Effects of salt stress on the growth, physiological responses, and glycoside contents of Stevia rebaudiana Bertoni

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.

Genome-wide transcriptional profiling and physiological investigation elucidating the molecular mechanism of multiple abiotic stress response in Stevia rebaudiana Bertoni

Considering the major source of plant-derived low/non-calorie steviol glycosides (SGs), comprehensive physiological, biochemical, and deep transcriptional investigations were conducted to explicit deeper insight into multiple abiotic stress responses in Stevia rebaudiana. The physiological indicators including photosynthesis, chlorophyll, relative water content, shoot growth, electrolyte leakage, and SG biosynthesis were negatively impacted under drought (DS), followed by salinity (SS) and waterlogging (WS). Global transcriptional analysis revealed significant upregulated expression of the genes encoding for ROS detoxification (GST, SOD, APX, glutathione peroxidase), osmotic adjustment (alpha-trehalose-phosphate and S-adenosylmethionine decarboxylase), ion transporters (CAX, NHX, CNGS, VPPase, VATPase), water channel (PIP1, TIP) and abiotic stress-responsive candidate genes (LEA, HSPs, and Dehydrins) regulating abiotic stress response in S. rebaudiana. These inferences were complemented with predicted interactome network that revealed regulation of energy metabolism by key stress-responsive genes (GST, HKT1, MAPKs, P5CSs, PIP), transcription factors (HSFA2, DREB1A, DREB2A), and abiotic stress responsive pathways (ABA, ethylene, ion stress). This is the first detailed study to comprehend the molecular regulation of stress response and their interplay under DS, SS, and WS. The key genes and regulators can be functionally validated, and will facilitate targeted gene editing for genetic improvement of crop sustainability under changing environmental conditions in S. rebaudiana.

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.

The Effect of Glycine Betaine on Nitrogen and Polyamine Metabolisms, Expression of Glycoside-Related Biosynthetic Enzymes, and K/Na Balance of Stevia under Salt Stress

The beneficial role of glycine betaine (GB) in the adaptation of plants to abiotic stresses is well known; therefore, the study of physiological and molecular responses induced by exogenous GB under NaCl stress can provide a suitable reference for the application of this compound to enhance the adaptation of plants to salinity. The present study was conducted under in vitro conditions to evaluate the effect of GB (25 and 50 mM) on the growth, physiological, and molecular traits of Stevia rebaudiana during NaCl toxicity (50 mM). The results showed that applying NaCl treatment increased Na accumulation, induced oxidative stress, and disrupted N metabolism and K/Na homeostasis, which, as a result, decreased the stevia plant's growth and biomass. However, application of GB improved the adaptation of NaCl-stressed plants by improving N metabolism and modulating the metabolism of polyamines. By increasing the activity of antioxidant enzymes, GB diminished oxidative stress, protected the plasma membrane, and restored photosynthetic pigments under NaCl toxicity. By reducing Na accumulation and increasing K accumulation, GB maintained the K/Na balance and reduced the effects of toxicity caused by the high Na concentration in stevia leaves. GB increased the leaf accumulation of rebaudioside A in NaCl-stressed plants by modulating the expression of genes (KAH, UGT74G1, UGT76G1, and UGT85C2) involved in the sugar compounds of the stevia plants. Our results provide a broad understanding of GB-induced responses in NaCl-stressed plants, which can help increase our knowledge of the role of GB in the defense mechanisms of plants under abiotic stresses.

The Content of Phenolic Compounds in Stevia rebaudiana (Bertoni) Plants Derived from Melatonin and NaCl Treated Seeds

Stevia is a plant with many beneficial properties. It contains not only steviol glycosides, which are used as non-caloric natural sweeteners, but also a number of metabolites with antioxidant properties. This study examined the content of both phenolic acids and flavonoids in stevia leaves as an effect of treating seeds with melatonin and conducting germination in NaCl conditions. The results of our research indicated higher amounts of phenolic acids compared to flavonoids in stevia leaves. Among these acids, isochlorogenic, rosmarinic, and chlorogenic acids were accumulated in the largest amounts, regardless of the germination conditions. For 5 and 100 µM of melatonin treatments, the content of both phenolic acids and flavonoids increased. However, in salinity conditions (50 mM NaCl), 500 µM of melatonin had the most favorable effect on the synthesis of phenolic acids. The phenolic acids in that case reached a level three-times higher than that in the samples with the same melatonin concentration but without NaCl. We also found that the content of phenolic compounds varied depending on the age of the leaves. To the best of our knowledge, this is the first study to describe the effect of melatonin and NaCl on the synthesis on phenolic acids and flavonoids in stevia.

Elicitation of Submerged Adventitious Root Cultures of Stevia rebaudiana with Cuscuta reflexa for Production of Biomass and Secondary Metabolites

Stevia rebaudiana is an important medicinal plant that belongs to the Asteraceae family. The leaves of Stevia rebaudiana are a rich source of many health-promoting agents such as polyphenols, flavonoids, and steviol glycoside, which play a key role in controlling obesity and diabetes. New strategies such as the elicitation of culture media are needed to enhance the productivity of active components. Herein, the Cuscuta reflexa extracts were exploited as elicitors to enhance the productivity of active components. Cuscuta reflexa is one of the parasitic plants that has the ability to elongate very fast and cover the host plant. Consequently, it may be possible that the addition of Cuscuta reflexa extracts to adventitious root cultures (ADR) of Stevia rebaudiana may elongate the root more than control cultures to produce higher quantities of the desired secondary metabolites. Therefore, the main objective of the current study was to investigate the effect of Cuscuta reflexa extract as a biotic elicitor on the biomass accumulation and production of antioxidant secondary metabolite in submerged adventitious root cultures of Stevia rebaudiana. Ten different concentrations of Cuscuta reflexa were added to liquid media containing 0.5 mg/L naphthalene acetic acid (NAA). The growth kinetics of adventitious roots was investigated for a period of 49 days with an interval of 7 days. The maximum biomass accumulation (7.83 g/3 flasks) was observed on medium containing 10 mg/L extract of Cuscuta reflexa on day 49. As the concentration of extract increases in the culture media, the biomass gradually decreases after 49 days of inoculation. In this study, the higher total phenolics content (0.31 mg GAE/g-DW), total flavonoids content (0.22 mg QE/g-DW), and antioxidant activity (85.54%) were observed in 100 mg/L treated cultures. The higher concentration (100 mg/L) of Cuscuta reflexa extract considerably increased the total phenolics content (TPC), total phenolics production (TPP), total flavonoids content (TFC), total flavonoids production (TFP), total polyphenolics content (TPPC), and total polyphenolics production (TPPP). It was concluded that the extract of Cuscuta reflexa moderately improved biomass accumulation but enhanced the synthesis of phenolics, flavonoids, and antioxidant activities. Here, biomass’s independent production of secondary metabolites was observed with the addition of extract. The present study will be helpful to scale up adventitious roots culture into a bioreactor for the production of secondary metabolites rather than biomass accumulation in medicinally important Stevia rebaudiana.

Salt Stress Mitigation via the Foliar Application of Chitosan-Functionalized Selenium and Anatase Titanium Dioxide Nanoparticles in Stevia (Stevia rebaudiana Bertoni)

High salt levels are one of the significant and major limiting factors on crop yield and productivity. Out of the available attempts made against high salt levels, engineered nanoparticles (NPs) have been widely employed and considered as effective strategies in this regard. Of these NPs, titanium dioxide nanoparticles (TiO₂ NPs) and selenium functionalized using chitosan nanoparticles (Cs–Se NPs) were applied for a quite number of plants, but their potential roles for alleviating the adverse effects of salinity on stevia remains unclear. Stevia (Stevia rebaudiana Bertoni) is one of the reputed medicinal plants due to their diterpenoid steviol glycosides (stevioside and rebaudioside A). For this reason, the current study was designed to investigate the potential of TiO₂ NPs (0, 100 and 200 mg L⁻¹) and Cs–Se NPs (0, 10 and 20 mg L⁻¹) to alleviate salt stress (0, 50 and 100 mM NaCl) in stevia. The findings of the study revealed that salinity decreased the growth and photosynthetic traits but resulted in substantial cell damage through increasing H₂O₂ and MDA content, as well as electrolyte leakage (EL). However, the application of TiO₂ NPs (100 mg L⁻¹) and Cs–Se NPs (20 mg L⁻¹) increased the growth, photosynthetic performance and activity of antioxidant enzymes, and decreased the contents of H₂O₂, MDA and EL under the saline conditions. In addition to the enhanced growth and physiological performance of the plant, the essential oil content was also increased with the treatments of TiO₂ (100 mg L⁻¹) and Cs–Se NPs (20 mg L⁻¹). In addition, the tested NPs treatments increased the concentration of stevioside (in the non-saline condition and under salinity stress) and rebaudioside A (under the salinity conditions) in stevia plants. Overall, the current findings suggest that especially 100 mg L⁻¹ TiO₂ NPs and 20 mg L⁻¹ Cs–Se could be considered as promising agents in combating high levels of salinity in the case of stevia.

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.

Salinity Effects on Gene Expression, Morphological, and Physio-Biochemical Responses of Stevia rebaudiana Bertoni In Vitro

Stevia rebaudiana Bertoni is a little bush, which is cultivated on a large scale in many countries for medicinal purposes and used as a natural sweetener in food products. The present work aims to conduct a protocol for stevia propagation in vitro to produce and introduce Stevia rebaudiana plants as a new sweetener crop to Egyptian agriculture. To efficiently maximize its propagation, it is important to study the influence of stress factors on the growth and development of Stevia rebaudiana grown in vitro. Two stevia varieties were investigated (Sugar High A3 and Spanti) against salt stress. Leaves were used as the source of explants for callus initiation, regeneration, multiplication and rooting. Some stress-related traits, i.e., photosynthetic pigments, proline contents, and enzyme activity for peroxidase (POD), polyphenol oxidase (PPO), and malate dehydrogenase (MDH) were studied. Murashig and Skoog (MS) medium was supplemented with four NaCl concentrations: 500, 1000, 2000, and 3000 mgL⁻¹, while a salt-free medium was used as the control. The data revealed that salinity negatively affected all studied characters: the number of surviving calli, regeneration%, shoot length, the number of multiple shoots, number of leaf plantlets⁻¹, number of root plantlets⁻¹, and root length. The data also revealed that Sugar High A3 is more tolerant than Spanti. The total chlorophyll content decreased gradually with increasing NaCl concentration. However, the opposite was true for proline content. Isozyme’s fractionation exhibited high levels of variability among the two varieties. Various biochemical parameters associated with salt tolerance were detected in POD. Namely, POD4, POD6, POD 9 at an Rf of 0.34, 0.57, and 0.91 in the Sugar High A3 variety under high salt concentration conditions, as well as POD 10 at an Rf of 0.98 in both varieties under high salt concentrations. In addition, the overexpression of POD 5 and POD 10 at Rf 0.52 and 0.83 was found in both varieties at high NaCl concentrations. Biochemical parameters associated with salt tolerance were detected in PPO (PPO1, PPO2 and PPO4 at an Rf of 0.38, 0.42 and 0.62 in the Sugar High A3 variety under high salt concentrations) and MDH (MDH 3 at an Rf of 0.40 in both varieties at high salt concentrations). Therefore, these could be considered as important biochemical markers associated with salt tolerance and could be applied in stevia breeding programs (marker-assisted selection). This investigation recommends stevia variety Sugar High A3 to be cultivated under salt conditions.

Variations in morphology, physiology, and multiple bioactive constituents of Lonicerae Japonicae Flos under salt stress

Lonicerae Japonicae Flos (LJF) is an important traditional Chinese medicine for the treatment of various ailments and plays a vital role in improving global human health. However, as unable to escape from adversity, the quality of sessile organisms is dramatically affected by salt stress. To systematically explore the quality formation of LJF in morphology, physiology, and bioactive constituents' response to multiple levels of salt stress, UFLC-QTRAP-MS/MS and multivariate statistical analysis were performed. Lonicera japonica Thunb. was planted in pots and placed in the field, then harvested after 35 days under salt stress. Indexes of growth, photosynthetic pigments, osmolytes, lipid peroxidation, and antioxidant enzymes were identified to evaluate the salt tolerance in LJF under different salt stresses (0, 100, 200, and 300 mM NaCl). Then, the total accumulation and dynamic variation of 47 bioactive constituents were quantitated. Finally, Partial least squares discrimination analysis and gray relational analysis were performed to systematically cluster, distinguish, and evaluate the samples, respectively. The results showed that 100 mM NaCl induced growth, photosynthetic, antioxidant activities, osmolytes, lipid peroxidation, and multiple bioactive constituents in LJF, which possessed the best quality. Additionally, a positive correlation was found between the accumulation of phenolic acids with antioxidant enzyme activity under salt stress, further confirming that phenolic acids could reduce oxidative damage. This study provides insight into the quality formation and valuable information to improve the LJF medicinal value under salt stress.

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.

Stevia rebaudiana Bertoni responses to salt stress and chitosan elicitor

This study examined the effect of chitosan elicitor with four different concentrations (0, 0.2, 0.4 and 0.6 g/l) on physiological and biochemical properties of stevia under four levels of salinity stress (0, 50, 100, 150 mM level of NaCl). Salt stress caused reduction of chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll, carotenoid and total protein content. The increment of malondialdehyde (MDA) content was not significant in all NaCl levels, while the CAT and POX activities were increased as well as stevioside and rebaudioside A under salinity stress. On one side, chitosan treatments could compensate the reduction of physiological traits such as photosynthetic pigments and protein content. On the other side, chitosan caused multiple increases in malondialdehyde content, antioxidant enzymes activity (catalase and peroxidase), steviol glycosides (stevioside and rebaudioside A) under salt stress. We report for the first time, the potential of chitosan to enhance salinity-tolerant abilities in stevia through increment of the salt-adaptive factors and to diminish harmful damages caused by NaCl stress.

Saline and Alkaline Tolerance of Wetland Plants—What are the Most Representative Evaluation Indicators?

The increasing discharge of wastewater containing inorganic salts, sometimes accompanied by high pH, has been a worldwide environmental problem. Constructed wetlands (CWs) are considered a viable technology for treating saline and/or alkaline wastewater provided that saline-alkaline tolerant plant species are selected and applied. The influence of both saline and alkaline stress on four wetland plant species during their seed germination, early growth, vegetative propagation and continued growth stages was evaluated by three experiments. Principal component analysis (PCA) was conducted for selecting representative indicators for evaluating the saline and alkaline tolerance of plants during vegetative propagation and plant growth stages. The saline and alkaline stress inhibited the vegetative propagation and plant growth of all tested plant species to varying degrees, therein the influences of saline-alkaline stress on plants were more marked than saline stress. The length of new roots, Na+ accumulation in plant tissue, Na+/K+ ratios in aerial tissue and the total dry biomass were selected as most representative indicators for evaluating the saline and alkaline tolerance of plants. Iris sibirica and Lythrum salicaria showed better saline and alkaline tolerance ability among tested species and could be grown in CWs for treating saline and/or alkaline wastewater.

Antioxidant Metabolism and Chlorophyll Fluorescence during the Acclimatisation to Ex Vitro Conditions of Micropropagated Stevia rebaudiana Bertoni Plants

In this study, the functioning of antioxidant metabolism and photosynthesis efficiency during the acclimatisation of Stevia rebaudiana plants to ex vitro conditions was determined. A high percentage of acclimatised plants (93.3%) was obtained after four weeks. According to the extent of lipid peroxidation, an oxidative stress occurred during the first hours of acclimatisation. A lower activity of monodehydroascorbate reductase (MDHAR) than dehydroascorbate reductase (DHAR) was observed after 2 days of acclimatisation. However, after 7 days of acclimatisation, stevia plants activated the MDHAR route to recycle ascorbate, which is much more efficient energetically than the DHAR route. Superoxide dismutase and catalase activities showed a peak of activity after 7 days of acclimatisation, suggesting a protection against reactive oxygen species. Peroxidase activity increased about 2-fold after 2 days of acclimatisation and remained high until day 14, probably linked to the cell wall stiffening and the lignification processes. In addition, a progressive increase in the photochemical quenching parameters and the electronic transport rate was observed, coupled with a decrease in the non-photochemical quenching parameters, which indicate a progressive photosynthetic efficiency during this process. Taken together, antioxidant enzymes, lipid peroxidation, and chlorophyll fluorescence are proven as suitable tools for the physiological state evaluation of micropropagated plants during acclimatisation to ex vitro conditions.

Simple and Efficient Green Extraction of Steviol Glycosides from Stevia rebaudiana Leaves

The food industry has currently shown great interest in alternative sweeteners to sugars with the aim of producing healthier products. In light of this, steviol glycosides are natural low-caloric sweeteners present in Stevia rebaudiana, which have additionally been described as bioactive components with potential therapeutic properties. In this work, a green method for the extraction of steviol glycosides from stevia leaves was optimized by applying a factorial screening design of five variables (time, temperature, agitation, grinding, and sample-solvent ratio) and the subsequent response surface design of Box-Behnken. The optimized extraction method allows for the recovery of stevia sweeteners in a simple and efficient manner by using tap water as the extractant, without the application of an auxiliary energy source to reduce costs, thus representing an interesting strategy for their industrial-scale production.

Stevia rebaudiana Bertoni bioactive effects: From in vivo to clinical trials towards future therapeutic approaches

Stevia rebaudiana Bertoni, a plant from South America and indigenous of Paraguay, has shown several biological effects and healthy properties, although it is especially used in South America and some Asiatic regions. In addition, it is a natural sweetener, almost 300 times sweeter than sucrose, being attributed to its phytoconstituents prominent antioxidant, antimicrobial, antidiabetic (antihyperglycemic, insulinotropic, and glucagonostatic), antiplatelet, anticariogenic, and antitumor effects. In this sense, this work aims to provide an extensive overview on the historical practices of stevia and its effects in human health based on its chemical composition and applications for both food and pharmaceutical industries.

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.

Stress and defense responses in plant secondary metabolites production

In the growth condition(s) of plants, numerous secondary metabolites (SMs) are produced by them to serve variety of cellular functions essential for physiological processes, and recent increasing evidences have implicated stress and defense response signaling in their production. The type and concentration(s) of secondary molecule(s) produced by a plant are determined by the species, genotype, physiology, developmental stage and environmental factors during growth. This suggests the physiological adaptive responses employed by various plant taxonomic groups in coping with the stress and defensive stimuli. The past recent decades had witnessed renewed interest to study abiotic factors that influence secondary metabolism during in vitro and in vivo growth of plants. Application of molecular biology tools and techniques are facilitating understanding the signaling processes and pathways involved in the SMs production at subcellular, cellular, organ and whole plant systems during in vivo and in vitro growth, with application in metabolic engineering of biosynthetic pathways intermediates.

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.

Variations in Physiology and Multiple Bioactive Constituents under Salt Stress Provide Insight into the Quality Evaluation of Apocyni Veneti Folium

As one of the major abiotic stresses, salinity stress may affect the physiology and biochemical components of Apocynum venetum L. To systematically evaluate the quality of Apocyni Veneti Folium (AVF) from the perspective of physiological and the wide variety of bioactive components response to various concentrations of salt stress, this experiment was arranged on the basis of ultra-fast liquid chromatography tandem triple quadrupole mass spectrometry (UFLC-QTRAP-MS/MS) technology and multivariate statistical analysis. Physiological characteristics of photosynthetic pigments, osmotic homeostasis, lipid peroxidation product, and antioxidative enzymes were introduced to investigate the salt tolerance mechanism of AVF under salinity treatments of four concentrations (0, 100, 200, and 300 mM NaCl, respectively). Furthermore, a total of 43 bioactive constituents, including 14 amino acids, nine nucleosides, six organic acids, and 14 flavonoids were quantified in AVF under salt stress. In addition, multivariate statistical analysis, including hierarchical clustering analysis, principal component analysis (PCA), and gray relational analysis (GRA) was employed to systematically cluster, distinguish, and evaluate the samples, respectively. Compared with the control, the results demonstrated that 200 mM and 100 mM salt stress contributed to maintain high quality of photosynthesis, osmotic balance, antioxidant enzyme activity, and the accumulation of metabolites, except for total organic acids, and the quality of AVF obtained by these two groups was better than others; however, under severe stress, the accumulation of the oxidative damage and the reduction of metabolite caused by inefficiently scavenging reactive oxygen species (ROS) lead to lower quality. In summary, the proposed method may provide integrated information for the quality evaluation of AVF and other salt-tolerant Chinese medicines.

Comparative metabolic and ionomic profiling of two cultivars of Stevia rebaudiana Bert. (Bertoni) grown under salinity stress

This study provides a comprehensive investigation on the impact of increasing NaCl concentrations on hydroponically grown Stevia rebaudiana cultivars (Shoutian-2 and Fengtian). Growth parameters including plant height, biomass and physiological responses including osmotic potential were measured. In addition, the levels of steviol glycosides, elements and primary metabolites were measured and statistically evaluated. The cultivar Fengtian grew faster, accumulated less Na⁺ and compatible organic solutes, and more K⁺ in the leaves, as compared to the cv. Shoutian-2. Metabolite analysis identified 81 differentially accumulated metabolites, indicating an alteration in the metabolite phenotype of both cultivars upon exposure to salinity A general increase in many amino acids, amines, sugars and sugar phosphates with a concurrent decrease in most organic acids; including tricarboxylic acid (TCA) cycle intermediates, was observed. In the more salt tolerant cv. Fengtian, the levels of hexose phosphates and metabolites involved in cellular protection increased in response to salinity. These metabolites remained unchanged in the sensitive cv. Shoutian-2. Interestingly, salt treatment notably increased the rebaudioside A concentration by 53% while at the same time stevioside decreased by 38% in Fengtian which has important implications for controlling the relative amounts of reboudioside A and stevioside. The findings of this study leads to the conclusion that mild salinity stress can increase the yield of sweetener compounds, which is dependent on the cultivar and the level of salinity stress.

Low-Molecular-Weightt Polysaccharides From Pyropia yezoensis Enhance Tolerance of Wheat Seedlings (Triticum aestivum L.) to Salt Stress

Soil salinity is one of the major issues worldwide that affects plant growth and reduces agricultural productivity. Seaweed polysaccharides have been shown to promote crop growth and improve the resistance of plant to abiotic stresses. Pyropia yezoensis is a commercially important edible red alga in Southeast Asia. However, there is little research on the application of polysaccharides from P. yezoensis in agriculture. The molecular weight (MW) of polysaccharides influences their properties. Therefore, in this study, four representative polysaccharides from P. yezoensis (PP) with different MWs (MW: 3.2, 10.5, 29.0, and 48.8 kDa) were prepared by microwave-assisted acid hydrolysis. The relationship between the degradation of polysaccharides from P. yezoensis (DPP) and their effects on plant salt tolerance was investigated. The results showed that exogenous PP and DPPs increased wheat seedling shoot and root lengths, and fresh and dry weights, alleviated membrane lipid peroxidation, increased the chlorophyll content and enhanced antioxidant activities. The expression level examination analysis of several Na⁺/K⁺ transporter genes suggested that DPPs could protect plants from the damage of salt stress by coordinating the efflux and compartmentation of Na⁺. The results demonstrated that polysaccharides could regulate antioxidant enzyme activities and modulate intracellular ion concentration, thereby to protect plants from salt stress damage. Furthermore, there was a significant correlation between the tolerance of wheat seedlings to salt stress and MW of polysaccharides. The results suggested that the lower-MW samples (DPP1, 3.2 kDa) most effectively protect wheat seedlings against salt stress.

Effects of salinity and short-term elevated atmospheric CO2 on the chemical equilibrium between CO2 fixation and photosynthetic electron transport of Stevia rebaudiana Bertoni

The effect of water salinity on plant growth and photosynthetic traits of Stevia rebaudiana was investigated to determine its level and mechanisms of salinity tolerance. It was also attempted to assess how short-term elevated CO₂ concentration would influence the boundaries and mechanisms of its photosynthetic capacity. The plants were grown in gravel/hydroponic system under controlled greenhouse conditions and irrigated with four different salinity levels (0, 25, 50 and 100 mol m^-3NaCl). Low salinity did not significantly alter the plant fresh weight, which was substantially decreased by 67% at high salinity treatment. Salinity tolerance threshold was reached at 50 mol m^-3 NaCl while C50 was between 50 and 100 mol m^-3 NaCl, indicating that S. rebaudiana is a moderate salt tolerant species. Salt-induced growth reduction was apparently linked to a significant decline of about 47% in the photosynthetic rates (A_net) at high salinity treatment, leading consequently to a disequilibrium between CO₂-assimilation and electron transport rates (indicated by enhanced ETR_max/A_gross ratio). Elevated atmospheric CO₂ enhanced CO₂ assimilation rates by 65% and 80% for control and high-salt-stressed plants respectively, likely due to significant increases in intercellular CO₂ concentration (indicated by enhanced C_i/C_a). The priority for Stevia under elevated atmospheric CO₂ was not to save water but to maximize photosynthesis so that the PWUE was progressively improved and the threat of oxidative stress was diminished (decline in ETR_max/A_gross). The results imply that elevated CO₂ level could ameliorate some of the detrimental effects of salinity, conferring higher tolerance and survival of S. rebaudiana, a highlydesired feature with the forthcoming era of global changes.

Salt-tolerance mechanisms induced in Stevia rebaudiana Bertoni: Effects on mineral nutrition, antioxidative metabolism and steviol glycoside content

In order to cope with challenges linked to climate change such as salinity, plants must develop a wide spectrum of physiological and molecular mechanisms to rapidly adapt. Stevia rebaudiana Bertoni plants are a case in point. According to our findings, salt stress has no significant effect on plant growth in these plants, which accumulate sodium (Na⁺) in their roots, thus avoiding excessive Na⁺ accumulation in leaves. Furthermore, salt stress (NaCl stress) increases the potassium (K⁺), calcium (Ca²⁺), chloride ion (Cl^-) and proline concentrations in Stevia leaves, which could contribute to osmotic adjustment. We also found that long-term NaCl stress does not produce changes in chlorophyll concentrations in Stevia leaves, reflecting a mechanism to protect the photosynthesis process. Interestingly, an increase in chlorophyll b (Chlb) content occured in the oldest plants studied. In addition, we found that NaCl induced reactive oxygen species (ROS) accumulation in Stevia leaves and that this accumulation was more evident in the presence of 5 g/L NaCl, the highest concentration used in the study. Nevertheless, Stevia plants are able to induce (16 d) or maintain (25 d) antioxidant enzymes to cope with NaCl-induced oxidative stress. Low salt levels did not affect steviolbioside and rebaudioside A contents. Our results suggest that Stevia plants induce tolerance mechanisms in order to minimize the deleterious effects of salt stress. We can thus conclude that saline waters can be used to grow Stevia plants and for Steviol glycosides (SGs) production.

Effect of Poly(γ-glutamic acid) on the Physiological Responses and Calcium Signaling of Rape Seedlings (Brassica napus L.) under Cold Stress

Cold stress adversely affects plant growth and development. Poly(γ-glutamic acid) (γ-PGA) is a potential plant growth regulator that may be an effective cryoprotectant that prevents crops from damage during cold weather. In this study, the effects of γ-PGA on the physiological responses of rape seedlings subject to cold stress were investigated using hydroponic experiments. We determined that the malondialdehyde content was decreased by 33.4% and the proline content was increased by 62.5% by γ-PGA after 144 h under cold stress. Antioxidant enzymes activities were also evidently enhanced after treatment with γ-PGA. These responses counteracted increases in the fresh weight and chlorophyll content of rape seedlings, which increased by 24.5 and 50.9%, respectively, after 144 h, which meant that growth inhibition caused by cold was mitigated by γ-PGA. Our results also showed that γ-PGA also regulated Ca(2+) concentrations in the cytoplasm and calcium-dependent protein kinases, which are associated with cold resistance. In conclusion, we suggest that the Ca(2+)/CPKs signal pathway is involved in the γ-PGA-mediated enhancement of cold resistance in rape seedlings.