Co-expression of anti-miR319g and miRStv_11 lead to enhanced steviol glycosides content in Stevia rebaudiana.

BACKGROUND: miRNAs are major regulators of gene expression and have proven their role in understanding the genetic regulation of biosynthetic pathways. Stevioside and rebaudioside-A, the two most abundant and sweetest compounds found in leaf extract of Stevia rebaudiana, have been used for many years in treatment of diabetes. It has been found that the crude extract is more potent than the purified extract. Stevioside, being accumulated in higher concentration, imparts licorice like aftertaste. Thus, in order to make the sweetener more potent and palatable, there is a need to increase the intrinsic concentration of steviol glycosides and to alter the ratio of rebaudioside-A to stevioside. Doing so would significantly increase the quality of the sweeteners, and the potential to be used on a wider scale. To do so, in previous report, miRNAs associated with genes of steviol glycosides biosynthetic pathway were identified in S. rebaudiana. In continuation to that in this study, the two miRNAs (miR319g and miRStv_11) targeting key genes of steviol glycosides biosynthetic pathway were modulated and their impact was evaluated on steviol glycosides contents. RESULTS: The over-expression results showed that miRStv_11 induced, while miR319g had repressive action on its target genes. The knock-down constructs for miR319g and miRStv_11 were then prepared and it was demonstrated that the expression of anti-miR319g produced inhibitory effect on its target miRNA, resulting in enhanced expression of its target genes. On the other hand, anti-miRStv_11 resulted in down-regulation of miRStv_11 and its target gene. Further miRStv_11 and anti-miR319gwere co-expressed which resulted in significant increase in stevioside (24.5%) and rebaudioside-A (51%) contents. CONCLUSION: In conclusion, the role of miR319g and miRStv_11 was successfully validated in steviol gycosides biosynthetic pathway gene regulation and their effect on steviol gycosides contents. In this study, we found the positively correlated miRNA-mRNA interaction network in plants, where miRStv_11 enhanced the expression of KAH gene. miRNAs knock-down was also successfully achieved using antisense precursors. Overall, this study thus reveals more complex nature and fundamental importance of miRNAs in biosynthetic pathway related gene networks and hence, these miRNAs can be successfully employed to enhance the ratio of rebaudioside-A to stevioside, thus enhancing the sweetening indices of this plant and making it more palatable.

Enzymatic Synthesis and Characterization of a Novel α-1→6-Glucosyl Rebaudioside C Derivative Sweetener.

Zero-calorie high-intensity sweeteners from natural sources perform very well in the market place. This has encouraged food scientists to continue the effort to search for novel natural ingredients to satisfy consumer demand. Rebaudioside C (reb C) is the third most prevalent steviol glycoside in the leaves of the Stevia rebaudiana Bertoni plant, but has limited applications in food and beverage products due to its low sweetness and high lingering bitterness compared to other major steviol glycosides, such as rebaudioside A (reb A). Here we present a new enzyme modification strategy to improve the taste profile of reb C by using Cargill's propriety enzyme and sucrose as a glucose donor. A novel α-1→6-glucosyl reb C derivative was produced and its structure was elucidated by mass spectrometry and NMR spectroscopy. Sensory analysis demonstrated that this new reb C derivative has improved sweetness, reduced bitterness, and enhanced solubility in water.

Study of gene expression and steviol glycosides accumulation in Stevia rebaudiana Bertoni under various mannitol concentrations.

Stevia rebaudiana produces sweet steviol glycosides that are 300 times sweeter than sugar and have the beneficial effects on human health including anti-hyperglycaemic. Tissue culture is the best method with high efficacy to propagate stevia. Abiotic stress has an impact on steviol glycoside contents in stevia. Therefore, we investigated the effect of mannitol on the expression of four genes involved in the biosynthesis of stevia including UGT74G1, UGT76G1, kaurene oxidase and kaurene synthase genes and steviol glycosides accumulation in stevia under in vitro conditions. The highest expression of UGT76G1 gene occurred in the plants grown under 20 g/l mannitol. While for the kaurene synthase gene, the highest amount of gene expression was observed at 40 g/l mannitol. The results were different about kaurene oxidase gene. As the highest and lowest gene expression were seen in 50 and 30 g/l mannitol conditions respectively. There were the same results for UGT74G1 that means the most appropriate and also the most inopportune treatment for the gene expression were same as the condition for the kaurene oxidase gene. Compared with control, adding mannitol to media in all concentrations increases the expression of UGT76G1 gene. Estimation of steviol glycosides contents under different treatments of mannitol carried out by HPLC. According to the results, the highest amount of stevioside was produced under 20 g/l mannitol treatment. However, rebaudioside A was accumulated in its maximum amounts under 30 g/l mannitol. It can be concluded that adding mannitol to media in the certain concentration increases steviol glycoside contents in the stevia.

A Combinatorial Approach To Study Cytochrome P450 Enzymes for De Novo Production of Steviol Glucosides in Baker's Yeast.

Biosynthesis of steviol glycosides in planta proceeds via two cytochrome P450 enzymes (CYPs): kaurene oxidase (KO) and kaurenoic acid hydroxylase (KAH). KO and KAH function in succession with the support of a NADPH-dependent cytochrome P450 reductase (CPR) to convert kaurene to steviol. This work describes a platform for recombinant production of steviol glucosides (SGs) in Saccharomyces cerevisiae, demonstrating the full reconstituted pathway from the simple sugar glucose to the SG precursor steviol. With a focus on optimization of the KO-KAH activities, combinations of functional homologues were tested in batch growth. Among the CYPs, novel KO75 (CYP701) and novel KAH82 (CYP72) outperformed their respective functional homologues from Stevia rebaudiana, SrKO (CYP701A5) and SrKAH (CYP81), in assays where substrate was supplemented to culture broth. With kaurene produced from glucose in the cell, SrCPR1 from S. rebaudiana supported highest turnover for KO-KAH combinations, besting two other CPRs isolated from S. rebaudiana, the Arabidopsis thaliana ATR2, and a new class I CPR12. Some coexpressions of ATR2 with a second CPR were found to diminish KAH activity, showing that coexpression of CPRs can lead to competition for CYPs with possibly adverse effects on catalysis.

Development of Photoaffinity Probe for the Discovery of Steviol Glycosides Biosynthesis Pathway in Stevia rebuadiana and Rapid Substrate Screening.

Functional discovery and characterization of the target enzymes responsible for the biosynthesis pathway coded for the genes is ongoing, and the unknown functional diversity of this class of enzymes has been revealed by genome sequencing. Commonly, it is feasible in annotating of biosynthetic genes of prokaryotes due to the existence of gene clusters of secondary metabolites. However, in eukaryotes, the biosynthetic genes are not compactly clustered in the way of prokaryotes. Hence, it remains challenging to identify the biosynthetic pathways of newly discovered natural products in plants. Steviol glycosides are one class of natural sweeteners found in high abundance in the herb Stevia rebaudiana. Here, we applied the chemoproteomic strategy for the proteomic profiling of the biosynthetic enzymes of steviol glycosides in Stevia rebaudiana. We not only identified a steviol-catalyzing UDP-glycosyltransferase (UGT) UGT73E1 involved in steviol glycoside biosynthesis but also built up a probe-based platform for the screening of potential substrates of functional uncharacterized UGT rapidly. This approach would be a complementary tool in mining novel synthetic parts for assembling of synthetic biological systems for the biosynthesis of other complex natural products.

Synthesis of rebaudioside D, using glycosyltransferase UGTSL2 and in situ UDP-glucose regeneration.

Steviol glycosides from Stevia rebaudiana leaves are used in stevia-based sweeteners for their intense sweetness and low calories. Rebaudioside D is present in leaves in minute quantities (∼0.4-0.5% w/w total dry weight), but it is ∼350 times sweeter than sucrose, and sweeter than the more abundant rebaudioside A and stevioside. In the present study, pathways for rebaudioside D synthesis and UDP-glucose recycling were developed by coupling recombinant UDP-glucosyltransferase UGTSL2 from Solanum lycopersicum and sucrose synthase StSUS1 from Solanum tuberosum. Reaction parameters, including substrate ratio, sucrose concentration, temperature, crude extract concentration, and reaction time, were evaluated, and 17.4 g/l of rebaudioside D (yield = 74.6%) was obtained from 20 g/l of rebaudioside A after 20 h, using UDP or UDP-glucose in recombinant cell crude extracts. Extending the reaction time generated rebaudioside M2 from further glycosylation of rebaudioside D. Km values for UGTSL2 indicated a higher affinity for rebaudioside D than for rebaudioside A.

Effects of life cycle and leaves location on gene expression and glycoside biosynthesis pathway in Stevia rebaudiana Bertoni.

Stevia rebaudiana Bertoni is One of the most important biologically sourced and low-calorie sweeteners that known as "Sweet Weed". It contains steviol glycosides that they are about 200-300 times sweeter than sucrose. Tissue culture is the best method with high efficiency that can overcome to problems of traditional methods, and it is the most useful tools for studying stress tolerance mechanisms under in vitro conditions to obtain drought tolerance. In the present research, we investigated the impact of life cycle, leaves location and the harvesting time on expression of UGT74G1 and UGT76G1 as well as steviol glycosides accumulation. The highest gene expression of both UGT74G1 and UGT76G1 (207.677 and 208.396 Total Lab unit, respectively) was observed in young leaves in the second vegetative year. Also, the highest amount of stevioside accumulation (13.04) was due to the old leaves in vegetative stage which had significant differences with other effects whereas the lowest accumulation (7.47) was seen at young leaves at vegetative stage. Interestingly, the highest level of rebaudioside a production (15.74) was occurred at the young leaves at vegetative stage. There was significant differences between life cycle and leaves location on steviol glycoside production in stevia.

Effect of nitrogen sources on gene expression of Stevia rebaudiana (Bertoni) under in vitro conditions.

Stevia rebaudiana (Bertoni) is a non-caloric sweetener belonging to Asteraceae family. Stevia compounds such as steviol glycosides (SGs) are 200 times sweeter than sugar. Stevioside and rebaudioside A are the two major steviol glycosides. Nitrogen is an essential element for plant growth and development. In this study the effects of nitrogen influenced by different concentrations of NH4NO3 (0, 825 and 1650 mg/l) and KNO3 (0, 950 and 1900 mg/l) is examined in MS medium. To analysis the UGT74G1 and UGT76G1 genes expression, involved in the synthesis of SGs, RT-qPCR technique was performed. Data showed that there were significant differences between all media. The shoot length, seedlings dry weight and leaf fresh weight of stevia increased with applying NH4NO3 along with KNO3. The highest expression of UGT74G1 gene, was observed in plantlets grown on MS medium with 0 mg/l NH4NO3 and 950 mg/l KNO3 (1.291 total lab unit) but the highest expression of UGT76G1 gene, was observed in plantlets grown on MS medium added by 1650 mg/l NH4NO3 +950 mg/l KNO3 (1.08 total lab unit). Moreover, the lowest value of UGT74G1 gene expression were revealed in MS medium added by 1650 mg/l NH4NO3 +0 mg/l KNO3 (0.80 total lab unit) and the lowest values of UGT76G1 gene expression seen in MS medium with 0 mg/l NH4NO3 +950 mg/l KNO3 (0.85 total lab unit) concentrations. The results of this study could be valuable in stevia breeding programs through glycosides biosynthesis pathways.

Metal oxides as a biostimulator for upregulation of genes involved in the biosynthesis of Rebaudioside- A.

Stevia rebaudiana Bertoni is a kind of perennial medicinal plant with sweetening properties which belongs to Asteraceae family. Its leaves with fundamental glycoside compounds consist of both a sugar part and a non-sugar sector. One of the glycoside compounds is Rebaudioside- A which has a greater importance in business. This experiment was conducted to evaluate the effects of Ag2O, CrO3, PbO, Fe2O3, BaO and TiO2 on the expression pattern of these genes in the Stevia rebaudiana. Rebaudioside- A biosynthesis was repeated 3 times with concentrations of 50, 100 and 200µM. Also, the results of the study pertaining to the expression pattern of these genes showed that metal oxides have led to an increase in the expression of the regulatory genes involved in biosynthesis of Rebaudioside- A. According to the expression profile, it was found that its effect on DXR, HDS, HDR, IDI and CPPS genes is more than other genes. The peak HPLC indicated for stevioside and Rebaudioside- A represents an increase in the production of this active ingredient under the influence of all treatments. In general, the expression profile of these genes and the results of HPLC show that whatever going to the end of the pathway of production of Rebaudioside- A, the activity of the enzymes increases under the influence of these treatments, and eventually a greater amount of Rebaudioside- A will be produced. This process shows that metal oxides will have a significant effect on the biosynthesis of Rebaudioside- A.

Molecular dissection of transcriptional reprogramming of steviol glycosides synthesis in leaf tissue during developmental phase transitions in Stevia rebaudiana Bert.

Stevia is a natural source of commercially important steviol glycosides (SGs), which share biosynthesis route with gibberellic acids (GAs) through plastidal MEP and cytosolic MVA pathways. Ontogeny-dependent deviation in SGs biosynthesis is one of the key factor for global cultivation of Stevia, has not been studied at transcriptional level. To dissect underlying molecular mechanism, we followed a global transcriptome sequencing approach and generated more than 100 million reads. Annotation of 41,262 de novo assembled transcripts identified all the genes required for SGs and GAs biosynthesis. Differential gene expression and quantitative analysis of important pathway genes (DXS, HMGR, KA13H) and gene regulators (WRKY, MYB, NAC TFs) indicated developmental phase dependent utilization of metabolic flux between SGs and GAs synthesis. Further, identification of 124 CYPs and 45 UGTs enrich the genomic resources, and their PPI network analysis with SGs/GAs biosynthesis proteins identifies putative candidates involved in metabolic changes, as supported by their developmental phase-dependent expression. These putative targets can expedite molecular breeding and genetic engineering efforts to enhance SGs content, biomass and yield. Futuristically, the generated dataset will be a useful resource for development of functional molecular markers for diversity characterization, genome mapping and evolutionary studies in Stevia.

Effect of sucrose concentrations on Stevia rebaudiana Bertoni tissue culture and gene expression.

Stevia rebaudiana (Bert.) Bertoni is known as sweet plant which it contains a high level of steviol glycosides in the leaves.  This plant has been used from centuries ago as a sweetener for tea. One of the most important steviol glycosides is stevioside that is attractive for diabetic persons. Tissue culture is the only rapid process for the mass propagation of stevia. One of the most important factors in the medium is sucrose that is a necessary for plant growth. In the present study, we use nodal segments of the stem as explants in mediums with different sucrose concentration (50 mM, 100mM and 150mM). Several morphological traits were measured in a 28 day period. Results analysis showed a significant variation between treatments. The highest growth rate, rooting and leaf production was obtained in medium with 100mM sucrose. The correlation between measured traits was significant at the 0.01 level. To investigation of UGT74G1, UGT76G1, UGT85C2 and KS genes expression that are involved in the synthesis of SGs, RT- PCR was done with the housekeeping gene of as internal control. There were significant differences between all media. The results showed thatsucrose 100 mM containing media was more desirable than others for expression of UGT76G1 and UGT85C2 genes. Whereas, the best medium for expression of UGT74G1 was sucrose 150 mM and sucrose 50 mM for KS gene. Totally, it seems that sucrose at a concentration of 100 mMprovides the best condition for stevia growth and steviol glycosides production.

Biosynthesis of the oxygenated diterpene nezukol in the medicinal plant Isodon rubescens is catalyzed by a pair of diterpene synthases.

Plants produce an immense diversity of natural products (i.e. secondary or specialized metabolites) that offer a rich source of known and potentially new pharmaceuticals and other desirable bioproducts. The Traditional Chinese Medicinal plant Isodon rubescens (Lamiaceae) contains an array of bioactive labdane-related diterpenoid natural products. Of these, the ent-kauranoid oridonin is the most prominent specialized metabolite that has been extensively studied for its potent antimicrobial and anticancer efficacy. Mining of a previously established transcriptome of I. rubescens leaf tissue identified seven diterpene synthase (diTPSs) candidates. Here we report the functional characterization of four I. rubescens diTPSs. IrTPS5 and IrTPS3 were identified as an ent-copalyl diphosphate (CPP) synthase and a (+)-CPP synthase, respectively. Distinct transcript abundance of IrTPS5 and the predicted ent-CPP synthase IrTPS1 suggested a role of IrTPS5 in specialized ent-kaurene metabolism possibly en route to oridonin. Nicotiana benthamiana co-expression assays demonstrated that IrTPS4 functions sequentially with IrTPS3 to form miltiradiene. In addition, IrTPS2 converted the IrTPS3 product (+)-CPP into the hydroxylated tricyclic diterpene nezukol not previously identified in I. rubescens. Metabolite profiling verified the presence of nezukol in I. rubescens leaf tissue. The proposed IrTPS2-catalyzed reaction mechanism proceeds via the common ionization of the diphosphate group of (+)-CPP, followed by formation of an intermediary pimar-15-en-8-yl+ carbocation and neutralization of the carbocation by water capture at C-8 to yield nezukol, as confirmed by nuclear magnetic resonance (NMR) analysis. Oxygenation activity is rare for the family of class I diTPSs and offers new catalysts for developing metabolic engineering platforms to produce a broader spectrum of bioactive diterpenoid natural products.

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.

Overview of Medicinally Important Diterpenoids Derived from Plastids.

BACKGROUND: FL 32608 Terpenoids are hydrocarbon compounds derived from common fivecarbon isoprene (C5H8) building blocks. They are formed through the condensation and subsequent modification of isoprene units in various ways including - among others - cyclization and/or oxygenation. Their synthesis is localized either to the chloroplast and/or to the cytoplasm/peroxisome/ endoplasmic reticulum and mitochondrion. Terpenoids represent a very large and diverse class of metabolites and play important roles in plant growth and development. In addition, they have been intensively used in human health care, disease treatment and in dietary supplements. Approximately 60% of natural products known so far are terpenoids. CONCLUSION: This review briefly summarizes the biosynthetic pathways of major plant terpenoids. Then, five well-known and medicinally important diterpenoids, including paclitaxel, tanshinone, ginkgolide, triptolide and oridonin are discussed in detail. Their structures, occurrence, extraction and identification methods, pharmacological properties and clinical uses are also reviewed. Finally, the prospects of using biotechnology to produce medicinally important terpenoids are also briefly discussed.

Microbial production of next-generation stevia sweeteners.

BACKGROUND: The glucosyltransferase UGT76G1 from Stevia rebaudiana is a chameleon enzyme in the targeted biosynthesis of the next-generation premium stevia sweeteners, rebaudioside D (Reb D) and rebaudioside M (Reb M). These steviol glucosides carry five and six glucose units, respectively, and have low sweetness thresholds, high maximum sweet intensities and exhibit a greatly reduced lingering bitter taste compared to stevioside and rebaudioside A, the most abundant steviol glucosides in the leaves of Stevia rebaudiana. RESULTS: In the metabolic glycosylation grid leading to production of Reb D and Reb M, UGT76G1 was found to catalyze eight different reactions all involving 1,3-glucosylation of steviol C 13- and C 19-bound glucoses. Four of these reactions lead to Reb D and Reb M while the other four result in formation of side-products unwanted for production. In this work, side-product formation was reduced by targeted optimization of UGT76G1 towards 1,3 glucosylation of steviol glucosides that are already 1,2-diglucosylated. The optimization of UGT76G1 was based on homology modelling, which enabled identification of key target amino acids present in the substrate-binding pocket. These residues were then subjected to site-saturation mutagenesis and a mutant library containing a total of 1748 UGT76G1 variants was screened for increased accumulation of Reb D or M, as well as for decreased accumulation of side-products. This screen was performed in a Saccharomyces cerevisiae strain expressing all enzymes in the rebaudioside biosynthesis pathway except for UGT76G1. CONCLUSIONS: Screening of the mutant library identified mutations with positive impact on the accumulation of Reb D and Reb M. The effect of the introduced mutations on other reactions in the metabolic grid was characterized. This screen made it possible to identify variants, such as UGT76G1Thr146Gly and UGT76G1His155Leu, which diminished accumulation of unwanted side-products and gave increased specific accumulation of the desired Reb D or Reb M sweeteners. This improvement in a key enzyme of the Stevia sweetener biosynthesis pathway represents a significant step towards the commercial production of next-generation stevia sweeteners.

Production of steviol from steviol glucosides using ß-glycosidase from Sulfolobus solfataricus.

Steviol is a diterpene isolated from the plant Stevia rebaudiana that has a potential role as an antihyperglycemic agent by stimulating insulin secretion from pancreatic beta cells and also has significant potential to diminish the renal clearance of anionic drugs and their metabolites. In this study, the lacS gene, which encodes a thermostable ß-glycosidase (SSbgly) enzyme from the extremely thermoacidophillic archaeon Sulfolobus solfataricus, was cloned and expressed in E. coli Rossetta BL21(DE3)pLyS using lactose as an inducer. Through fermentation, SSbgly was expressed as a 61kDa protein with activity of 24.3U/mg and the OD600 of 23 was reached after 18h induction with 10mM lactose. Purified protein was obtained by Ni-Sepharose chromatography with a yield of 92.3%. SSbgly hydrolyzed steviol glycosides to produce steviol with a yield of 99.2%. The optimum conditions for steviol production were 50U/ml SSbgly and 90mg/ml Ste at 75°C as determined by the response surface method.

The decision to germinate is regulated by divergent molecular networks in spores and seeds.

Dispersal is a key step in land plant life cycles, usually via formation of spores or seeds. Regulation of spore- or seed-germination allows control over the timing of transition from one generation to the next, enabling plant dispersal. A combination of environmental and genetic factors determines when seed germination occurs. Endogenous hormones mediate this decision in response to the environment. Less is known about how spore germination is controlled in earlier-evolving nonseed plants. Here, we present an in-depth analysis of the environmental and hormonal regulation of spore germination in the model bryophyte Physcomitrella patens (Aphanoregma patens). Our data suggest that the environmental signals regulating germination are conserved, but also that downstream hormone integration pathways mediating these responses in seeds were acquired after the evolution of the bryophyte lineage. Moreover, the role of abscisic acid and diterpenes (gibberellins) in germination assumed much greater importance as land plant evolution progressed. We conclude that the endogenous hormone signalling networks mediating germination in response to the environment may have evolved independently in spores and seeds. This paves the way for future research about how the mechanisms of plant dispersal on land evolved.

Engineering Aspergillus nidulans for heterologous ent-kaurene and gamma-terpinene production.

Terpenes are a large and varied group of natural products with a wide array of bioactivities and applications. The chemical production of industrially relevant terpenes can be expensive and time-consuming due to the structural complexity of these compounds. Here, we studied Aspergillus nidulans as a heterologous host for monoterpene and diterpene production. Previously, we identified a novel diterpene gene cluster in A. nidulans and showed that overexpression of the cluster-specific transcription factor (pbcR) led to ent-pimara-8(14),15-diene (PD) production. We report further characterization of the A. nidulans PD synthase gene (pbcA). In A. nidulans, overexpression of pbcA resulted in PD production, while deletion of pbcA abolished PD production. Overexpression of Fusarium fujikuroi ent-kaurene synthase (cps/ks) and Citrus unshiu gamma-terpinene synthase resulted in ent-kaurene and gamma-terpinene production, respectively. A. nidulans is a fungal model organism and a close relative to other industrially relevant Aspergillus species. A. nidulans is a known producer of many secondary metabolites, but its ability to produce heterologous monoterpene and diterpene compounds has not been characterized. Here, we show that A. nidulans is capable of heterologous terpene production and thus has potential as a production host for industrially relevant compounds. The genetic engineering principles reported here could also be applied to other Aspergilli.

Production of Rebaudioside A from Stevioside Catalyzed by the Engineered Saccharomyces cerevisiae.

Rebaudioside A has superior taste quality among the steviol glycosides extracted from Stevia rebaudiana leaves. Given its high purity as a general-purpose sweetener, rebaudioside A has received significant attention and has been widely applied in food and beverages in recent decades. Stevioside is one of the major steviol glycosides and can be converted to rebaudioside A by the uridine-diphosphate dependent glucosyltransferase UGT76G1 in S. rebaudiana. To explore the applicability of and limits in producing rebaudioside A from stevioside through whole-cell biocatalysis, the engineered Saccharomyces cerevisiae expressing UGT76G1, using a newly constructed constitutive expression vector, was used as the whole-cell biocatalyst. Citrate was added to the reaction mixture to allow metabolic regulation when glucose was fed to provide the activated sugar donor UDP-glucose for glycosylation of stevioside in vivo. In an evaluation of the whole-cell reaction parameters involving cell permeability, temperature, pH, citrate and Mg(2+) concentrations, and glucose feeding, production of 1160.5 mg/L rebaudioside A from 2 g/L stevioside was achieved after 48 h without supplementation of extracellular UDP-glucose.