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

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.

Rhamnose synthase activity is required for pathogenicity of the vascular wilt fungus Verticillium dahliae

The initial interaction of a pathogenic fungus with its host is complex and involves numerous metabolic pathways and regulatory proteins. Considerable attention has been devoted to proteins that play a crucial role in these interactions, with an emphasis on so-called effector molecules that are secreted by the invading microbe to establish the symbiosis. However, the contribution of other types of molecules, such as glycans, is less well appreciated. Here, we present a random genetic screen that enabled us to identify 58 novel candidate genes that are involved in the pathogenic potential of the fungal pathogen Verticillium dahliae, which causes vascular wilt diseases in over 200 dicotyledonous plant species, including economically important crops. One of the candidate genes that was identified concerns a putative biosynthetic gene involved in nucleotide sugar precursor formation, as it encodes a putative nucleotide-rhamnose synthase/epimerase-reductase (NRS/ER). This enzyme has homology to bacterial enzymes involved in the biosynthesis of the nucleotide sugar deoxy-thymidine diphosphate (dTDP)-rhamnose, a precursor of L-rhamnose, which has been shown to be required for virulence in several human pathogenic bacteria. Rhamnose is known to be a minor cell wall glycan in fungi and has therefore not been suspected as a crucial molecule in fungal-host interactions. Nevertheless, our study shows that deletion of the VdNRS/ER gene from the V. dahliae genome results in complete loss of pathogenicity on tomato and Nicotiana benthamiana plants, whereas vegetative growth and sporulation are not affected. We demonstrate that VdNRS/ER is a functional enzyme in the biosynthesis of uridine diphosphate (UDP)-rhamnose, and further analysis has revealed that VdNRS/ER deletion strains are impaired in the colonization of tomato roots. Collectively, our results demonstrate that rhamnose, although only a minor cell wall component, is essential for the pathogenicity of V. dahliae.

TRANSPARENT TESTA GLABRA 1 ubiquitously regulates plant growth and development from Arabidopsis to foxtail millet (Setaria italica)

TRANSPARENT TESTA GLABRA 1 of Arabidopsis thaliana (AtTTG1) is a WD40 repeat transcription factor that plays multiple roles in plant growth and development, particularly in seed metabolite production. In the present study, to determine whether SiTTG1 of the phylogenetically distant monocot foxtail millet (Setaria italica) has similar functions, we used transgenic Arabidopsis and Nicotiana systems to explore its activities. We found that SiTTG1 functions as a transcription factor. Overexpression of the SiTTG1 gene rescued many of the mutant phenotypes in Arabidopsis ttg1-13 plants. Additionally, SiTTG1 overexpression fully corrected the reduced expression of mucilage biosynthetic genes, and the induced expression of genes involved in accumulation of seed fatty acids and storage proteins in developing seeds of ttg1-13 plants. Ectopic expression of SiTTG1 restored the sensitivity of the ttg1-13 mutant to salinity and high glucose stresses during germination and seedling establishment, and restored altered expression levels of some stress-responsive genes in ttg1-13 seedlings to the wild type level under salinity and glucose stresses. Our results provide information that will be valuable for understanding the function of TTG1 from monocot to dicot species and identifying a promising target for genetic manipulation of foxtail millet to improve the amount of seed metabolites.

Functional characterization of UDP-rhamnose-dependent rhamnosyltransferase involved in anthocyanin modification, a key enzyme determining blue coloration in Lobelia erinus

Because structural modifications of flavonoids are closely related to their properties, such as stability, solubility, flavor and coloration, characterizing the enzymes that catalyze the modification reactions can be useful for engineering agriculturally beneficial traits of flavonoids. In this work, we examined the enzymes involved in the modification pathway of highly glycosylated and acylated anthocyanins that accumulate in Lobelia erinus. Cultivar Aqua Blue (AB) of L. erinus is blue-flowered and accumulates delphinidin 3-O-p-coumaroylrutinoside-5-O-malonylglucoside-3'5'-O-dihydroxycinnamoylglucoside (lobelinins) in its petals. Cultivar Aqua Lavender (AL) is mauve-flowered, and LC-MS analyses showed that AL accumulated delphinidin 3-O-glucoside (Dp3G), which was not further modified toward lobelinins. A crude protein assay showed that modification processes of lobelinin were carried out in a specific order, and there was no difference between AB and AL in modification reactions after rhamnosylation of Dp3G, indicating that the lack of highly modified anthocyanins in AL resulted from a single mutation of rhamnosyltransferase catalyzing the rhamnosylation of Dp3G. We cloned rhamnosyltransferase genes (RTs) from AB and confirmed their UDP-rhamnose-dependent rhamnosyltransferase activities on Dp3G using recombinant proteins. In contrast, the RT gene in AL had a 5-bp nucleotide deletion, resulting in a truncated polypeptide without the plant secondary product glycosyltransferase box. In a complementation test, AL that was transformed with the RT gene from AB produced blue flowers. These results suggest that rhamnosylation is an essential process for lobelinin synthesis, and thus the expression of RT has a great impact on the flower color and is necessary for the blue color of Lobelia flowers.

Functional analyses of OcRhS1 and OcUER1 involved in UDP-L-rhamnose biosynthesis in Ornithogalum caudatum

UDP-L-rhamnose (UDP-Rha) is an important sugar donor for the synthesis of rhamnose-containing compounds in plants. However, only a few enzymes and their encoding genes involved in UDP-Rha biosynthesis are available in plants. Here, two genes encoding rhamnose synthase (RhS) and bi-functional UDP-4-keto-6-deoxy-D-glucose (UDP-4K6DG) 3, 5-epimerase/UDP-4-keto-L-rhamnose (UDP-4KR) 4-keto-reductase (UER) were isolated from Ornithogalum caudatum based on the RNA-Seq data. The OcRhS1 gene has an ORF (open reading frame) of 2019 bp encoding a tri-functional RhS enzyme. In vitro enzymatic assays revealed OcRhS1 can really convert UDP-D-glucose (UDP-Glc) into UDP-Rha via three consecutive reactions. Biochemical evidences indicated that the recombinant OcRhS1 was active in the pH range of 5-11 and over the temperature range of 0-60 °C. The K_m value of OcRhS1 for UDP-Glc was determined to be 1.52 × 10^-4 M. OcRhS1 is a multi-domain protein with two sets of cofactor-binding motifs. The cofactors dependent properties of OcRhS1 were thus characterized in this research. Moreover, the N-terminal portion of OcRhS1 (OcRhS1-N) was observed to metabolize UDP-Glc to form intermediate UDP-4K6DG. OcUER1 contains an ORF of 906 bp encoding a polypeptide of 301 aa. OcUER1 shared high similarity with the carboxy-terminal domain of OcRhS1 (OcRhS1-C), suggesting its intrinsic ability of converting UDP-4K6DG into UDP-Rha. It was thus reasonably inferred that UDP-Glc could be bio-transformed into UDP-Rha under the collaborating action of OcRhS1-N and OcUER1. The subsequently biochemical assay verified this notion. Importantly, expression profiles of OcRhS1 and OcUER1 revealed their possible involvement in the biosynthesis of rhamnose-containing polysaccharides in O. caudatum.

Metabolic engineering of Saccharomyces cerevisiae for de novo production of dihydrochalcones with known antioxidant, antidiabetic, and sweet tasting properties

Dihydrochalcones are plant secondary metabolites comprising molecules of significant commercial interest as antioxidants, antidiabetics, or sweeteners. To date, their heterologous biosynthesis in microorganisms has been achieved only by precursor feeding or as minor by-products in strains engineered for flavonoid production. Here, the native ScTSC13 was overexpressed in Saccharomyces cerevisiae to increase its side activity in reducing p-coumaroyl-CoA to p-dihydrocoumaroyl-CoA. De novo production of phloretin, the first committed dihydrochalcone, was achieved by co-expression of additional relevant pathway enzymes. Naringenin, a major by-product of the initial pathway, was practically eliminated by using a chalcone synthase from barley with unexpected substrate specificity. By further extension of the pathway from phloretin with decorating enzymes with known specificities for dihydrochalcones, and by exploiting substrate flexibility of enzymes involved in flavonoid biosynthesis, de novo production of the antioxidant molecule nothofagin, the antidiabetic molecule phlorizin, the sweet molecule naringin dihydrochalcone, and 3-hydroxyphloretin was achieved.

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De novo biosynthesis of phloretin in S. cerevisiae.

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De novo pathway extended to various dihydrochalcones of commercial interest.

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A barley CHS exhibits very high specificity for phloretin production.

A Novel FC116/BC10 Mutation Distinctively Causes Alteration in the Expression of the Genes for Cell Wall Polymer Synthesis in Rice

We report isolation and characterization of a fragile culm mutant fc116 that displays reduced mechanical strength caused by decreased cellulose content and altered cell wall structure in rice. Map-based cloning revealed that fc116 was a base substitution mutant (G to A) in a putative beta-1,6-N-acetylglucosaminyltransferase (C2GnT) gene (LOC_Os05g07790, allelic to BC10). This mutation resulted in one amino acid missing within a newly-identified protein motif "R, RXG, RA." The FC116/BC10 gene was lowly but ubiquitously expressed in the all tissues examined across the whole life cycle of rice, and slightly down-regulated during secondary growth. This mutant also exhibited a significant increase in the content of hemicelluloses and lignins, as well as the content of pentoses (xylose and arabinose). But the content of hexoses (glucose, mannose, and galactose) was decreased in both cellulosic and non-cellulosic (pectins and hemicelluloses) fractions of the mutant. Transcriptomic analysis indicated that the typical genes in the fc116 mutant were up-regulated corresponding to xylan biosynthesis, as well as lignin biosynthesis including p-hydroxyphenyl (H), syringyl (S), and guaiacyl (G). Our results indicate that FC116 has universal function in regulation of the cell wall polymers in rice.

The inside and outside: topological issues in plant cell wall biosynthesis and the roles of nucleotide sugar transporters

The cell wall is a complex extracellular matrix composed primarily of polysaccharides. Noncellulosic polysaccharides, glycoproteins and proteoglycans are synthesized in the Golgi apparatus by glycosyltransferases (GTs), which use nucleotide sugars as donors to glycosylate nascent glycan and glycoprotein acceptors that are subsequently exported to the extracellular space. Many nucleotide sugars are synthesized in the cytosol, leading to a topological issue because the active sites of most GTs are located in the Golgi lumen. Nucleotide sugar transporters (NSTs) overcome this problem by translocating nucleoside diphosphate sugars from the cytosol into the lumen of the organelle. The structures of the cell wall components synthesized in the Golgi are diverse and complex; therefore, transporter activities are necessary so that the nucleotide sugars can provide substrates for the GTs. In this review, we describe the topology of reactions involved in polysaccharide biosynthesis in the Golgi and focus on the roles of NSTs as well as their impacts on cell wall structure when they are altered.

Characterization of ubiquitin ligase SlATL31 and proteomic analysis of 14-3-3 targets in tomato fruit tissue (Solanum lycopersicum L.)

The 14-3-3 proteins participate in many aspects of plant physiology by interacting with phosphorylated proteins and thereby regulating target protein functions. In Arabidopsis plant, the ubiquitin ligase ATL31 controls 14-3-3 stability via both direct interaction and ubiquitination, and this consequently regulates post-germinative growth in response to carbon and nitrogen nutrient availability. Since 14-3-3 proteins regulate the activities of many key enzymes related to nutrient metabolism, one would anticipate that they should play an essential role not only in vegetative but also in reproductive tissue. Because fruit yield largely depends on carbon and nitrogen availability and their utilization, the function of 14-3-3 proteins was analyzed in tomato fruit tissue. Here, we isolated and characterized an ubiquitin ligase SlATL31 (Solyc03g112340) from tomato and demonstrated that SlATL31 has ubiquitin ligase activity as well as interaction with tomato 14-3-3 proteins, suggesting the possibility that the SlATL31 functions as an ubiquitin ligase for 14-3-3 similarly to its Arabidopsis ortholog. Furthermore, we performed proteomic analysis of 14-3-3 interacting proteins and identified 106 proteins as putative 14-3-3 targets including key enzymes for carbon metabolism and photosynthesis. This 14-3-3 interactome result and available transcriptome profile suggest a considerable yet complex role of 14-3-3 proteins in tomato fruit tissue.

BIOLOGICAL SIGNIFICANCE

Considerable cumulative evidence exists which implies that 14-3-3 proteins are involved in the regulation of plant primary metabolism. Here we provide the first report of 14-3-3 interactome analysis and identify putative 14-3-3 targets in tomato fruit tissue, which may be highly important given the documented metabolic shifts, which occur during fruit development and ripening. These data open future research avenues by which to understand the regulation of the role of post-translational regulation in tomato fruit development.

Identification of a dTDP-rhamnose biosynthetic pathway that oscillates with the molting cycle in Caenorhabditis elegans

The rhamnose biosynthetic pathway, which is highly conserved across nematode species, was characterized in the nematode Caenorhabditis elegans. The pathway is up-regulated before each larval molt, suggesting that rhamnose biosynthesis plays a role in cuticle or surface coat synthesis.

L-Rhamnose is a common component of cell-wall polysaccharides, glycoproteins and some natural products in bacteria and plants, but is rare in fungi and animals. In the present study, we identify and characterize a biosynthetic pathway for dTDP-rhamnose in Caenorhabditis elegans that is highly conserved across nematode species. We show that RML-1 activates glucose 1-phosphate (Glc-1-P) in the presence of either dTTP or UTP to yield dTDP-glucose or UDP-glucose, respectively. RML-2 is a dTDP-glucose 4,6-dehydratase, converting dTDP-glucose into dTDP-4-keto-6-deoxyglucose. Using mass spectrometry and NMR spectroscopy, we demonstrate that coincubation of dTDP-4-keto-6-deoxyglucose with RML-3 (3,5-epimerase) and RML-4 (4-keto-reductase) produces dTDP-rhamnose. RML-4 could only be expressed and purified in an active form through co-expression with a co-regulated protein, RML-5, which forms a complex with RML-4. Analysis of the sugar nucleotide pool in C. elegans established the presence of dTDP-rhamnose in vivo. Targeting the expression of the rhamnose biosynthetic genes by RNAi resulted in significant reductions in dTDP-rhamnose, but had no effect on the biosynthesis of a closely related sugar, ascarylose, found in the ascaroside pheromones. Therefore, the rhamnose and ascarylose biosynthetic pathways are distinct. We also show that transcriptional reporters for the rhamnose biosynthetic genes are expressed highly in the embryo, in the hypodermis during molting cycles and in the hypodermal seam cells specifically before the molt to the stress-resistant dauer larval stage. These expression patterns suggest that rhamnose biosynthesis may play an important role in hypodermal development or the production of the cuticle or surface coat during molting.

Expanded investigations of the aglycon promiscuity and catalysis characteristic of flavonol 3-O-rhamnosyltransferase AtUGT78D1 from Arabidopsis thaliana

Rhamnosides usually possess better bioavailabilities and improved solubilities compared with their aglycons and are a major source of bioactive natural products.

The Autonomous Glycosylation of Large DNA Viruses

Glycosylation of surface molecules is a key feature of several eukaryotic viruses, which use the host endoplasmic reticulum/Golgi apparatus to add carbohydrates to their nascent glycoproteins. In recent years, a newly discovered group of eukaryotic viruses, belonging to the Nucleo-Cytoplasmic Large DNA Virus (NCLDV) group, was shown to have several features that are typical of cellular organisms, including the presence of components of the glycosylation machinery. Starting from initial observations with the chlorovirus PBCV-1, enzymes for glycan biosynthesis have been later identified in other viruses; in particular in members of the Mimiviridae family. They include both the glycosyltransferases and other carbohydrate-modifying enzymes and the pathways for the biosynthesis of the rare monosaccharides that are found in the viral glycan structures. These findings, together with genome analysis of the newly-identified giant DNA viruses, indicate that the presence of glycogenes is widespread in several NCLDV families. The identification of autonomous viral glycosylation machinery leads to many questions about the origin of these pathways, the mechanisms of glycan production, and eventually their function in the viral replication cycle. The scope of this review is to highlight some of the recent results that have been obtained on the glycosylation systems of the large DNA viruses, with a special focus on the enzymes involved in nucleotide-sugar production.

Arabidopsis seed mucilage secretory cells: regulation and dynamics

Seeds from various angiosperm species produce polysaccharide mucilage facilitating germination and, therefore, conferring major evolutionary advantages. The seed epidermal mucilage secretory cells (MSCs) undergo numerous tightly controlled changes of their extracellular matrixes (ECMs) throughout seed development. Recently, major progress based on the model species Arabidopsis thaliana was published, including the identification of 54 genes necessary for mucilage synthesis and release. Here, we review these genes that constitute the so-called 'MSC toolbox', within which transcription factors and proteins related to polysaccharide production, secretion, modification, and stabilization are the most abundant and belong to complex regulatory networks. We also discuss how seed coat 'omics data-mining, comparative genomics, and operon-like gene cluster studies will provide means to identify new members of the MSC toolbox.

Structural and biochemical insights into nucleotide-rhamnose synthase/epimerase-reductase from Arabidopsis thaliana

L-Rhamnose (Rha) is synthesized via a similar enzymatic pathway in bacteria, plants and fungi. In plants, nucleotide-rhamnose synthase/epimerase-reductase (NRS/ER) catalyzes the final step in the conversion of dTDP/UDP-α-D-Glc to dTDP/UDP-β-L-Rha in an NAD(P)H dependent manner. Currently, only biochemical evidence for the function of NRS/ER has been described. In this study, a crystal structure for Arabidopsis thaliana NRS/ER was determined, which is the first report of a eukaryotic rhamnose synthase with both epimerase and reductase activities. NRS/ER functions as a metal ion independent homodimer that forms through hydrophobic interactions via a four-helix bundle. Each monomer exhibits α/β folding that can be divided into two regions, nucleotide cofactor binding domain and sugar substrate binding domain. The affinities of ligands with NRS/ER were measured using isothermal titration calorimetry, which showed that NRS/ER has a preference for dTDP over UDP, while the cofactor binding site has a similar affinity for NADH and NADPH. Structural analysis coupled to site-directed mutagenesis suggested C115 and K183 as the acid/base pair responsible for epimerization, while T113, Y144 and K148 are the conserved residues in reduction. These findings shed light on the molecular mechanism of NRS/ER and were helpful to explore other eukaryotic enzymes involved in L-Rha synthesis.

Biosynthesis and production of glycosylated flavonoids in Escherichia coli: current state and perspectives

Flavonoids are plant secondary metabolites containing several hydroxyl groups that are targets for modification reactions such as methylation and glycosylation. In plants, flavonoids are present as glycones. Although glucose is the most common sugar attached to flavonoids, arabinose, galactose, glucuronic acid, rhamnose, and xylose are also linked to flavonoids. Depending on the kind and the position of the attached sugar, flavonoid glycones show different biological properties. Flavonoid-O-glycosides are synthesized by uridine diphosphate-dependent glycosyltransferases (UGTs), which use nucleotide sugar as a sugar donor and a diverse compound as a sugar acceptor. Recently, diverse flavonoid-O-glycosides have been synthesized in Escherichia coli by introducing UGTs from plants and bacteria and modifying endogenous pathways. The nucleotide sugar biosynthesis pathway in E. coli has been engineered to provide the proper nucleotide sugar for flavonoid-O-glycoside biosynthesis. In this review, we will discuss recent advances in flavonoid-O-glycoside biosynthesis using engineered E. coli.

Biotechnological advances in UDP-sugar based glycosylation of small molecules

Glycosylation of small molecules like specialized (secondary) metabolites has a profound impact on their solubility, stability or bioactivity, making glycosides attractive compounds as food additives, therapeutics or nutraceuticals. The subsequently growing market demand has fuelled the development of various biotechnological processes, which can be divided in the in vitro (using enzymes) or in vivo (using whole cells) production of glycosides. In this context, uridine glycosyltransferases (UGTs) have emerged as promising catalysts for the regio- and stereoselective glycosylation of various small molecules, hereby using uridine diphosphate (UDP) sugars as activated glycosyldonors. This review gives an extensive overview of the recently developed in vivo production processes using UGTs and discusses the major routes towards UDP-sugar formation. Furthermore, the use of interconverting enzymes and glycorandomization is highlighted for the production of unusual or new-to-nature glycosides. Finally, the technological challenges and future trends in UDP-sugar based glycosylation are critically evaluated and summarized.

Starting to gel: how Arabidopsis seed coat epidermal cells produce specialized secondary cell walls

For more than a decade, the Arabidopsis seed coat epidermis (SCE) has been used as a model system to study the synthesis, secretion and modification of cell wall polysaccharides, particularly pectin. Our detailed re-evaluation of available biochemical data highlights that Arabidopsis seed mucilage is more than just pectin. Typical secondary wall polymers such as xylans and heteromannans are also present in mucilage. Despite their low abundance, these components appear to play essential roles in controlling mucilage properties, and should be further investigated. We also provide a comprehensive community resource by re-assessing the mucilage phenotypes of almost 20 mutants using the same conditions. We conduct an in-depth functional evaluation of all the SCE genes described in the literature and propose a revised model for mucilage production. Further investigation of SCE cells will improve our understanding of plant cell walls.

A novel polysaccharide in insects activates the innate immune system in mouse macrophage RAW264 cells

A novel water-soluble polysaccharide was identified in the pupae of the melon fly (Bactrocera cucurbitae) as a molecule that activates the mammalian innate immune response. We attempted to purify this innate immune activator using nitric oxide (NO) production in mouse RAW264 macrophages as an indicator of immunostimulatory activity. A novel acidic polysaccharide was identified, which we named “dipterose”, with a molecular weight of 1.01×10⁶ and comprising nine monosaccharides. Dipterose was synthesized in the melon fly itself at the pupal stage. The NO-producing activity of dipterose was approximately equal to that of lipopolysaccharide, a potent immunostimulator. Inhibition of Toll-like receptor 4 (TLR4) led to the suppression of NO production by dipterose. Furthermore, dipterose induced the expression of proinflammatory cytokines and interferon β (IFNβ) and promoted the activation of nuclear factor kappa B (NF-κB) in macrophages, indicating that it stimulates the induction of various cytokines in RAW264 cells via the TLR4 signaling pathway. Our results thus suggest that dipterose activates the innate immune response against various pathogenic microorganisms and viral infections. This is the first identification of an innate immune-activating polysaccharide from an animal.

Transcriptional evidence for inferred pattern of pollen tube-stigma metabolic coupling during pollination

It is difficult to derive all qualitative proteomic and metabolomic experimental data in male (pollen tube) and female (pistil) reproductive tissues during pollination because of the limited sensitivity of current technology. In this study, genome-scale enzyme correlation network models for plants (Arabidopsis/maize) were constructed by analyzing the enzymes and metabolic routes from a global perspective. Then, we developed a data-driven computational pipeline using the "guilt by association" principle to analyze the transcriptional coexpression profiles of enzymatic genes in the consecutive steps for metabolic routes in the fast-growing pollen tube and stigma during pollination. The analysis identified an inferred pattern of pollen tube-stigma ethanol coupling. When the pollen tube elongates in the transmitting tissue (TT) of the pistil, this elongation triggers the mobilization of energy from glycolysis in the TT cells of the pistil. Energy-rich metabolites (ethanol) are secreted that can be taken up by the pollen tube, where these metabolites are incorporated into the pollen tube's tricarboxylic acid (TCA) cycle, which leads to enhanced ATP production for facilitating pollen tube growth. In addition, our analysis also provided evidence for the cooperation of kaempferol, dTDP-alpha-L-rhamnose and cell-wall-related proteins; phosphatidic-acid-mediated Ca2+ oscillations and cytoskeleton; and glutamate degradation IV for γ-aminobutyric acid (GABA) signaling activation in Arabidopsis and maize stigmas to provide the signals and materials required for pollen tube tip growth. In particular, the "guilt by association" computational pipeline and the genome-scale enzyme correlation network models (GECN) developed in this study was initiated with experimental "omics" data, followed by data analysis and data integration to determine correlations, and could provide a new platform to assist inachieving a deeper understanding of the co-regulation and inter-regulation model in plant research.

Giant virus Megavirus chilensis encodes the biosynthetic pathway for uncommon acetamido sugars

Giant viruses mimicking microbes, by the sizes of their particles and the heavily glycosylated fibrils surrounding their capsids, infect Acanthamoeba sp., which are ubiquitous unicellular eukaryotes. The glycans on fibrils are produced by virally encoded enzymes, organized in gene clusters. Like Mimivirus, Megavirus glycans are mainly composed of virally synthesized N-acetylglucosamine (GlcNAc). They also contain N-acetylrhamnosamine (RhaNAc), a rare sugar; the enzymes involved in its synthesis are encoded by a gene cluster specific to Megavirus close relatives. We combined activity assays on two enzymes of the pathway with mass spectrometry and NMR studies to characterize their specificities. Mg534 is a 4,6-dehydratase 5-epimerase; its three-dimensional structure suggests that it belongs to a third subfamily of inverting dehydratases. Mg535, next in the pathway, is a bifunctional 3-epimerase 4-reductase. The sequential activity of the two enzymes leads to the formation of UDP-l-RhaNAc. This study is another example of giant viruses performing their glycan synthesis using enzymes different from their cellular counterparts, raising again the question of the origin of these pathways.

Haloferax volcanii N-glycosylation: delineating the pathway of dTDP-rhamnose biosynthesis

In the halophilic archaea Haloferax volcanii, the surface (S)-layer glycoprotein can be modified by two distinct N-linked glycans. The tetrasaccharide attached to S-layer glycoprotein Asn-498 comprises a sulfated hexose, two hexoses and a rhamnose. While Agl11-14 have been implicated in the appearance of the terminal rhamnose subunit, the precise roles of these proteins have yet to be defined. Accordingly, a series of in vitro assays conducted with purified Agl11-Agl14 showed these proteins to catalyze the stepwise conversion of glucose-1-phosphate to dTDP-rhamnose, the final sugar of the tetrasaccharide glycan. Specifically, Agl11 is a glucose-1-phosphate thymidylyltransferase, Agl12 is a dTDP-glucose-4,6-dehydratase and Agl13 is a dTDP-4-dehydro-6-deoxy-glucose-3,5-epimerase, while Agl14 is a dTDP-4-dehydrorhamnose reductase. Archaea thus synthesize nucleotide-activated rhamnose by a pathway similar to that employed by Bacteria and distinct from that used by Eukarya and viruses. Moreover, a bioinformatics screen identified homologues of agl11-14 clustered in other archaeal genomes, often as part of an extended gene cluster also containing aglB, encoding the archaeal oligosaccharyltransferase. This points to rhamnose as being a component of N-linked glycans in Archaea other than Hfx. volcanii.

TRANSPARENT TESTA2 regulates embryonic fatty acid biosynthesis by targeting FUSCA3 during the early developmental stage of Arabidopsis seeds

TRANSPARENT TESTA2 (TT2) regulates the biosynthesis of proanthocyanidins in the seed coat of Arabidopsis. We recently found that TT2 also participates in inhibition of fatty acid (FA) biosynthesis in the seed embryo. However, the mechanism by which TT2 suppresses the accumulation of seed FA remains unclear. In this study, we show that TT2 is expressed in embryos at an early developmental stage. TT2 is directly bound to the regulatory region of FUSCA3 (FUS3), and mediates the expression of numerous genes in the FA biosynthesis pathway. These genes include BCCP2, CAC2, MOD1 and KASII, which encode proteins involved in the initial steps of FA chain formation, FAD2 and FAD3, which are responsible for FA desaturation, and FAE1, which catalyzes very-long-chain FA elongation. Loss of function of TT2 results in reduced expression of GLABRA2 but does not cause a significant reduction in the mucilage attached to the seed coats, which competes with FA for photosynthates. TT2 is expressed in both maternal seed coats and embryonic tissues, but proanthocyanidins are only found in wild-type seed coats and not in embryonic tissues. The amount of proanthocyanidins in the seed coat is negatively correlated with the amount of FAs in the embryo.

The flavonoid biosynthetic pathway in Arabidopsis: structural and genetic diversity

Flavonoids are representative plant secondary products. In the model plant Arabidopsis thaliana, at least 54 flavonoid molecules (35 flavonols, 11 anthocyanins and 8 proanthocyanidins) are found. Scaffold structures of flavonoids in Arabidopsis are relatively simple. These include kaempferol, quercetin and isorhamnetin for flavonols, cyanidin for anthocyanins and epicatechin for proanthocyanidins. The chemical diversity of flavonoids increases enormously by tailoring reactions which modify these scaffolds, including glycosylation, methylation and acylation. Genes responsible for the formation of flavonoid aglycone structures and their subsequent modification reactions have been extensively characterized by functional genomic efforts - mostly the integration of transcriptomics and metabolic profiling followed by reverse genetic experimentation. This review describes the state-of-art of flavonoid biosynthetic pathway in Arabidopsis regarding both structural and genetic diversity, focusing on the genes encoding enzymes for the biosynthetic reactions and vacuole translocation.

Crystal structure of the capsular polysaccharide synthesizing protein CapE of Staphylococcus aureus

Enzymes synthesizing the bacterial CP (capsular polysaccharide) are attractive antimicrobial targets. However, we lack critical information about the structure and mechanism of many of them. In an effort to reduce that gap, we have determined three different crystal structures of the enzyme CapE of the human pathogen Staphylococcus aureus. The structure reveals that CapE is a member of the SDR (short-chain dehydrogenase/reductase) super-family of proteins. CapE assembles in a hexameric complex stabilized by three major contact surfaces between protein subunits. Turnover of substrate and/or coenzyme induces major conformational changes at the contact interface between protein subunits, and a displacement of the substrate-binding domain with respect to the Rossmann domain. A novel dynamic element that we called the latch is essential for remodelling of the protein–protein interface. Structural and primary sequence alignment identifies a group of SDR proteins involved in polysaccharide synthesis that share the two salient features of CapE: the mobile loop (latch) and a distinctive catalytic site (MxxxK). The relevance of these structural elements was evaluated by site-directed mutagenesis.

Regioselective synthesis of flavonoid bisglycosides using Escherichia coli harboring two glycosyltransferases

Regioselective glycosylation of flavonoids cannot be easily achieved due to the presence of several hydroxyl groups in flavonoids. This hurdle could be overcome by employing uridine diphosphate-dependent glycosyltransferases (UGTs), which use nucleotide sugars as sugar donors and diverse compounds including flavonoids as sugar acceptors. Quercetin rhamnosides contain antiviral activity. Two quercetin diglycosides, quercetin 3-O-glucoside-7-O-rhamnoside and quercetin 3,7-O-bisrhamnoside, were synthesized using Escherichia coli expressing two UGTs. For the synthesis of quercetin 3-O-glucoside-7-O-rhamnoside, AtUGT78D2, which transfers glucose from UDP-glucose to the 3-hydroxyl group of quercetin, and AtUGT89C1, which transfers rhamnose from UDP-rhamnose to the 7-hydroxyl group of quercetin 3-O-glucoside, were transformed into E. coli. Using this approach, 67 mg/L of quercetin 3-O-glucoside-7-O-rhamnoside was synthesized. For the synthesis of quercetin 3,7-O-bisrhamnoside, AtUGT78D1, which transfers rhamnose to the 3-hydroxy group of quercetin, and AtUGT89C1 were used. The RHM2 gene from Arabidopsis thaliana was coexpressed to supply the sugar donor, UDP-rhamnose. E. coli expressing AtUGT78D1, AtUGT89C1, and RHM2 was used to obtain 67.4 mg/L of quercetin 3,7-O-bisrhamnoside.

Flying saucer1 is a transmembrane RING E3 ubiquitin ligase that regulates the degree of pectin methylesterification in Arabidopsis seed mucilage

Pectins are complex polysaccharides that form the gel matrix of the primary cell wall and are abundant in the middle lamella that holds plant cells together. Their degree of methylesterification (DM) impacts wall strength and cell adhesion since unesterified pectin regions can cross-link via Ca(2+) ions to form stronger gels. Here, we characterize flying saucer1 (fly1), a novel Arabidopsis thaliana seed coat mutant, which displays primary wall detachment, reduced mucilage extrusion, and increased mucilage adherence. These defects appear to result from a lower DM in mucilage and are enhanced by the addition of Ca(2+) or completely rescued using alkaline Ca(2+) chelators. FLY1 encodes a transmembrane protein with a RING-H2 domain that has in vitro E3 ubiquitin ligase activity. FLY1 is orthologous to TRANSMEMBRANE UBIQUITIN LIGASE1, a Golgi-localized E3 ligase involved in the quality control of membrane proteins in yeast. However, FLY1-yellow fluorescent protein (YFP) fusions are localized in punctae that are predominantly distinct from the Golgi and the trans-Golgi network/early endosome in the seed coat epidermis. Wortmannin treatment, which induces the fusion of late endosomes in plants, resulted in enlarged FLY1-YFP bodies. We propose that FLY1 regulates the DM of pectin in mucilage, potentially by recycling pectin methylesterase enzymes in the endomembrane system of seed coat epidermal cells.

The evolution of phenylpropanoid metabolism in the green lineage

Phenolic secondary metabolites are only produced by plants wherein they play important roles in both biotic and abiotic defense in seed plants as well as being potentially important bioactive compounds with both nutritional and medicinal benefits reported for animals and humans as a consequence of their potent antioxidant activity. During the long evolutionary period in which plants have adapted to the environmental niches in which they exist (and especially during the evolution of land plants from their aquatic algal ancestors), several strategies such as gene duplication and convergent evolution have contributed to the evolution of this pathway. In this respect, diversity and redundancy of several key genes of phenolic secondary metabolism such as polyketide synthases, cytochrome P450s, Fe(2+)/2-oxoglutarate-dependent dioxygenases and UDP-glycosyltransferases have played an essential role. Recent technical developments allowing affordable whole genome sequencing as well as a better inventory of species-by-species chemical diversity have resulted in a dramatic increase in the number of tools we have to assess how these pathways evolved. In parallel, reverse genetics combined with detailed molecular phenotyping is allowing us to elucidate the functional importance of individual genes and metabolites and by this means to provide further mechanistic insight into their biological roles. In this review, phenolic metabolite-related gene sequences (for a total of 65 gene families including shikimate biosynthetic genes) are compared across 23 independent species, and the phenolic metabolic complement of various plant species are compared with one another, in attempt to better understand the evolution of diversity in this crucial pathway.

PECTIN METHYLESTERASE INHIBITOR6 promotes Arabidopsis mucilage release by limiting methylesterification of homogalacturonan in seed coat epidermal cells

Imbibed seeds of the Arabidopsis thaliana accession Djarly are affected in mucilage release from seed coat epidermal cells. The impaired locus was identified as a pectin methylesterase inhibitor gene, PECTIN METHYLESTERASE INHIBITOR6 (PMEI6), specifically expressed in seed coat epidermal cells at the time when mucilage polysaccharides are accumulated. This spatio-temporal regulation appears to be modulated by GLABRA2 and LEUNIG HOMOLOG/MUCILAGE MODIFIED1, as expression of PMEI6 is reduced in mutants of these transcription regulators. In pmei6, mucilage release was delayed and outer cell walls of epidermal cells did not fragment. Pectin methylesterases (PMEs) demethylate homogalacturonan (HG), and the majority of HG found in wild-type mucilage was in fact derived from outer cell wall fragments. This correlated with the absence of methylesterified HG labeling in pmei6, whereas transgenic plants expressing the PMEI6 coding sequence under the control of the 35S promoter had increased labeling of cell wall fragments. Activity tests on seeds from pmei6 and 35S:PMEI6 transgenic plants showed that PMEI6 inhibits endogenous PME activities, in agreement with reduced overall methylesterification of mucilage fractions and demucilaged seeds. Another regulator of PME activity in seed coat epidermal cells, the subtilisin-like Ser protease SBT1.7, acts on different PMEs, as a pmei6 sbt1.7 mutant showed an additive phenotype.

Evolving views of pectin biosynthesis

Recent progress in the identification and characterization of pectin biosynthetic proteins and the discovery of pectin domain-containing proteoglycans are changing our view of how pectin, the most complex family of plant cell wall polysaccharides, is synthesized. The functional confirmation of four types of pectin biosynthetic glycosyltransferases, the identification of multiple putative pectin glycosyl- and methyltransferases, and the characteristics of the GAUT1:GAUT7 homogalacturonan biosynthetic complex with its novel mechanism for retaining catalytic subunits in the Golgi apparatus and its 12 putative interacting proteins are beginning to provide a framework for the pectin biosynthetic process. We propose two partially overlapping hypothetical and testable models for pectin synthesis: the consecutive glycosyltransferase model and the domain synthesis model.

Effects of exogenous auxin and ethylene on the Arabidopsis root proteome

The phytohormones, auxin and ethylene, together control a wide range of physiological and developmental processes in plants. The lack of knowledge regarding how the underlying signaling processes are reflected at the protein level represents a major gap in understanding phytohormone signaling, including that mediated by crosstalk between auxin and ethylene. Herein is a parallel comparison of the effects of these two hormones on the Arabidopsis root proteome. Arabidopsis seedlings were exposed to 1 μm indole-3-acetic acid (IAA, auxin) or 1 μm 1-amino-cyclopropane carboxylic acid (ACC) for 24h. Root protein extracts were fractionated using two-dimensional gel electrophoresis and the proteins that changed the most were analyzed by MALDI TOF/TOF mass spectrometry. Of the 500 total spots that were matched across all gels, 24 were significantly different after IAA exposure, while seven others were different after ACC exposure. Using rigorous criteria, identities of eight proteins regulated by IAA and five regulated by ACC were assigned. Interestingly, although both hormones affected proteins associated with fundamental cellular processes, no overlap was observed among the proteins affected by auxin or ethylene treatment. This report provides a comparison of the effects of these two hormones relative to a control utilizing equivalent treatment regimes and suggests that, while these hormones communicate to control similar physiological and transcriptional processes, they have different effects on the most abundant proteins in Arabidopsis roots.

Proteomic analysis of dark response in Arabidopsis cell suspension cultures

Despite intense research on light responses in plants, the consequences of a simple shift from light to darkness are largely unexplored. In this research, the physiological outcome and proteomic changes in Arabidopsis cell suspension cultures after switching from light to total darkness were examined. Deprivation of light led to a visible loss of chlorophyll and failure to develop functional chloroplasts that are present in light-grown cells. This response was accompanied by a significant increase in the cell multiplication rate, most likely due to decreased formation of the damaging reactive oxygen species in the dark. Additionally, there were significant changes in the abundance of 46 protein spots (mostly assigned to chloroplasts, mitochondria and membranes) which were resolved by two-dimensional difference gel electrophoresis and mass spectrometric analysis. All identified chloroplast proteins were down-regulated in response to sustained darkness. In contrast, all differentially expressed proteins associated with cell wall biosynthesis were up-regulated by the dark treatment. Changes in the levels of these proteins were consistent with the observed morphological and physiological changes of the cells. These results reveal a comprehensive picture of the dark response in Arabidopsis cells and provide a useful platform for further characterization of gene function and regulation in plant responses to light.

Production of bioactive flavonol rhamnosides by expression of plant genes in Escherichia coli

Biotransformation of flavonoids using Escherichia coli harboring specific glycosyltransferases is an excellent method for the regioselective synthesis of flavonoid glycosides. Flavonol rhamnosides have been shown to contain better antiviral and antibacterial activities compared to flavonol aglycones. To synthesize flavonoid rhamnoside, a strain of E. coli expressing UDP-rhamnose flavonol glycosyltransferase (AtUGT78D1) from Arabidopsis thaliana was used to produce quercetin 3-O-rhamnoside. The biotransformation of quercetin using this E. coli transformant resulted in the production of quercetin 3-O-rhamnoside as a major product. A strain of E. coli rfbD (encoding dTDP-4-dehydrorhamnose reductase) expressing AtUGT78D1, which is involved in the final step of thymidine diphosphate rhamnose (TDP-rhamnose) biosynthesis, did not produce quercetin 3-O-rhamnoside, meaning that AtUGT78D1 used endogenous TDP-rhamnose as a sugar donor to produce quercetin 3-O-rhamnoside. The production of quercetin 3-O-rhamnoside could be increased by up to 160% by co-expressing AtUGT78D1 and rhamnose synthase gene 2 (RHM2), which catalyzes the conversion of UDP-glucose into UDP-rhamnose. Using an E. coli strain harboring AtUGT78D1 and RHM2, 150 mg/L quercetin 3-O-rhamnoside and 200 mg/L kaempferol 3-O-rhamnoside were produced in 48 h.

Pathways for epidermal cell differentiation via the homeobox gene GLABRA2: update on the roles of the classic regulator

Recent plant development studies have identified regulatory pathways for epidermal cell differentiation in Arabidopsis thaliana. Interestingly, some of such pathways contain transcriptional networks with a common structure in which the homeobox gene GLABLA2 (GL2) is downstream of the transactivation complex consisting of MYB, bHLH, and WD40 proteins. Here, we review the role of GL2 as an output device of the conserved network, and update the knowledge of epidermal cell differentiation pathways downstream of GL2. Despite the consistent position of GL2 within the network, its role in epidermal tissues varies; in the root epidermis, GL2 promotes non-hair cell differentiation after cell pattern formation, whereas in the leaf epidermis, it is likely to be involved in both pattern formation and differentiation of trichomes. GL2 expression levels act as quantitative factors for initiation of cell differentiation in the root and leaf epidermis; the quantity of hairless cells in non-root hair cell files is reduced by gl2 mutations in a semi-dominant manner, and entopically additive expression of GL2 and a heterozygous gl2 mutation increase and decrease the number of trichomes, respectively. Although few direct target genes have been identified, evidence from genetic and expression analyses suggests that GL2 directly regulates genes with various hierarchies in epidermal cell differentiation pathways.

Crystal structure of the enzyme CapF of Staphylococcus aureus reveals a unique architecture composed of two functional domains

CP (capsular polysaccharide) is an important virulence factor during infections by the bacterium Staphylococcus aureus. The enzyme CapF is an attractive therapeutic candidate belonging to the biosynthetic route of CP of pathogenic strains of S. aureus. In the present study, we report two independent crystal structures of CapF in an open form of the apoenzyme. CapF is a homodimer displaying a characteristic dumb-bell-shaped architecture composed of two domains. The N-terminal domain (residues 1-252) adopts a Rossmann fold belonging to the short-chain dehydrogenase/reductase family of proteins. The C-terminal domain (residues 252-369) displays a standard cupin fold with a Zn2+ ion bound deep in the binding pocket of the β-barrel. Functional and thermodynamic analyses indicated that each domain catalyses separate enzymatic reactions. The cupin domain is necessary for the C3-epimerization of UDP-4-hexulose. Meanwhile, the N-terminal domain catalyses the NADPH-dependent reduction of the intermediate species generated by the cupin domain. Analysis by ITC (isothermal titration calorimetry) revealed a fascinating thermodynamic switch governing the attachment and release of the coenzyme NADPH during each catalytic cycle. These observations suggested that the binding of coenzyme to CapF facilitates a disorder-to-order transition in the catalytic loop of the reductase (N-terminal) domain. We anticipate that the present study will improve the general understanding of the synthesis of CP in S. aureus and will aid in the design of new therapeutic agents against this pathogenic bacterium.

Arabidopsis Seed Coat Mucilage is a Specialized Cell Wall that Can be Used as a Model for Genetic Analysis of Plant Cell Wall Structure and Function

Arabidopsis seed coat epidermal cells produce a large quantity of mucilage that is extruded upon exposure to water. Chemical analyses and cell biological techniques suggest that this mucilage represents a specialized type of secondary cell wall composed primarily of pectin with lesser amounts of cellulose and xyloglucan. Once extruded, the mucilage capsule has a distinctive structure with an outer non-adherent layer that is easily removed by shaking in water, and an inner adherent layer that can only be removed with strong acid or base. Most of the cellulose in the mucilage is present in the inner layer and is responsible at least in part for its adherence to the seed. There are also differences in the pectin composition between the two layers that could contribute to the difference in adherence. The Arabidopsis seed coat epidermis and its mucilage are not essential for seed viability or germination. This dispensability, combined with the fact that the epidermal cells synthesize an accessible pectin-rich cell wall at a specific time in development, makes them well suited as a genetic model for studying cell wall biogenesis, function, and regulation. Mutants defective in seed mucilage identified by both forward and reverse genetic analyses are proving useful in establishing connections between carbohydrate structure and cell wall properties in vivo. In the future, genetic engineering of seed coat mucilage carbohydrates should prove useful for testing hypotheses concerning cell wall structure and function.

Arabidopsis glabra2 mutant seeds deficient in mucilage biosynthesis produce more oil

Seed oil, one of the major seed storage compounds in plants, is of great economic importance for human consumption, as an industrial raw material and as a source of biofuels. Thus, improving the seed oil yield in crops is an important objective. The GLABRA2 (GL2) gene in Arabidopsis thaliana encodes a transcription factor that is required for the proper differentiation of several epidermal cell types. GL2 has also been shown to regulate seed oil levels, as a loss-of-function mutation in the GL2 gene results in plants with a higher seed oil content than wild-type. We have extended this observation by showing that loss-of-function mutations in several positive regulators of GL2 also result in a high seed oil phenotype. The GL2 gene is expressed in both the seed coat and embryo, but the embryo is the main site of seed oil accumulation. Surprisingly, our results indicate that it is loss of GL2 activity in the seed coat, not the embryo, that contributes to the high seed oil phenotype. One target of GL2 in the seed coat is the gene MUCILAGE MODIFIED 4 (MUM4), which encodes a rhamnose synthase that is required for seed mucilage biosynthesis. We found that mum4 mutant seeds, like those of gl2 mutants, have an increased seed oil content in comparison with wild-type. Therefore, GL2 regulates seed oil production at least partly through its influence on MUM4 expression in the seed coat. We propose that gl2 mutant seeds produce more oil due to increased carbon allocation to the embryo in the absence of seed coat mucilage biosynthesis.

Increase in pectin deposition by overexpression of an ERF gene in cultured cells of Arabidopsis thaliana

Ethylene-responsive transcription factor (ERF) family genes, which are involved in regulation of metabolic pathways and/or are useful for metabolic engineering, were investigated in the cultured cells of Arabidopsis thaliana. The pectin content in the gelatinous precipitates after the ethanol precipitation of extracts derived from calli of a transgenic cell line, A17, overexpressing an ERF gene (At1g44830), increased in comparison with the control. Expression of genes involved in pectin biosynthesis was up-regulated in the A17 calli. Overexpression of the ERF gene coordinately activates the pectin biosynthetic pathway genes and increases the content of pectin. These results therefore will be useful as a genetic resource for engineering pectin biosynthesis in plants.

Biosynthesis of UDP-4-keto-6-deoxyglucose and UDP-rhamnose in pathogenic fungi Magnaporthe grisea and Botryotinia fuckeliana

There is increasing evidence that in several fungi, rhamnose-containing glycans are involved in processes that affect host-pathogen interactions, including adhesion, recognition, virulence, and biofilm formation. Nevertheless, little is known about the pathways for the synthesis of these glycans. We show that rhamnose is present in glycans isolated from the rice pathogen Magnaporthe grisea and from the plant pathogen Botryotinia fuckeliana. We also provide evidence that these fungi produce UDP-rhamnose. This is in contrast to bacteria where dTDP-rhamnose is the activated form of this sugar. In bacteria, formation of dTDP-rhamnose requires three enzymes. Here, we demonstrate that in fungi only two genes are required for UDP-Rha synthesis. The first gene encodes a UDP-glucose-4,6-dehydratase that converts UDP-glucose to UDP-4-keto-6-deoxyglucose. The product was shown by time-resolved (1)H NMR spectroscopy to exist in solution predominantly as a hydrated form along with minor amounts of a keto form. The second gene encodes a bifunctional UDP-4-keto-6-deoxyglucose-3,5-epimerase/-4-reductase that converts UDP-4-keto-6-deoxyglucose to UDP-rhamnose. Sugar composition analysis and gene expression studies at different stages of growth indicate that the synthesis of rhamnose-containing glycans is under tissue-specific regulation. Together, our results provide new insight into the formation of rhamnose-containing glycans during the fungal life cycle. The role of these glycans in the interactions between fungal pathogens and their hosts is discussed. Knowledge of the metabolic pathways involved in the formation of rhamnose-containing glycans may facilitate the development of drugs to combat fungal diseases in humans, as to the best of our knowledge mammals do not make these types of glycans.

Evolution of plant nucleotide-sugar interconversion enzymes

Nucleotide-diphospho-sugars (NDP-sugars) are the building blocks of diverse polysaccharides and glycoconjugates in all organisms. In plants, 11 families of NDP-sugar interconversion enzymes (NSEs) have been identified, each of which interconverts one NDP-sugar to another. While the functions of these enzyme families have been characterized in various plants, very little is known about their evolution and origin. Our phylogenetic analyses indicate that all the 11 plant NSE families are distantly related and most of them originated from different progenitor genes, which have already diverged in ancient prokaryotes. For instance, all NSE families are found in the lower land plant mosses and most of them are also found in aquatic algae, implicating that they have already evolved to be capable of synthesizing all the 11 different NDP-sugars. Particularly interesting is that the evolution of RHM (UDP-L-rhamnose synthase) manifests the fusion of genes of three enzymatic activities in early eukaryotes in a rather intriguing manner. The plant NRS/ER (nucleotide-rhamnose synthase/epimerase-reductase), on the other hand, evolved much later from the ancient plant RHMs through losing the N-terminal domain. Based on these findings, an evolutionary model is proposed to explain the origin and evolution of different NSE families. For instance, the UGlcAE (UDP-D-glucuronic acid 4-epimerase) family is suggested to have evolved from some chlamydial bacteria. Our data also show considerably higher sequence diversity among NSE-like genes in modern prokaryotes, consistent with the higher sugar diversity found in prokaryotes. All the NSE families are widely found in plants and algae containing carbohydrate-rich cell walls, while sporadically found in animals, fungi and other eukaryotes, which do not have or have cell walls with distinct compositions. Results of this study were shown to be highly useful for identifying unknown genes for further experimental characterization to determine their functions in the synthesis of diverse glycosylated molecules.

UDP-glucose 4, 6-dehydratase activity plays an important role in maintaining cell wall integrity and virulence of Candida albicans

Candida albicans, a human fungal pathogen, undergoes morphogenetic changes that are associated with virulence. We report here that GAL102 in C. albicans encodes a homolog of dTDP-glucose 4,6-dehydratase, an enzyme that affects cell wall properties as well as virulence of many pathogenic bacteria. We found that GAL102 deletion leads to greater sensitivity to antifungal drugs and cell wall destabilizing agents like Calcofluor white and Congo red. The mutant also formed biofilms consisting mainly of hyphal cells that show less turgor. The NMR analysis of cell wall mannans of gal102 deletion strain revealed that a major constituent of mannan is missing and the phosphomannan component known to affect virulence is greatly reduced. We also observed that there was a substantial reduction in the expression of genes involved in biofilm formation but increase in the expression of genes encoding glycosylphosphatidylinositol-anchored proteins in the mutant. These, along with altered mannosylation of cell wall proteins together might be responsible for multiple phenotypes displayed by the mutant. Finally, the mutant was unable to grow in the presence of resident peritoneal macrophages and elicited a weak pro-inflammatory cytokine response in vitro. Similarly, this mutant elicited a poor serum pro-inflammatory cytokine response as judged by IFNγ and TNFα levels and showed reduced virulence in a mouse model of systemic candidiasis. Importantly, an Ala substitution for a conserved Lys residue in the active site motif YXXXK, that abrogates the enzyme activity also showed reduced virulence and increased filamentation similar to the gal102 deletion strain. Since inactivating the enzyme encoded by GAL102 makes the cells sensitive to antifungal drugs and reduces its virulence, it can serve as a potential drug target in combination therapies for C. albicans and related pathogens.

CGR3: a Golgi-localized protein influencing homogalacturonan methylesterification

Plant cell walls are complex structures that offer structural and mechanical support to plant cells and are ultimately responsible for plant architecture and form. Pectins are a large group of complex polysaccharides of the plant cell wall that are made in the Golgi and secreted to the wall. The methylesterification of pectins is believed to be an important factor for the dynamic properties of the cell wall. Here, we report on a protein of unknown function discovered using an extensive proteomics analysis of cotton Golgi. Through bioinformatic analyses, we identified the ortholog of such protein, here named cotton Golgi-related 3 (CGR3) in Arabidopsis and found that it shares conserved residues with S-adenosylmethionine methyltransferases. We established that CGR3 is localized at the Golgi apparatus and that the expression of the CGR3 gene is correlated with that of several cell wall biosynthetic genes, suggesting a possible role of the protein in pectin modifications. Consistent with this hypothesis, immunofluorescence microscopy with antibodies for homogalacturonan pectins (HG) indicated that the cell walls of cgr3 knockout mutants and plants overexpressing CGR3 are decreased and increased in HG methylesterification, respectively. Our results suggest that CGR3 plays a role in the methylesterification of homogalacturonan in Arabidopsis.

CESA5 is required for the synthesis of cellulose with a role in structuring the adherent mucilage of Arabidopsis seeds

Imbibed Arabidopsis (Arabidopsis thaliana) seeds are encapsulated by mucilage that is formed of hydrated polysaccharides released from seed coat epidermal cells. The mucilage is structured with water-soluble and adherent layers, with cellulose present uniquely in an inner domain of the latter. Using a reverse-genetic approach to identify the cellulose synthases (CESAs) that produce mucilage cellulose, cesa5 mutants were shown to be required for the correct formation of these layers. Expression of CESA5 in the seed coat was specific to epidermal cells and coincided with the accumulation of mucilage polysaccharides in their apoplast. Analysis of sugar composition showed that although total sugar composition or amounts were unchanged, their partition between layers was different in the mutant, with redistribution from adherent to water-soluble mucilage. The macromolecular characteristics of the water-soluble mucilage were also modified. In accordance with a role for CESA5 in mucilage cellulose synthesis, crystalline cellulose contents were reduced in mutant seeds and birefringent microfibrils were absent from adherent mucilage. Although the mucilage-modified5 mutant showed similar defects to cesa5 in the distribution of sugar components between water-soluble and adherent mucilage, labeling of residual adherent mucilage indicated that cesa5 contained less cellulose and less pectin methyl esterification. Together, the results demonstrate that CESA5 plays a major and essential role in cellulose production in seed mucilage, which is critical for the establishment of mucilage structured in layers and domains.