Agrobacterium mediated transient gene silencing (AMTS) in Stevia rebaudiana: insights into steviol glycoside biosynthesis pathway

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.

Oligosaccharides increased both leaf biomass and steviol glycosides content of Stevia rebaudiana

Steviol glycosides (SGs) are a variety of important natural sweeteners. They are 200-350 times sweeter than sucrose without calories. Currently, their production is still mainly dependent on extraction from Stevia rebaudiana Bertoni (stevia). Oligosaccharides are environmentally friendly elicitors that promote plant growth and accumulation of secondary metabolites. In the present study, different concentrations of chitosan oligosaccharides (COS) and alginate oligosaccharides (AOS) were applied to stevia to explore their effect on growth and SGs biosynthesis. It was found that both COS and AOS promoted biomass production by increasing the leaf number and photosynthetic efficiency, which may be related to the decreased content of abscisic acid. The content of SGs was significantly increased after 50 mg/L AOS treatment, which not only increased the contents of stevioside (STV) and rebaudioside A (Reb A) significantly, but some important minority glucosides, like Reb E, Reb D, and Reb M. The increased SGs contents were the combined effect of the higher expression of SGs biosynthesis related genes, including KAH, UGT74G1, UGT85C2, and UGT91D2. The geometry changes of stem induced by COS and AOS may help to increase the lodging resistance of stevia. Thus, COS and AOS can be used in the field planting of stevia to increase the yield of SGs for industrial purposes.

Seeds of Stevia rebaudiana Bertoni as a Source of Plant Growth-Promoting Endophytic Bacteria with the Potential to Synthesize Rebaudioside A

In this study, a new strain of Pantoea vagans, SRS89, was isolated from surface-sterilized stevia seeds. The isolate was evaluated using morphological, molecular, and biochemical methods. The bacterium was 1.5 μm long, yellowish in color, and classified as Gram-negative. Whole genome sequencing of our strain revealed the presence of a 4,610,019 bp chromosome, and genome annotation resulted in the detection of 4283 genes encoding 4204 putative coding sequences. Phylogenic analysis classified the genome of our strain close to the MP7 and LMG 24199 strains of P. vagans. Functional analysis showed that the highest number of genes within the analyzed bacterium genome were involved in transcription, amino acid transport and metabolism, and carbohydrate transport and metabolism. We also identified genes for enzymes involved in the biosynthesis of carotenoids and terpenoids. Furthermore, we showed the presence of growth regulators, with the highest amount noted for gibberellic acid A₃, indole-3-acetic acid, and benzoic acid. However, the most promising property of this strain is its ability to synthesize rebaudioside A; the estimated amount quantified using reversed-phase (RP)-HPLC was 4.39 mg/g of the dry weight of the bacteria culture. The isolated endophytic bacterium may be an interesting new approach to the production of this valuable metabolite.

An interdisciplinary approach towards sustainable and higher steviol glycoside production from in vitro cultures of Stevia rebaudiana

Stevia rebaudiana is one of the vastly acclaimed commercial plant in the world and belongs to Asteraceae family. The exclusive advantage of Stevia over artificial sweeteners is impeccable and targets its potentiality to the presence of diterpene glycosides. Moreover, the flaunting sweetness of steviol glycosides with associated medicinal benefits, turns the plant to be one of the most economic assets, globally. As compared to vegetative propagation through stem-cuttings, plant tissue culture is the most suitable approach in obtaining true-to-type plants of superior quality. During last few decades, significant in vitro propagation methods have been developed and still the research is ongoing. The present review discusses the tissue culture perspectives of S. rebaudiana, primarily focusing on the mineral nutrition, growth regulators and other accessory factors, motioning the optimum growth and development of the plant. Another crucial aspect is the generation of sweeter varieties in order to reduce the bitter-off taste, which is noticed after the consumption of the leaves. The in vitro cultures pose an efficient alternative system for production of steviol glycosides, with higher rebaudioside(s) content. Moreover, the review also covers the recent approaches pertaining to scale-up studies and genome editing perspectives.

PhUGT78A22, a novel glycosyltransferase in Paeonia 'He Xie', can catalyze the transfer of glucose to glucosylated anthocyanins during petal blotch formation

BACKGROUND

Flower color patterns play an important role in the evolution and subsequent diversification of flowers by attracting animal pollinators. This interaction can drive the diversity observed in angiosperms today in many plant families such as Liliaceae, Paeoniaceae, and Orchidaceae, and increased their ornamental values. However, the molecular mechanism underlying the differential distribution of anthocyanins within petals remains unclear in Paeonia.

RESULTS

In this study, we used an intersectional hybrid between the section Moutan and Paeonia, hereafter named Paeonia 'He Xie', which has purple flowers with dark purple blotches. After Ultra-high performance liquid chromatography-diode array detector (UPLC-DAD) analysis of blotched and non-blotched parts of petals, we found the anthocyanin content in the blotched part was always higher than that in the non-blotched part. Four kinds of anthocyanins, namely cyanidin-3-O-glucoside (Cy3G), cyanidin-3,5-O-glucoside (Cy3G5G), peonidin-3-O-glucoside (Pn3G), and peonidin-3,5-O-glucoside (Pn3G5G) were detected in the blotched parts, while only Cy3G5G and Pn3G5G were detected in the non-blotched parts. This suggests that glucosyltransferases may play a vital role in the four kinds of glucosylated anthocyanins in the blotched parts. Moreover, 2433 differentially expressed genes (DEGs) were obtained from transcriptome analysis of blotched and non-blotched parts, and a key UDP-glycosyltransferase named PhUGT78A22 was identified, which could use Cy3G and Pn3G as substrates to produce Cy3G5G and Pn3G5G, respectively, in vitro. Furthermore, silencing of PhUGT78A22 reduced the content of anthocyanidin 3,5-O-diglucoside in P. 'He Xie'.

CONCLUSIONS

A UDP-glycosyltransferase, PhUGT78A22, was identified in P. 'He Xie', and the molecular mechanism underlying differential distribution of anthocyanins within petals was elucidated. This study provides new insights on the biosynthesis of different kinds of anthocyanins within colorful petals, and helps to explain petal blotch formation, which will facilitate the cultivar breeding with respect to increasing ornamental value. Additionally, it provides a reference for understanding the molecular mechanisms responsible for precise regulation of anthocyanin biosynthesis and distribution patterns.

The role of metabolites of steviol glycosides and their glucosylated derivatives against diabetes-related metabolic disorders

Diabetes mellitus (DM), characterized by abnormal carbohydrate, lipid, and protein metabolism, is a metabolic disorder caused by a shortage of insulin secretion or decreased sensitivity of target cells to insulin. In addition to changes in lifestyle, a low-calorie diet is recommended to reduce the development of DM. Steviol glycosides (SGs), as natural sweeteners, have gained attention as sucrose alternatives because of their advantages of high sweetness and being low calorie. Most SGs with multiple bioactivities are beneficial to regulate physiological functions. Though SGs have been widely applied in food industry, there is little data on their glucosylated derivatives that are glucosylated steviol glycosides (GSGs). In this review, we have discussed the metabolic fate of GSGs in contrast to SGs, and the molecular mechanisms of glycoside metabolites against diabetes-related metabolic disorders are also summarized. SGs are generally extracted from the Stevia leaf, while GSGs are mainly manufactured using enzymes that transfer glucose units from a starch source to SGs. Results from this study suggest that SGs and GSGs share same bioactive metabolites, steviol and steviol glucuronide (SVG), which exhibit anti-hyperglycemic effects by activating glucose-induced insulin secretion to enhance pancreatic β-cell function. In addition, steviol and SVG have been found to ameliorate the inflammatory response, lipid imbalance, myocardial fibrosis and renal functions to modulate diabetes-related metabolic disorders. Therefore, both SGs and GSGs may be used as potential sucrose alternatives and/or pharmacological alternatives for preventing and treating metabolic disorders.

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.

Exogenous naringenin improved digestible protein accumulation and altered morphology via VrPIN and auxin redistribution in Vigna radiata

Naringenin exposure altered auxin redistribution via VrPIN1 leading to morphological alterations and significantly reduced the protein precipitable tannins that further enhanced the protein accumulation and bioavailability. Flavonoid exposure is known to affect the antioxidant profile of legumes. However, a detailed study evaluating the effect of flavonoid naringenin on morphology and biochemical profile of legume is lacking. The present study is a novel report of improved in planta protein bioavailability and antioxidant potential of legume mungbean on naringenin exposure. The quantitative evaluation revealed significant protein accumulation (64-122 μg/g FW) on naringenin exposure. Further, an increase in protein solubility and digestibility compared to control was evident. Naringenin mediated altered α-amylase activity improved the mungbean seed germination rate. Naringenin induced auxin redistribution and altered PIN formed transcript expression reduced lateral root density and increased stem length that was subsequently reverted on exogenous indole acetic acid application. Naringenin enhanced polyphenolic accumulation and improved the antioxidant potential of mungbean. Additionally, the responsiveness of the early gene of the flavonoid biosynthetic pathway, Chalcone isomerase to naringenin concentration was revealed indicating a probable feedback regulation. Further, the presence of alternate liquiritigenin biosynthesis was also evident. The present study, thus reveals the probable potential of phytochemical naringenin towards agricultural sustainability in the changing environmental conditions.

PkGPPS.SSU interacts with two PkGGPPS to form heteromeric GPPS in Picrorhiza kurrooa: Molecular insights into the picroside biosynthetic pathway

Geranyl geranyl pyrophosphate synthase (GGPPS) is known to form an integral subunit of the heteromeric GPPS (geranyl pyrophosphate synthase) complex and catalyzes the biosynthesis of monoterpene in plants. Picrorhiza kurrooa Royle ex Benth., a medicinally important high altitude plant is known for picroside biomolecules, the monoterpenoids. However, the significance of heteromeric GPPS in P. kurrooa still remains obscure. Here, transient silencing of PkGGPPS was observed to reduce picroside-I (P-I) content by more than 60% as well as picroside-II (P-II) by more than 75%. Thus, PkGGPPS was found to be involved in the biosynthesis of P-I and P-II besides other terpenoids. To unravel the mechanism, small subunit of GPPS (PkGPPS.SSU) was identified from P. kurrooa. Protein-protein interaction studies in yeast as well as bimolecular fluorescence complementation (BiFC) in planta have indicated that large subunit of GPPS PkGPPS.LSUs (PkGGPPS1 and PkGGPPS2) and PkGPPS.SSU form a heteromeric GPPS. Presence of similar conserved domains such as light responsive motifs, low temperature responsive elements (LTRE), dehydration responsive elements (DREs), W Box and MeJA responsive elements in the promoters of PkGPPS.LSU and PkGPPS.SSU documented their involvement in picroside biosynthesis. Further, the tissue specific transcript expression analysis vis-à-vis epigenetic regulation (DNA methylation) of promoters as well as coding regions of PkGPPS.LSU and PkGPPS.SSU has strongly suggested their role in picroside biosynthesis. Taken together, the newly identified PkGPPS.SSU formed the heteromeric GPPS by interacting with PkGPPS.LSUs to synthesize P-I and P-II in P. kurrooa.

Steviol glycosides profile in Stevia rebaudiana Bertoni hairy roots cultured under oxidative stress-inducing conditions

The ability to synthesize particular steviol glycosides (SvGls) was studied in Stevia rebaudiana Bertoni hairy roots (HR) grown in the light or in the dark under the influence of different osmotic active compounds. Manipulation of culture conditions led to changes in the morphology and growth rate of HR, as well as to an increase in oxidative stress manifested as an enhancement in endogenous hydrogen peroxide concentration in the cultured samples. The highest level of H₂O₂ was noted in HR cultured under light or in the medium with the highest osmotic potential. This correlated with the highest increase in the expression level of ent-kaurenoic acid hydroxylase, responsible for the redirection of metabolic route to SvGls biosynthesis pathway. An analysis of transcriptional activity of some UDPglucosyltransferase (UGT85c2, UGT74g1, UGT76g1) revealed that all of them were upregulated due to the manipulation of culture conditions. However, the level of their upregulation depended on the type of stress factor used in our experiment. Analysis of SvGls content revealed that HR grown under all applied conditions were able to synthesize and accumulate several SvGls but their concentration differed between the samples across the different conditions. The level of rebaudioside A concentration exceeded the content of stevioside in HR in all tested conditions. Concomitantly, the presence of some minor SvGls, such as steviolbioside and rebaudioside F, was confirmed only in HR cultured in the lowest osmotic potential of the medium while rebaudioside D was also detected in the samples cultured in the media supplemented with NaCl or PEG.Key Points● Several steviol glycosides are synthesized in hairy roots of S. rebaudiana.● Light or osmotic factors cause enhancement in oxidative stress level in hairy roots.● It correlates with a significant increase in the level of KAH expression.● UGTs expression and steviol glycosides content depends on culture conditions.

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.

An Efficient Stevia rebaudiana Transformation System and In vitro Enzyme Assays Reveal Novel Insights into UGT76G1 Function

Stevia rebaudiana (Bertoni) is one of a very few plant species that produce zero calorie, sweet compounds known as steviol glycosides (SG). SGs differ in their sweetness and organoleptic properties depending on the number and positioning of sugar groups on the core steviol backbone. There is great interest of modulating the SG profiles of the Stevia plant to enhance the flavor profile for a given application in the food and beverage industries. Here, we report a highly efficient Agrobacterium-mediated stable transformation system using axillary shoots as the initial explant. Using this system, we generated over 200 transgenic Stevia plants overexpressing a specific isoform of UGT76G1. By comparing the SG profiles among independent transgenic events, we demonstrated that altering UGT76G1 expression can change the ratios of specific SG species. Furthermore, using recombinant proteins produced in E. coli, we show that two closely related UGT76G1 isoforms differ in their substrate specificities, providing new insights into mechanisms underlying the diversity of SG profiles that are observed across Stevia germplasm. Finally, we found evidence suggesting that alternative and/or aberrant splicing may serve to influence the ability of the plant to produce functional UGT76G1 transcripts, and possibly produce enzyme variants within the plant.

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.

Overexpression of SrUGT76G1 in Stevia alters major steviol glycosides composition towards improved quality

Steviol glycosides (SGs) are extracted from Stevia leaves for use as a natural sweetener. Among SGs, stevioside is most abundant in leaf extracts followed by rebaudioside A (Reb A). However, Reb A is of particular interest because of its sweeter and more pleasant taste compared to stevioside. Therefore, the development of new Stevia varieties with a higher Reb A to stevioside ratio would be desirable for the production of higher quality natural sweeteners. Here, we generated transgenic Stevia plants overexpressing Stevia UDP-glycosyltransferase 76G1 (SrUGT76G1) that is known to convert stevioside to Reb A through 1,3-β-d-glucosylation in vitro. Interestingly, by overexpressing SrUGT76G1, the Reb A to stevioside ratio was drastically increased from 0.30 in wild-type (WT) plants up to 1.55 in transgenic lines without any significant changes in total SGs content. This was contributed by a concurrent increase in Reb A content and a decrease in stevioside content. Additionally, we were able to find an increase in the Reb C to dulcoside A ratio in transgenic lines. Using the glutathione S-transferase-tagged SrUGT76G1 recombinant protein for an in vitro glucosyltransferase assay, we further demonstrated that Reb C can be produced from the glucosylation of dulcoside A by SrUGT76G1. Transgenic Stevia plants having higher Reb A to stevioside ratio were visually indistinguishable from WT plants. Taken together, our results demonstrate that the overexpression of SrUGT76G1 in Stevia is an effective way to generate new Stevia varieties with higher proportion of the more preferred Reb A without compromising on plant development.

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.

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.

Isoprenoid generating systems in plants - A handy toolbox how to assess contribution of the mevalonate and methylerythritol phosphate pathways to the biosynthetic process

Isoprenoids comprise an astonishingly diverse group of metabolites with numerous potential and actual applications in medicine, agriculture and the chemical industry. Generation of efficient platforms producing isoprenoids is a target of numerous laboratories. Such efforts are generally enhanced if the native biosynthetic routes can be identified, and if the regulatory mechanisms responsible for the biosynthesis of the compound(s) of interest can be determined. In this review a critical summary of the techniques applied to establish the contribution of the two alternative routes of isoprenoid production operating in plant cells, the mevalonate and methylerythritol pathways, with a focus on their co-operation (cross-talk) is presented. Special attention has been paid to methodological aspects of the referred studies, in order to give the reader a deeper understanding for the nuances of these powerful techniques. This review has been designed as an organized toolbox, which might offer the researchers comments useful both for project design and for interpretation of results obtained.

Assessment of RNAi-induced silencing in banana (Musa spp.)

BACKGROUND

In plants, RNA- based gene silencing mediated by small RNAs functions at the transcriptional or post-transcriptional level to negatively regulate target genes, repetitive sequences, viral RNAs and/or transposon elements. Post-transcriptional gene silencing (PTGS) or the RNA interference (RNAi) approach has been achieved in a wide range of plant species for inhibiting the expression of target genes by generating double-stranded RNA (dsRNA). However, to our knowledge, successful RNAi-application to knock-down endogenous genes has not been reported in the important staple food crop banana.

RESULTS

Using embryogenic cell suspension (ECS) transformed with ß-glucuronidase (GUS) as a model system, we assessed silencing of gusAINT using three intron-spliced hairpin RNA (ihpRNA) constructs containing gusAINT sequences of 299-nt, 26-nt and 19-nt, respectively. Their silencing potential was analysed in 2 different experimental set-ups. In the first, Agrobacterium-mediated co-transformation of banana ECS with a gusAINT containing vector and an ihpRNA construct resulted in a significantly reduced GUS enzyme activity 6-8 days after co-cultivation with either the 299-nt and 19-nt ihpRNA vectors. In the second approach, these ihpRNA constructs were transferred to stable GUS-expressing ECS and their silencing potential was evaluated in the regenerated in vitro plants. In comparison to control plants, transgenic plants transformed with the 299-nt gusAINT targeting sequence showed a 4.5 fold down-regulated gusA mRNA expression level, while GUS enzyme activity was reduced by 9 fold. Histochemical staining of plant tissues confirmed these findings. Northern blotting used to detect the expression of siRNA in the 299-nt ihpRNA vector transgenic in vitro plants revealed a negative relationship between siRNA expression and GUS enzyme activity. In contrast, no reduction in GUS activity or GUS mRNA expression occurred in the regenerated lines transformed with either of the two gusAINT oligo target sequences (26-nt and 19-nt).

CONCLUSIONS

RNAi-induced silencing was achieved in banana, both at transient and stable level, resulting in significant reduction of gene expression and enzyme activity. The success of silencing was dependent on the targeted region of the target gene. The successful generation of transgenic ECS for second transformation with (an)other construct(s) can be of value for functional genomics research in banana.