Base substitution mutations in uridinediphosphate-dependent glycosyltransferase 76G1 gene of Stevia rebaudiana causes the low levels of rebaudioside A: mutations in UGT76G1, a key gene of steviol glycosides synthesis

Enhanced Production of Rebaudioside D and Rebaudioside M through V155T Substitution in the Glycosyltransferase UGT91D2 from Stevia rebaudiana

Steviol glycosides (SGs) are noncaloric natural sweeteners found in the leaves of stevia (Stevia rebaudiana). These diterpene glycosides are biosynthesized by attaching varying numbers of monosaccharides, primarily glucose, to steviol aglycone. Rebaudioside (Reb) D and Reb M are highly glucosylated SGs that are valued for their superior sweetness and organoleptic properties, yet they are present in limited quantities in stevia leaves. This study aims to improve the substrate preference and catalytic efficiency of UDP-sugar-dependent glycosyltransferase UGT91D2 from stevia, which acts as a bottleneck in the biosynthesis of Reb D and Reb M. We modeled the structure of UGT91D2 and substituted two amino acid residues, Y134 and V155, which are located near the glycosyl acceptor and donor, respectively. Expression of the UGT91D2V155T in budding yeast significantly enhanced the production of Reb D and Reb M. Furthermore, transient expression in Nicotiana benthamiana revealed that the V155T substitution improved the glucosylation activity of UGT91D2, suggesting that this substitution enhances UDP-glucose binding and reduces side reactions involving nonglucose donors. By coexpressing multiple stevia UGT genes in N. benthamiana, we successfully produced highly glucosylated SGs from steviol. Our results provide insights into the substrate specificity of UGT91D2 and contribute to the engineering of SG biosynthesis.

Multi-walled carbon nanotubes interact with light intensity to affect morpho-biochemical, nutrient uptake, DNA damage, and secondary metabolism of Stevia rebaudiana

In this study, the interaction between nanoparticles (0, 50, 100, and 150 mg L^-1) and light intensity (100, 200, and 400 μmol·m^-2·s^-1) was evaluated for effectiveness in improving stevia shoot induction by measuring morphological traits, nutrient absorption, total carbohydrates, steviol glycosides (SVglys), and DNA damage in two DNA sequence regions (promoter and sequence of the UGT76G1 gene). MWCNTs at a concentration of 50 mg L^-1 in interaction with the light intensity of 200 μmol·m^-2·s^-1 improved the morphological traits and absorption of nutrients such as nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), iron (Fe), and Manganese (Mn), compared to other treatments. Also, under this interaction, the accumulation of total carbohydrates and SVglys was elevated. Moreover, DNA damage in both regions of the DNA sequence under light intensity at low concentrations of MWCNTs (0 and 50 mg L^-1) did not show a significant change but increased with increasing MWCNT concentration at high light intensities (200 and 400 μmol·m^-2·s^-1). These results demonstrate that the advantages and phytotoxicity of MWCNTs in the in vitro culture of stevia are dose-dependent and are affected by light intensity. Based on this, the interaction of 50 mg L^-1 of MWCNTs with the light intensity of 200 μmol·m^-2·s^-1 is recommended to improve stevia micropropagation and subsequent growth and metabolism.

Enhancement of diterpenoid steviol glycosides by co-overexpressing SrKO and SrUGT76G1 genes in Stevia rebaudiana Bertoni

Stevia rebaudiana (stevia) contains commercially important steviol glycosides, stevioside and rebaudioside A, these compounds have insulinotropic and anti-hyperglycemic effect. Steviol, stevioside and rebaudioside-A have taste modulation and insulin potentiation activity. Stevia leaves are composed of steviol (2-5%), stevioside (4-13%) and rebaudioside-A (1-6%). Stevioside has after-taste bitterness, rebaudioside-A is sweetest in taste among all the glycosides present. Therefore, lower ratio of rebaudioside-A to stevioside has bitter after-taste, which makes stevia plants unpalatable. By over-expressing the genes, SrUGT76G1 and SrKO, we propose to increase the ratio of RebA to stevioside in stevia. Various lines were generated and amongst them, seven lines had both the transgenes present. Co-overxpresion of SrUGT76G1 and SrKO led to the increased concentration of RebA in all the seven transgenic lines (KU1-KU7) than control plant and RebA to stevioside ratio also increased significantly. Steviol, stevioside and RebA showed a differential concentration in all the seven lines, but the pattern was the same in all of them and the ratio of RebA to stevioside increased dramatically. In transgenic line 2 (KU2), RebA showed a steep increase in concentration 52% the rebaudioside-A to stevioside ratio increased from 0.74 (control) to 2.83. In overall all the lines, RebA showed a positive correlation with steviol and stevioside. Overexpression of SrKO led to an increase in steviol which increased the stevioside, overexpression of SrUGT76G1 ultimately increased RebA concentration. In conclusion, concentration of RebA increased significantly with co- overexpression of SrUGT6G1 and SrKO genes. Lines with increased RebA are more palatable and commercially viable.

Natural and low-caloric rebaudioside A as a substitute for dietary sugars: A comprehensive review

For health and safety concerns, traditional high-calorie sweeteners and artificial sweeteners are gradually replaced in food industries by natural and low-calorie sweeteners. As a natural and high-quality sugar substitute, steviol glycosides (SvGls) are continually scrutinized regarding their safety and application. Recently, the cultivation of organic stevia has been increasing in many parts of Europe and Asia, and it is obvious that there is a vast market for sugar substitutes in the future. Rebaudioside A, the main component of SvGls, is gradually accepted by consumers due to its safe, zero calories, clear, and sweet taste with no significant undesirable characteristics. Hence, it can be used in various foods or dietary supplements as a sweetener. In addition, rebaudioside A has been demonstrated to have many physiological functions, such as antihypertension, anti-diabetes, and anticaries. But so far, there are few comprehensive reviews of rebaudioside A. In this review article, we discuss the physicochemical properties, metabolic process, safety, regulatory, health benefits, and biosynthetic pathway of rebaudioside A and summarize the modification methods and state-of-the-art production and purification techniques of rebaudioside A. Furthermore, the current problems hindering the future production and application of rebaudioside A are analyzed, and suggestions are provided.

DNA Free CRISPR/DCAS9 Based Transcriptional Activation System for UGT76G1 Gene in Stevia rebaudiana Bertoni Protoplasts

The UDP-glycosyltransferase 76G1 (UGT76G1) is responsible for the conversion of stevioside to rebaudioside A. Four single guide RNAs (sgRNAs) were designed from the UGT76G1 proximal promoter region of stevia by using the online-based tool, benchling. The dCas9 fused with VP64 as a transcriptional activation domain (TAD) was produced and purified for the formation of ribonucleoproteins (RNPs) by mixing with the in vitro transcribed sgRNAs. Protoplast yield was the highest from leaf mesophyll of in vitro grown stevia plantlets (3.16 × 10⁶/g of FW) using ES5 (1.25% cellulase R-10 and 0.75% macerozyme R-10). The RNPs were delivered into the isolated protoplasts through the Polyethylene glycol (PEG)-mediated transfection method. The highest endogenous activation of the UGT76G1 gene was detected at 27.51-fold after 24 h of transfection with RNP30 consisting of CRISPR/dCas9-TAD with sgRNA30 and a similar activation level was obtained using RNP18, RNP33, and RNP34, produced using sgRNA18, sgRNA33, and sgRNA34, respectively. Activation of UGT76G1 by RNP18 led to a significant increase in the expression of the rate-limiting enzyme UGT85C2 by 2.37-fold and there was an increasing trend in the expression of UGT85C2 using RNP30, RNP33, and RNP34. Successful application of CRISPR/dCas9-TAD RNP in activating specific genes can avoid the negative integration effects of introduced DNA in the host genome.

Metabolic engineering for the synthesis of steviol glycosides: current status and future prospects

With the pursuit of natural non-calorie sweeteners, steviol glycosides (SGs) have become one of the most popular natural sweeteners in the market. The SGs in Stevia are a mixture of SGs synthesized from steviol (a terpenoid). SGs are diterpenoids. Different SGs depend on the number and position of sugar groups on the core steviol backbone. This diversity comes from the processing of glycoside steviol by various glycosyltransferases. Due to the differences in glycosylation, each SG has unique sensory properties. At present, it is more complicated to extract high-quality SGs from plants, so the excavation of the metabolic pathways of engineered microorganisms to synthesize SGs has been extensively studied. Specifically, the expression of different glycosyltransferases in microbes is key to the synthesis of various SGs by engineered microorganisms. To trigger more researches on the functional characterization of the enzymes encoded by these genes, this review describes the latest research progresses of the related enzymes involved in SG biosynthesis and metabolic engineering.Key points• Outlines the research progress of key enzymes in the biosynthetic pathway of SGs• Factors affecting the catalytic capacity of stevia glucosyltransferase• Provide guidance for the efficient synthesis of SGs in microbial cell factories.

Co-immobilized recombinant glycosyltransferases efficiently convert rebaudioside A to M in cascade

Rebaudioside M (Reb M), as a natural and healthy Stevia sweetener, is produced by two glycosyltransferases that catalyze the serial glycosylation of Rebaudioside A (Reb A) and Rebaudioside D (Reb D) in cascade. Meanwhile, it is of great importance in developing an immobilization strategy to improve the reusability of glycosyltransferases in reducing the production cost of Reb M. Here, the recombinant glycosyltransferases, i.e., OsEUGT11 (UGT1) and SrUGT76G1 (UGT2), were expressed in Escherichia coli and covalently immobilized onto chitosan beads. UGT1 and UGT2 were individually immobilized and co-immobilized onto the beads that catalyze Reb A to Reb M in one-pot. The co-immobilized enzymes system exhibited ∼3.2-fold higher activity than that of the mixed immobilized enzymes system. A fairly high Reb A conversion rate (97.3%) and a high Reb M yield of 72.2% (4.82 ± 0.11 g L-1) were obtained with a feeding Reb A concentration of 5 g L-1. Eventually, after 4 and 8 reused cycles, the co-immobilized enzymes retained 72.5% and 53.1% of their original activity, respectively, showing a high stability to minimize the total cost of enzymes and suggesting that the co-immobilized UGTs is of potentially signficant value for the production of Reb M.

Identification of the Key Residues of the Uridine Diphosphate Glycosyltransferase 91D2 and its Effect on the Accumulation of Steviol Glycosides in Stevia rebaudiana

Stevia (Stevia rebaudiana Bertoni) possesses substantial value for its unique sweet compounds-steviol glycosides (SGs). In the metabolic glycosylation grid of SGs, SrUGT91D2 has been shown to catalyze formation of 1,2-β-d-glucoside linkages at the C₁₃- and C₁₉-positions and play a crucial role in the synthesis of SGs, including the formation of stevioside (ST), rebaudioside E (RE), and rebaudioside D (RD). However, the key residues of the SrUGT91D2 enzyme and how SrUGT91D2 affects the accumulation of SGs in S. rebaudiana remain unclear. In the present study, cloning and functional analysis of full-length SrUGT91D2 gene sequences were performed in 10 different S. rebaudiana genotypes with divergent SG compositions. After sequence analysis, it was found that most of the sequences of this gene (more than 50%) in each genotype were consistent with the UGT91D2e_No.5 allele, which has been reported to exert catalytic activity on 1,2-β-d-glucoside. Moreover, six variants (UGT91D2e_No.5, SrUGT91D2-11-14, SrUGT91D2-110, SrUGT91D2-023, SrUGT91D2-N01, and SrUGT91D2-N04) of this gene were obtained, and their activities were identified. Although there were some differences among these variants, the only type of mutation was partial base substitution at a very low level. In addition, the expression analysis of SrUGT91D2 in each genotype showed that the expression level of the gene was significantly different among genotypes, and a significant positive correlation was found between the content of RD (which was closely influenced by SrUGT91D2) and the expression level of SrUGT91D2 in each genotype (correlation coefficient = 0.91). Thus, it was indicated that SrUGT91D2 was relatively conserved in S. rebaudiana, and the differential effect of SrUGT91D2 on the accumulation of related SGs mainly derived from its expression level. Furthermore, based on homologous modeling and molecular docking analysis, T84, T144, A194, S284, E285, V286, G365, E369, R404, and G409 were predicted to be key residues in the glucosylation of SGs by SrUGT91D2. After site-mutation and enzyme assays, it was confirmed that T84, T144, R404, A194, and G409 are the key residues in the SrUGT91D2 protein, especially T144 and G409. This work provided valuable information for understanding the structure-activity relationship of the SrUGT91D2 protein and the molecular mechanism of SG accumulation in stevia.

Development of screening methods for functional characterization of UGTs from Stevia rebaudiana

Glycosylation is a key modification that contributes to determine bioactivity and bioavailability of plant natural products, including that of terpenoids and steviol glycosides (SVglys). It is mediated by uridine-diphosphate glycosyltransferases (UGTs), that achieve their activity by transferring sugars on small molecules. Thus, the diversity of SVglys is due to the number, the position and the nature of glycosylations on the hydroxyl groups in C-13 and C-19 of steviol. Despite the intense sweetener property of SVglys and the numerous studies conducted, the SVglys biosynthetic pathway remains largely unknown. More than 60 SVglys and 68 putative UGTs have been identified in Stevia rebaudiana. This study aims to provide methods to characterize UGTs putatively involved in SVglys biosynthesis. After agroinfiltration-based transient gene expression in Nicotiana benthamiana, functionality of the recombinant UGT can be tested simply and directly in plants expressing it or from a crude extract. The combined use of binary vectors from pGWBs series to produce expression vectors containing the stevia's UGT, enables functionality testing with many substrates as well as other applications for further analysis, including subcellular localization.

A beta-glucosidase gene from Stevia rebaudiana may be involved in the steviol glycosides catabolic pathway

We herein report the preparation of a full-length raucaffricine-O-beta-D-glucosidase gene of stevia rebaudiana Bertoni (named SrRG1, GenBank accession number MK920450). Sequence analysis indicated SrRG1 consists of a 1650 bp open reading frame encoding a protein of 549 amino acids. Its deduced amino acid sequence showed a high identity of 82% with a raucaffricine-O-beta-D-glucosidase from H. annuus of glycoside hydrolase family 1. The expression pattern analyzed by real-time quantitative PCR showed no significant difference among different tissues, developmental stages, and cultivars under normal growth conditions. Furthermore, the gene function of SrRG1 was preliminarily studied by agrobacterium-mediated transformation on instantaneous expression. In the test of agrobacterium-mediated transformation on instantaneous expression, it was observed that overexpression of SrRG1 increased the accumulation of steviol content and decreased the major components and total SGs contents. Such results demonstrated that SrRG1 may participate in the steviol glycosides catabolic pathway. However, the effect of silencing construct infiltration on steviol and SGs content was not significant and its expression pattern was constitutive, which most probably, attributed the hydrolysis of SGs to the secondary activity of SrRG1. This study firstly identified the bate-glucosidase in stevia and advances our understanding of steviol glycosides hydrolyzation.

The SrWRKY71 transcription factor negatively regulates SrUGT76G1 expression in Stevia rebaudiana

SrUGT76G1 is vital for the biosynthesis of rebaudioside A, D and M in Stevia rebaudiana Bertoni; however, its transcriptional regulatory mechanism remains unknown. In this study, the 2050-bp promoter region of SrUGT76G1 was isolated by the TAIL-PCR method, and sequence analysis revealed the presence of several W-box cis-elements, which are the recognition motifs of WRKY transcription factors. Furthermore, SrWRKY71, characterized by a typical WRKY domain and a C2H2 zinc finger-like motif, was identified as a putative transcriptional regulator of SrUGT76G1. The transcript of SrWRKY71 predominantly accumulated in leaves and was present at a lower level in stems, roots and flowers. The SrWRKY71-GFP fusion protein was specifically localized to the nucleus in tobacco epidermal cells. In addition, the N and C terminal regions of SrWRKY71 contributed to its transactivation activity. Y1H and EMSA assays validated that SrWRKY71 binds directly to W-box1 and W-box2 in the proximal promoter region of SrUGT76G1. Moreover, SrWRKY71 represses the expression level of SrUGT76G1 in both tobacco leaves and stevia callus. Taken together, the data in this study represent the first identification of an essential upstream transcription factor of SrUGT76G1 and provides new insight into the regulatory network of steviol glycoside biosynthesis in Stevia rebaudiana.

Comparison of uridine diphosphate-glycosyltransferase UGT76G1 genes from some varieties of Stevia rebaudiana Bertoni

Stevia leaves contain various components, such as flavonoids, labdanes, chlorophylls, sterols, triterpenoids, mono-disaccharides, organic acids and inorganic salts. Stevia is known to accumulate diterpenoid steviol glycosides, which are approximately 300 times sweeter than regular sugar. Stevioside and rebaudioside A are the main diterpenic glycosides in stevia. Steviol glycosides are the secondary metabolites responsible for the sweetness of stevia. The main objectives of the present study were to determine the concentrations of diterpenic glycosides (stevioside and rebaudioside A) in three stevia varieties (Stevia rebaudiana) via the HPLC-UV technique and to amplify the UGT76G1 gene by PCR using gene-specific primers. The expression levels of the UGT76G1 gene were determined in the three stevia varieties. The PCR products were sequenced and analyzed, and the nucleotide sequences of the UGT76G1 gene were submitted to GenBank and assigned to the following three varieties: Egy1 (MH087463), China1 (MH087464) and Sponti (MH087465). Cluster analysis was used to separate the three varieties into two major clusters based on their phylogenetic relationship. In addition, chemical analysis was carried out to evaluate stevioside and rebaudioside A. The present study concluded that Egy1 and Sponti are closely related varieties as they fall in the same cluster, while China1 forms a separate cluster. Bioprospecting studies could be useful for selection of superior ecotypes of Stevia rebaudiana.

Molecular basis for branched steviol glucoside biosynthesis

Steviol glucosides, such as stevioside and rebaudioside A, are natural products roughly 200-fold sweeter than sugar and are used as natural, noncaloric sweeteners. Biosynthesis of rebaudioside A, and other related stevia glucosides, involves formation of the steviol diterpenoid followed by a series of glycosylations catalyzed by uridine diphosphate (UDP)-dependent glucosyltransferases. UGT76G1 from Stevia rebaudiana catalyzes the formation of the branched-chain glucoside that defines the stevia molecule and is critical for its high-intensity sweetness. Here, we report the 3D structure of the UDP-glucosyltransferase UGT76G1, including a complex of the protein with UDP and rebaudioside A bound in the active site. The X-ray crystal structure and biochemical analysis of site-directed mutants identifies a catalytic histidine and how the acceptor site of UGT76G1 achieves regioselectivity for branched-glucoside synthesis. The active site accommodates a two-glucosyl side chain and provides a site for addition of a third sugar molecule to the C3' position of the first C13 sugar group of stevioside. This structure provides insight on the glycosylation of other naturally occurring sweeteners, such as the mogrosides from monk fruit, and a possible template for engineering of steviol biosynthesis.

Mutations in the uridine diphosphate glucosyltransferase 76G1 gene result in different contents of the major steviol glycosides in Stevia rebaudiana

In the metabolic glycosylation grid of steviol glycosides, UGT76G1 was shown to catalyze at least eight different glucosylation steps, including the formation of rebaudioside B (Reb B) and rebaudioside A (Reb A) (Olsson et al., 2016). In this study, the accumulation of steviolbioside, Reb B, stevioside (ST) and Reb A in more than 140 samples of stevia leaves collected from different regions in China were analyzed by high-performance liquid chromatography (HPLC), and five genotypes, 'N01-N05', with significantly different levels of the abovementioned glycosides were discovered. Mutations in the UGT76G1 gene cloned from cDNAs from these five genotypes were identified, and the functions of the recombinant UGT76G1 variants were ascertained by adding steviolbioside and ST substrates. In addition, homology modeling and molecular docking were used to elucidate the functional differences between variants and UGT76G1. Comparing the sequences of the five variants 'N01-N05' with UGT76G1 (AY345974.1) revealed that base substitutions were not observed in 'N01'. By contrast, 'N02' exhibited 9 single nucleotide polymorphisms (SNPs) and 9 associated amino acid substitutions or insertions with notable variations in the protein structure; however, an enzyme assay showed similar functionalities between the variant and UGT76G1. In 'N03', 49 SNPs and 29 associated amino acid substitutions or insertions were identified and shown to induce significant variations in the protein structure, especially in the binding pocket, resulting in the lack of functionality of this variant in the enzyme assay. These results were in agreement with the docking profiles. Moreover, a nonsense mutation of p.1090T > G in 'N04' and an insertion of a 68 base fragment in 'N05' were found, and both produced a premature protein without any catalytic activity. Therefore, UGT76G1, which is vital to the content of main steviol glycosides, should be a key gene marker for the molecular breeding of Stevia rebaudiana. Our investigations also revealed the location and orientation of active groups of the receptors and donors in the UGT76G1 enzyme, which play key roles in determining whether the enzyme has any enzymatic activity.

UGT76G1 polymorphism in Stevia rebaudiana: New variants for steviol glycosides conjugation

Steviol glycosides (SVglys) are secondary metabolites derived from terpenoids exhibiting high-sweetening properties produced in Stevia rebaudiana leaves. Their great diversity is due to the number, the position and the nature of glycosylations on the steviol aglycone. Steviol conjugation is mediated by uridine-diphosphate glycosyltransferases (UGTs). Four UGTs have been clearly identified as involved in SVglys metabolism: UGT74G1, UGT85C2, UGT76G1 and UGT73E1. Natural non-functional mutants with nonsense codon have yet been observed for UGT76G1. To investigate the variability of UGT76G1 functionality, natural mutants with low or no content of rebaudioside A and C were identified in a germplasm collection of Stevia rebaudiana. These compounds are known to be the direct products of UGT76G1 and their biosynthesis is governed by a single gene at the locus Rae (Rebaudioside A enablement). Crosses were done with remarkable accessions including phenotypes with low (0-3%) and high proportions (70%) of rebaudioside A and C, to investigate the functionality of the Rae locus in the parents. Seven variants of UGT76G1 were found, among them 4 lead to a functional protein and 3 lead to non-functional isoforms. Five of these variants are new. We found that non-functionality of UGT76G1 towards SVglys is not due to a premature nonsense codon, which appears to be an extreme case to explain the loss of functionality of an UGT. Variations in steviol glycoside profile in stevia leaves is partly due to UGT76G1 polymorphism: amino acid substitutions in parts of the protein involved in the substrate specificity can be found by sequence comparison.

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.

Potential applications of glucosyltransferases in terpene glucoside production: impacts on the use of aroma and fragrance

The detection of glucoconjugated forms of monoterpene alcohols in rose petals in the late 1960s opened the new field of nonvolatile aroma precursors in flavor research. It is now well established that odorless glycosides represent a significant pool of aroma precursors in plants where they act as preformed but inactivated defense or attractive chemicals. Technical improvements in the separation and identification of plant secondary metabolites have provided a multitude of chemical structures, but functional characterization of glycosyltransferases that catalyze their formation lags behind. As technical efforts and costs for DNA sequencing dramatically dropped during the last decade, the number of plant genome sequences increased significantly, thus providing opportunities to functionally characterize the glycosyltransferase gene families in plants. These studies yielded the first glycosyltransferase genes that encode efficient biocatalysts for the production of monoterpene glucosides. They have applications in the food, feed, chemical, cosmetic, and pharmaceutical industries as slow release aroma chemicals.