation time of 60 min, providing an AC_op with S_BET of 874 m² g^-1. The AC_op was characterized from several analytical techniques, which showed that it has heterogeneous morphology features and different surface chemical groups, predominating the acidic character. The adsorption performance of AC_op for triclosan (TCS) removal from solution was investigated by kinetic, equilibrium and thermodynamic studies. The results showed that TCS adsorption process onto AC_op is spontaneous and endothermic, wherein the mechanism occurs by different steps, which equally play important roles. Additionally, the monolayer adsorption capacity (Q_m) was found to be 117.00 mg g^-1.

Determination of steviol glycosides in commercial extracts of Stevia rebaudiana and sweeteners by ultra-high performance liquid chromatography Orbitrap mass spectrometry

Stevia rebaudiana extracts are used as sweeteners in several countries worldwide. Several extracts of diverse composition are available on the market, and their taste depends on the contents of the various steviol glycosides. This study presents an accurate method for the qualitative and quantitative determination of steviol glycosides in 40 Stevia extracts, 7 sweeteners and 3 Stevia-sweetened beverages by a UHPLC coupled to an Orbitrap mass spectrometer. The sub-2 μm amide column provided the separation of all the target analytes in a run time of 30 min with high resolution. The effect of different eluent compositions on the ionisation efficiency of the steviol glycosides was studied. The optimal ionisation conditions were achieved in negative mode using 0.05% formic acid. Under this condition, adducts were not found, [M-H]^- were the main ions and the spontaneous loss of a glucose residue at C19 was reduced. The %RSD for intra- and inter-day precision for all eleven analytes varied from 2.1 to 4.2% and 3.0-5.1%, respectively. The recoveries from spiked Stevia extract samples were greater than 95% for all analytes. Rebaudioside A was the most abundant, ranging from 23 to 102%. Nine Stevia extracts and one drink were not compliant with the European Regulation. Isosteviol was under the LOD in all samples and steviol was found in four samples in quantities in the range 0.01-0.03%.

Effect of Cold Stress on Photosynthetic Traits, Carbohydrates, Morphology, and Anatomy in Nine Cultivars of Stevia rebaudiana

Stevia rebaudiana Bertoni is a sweet medicinal herb that is cultivated worldwide. This study aimed to identify the genotypic responses and function of nine cultivars of S. rebaudiana (accession numbers 1-9 from the EUSTAS Stevia Gene Bank) to low temperature. Plants were grown in vitro and incubated under controlled conditions at 5° or 25°C for 1 month. Cold stress significantly decreased the maximum quantum yield of photosystem II (Fv/Fm) in all cultivars, which was more pronounced in cultivars 5, 6, 8, and 9. The efficiency of photosystems I and II (PIABS) also declined in cold-stressed plants and was accompanied by reductions in net photosynthesis (PN), intercellular CO2 (Ci), water use efficiency (WUE), and chlorophyll a, chlorophyll b and carotenoid contents, more so in cultivars 5, 6, 8, and 9. Regardless of the downregulation of photosynthetic capacity, the cold stress increased water-soluble carbohydrates in all cultivars, which was accompanied by an increase in fresh leaf mass and area, more so in cultivars 5, 6, 8, and 9. Furthermore, cold stress increased the stomatal index and density, epidermal cell density, stem diameter, xylem vessel width, phloem tissue width, and number of sclerenchyma in all cultivars. Even though the nine cultivars of S. rebaudiana had lower PSII efficiencies at low temperatures, the increase in carbohydrates and leaf mass suggests that damage to PSII is not responsible for the reduction in its efficiency.

Beaming steviol glycoside analysis into the next dimension

Nine state-of-the-art reversed phase (RP) columns for ultra-high performance liquid chromatography were tested for the separation of steviol glycosides. The main criteria were resolution of the critical peak pair rebaudioside A and stevioside and the retention time of rebaudioside D. Three columns yielded a resolution of 2 or more of the critical peak pair and two of them showed sufficient retention of rebaudioside D, namely 1.62 and 1.84min corresponding to retention factors of 0.98 and 1.24. The separation of nine steviol glycosides was possible in 11min with UV and MS compatible, buffer-free eluents at moderate temperature. The presented method is proposed to be adopted as a new official method.

Abiotic stress of ZnO‐PEG, ZnO‐PVP, CuO‐PEG and CuO‐PVP nanoparticles enhance growth, sweetener compounds and antioxidant activities in shoots of Stevia rebaudiana Bertoni

Nanoparticles are known to play remarkable role as abiotic stress elicitors in plants. This study reports the comparative analysis of effects produced by capped [zinc oxide (ZnO)‐polyethylene glycol (PEG), ZnO‐polyvinyl pyrrolidone (PVP), copper oxide (CuO)‐PEG, CuO‐PVP] and uncapped (ZnO and CuO) nanoparticles on the medicinal plant, Stevia rebaudiana raised in vitro for the production of commercially important sweetener compounds. In context of shoot organogenesis, ZnO‐PEG, ZnO‐PVP, CuO‐PEG, CuO‐PVP were employed to the growth medium that resulted in increased growth parameters, and larger content of steviol glycosides as compared to the shoots raised in medium containing ZnO and CuO, revealed by high‐performance liquid chromatography. In the meanwhile, non‐enzymatic antioxidant activities including total phenolic content, total flavonoid content, total antioxidant capacity, total reducing power and 2,2‐diphenyl‐1‐picryl hydrazyl‐free radical scavenging activity were calculated and showed comparatively greater amounts in shoots grown in medium containing capped ZnO or CuO nanoparticles. Furthermore, the ZnO and its derivatives revealed to be more reactive at 1 mg/l of concentration. Whereas, the CuO and its derivatives produced greater response on Stevia at 10 mg/l concentration of nanoparticles. This study paves the way for more such studies encompassing capped and uncapped nanoparticles and their ultimate effect on in‐vitro grown plant tissues for the production of active metabolites on industrial scale.

Targeted and untargeted mass spectrometric approaches in discrimination between Myrtus communis cultivars from Sardinia region

In the present study, the discrimination of phytochemical content of Myrtus communis berries from different geographical origin and cultivars was explored by Liquid Chromatography-Electrospray Ionization-Fourier Transform-Mass Spectrometry (LC-ESI-FT-MS) metabolic profiling and quantitative analysis. Experiments were carried on myrtle plants grown in an experimental area of Sardinia region, obtained by the germination of seeds taken from berries collected in each part of the region. A preliminary untargeted approach on fruit's extracts was realized by collecting LC-ESI-FT-(Orbitrap)-MS data obtained by operating in negative ion mode and performing principal component analysis with the result of differentiation of samples. In a second step, targeted analysis with a reduced number of variables was realized. A data matrix was obtained by the data fusion of positive and negative ionization LC-ESI-MS results, by using as variables the peak areas of each known compounds. By the observation of principal component analysis, results found that anthocyanins, and mainly derivatives of cyanidin, are the principal marker compounds responsive for the discrimination of samples based on the geographical origin of the seeds. Based on this finding, finally, an LC-diode array detector method was developed, validated and applied for the quantitative analysis of berries' extracts based on 11 commercial standard compounds corresponding to the identified markers. Copyright © 2016 John Wiley & Sons, Ltd.