Functional analysis of Arabidopsis thaliana RHM2/MUM4, a multidomain protein involved in UDP-D-glucose to UDP-L-rhamnose conversion

Advances in Engineering Nucleotide Sugar Metabolism for Natural Product Glycosylation in Saccharomyces cerevisiae

Glycosylation is a ubiquitous modification present across all of biology, affecting many things such as physicochemical properties, cellular recognition, subcellular localization, and immunogenicity. Nucleotide sugars are important precursors needed to study glycosylation and produce glycosylated products. Saccharomyces cerevisiae is a potentially powerful platform for producing glycosylated biomolecules, but it lacks nucleotide sugar diversity. Nucleotide sugar metabolism is complex, and understanding how to engineer it will be necessary to both access and study heterologous glycosylations found across biology. This review overviews the potential challenges with engineering nucleotide sugar metabolism in yeast from the salvage pathways that convert free sugars to their associated UDP-sugars to de novo synthesis where nucleotide sugars are interconverted through a complex metabolic network with governing feedback mechanisms. Finally, recent examples of engineering complex glycosylation of small molecules in S. cerevisiae are explored and assessed.

Seed coating with phages for sustainable plant biocontrol of plant pathogens and influence of the seed coat mucilage

Pathogens resistant to classical control strategies pose a significant threat to crop yield, with seeds being a major transmission route. Bacteriophages, viruses targeting bacteria, offer an environmentally sustainable biocontrol solution. In this study, we isolated and characterized two novel phages, Athelas and Alfirin, which infect Pseudomonas syringae and Agrobacterium fabrum, respectively, and included the recently published Pfeifenkraut phage infecting Xanthomonas translucens. Using a simple immersion method, phages coated onto seeds successfully lysed bacteria post air-drying. The seed coat mucilage (SCM), a polysaccharide-polymer matrix exuded by seeds, plays a critical role in phage binding. Seeds with removed mucilage formed five to 10 times less lysis zones compared to those with mucilage. The podovirus Athelas showed the highest mucilage dependency. Phages from the Autographiviridae family also depended on mucilage for seed adhesion. Comparative analysis of Arabidopsis SCM mutants suggested the diffusible cellulose as a key component for phage binding. Long-term activity tests demonstrated high phage stability on seed surfaces and significantly increasing seedling survival rates in the presence of pathogens. Using non-virulent host strains enhanced phage presence on seeds but also has potential limitations. These findings highlight phage-based interventions as promising, sustainable strategies for combating pathogen resistance and improving crop yield.

Brassica napus BnaA09.MYB52 enhances seed coat mucilage accumulation and tolerance to osmotic stress during seed germination in Arabidopsis thaliana

Seed coat mucilage plays an important role in promoting seed germination under adversity. Previous studies have shown that Arabidopsis thaliana MYB52 (AtMYB52) can positively regulate seed coat mucilage accumulation. However, the role of Brassica napus MYB52 (BnaMYB52) in accumulation of seed coat mucilage and tolerance to osmotic stress during seed germination remains largely unknown. We cloned the BnaA09.MYB52 coding domain sequence from B. napus cv ZS11, identified its conserved protein domains and elucidated its relationship with homologues from a range of plant species. Transgenic plants overexpressing BnaA09.MYB52 in the A. thaliana myb52-1 mutant were generated through Agrobacterium-mediated transformation and used to assess the possible roles of BnaA09.MYB52 in accumulation of seed coat mucilage and tolerance to osmotic stress during seed germination. Subcellular localization and transcriptional activity assays demonstrated that BnaA09.MYB52 functions as a transcription factor. RT-qPCR results indicate that BnaA09.MYB52 is predominantly expressed in roots and developing seeds of B. napus cv ZS11. Introduction of BnaA09.MYB52 into myb52-1 restored thinner seed coat mucilage in this mutant to levels in the wild type. Consistently, expression levels of three key genes participating in mucilage formation in developing seeds of myb52-1 were also restored to wild type levels by overexpressing BnaA09.MYB52. Furthermore, BnaA09.MYB52 was induced by osmotic stress during seed germination in B. napus, and ectopic expression of BnaA09.MYB52 successfully corrected sensitivity of the myb52-1 mutant to osmotic stress during seed germination. These findings enhance our understanding of the functions of BnaA09.MYB52 and provide a novel strategy for future B. napus breeding.

Discovery of α-(1→6)-linked mannan structures resembling yeast N-glycan outer chains in Aspergillus fumigatus mycelium

The cellular surface of the pathogenic filamentous fungus Aspergillus fumigatus is enveloped in a mannose layer, featuring well-established fungal-type galactomannan and O-mannose-type galactomannan. This study reports the discovery of cell wall component in A. fumigatus mycelium, which resembles N-glycan outer chains found in yeast. The glycosyltransferases involved in its biosynthesis in A. fumigatus were identified, with a focus on two key α-(1→2)-mannosyltransferases, Mnn2 and Mnn5, and two α-(1→6)-mannosyltransferases, Mnn9 and Van1. In vitro examination revealed the roles of recombinant Mnn2 and Mnn5 in transferring α-(1→2)-mannosyl residues. Proton nuclear magnetic resonance (1H-NMR) analysis of cell wall extracts from the ∆mnn2∆mnn5 strain indicated the existence of an α-(1→6)-linked mannan backbone in the A. fumigatus mycelium, with Mnn2 and Mnn5 adding α-(1→2)-mannosyl residues to this backbone. The α-(1→6)-linked mannan backbone was absent in strains where mnn9 or van1 was disrupted in the parental ∆mnn2∆mnn5 strain in A. fumigatus. Mnn9 and Van1 functioned as α-(1→6)-linked mannan polymerases in heterodimers when co-expressed in Escherichia coli, indicating their crucial role in biosynthesizing the α-(1→6)-linked mannan backbone. Disruptions of these mannosyltransferases did not affect fungal-type galactomannan biosynthesis. This study provides insights into the complexity of fungal cell wall architecture and a better understanding of mannan biosynthesis in A. fumigatus.

IMPORTANCE

This study unravels the complexities of mannan biosynthesis in A. fumigatus, a key area for antifungal drug discovery. It reveals the presence of α-(1→6)-linked mannan structures resembling yeast N-glycan outer chains in A. fumigatus mycelium, offering fresh insights into the fungal cell wall's design. Key enzymes, Mnn2, Mnn5, Mnn9, and Van1, are instrumental in this process, with Mnn2 and Mnn5 adding specific mannose residues and Mnn9 and Van1 assembling the α-(1→6)-linked mannan structures. Although fungal-type galactomannan's presence in the cell wall is known, the existence of an α-(1→6)-linked mannan adds a new dimension to our understanding. This intricate web of mannan biosynthesis opens avenues for further exploration and enhances our understanding of fungal cell wall dynamics, paving the way for targeted drug development.

Engineered Saccharomyces cerevisiae as a Biosynthetic Platform of Nucleotide Sugars

Glycosylation of biomolecules can greatly alter their physicochemical properties, cellular recognition, subcellular localization, and immunogenicity. Glycosylation reactions rely on the stepwise addition of sugars using nucleotide diphosphate (NDP)-sugars. Making these substrates readily available will greatly accelerate the characterization of new glycosylation reactions, elucidation of their underlying regulation mechanisms, and production of glycosylated molecules. In this work, we engineered Saccharomyces cerevisiae to heterologously express nucleotide sugar synthases to access a wide variety of uridine diphosphate (UDP)-sugars from simple starting materials (i.e., glucose and galactose). Specifically, activated glucose, uridine diphosphate d-glucose (UDP-d-Glc), can be converted to UDP-d-glucuronic acid (UDP-d-GlcA), UDP-d-xylose (UDP-d-Xyl), UDP-d-apiose (UDP-d-Api), UDP-d-fucose (UDP-d-Fuc), UDP-l-rhamnose (UDP-l-Rha), UDP-l-arabinopyranose (UDP-l-Arap), and UDP-l-arabinofuranose (UDP-l-Araf) using the corresponding nucleotide sugar synthases of plant and microbial origins. We also expressed genes encoding the salvage pathway to directly activate free sugars to achieve the biosynthesis of UDP-l-Arap and UDP-l-Araf. We observed strong inhibition of UDP-d-Glc 6-dehydrogenase (UGD) by the downstream product UDP-d-Xyl, which we circumvented using an induction system (Tet-On) to delay the production of UDP-d-Xyl to maintain the upstream UDP-sugar pool. Finally, we performed a time-course study using strains containing the biosynthetic pathways to produce five non-native UDP-sugars to elucidate their time-dependent interconversion and the role of UDP-d-Xyl in regulating UDP-sugar metabolism. These engineered yeast strains are a robust platform to (i) functionally characterize sugar synthases in vivo, (ii) biosynthesize a diverse selection of UDP-sugars, (iii) examine the regulation of intracellular UDP-sugar interconversions, and (iv) produce glycosylated secondary metabolites and proteins.

Deciphering triterpenoid saponin biosynthesis by leveraging transcriptome response to methyl jasmonate elicitation in Saponaria vaccaria

Methyl jasmonate (MeJA) is a known elicitor of plant specialized metabolism, including triterpenoid saponins. Saponaria vaccaria is an annual herb used in traditional Chinese medicine, containing large quantities of oleanane-type triterpenoid saponins with anticancer properties and structural similarities to the vaccine adjuvant QS-21. Leveraging the MeJA-elicited saponin biosynthesis, we identify multiple enzymes catalyzing the oxidation and glycosylation of triterpenoids in S. vaccaria. This exploration is aided by Pacbio full-length transcriptome sequencing and gene expression analysis. A cellulose synthase-like enzyme can not only glucuronidate triterpenoid aglycones but also alter the product profile of a cytochrome P450 monooxygenase via preference for the aldehyde intermediate. Furthermore, the discovery of a UDP-glucose 4,6-dehydratase and a UDP-4-keto-6-deoxy-glucose reductase reveals the biosynthetic pathway for the rare nucleotide sugar UDP-D-fucose, a likely sugar donor for fucosylation of plant natural products. Our work enables the production and optimization of high-value saponins in microorganisms and plants through synthetic biology approaches.

Cellulose assembles into helical bundles of uniform handedness in cell walls with abnormal pectin composition

Plant cells and organs grow into a remarkable diversity of shapes, as directed by cell walls composed primarily of polysaccharides such as cellulose and multiple structurally distinct pectins. The properties of the cell wall that allow for precise control of morphogenesis are distinct from those of the individual polysaccharide components. For example, cellulose, the primary determinant of cell morphology, is a chiral macromolecule that can self-assemble in vitro into larger-scale structures of consistent chirality, and yet most plant cells do not display consistent chirality in their growth. One interesting exception is the Arabidopsis thaliana rhm1 mutant, which has decreased levels of the pectin rhamnogalacturonan-I and causes conical petal epidermal cells to grow with a left-handed helical twist. Here, we show that in rhm1 the cellulose is bundled into large macrofibrils, unlike the evenly distributed microfibrils of the wild type. This cellulose bundling becomes increasingly severe over time, consistent with cellulose being synthesized normally and then self-associating into macrofibrils. We also show that in the wild type, cellulose is oriented transversely, whereas in rhm1 mutants, the cellulose forms right-handed helices that can account for the helical morphology of the petal cells. Our results indicate that when the composition of pectin is altered, cellulose can form cellular-scale chiral structures in vivo, analogous to the helicoids formed in vitro by cellulose nano-crystals. We propose that an important emergent property of the interplay between rhamnogalacturonan-I and cellulose is to permit the assembly of nonbundled cellulose structures, providing plants flexibility to orient cellulose and direct morphogenesis.

Determination of the effects of pre-harvest bagging treatment on kiwifruit appearance and quality via transcriptome and metabolome analyses

Bagging is an effective cultivation strategy to produce attractive and pollution-free kiwifruit. However, the effect and metabolic regulatory mechanism of bagging treatment on kiwifruit quality remain unclear. In this study, transcriptome and metabolome analyses were conducted to determine the regulatory network of the differential metabolites and genes after bagging. Using outer and inner yellow single-layer fruit bags, we found that bagging treatment improved the appearance of kiwifruit, increased the soluble solid content (SSC) and carotenoid and anthocyanin levels, and decreased the chlorophyll levels. We also identified 41 differentially expressed metabolites and 897 differentially expressed genes (DEGs) between the bagged and control 'Hongyang' fruit. Transcriptome and metabolome analyses revealed that the increase in SSC after bagging treatment was mainly due to the increase in D-glucosamine metabolite levels and eight DEGs involved in amino sugar and nucleotide sugar metabolic pathways. A decrease in glutamyl-tRNA reductase may be the main reason for the decrease in chlorophyll. Downregulation of lycopene epsilon cyclase and 9-cis-epoxycarotenoid dioxygenase increased carotenoid levels. Additionally, an increase in the levels of the taxifolin-3'-O-glucoside metabolite, flavonoid 3'-monooxygenase, and some transcription factors led to the increase in anthocyanin levels. This study provides novel insights into the effects of bagging on the appearance and internal quality of kiwifruit and enriches our theoretical knowledge on the regulation of color pigment synthesis in kiwifruit.

Functional characterization of a Flavonol 3-O-rhamnosyltransferase and two UDP-rhamnose synthases from Hypericum monogynum

Rhamnosyltransferase (RT) and rhamnose synthase (Rhs) are the key enzymes that are responsible for the biosynthesis of rhamnosides and UDP-l-rhamnose (UDP-Rha) in plants, respectively. How to discover such enzymes efficiently for use is still a problem to be solved. Here, we identified HmF3RT, HmRhs1, and HmRhs2 from Hypericum monogynum, which is abundant in flavonol rhamnosides, with the help of a full-length and high throughput transcriptome sequencing platform. HmF3RT could regiospecifically transfer the rhamnose moiety of UDP-Rha onto the 3-OH position of flavonols and has weakly catalytic for UDP-xylose (UDP-Xyl) and UDP-glucose (UDP-Glc). HmF3RT showed well quercetin substrate affinity and high catalytic efficiency with K_m of 5.14 μM and k_cat/K_m of 2.21 × 10⁵ S^-1 M^-1, respectively. Docking, dynamic simulation, and mutagenesis studies revealed that V129, D372, and N373 are critical residues for the activity and sugar donor recognition of HmF3RT, mutant V129A, and V129T greatly enhance the conversion rate of catalytic flavonol glucosides. HmRhs1 and HmRhs2 convert UDP-Glc to UDP-Rha, which could be further used by HmF3RT. The HmF3RT and HmRhs1 co-expressed strain RTS1 could produce quercetin 3-O-rhamnoside (quercitrin), kaempferol 3-O-rhamnoside (afzelin), and myricetin 3-O-rhamnoside (myricitrin) at yields of 85.1, 110.7, and 77.6 mg L^-1, respectively. It would provide a valuable reference for establishing a better and more efficient biocatalyst for preparing bioactive flavonol rhamnosides by identifying HmF3RT and HmRhs.

Elucidation of the pathway for biosynthesis of saponin adjuvants from the soapbark tree

The Chilean soapbark tree (Quillaja saponaria) produces soap-like molecules called QS saponins that are important vaccine adjuvants. These highly valuable compounds are sourced by extraction from the bark, and their biosynthetic pathway is unknown. Here, we sequenced the Q. saponaria genome. Through genome mining and combinatorial expression in tobacco, we identified 16 pathway enzymes that together enable the production of advanced QS pathway intermediates that represent a bridgehead for adjuvant bioengineering. We further identified the enzymes needed to make QS-7, a saponin with excellent therapeutic properties and low toxicity that is present in low abundance in Q. saponaria bark extract. Our results enable the production of Q. saponaria vaccine adjuvants in tobacco and open the way for new routes to access and engineer natural and new-to-nature immunostimulants.

Complete biosynthesis of the phenylethanoid glycoside verbascoside

Verbascoside, which was first discovered in 1963, is a well-known phenylethanoid glycoside (PhG) that exhibits antioxidant, anti-inflammatory, antimicrobial, and neuroprotective activities and contributes to the therapeutic effects of many medicinal plants. However, the biosynthetic pathway of verbascoside remains to be fully elucidated. Here, we report the identification of two missing enzymes in the verbascoside biosynthesis pathway by transcriptome mining and in vitro enzymatic assays. Specifically, a BAHD acyltransferase (hydroxycinnamoyl-CoA:salidroside hydroxycinnamoyltransferase [SHCT]) was shown to catalyze the regioselective acylation of salidroside to form osmanthuside A, and a CYP98 hydroxylase (osmanthuside B 3,3'-hydroxylase [OBH]) was shown to catalyze meta-hydroxylations of the p-coumaroyl and tyrosol moieties of osmanthuside B to complete the biosynthesis of verbascoside. Because SHCTs and OBHs are found in many Lamiales species that produce verbascoside, this pathway may be general. The findings from the study provide novel insights into the formation of caffeoyl and hydroxytyrosol moieties in natural product biosynthetic pathways. In addition, with the newly acquired enzymes, we achieved heterologous production of osmanthuside B, verbascoside, and ligupurpuroside B in Escherichia coli; this work lays a foundation for sustainable production of verbascoside and other PhGs in micro-organisms.

Regulation of seed coat mucilage production and modification in Arabidopsis

The Arabidopsis seed coat mucilage is a polysaccharide-rich matrix synthesized by the seed coat epidermal cells. It is a specialized cell wall mainly composed of three types of polysaccharides (i. e. pectin, hemicellulose, and cellulose), and represents as an ideal model system for plant cell wall research. A large number of genes responsible for the synthesis and modification of cell wall polysaccharides have been identified using this model system. Moreover, a subset of regulators controlling mucilage production and modification have been characterized, and the underlying transcriptional regulatory mechanisms have been elucidated. This substantially contributes to the understanding of the molecular mechanisms underlying mucilage synthesis and modification. In this review, we concisely summarize the various genes and regulators involved in seed coat cell differentiation, mucilage biosynthesis and modification, and secondary cell wall formation. In particular, we put emphasis on the latest knowledge gained regarding the transcriptional regulation of mucilage production, which is composed of a hierarchal cascade with three-layer transcriptional regulators. Collectively, we propose an updated schematic framework of the genetic regulatory network controlling mucilage production and modification in the Arabidopsis mucilage secretory cells.

The L-Rhamnose Biosynthetic Pathway in Trichomonas vaginalis: Identification and Characterization of UDP-D-Glucose 4,6-dehydratase

Trichomonas vaginalis is the causative agent of one of the most widespread sexually transmitted diseases in the world. The adhesion of the parasite to the vaginal epithelial cells is mediated by specific proteins and by a complex glycan structure, the lipoglycan (TvLG), which covers the pathogen surface. L-rhamnose is an important component of TvLG, comprising up to 40% of the monosaccharides. Thus, the inhibition of its production could lead to a severe alteration in the TvLG structure, making the L-rhamnose biosynthetic pathway an attractive pharmacologic target. We report the identification and characterization of the first committed and limiting step of the L-rhamnose biosynthetic pathway, UDP-D-glucose 4,6-dehydratase (UGD, EC 4.2.1.76). The enzyme shows a strong preference for UDP-D-glucose compared to dTDP-D-glucose; we propose that the mechanism underlying the higher affinity for the UDP-bound substrate is mediated by the differential recognition of ribose versus the deoxyribose of the nucleotide moiety. The identification of the enzymes responsible for the following steps of the L-rhamnose pathway (epimerization and reduction) was more elusive. However, sequence analyses suggest that in T. vaginalis L-rhamnose synthesis proceeds through a mechanism different from the typical eukaryotic pathways, displaying intermediate features between the eukaryotic and prokaryotic pathways and involving separate enzymes for the epimerase and reductase activities, as observed in bacteria. Altogether, these results form the basis for a better understanding of the formation of the complex glycan structures on TvLG and the possible use of L-rhamnose biosynthetic enzymes for the development of selective inhibitors.

Mnt1, an α-(1 → 2)-mannosyltransferase responsible for the elongation of N-glycans and O-glycans in Aspergillus fumigatus

The fungal cell wall is necessary for survival as it serves a barrier for physical protection. Therefore, glycosyltransferases responsible for the synthesis of cell wall polysaccharides may be suitable targets for drug development. Mannose is a monosaccharide that is commonly found in sugar chains in the walls of fungi. Mannose residues are present in fungal-type galactomannan, O-glycans, N-glycans, glycosylphosphatidylinositol anchors, and glycosyl inositol phosphorylceramides in Aspergillus fumigatus. Three genes that are homologous to α-(1 → 2)-mannosyltransferase genes and belong to the glycosyltransferase family 15 were found in the A. fumigatus strain, Af293/A1163, genome: cmsA/ktr4, cmsB/ktr7, and mnt1. It is reported that the mutant ∆mnt1 strain exhibited a wide range of properties that included high temperature and drug sensitivity, reduced conidia formation, leakage at the hyphal tips, and attenuation of virulence. However, it is unclear whether Mnt1 is a bona fide α-(1 → 2)-mannosyltransferase and which mannose residues are synthesized by Mnt1 in vivo. In this study, we elucidated the structure of the Mnt1 reaction product, the structure of O-glycan in the Δmnt1 strain. In addition, the length of N-glycans attached to invertase was evaluated in the Δmnt1 strain. The results indicated that Mnt1 functioned as an α-(1 → 2)-mannosyltransferase involved in the elongation of N-glycans and synthesis of the second mannose residue of O-glycans. The widespread abnormal phenotype caused by the disruption of the mnt1 gene is the combined result of the loss of mannose residues from O-glycans and N-glycans. We also clarified the enzymatic properties and substrate specificity of Mnt1 based on its predicted protein structure.

PhRHMs play important roles in leaf and flower development and anthocyanin synthesis in petunia

Anthocyanins, vital metabolites in plants, are formed by anthocyanidins combined with various monosaccharides, including glucose, rhamnose, and arabinose. Rhamnose contributes greatly to the glycosylation of anthocyanidins. There are two kinds of rhamnose synthase (RS): rhamnose biosynthesis (RHM), and nucleotide-RS/epimerase-reductase (UER1). Nevertheless, no RS isoform was reported to be involved in anthocyanin synthesis. Here, three homologous PhRHM genes, namely PhRHM1, PhRHM2, and PhRHM3, and one PhUER1 gene from petunia were cloned and characterized. Green fluorescent protein fusion protein assays revealed that PhRHMs and PhUER1 are localized in the cytoplasm. We obtained PhRHM1 or/and PhRHM2 or PhUER1 silenced petunia plants and did not attempt to obtain PhRHM3 silenced plants since PhRHM3 mRNA was not detected in petunia organs examined. PhRHM1 and PhRHM2 (PhRHM1-2) silencing induced abnormal plant growth and decreased the contents of l-rhamnose, photosynthetic pigments and total anthocyanins, while PhUER1 silencing did not cause any visible phenotypic changes. Flavonoid metabolome analysis further revealed that PhRHM1-2 silencing reduced the contents of anthocyanins with rhamnose residue. These results revealed that PhRHMs contribute to the biosynthesis of rhamnose and that PhRHMs participate in the anthocyanin rhamnosylation in petunia, while PhUER1 does not.

Rhamnose-Containing Compounds: Biosynthesis and Applications

Rhamnose-associated molecules are attracting attention because they are present in bacteria but not mammals, making them potentially useful as antibacterial agents. Additionally, they are also valuable for tumor immunotherapy. Thus, studies on the functions and biosynthetic pathways of rhamnose-containing compounds are in progress. In this paper, studies on the biosynthetic pathways of three rhamnose donors, i.e., deoxythymidinediphosphate-L-rhamnose (dTDP-Rha), uridine diphosphate-rhamnose (UDP-Rha), and guanosine diphosphate rhamnose (GDP-Rha), are firstly reviewed, together with the functions and crystal structures of those associated enzymes. Among them, dTDP-Rha is the most common rhamnose donor, and four enzymes, including glucose-1-phosphate thymidylyltransferase RmlA, dTDP-Glc-4,6-dehydratase RmlB, dTDP-4-keto-6-deoxy-Glc-3,5-epimerase RmlC, and dTDP-4-keto-Rha reductase RmlD, are involved in its biosynthesis. Secondly, several known rhamnosyltransferases from Geobacillus stearothermophilus, Saccharopolyspora spinosa, Mycobacterium tuberculosis, Pseudomonas aeruginosa, and Streptococcus pneumoniae are discussed. In these studies, however, the functions of rhamnosyltransferases were verified by employing gene knockout and radiolabeled substrates, which were almost impossible to obtain and characterize the products of enzymatic reactions. Finally, the application of rhamnose-containing compounds in disease treatments is briefly described.

Pectin methyltransferase QUASIMODO2 functions in the formation of seed coat mucilage in Arabidopsis

Pectin, cellulose, and hemicelluloses are major components of primary cell walls in plants. In addition to cell adhesion and expansion, pectin plays a central role in seed mucilage. Seed mucilage contains abundant pectic rhamnogalacturonan-I (RG-I) and lower amounts of homogalacturonan (HG), cellulose, and hemicelluloses. Previously, accumulated evidence has addressed the role of pectin RG-I in mucilage production and adherence. However, less is known about the function of pectin HG in seed coat mucilage formation. In this study, we analyzed a novel mutant, designated things fall apart2 (tfa2), which contains a mutation in HG methyltransferase QUASIMODO2 (QUA2). Etiolated tfa2 seedlings display short hypocotyls and adhesion defects similar to qua2 and tumorous shoot development2 (tsd2) alleles, and show seed mucilage defects. The diminished uronic acid content and methylesterification degree of HG in mutant seed mucilage indicate the role of HG in the formation of seed mucilage. Cellulosic rays in mutant mucilage are collapsed. The epidermal cells of seed coat in tfa2 and tsd2 display deformed columellae and reduced radial wall thickness. Under polyethylene glycol treatment, seeds from these three mutant alleles exhibit reduced germination rates. Together, these data emphasize the requirement of pectic HG biosynthesis for the synthesis of seed mucilage, and the functions of different pectin domains together with cellulose in regulating its formation, expansion, and release.

Receptor-like kinase HAESA-like 1 positively regulates seed longevity in Arabidopsis

Based on the phenotypic, physiological and transcriptomic analysis, receptor-like kinase HAESA-like 1 was demonstrated to positively affect seed longevity in Arabidopsis. Seed longevity is very important for both genetic resource conservation and crop production. Receptor-like kinases (RLKs) are widely involved in plant growth, development and stress responses. However, the role of most RLKs, especially in seed longevity, is largely unknown. In this study, we report that Arabidopsis HAESA-like 1 (AtHSL1) positively regulated seed longevity. Disruption of HSL1 significantly decreased the germination rate to 50% at 7 days after cold stratification (DAC), compared with that of the wild type (93.5% at 7 DAC), after accelerated aging treatment. Expression of the HSL1 gene in hsl1 basically restored the defective phenotype (86.3%), while HSL1-overexpressing lines (98.3%) displayed slower accelerated aging than WT (93.5%). GUS staining revealed HSL1 was highly expressed universally, especially in young seedlings, mature seeds and embryos of imbibed seeds, and its expression could be induced by accelerated aging. No difference in the dyeing color and area of mucilage were identified between WT and hsl1. The soluble pectin content also was not different, while the adherent pectin content was significantly increased in hsl1. Global transcriptomics revealed that disruption of HSL1 mainly downregulated genes involved in trehalose synthesis, nucleotide sugar metabolism and protection and repair mechanisms. Therefore, an increase in adherent pectin content and downregulation of genes involved in trehalose synthesis may be the main reasons for decreasing seed longevity owing to disruption of HSL1 in Arabidopsis. Our work provides valuable information for understanding the function and mechanism of a receptor-like kinase, AtHSL1, in seed longevity.

Creating diverse glycosides of 2'-hydroxyflavone through microbial glycosylation

Four new 2'-hydroxyflavone glycosides, namely hydroxyflavone-2'-O-β-D-glucuronide (1), hydroxyflavone-2'-O-α-L-rhamnoside (2), hydroxyflavone-2'-O-β-D-glucoside (3), and hydroxyflavone-2'-O-4"-O-methyl-β-D-glucoside (4), were biosynthesized through microbial glycosylation using Streptomyces coeruleorubidus NRRL B-2569, Streptomyces toxytricini NRRL 15443, Escherichia coli BL21(DE3)/pWZ8, and Beauveria bassiana ATCC 7159, respectively. Compounds 1-4 were structurally characterized through extensive analysis of 1D and 2D NMR spectroscopic data. The water solubility of glycosylated products 1-4 were enhanced by 7 to 15 times compared to the substrate 2'-hydroxyflavone. Moreover, antioxidant assays revealed that compounds 1 and 2 exhibited stronger 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity than the substrate, decreasing the logIC₅₀ by 68.7% and 80.7%, respectively. Therefore, this research provides several effective biocatalysts that can be used for structural modification of flavonoids for enhanced water solubility and biological activities.

The Class II KNOX family members KNAT3 and KNAT7 redundantly participate in Arabidopsis seed coat mucilage biosynthesis

The production of Arabidopsis seed mucilage involves complex polysaccharide biosynthetic pathways and developmental processes in seed epidermal cells. Although the polysaccharide components of Arabidopsis seed mucilage have been identified, their regulatory mechanism requires further investigation. Here, we show that Class II KNOX gene family members KNAT3 and KNAT7 play an essential role in regulating mucilage production in the early developmental stages of Arabidopsis seeds. Double mutant knat3knat7 resulted in defective seed mucilage production and columellae formation, whereas knat3 showed a normal phenotype compared with wild type, and the mucilage thickness in knat7 was slightly disturbed. Rhamnogalacturonan I (RG-I) and its biosynthetic substrates galacturonic acid and rhamnose were reduced in both the adherent and soluble mucilage of knat3knat7. Comparative transcriptome analysis on whole seeds suggested that polysaccharide, glucosinolate and anthocyanin biosynthetic pathways were specifically repressed in knat3knat7. Transient co-expression of KNAT3 and KNAT7 with promoter regions of candidate genes in Arabidopsis protoplasts revealed that both KNAT3 and KNAT7 act as positive regulators of the RG-I biosynthetic gene MUCILAGE-MODIFIED 4 (MUM4, AT1G53500). Collectively, our results demonstrate that KNAT3 and KNAT7 are multifunctional transcription factors in secondary cell wall development and redundantly modulate mucilage biosynthesis in Arabidopsis seeds.

A DE1 BINDING FACTOR 1-GLABRA2 module regulates rhamnogalacturonan I biosynthesis in Arabidopsis seed coat mucilage

The mucilage surrounding hydrated Arabidopsis thaliana seeds is a specialized extracellular matrix composed mainly of the pectic polysaccharide rhamnogalacturonan I (RG-I). Although, several genes responsible for RG-I biosynthesis have been identified, the transcriptional regulatory mechanisms controlling RG-I production remain largely unknown. Here we report that the trihelix transcription factor DE1 BINDING FACTOR 1 (DF1) is a key regulator of mucilage RG-I biosynthesis. RG-I biosynthesis is significantly reduced in loss-of-function mutants of DF1. DF1 physically interacts with GLABRA2 (GL2) and both proteins transcriptionally regulate the expression of the RG-I biosynthesis genes MUCILAGE MODIFIED 4 (MUM4) and GALACTURONOSYLTRANSFERASE-LIKE5 (GATL5). Through chromatin immunoprecipitation-quantitative PCR and transcriptional activation assays, we uncover a cooperative mechanism of the DF1-GL2 module in activating MUM4 and GATL5 expression, in which DF1 binds to the promoters of MUM4 and GATL5 through interacting with GL2 and facilitates the transcriptional activity of GL2. The expression of DF1 and GL2 is directly regulated by TRANSPARENT TESTA GLABRA2 (TTG2) and, in turn, DF1 directly represses the expression of TTG2. Taken together, our data reveal that the transcriptional regulation of mucilage RG-I biosynthesis involves a regulatory module, comprising DF1, GL2, and TTG2.

Nucleotide sugar dehydratases: Structure, mechanism, substrate specificity, and application potential

Nucleotide sugar (NS) dehydratases play a central role in the biosynthesis of deoxy and amino sugars, which are involved in a variety of biological functions in all domains of life. Bacteria are true masters of deoxy sugar biosynthesis as they can produce a wide range of highly specialized monosaccharides. Indeed, deoxy and amino sugars play important roles in the virulence of gram-positive and gram-negative pathogenic species and are additionally involved in the biosynthesis of diverse macrolide antibiotics. The biosynthesis of deoxy sugars relies on the activity of NS dehydratases, which can be subdivided into three groups based on their structure and reaction mechanism. The best-characterized NS dehydratases are the 4,6-dehydratases that, together with the 5,6-dehydratases, belong to the NS-short-chain dehydrogenase/reductase superfamily. The other two groups are the less abundant 2,3-dehydratases that belong to the Nudix hydrolase superfamily and 3-dehydratases, which are related to aspartame aminotransferases. 4,6-Dehydratases catalyze the first step in all deoxy sugar biosynthesis pathways, converting nucleoside diphosphate hexoses to nucleoside diphosphate-4-keto-6-deoxy hexoses, which in turn are further deoxygenated by the 2,3- and 3-dehydratases to form dideoxy and trideoxy sugars. In this review, we give an overview of the NS dehydratases focusing on the comparison of their structure and reaction mechanisms, thereby highlighting common features, and investigating differences between closely related members of the same superfamilies.

Two UDP-Glycosyltransferases Catalyze the Biosynthesis of Bitter Flavonoid 7-O-Neohesperidoside through Sequential Glycosylation in Tea Plants

Flavonoid glycosides are typical bitter and astringent tasting compounds that contribute to the taste of tea beverages. However, the genes that contribute to the biosynthesis of bitter compounds (e.g., flavanone 7-O-neohesperidoside) in tea plants have yet to be identified. In this study, we identified 194 UDP-glycosyltransferases (UGTs) from the tea transcriptome database. Among them, two genes, CsUGT75L12 and CsUGT79B28, encoding flavonoid 7-O-glycosyltransferase and 7-O-glucoside(1→2)rhamnosyltransferase, respectively, were identified from Camellia sinensis. In vitro, the purified recombinant enzyme rCsUGT75L12 specifically transports the glucose unit from UDP-glucose to the 7-OH position of the flavonoid to produce the respective 7-O-glucoside. rCsUGT79B28 regiospecifically transfers a rhamnose unit from UDP-rhamnose to the 2″-OH position of flavonoid 7-O-glucosides to produce flavonoid 7-O-di-glycosides. Additionally, the expression profiles of the two CsUGTs were correlated with the accumulation patterns of 7-O-glucoside and 7-O-neohesperidoside, respectively, in tea plants. These results indicated that the two CsUGTs are involved in the biosynthesis of bitter flavonoid 7-O-neohesperidoside through the sequential glucosylation and rhamnosylation of flavonoids in C. sinensis. Taken together, our findings provided not only molecular insights into flavonoid di-glycoside metabolism in tea plants but also crucial molecular markers for controlling the bitterness and astringent taste of tea.

Pectin Modification in Seed Coat Mucilage by In Vivo Expression of Rhamnogalacturonan-I- and Homogalacturonan-Degrading Enzymes

The cell wall is essential for plant survival. Determining the relationship between cell wall structure and function using mutant analysis or overexpressing cell wall-modifying enzymes has been challenging due to the complexity of the cell wall and the appearance of secondary, compensatory effects when individual polymers are modified. In addition, viability of the plants can be severely impacted by wall modification. A useful model system for studying structure-function relationships among extracellular matrix components is the seed coat epidermal cells of Arabidopsis thaliana. These cells synthesize relatively simple, easily accessible, pectin-rich mucilage that is not essential for plant viability. In this study, we expressed enzymes predicted to modify polysaccharide components of mucilage in the apoplast of seed coat epidermal cells and explored their impacts on mucilage. The seed coat epidermal-specific promoter TESTA ABUNDANT2 (TBA2) was used to drive expression of these enzymes to avoid adverse effects in other parts of the plant. Mature transgenic seeds expressing Rhamnogalacturonate lyase A (RglA) or Rhamnogalacturonate lyase B (RglB) that degrade the pectin rhamnogalacturonan-I (RG-I), a major component of mucilage, had greatly reduced mucilage capsules surrounding the seeds and concomitant decreases in the monosaccharides that comprise the RG-I backbone. Degradation of the minor mucilage component homogalacturonan (HG) using the HG-degrading enzymes Pectin lyase A (PLA) or ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE2 (ADPG2) resulted in developing seed coat epidermal cells with disrupted cell-cell adhesion and signs of early cell death. These results demonstrate the feasibility of manipulating the seed coat epidermal cell extracellular matrix using a targeted genetic engineering approach.

Galactoglucomannan structure of Arabidopsis seed-coat mucilage in GDP-mannose synthesis impaired mutants

Cell-wall polysaccharides are synthesized from nucleotide sugars by glycosyltransferases. However, in what way the level of nucleotide sugars affects the structure of the polysaccharides is not entirely clear. guanosine diphosphate (GDP)-mannose (GDP-Man) is one of the major nucleotide sugars in plants and serves as a substrate in the synthesis of mannan polysaccharides. GDP-Man is synthesized from mannose 1-phosphate and GTP by a GDP-Man pyrophosphorylase, VITAMIN C DEFECTIVE1 (VTC1), which is positively regulated by the interacting protein KONJAC1 (KJC1) in Arabidopsis. Since seed-coat mucilage can serve as a model of the plant cell wall, we examined the influence of vtc1 and kjc1 mutations on the synthesis of mucilage galactoglucomannan. Sugar composition analysis showed that mannose content in adherent mucilage of kjc1 and vtc1 mutants was only 42% and 11% of the wild-type, respectively, indicating a drastic decrease of galactoglucomannan. On the other hand, structural analysis based on specific oligosaccharides released by endo-β-1,4-mannanase indicated that galactoglucomannan had a patterned glucomannan backbone consisting of alternating residues of glucose and mannose and the frequency of α-galactosyl branches was also similar to the wild type structure. These results suggest that the structure of mucilage galactoglucomannan is mainly determined by properties of glycosyltransferases rather than the availability of nucleotide sugars.

Integument-Specific Transcriptional Regulation in the Mid-Stage of Flax Seed Development Influences the Release of Mucilage and the Seed Oil Content

Flax (Linum usitatissimum L.) seed oil, which accumulates in the embryo, and mucilage, which is synthesized in the seed coat, are of great economic importance for food, pharmaceutical as well as chemical industries. Theories on the link between oil and mucilage production in seeds consist in the spatio-temporal competition of both compounds for photosynthates during the very early stages of seed development. In this study, we demonstrate a positive relationship between seed oil production and seed coat mucilage extrusion in the agronomic model, flax. Three recombinant inbred lines were selected for low, medium and high mucilage and seed oil contents. Metabolite and transcript profiling (1H NMR and DNA oligo-microarrays) was performed on the seeds during seed development. These analyses showed main changes in the seed coat transcriptome during the mid-phase of seed development (25 Days Post-Anthesis), once the mucilage biosynthesis and modification processes are thought to be finished. These transcriptome changes comprised genes that are putatively involved in mucilage chemical modification and oil synthesis, as well as gibberellic acid (GA) metabolism. The results of this integrative biology approach suggest that transcriptional regulations of seed oil and fatty acid (FA) metabolism could occur in the seed coat during the mid-stage of seed development, once the seed coat carbon supplies have been used for mucilage biosynthesis and mechanochemical properties of the mucilage secretory cells.

In through the out door: Biochemical mechanisms affecting flavonoid glycoside catabolism in plants

Plants are the sole source of flavonoids, a chemical category that includes flavonols. For the most part, flavonols occur as glycosides with numerous postulated biological roles in plants, including photoprotection, modulation of hormone translocation, and sequestration of reactive oxygen species. Flavonol glycosides are often considered as dead-end metabolites because related flavonoids (i.e., anthocyanins) occur in terminal tissues such as flowers and fruit, but recent evidence points to their turnover in planta, including developing photosynthetic tissues. Although microbial degradation pathways for flavonol glycosides of plant origin are well described, plant catabolic pathways are little studied by comparison. This review will address our current understanding of biochemical processes leading to the loss of flavonol glycosides in plants, with a specific emphasis on the evidence for flavonol-specific β-glucosidases. Complete elucidation of these catabolic pathways is dependent on the identification of regiospecific modifying steps, including enzymes associated with the hydrolysis of rhamnosylated flavonols, as well as flavonol peroxidation and their encoding genes. Herein, we highlight challenges for the identification of hypothetical plant α-rhamnosidases and peroxidases involved in flavonol glycoside degradation, and the potential biological role of this catabolism in mitigating oxidative stress in developing and postharvest plant tissues.

Nucleotide-sugar metabolism in plants: the legacy of Luis F. Leloir

This review commemorates the 50th anniversary of the Nobel Prize in Chemistry awarded to Luis F. Leloir 'for his discovery of sugar-nucleotides and their role in the biosynthesis of carbohydrates'. He and his co-workers discovered that activated forms of simple sugars, such as UDP-glucose and UDP-galactose, are essential intermediates in the interconversion of sugars. They elucidated the biosynthetic pathways for sucrose and starch, which are the major end-products of photosynthesis, and for trehalose. Trehalose 6-phosphate, the intermediate of trehalose biosynthesis that they discovered, is now a molecule of great interest due to its function as a sugar signalling metabolite that regulates many aspects of plant metabolism and development. The work of the Leloir group also opened the doors to an understanding of the biosynthesis of cellulose and other structural cell wall polysaccharides (hemicelluloses and pectins), and ascorbic acid (vitamin C). Nucleotide-sugars also serve as sugar donors for a myriad of glycosyltransferases that conjugate sugars to other molecules, including lipids, phytohormones, secondary metabolites, and proteins, thereby modifying their biological activity. In this review, we highlight the diversity of nucleotide-sugars and their functions in plants, in recognition of Leloir's rich and enduring legacy to plant science.

NDP-rhamnose biosynthesis and rhamnosyltransferases: building diverse glycoconjugates in nature

Rhamnose is an important 6-deoxy sugar present in many natural products, glycoproteins, and structural polysaccharides. Whilst predominantly found as the l-enantiomer, instances of d-rhamnose are also found in nature, particularly in the Pseudomonads bacteria. Interestingly, rhamnose is notably absent from humans and other animals, which poses unique opportunities for drug discovery targeted towards rhamnose utilizing enzymes from pathogenic bacteria. Whilst the biosynthesis of nucleotide-activated rhamnose (NDP-rhamnose) is well studied, the study of rhamnosyltransferases that synthesize rhamnose-containing glycoconjugates is the current focus amongst the scientific community. In this review, we describe where rhamnose has been found in nature, as well as what is known about TDP-β-l-rhamnose, UDP-β-l-rhamnose, and GDP-α-d-rhamnose biosynthesis. We then focus on examples of rhamnosyltransferases that have been characterized using both in vivo and in vitro approaches from plants and bacteria, highlighting enzymes where 3D structures have been obtained. The ongoing study of rhamnose and rhamnosyltransferases, in particular in pathogenic organisms, is important to inform future drug discovery projects and vaccine development.

Biosynthesis and Transport of Nucleotide Sugars for Plant Hemicellulose

Hemicellulose is entangled with cellulose through hydrogen bonds and meanwhile acts as a bridge for the deposition of lignin monomer in the secondary wall. Therefore, hemicellulose plays a vital role in the utilization of cell wall biomass. Many advances in hemicellulose research have recently been made, and a large number of genes and their functions have been identified and verified. However, due to the diversity and complexity of hemicellulose, the biosynthesis and regulatory mechanisms are yet unknown. In this review, we summarized the types of plant hemicellulose, hemicellulose-specific nucleotide sugar substrates, key transporters, and biosynthesis pathways. This review will contribute to a better understanding of substrate-level regulation of hemicellulose synthesis.

Rhamnose in plants - from biosynthesis to diverse functions

In plants, the deoxy sugar l-rhamnose is widely present as rhamnose-containing polymers in cell walls and as part of the decoration of various specialized metabolites. Here, we review the current knowledge on the distribution of rhamnose, highlighting the differences between what is known in dicotyledoneuos compared to commelinid monocotyledoneous (grasses) plants. We discuss the biosynthesis and transport of UDP-rhamnose, as well as the transfer of rhamnose from UDP-rhamnose to various primary and specialized metabolites. This is carried out by rhamnosyltransferases, enzymes that can use a large variety of substrates. Some unique characteristics of rhamnose synthases, the multifunctional enzymes responsible for the conversion of UDP-glucose into UDP-rhamnose, are considered, particularly from the perspective of their ability to convert glucose present in flavonoids. Finally, we discuss how little is still known with regards to how plants rescue rhamnose from the many compounds to which it is linked, or how rhamnose is catabolized.

Functional analysis of PpRHM1 and PpRHM2 involved in UDP-l-rhamnose biosynthesis in Prunus persica

UDP-l-rhamnose (UDP-Rha) is an important sugar donor for glycosylation of various cell molecules in plant. Rhamnosides are widely present in different plant tissues and play important biological roles under different developmental or environmental conditions. However, enzymes involved in UDP-Rha biosynthesis and their encoding genes have been identified in few plants, which limits the functional analysis of plant rhamnosides. Here, two UDP-Rha biosynthesis genes, named PpRHM1 (2028 bp) and PpRHM2 (2016 bp), were isolated and characterized from Prunus persica, which is rich sources of flavonol rhamnosides. Both recombinant RHM proteins can catalyze the transformation from UDP-d-glucose (UDP-Glc) to UDP-Rha, which was confirmed by LC-MS and formation of flavonol rhamnosides. Biochemical analysis showed that both recombinant RHM proteins preferred alkaline conditions in pH range of 8.0-9.0 and had optimal reaction temperature between 25 and 30 °C. PpRHM1 showed the better UDP-Glc substrate affinity with K_m of 360.01 μM. Gene expression analysis showed different transcript levels of both RHMs in all plant tissues tested, indicating the involvement of rhamnosides in various tissues in plant. Such results provide better understanding of UDP-Rha biosynthesis in fruit tree and may be helpful for further investigation of various rhamnose derivatives and their biological functions.

The Dipterose of Black Soldier Fly (Hermetia illucens) Induces Innate Immune Response through Toll-Like Receptor Pathway in Mouse Macrophage RAW264.7 Cells

In our study, a novel bioactive polysaccharide was identified in the larvae of the black soldier fly (BSF) (Hermetia illucens) as a molecule that activates the mammalian innate immune response. We attempted to isolate this molecule, which was named dipterose-BSF, by gel-filtration and anion-exchange chromatography, followed by nitric oxide (NO) production in mouse RAW264.7 macrophage cells as a marker of immunomodulatory activity. Dipterose-BSF had an average molecular weight of 1.47 × 10⁵ and consisted of ten monosaccharides. Furthermore, in vitro assays demonstrated that dipterose-BSF enhanced the expression of proinflammatory cytokines and interferon β (IFNβ) in RAW264.7 cells. The inhibition of Toll-like receptor 2 (TLR2) and 4 (TLR4) significantly attenuated NO production by dipterose-BSF, indicating that dipterose-BSF stimulates the induction of various cytokines in macrophages via the TLR signaling pathway. This observation was analogous with the activation of nuclear factor kappa B in RAW264.7 cells after exposure to dipterose-BSF. Our results suggest that dipterose-BSF has immunomodulatory potential through activating the host innate immune system, which allows it to be a novel immunomodulator for implementation as a functional food supplement in poultry, livestock, and farmed fish.

Production of isoorientin and isovitexin from luteolin and apigenin using coupled catalysis of glycosyltransferase and sucrose synthase

Isoorientin and isovitexin, kinds of flavone C-glycosides, exhibit a number of biological properties. In this work, The C-glucosyltransferase (Gt6CGT) gene from Gentiana triflora was cloned and expressed in Escherichia coli BL21(DE3). The optimal activity of Gt6CGT was at pH 7.5 and 50° C. The enzyme was stable over pH range of 6.5-9.0, and had a 1-h half-life at 50° C. The V_max for luteolin and apigenin was 21.1 nmol/min/mg and 31.7 nmol/min/mg, while the K_m was 0.21 mM and 0.22 mM, respectively. Then, we developed an environmentally safe and efficient method for isoorientin and isovitexin production using the coupled catalysis of Gt6CGT and Glycine max sucrose synthase (GmSUS). By optimizing coupled reaction conditions, the titer of isoorientin and isovitexin reached 3820 mg/L with a corresponding molar conversion of 94.7% and 3772 mg/L with a corresponding molar conversion of 97.1%, respectively. The maximum number of UDP-glucose regeneration cycles (RC_max) reached 28.4 for isoorientin and 29.1 for isovitexin. The coupled catalysis reported herein represents a promising method to meet industrial requirements for large-scale isoorientin and isovitexin production in the future. Graphical Abstract.

Discovery of an RmlC/D fusion protein in the microalga Prymnesium parvum and its implications for NDP-β-l-rhamnose biosynthesis in microalgae

The 6-deoxy sugar l-rhamnose (l-Rha) is found widely in plant and microbial polysaccharides and natural products. The importance of this and related compounds in host-pathogen interactions often means that l-Rha plays an essential role in many organisms. l-Rha is most commonly biosynthesized as the activated sugar nucleotide uridine 5'-diphospho-β-l-rhamnose (UDP-β-l-Rha) or thymidine 5'-diphospho-β-l-rhamnose (TDP-β-l-Rha). Enzymes involved in the biosynthesis of these sugar nucleotides have been studied in some detail in bacteria and plants, but the activated form of l-Rha and the corresponding biosynthetic enzymes have yet to be explored in algae. Here, using sugar-nucleotide profiling in two representative algae, Euglena gracilis and the toxin-producing microalga Prymnesium parvum, we show that levels of UDP- and TDP-activated l-Rha differ significantly between these two algal species. Using bioinformatics and biochemical methods, we identified and characterized a fusion of the RmlC and RmlD proteins, two bacteria-like enzymes involved in TDP-β-l-Rha biosynthesis, from P. parvum Using this new sequence and also others, we explored l-Rha biosynthesis among algae, finding that although most algae contain sequences orthologous to plant-like l-Rha biosynthesis machineries, instances of the RmlC-RmlD fusion protein identified here exist across the Haptophyta and Gymnodiniaceae families of microalgae. On the basis of these findings, we propose potential routes for the evolution of nucleoside diphosphate β-l-Rha (NDP-β-l-Rha) pathways among algae.

The N-glycan structures of the antigenic variants of chlorovirus PBCV-1 major capsid protein help to identify the virus-encoded glycosyltransferases

The chlorovirus Paramecium bursaria chlorella virus 1 (PBCV-1) is a large dsDNA virus that infects the microalga Chlorella variabilis NC64A. Unlike most other viruses, PBCV-1 encodes most, if not all, of the machinery required to glycosylate its major capsid protein (MCP). The structures of the four N-linked glycans from the PBCV-1 MCP consist of nonasaccharides, and similar glycans are not found elsewhere in the three domains of life. Here, we identified the roles of three virus-encoded glycosyltransferases (GTs) that have four distinct GT activities in glycan synthesis. Two of the three GTs were previously annotated as GTs, but the third GT was identified in this study. We determined the GT functions by comparing the WT glycan structures from PBCV-1 with those from a set of PBCV-1 spontaneous GT gene mutants resulting in antigenic variants having truncated glycan structures. According to our working model, the virus gene a064r encodes a GT with three domains: domain 1 has a β-l-rhamnosyltransferase activity, domain 2 has an α-l-rhamnosyltransferase activity, and domain 3 is a methyltransferase that decorates two positions in the terminal α-l-rhamnose (Rha) unit. The a075l gene encodes a β-xylosyltransferase that attaches the distal d-xylose (Xyl) unit to the l-fucose (Fuc) that is part of the conserved N-glycan core region. Last, gene a071r encodes a GT that is involved in the attachment of a semiconserved element, α-d-Rha, to the same l-Fuc in the core region. Our results uncover GT activities that assemble four of the nine residues of the PBCV-1 MCP N-glycans.

TRANSPARENT TESTA 4-mediated flavonoids negatively affect embryonic fatty acid biosynthesis in Arabidopsis

Flavonoids are involved in many physiological processes in plants. TRANSPARENT TESTA 4 (TT4) acts at the first step of flavonoid biosynthesis, and the loss of TT4 function causes a lack of flavonoid. Flavonoid deficiency is reportedly the main cause of increased fatty acid content in pale-coloured oilseeds, but details regarding the relationship between seed flavonoids and fatty acid biosynthesis are elusive. In this work, we applied a genetic strategy combined with biochemical and cytological assays to determine the effect of seed flavonoids on the biosynthesis of fatty acids in Arabidopsis thaliana. We showed that TT4-mediated flavonoids negatively affect embryonic fatty acid biosynthesis. A crossing experiment indicated that seed flavonoid biosynthesis and the impact of this process on fatty acid biosynthesis were controlled in a maternal line-dependent manner. Loss of TT4 function activated glycolysis in seed embryos, thereby enhancing fatty acid biosynthesis, but did not improve seed mucilage production. Moreover, loss of TT4 function reduced PIN-FORMED 4 expression and subsequently increased auxin accumulation in embryos. Pharmacologically and genetically elevated auxin levels enhanced seed fatty acid biosynthesis. These results indicated that flavonoids affect fatty acid biosynthesis by carbon source reallocation via regulation of WRINKLE1 and auxin transport.

GC-MS metabolite profiling for specific detection of dwarf somaclonal variation in banana plants

PREMISE OF THE STUDY

The production of banana (Musa spp.; Musaceae) plants is affected by various types of somaclonal variations (SV), including dwarfism. However, methods for specific detection of SV are still scarce. To overcome this, a metabolite-based method for detection of dwarf variants was evaluated.

METHODS

The gas chromatography-mass spectrometry (GC-MS) metabolite profile of dwarf banana variants was investigated and compared to that of normal-healthy (N) and cucumber mosaic virus (CMV)-infected plants using principal components analysis and partial least squares discriminant analysis (PLS-DA).

RESULTS

Significant differences among the sample groups were observed in 82 metabolites. Rhamnose was exclusively present in dwarf plants but allothreonine and trehalose were present in all but SV samples. Cellobiose was only detected in N plants, while 45 other metabolites, including methyl-glucopyranoside, allopyranose, lactose, phenylalanine, and l-lysine were detected in all but CMV-infected samples. PLS-DA models were able to detect SV, CMV, and N plants with 100% accuracy and specificity.

DISCUSSION

The GC-MS metabolite profile can be used for the rapid, specific detection of SV at early plant production stages. This is the first metabolite-based characterization and detection of somaclonal variation in plants.

Identification and characterization of a rhamnosyltransferase involved in rutin biosynthesis in Fagopyrum esculentum (common buckwheat)

Rutin, a 3-rutinosyl quercetin, is a representative flavonoid distributed in many plant species, and is highlighted for its therapeutic potential. In this study, we purified uridine diphosphate-rhamnose: quercetin 3-O-glucoside 6″-O-rhamnosyltransferase and isolated the corresponding cDNA (FeF3G6″RhaT) from seedlings of common buckwheat (Fagopyrum esculentum). The recombinant FeF3G6″RhaT enzyme expressed in Escherichia coli exhibited 6″-O-rhamnosylation activity against flavonol 3-O-glucoside and flavonol 3-O-galactoside as substrates, but showed only faint activity against flavonoid 7-O-glucosides. Tobacco cells expressing FeF3G6″RhaT converted the administered quercetin into rutin, suggesting that FeF3G6″RhaT can function as a rhamnosyltransferase in planta. Quantitative PCR analysis on several organs of common buckwheat revealed that accumulation of FeF3G6″RhaT began during the early developmental stages of rutin-accumulating organs, such as flowers, leaves, and cotyledons. These results suggest that FeF3G6″RhaT is involved in rutin biosynthesis in common buckwheat.

Involvement of Three CsRHM Genes from Camellia sinensis in UDP-Rhamnose Biosynthesis

UDP-Rhamnose synthase (RHM), the branch-point enzyme controlling the nucleotide sugar interconversion pathway, converts UDP-d-glucose into UDP-rhamnose. As a rhamnose residue donor, UDP-l-rhamnose is essential for the biosynthesis of pectic polysaccharides and secondary metabolites in plants. In this study, three CsRHM genes from tea plants ( Camellia sinensis) were cloned and characterized. Enzyme assays showed that three recombinant proteins displayed RHM activity and were involved in the biosynthesis of UDP-rhamnose in vitro. The transcript profiles, metabolite profiles, and mucilage location suggest that the three CsRHM genes likely contribute to UDP-rhamnose biosynthesis and may be involved in primary wall formation in C. sinensis. These analyses of CsRHM genes and metabolite profiles provide a comprehensive understanding of secondary metabolite biosynthesis and regulation in tea plants. Moreover, our results can be applied for the synthesis of the secondary metabolite rhamnoside in future studies.

Flavonol rhamnosylation indirectly modifies the cell wall defects of RHAMNOSE BIOSYNTHESIS1 mutants by altering rhamnose flux

Rhamnose is required in Arabidopsis thaliana for synthesizing pectic polysaccharides and glycosylating flavonols. RHAMNOSE BIOSYNTHESIS1 (RHM1) encodes a UDP-l-rhamnose synthase, and rhm1 mutants exhibit many developmental defects, including short root hairs, hyponastic cotyledons, and left-handed helically twisted petals and roots. It has been proposed that the hyponastic cotyledons observed in rhm1 mutants are a consequence of abnormal flavonol glycosylation, while the root hair defect is flavonol-independent. We have recently shown that the helical twisting of rhm1 petals results from decreased levels of rhamnose-containing cell wall polymers. In this study, we found that flavonols indirectly modify the rhm1 helical petal phenotype by altering rhamnose flux to the cell wall. Given this finding, we further investigated the relationship between flavonols and the cell wall in rhm1 cotyledons. We show that decreased flavonol rhamnosylation is not responsible for the cotyledon phenotype of rhm1 mutants. Instead, blocking flavonol synthesis or rhamnosylation can suppress rhm1 defects by diverting UDP-l-rhamnose to the synthesis of cell wall polysaccharides. Therefore, rhamnose is required in the cell wall for normal expansion of cotyledon epidermal cells. Our findings suggest a broad role for rhamnose-containing cell wall polysaccharides in the morphogenesis of epidermal cells.

Site-specific phosphorylation of TRANSPARENT TESTA GLABRA1 mediates carbon partitioning in Arabidopsis seeds

Seed development is dependent on nutrients, such as a source of carbon, supplied by the parent plant. It remains largely unknown how these nutrients are distributed to zygotic and maternal tissues to coordinate storage of reserve compounds and development of protective tissues like seed coat. Here we show that phosphorylation of TRANSPARENT TESTA GLABRA1 (TTG1) is regulated by SHAGGY-like kinases 11/12 (SK11/12) and that this mediates carbon flow to fatty acid synthesis and seed coat traits in Arabidopsis seeds. SK11/12 phosphorylate TTG1 at serine 215, thus preventing TTG1 interaction with TRANSPARENT TESTA2. This compromises recruitment of TTG1 to the GLABRA2 locus and downregulates GLABRA2 expression, which enhances biosynthesis of fatty acids in the embryo, but reduces production of mucilage and flavonoid pigments in the seed coat. Therefore, site-specific phosphorylation of TTG1 by SK11/SK12 regulates carbon partitioning between zygotic and maternal sinks in seeds.

Carbon Supply and the Regulation of Cell Wall Synthesis

All plant cells are surrounded by a cell wall that determines the directionality of cell growth and protects the cell against its environment. Plant cell walls are comprised primarily of polysaccharides and represent the largest sink for photosynthetically fixed carbon, both for individual plants and in the terrestrial biosphere as a whole. Cell wall synthesis is a highly sophisticated process, involving multiple enzymes and metabolic intermediates, intracellular trafficking of proteins and cell wall precursors, assembly of cell wall polymers into the extracellular matrix, remodeling of polymers and their interactions, and recycling of cell wall sugars. In this review we discuss how newly fixed carbon, in the form of UDP-glucose and other nucleotide sugars, contributes to the synthesis of cell wall polysaccharides, and how cell wall synthesis is influenced by the carbon status of the plant, with a focus on the model species Arabidopsis (Arabidopsis thaliana).

A regiospecific rhamnosyltransferase from Epimedium pseudowushanense catalyzes the 3-O-rhamnosylation of prenylflavonols

A novel rhamnosyltransferase and a UDP-rhamnose synthase were characterized and applied to the biosynthesis of bioactive prenylflavonol rhamnosides.

Identification of a seed coat-specific promoter fragment from the Arabidopsis MUCILAGE-MODIFIED4 gene

The Arabidopsis seed coat-specific promoter fragment described is an important tool for basic and applied research in Brassicaceae species. During differentiation, the epidermal cells of the Arabidopsis seed coat produce and secrete large quantities of mucilage. On hydration of mature seeds, this mucilage becomes easily accessible as it is extruded to form a tightly attached halo at the seed surface. Mucilage is composed mainly of pectin, and also contains the key cell wall components cellulose, hemicellulose, and proteins, making it a valuable model for studying numerous aspects of cell wall biology. Seed coat-specific promoters are an important tool that can be used to assess the effects of expressing biosynthetic enzymes and diverse cell wall-modifying proteins on mucilage structure and function. Additionally, they can be used for production of easily accessible recombinant proteins of commercial interest. The MUCILAGE-MODIFIED4 (MUM4) gene is expressed in a wide variety of plant tissues and is strongly up-regulated in the seed coat during mucilage synthesis, implying the presence of a seed coat-specific region in its promoter. Promoter deletion analysis facilitated isolation of a 308 base pair sequence (MUM4 _0.3Pro ) that directs reporter gene expression in the seed coat cells of both Arabidopsis and Camelina sativa, and is regulated by the same transcription factor cascade as endogenous MUM4. Therefore, MUM4 _0.3Pro is a promoter fragment that serves as a new tool for seed coat biology research.

Engineering and Dissecting the Glycosylation Pathway of a Streptococcal Serine-rich Repeat Adhesin

Serine-rich repeat glycoproteins (SRRPs) are conserved in Gram-positive bacteria. They are crucial for modulating biofilm formation and bacterial-host interactions. Glycosylation of SRRPs plays a pivotal role in the process; thus understanding the glycosyltransferases involved is key to identifying new therapeutic drug targets. The glycosylation of Fap1, an SRRP of Streptococcus parasanguinis, is mediated by a gene cluster consisting of six genes: gtf1, gtf2, gly, gtf3, dGT1, and galT2. Mature Fap1 glycan possesses the sequence of Rha1–3Glc1-(Glc1–3GlcNAc1)-2,6-Glc1–6GlcNAc. Gtf12, Gtf3, and dGT1 are responsible for the first four steps of the Fap1 glycosylation, catalyzing the transfer of GlcNAc, Glc, Glc, and GlcNAc residues to the protein backbone sequentially. The role of GalT2 and Gly in the Fap1 glycosylation is unknown. In the present study, we synthesized the fully modified Fap1 glycan in Escherichia coli by incorporating all six genes from the cluster. This study represents the first reconstitution of an exogenous stepwise O-glycosylation synthetic pathway in E. coli. In addition, we have determined that GalT2 mediates the fifth step of the Fap1 glycosylation by adding a rhamnose residue, and Gly mediates the final glycosylation step by transferring glucosyl residues. Furthermore, inactivation of each glycosyltransferase gene resulted in differentially impaired biofilms of S. parasanguinis, demonstrating the importance of Fap1 glycosylation in the biofilm formation. The Fap1 glycosylation system offers an excellent model to engineer glycans using different permutations of glycosyltransferases and to investigate biosynthetic pathways of SRRPs because SRRP genetic loci are highly conserved.