Stimulation of steviol glycoside accumulation in Stevia rebaudiana by red LED light

LED light improves shoot multiplication, steviol glycosides and phenolic compounds biosynthesis in Stevia rebaudiana Bertoni in vitro culture

Light-emitting diode (LED) lamps are efficient elicitors of secondary metabolites. To investigate the influence of LED light on steviol glycosides (SGs) and phenolic compounds biosynthesis, stevia shoots were cultured under the following LED lights: white-WL, blue-B, red-R, 70% red and 30% blue-RB, 50% UV, 35% red and 15% blue-RBUV, 50% green, 35% red and 15% blue-RBG, 50% yellow, 35% red and 15% blue-RBY, 50% far-red, 35% red and 15% blue-RBFR and white fluorescent light (WFl, control). RBG light stimulated shoots' biomass production. RBFR had a beneficial impact on stevioside biosynthesis (1.62 mg/g dry weight, DW), while RBUV favoured the production of rebaudioside A (3.15 mg/g DW). Neochlorogenic, chlorogenic, caffeic, 4-feruloylquinic, isochlorogenic A, rosmarinic acids and the flavonoid quercitrin were identified in the obtained material. A stimulatory effect of RBFR and RBUV on the biosynthesis of phenolic compounds was noted. LED light also influenced stomata appearance, stomata density, photosynthetic pigments, soluble sugar content and antioxidant enzyme activities in stevia shoots. This is the first report to provide evidence of the stimulating effect of LED light on biomass yield, SGs production and phenolic compounds in stevia shoot cultures.

Comparison between bacterial bio-formulations and gibberellic acid effects on Stevia rebaudiana growth and production of steviol glycosides through regulating their encoding genes

Stevia rebaudiana is associated with the production of calorie-free steviol glycosides (SGs) sweetener, receiving worldwide interest as a sugar substitute for people with metabolic disorders. The aim of this investigation is to show the promising role of endophytic bacterial strains isolated from Stevia rebaudiana Egy1 leaves as a biofertilizer integrated with Azospirillum brasilense ATCC 29,145 and gibberellic acid (GA3) to improve another variety of stevia (S. rebaudiana Shou-2) growth, bioactive compound production, expression of SGs involved genes, and stevioside content. Endophytic bacteria isolated from S. rebaudiana Egy1 leaves were molecularly identified and assessed in vitro for plant growth promoting (PGP) traits. Isolated strains Bacillus licheniformis SrAM2, Bacillus paralicheniformis SrAM3 and Bacillus paramycoides SrAM4 with accession numbers MT066091, MW042693 and MT066092, respectively, induced notable variations in the majority of PGP traits production. B. licheniformis SrAM2 revealed the most phytohormones and hydrogen cyanide (HCN) production, while B. paralicheniformis SrAM3 was the most in exopolysaccharides (EPS) and ammonia production 290.96 ± 10.08 mg/l and 88.92 ± 2.96 mg/ml, respectively. Treated plants significantly increased in performance, and the dual treatment T7 (B. paramycoides SrAM4 + A. brasilense) exhibited the highest improvement in shoot and root length by 200% and 146.7%, respectively. On the other hand, T11 (Bacillus cereus SrAM1 + B. licheniformis SrAM2 + B. paralicheniformis SrAM3 + B. paramycoides SrAM4 + A. brasilense + GA3) showed the most elevation in number of leaves, total soluble sugars (TSS), and up-regulation in the expression of the four genes ent-KO, UGT85C2, UGT74G1 and UGT76G1 at 2.7, 3.3, 3.4 and 3.7, respectively. In High-Performance Liquid Chromatography (HPLC) analysis, stevioside content showed a progressive increase in all tested samples but the maximum was exhibited by dual and co-inoculations at 264.37% and 289.05%, respectively. It has been concluded that the PGP endophytes associated with S. rebaudiana leaves improved growth and SGs production, implying the usability of these strains as prospective tools to improve important crop production individually or in consortium.

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.

Nano-stevia interaction: Past, present, and future

Nanotechnology has recently been emerged as a transformative technology that offers efficient and sustainable options for nano-bio interface. There has been a considerable interest in exploring the factors affecting elicitation mechanism and nanomaterials have been emerged as strong elicitors in medicinal plants. Stevia rebaudiana is well-known bio-sweetener and the presence of zero calorie, steviol glycosides (SGs) in the leaves of S. rebaudiana have made it a desirable crop to be cultivated on large scale to obtain its higher yield and maximal content of high quality natural sweeteners. Besides, phenolics, flavonoids, and antioxidants are abundant in stevia which contribute to its medicinal importance. Currently, scientists are trying to increase the market value of stevia by the enhancement in production of its bioactive compounds. As such, various in vitro and cell culture strategies have been adopted. In stevia agronanotechnology, nanoparticles behave as elicitors for the triggering of its secondary metabolites, specifically rebaudioside A. This review article discusses the importance of S. rebaudiana and SGs, conventional approaches that have failed to increase the desired yield and quality of stevia, modern approaches that are currently being applied to obtain utmost benefits of SGs, and future needs of advanced technologies for further exploitation of this wonder of nature.

Effect of light intensity on steviol glycosides production in leaves of Stevia rebaudiana plants

Stevia rebaudiana leaf extracts contain stevioside and rebaudioside A, two steviol glycosides (SGs) used as natural sweeteners because of their non-toxic, thermally stable and non-caloric properties. Indeed, leaf extracts can be up to 300 times sweeter than sucrose. Stevioside and rebaudioside A have organoleptic differences, the first one having an undesirable bitterness and the second one a higher sweetener capacity. Selection of the S. rebaudiana varieties and the best environmental conditions that elicit higher SGs content and the appropriate composition is an important goal. In this study we quantified and compared the amount of stevioside and rebaudioside A in two of the most used S. rebaudiana cultivars, Morita II and Criolla. Our results show a strong differential ratio of stevioside and rebaudioside A accumulated in the leaf between these cultivars. The Criolla cultivar showed about 3 times more stevioside per mg of dry weight than Morita II, whereas the Morita II accumulated almost 10 times more rebaudioside A than that produced in Criolla. We observed an enhanced expression in Morita II of three genes (SrKA13H, SrUGT74G1 and SrUGT76G1) known to encode three enzymes that participate in SGs biosynthesis, likely contributing to the differences in the stevioside and rebaudioside A accumulation. Not only genetic variation can affect SGs composition, but also environmental factors and crop management. Numerous studies have shown that the light regime in which S. rebaudiana cultivars grow can affect SGs accumulation. However, the optimal light regime to increase total SGs content is currently controversial. By applying various light intensities, we detected an increase of expression of these three biosynthetic genes at higher light intensity, accompanied by higher levels of stevioside and rebaudioside A, demonstrating that light intensity influences the synthesis of SGs.

Beyond the PAR spectra: impact of light quality on the germination, flowering, and metabolite content of Stevia rebaudiana (Bertoni)

BACKGROUND

Stevia rebaudiana is a high value crop due to the strong commercial demand for its metabolites (steviol glycosides) but has limited geographical cultivation range. In non-native environments with different daylength and light quality, Stevia has low germination rates and early flowering resulting in lower biomass and poor yield of the desired metabolites. In this study, artificial lighting with light-emitting diodes (LEDs) was used to determine if different light quality within and outside of the photosynthetically active radiation (PAR) range can be used to improve germination rates and yields for production of steviol glycosides for the herbal supplement and food industry.

RESULTS

Plants treated with red and blue light at an intensity of 130 μmol m^-2  s^-1 supplemented with 5% of UV-A light under a 16-h photoperiod produced the most desirable overall results with a high rate of germination, low percentage of early flowering, and high yields of dry leaf, stevioside and rebaudioside A, 175 days after planting.

CONCLUSION

While red and blue light combinations are effective for plant growth, the use of supplemental non-PAR irradiation of UV-A wavelength significantly and desirably delayed flowering, enhanced germination, biomass, rebaudioside A and stevioside yields, while supplemental green light improved yield of biomass and rebaudioside A, but not stevioside. Overall, the combination of red, blue and UV-A light resulted in the best overall productivity for Stevia rebaudiana. © 2021 Society of Chemical Industry.

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 Light Conditions and Medium Composition on Morphophysiological Characteristics of Stevia rebaudiana Bertoni In Vitro and In Vivo

We investigated the influence of different conditions (light composition and plant growth regulators (PGRs) in culture media) on the morphophysiological parameters of Stevia rebaudiana Bertoni in vitro and in vivo. Both PGRs and the light spectra applied were found to significantly affect plant morphogenesis. During the micropropagation stage of S. rebaudiana, optimal growth, with a multiplication coefficient of 15, was obtained in an MS culture medium containing 2,4-epibrassinolide (Epin) and indole-3-acetic acid (IAA) at concentrations of 0.1 and 0.5 mg L−1, respectively. During the rooting stage, we found that the addition of 0.5 mg L−1 hydroxycinnamic acid (Zircon) to the MS medium led to an optimal root formation frequency of 85% and resulted in the formation of strong plants with well-developed leaf blades. Cultivation on media containing 0.1 mg L−1 Epin and 0.5 mg L−1 IAA and receiving coherent light irradiation on a weekly basis resulted in a 100% increase in the multiplication coefficient, better adventitious shoot growth, and a 33% increase in the number of leaves. S. rebaudiana microshoots, cultured on MS media containing 1.0 mg L−1 6-benzylaminopurine (BAP) and 0.5 mg L−1 IAA with red monochrome light treatments, increased the multiplication coefficient by 30% compared with controls (white light, media without PGRs).

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.

Synthesis and production of steviol glycosides: recent research trends and perspectives

Abstract

Steviol glycosides (SvGls) are plant secondary metabolites belonging to a class of chemical compounds known as diterpenes. SvGls have been discovered only in a few plant species, including in the leaves of Stevia rebaudiana Bertoni. Over the last few decades, SvGls have been extensively researched for their extraordinary sweetness. As a result, the nutritional and pharmacological benefits of these secondary metabolites have grown increasingly apparent. In the near future, SvGls may become a basic, low-calorie, and potent sweetener in the growing natural foods market, and a natural anti-diabetic remedy, a highly competitive alternative to commercially available synthetic drugs. Commercial cultivation of stevia plants and the technologies of SvGls extraction and purification from plant material have already been introduced in many countries. However, new conventional and biotechnological solutions are still being sought to increase the level of SvGls in plants. Since many aspects related to the biochemistry and metabolism of SvGls in vivo, as well as their relationship to the overall physiology of S. rebaudiana are not yet understood, there is also a great need for in-depth scientific research on this topic. Such research may have positive impact on optimization of the profile and SvGls concentration in plants and thus lead to obtaining desired yield. This research summarizes the latest approaches and developments in SvGls production.

Key points

• Steviol glycosides (SvGls) are found in nature in S. rebaudiana plants.

• They exhibit nutraceutical properties.

• This review provides an insight on different approaches to produce SvGls.

• The areas of research that still need to be explored have been identified.

Comparative transcriptome analysis revealed gamma-irradiation mediated disruption of floral integrator gene(s) leading to prolonged vegetative phase in Stevia rebaudiana Bertoni

Stevia rebaudiana Bert. is getting global attention because of its ability to synthesize commercially important low/no calorie natural sweeteners (LNCSs) steviol glycosides (SGs). Considering, higher accumulation of SGs in vegetative phase followed by decrement during reproductive phase necessitate the understanding of different molecular components of floral transition to develop superior varieties/cultivars with prolonged vegetative phase in Stevia. Current comparative transcriptional analysis of low dose (5 kR) gamma-irradiated mutant genotype (SMG) with prolonged vegetative phase vis-à-vis background genotype (SBG) identified DGEs of major floral transition pathways, and expressed according to their physiological fate irrespective to SMG & SBG. Contrarily, reduced expression of floral integrator genes (FT and LEAFY) in mutant genotype suggests their involvement in prolonged vegetative phase phenotype. Likewise, GO and KEGG enrichment of photosynthesis and carbon assimilation efficiency might be associated with prolonged vegetative phase and higher accumulation of Stevioside content in mutant genotype. Furthermore, deviation of flowering related transcription factors (higher expressions except MIKS-type MADS-box SMG_PV compared to SBG_F) may possibly be correlated with low expression of floral integrator genes. Findings of current studies will facilitate the genetic manipulations and crop improvement efforts in Stevia through conventional breeding and genome editing approaches for increased SGs biosynthesis.

The influence of light quality on the production of bioactive metabolites - verbascoside, isoverbascoside and phenolic acids and the content of photosynthetic pigments in biomass of Verbena officinalis L. cultured in vitro

In vitro callus cultures of Verbena officinalis L. were maintained on solid Murashige and Skoog medium, enriched with 1 mg dm^-3 BA and 1 mg dm^-3 IBA under LED lights (red, blue, red/blue 70%/30%), in darkness and under control fluorescent lamps. The measurements of 2 phenylpropanoid glycosides (verbascoside and isoverbascoside) and 23 phenolic acids were performed in methanolic extracts from the biomass collected after 2-, 3- and 4-week growth cycles using the HPLC-DAD method. The presence of verbascoside, isoverbascoside and additionaly 7 phenolic acids (protocatechuic, chlorogenic, vanillic, caffeic, ferulic, o-coumaric and m-coumaric acids) was confirmed in all extracts. Blue and red/blue lights stimulated the accumulation of verbascoside (max. of 6716 and 6023 mg 100 g^-1 DW after a 4-week growth cycle) and isoverbascoside (max. 333 and 379 mg 100 g^-1 DW also after 4 weeks). The maximum amounts of verbascoside and isoverbascoside were respectively 1.8- and 7.0-fold higher than under the control conditions. Phenolic acids were accumulated in different amounts, and the maximum total amounts ranged from 36 to 65 mg 100 g^-1 DW. LED lights also stimulated their accumulation in comparison with darkness and control. The main phenolic acids included: m-coumaric acid (max. 39 mg 100 g^-1 DW), ferulic acid (max. 12 mg 100 g^-1 DW), and protocatechuic acid (max. 13 mg 100 g^-1 DW). Additionally, the quantities of photosynthetic pigments (chlorophyll a, b and carotenoids) were estimated in acetonic extracts using spectrophotometry. Red/blue light stimulated the biosynthesis of pigments (max. total content 287 μg g^-1 FW after 4-week growth cycles). This is the first study describing the effect of LED lights on the production of phenylpropanoid glycosides and phenolic acids in V. officinalis callus cultures. Very high amounts of verbascoside and isoverbascoside are interesting from a practical point of view.

Nitrogen drives plant growth to the detriment of leaf sugar and steviol glycosides metabolisms in Stevia (Stevia rebaudiana Bertoni)

Steviol glycosides (SGs) in Stevia (Stevia rebaudiana Bertoni) leaves are important due to their high sweetness and low calorific value. The yield of SGs is dependent on fertilization regimes, but the relationship between nitrogen (N) administration and SGs synthesis is still unclear. In this study, we investigate the effects of N rates on SGs production through hydroponic and plot experiments. The SGs yield was not significantly changed by N fertilization, but leaf SGs concentrations were significantly reduced due to the "dilution effect". Additionally, N addition decreased leaf carbon (C)/N ratio and soluble sugar concentration, accompanied with the inhibited phosphoenolpyruvate carboxylase and L-phenylalanine ammonia_lyase activities. A significant positive correlation between leaf SGs concentrations, C/N ratio and soluble sugar concentration was observed. Overall, we suggest that N-driven Stevia growth negatively affects SGs concentrations. The leaf C/N ratio and soluble sugar changes indicated the occurrence of metabolic reprogramming.

Effects of cytokinins, gibberellic acid 3, and gibberellic acid 4/7 on in vitro growth, morphological traits, and content of steviol glycosides in Stevia rebaudiana

Steviol glycosides (SGs) and gibberellic acids share a part of their biosynthesis pathways. Despite the widespread studies on the effect of gibberellic acid 3 (GA₃), the effect of gibberellic acid 4 and 7 (GA_4/7) on Stevia rebaudiana has never been studied. This study aimed at a comparative evaluation of different hormone effects, i.e., 1 mg L^-1 GA_4/7, 1 mg L^-1 GA₃, or 0.5 mg L^-1 kinetin and 0.5 mg L^-1 6-benzylaminopurine (BAP) (KB 0.5), on in vitro propagation, growth, morphological properties, and content of SGs in leaf samples of stevia. In comparison with the control group (hormone-free), the treatments of KB 0.5 or GA₃ produced the highest biomasses and largest leaf areas. The three hormonal treatments produced a similar number of leaves, the ratio of fresh to dry weight, and leaf length. GA_4/7-treated explants produced the highest ratio of leaf area to leaf length. The effect of GA_4/7 on shoot elongation was greater than that of the control or even GA₃. While the effect of GA₃ on rebaudioside-A (Reb-A) production was similar to that of the control (16.2 and 18.04 mg g^-1, respectively), GA_4/7 resulted in a lower amount of it (13.31 mg g^-1). Except for GA_4/7, which induced more stevioside accumulation, the treatments' effects were comparable to that of the control. The ratio of stevioside to Reb-A was the highest for GA_4/7 (2.62), followed by GA₃ (1.93), and then the two others. Sum of Reb-A and stevioside content was not changed by the use of any of the treatments.

Do Steviol Glycosides Act Either as a Carbon Storage Pool or in Osmoregulation within Leaves of Stevia rebaudiana?

Steviol glycosides (SG) (with stevioside and rebaudioside A predominating) are present in wild-type Stevia rebaudiana, at approximately 10% of dry weight (dw), prompting a consideration of the autoecological role played by these compounds in terms of energy (C) storage and/or osmoregulation. The leaf starch pool was observed to change diurnally with respect to the light cycle (from 3.29% to 0.73% of leaf dw between dusk and dawn) and also to increase under constant light treatment (from 1.53% to 6.25% of leaf dw), while SG pools were relatively constant (around 6% w/dw). A similar trend was observed during exposure to elevated CO₂ (800 ppm), with starch increasing (from 10% to 15% of leaf dw), while SG pool size was constant (around 12% w/dw). For plants subject to increasing water stress over several days, an increase in leaf sap osmolality was observed in the leaves of a severely stressed group (from -1 MPa to -3 MPa, after 2 days of treatment), while stevioside and rebaudioside A leaf concentration was relatively constant (around 16% w/dw). These results are not consistent with a role for SG as either a short-term C store or osmoregulator in S. rebaudiana.

Impact of blue, red, and far-red light treatments on gene expression and steviol glycoside accumulation in Stevia rebaudiana

Stevia rebaudiana (Bertoni) Bertoni is a plant that biosynthesizes a group of natural sweeteners that are up to approximately 400 times sweeter than sucrose. The sweetening components of S. rebaudiana are steviol glycosides (SGs) that partially share their biosynthesis pathway with gibberellins (GAs). However, the molecular mechanisms through which SGs levels can be improved have not been studied. Therefore, transcription levels of several SG biosynthesis-related genes were analyzed under several light treatments involved in GA biosynthesis. We detected higher transcription of UGT85C2, which is one of the UDP-glycosyltransferases (UGTs) involved in catalyzing the sugar-transfer reaction, under red/far-red (R/FR) 1.22 light-emitting diodes (LEDs) and blue LEDs treatment. In this study, it was demonstrated that transcription levels of SG-related genes and the SGs content are affected by light treatments known to affect the GA contents. It is expected that this approach could serve as a practical way to increase SG contents using specific light treatments.

Gene transcription and steviol glycoside accumulation in Stevia rebaudiana under polyethylene glycol-induced drought stress in greenhouse cultivation

Stevia rebaudiana is a sweet herb of the Astraceae family, which is cultivated for the natural sweeteners it contains. The aim of this study was to assess the effect of drought, simulated by the application of polyethylene glycol (5%, 10%, and 15% w/v), on the content of steviol glycosides (SVglys) and transcription levels of six genes involved in the biosynthesis of these natural sweeteners. The transcription levels of ent-kaurene synthase, ent-kaurene oxidase, ent-kaurenoic acid hydroxylase, and three UDP-dependent glycosyltransferases, UGT85C2,UGT74G1 and UGT76G1 were downregulated under polyethylene glycol treatment. Polyethylene glycol treatment significantly decreased the amount of stevioside, rebaudioside A, B, C and F, steviolbioside, dulcoside A, rubusoside, and total SVglys. These results strongly suggest a close relationship of SVglys content with the transcription of genes involved in the SVglys biosynthesis pathway. Comparing the observations of the present study with other reports provided the knowledge that the Stevia response to drought stress can be influenced by different environmental and experimental factors, in addition to intensity of drought stress. In conclusion, these results strongly suggest that polyethylene glycol-induced drought stress has a negative effect on the content of SVglys and transcription of SVglys biosynthetic genes and that this should be investigated further. We recommend that sufficient irrigation of Stevia is required to obtain a high content of SVglys.

Stevia Glycosides: Chemical and Enzymatic Modifications of Their Carbohydrate Moieties to Improve the Sweet-Tasting Quality

Stevia glycosides, extracted from the leaves of the plant Stevia rebaudiana Bertoni, display an amazing high degree of sweetness. As processed plant products, they are considered as excellent bio-alternatives for sucrose and artificial sweeteners. Being noncaloric and having beneficial properties for human health, they are the subject of an increasing number of studies for applications in food and pharmacy. However, one of the main obstacles for the successful commercialization of Stevia sweeteners, especially in food, is their slight bitter aftertaste and astringency. These undesirable properties may be reduced or eliminated by modifying the carbohydrate moieties of the steviol glycosides. A promising procedure is to subject steviol glycosides to enzymatic glycosylation, thereby introducing additional monosaccharide residues into the molecules. Depending on the number and positions of the monosaccharide units, the taste quality and sweetness potency of the compounds will vary. Many studies have been performed already, and this review summarizes the structures of native steviol glycosides and the recent data of modifications of the carbohydrate moieties that have been published to provide an overview of the current progress.

Comprehensive review on agro technologies of low-calorie natural sweetener stevia (Stevia rebaudiana Bertoni): a boon to diabetic patients

Stevia rebaudiana Bertoni is a low-calorie natural sweetener plant native to Paraguay. The leaves of stevia have sweetening compounds called steviol glycosides (SGs), which contain different marker compounds, i.e. stevioside (St), rebaudioside (Rb) A, B, C, D and E, dulcoside A and steviol biosides, which are nearly 300 times sweeter than sugar. Stevia is a better alternative to sugar in formulating food products, reducing the harmful effect of sugar and improving the nutrient properties. We have tried to compile a literature on various agronomic and management aspects which are helpful in increasing the yield and quality of stevia to be grown as a crop that will benefit farmers and industrialists. The stevioside thus obtained can be used to make different food products for sweetening purposes, which could be a boon to diabetic patients. Incorporation of different agronomic techniques like propagation method, transplanting time, intercropping, irrigation, mulching, plant geometry, pinching and harvesting time not only improve the biomass but also increase the quality of stevia. Therefore, agronomic considerations are of high priority to utilize its maximum potential. © 2015 Society of Chemical Industry.

Effect of Nitrogen Fertilization and Harvest Time on Steviol Glycosides, Flavonoid Composition, and Antioxidant Properties in Stevia rebaudiana Bertoni

This work investigated the effect of nitrogen fertilization and harvest time on the flavonoid composition and antioxidant properties of Stevia rebaudiana leaves. At the same time, changes in stevioside (Stev) and rebaudioside A (RebA) contents were recorded. A pot trial under open air conditions was set up, testing five N rates and three harvest times. The results showed that, by using an adequate N rate and choosing an appropriate harvest time, it was possible to significantly increase and optimize the bioactive compound levels. In particular, higher RebA, RebA/Stev ratio, total phenols and flavonoids, luteolin-7-O-glucoside, and apigenin-7-O-glucoside levels and antioxidant capacity were recorded by supplying 150 kg N ha(-1). Reduced or increased N availability in comparison with N150 had no consistent effect on Stevia phytochemicals content. Significant correlations were also found between stevioside and some of the flavonoids, indicating a possible role of flavonoids in the stevioside metabolic pathway, which deserves more investigations.

Enhanced production of steviol glycosides in mycorrhizal plants: a concerted effect of arbuscular mycorrhizal symbiosis on transcription of biosynthetic genes

Stevia rebaudiana (Bertoni) produces steviol glycosides (SGs)--stevioside (stev) and rebaudioside-A (reb-A) that are valued as low calorie sweeteners. Inoculation with arbuscular mycorrhizal fungi (AMF) augments SGs production, though the effect of this interaction on SGs biosynthesis has not been studied at molecular level. In this study transcription profiles of eleven key genes grouped under three stages of the SGs biosynthesis pathway were compared. The transcript analysis showed upregulation of genes encoding 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway enzymes viz.,1-deoxy-D-xylulose 5-phospate synthase (DXS), 1-deoxy-D-xylulose 5-phospate reductoisomerase (DXR) and 2-C-methyl-D-erytrithol 2,4-cyclodiphosphate synthase (MDS) in mycorrhizal (M) plants. Zn and Mn are imperative for the expression of MDS and their enhanced uptake in M plants could be responsible for the increased transcription of MDS. Furthermore, in the second stage of SGs biosynthesis pathway, mycorrhization enhanced the transcription of copalyl diphosphate synthase (CPPS) and kaurenoic acid hydroxylase (KAH). Their expression is decisive for SGs biosynthesis as CPPS regulates flow of metabolites towards synthesis of kaurenoid precursors and KAH directs these towards steviol synthesis instead of gibberellins. In the third stage glucosylation of steviol to reb-A by four specific uridine diphosphate (UDP)-dependent glycosyltransferases (UGTs) occurs. While higher transcription of all the three characterized UGTs in M plants explains augmented production of SGs; higher transcript levels of UGT76G1, specifically improved reb-A to stev ratio implying increased sweetness. The work signifies that AM symbiosis upregulates the transcription of all eleven SGs biosynthesis genes as a result of improved nutrition and enhanced sugar concentration due to increased photosynthesis in M plants.

Crop-ecology and nutritional variability influence growth and secondary metabolites of Stevia rebaudiana Bertoni

BACKGROUND

Plant nutrition and climatic conditions play important roles on the growth and secondary metabolites of stevia (Stevia rebaudiana Bertoni); however, the nutritional dose is strongly governed by the soil properties and climatic conditions of the growing region. In northern India, the interactive effects of crop ecology and plant nutrition on yield and secondary metabolites of stevia are not yet properly understood. Thus, a field experiment comprising three levels of nitrogen, two levels of phosphorus and three levels of potassium was conducted at three locations to ascertain whether the spatial and nutritional variability would dominate the leaf yield and secondary metabolites profile of stevia.

RESULTS

Principal component analysis (PCA) indicates that the applications of 90 kg N, 40 kg P2O5 and 40 kg K2O ha-1 are the best nutritional conditions in terms of dry leaf yield for CSIR-IHBT (Council of Scientific and Industrial Research- Institute Himalayan Bioresource Technology) and RHRS (Regional Horticultural Research Station) conditions. The spatial variability also exerted considerable effect on the leaf yield and stevioside content in leaves. Among the three locations, CSIR-IHBT was found most suitable in case of dry leaf yield and secondary metabolites accumulation in leaves.

CONCLUSIONS

The results suggest that dry leaf yield and accumulation of stevioside are controlled by the environmental factors and agronomic management; however, the accumulation of rebaudioside-A (Reb-A) is not much influenced by these two factors. Thus, leaf yield and secondary metabolite profiles of stevia can be improved through the selection of appropriate growing locations and proper nutrient management.

Environmental cues induce changes of steviol glycosides contents and transcription of corresponding biosynthetic genes in Stevia rebaudiana

Plant growth and secondary metabolism are commonly regulated by external cues such as light, temperature and water availability. In this study, the influences of low and high temperatures, dehydration, photoperiods, and different growing stages on the changes of steviol glycosides (SGs) contents and transcription levels of fifteen genes involved in SGs biosynthesis of Stevia rebaudiana Bertoni were examined using HPLC and RT-PCR. The observations showed that the transcript levels of all the fifteen genes were maximum under 25 °C treatment, and the transcription of SrDXS, SrDXR, SrMCT, SrCMK, SrMDS, SrHDS, SrHDR, SrIDI, SrGGDPS, SrCPPS1, SrUGT85C2 and SrUGT76G1 were restrained both in low temperature (15 °C) and high temperature (35 °C). Most genes in SGs biosynthesis pathway exhibited down-regulation in dehydration. To elucidate the effect of photoperiods, the plants were treated by different simulated photoperiods (8 L/16 D, 1 0L/14 D, 14 L/10 D and 16 L/8 D), but no significant transcription changes were observed. In the study of growing stages, there were evident changes of SGs contents, and the transcript levels of all the fifteen genes were minimal in fast growing period, and exhibited evident increase both in flower-bud appearing stage and flowering stage. The obtained results strongly suggest that the effect of environmental cues on steviol glycosides contents and transcription of corresponding biosynthetic genes in S. rebaudiana is significant. It is worth to study deeply.

A functional (E)-4-hydroxy-3-methylbut-2-enyl diphosphate reductase exhibits diurnal regulation of expression in Stevia rebaudiana (Bertoni)

The leaves of stevia [Stevia rebaudiana (Bertoni)] are a rich source of steviol glycosides that are used as non-calorific sweetener in many countries around the world. Steviol moiety of steviol glycosides is synthesized via plastidial 2C-methyl-D-erythritol 4-phosphate pathway, where (E)-4-hydroxy-3-methylbut-2-enyl diphosphate reductase (HDR) is the key enzyme. HDR catalyzes the simultaneous conversion of (E)-4-hydroxy-3-methylbut-2-enyl diphosphate into five carbon isoprenoid units, isopentenyl diphosphate and dimethylallyl diphosphate. Stevia HDR (SrHDR) successfully rescued HDR lethal mutant strain MG1655 ara<>ispH upon genetic complementation, suggesting SrHDR to encode a functional protein. The gene exhibited diurnal variation in expression. To identify the possible regulatory elements, upstream region of the gene was cloned and putative cis-acting elements were detected by in silico analysis. Electrophoretic mobility shift assay, using a putative light responsive element GATA showed the binding of nuclear proteins (NP) isolated from leaves during light period of the day, but not with the NP from leaves during the dark period. Data suggested the involvement of GATA box in light mediated gene regulation of SrHDR in stevia.

Steviol glycosides: chemical diversity, metabolism, and function

Steviol glycosides are a group of highly sweet diterpene glycosides discovered in only a few plant species, most notably the Paraguayan shrub Stevia rebaudiana. During the past few decades, the nutritional and pharmacological benefits of these secondary metabolites have become increasingly apparent. While these properties are now widely recognized, many aspects related to their in vivo biochemistry and metabolism and their relationship to the overall plant physiology of S. rebaudiana are not yet understood. Furthermore, the large size of the steviol glycoside pool commonly found within S. rebaudiana leaves implies a significant metabolic investment and poses questions regarding the benefits S. rebaudiana might gain from their accumulation. The current review intends to thoroughly discuss the available knowledge on these issues.

Spatio-temporal variation of the diterpene steviol in Stevia rebaudiana grown under different photoperiods

As part of an ongoing study on the effects of photoperiodism on the metabolism of steviol glycosides (SVglys) in Stevia rebaudiana, the spatio-temporal variations of free steviol (SV) have now been evaluated. For its quantitation, an internal standard method was used, based upon a specific fluorometric detection of SV as its methoxycoumarinyl derivative. The level of free SV in leaves did not exceed 30 μg/g dry wt and was at least 1000-fold smaller than that of its glycosidic conjugates. In other organs, free SV was mainly measured in stem tissue and apices, with relatively large amounts measured in the latter. Similarly to SVglys, the content of free SV was influenced by photoperiod and genotype. In plants grown under long-days (LD) of 16 h, more spatial variations were seen compared to those under short-days (SD) of 8h. In the former, upper leaves contained almost four times more free SV compared to lower ones near the end of vegetative growth. In addition, the correlation between SV and its glycosidic conjugates was more linear under SD. Despite the variability of SV levels, a decrease was noted in all conditions after flower opening, which can be related a decreased transcription of the biosynthetic genes involved.