Structural characterization and cytotoxic properties of an apiose-rich pectic polysaccharide obtained from the cell wall of the marine phanerogam Zostera marina

Seagrass genomes reveal ancient polyploidy and adaptations to the marine environment

We present chromosome-level genome assemblies from representative species of three independently evolved seagrass lineages: Posidonia oceanica, Cymodocea nodosa, Thalassia testudinum and Zostera marina. We also include a draft genome of Potamogeton acutifolius, belonging to a freshwater sister lineage to Zosteraceae. All seagrass species share an ancient whole-genome triplication, while additional whole-genome duplications were uncovered for C. nodosa, Z. marina and P. acutifolius. Comparative analysis of selected gene families suggests that the transition from submerged-freshwater to submerged-marine environments mainly involved fine-tuning of multiple processes (such as osmoregulation, salinity, light capture, carbon acquisition and temperature) that all had to happen in parallel, probably explaining why adaptation to a marine lifestyle has been exceedingly rare. Major gene losses related to stomata, volatiles, defence and lignification are probably a consequence of the return to the sea rather than the cause of it. These new genomes will accelerate functional studies and solutions, as continuing losses of the 'savannahs of the sea' are of major concern in times of climate change and loss of biodiversity.

The apiosyltransferase celery UGT94AX1 catalyzes the biosynthesis of the flavone glycoside apiin

Apiose is a unique branched-chain pentose found in plant glycosides and a key component of the cell wall polysaccharide pectin and other specialized metabolites. More than 1,200 plant-specialized metabolites contain apiose residues, represented by apiin, a distinctive flavone glycoside found in celery (Apium graveolens) and parsley (Petroselinum crispum) in the family Apiaceae. The physiological functions of apiin remain obscure, partly due to our lack of knowledge on apiosyltransferase during apiin biosynthesis. Here, we identified UGT94AX1 as an A. graveolens apiosyltransferase (AgApiT) responsible for catalyzing the last sugar modification step in apiin biosynthesis. AgApiT showed strict substrate specificity for the sugar donor, UDP-apiose, and moderate specificity for acceptor substrates, thereby producing various apiose-containing flavone glycosides in celery. Homology modeling of AgApiT with UDP-apiose, followed by site-directed mutagenesis experiments, identified unique Ile139, Phe140, and Leu356 residues in AgApiT, which are seemingly crucial for the recognition of UDP-apiose in the sugar donor pocket. Sequence comparison and molecular phylogenetic analysis of celery glycosyltransferases suggested that AgApiT is the sole apiosyltransferase-encoding gene in the celery genome. Identification of this plant apiosyltransferase gene will enhance our understanding of the physioecological functions of apiose and apiose-containing compounds.

Seasonal Variation in Cell Wall Composition and Carbohydrate Metabolism in the Seagrass Posidonia oceanica Growing at Different Depths

Posidonia oceanica is a common seagrass in the Mediterranean Sea that is able to sequester large amounts of carbon. The carbon assimilated during photosynthesis can be partitioned into non-structural sugars and cell-wall polymers. In this study, we investigated the distribution of carbon in starch, soluble carbohydrates and cell-wall polymers in leaves and rhizomes of P. oceanica. Analyses were performed during summer and winter in meadows located south of the Frioul archipelago near Marseille, France. The leaves and rhizomes were isolated from plants collected in shallow (2 m) and deep water (26 m). Our results showed that P. oceanica stores more carbon as starch, sucrose and cellulose in summer and that this is more pronounced in rhizomes from deep-water plants. In winter, the reduction in photoassimilates was correlated with a lower cellulose content, compensated with a greater lignin content, except in rhizomes from deep-water plants. The syringyl-to-guaiacyl (S/G) ratio in the lignin was higher in leaves than in rhizomes and decreased in rhizomes in winter, indicating a change in the distribution or structure of the lignin. These combined data show that deep-water plants store more carbon during summer, while in winter the shallow- and deep-water plants displayed a different cell wall composition reflecting their environment.

Pectins from the sea grass Enhalus acoroides (L.f.) Royle: Structure, biological activity and ability to form nanoparticles

Two pectins from the seagrass Enhalus acoroides (L.f.) Royle were isolated for the first time. Their structures and biological activities were investigated. NMR spectroscopy showed one of them to consist exclusively from the repeating →4-α-d-GalpUA→ residue (Ea1), while the other had a much more complex structure that also included 1→3-linked α-d-GalpUA residues, 1→4-linked β-apiose residues and small amounts of galactose and rhamnose (Ea2). The pectin Ea1 showed noticeable dose-dependent immunostimulatory activity, the Ea2 fraction was less effective. Both pectins were used to create pectin-chitosan nanoparticles for the first time, and the influence of pectin/chitosan mass ratio on their size and zeta potential was investigated. Ea1 particles were slightly smaller than Ea2 particles (77 ± 16 nm vs 101 ± 12 nm) and less negatively charged (-23 mV vs -39 mV). Assessment of their thermodynamic parameters showed that only the second pectin could form nanoparticles at room temperature.

Pectin from Three Vietnamese Seagrasses: Isolation, Characterization, and Antioxidant Activity

This study focused on the isolation and structural characterization of pectin from three distinct species of Vietnamese seagrass including Enhalus acoroides, Thalassia hemprichii, and Halophila ovalis. The pectin yield obtained from Enhalus acoroides was the highest, corresponding to 24.15%, followed by those from Thalassia hemprichii (20.04%) and Halophila ovalis (19.14%). The physicochemical properties of pectin including total carbohydrate content, anhydrouronic acid (AUA) content, equivalent weight (EW), methoxyl content (MeO), and degree of esterification (DE) were determined using various analysis techniques. The pectin obtained from all three species were found to be low-methyl-esterified pectin, with the MeO content and DE for E. acoroides, T. hemprichii, and H. ovalis being 6.15% and 27.18%, 3.26% and 43.31%, and 4.65% and 33.25%, respectively. The average molecular weight (MW) of pectin was analyzed by size-exclusion chromatography. Pectin from T. hemprichii had the highest MW of 173.01 kDa, followed by pectin from E. acoroides, with a MW of 127.32 kDa, and that from H. ovalis, with a MW of 56.06 kDa. Furthermore, the pectins from all three seagrass species exhibited high antioxidant activity and might be promising as antioxidants.

Profiling the cell walls of seagrasses from A (Amphibolis) to Z (Zostera)

BACKGROUND

The polyphyletic group of seagrasses shows an evolutionary history from early monocotyledonous land plants to the marine environment. Seagrasses form important coastal ecosystems worldwide and large amounts of seagrass detritus washed on beaches might also be valuable bioeconomical resources. Despite this importance and potential, little is known about adaptation of these angiosperms to the marine environment and their cell walls.

RESULTS

We investigated polysaccharide composition of nine seagrass species from the Mediterranean, Red Sea and eastern Indian Ocean. Sequential extraction revealed a similar seagrass cell wall polysaccharide composition to terrestrial angiosperms: arabinogalactans, pectins and different hemicelluloses, especially xylans and/or xyloglucans. However, the pectic fractions were characterized by the monosaccharide apiose, suggesting unusual apiogalacturonans are a common feature of seagrass cell walls. Detailed analyses of four representative species identified differences between organs and species in their constituent monosaccharide composition and lignin content and structure. Rhizomes were richer in glucosyl units compared to leaves and roots. Enhalus had high apiosyl and arabinosyl abundance, while two Australian species of Amphibolis and Posidonia, were characterized by high amounts of xylosyl residues. Interestingly, the latter two species contained appreciable amounts of lignin, especially in roots and rhizomes whereas Zostera and Enhalus were lignin-free. Lignin structure in Amphibolis was characterized by a higher syringyl content compared to that of Posidonia.

CONCLUSIONS

Our investigations give a first comprehensive overview on cell wall composition across seagrass families, which will help understanding adaptation to a marine environment in the evolutionary context and evaluating the potential of seagrass in biorefinery incentives.

The Cell Wall of Seagrasses: Fascinating, Peculiar and a Blank Canvas for Future Research

Seegrasses are a polyphyletic group of angiosperm plants, which evolved from early monocotyledonous land plants and returned to the marine environment around 140 million years ago. Today, seagrasses comprise the five families Zosteraceae, Hydrocharitaceae, Posidoniaceae, Cymodoceaceae, and Ruppiaceae and form important coastal ecosystems worldwide. Despite of this ecological importance, the existing literature on adaption of these angiosperms to the marine environment and especially their cell wall composition is limited up to now. A unique feature described for some seagrasses is the occurrence of polyanionic, low-methylated pectins mainly composed of galacturonic acid and apiose (apiogalacturonans). Furthermore, sulfated galactans have been detected in some species. Recently, arabinogalactan-proteins (AGPs), highly glycosylated proteins of the cell wall of land plants, have been isolated for the first time from a seagrass of the baltic sea. Obviously, seagrass cell walls are characterized by new combinations of structural polysaccharide and glycoprotein elements known from macroalgae and angiosperm land plants. In this review, current knowledge on cell walls of seagrasses is summarized and suggestions for future investigations are given.

Arabinogalactan-proteins of Zostera marina L. contain unique glycan structures and provide insight into adaption processes to saline environments

Seagrasses evolved from monocotyledonous land plants that returned to the marine habitat. This transition was accomplished by substantial changes in cell wall composition, revealing habitat-driven adaption to the new environment. Whether arabinogalactan-proteins (AGPs), important signalling molecules of land plants, are present in seagrass cell walls is of evolutionary and plant development interest. AGPs of Zostera marina L. were isolated and structurally characterised by analytical and bioinformatics methods as well as by ELISA with different anti-AGP antibodies. Calcium-binding capacity of AGPs was studied by isothermal titration calorimetry (ITC) and microscopy. Bioinformatic searches of the Z. marina proteome identified 9 classical AGPs and a large number of chimeric AGPs. The glycan structures exhibit unique features, including a high degree of branching and an unusually high content of terminating 4-O-methyl-glucuronic acid (4-OMe GlcA) residues. Although the common backbone structure of land plant AGPs is conserved in Z. marina, the terminating residues are distinct with high amounts of uronic acids. These differences likely result from the glycan-active enzymes (glycosyltransferases and methyltransferases) and are essential for calcium-binding properties. The role of this polyanionic surface is discussed with regard to adaption to the marine environment.

Reaction mechanism of β-apiosidase from Aspergillus aculeatus

Apiosidases are glycosidases relevant for aroma development during fermentation of wines and black tea. Reaction mechanism of apiosidase from Aspergillus aculeatus in commercial glycanase Viscozyme L was studied by ¹H NMR technique. Study of hydrolysis of 4-nitrophenyl β-D-apiofuranoside revealed that this reaction proceeds with inversion of hydroxyl group in the anomeric center, which confirms inverting mechanism of the enzyme and its inability to catalyze transapiosylation in syntheses of apiosides.

Evolution of Cell Wall Polymers in Tip-Growing Land Plant Gametophytes: Composition, Distribution, Functional Aspects and Their Remodeling

During evolution of land plants, the first colonizing species presented leafy-dominant gametophytes, found in non-vascular plants (bryophytes). Today, bryophytes include liverworts, mosses, and hornworts. In the first seedless vascular plants (lycophytes), the sporophytic stage of life started to be predominant. In the seed producing plants, gymnosperms and angiosperms , the gametophytic stage is restricted to reproduction. In mosses and ferns, the haploid spores germinate and form a protonema, which develops into a leafy gametophyte producing rhizoids for anchorage, water and nutrient uptakes. The basal gymnosperms (cycads and Ginkgo) reproduce by zooidogamy. Their pollen grains develop a multi-branched pollen tube that penetrates the nucellus and releases flagellated sperm cells that swim to the egg cell. The pollen grain of other gymnosperms (conifers and gnetophytes) as well as angiosperms germinates and produces a pollen tube that directly delivers the sperm cells to the ovule (siphonogamy). These different gametophytes, which are short or long-lived structures, share a common tip-growing mode of cell expansion. Tip-growth requires a massive cell wall deposition to promote cell elongation, but also a tight spatial and temporal control of the cell wall remodeling in order to modulate the mechanical properties of the cell wall. The growth rate of these cells is very variable depending on the structure and the species, ranging from very slow (protonemata, rhizoids, and some gymnosperm pollen tubes), to a slow to fast-growth in other gymnosperms and angiosperms. In addition, the structural diversity of the female counterparts in angiosperms (dry, semi-dry vs wet stigmas, short vs long, solid vs hollow styles) will impact the speed and efficiency of sperm delivery. As the evolution and diversity of the cell wall polysaccharides accompanied the diversification of cell wall structural proteins and remodeling enzymes, this review focuses on our current knowledge on the biochemistry, the distribution and remodeling of the main cell wall polymers (including cellulose, hemicelluloses, pectins, callose, arabinogalactan-proteins and extensins), during the tip-expansion of gametophytes from bryophytes, pteridophytes (lycophytes and monilophytes), gymnosperms and the monocot and eudicot angiosperms.

Biochemical Reconstruction of a Metabolic Pathway from a Marine Bacterium Reveals Its Mechanism of Pectin Depolymerization

Pectin is a complex uronic acid-containing polysaccharide typically found in plant cell walls, though forms of pectin are also found in marine diatoms and seagrasses. Genetic loci that target pectin have recently been identified in two phyla of marine bacteria. These loci appear to encode a pectin saccharification pathway that is distinct from the canonical pathway typically associated with phytopathogenic terrestrial bacteria. However, very few components of the marine pectin metabolism pathway have been experimentally validated. Here, we biochemically reconstructed the pectin saccharification pathway from a marine Pseudoalteromonas sp. in vitro and show that it results in the production of galacturonate and the key metabolic intermediate 5-keto-4-deoxyuronate (DKI). We demonstrate the sequential de-esterification and depolymerization of pectin into oligosaccharides and the synergistic action of glycoside hydrolases (GHs) to fully degrade these oligosaccharides into monosaccharides. Furthermore, we show that this pathway relies on enzymes belonging to GH family 105 to carry out the equivalent chemistry afforded by an exolytic polysaccharide lyase (PL) and KdgF in the canonical pectin pathway. Finally, we synthesize our findings into a model of marine pectin degradation and compare it with the canonical pathway. Our results underline the shifting view of pectin as a solely terrestrial polysaccharide and highlight the importance of marine pectin as a carbon source for suitably adapted marine heterotrophs. This alternate pathway has the potential to be exploited in the growing field of biofuel production from plant waste.IMPORTANCE Marine polysaccharides, found in the cell walls of seaweeds and other marine macrophytes, represent a vast sink of photosynthetically fixed carbon. As such, their breakdown by marine microbes contributes significantly to global carbon cycling. Pectin is an abundant polysaccharide found in the cell walls of terrestrial plants, but it has recently been reported that some marine bacteria possess the genetic capacity to degrade it. In this study, we biochemically characterized seven key enzymes from a marine bacterium that, together, fully degrade the backbone of pectin into its constituent monosaccharides. Our findings highlight the importance of pectin as a marine carbon source available to bacteria that possess this pathway. The characterized enzymes also have the potential to be utilized in the production of biofuels from plant waste.

Changes in the abundance of cell wall apiogalacturonan and xylogalacturonan and conservation of rhamnogalacturonan II structure during the diversification of the Lemnoideae

The diversification of the Lemnoideae was accompanied by a reduction in the abundance of cell wall apiogalacturonan and an increase in xylogalacturonan whereas rhamnogalacturonan II structure and cross-linking are conserved. The subfamily Lemnoideae is comprised of five genera and 38 species of small, fast-growing aquatic monocots. Lemna minor and Spirodela polyrhiza belong to this subfamily and have primary cell walls that contain large amounts of apiogalacturonan and thus are distinct from the primary walls of most other flowering plants. However, the pectins in the cell walls of other members of the Lemnoideae have not been investigated. Here, we show that apiogalacturonan decreased substantially as the Lemnoideae diversified since Wolffiella and Wolffia walls contain between 63 and 88% less apiose than Spirodela, Landoltia, and Lemna walls. In Wolffia, the most derived genus, xylogalacturonan is far more abundant than apiogalacturonan, whereas in Wolffiella pectic polysaccharides have a high arabinose content, which may arise from arabinan sidechains of RG I. The apiose-containing pectin rhamnogalacturonan II (RG-II) exists in Lemnoideae walls as a borate cross-linked dimer and has a glycosyl sequence similar to RG-II from terrestrial plants. Nevertheless, species-dependent variations in the extent of methyl-etherification of RG-II sidechain A and arabinosylation of sidechain B are discernible. Immunocytochemical studies revealed that pectin methyl-esterification is higher in developing daughter frond walls than in mother frond walls, indicating that methyl-esterification is associated with expanding cells. Our data support the notion that a functional cell wall requires conservation of RG-II structure and cross-linking but can accommodate structural changes in other pectins. The Lemnoideae provide a model system to study the mechanisms by which wall structure and composition has changed in closely related plants with similar growth habits.

Functional Characterization of UDP-apiose Synthases from Bryophytes and Green Algae Provides Insight into the Appearance of Apiose-containing Glycans during Plant Evolution

Apiose is a branched monosaccharide that is present in the cell wall pectic polysaccharides rhamnogalacturonan II and apiogalacturonan and in numerous plant secondary metabolites. These apiose-containing glycans are synthesized using UDP-apiose as the donor. UDP-apiose (UDP-Api) together with UDP-xylose is formed from UDP-glucuronic acid (UDP-GlcA) by UDP-Api synthase (UAS). It was hypothesized that the ability to form Api distinguishes vascular plants from the avascular plants and green algae. UAS from several dicotyledonous plants has been characterized; however, it is not known if avascular plants or green algae produce this enzyme. Here we report the identification and functional characterization of UAS homologs from avascular plants (mosses, liverwort, and hornwort), from streptophyte green algae, and from a monocot (duckweed). The recombinant UAS homologs all form UDP-Api from UDP-glucuronic acid albeit in different amounts. Apiose was detected in aqueous methanolic extracts of these plants. Apiose was detected in duckweed cell walls but not in the walls of the avascular plants and algae. Overexpressing duckweed UAS in the moss Physcomitrella patens led to an increase in the amounts of aqueous methanol-acetonitrile-soluble apiose but did not result in discernible amounts of cell wall-associated apiose. Thus, bryophytes and algae likely lack the glycosyltransferase machinery required to synthesize apiose-containing cell wall glycans. Nevertheless, these plants may have the ability to form apiosylated secondary metabolites. Our data are the first to provide evidence that the ability to form apiose existed prior to the appearance of rhamnogalacturonan II and apiogalacturonan and provide new insights into the evolution of apiose-containing glycans.

Secondary metabolites of seagrasses (Alismatales and Potamogetonales; Alismatidae): Chemical diversity, bioactivity, and ecological function

Seagrasses are the only higher plants living in fully marine environments; they play a significant role in coastal ecosystems. Seagrasses inhabit the coastal shelves of all continents except Antarctica and can grow in depths of up to 90 m. Because of their eminent ecological importance, innumerous studies have been dedicated to seagrasses and their ecology. However, the phytochemistry has not been equally well investigated yet and many of the existing studies in chemical ecology are only investigating the chemistry at the level of compound classes, e.g. phenolics, and not at the level of chemically defined metabolites. In the present review, the existing literature on secondary metabolites of seagrasses, their known source seagrasses, their bioactivity, and ecological function are compiled and critically assessed. Moreover, research gaps are highlighted and avenues for future research are discussed. Currently, a total of 154 chemically defined natural products have been reported from the about 70 seagrass species known worldwide. Compounds reported include simple phenols derivatives (four compounds), phenylmethane derivatives (14 compounds), phenylethane derivatives (four compounds), phenylpropane derivatives including their esters and dimers (20 compounds), chalkones (four compounds), flavonoids including catechins (57 compounds), phenylheptanoids (four compounds), one monoterpene derivative, one sesquiterpene, diterpenoids (13 compounds), steroids (31 compounds), and one alkaloid. Most of the existing bioactivity studies of seagrass metabolites and extracts have been directed to potential cytotoxic, antimicrobial, or antimacrofouling activity. Antimicrobial studies have been performed towards panels of both human pathogens and ecologically relevant pathogens. In the antimacrofouling studies, investigations of the potential of zosteric acid from the genus Zostera are the most numerous and have yielded so far the most interesting results. Studies on the chemical ecology of seagrasses often have been focused on variation in phenolic compounds and include but are not limited to studies on variation due to abiotic factors, seasonal variation, variation in response to grazing by fish or sea urchins, or following microbial attack.

Extraction, Isolation, Structural Characterization and Anti-Tumor Properties of an Apigalacturonan-Rich Polysaccharide from the Sea Grass Zostera caespitosa Miki

An apigalacturonan (AGA)-rich polysaccharide, ZCMP, was isolated from the sea grass Zostera caespitosa Miki. The depolymerized fragments derived from ZCMP were obtained by either acidic degradation or pectinase degradation, and their structures were characterized by electrospray ionization collision-induced-dissociation mass spectrometry (ESI-CID-MS2) and nuclear magnetic resonance (NMR) spectroscopy. The average molecular weight of ZCMP was 77.2 kD and it consisted of galacturonic acid (GalA), apiosefuranose (Api), galactose (Gal), rhamnose (Rha), arabinose (Ara), xylose (Xyl), and mannose (Man), at a molar ratio of 51.4꞉15.5꞉6.0꞉11.8꞉4.2꞉4.4꞉4.2. There were two regions of AGA (70%) and rhamnogalacturonan-I (RG-Ι, 30%) in ZCMP. AGA was composed of an α-1,4-D-galactopyranosyluronan backbone mainly substituted at the O-3 position by single Api residues. RG-Ι possessed a backbone of repeating disaccharide units of →4GalAα1,2Rhaα1→, with a few α-L-arabinose and β-D-galactose residues as side chains. The anti-angiogenesis assay showed that ZCMP inhibited the migratory activity of human umbilical vein endothelial cell (HUVECs), with no influence on endothelial cells growth. ZCMP also promoted macrophage phagocytosis. These findings of the present study demonstrated the potential anti-tumor activity of ZCMP through anti-angiogenic and immunoregulatory pathways.

Polypotency of the immunomodulatory effect of pectins

Pectins are the major component of plant cell walls, and they display diverse biological activities including immunomodulation. The pectin macromolecule contains fragments of linear and branched regions of polysaccharides such as homogalacturonan, rhamnogalacturonan-I, xylogalacturonan, and apiogalacturonan. These structural features determine the effect of pectins on the immune system. The backbones of pectic macromolecules have immunosuppressive activity. Pectins containing greater than 80% galacturonic acid residues were found to decrease macrophage activity and inhibit the delayed-type hypersensitivity reaction. Branched galacturonan fragments result in a biphasic immunomodulatory action. The branched region of pectins mediates both increased phagocytosis and antibody production. The fine structure of the galactan, arabinan, and apiogalacturonan side chains determines the stimulating interaction between pectin and immune cells. This review summarizes data regarding the relationship between the structure and immunomodulatory activity of pectins isolated from the plants of the European north of Russia and elucidates the concept of polypotency of pectins in native plant cell walls to both stimulate and suppress the immune response. The possible mechanisms of the immunostimulatory and anti-inflammatory effects of pectins are also discussed.

Cerium binding activity of pectins isolated from the seagrasses Zostera marina and Phyllospadix iwatensis

Cerium binding activity of three different water soluble pectin compounds of different origin was studied in a batch sorption system. The Langmuir, Freundlich and BET sorption models were adopted to describe the binding reactions between metal ions and pectin molecules. The Langmuir model provided the best fit. Within the pH range from 4.0 to 6.0, the largest amount of the cerium ions was bound by pectin isolated from the seagrass Phylospadix iwatensis in comparison to pectin extracted from the seagrass Zostera marina and pectin obtained from citrus peel (commercial grade). The Langmuir constants were also highest for the pectin samples isolated from the seagrass P. iwatensis. The results obtained from this study suggest that pectin is a prospective source for the development of radioisotope-removing pharmaceuticals.

The Synthesis and Origin of the Pectic Polysaccharide Rhamnogalacturonan II - Insights from Nucleotide Sugar Formation and Diversity

There is compelling evidence showing that the structurally complex pectic polysaccharide rhamnogalacturonan II (RG-II) exists in the primary cell wall as a borate cross-linked dimer and that this dimer is required for the assembly of a functional wall and for normal plant growth and development. The results of several studies have also established that RG-II structure and cross-linking is conserved in vascular plants and that RG-II likely appeared early in the evolution of land plants. Two features that distinguish RG-II from other plant polysaccharides are that RG-II is composed of 13 different glycoses linked to each other by up to 22 different glycosidic linkages and that RG-II is the only polysaccharide known to contain both apiose and aceric acid. Thus, one key event in land plant evolution was the emergence of genes encoding nucleotide sugar biosynthetic enzymes that generate the activated forms of apiose and aceric acid required for RG-II synthesis. Many of the genes involved in the generation of the nucleotide sugars used for RG-II synthesis have been functionally characterized. By contrast, only one glycosyltransferase involved in the assembly of RG-II has been identified. Here we provide an overview of the formation of the activated sugars required for RG-II synthesis and point to the possible cellular and metabolic processes that could be involved in assembling and controlling the formation of a borate cross-linked RG-II molecule. We discuss how nucleotide sugar synthesis is compartmentalized and how this may control the flux of precursors to facilitate and regulate the formation of RG-II.

Plant nucleotide sugar formation, interconversion, and salvage by sugar recycling

Nucleotide sugars are the universal sugar donors for the formation of polysaccharides, glycoproteins, proteoglycans, glycolipids, and glycosylated secondary metabolites. At least 100 genes encode proteins involved in the formation of nucleotide sugars. These nucleotide sugars are formed using the carbohydrate derived from photosynthesis, the sugar generated by hydrolyzing translocated sucrose, the sugars released from storage carbohydrates, the salvage of sugars from glycoproteins and glycolipids, the recycling of sugars released during primary and secondary cell wall restructuring, and the sugar generated during plant-microbe interactions. Here we emphasize the importance of the salvage of sugars released from glycans for the formation of nucleotide sugars. We also outline how recent studies combining biochemical, genetic, molecular and cellular approaches have led to an increased appreciation of the role nucleotide sugars in all aspects of plant growth and development. Nevertheless, our understanding of these pathways at the single cell level is far from complete.