Sucrose synthase: A unique glycosyltransferase for biocatalytic glycosylation process development

Basal ABA signaling balances transpiration and photosynthesis

The balance between the CO2 entry for photosynthesis and transpiration water loss is crucial for plant growth, and ABA signaling can affect this equilibrium. To test how ABA balances plant growth and environmental adaptation, we performed molecular genetics studies in the biotech crop Nicotiana benthamiana under well-watered or drought conditions. Studies on ABA signaling in crops are complicated by the multigenic nature of the PYR/PYL/RCAR ABA receptor family and its functional redundancy, which is particularly challenging in polyploid plants. We have generated a pentuple pyl mutant in the allotetraploid Nicotiana benthamiana through CRISPR/Cas9 gene editing. The pentuple mutant is impaired in 2 NbPYL1-like and 3 NbPYL8-like receptors, affecting the regulation of transpiration and several ABA-dependent transcriptional processes. RNA-seq and metabolite analysis revealed that the synthesis of galactinol, an essential precursor for the osmoprotective raffinose family of oligosaccharides, is ABA-dependent and impaired in the mutant under osmotic stress. In contrast, our results show that, under well-watered conditions, partial inactivation of ABA signaling leads to higher CO2 entry and photosynthesis in the mutant than in WT. Photosynthesis analyses revealed an increased CO2 diffusion capacity mediated by higher stomatal and mesophyll conductances, and higher substomatal CO2 concentration in the pentuple mutant. RNA-seq analyses revealed that genes associated with cell wall loosening (e.g., expansins) and porosity were strongly downregulated by ABA in WT. In summary, a partial relief of the ABA control on transpiration mediated by ABA receptors positively affects photosynthesis when water is not limited, at the expense of reduced water use efficiency.

The effect of exogenous gibberellin and its synthesis inhibitor treatments for morphological and physiological characteristics of Tartary buckwheat

Gibberellin (GA3) is an important plant hormone involved in many physiological and developmental processes in plants. However, the physiological mechanism of GA3 on the regulation yield and grain shell thickness of Tartary buckwheat is still unclear. In this study, the thick-shelled cultivar "Jinqiao 2" and thin-shelled cultivar "Miku 18" were used to study the effects of different concentrations (0, 50, and 100 mg L-1) of exogenous GA3 and chlorocholine chloride (CCC, GA3 synthesis inhibitor) on the cellulose content, amylase, and sucrose synthase (SS) activity in grain shell and the yield of Tartary buckwheat. The application of exogenous GA3 can improve the cellulose content and the activity of amylase and SS in the grain shell of the two Tartary buckwheat varieties. It can also increase the main stem node number, main stem branch number, grains per plant, and yield. Compared with the control treatment (CK, 0 mg L-1), the 100 mg/L exogenous GA3 treatment increased the number of grains per plant, grain weight per plant, 1000-grain weight, and yield of Jinqiao 2 by 20.1%, 41.9%, 13%, and 34.7%, respectively. These items of Miku 18 were increased by 26%, 15.2%, 10.2%, and 23.8%. The application of CCC reduced the activity of amylase and SS and cellulose content in grain shell. In addition, it decreased the main stem node number, main stem branch number, grains per plant, and yield of Tartary buckwheat. In summary, exogenous GA3 treatment not only improved the yield of Tartary buckwheat but also increased the thickness of grain shell by enhancing the activity of amylase and SS and promoting the synthesis and accumulation of cellulose. The results can provide theoretical references for clarifying the physiological mechanism of the difference in shell thickness between Tartary buckwheat varieties.

Tandem mass tag (TMT)-based quantitative proteomics analysis reveals the different responses of contrasting alfalfa varieties to drought stress

BACKGROUND

Drought stress restricts the growth, distribution and productivity of alfalfa (Medicago sativa L.). In order to study the response differences of alfalfa cultivars to drought stress, we previously carried out physiological and molecular comparative analysis on two alfalfa varieties with contrasting drought resistance (relatively drought-tolerant Longdong and drought-sensitive Algonquin). However, the differences in proteomic factors of the two varieties in response to drought stress still need to be further studied. Therefore, TMT-based quantitative proteomic analysis was performed using leaf tissues of the two alfalfa cultivars to identify and uncover differentially abundant proteins (DAPs).

RESULTS

In total, 677 DAPs were identified in Algonquin and 277 in Longdong under drought stress. Subsequently, we conducted various bioinformatics analysis on these DAPs, including subcellular location, functional classification and biological pathway enrichment. The first two main COG functional categories of DAPs in both alfalfa varieties after drought stress were 'Translation, ribosomal structure and biogenesis' and 'Posttranslational modification, protein turnover, chaperones'. According to KEGG database, the DAPs of the two alfalfa cultivars after drought treatment were differentially enriched in different biological pathways. The DAPs from Algonquin were enriched in 'photosynthesis' and 'ribosome'. The pathways of 'linoleic acid metabolism', 'protein processing in endoplasmic reticulum' and 'RNA transport' in Longdong were significantly enriched. Finally, we found significant differences in DAP enrichment and expression patterns between Longdong and Algonquin in glycolysis/glycogenesis, TCA cycle, photosynthesis, protein biosynthesis, flavonoid and isoflavonoid biosynthesis, and plant-pathogen interaction pathway after drought treatment.

CONCLUSIONS

The differences of DAPs involved in various metabolic pathways may explain the differences in the resistance of the two varieties to drought stress. These DAPs can be used as candidate proteins for molecular breeding of alfalfa to cultivate new germplasm with more drought tolerance to adapt to unfavorable environments.

Development of New Multi-Glycosylation Routes to Facilitate the Biosynthesis of Sweetener Mogrosides from Bitter Immature Siraitia Grosvenorii Using Engineered Escherichia coli

Mogrosides, which have various pharmacological activities, are mainly extracted from Siraitia grosvenorii (Luo Han Guo) and are widely used as natural zero-calorie sweeteners. Unfortunately, the difficult cultivation and long maturation time of Luo Han Guo have contributed to a shortage of mogrosides. To overcome this obstacle, we developed a highly efficient biosynthetic method using engineered Escherichia coli to synthesize sweet mogrosides from bitter mogrosides. Three UDP-glycosyltransferase (UGT) genes with primary/branched glycosylation catalytic activity at the C3/C24 sites of mogrosides were screened and tested. Mutant M3, which could catalyze the glycosylation of nine types of mogrosides, was obtained through enhanced catalytic activity. This improvement in β-(1,6)-glycosidic bond formation was achieved through single nucleotide polymorphisms and direct evolution, guided by 3D structural analysis. A new multienzyme system combining three UGTs and UDP-glucose (UDPG) regeneration was developed to avoid the use of expensive UDPG. Finally, the content of sweet mogrosides in the immature Luo Han Guo extract increased significantly from 57% to 95%. This study not only established a new multienzyme system for the highly efficient production of sweet mogrosides from immature Luo Han Guo but also provided a guideline for the high-value utilization of rich bitter mogrosides from agricultural waste and residues.

Engineering UDP-Glycosyltransferase UGTPg29 for the Efficient Synthesis of Ginsenoside Rg3 from Protopanaxadiol

Rare ginsenosides Rg3 and Rh2, which exhibit diverse pharmacological effects, are derivatives of protopanaxadiol (PPD). UDP-glycosyltransferases, such as the M315F variant of Bs-YjiC (Bs-YjiCm) from Bacillus subtilis and UGTPg29 from Panax ginseng, can efficiently convert PPD into Rh2 and Rh2 into Rg3, respectively. In the present study, the N178I mutation of Bs-YjiCm was introduced, resulting in an increase in Rh2 production. UDP-glycosyltransferase UGTPg29 was then engineered to improve its robustness through semi-rational design. The variant R91M/D184M/A287V/A342L, which indicated desirable stability and activity, was utilized in coupling with the N178I variant of Bs-YjiCm and sucrose synthase AtSuSy from Arabidopsis thaliana to set up a "one-pot" three-enzyme reaction for the biosynthesis of Rg3. The influential factors, including the ratio and concentration of UDP-glycosyltransferases, pH, and the concentrations of UDP, sucrose, and DMSO, were optimized. On this basis, a fed-batch strategy was adopted to achieve a Rg3 yield as high as 12.38 mM (9.72 g/L) with a final yield of 68.78% within 24 h. This work may provide promising UDP-glycosyltransferase candidates for ginsenoside biosynthesis.

Bottom-Up Synthesized Glucan Materials: Opportunities from Applied Biocatalysis

Linear d-glucans are natural polysaccharides of simple chemical structure. They are comprised of d-glucosyl units linked by a single type of glycosidic bond. Noncovalent interactions within, and between, the d-glucan chains give rise to a broad variety of macromolecular nanostructures that can assemble into crystalline-organized materials of tunable morphology. Structure design and functionalization of d-glucans for diverse material applications largely relies on top-down processing and chemical derivatization of naturally derived starting materials. The top-down approach encounters critical limitations in efficiency, selectivity, and flexibility. Bottom-up approaches of d-glucan synthesis offer different, and often more precise, ways of polymer structure control and provide means of functional diversification widely inaccessible to top-down routes of polysaccharide material processing. Here the natural and engineered enzymes (glycosyltransferases, glycoside hydrolases and phosphorylases, glycosynthases) for d-glucan polymerization are described and the use of applied biocatalysis for the bottom-up assembly of specific d-glucan structures is shown. Advanced material applications of the resulting polymeric products are further shown and their important role in the development of sustainable macromolecular materials in a bio-based circular economy is discussed.

A growth selection system for sucrose synthases (SuSy): design and test

High throughput screening (HTS) methods of enzyme variants are essential for the development of robust biocatalysts suited for low impact, industrial scale, biobased synthesis of a myriad of compounds. However, for the majority of enzyme classes, current screening methods have limited throughput, or need expensive substrates in combination with sophisticated setups. Here, we present a straightforward, high throughput selection system that couples sucrose synthase activity to growth. Enabling high throughput screening of this enzyme class holds the potential to facilitate the creation of robust variants, which in turn can significantly impact the future of cost effective industrial glycosylation.

Biosynthesis of Apigenin Glucosides in Engineered Corynebacterium glutamicum

Glucosylation is a well-known approach to improve the solubility, pharmacological, and biological properties of flavonoids, making flavonoid glucosides a target for large-scale biosynthesis. However, the low yield of products coupled with the requirement of expensive UDP-sugars limits the application of enzymatic systems for large-scale. C. glutamicum is a Gram-positive and generally regarded as safe (GRAS) bacteria frequently employed for the large-scale production of amino acids and bio-fuels. Due to the versatility of its cell factory system and its non-endotoxin producing properties, it has become an attractive system for the industrial-scale biosynthesis of alternate products. Here, we explored the cell factory of C. glutamicum for efficient glucosylation of flavonoids using apigenin as a model flavonoid, with the heterologous expression of a promiscuous glycosyltransferase, YdhE from Bacillus licheniformis and the endogenous overexpression of C. glutamicum genes galU1 encoding UDP-glucose pyrophosphorylase and pgm encoding phosphoglucomutase involved in the synthesis of UDP-glucose to create a C. glutamicum cell factory system capable of efficiently glucosylation apigenin with a high yield of glucosides production. Consequently, the production of various apigenin glucosides was controlled under different temperatures yielding almost 4.2 mM of APG1(apigenin-4'-O-β-glucoside) at 25°C, and 0.6 mM of APG2 (apigenin-7-O-β-glucoside), 1.7 mM of APG3 (apigenin-4',7-O-β-diglucoside) and 2.1 mM of APG4 (apigenin-4',5-O-β-diglucoside) after 40 h of incubation with the supplementation of 5 mM of apigenin and 37°C. The cost-effective developed system could be used to modify a wide range of plant secondary metabolites with increased pharmacokinetic activities on a large scale without the use of expensive UDP-sugars.

Sugar delivery at the tomato root and root galls after Meloidogyne incognita infestation

Root-knot nematodes (RKNs) infect host plants and obtain nutrients such as sugars for their own development. Therefore, inhibiting the nutrient supply to RKNs may be an effective method for alleviating root-knot nematode disease. At present, the pathway by which sucrose is unloaded from the phloem cells to giant cells (GCs) in root galls and which genes related to sugar metabolism and transport play key roles in this process are unclear. In this study, we found that sugars could be unloaded into GCs only from neighboring phloem cells through the apoplastic pathway. With the development of galls, the contents of sucrose, fructose and glucose in the galls and adjacent tissue increased gradually. SUT1, SUT2, SWEET7a, STP10, SUS3 and SPS1 may provide sugar sources for GCs, while STP1, STP2 and STP12 may transport more sugar to phloem parenchyma cells. At the early stage of Meloidogyne incognita infestation, the sucrose content in tomato roots and leaves increased, while the glucose and fructose contents decreased. SWEET7a, SPS1, INV-INH1, INV-INH2, SUS1 and SUS3 likely play key roles in root sugar delivery. These results elucidated the pathway of sugar unloading in tomato galls and provided an important theoretical reference for eliminating the sugar source of RKNs and preventing root-knot nematode disease.

NnSUS1 encodes a sucrose synthase involved in sugar accumulation in lotus seed cotyledons

Fresh lotus seeds are gaining favor with consumers for their crunchy texture and natural sweetness. However, the intricacies of sugar accumulation in lotus seeds remain elusive, which greatly hinders the quality improvement of fresh lotus seeds. This study endeavors to elucidate this mechanism by identifying and characterizing the sucrose synthase (SUS) gene family in lotus. Comprising five distinct members, namely NnSUS1 to NnSUS5, each gene within this family features a C-terminal glycosyl transferase1 (GT1) domain. Among them, NnSUS1 is the predominately expressed gene, showing high transcript abundance in the floral organs and cotyledons. NnSUS1 was continuously up-regulated from 6 to 18 days after pollination (DAP) in lotus cotyledons. Furthermore, NnSUS1 demonstrates co-expression relationships with numerous genes involved in starch and sucrose metabolism. To investigate the function of NnSUS1, a transient overexpression system was established in lotus cotyledons, which confirmed the gene's contribution to sugar accumulation. Specifically, transient overexpression of NnSUS1 in seed cotyledons leads to a significant increase in the levels of total soluble sugar, including sucrose and fructose. These findings provide valuable theoretical insights for improving sugar content in lotus seeds through molecular breeding methods.

Investigation on production and reaction conditions of sucrose synthase based glucosylation cascade towards flavonoid modification

Enzyme-based glycosylation is of great interest in the context of natural products decoration. Yet, its industrial application is hindered by optimisation difficulties and hard-to-standardise productivities. In this study, five sugar nucleotide-dependent glucosyltransferases from different origins (bacterial, plant and fungal) were coupled with soy sucrose synthase (GmSuSy) to create a set of diverse cascade biocatalysts for flavonoid glucosylation, which evaluation brought new insights into the field. Investigations into co-expression conditions and reaction settings enabled to define optimal induction temperature (25 °C) and uridine diphosphate (UDP) concentration (0.5 mM) for all tested pairs of enzymes. Moreover, the influence of pH and substrate concentration on the monoglucosylated product distribution was detected and analysed. The utilisation of crude protein extracts as a cost-effective source of catalysts unveiled their glycosidase activity against flavonoid glucosides, resulting in decreased productivity, which, to our knowledge, has not previously been discussed in such a context. Additionally, examination of the commercially available EziG immobilisation resins showed that selection of suitable carrier for solid catalyst production can be problematic and not only enzyme's but also reagent's properties have to be considered. Flavonoids, due to their complexation and hydrophobic properties, can adsorb on different types of surfaces, including divalent metal ions required for IMAC based immobilisation, necessitating detailed examination of the resins while the catalysis design.

In-Silico Identification, Characterization and Expression Analysis of Genes Involved in Resistant Starch Biosynthesis in Potato (Solanum tuberosum L.) Varieties

Potato (Solanum tuberosum L.), an important horticultural crop is a member of the family Solanaceae and is mainly grown for consumption at global level. Starch, the principal component of tubers, is one of the significant elements for food and non-food-based applications. The genes associated with biosynthesis of starch have been investigated extensively over the last few decades. However, a complete regulation pathway of constituent of amylose and amylopectin are still not deeply explored. The current in-silico study of genes related to amylose and amylopectin synthesis and their genomic organization in potato is still lacking. In the current study, the nucleotide and amino acid arrangement in genome and twenty-two genes linked to starch biosynthesis pathway in potato were analysed. The genomic structure analysis was also performed to find out the structural pattern and phylogenetic relationship of genes. The genome mining and structure analysis identified ten specific motifs and phylogenetic analysis of starch biosynthesis genes divided them into three different clades on the basis of their functioning and phylogeny. Quantitative real-time PCR (qRT-PCR) of amylose biosynthesis pathway genes in three contrast genotypes revealed the down-gene expression that leads to identify potential cultivar for functional genomic approaches. These potential lines may help to achieve higher content of resistant starch.

Current perspectives on the regulatory mechanisms of sucrose accumulation in sugarcane

Sugars transported from leaves (source) to stems (sink) energize cell growth, elongation, and maintenance. which are regulated by a variety of genes. This review reflects progress and prospects in the regulatory mechanism for maximum sucrose accumulation, including the role of sucrose metabolizing enzymes, sugar transporters and the elucidation of post-transcriptional control of sucrose-induced regulation of translation (SIRT) in the accumulation of sucrose. The current review suggests that SIRT is emerging as a significant mechanism controlling Scbzip44 activities in response to endogenous sugar signals (via the negative feedback mechanism). Sucrose-controlled upstream open reading frame (SC-uORF) exists at the 5' leader region of Scbzip44's main ORF, which inhibits sucrose accumulation through post-transcriptional regulatory mechanisms. Sucrose transporters (SWEET1a/4a/4b/13c, TST, SUT1, SUT4 and SUT5) are crucial for sucrose translocation from source to sink. Particularly, SWEET13c was found to be a major contributor to the efflux in the transportation of stems. Tonoplast sugar transporters (TSTs), which import sucrose into the vacuole, suggest their tissue-specific role from source to sink. Sucrose cleavage has generally been linked with invertase isozymes, whereas sucrose synthase (SuSy)-catalyzed metabolism has been associated with biosynthetic processes such as UDP-Glc, cellulose, hemicellulose and other polymers. However, other two key sucrose-metabolizing enzymes, such as sucrose-6-phosphate phosphohydrolase (S6PP) and sucrose phosphate synthase (SPS) isoforms, have been linked with sucrose biosynthesis. These findings suggest that manipulation of genes, such as overexpression of SPS genes and sucrose transporter genes, silencing of the SC-uORF of Scbzip44 (removing the 5' leader region of the main ORF that is called SIRT-Insensitive) and downregulation of the invertase genes, may lead to maximum sucrose accumulation. This review provides an overview of sugarcane sucrose-regulating systems and baseline information for the development of cultivars with higher sucrose accumulation.

PbrbZIP15 promotes sugar accumulation in pear via activating the transcription of the glucose isomerase gene PbrXylA1

The composition and abundance of soluble sugars in mature pear (Pyrus) fruit are important for its acceptance by consumers. However, our understanding of the genes responsible for soluble sugar accumulation remains limited. In this study, a S1-group member of bZIP gene family, PbrbZIP15, was characterized from pear genome through the combined analyses of metabolite and transcriptome data followed by experimental validation. PbrbZIP15, located in nucleus, was found to function in fructose, sucrose, and total soluble sugar accumulation in pear fruit and calli. After analyzing the expression profiles of sugar-metabolism-related genes and the distribution of cis-acting elements in their promoters, the glucose isomerase 1 gene (PbrXylA1), whose corresponding protein catalyzed the isomerization of glucose and fructose in vitro, was identified as a downstream target gene of PbrbZIP15. PbrbZIP15 could directly bind to the G-box element in PbrXylA1 promoter and activate its transcription, as evidenced by chromatin immunoprecipitation-quantitative PCR, yeast one-hybrid, electrophoretic mobility shift assay, and dual-luciferase assay. PbrXylA1, featuring a leucine-rich signal peptide in its N-terminal, was localized to the endoplasmic reticulum. It was validated to play a significant role in fructose, sucrose, and total soluble sugar accumulation in pear fruit and calli, which was associated with the upregulated fructose/glucose ratio. Further studies revealed a positive correlation between the sucrose content and the expression levels of several sucrose-biosynthesis-related genes (PbrFRK3/8, PbrSPS1/3/4/8, and PbrSPP1) in PbrbZIP15-/PbrXylA1-transgenic fruit/calli. In conclusion, our results suggest that PbrbZIP15-induced soluble sugar accumulation during pear development is at least partly attributed to the activation of PbrXylA1 transcription.

Chemoenzymatic indican for light-driven denim dyeing

Blue denim, a billion-dollar industry, is currently dyed with indigo in an unsustainable process requiring harsh reducing and alkaline chemicals. Forming indigo directly in the yarn through indican (indoxyl-β-glucoside) is a promising alternative route with mild conditions. Indican eliminates the requirement for reducing agent while still ending as indigo, the only known molecule yielding the unique hue of blue denim. However, a bulk source of indican is missing. Here, we employ enzyme and process engineering guided by techno-economic analyses to develop an economically viable drop-in indican synthesis technology. Rational engineering of PtUGT1, a glycosyltransferase from the indigo plant, alleviated the severe substrate inactivation observed with the wildtype enzyme at the titers needed for bulk production. We further describe a mild, light-driven dyeing process. Finally, we conduct techno-economic, social sustainability, and comparative life-cycle assessments. These indicate that the presented technologies have the potential to significantly reduce environmental impacts from blue denim dyeing with only a modest cost increase.

Selective synthesis of rebaudioside M2 through structure-guided engineering of glycosyltransferase UGT94D1

Rebaudioside M2 (Reb M2), a novel steviol glycoside derivative, has limited industrial applications due to its low synthetic yield and selectivity. Herein, we identify UGT94D1 as a selective glycosyltransferase for rebaudioside D (Reb D), leading to the production of a mono β-1,6-glycosylated derivative, Reb M2. A variant UGT94D1-F119I/D188P was developed through protein engineering. This mutant exhibited a 6.33-fold improvement in catalytic efficiency, and produced Reb M2 with 92% yield. Moreover, molecular dynamics simulations demonstrated that UGT94D1-F119I/D188P exhibited a shorter distance between the nucleophilic oxygen (OH6) of the substrate Reb D and uridine diphosphate glucose, along with an increased Ophosphate-C1-Oacceptor angle, thus improving the catalytic activity of the enzyme. Therefore, this study provides an efficient method for the selective synthesis of Reb M2 and paves the way for its applications in various fields.

A sustainable strategy for biosynthesis of Rebaudioside D using a novel glycosyltransferase of Solanum tuberosum

Bioconversion of Rebaudioside D faces high-cost obstacles. Herein, a novel glycosyltransferase StUGT converting Rebaudioside A to Rebaudioside D was screened and characterized, which exhibits stronger affinity and substrate specificity for Rebaudioside A than previously reported enzymes. A whole-cell catalytic system was thus developed using the StUGT strain. The production of Rebaudioside D was enhanced significantly by enhancing cell permeability, and the maximum production of 6.12 g/L and the highest yield of 98.08% by cell catalyst was obtained by statistical-based optimization. A new cascade process utilizing this recombinant strain and E. coli expressing sucrose synthase was further established to reduce cost through replacing expensive UDPG with sucrose. A StUGT-GsSUS1 system exhibited high catalytic capability, and 5.27 g L-1 Rebaudioside D was achieved finally without UDPG addition by systematic optimization. This is the best performance reported in cell-cascaded biosynthesis, which paves a new cost-effective strategy for sustainable synthesis of scarce premium sweeteners from biomass.

Cyclic variable temperature conditioning induces the rapid sweetening of sweet potato tuberous roots by regulating the sucrose metabolism

This study aimed to investigate the influence of cyclic variable temperature conditioning (CVTC) on the rapid sweetening of sweet potato tuberous roots, as assessed through the analysis of sugar metabolism-related compounds and enzyme activities of tubers during storage. The results showed that CVTC effectively preserved the quality of sweet potato tuberous roots, leading to a significant elevation in soluble solids and soluble sugars. The CVTC group displayed sucrose and fructose levels that were 1.72 and 1.46 times higher, respectively, compared to the control group at the 8 d. Additionally, after storage, the activities of β-amylase, sucrose phosphate synthase (SPS), and sucrose synthase (SS) in the CVTC group were increased by 19.85 %, 60.74 %, and 82.48 %, respectively. Conversely, acid convertase (AI) activity showed inhibition of 64.72 %. In conclusion, implementing CVTC enhanced enzymatic activity in β-amylase, SPS, and SS, facilitating starch degradation and sucrose synthesis, which contributed to the overall improvement in the sweetness of sweet potato tubers.

Overexpression of ZmSUS1 increased drought resistance of maize (Zea mays L.) by regulating sucrose metabolism and soluble sugar content

MAIN CONCLUSION

ZmSUS1 improved drought tolerance of maize by regulating sucrose metabolism and increasing soluble sugar content, and endowing transgenic maize with higher relative water content and photosynthesis levels. Sucrose synthase (SUS), a key enzyme of sugar metabolism, plays an important role in the regulation of carbon partitioning in plant, and affects important agronomic traits and abiotic responses to adversity. However, the function of ZmSUS1 in plant drought tolerance is still unknown. In this study, the expression patterns of ZmSUS1 in different tissues and under drought stress were analyzed in maize (Zea mays L.). It was found that ZmSUS1 was highly expressed during kernel development but also in leaves and roots of maize, and ZmSUS1 was induced by drought stress. Homozygous transgenic maize lines overexpressing ZmSUS1 increased the content and activity of SUS under drought stress and exhibited higher relative water content, proline and abscisic acid content in leaves. Specifically, the net photosynthetic rate and the soluble sugar contents including sucrose, glucose, fructose and SUS decomposition products including UDP-glucose (UDP-G) and ADP-glucose (ADP-G) in transgenic plants were significantly improved after drought stress. RNA-seq analysis showed that overexpressing of ZmSUS1 mainly affected the expression level of carbon metabolism-related genes. Especially the expression level of sucrose metabolism-related genes including sucrose phosphatase gene (SPP), sucrose phosphate synthase gene (SPS) and invertase gene (INV) were significantly up-regulated in transgenic maize. Overall, these results suggested that ZmSUS1 improved drought tolerance by regulating sucrose metabolism and increasing the soluble sugar content, and endowing transgenic maize with higher relative water content and photosynthesis levels, which can serve as a new gene candidate for cultivating drought-resistant maize varieties.

Proteomic Analysis Revealed the Antagonistic Effect of Decapitation and Strigolactones on the Tillering Control in Rice

Removing the panicle encourages the growth of buds on the elongated node by getting rid of apical dominance. Strigolactones (SLs) are plant hormones that suppress tillering in rice. The present study employed panicle removal (RP) and external application of synthesized strigolactones (GR) to modulate rice bud growth at node 2. We focused on the full-heading stage to investigate proteomic changes related to bud germination (RP-Co) and suppression (GR-RP). A total of 434 represented differentially abundant proteins (DAPs) were detected, with 272 DAPs explicitly specified in the bud germination process, 106 in the bud suppression process, and 28 in both. DAPs in the germination process were most associated with protein processing in the endoplasmic reticulum and ribosome biogenesis. DAPs were most associated with metabolic pathways and glycolysis/gluconeogenesis in the bud suppression process. Sucrose content and two enzymes of sucrose degradation in buds were also determined. Comparisons of DAPs between the two reversed processes revealed that sucrose metabolism might be a key to modulating rice bud growth. Moreover, sucrose or its metabolites should be a signal downstream of the SLs signal transduction that modulates rice bud outgrowth. Contemplating the result so far, it is possible to open new vistas of research to reveal the interaction between SLs and sucrose signaling in the control of tillering in rice.

Enhancing Glycosylation of Flavonoids by Engineering the Uridine Diphosphate Glucose Supply in Escherichia coli

Glycosylation can enhance the solubility and stability of flavonoids. The main limitation of the glycosylation process is low intracellular uridine diphosphate glucose (UDPG) availability. This study aimed to create a glycosylation platform strain in Escherichia coli BL21(DE3) by multiple metabolic engineering of the UDPG supply. Glycosyltransferase TcCGT1 was introduced to synthesize vitexin and orientin from apigenin and luteolin, respectively. To further expand this glycosylation platform strain, not only were UDP rhamnose and UDP galactose synthesis pathways constructed, but rhamnosyltransferase (GtfC) and galactosyltransferase (PhUGT) were also introduced, respectively. In a 5 L bioreactor with apigenin, luteolin, kaempferol, and quercetin as glycosyl acceptors, vitexin, orientin, afzelin, quercitrin, hyperoside, and trifolin glycosylation products reached 17.2, 36.5, 5.2, 14.1, 6.4, and 11.4 g/L, respectively, the highest titers reported to date for all. The platform strain has great potential for large-scale production of glycosylated flavonoids.

Interactions between sulfonamide homologues and glycosyltransferase induced metabolic disorders in rice (Oryza sativa L.)

Sulfadiazine and its derivatives (sulfonamides, SAs) could induce distinct biotoxic, metabolic and physiological abnormalities, potentially due to their subtle structural differences. This study conducted an in-depth investigation on the interactions between SA homologues, i.e. sulfadiazine (SD), sulfamerazine (SD1), and sulfamethazine (SD2), and the key metabolic enzyme (glycosyltransferase, GT) in rice (Oryza sativa L.). Untargeted screening of SA metabolites revealed that GT-catalyzed glycosylation was the primary transformation pathway of SAs in rice. Molecular docking identified that the binding sites of SAs on GT (D0TZD6) were responsible for transferring sugar moiety to synthesize polysaccharides and detoxify SAs. Specifically, amino acids in the GT-binding cavity (e.g., GLY487 and CYS486) formed stable hydrogen bonds with SAs (e.g., the sulfonamide group of SD). Molecular dynamics simulations revealed that SAs induced conformational changes in GT ligand binding domain, which was supported by the significantly decreased GT activity and gene expression level. As evidenced by proteomics and metabolomics, SAs inhibited the transfer and synthesis of sugar but stimulated sugar decomposition in rice leaves, leading to the accumulation of mono- and disaccharides in rice leaves. While the differences in the increased sugar content by SD (24.3%, compared with control), SD1 (11.1%), and SD2 (6.24%) can be attributed to their number of methyl groups (0, 1, 2, respectively), which determined the steric hindrance and hydrogen bonds formation with GT. This study suggested that the disturbances on crop sugar metabolism by homologues contaminants are determined by the interaction between the contaminants and the target enzyme, and are greatly dependent on the steric hindrance effects contributed by their side chains. The results are of importance to identify priority pollutants and ensure crop quality in contaminated fields.

Glycosylation of polyphenolic compounds: Design of a self-sufficient biocatalyst by co-immobilization of a glycosyltransferase, a sucrose synthase and the cofactor UDP

Glycosyltransferases catalyze the regioselective glycosylation of polyphenolic compounds, increasing their solubility without altering their antioxidant properties. Leloir-type glycosyltransferases require UDP-glucose as a cofactor to glycosylate a hydroxyl of the polyphenol, which is expensive and unstable. To simplify these processes for industrial implementation, the preparation of self-sufficient heterogeneous biocatalysts is needed. In this study, a glycosyltransferase and a sucrose synthase (as an UDP-regenerating enzyme) were co-immobilized onto porous agarose-based supports coated with polycationic polymers: polyethylenimine and polyallylamine. In addition, the UDP cofactor was strongly ionically adsorbed and co-immobilized with the enzymes, eliminating the need to add it separately. Thus, the optimal self-sufficient heterogeneous biocatalyst was able to catalyze the glycosylation of three polyphenolic compounds (piceid, phloretin and quercetin) with in situ regeneration of the UDP-glucose, allowing multiple consecutive reaction cycles without the addition of exogenous cofactor. A TTN value of 50 (theoretical maximum) was obtained in the reaction of piceid glycosylation, after 5 reaction cycles, using the self-sufficient biocatalyst based on an improved sucrose synthase variant. This result was 5-fold higher than the obtained using soluble cofactor and the co-immobilized enzymes, and much higher than those reported in the literature for similar processes.

Sugar metabolism and 14-3-3 protein genes expression induced by arbuscular mycorrhizal fungi and phosphorus addition to response drought stress in Populus cathayana

Sugar, as a nutrient exchange substance between arbuscular mycorrhizal (AM) fungi and host plants, plays an important role in the abiotic stress response of mycorrhizal plants. This experiment aimed to study the effects of AM fungi and phosphorus (P) addition on the sugar metabolism and 14-3-3 gene expression of Populus cathayana under drought stress. The results showed that drought affects the process of sugar metabolism by increasing the activities of amylase and invertase, resulting in the decrease of starch content in leaves and roots and the accumulation of soluble sugars (including reducing sugar and sucrose) in roots. Under drought stress, the activity or content of sucrose synthetase, sucrose phosphate synthase, acid invertase, alkaline invertase, reducing sugar, soluble sugar, sucrose, and starch in the root showed the best mycorrhizal effect at the 100 mg P level. The expression levels of the 14-3-3 genes (PcGRF10 and PcGRF11) were significantly increased by mycorrhizal induction under drought stress. These levels were positively correlated with SS, SPS, sucrose, and starch phosphorylase in leaves, as well as with almost all sugar metabolism indicators in roots. However, they were negatively correlated with starch content in both leaves and roots. Sugar metabolism and 14-3-3 protein gene expression were induced by AM fungi and P addition in response to drought stress. The 14-3-3 genes induced by AM fungi may be involved in participating in osmotic regulation during drought stress. This study provides a new idea for the mechanism of sugar metabolism of mycorrhizal plants in arid regions.

Highly Efficient Production of UDP-Glucose from Sucrose via the Semirational Engineering of Sucrose Synthase and a Cascade Route Design

Nucleotide sugars are essential precursors for carbohydrate synthesis but are in scarce supply. Uridine diphosphate (UDP)-glucose is a core building block in nucleotide sugar preparation, making its efficient synthesis critical. Here, a process for producing valuable UDP-glucose and functional mannose from sucrose was established and improved via a semirational sucrose synthase (SuSy) design and the accurate D-mannose isomerase (MIase) cascade. Engineered SuSy exhibited enzyme activity 2.2-fold greater than that of the WT. The structural analysis identified a latch-hinge combination as the hotspot for enhancing enzyme activity. Coupling MIase, process optimization, and reaction kinetic analysis revealed that MIase addition during the high-speed UDP-glucose synthesis phase distinctly accelerated the entire process. The simultaneous triggering of enzyme modules halved the reaction time and significantly increased the UDP-glucose yield. A maximum UDP-glucose yield of 83%, space-time yield of 70 g/L/h, and mannose yield of 32% were achieved. This novel and efficient strategy for sucrose value-added exploitation has industrial promise.

Identification of sucrose synthase from Micractinium conductrix to favor biocatalytic glycosylation

Sucrose synthase (SuSy, EC 2.4.1.13) is a unique glycosyltransferase (GT) for developing cost-effective glycosylation processes. Up to now, some SuSys derived from plants and bacteria have been used to recycle uridine 5'-diphosphate glucose in the reactions catalyzed by Leloir GTs. In this study, after sequence mining and experimental verification, a SuSy from Micractinium conductrix (McSuSy), a single-cell green alga, was overexpressed in Escherichia coli, and its enzymatic properties were characterized. In the direction of sucrose cleavage, the specific activity of the recombinant McSuSy is 9.39 U/mg at 37°C and pH 7.0, and the optimum temperature and pH were 60°C and pH 7.0, respectively. Its nucleotide preference for uridine 5'-diphosphate (UDP) was similar to plant SuSys, and the enzyme activity remained relatively high when the DMSO concentration below 25%. The mutation of the predicted N-terminal phosphorylation site (S31D) significantly stimulated the activity of McSuSy. When the mutant S31D of McSuSy was applied by coupling the engineered Stevia glycosyltransferase UGT76G1 in a one-pot two-enzyme reaction at 10% DMSO, 50 g/L rebaudioside E was transformed into 51.06 g/L rebaudioside M in 57 h by means of batch feeding, with a yield of 76.48%. This work may reveal the lower eukaryotes as a promising resource for SuSys of industrial interest.

Genome-Wide Identification and Expression Analysis of the Sucrose Synthase Gene Family in Sweet Potato and Its Two Diploid Relatives

Sucrose synthases (SUS; EC 2.4.1.13) encoded by a small multigene family are the central system of sucrose metabolism and have important implications for carbon allocation and energy conservation in nonphotosynthetic cells of plants. Though the SUS family genes (SUSs) have been identified in several plants, they have not been explored in sweet potato. In this research, nine, seven and seven SUSs were identified in the cultivated sweet potato (Ipomoea batatas, 2n = 6x = 90) as well as its two diploid wild relatives I. trifida (2n = 2x = 30) and I. triloba (2n = 2x = 30), respectively, and divided into three subgroups according to their phylogenetic relationships. Their protein physicochemical properties, chromosomal localization, phylogenetic relationship, gene structure, promoter cis-elements, protein interaction network and expression patterns were systematically analyzed. The results indicated that the SUS gene family underwent segmental and tandem duplications during its evolution. The SUSs were highly expressed in sink organs. The IbSUSs especially IbSUS2, IbSUS5 and IbSUS7 might play vital roles in storage root development and starch biosynthesis. The SUSs could also respond to drought and salt stress responses and take part in hormone crosstalk. This work provides new insights for further understanding the functions of SUSs and candidate genes for improving yield, starch content, and abiotic stress tolerance in sweet potatoes.

Overexpression of the ZmSUS1 gene alters the content and composition of endosperm starch in maize (Zea mays L.)

ZmSUS1 increases the amylose content of maize by regulating the expression of Shrunken2 (Sh2) and Brittle2 (Bt2) which encode the size subunits of endosperm ADP-glucose pyrophosphorylase, and Granule bound starchsynthase1 (GBSS1) and Starch synthase1 (SS1). Cereal crops accumulate starch in seeds as an energy reserve. Sucrose Synthase (SuSy) plays an important role in grain starch synthesis. In this study, ZmSUS1 was transformed into maize inbred line KN5585, and transgenic plants were obtained. Compared with the non-transgenic negative control, the content and activity of SuSy were significantly increased, the amylose content in mature seeds of transgenic maize increased by 41.1-69.2%, the total starch content increased by 5.0-13.5%, the 100-grain weight increased by 19.0-26.2% and the average diameter of starch granules increased by 10.8-17.2%. These results indicated that overexpression of ZmSUS1 can significantly improve the traits of maize seeds and obtain new lines with high amylose content. It was also found that the overexpression of ZmSUS1 may increase the amylose content by altering the expression of endosperm ADP-glucose pyrophosphorylase (AGPase) subunits Shrunken2 (Sh2) and Brittle2 (Bt2). Moreover, the ectopic expression of ZmSUS1 also affected the expression of Granule bound starch synthase1 (GBSS1) and Starch synthase1 (SS1) which encode starch synthase. This study proved the important role of ZmSUS1 in maize starch synthesis and provided a new technology strategy for improving maize starch content and yield.

Genome-Wide Identification and Analysis of Stress Response of Trehalose-6-Phosphate Synthase and Trehalose-6-Phosphate Phosphatase Genes in Quinoa

Saline-alkali stress seriously affects the yield and quality of crops, threatening food security and ecological security. Improving saline-alkali land and increasing effective cultivated land are conducive to sustainable agricultural development. Trehalose, a nonreducing disaccharide, is closely related to plant growth and development and stress response. Trehalose 6-phosphate synthase (TPS) and trehalose-6-phosphate phosphatase (TPP) are key enzymes catalyzing trehalose biosynthesis. To elucidate the effects of long-term saline-alkali stress on trehalose synthesis and metabolism, we conducted an integrated transcriptome and metabolome analysis. As a result, 13 TPS and 11 TPP genes were identified in quinoa (Chenopodium quinoa Willd.) and were named CqTPS1-13 and CqTPP1-11 according to the order of their Gene IDs. Through phylogenetic analysis, the CqTPS family is divided into two classes, and the CqTPP family is divided into three classes. Analyses of physicochemical properties, gene structures, conservative domains and motifs in the proteins, and cis-regulatory elements, as well as evolutionary relationships, indicate that the TPS and TPP family characteristics are highly conserved in quinoa. Transcriptome and metabolome analyses of the sucrose and starch metabolism pathway in leaves undergoing saline-alkali stress indicate that CqTPP and Class II CqTPS genes are involved in the stress response. Moreover, the accumulation of some metabolites and the expression of many regulatory genes in the trehalose biosynthesis pathway changed significantly, suggesting the metabolic process is important for the saline-alkali stress response in quinoa.

Sucrose promotes branch-thorn occurrence of Lycium ruthenicum through dual effects of energy and signal

Lycium ruthenicum is an important eco-economic thorny shrub. In this study, L. ruthenicum plants of a clone showed two types of 'less leaves without thorn' and 'more leaves with thorns' under the same condition after transplanting. Microscopic observation revealed that apical buds of the thornless (Thless) and thorny (Thorny) branches should be selected as materials for further study. RNA-Seq analysis showed that KEGG pathway of Starch and sucrose metabolism and DEGs of SUT13, SUS, TPP and TPS were significantly up-regulated in the Thorny. The results of qRT-PCR confirmed the accuracy and credibility of the RNA-Seq. The content of sucrose in the Thorny was significantly higher than that in the Thless, but the content of trehalose-6-phosphate was opposite. Leaf-clipping treatments reduced sucrose content and inhibited the occurrence/development of branch-thorns; exogenous sucrose of 16 g/L significantly promoted the occurrence and growth of branch-thorns and the promotion effects were significantly higher than those treated with non-metabolizable sucrose analogs (isomaltolose, melitose). These findings suggested that sucrose might play a dual role of energy and signal in the occurrence of branch-thorns. Higher sucrose supply in apical buds from more leaves promoted the occurrence of branch-thorns via lower content of trehalose-6-phosphate and higher expression levels of SUS, TPP and TPS, whereas less leaves inhibited the occurrence. Molecular hypothesis model of leaf number/sucrose supply regulating the occurrence of branch-thorns in L. ruthenicum was established in the study, which provides foundation for breeding both thornless L. ruthenicum and thornless types of other species.

Rethinking Biosynthesis of Aclacinomycin A

Aclacinomycin A (ACM-A) is an anthracycline antitumor agent widely used in clinical practice. The current industrial production of ACM-A relies primarily on chemical synthesis and microbial fermentation. However, chemical synthesis involves multiple reactions which give rise to high production costs and environmental pollution. Microbial fermentation is a sustainable strategy, yet the current fermentation yield is too low to satisfy market demand. Hence, strain improvement is highly desirable, and tremendous endeavors have been made to decipher biosynthesis pathways and modify key enzymes. In this review, we comprehensively describe the reported biosynthesis pathways, key enzymes, and, especially, catalytic mechanisms. In addition, we come up with strategies to uncover unknown enzymes and improve the activities of rate-limiting enzymes. Overall, this review aims to provide valuable insights for complete biosynthesis of ACM-A.

One-pot biosynthesis of gastrodin using UDP-glycosyltransferase itUGT2 with an in situ UDP-glucose recycling system

Gastrodin, the major effective ingredient in Tianma (Gastrodia elata), is a p-hydroxybenzoic acid derivative with various activities. Gastrodin has been widely investigated for food and medical applications. The last biosynthetic step for gastrodin is UDP-glycosyltransferase (UGT)-mediated glycosylation with UDP-glucose (UDPG) as glycosyl donor. In this study, we performed a one-pot reaction both in vitro and in vivo to synthesize gastrodin from p-hydroxybenzyl alcohol (pHBA) by coupling UDP-glucosyltransferase from Indigofera tinctoria (itUGT2) to sucrose synthase from Glycine max (GmSuSy) for regeneration of UDPG. The in vitro results showed that itUGT2 transferred a glucosyl group to pHBA to generate gastrodin. After 37 UDPG regeneration cycles with 2.5% (molar ratio) UDP, the pHBA conversion reached 93% at 8 h. Furthermore, a recombinant strain with itUGT2 and GmSuSy genes was constructed. Through optimizing the incubation conditions, a 95% pHBA conversion rate (220 mg/L gastrodin titer) was achieved in vivo without addition of UDPG, which was 2.6-fold higher than that without GmSuSy. This in situ system for gastrodin biosynthesis provides a highly efficient strategy for both in vitro gastrodin synthesis and in vivo biosynthesis of gastrodin in E. coli with UDPG regeneration.

Engineering the Thermostability of Sucrose Synthase by Reshaping the Subunit Interaction Contributes to Efficient UDP-Glucose Production

The restricted availability of UDP-glucose, an essential precursor that targets oligo/polysaccharide and glycoside synthesis, makes its practical application difficult. Sucrose synthase (Susy), which catalyzes one-step UDP-glucose synthesis, is a promising candidate. However, due to poor thermostability of Susy, mesophilic conditions are required for synthesis, which slow down the process, limit productivity, and prevent scaled and efficient UDP-glucose preparation. Here, we obtained an engineered thermostable Susy (mutant M4) from Nitrosospira multiformis through automated prediction and greedy accumulation of beneficial mutations. The mutant improved the T_1/2 value at 55 °C by 27-fold, resulting in UDP-glucose synthesis at 37 g/L/h of space-time yield that met industrial biotransformation standards. Furthermore, global interaction between mutant M4 subunits was reconstructed by newly formed interfaces according to molecular dynamics simulations, with residue Trp162 playing an important role in strengthening the interface interaction. This work enabled effective, time-saving UDP-glucose production and paved the way for rational thermostability engineering of oligomeric enzymes.

Mechanistic and evolutionary insights into alkaline phosphatase superfamily through structure-function studies on Sphingomonas alkaline phosphatase

Alkaline phosphatases (APs), represented by E. coli AP (ECAP), employ an arginine residue to stabilize the phosphoryl group in the active site; whereas, AP from Sphingomonas (SPAP) shows a unique combination of substrate-binding residues; Thr89, Asn110, Lys171, and Arg173. Although such combination has been observed only in SPAP, these residues are present separately in different members of the AP superfamily. Here, we establish the presence of two distinct classes of APs; ECAP-type and SPAP-type. Bioinformatic analyses show that SPAP-type of APs are widely distributed in the bacterial kingdom. The role of active site residues in the catalytic mechanism has been delineated through a set of crystal structures reported here. These structures, representing different stages of the reaction pathway provide wealth of information for the catalytic mechanism. Despite critical differences in the substrate binding residues, SPAP follows a mechanism similar to that of ECAP-type of APs. Structure-based phylogenetic analysis suggests that SPAP and ECAP may have diverged very early during the evolution from a common ancestor. Moreover, it is proposed that the SPAP-type of APs are fundamental members of the AP superfamily and are more closely related to other members of the superfamily as compared to the ECAP-type of APs.

VaSUS2 confers cold tolerance in transgenic tomato and Arabidopsis by regulation of sucrose metabolism and ROS homeostasis

VaSUS2 enhances cold tolerance of transgenic tomato and Arabidopsis by regulating sucrose metabolism and improving antioxidant enzymes activity. Sucrose synthetase (SUS) is a key enzyme of sugar metabolism, and plays an important role in response to abiotic stress in plant. However, the function of VaSUS2 remains unknown in cold tolerance. Here, the cloning and functional characterization of the plasma membrane-localized VaSUS2 gene isolated from Vitis amurensis was studied. The transcript level of VaSUS2 was up-regulated under cold stress in Vitis amurensis. Heterologous expression of VaSUS2 in tomato increased SUS activity, which promoted the accumulation of glucose and fructose under cold treatment. The transgenic tomato and Arabidopsis exhibited higher levels of antioxidant enzymes activity, lower relative electrolyte leakage (REL), malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) content compared to wild type under cold stress. Importantly, the ability of scavenging reactive oxygen species (ROS) in transgenic plants was significantly improved. Moreover, yeast two-hybrid (Y2H) indicated that VaSnRK1 might be a potential interaction protein of VaSUS2. qRT-PCR showed that sucrose metabolism-related genes SlSUS, SlSPS and SlINV were significantly up-regulated in transgenic tomatoes. Meanwhile, the expression levels of antioxidant enzyme genes and cold-related genes CBF1, COR47 and ICE1 were up-regulated in transgenic plants. Taken together, these results suggested that VaSUS2 was involved in cold tolerance by increasing the levels of soluble sugars, improving the activity of antioxidant enzymes, and up-regulating the expression of cold-related genes in transgenic tomatoes and Arabidopsis.

Direct Yeast Surface Codisplay of Sequential Enzymes with Complementary Anchor Motifs: Enabling Enhanced Glycosylation of Natural Products

Yeast surface display is an appealing technique for constructing multienzyme cascades. This technique is commonly achieved using a scaffold for the ordered arrangement of various enzymes. However, this method is typically complicated because scaffold use may engender extra metabolic burden on the cell host. Here, we established a direct yeast surface codisplay strategy by employing two complementary anchor motifs, Agα1 and Pir1. These motifs allow for the codisplay of sequential uridine diphosphate-glycosyltransferase (UGT) and sucrose synthase (SUS) on the surface of Pichia pastoris (syn. Komagataella phaffii) for the glycosylation of natural products. We manipulated the displayed stoichiometry, amount, and assembly order of UGT and SUS by coupling them with anchor motifs. Furthermore, their effect on enzyme activity was thoroughly investigated. The surface-codisplayed strain UGT-Pir-SUS-Agα exhibited greater thermostability than the single-displayed strains and their free counterparts. Moreover, the strain UGT-Pir-SUS-Agα was successfully applied to glycyrrhetinic acid (GA) glycosylation to produce GA-3-O-Glc, with sucrose being the sugar donor in this process. This generated 7.5- to 20- and 5.3-fold higher GA-3-O-Glc concentration compared with the free counterparts (enzyme mass loading of 20-fold in excess) and mixed single-displayed strains of UGT-Agα and SUS-Pir, respectively. This increase was due to the improved biochemical properties and substrate channeling effect of strain UGT-Pir-SUS-Agα. This controllable direct surface codisplay strategy, based on complementary anchor motifs, is readily extendable to other enzyme cascades.

Highly efficient synthesis of mono-β-1,6-Glucosylated Rebaudioside A derivative catalyzed by glycosyltransferase YjiC

Steviol glycosides have attracted great interest because of their high levels of sweetness and safety, and absence of calories. Improvement of their sensory qualities via glycosylation modification by glycosyltransferase is a research hotspot. In this study, YjiC, a uridine diphosphate-dependent glycosyltransferase from Bacillus subtilis 168, was found with the ability to glycosylate rebaudioside A (Reb A) to produce a novel mono β-1, 6-glycosylated Reb A derivative rebaudioside L2 (Reb L2). It has an improved sweetness compared with Reb A. Next, a cascade reaction was established by combining YjiC with sucrose synthase AtSuSy from Arabidopsis thaliana for scale-up preparation of Reb L2. It shows that Reb L2 (30.94 mg/mL) could be efficiently synthesized with an excellent yield of 91.34% within 12 h. Therefore, this study provides a potential approach for the production and application of new steviol glycoside Reb L2, expanding the scope of steviol glycosides.

Exogenous brassinosteroids promotes root growth, enhances stress tolerance, and increases yield in maize

Brassinosteroids (BRs) regulate of maize (Zea mays L.) growth, but the underlying molecular mechanism remains unclear. In this study, we used a multi-disciplinary approach to determine how BRs regulate maize morphology and physiology during development. Treatment with the BRs promoted primary root the elongation and growth during germination, and the early development of lateral roots. BRs treatment during the middle growth stage increased the levels of various stress resistance factors, and enhanced resistance to lodging, likely by protecting the plant against stem rot and sheath rot. BRs had no significant effect on plant height during late growth, but it increased leaf angle and photosynthetic efficiency, as well as yield and quality traits. Our findings increase our understanding of the regulatory effects of BR on maize root growth and development and the mechanism by which BR improves disease resistance, which could further the potential for using BR to improve maize yield.

Characteristics and Expression Analysis of Invertase Gene Family in Common Wheat (Triticum aestivum L.)

Invertase (INV) irreversibly catalyzes the conversion of sucrose into glucose and fructose, playing important role in plant development and stress tolerance. However, the functions of INV genes in wheat have been less studied. In this study, a total of 126 TaINV genes were identified using a genome-wide search method, which could be classified into five classes (TaCWI-α, TaCWI-β, TaCI-α, TaCI-β, and TaVI) based on phylogenetic relationship. A total of 101 TaINVs were collinear with their ancestors in the synteny analysis, and we speculated that polyploidy events were the main force in the expansion of the TaINV gene family. Compared with TaCI, TaCWI and TaVI are more similar in gene structure and protein properties. Transcriptome sequencing analysis showed that TaINVs expressed in multiple tissues with different expression levels. Among 19 tissue-specific expressed TaINVs, 12 TaINVs showed grain-specific expression pattern and might play an important role in wheat grain development. In addition, qRT-PCR results further confirmed that TaCWI50 and TaVI27 show different expression in grain weight NILs. Our results demonstrated that the high expression of TaCWI50 and TaVI27 may be associated with a larger TGW phenotype. This work provides the foundations for understanding the grain development mechanism.

3D Printed Porous Nanocellulose-Based Scaffolds As Carriers for Immobilization of Glycosyltransferases

Biocatalysis is increasingly becoming an alternative method for the synthesis of industrially relevant complex molecules. This can be realized by using enzyme immobilized polysaccharide-based 3D scaffolds as compatible carriers, with defined properties. Especially, immobilization of either single or multiple enzymes on a 3D printed polysaccharide scaffold, exhibiting well-organized interconnected porous structure and morphology, is a versatile approach to access the performance of industrially important enzymes. Here, we demonstrated the use of nanocellulose-based 3D porous scaffolds for the immobilization of glycosyltransferases, responsible for glycosylation in natural biosynthesis. The scaffolds were produced using an ink containing nanofibrillated cellulose (NFC), carboxymethyl cellulose (CMC), and citric acid. Direct-ink-writing 3D printing followed by freeze-drying and dehydrothermal treatment at elevated temperature resulted in chemically cross-linked scaffolds, featuring tunable negative charges (2.2-5.0 mmol/g), pore sizes (10-800 μm), fluid uptake capacity, and exceptional dimensional and mechanical stability in the wet state. The negatively charged scaffolds were applied to immobilize two sugar nucleotide-dependent glycosyltransferases (C-glycosyltransferase, Z_basic2-CGT; sucrose synthase, Z_basic2-SuSy), each harboring a cationic binding module (Z_basic2) to promote charge-based enzyme adsorption. Both enzymes were immobilized at ∼30 mg of protein/g of dry carrier (∼20% yield), independent of the scaffold used. Their specific activities were 0.50 U/mg (Z_basic2-CGT) and 0.19 U/mg (Z_basic2-SuSy), corresponding to an efficacy of 37 and 18%, respectively, compared to the soluble enzymes. The glycosyltransferases were coimmobilized and shown to be active in a cascade reaction to give the natural C-glycoside nothofagin from phloretin (1.0 mM; ∼95% conversion). All enzyme bound scaffolds showed reusability of a maximum of 5 consecutive reactions. These results suggest that the 3D printed and cross-linked NFC/CMC-based scaffolds could present a class of solid carriers for enzyme (co)-immobilization, with promising applications in glycosyltransferase-catalyzed synthesis and other fields of biocatalysis.

Giant Viruses as a Source of Novel Enzymes for Biotechnological Application

The global demand for industrial enzymes has been increasing in recent years, and the search for new sources of these biological products is intense, especially in microorganisms. Most known viruses have limited genetic machinery and, thus, have been overlooked by the enzyme industry for years. However, a peculiar group of viruses breaks this paradigm. Giant viruses of the phylum Nucleocytoviricota infect protists (i.e., algae and amoebae) and have complex genomes, reaching up to 2.7 Mb in length and encoding hundreds of genes. Different giant viruses have robust metabolic machinery, especially those in the Phycodnaviridae and Mimiviridae families. In this review, we present some peculiarities of giant viruses that infect protists and discuss why they should be seen as an outstanding source of new enzymes. We revisited the genomes of representatives of different groups of giant viruses and put together information about their enzymatic machinery, highlighting several genes to be explored in biotechnology involved in carbohydrate metabolism, DNA replication, and RNA processing, among others. Finally, we present additional evidence based on structural biology using chitinase as a model to reinforce the role of giant viruses as a source of novel enzymes for biotechnological application.

Evaluation of double expression system for co-expression and co-immobilization of flavonoid glucosylation cascade

Glucosylation cascade consisting of Leloir glycosyltransferase and sucrose synthase with in situ regeneration system of expensive and low available nucleotide sugars is a game-changing strategy for enzyme-based production of glycoconjugates of relevant natural products. We designed a stepwise approach including co-expression and one-step purification and co-immobilization on glass-based EziG resins of sucrose synthase from Glycine max (GmSuSy) with promiscuous glucosyltransferase YjiC from Bacillus licheniformis to produce efficient, robust, and versatile biocatalyst suited for preparative scale flavonoid glucosylation. The undertaken investigations identified optimal reaction conditions (30 °C, pH 7.5, and 10 mM Mg2+) and the best-suited carrier (EziG Opal). The prepared catalyst exhibited excellent reusability, retaining up to 96% of initial activity after 12 cycles of reactions. The semi-preparative glucosylation of poorly soluble isoflavone Biochanin A resulted in the production of 73 mg Sissotrin (Biochanin A 7-O-glucoside). Additionally, the evaluation of the designed double-controlled, monocistronic expression system with two independently induced promoters (rhaBAD and trc) brought beneficial information for dual-expression plasmid design. KEY POINTS: • Simultaneous and titratable expression from two independent promoters is possible, although full control over the expression is limited. • Designed catalyst managed to glucosylate poorly soluble isoflavone. • The STY of Sissotrin using the designed catalyst reached 0.26 g/L∙h∙g of the resin.

Nano-fibrillated cellulose-based scaffolds for enzyme (co)-immobilization: Application to natural product glycosylation by Leloir glycosyltransferases

Polysaccharide-based scaffolds are promising carriers for enzyme immobilization. Here, we demonstrate a porous scaffold prepared by direct-ink-writing 3D printing of an ink consisting of nanofibrillated cellulose, carboxymethyl cellulose and citric acid for immobilization application. Negative surface charge introduced by the components made the scaffold amenable for an affinity-like immobilization via the cationic protein module Z_basic2. Z_basic2 fusions of two sugar nucleotide-dependent glycosyltransferases (C-glycosyltransferase, Z-CGT; sucrose synthase, Z-SuSy) were immobilized individually, or co-immobilized, and applied to synthesize the natural C-glycoside nothofagin. The cascade reaction involved β-C-glycosylation of phloretin (10 mM, ~90 % conversion) from UDP-glucose, provided from sucrose and catalytic amounts of UDP (1.0 mM). Enzymes were co-immobilized at ~65 mg protein/g carrier to receive activities of 9.5 U/g (Z-CGT) and 4.5 U/g (Z-SuSy) in 22-33 % yield (protein) and an effectiveness of 23 % (Z-CGT) and 13 % (Z-SuSy). The scaffold-bound enzymes were recyclable for 5 consecutive reactions.

Highly efficient production of rebaudioside D enabled by structure-guided engineering of bacterial glycosyltransferase YojK

Owing to zero-calorie, high-intensity sweetness and good taste profile, the plant-derived sweetener rebaudioside D (Reb D) has attracted great interest to replace sugars. However, low content of Reb D in stevia rebaudiana Bertoni as well as low soluble expression and enzymatic activity of plant-derived glycosyltransferase in Reb D preparation restrict its commercial usage. To address these problems, a novel glycosyltransferase YojK from Bacillus subtilis 168 with the ability to glycosylate Reb A to produce Reb D was identified. Then, structure-guided engineering was performed after solving its crystal structure. A variant YojK-I241T/G327N with 7.35-fold increase of the catalytic activity was obtained, which allowed to produce Reb D on a scale preparation with a great yield of 91.29%. Moreover, based on the results from molecular docking and molecular dynamics simulations, the improvement of enzymatic activity of YojK-I241T/G327N was ascribed to the formation of new hydrogen bonds between the enzyme and substrate or uridine diphosphate glucose. Therefore, this study provides an engineered bacterial glycosyltransferase YojK-I241T/G327N with high solubility and catalytic efficiency for potential industrial scale-production of Reb D.

Family 1 glycosyltransferases (GT1, UGTs) are subject to dilution-induced inactivation and low chemo stability toward their own acceptor substrates

Glycosylation reactions are essential but challenging from a conventional chemistry standpoint. Conversely, they are biotechnologically feasible as glycosyltransferases can transfer sugar to an acceptor with perfect regio- and stereo-selectivity, quantitative yields, in a single reaction and under mild conditions. Low stability is often alleged to be a limitation to the biotechnological application of glycosyltransferases. Here we show that these enzymes are not necessarily intrinsically unstable, but that they present both dilution-induced inactivation and low chemostability towards their own acceptor substrates, and that these two phenomena are synergistic. We assessed 18 distinct GT1 enzymes against three unrelated acceptors (apigenin, resveratrol, and scopoletin—respectively a flavone, a stilbene, and a coumarin), resulting in a total of 54 enzymes: substrate pairs. For each pair, we varied catalyst and acceptor concentrations to obtain 16 different reaction conditions. Fifteen of the assayed enzymes (83%) displayed both low chemostability against at least one of the assayed acceptors at submillimolar concentrations, and dilution-induced inactivation. Furthermore, sensitivity to reaction conditions seems to be related to the thermal stability of the enzymes, the three unaffected enzymes having melting temperatures above 55°C, whereas the full enzyme panel ranged from 37.4 to 61.7°C. These results are important for GT1 understanding and engineering, as well as for discovery efforts and biotechnological use.

Systematic analysis of the sugar accumulation mechanism in sucrose- and hexose- accumulating cherry tomato fruits

BACKGROUND

Sugar content is an important indicator of fruit quality. Except for a few wild tomato species that accumulate sucrose in the fruits, most cultivated tomato species accumulate hexose. Although several studies have focused on wild sucrose-accumulating tomato, the sucrose accumulation mechanism is still unclear.

RESULTS

Here, two homozygous inbred cherry tomato lines ('TB0023' and 'TB0278', which accumulated sucrose and hexose, respectively) were selected to analyze the sugar accumulation mechanism. Carbohydrate analysis, cytological observation, gene expression and enzyme activity analysis and proteomics methods were used in this study. The results indicated that glucose and fructose were absolutely dominant in the soluble sugar content of hexose-accumulating cherry tomato fruit, while sucrose and a certain proportion of hexose were the main forms of soluble sugar in sucrose-accumulating cherry tomato fruit. The phloem unloading pathway of the hexose-accumulating cherry tomato fruit switched from symplastic to apoplastic during fruit development, and the sucrose-accumulating cherry tomato probably had a mixed unloading pathway involving the symplastic and apoplastic. High activity of acid invertase (AI), sucrose phosphate synthase (SPS), sucrose synthase (SS) and sugar transporters LeSUT1, SlSWEET2a and SlSWEET12c were important factors for hexose accumulation in the hexose-accumulating cherry tomato fruit, while LeSUT2, SPS, SS, SlSWEET1b, SlSWEET5b, SlSWEET11b, SlSWEET7a, SlSWEET14 were responsible for solute sugar accumulation in the sucrose-accumulating cherry tomato.

CONCLUSIONS

This study provides detailed evidence for elucidation of the tomato sugar accumulation mechanism from the perspective of cell structure, physiology and molecular biology, providing a theoretical basis for the improvement of tomato quality and aiding the utilization of tomato genetic resources.

Comprehensive utilization of sucrose resources via chemical and biotechnological processes: A review

Sucrose, one of the most widespread disaccharides in nature, has been available in daily human life for many centuries. As an abundant and cheap sweetener, sucrose plays an essential role in our diet and the food industry. However, it has been determined that many diseases, such as obesity, diabetes, hyperlipidemia, etc., directly relate to the overconsumption of sucrose. It arouses many explorations for the conversion of sucrose to high-value chemicals. Production of valuable substances from sucrose by chemical methods has been studied since a half-century ago. Compared to chemical processes, biotechnological conversion approaches of sucrose are more environmentally friendly. Many enzymes can use sucrose as the substrate to generate functional sugars, especially those from GH68, GH70, GH13, and GH32 families. In this review, enzymatic catalysis and whole-cell fermentation of sucrose for the production of valuable chemicals were reviewed. The multienzyme cascade catalysis and metabolic engineering strategies were addressed.