A β-glucuronosyltransferase from Arabidopsis thaliana involved in biosynthesis of type II arabinogalactan has a role in cell elongation during seedling growth

In vivo and ex vivo study on cell wall components as part of the network in tomato fruit during the ripening process

Ripening is a process involving various morphological, physiological, and biochemical changes in fruits. This process is affected by modifications in the cell wall structure, particularly in the composition of polysaccharides and proteins. The cell wall assembly is a network of polysaccharides and proteoglycans named the arabinoxylan pectin arabinogalactan protein1 (APAP1). The complex consists of the arabinogalactan protein (AGP) core with the pectin domain including arabinogalactan (AG) type II, homogalacturonan (HG), and rhamnogalacturonan I (RG-I). The present paper aims to determine the impact of a disturbance in the synthesis of one constituent on the integrity of the cell wall. Therefore, in the current work, we have tested the impact of modified expression of the SlP4H3 gene connected with proline hydroxylase (P4H) activity on AGP presence in the fruit matrix. Using an immunolabelling technique (CLSM), an immunogold method (TEM), molecular tools, and calcium mapping (SEM-EDS), we have demonstrated that disturbances in AGP synthesis affect the entire cell wall structure. Changes in the spatio-temporal AGP distribution may be related to the formation of a network between AGPs with other cell wall components. Moreover, the modified structure of the cell wall assembly induces morphological changes visible at the cellular level during the progression of the ripening process. These results support the hypothesis that AGPs and pectins are required for the proper progression of the physiological processes occurring in fruits.

Antagonistic functions of CTL1 and SUH1 mediate cell wall assembly in Arabidopsis

Plant genomes contain numerous genes encoding chitinase-like (CTL) proteins, which have a similar protein structure to chitinase belonging to the glycoside hydrolase (GH) family but lack the chitinolytic activity to cleave the β-1,4-glycosidic bond in chitins, polymers of N-acetylglucosamine. CTL1 mutations found in rice and Arabidopsis have caused pleiotropic developmental defects, including altered cell wall composition and decreased abiotic stress tolerance, likely due to reduced cellulose content. In this study, we identified suppressor of hot2 1 (suh1) as a genetic suppressor of the ctl1 hot2-1 mutation in Arabidopsis. The mutation in SUH1 restored almost all examined ctl1 hot2-1 defects to nearly wild-type levels or at least partially. SUH1 encodes a Golgi-located type II membrane protein with glycosyltransferase (GT) activity, and its mutations lead to a reduction in cellulose content and hypersensitivity to cellulose biosynthesis inhibitors, although to a lesser extent than ctl1 hot2-1 mutation. The SUH1 promoter fused with the GUS reporter gene exhibited GUS activity in interfascicular fibers and xylem in stems; meanwhile, the ctl1 hot2-1 mutation significantly increased this activity. Our findings provide genetic and molecular evidence that the antagonistic activities of CTL1 and SUH1 play an essential role in assembling the cell wall in Arabidopsis.

Disruption of a DUF247 Containing Protein Alters Cell Wall Polysaccharides and Reduces Growth in Arabidopsis

Plant cell wall biosynthesis is a complex process that requires proteins and enzymes from glycan synthesis to wall assembly. We show that disruption of At3g50120 (DUF247-1), a member of the DUF247 multigene family containing 28 genes in Arabidopsis, results in alterations to the structure and composition of cell wall polysaccharides and reduced growth and plant size. An ELISA using cell wall antibodies shows that the mutants also exhibit ~50% reductions in xyloglucan (XyG), glucuronoxylan (GX) and heteromannan (HM) epitopes in the NaOH fraction and ~50% increases in homogalacturonan (HG) epitopes in the CDTA fraction. Furthermore, the polymer sizes of XyGs and GXs are reduced with concomitant increases in short-chain polymers, while those of HGs and mHGs are slightly increased. Complementation using 35S:DUF247-1 partially recovers the XyG and HG content, but not those of GX and HM, suggesting that DUF247-1 is more closely associated with XyGs and HGs. DUF247-1 is expressed throughout Arabidopsis, particularly in vascular and developing tissues, and its disruption affects the expression of other gene members, indicating a regulatory control role within the gene family. Our results demonstrate that DUF247-1 is required for normal cell wall composition and structure and Arabidopsis growth.

Arabinogalactan protein polysaccharide chains are required for normal biogenesis of plasmodesmata

Arabinogalactan proteins (AGPs) are a plant-specific family of extracellular proteoglycans characterized by large and complex galactose-rich polysaccharide chains. Functional elucidation of AGPs, however, has been hindered by the high degree of redundancy of AGP genes. To uncover as yet unexplored roles of AGPs in Arabidopsis, a mutant of Hyp O-galactosyltransferase (HPGT), a critical enzyme that catalyzes the common initial step of Hyp-linked arabinogalactan chain biosynthesis, was used. Here we show, using the hpgt1,2,3 triple mutant, that a reduction in functional AGPs leads to a stomatal patterning defect in which two or more stomata are clustered together. This defect is attributed to increased and dysregulated symplastic transport following changes in plasmodesmata structure, such that highly permeable complex branched plasmodesmata with cavities in branching parts increased in the mutant. We also found that the hpgt1,2,3 mutation causes a reduction of cellulose in the cell wall and accumulation of pectin, which controls cell wall porosity. Our results highlight the importance of AGPs in the correct biogenesis of plasmodesmata, possibly acting through the regulation of cell wall properties surrounding the plasmodesmata.

Review: structure and modifications of arabinogalactan proteins (AGPs)

The aim of this report is to provide general information on the molecular structure and synthesis of arabinogalactan proteins (AGPs) in association to their physiological significance. Assessment of genetic modifications of the activity of enzymes involved in the AGP biosynthesis is an efficient tool to study AGP functions. Thus, P4H (prolyl 4 hydroxylase) mutants, GLCAT (β-glucuronosyltransferase) mutants, and GH43 (glycoside hydrolase family 43) mutants have been described. We focused on the overview of AGPs modifications observed at the molecular, cellular, and organ levels. Inhibition of the hydroxylation process results in an increase in the intensity of cell divisions and thus, has an impact on root system length and leaf area. In turn, overexpression of P4H genes stimulates the density of root hairs. A mutation in GLCAT genes responsible for the transfer of glucuronic acid to the AGP molecule revealed that the reduction of GlcA in AGP disrupts the substantial assembly of the primary cell wall. Furthermore, silencing of genes encoding GH43, which has the ability to hydrolyze the AGP glycan by removing incorrectly synthesized β-1,3-galactans, induces changes in the abundance of other cell wall constituents, which finally leads to root growth defects. This information provides insight into AGPs as a crucial players in the structural interactions present in the plant extracellular matrix.

A Golgi-localized glycosyltransferase, OsGT14;1, is required for growth of both roots and shoots in rice

Glycosyltransferases (GTs) form a large family in plants and are important enzymes for the synthesis of various polysaccharides, but only a few members have been functionally characterized. Here, through mutant screening with gene mapping, we found that an Oryza sativa (rice) mutant with a short-root phenotype was caused by a frame-shift mutation of a gene (OsGT14;1) belonging to the glycosyltransferase gene family 14. Further analysis indicated that the mutant also had a brittle culm and produced lower grain yield compared with wild-type rice, but the roots showed similar root structure and function in terms of the uptake of mineral nutrients. OsGT14;1 was broadly expressed in all organs throughout the entire growth period, with a relatively high expression in the roots, stems, node I and husk. Furthermore, OsGT14;1 was expressed in all tissues of these organs. Subcellular observation revealed that OsGT14;1 encoded a Golgi-localized protein. Mutation of OsGT14;1 resulted in decreased cellulose content and increased hemicellulose, but did not alter pectin in the cell wall of roots and shoots. The knockout of OsGT14;1 did not affect the tolerance to toxic mineral elements, including Al, As, Cd and salt stress, but did increase the sensitivity to low pH. Taken together, OsGT14;1 located at the Golgi is required for growth of both roots and shoots in rice through affecting cellulose synthesis.

Arabinogalactan Proteins: Focus on the Role in Cellulose Synthesis and Deposition during Plant Cell Wall Biogenesis

Arabinogalactan proteins (AGPs) belong to a family of glycoproteins that are widely present in plants. AGPs are mostly composed of a protein backbone decorated with complex carbohydrate side chains and are usually anchored to the plasma membrane or secreted extracellularly. A trickle of compelling biochemical and genetic evidence has demonstrated that AGPs make exciting candidates for a multitude of vital activities related to plant growth and development. However, because of the diversity of AGPs, functional redundancy of AGP family members, and blunt-force research tools, the precise functions of AGPs and their mechanisms of action remain elusive. In this review, we put together the current knowledge about the characteristics, classification, and identification of AGPs and make a summary of the biological functions of AGPs in multiple phases of plant reproduction and developmental processes. In addition, we especially discuss deeply the potential mechanisms for AGP action in different biological processes via their impacts on cellulose synthesis and deposition based on previous studies. Particularly, five hypothetical models that may explain the AGP involvement in cellulose synthesis and deposition during plant cell wall biogenesis are proposed. AGPs open a new avenue for understanding cellulose synthesis and deposition in plants.

Dietary Fiber in Plant Cell Walls—The Healthy Carbohydrates

Dietary fiber (DF) is one of the major classes of nutrients for humans. It is widely distributed in the edible parts of natural plants, with the cell wall being the main DF-containing structure. The DF content varies significantly in different plant species and organs, and the processing procedure can have a dramatic effect on the DF composition of plant-based foods. Given the considerable nutritional value of DF, a deeper understanding of DF in food plants, including its composition and biosynthesis, is fundamental to the establishment of a daily intake reference of DF and is also critical to molecular breeding programs for modifying DF content. In the past decades, plant cell wall biology has seen dramatic progress, and such knowledge is of great potential to be translated into DF-related food science research and may provide future research directions for improving the health benefits of food crops. In this review, to spark interdisciplinary discussions between food science researchers and plant cell wall biologists, we focus on a specific category of DF—cell wall carbohydrates. We first summarize the content and composition of carbohydrate DF in various plant-based foods, and then discuss the structure and biosynthesis mechanism of each carbohydrate DF category, in particular the respective biosynthetic enzymes. Health impacts of DF are highlighted, and finally, future directions of DF research are also briefly outlined.

Glucuronidation of type II arabinogalactan polysaccharides function in sexual reproduction of Arabidopsis

Arabinogalactan proteins (AGPs) are complex, hyperglycosylated plant cell wall proteins with little known about the biological roles of their glycan moieties in sexual reproduction. Here, we report that GLCAT14A, GLCAT14B, and GLCAT14C, three enzymes responsible for the addition of glucuronic acid residues to AGPs, function in pollen development, polytubey block, and normal embryo development in Arabidopsis. Using biochemical and immunolabeling techniques, we demonstrated that the loss of function of the GLCAT14A, GLCAT14B, and GLCAT14C genes resulted in disorganization of the reticulate structure of the exine wall, abnormal development of the intine layer, and collapse of pollen grains in glcat14a/b and glcat14a/b/c mutants. Synchronous development between locules within the same anther was also lost in some glcat14a/b/c stamens. In addition, we observed excessive attraction of pollen tubes targeting glcat14a/b/c ovules, indicating that the polytubey block mechanism was compromised. Monosaccharide composition analysis revealed significant reductions in all sugars in glcat14a/b and glcat14a/b/c mutants except for arabinose and galactose, while immunolabeling showed decreased amounts of AGP sugar epitopes recognized by glcat14a/b and glcat14a/b/c mutants compared with the wild type. This work demonstrates the important roles that AG glucuronidation plays in Arabidopsis sexual reproduction and reproductive development.

Transcriptomics Reveals Host-Dependent Differences of Polysaccharides Biosynthesis in Cynomorium songaricum

Cynomorium songaricum is a root holoparasitic herb that is mainly hosted in the roots of Nitraria roborowskii and Nitraria sibirica distributed in the arid desert and saline-alkaline regions. The stem of C. songaricum is widely used as a traditional Chinese medicine and applied in anti-viral, anti-obesity and anti-diabetes, which largely rely on the bioactive components including: polysaccharides, flavonoids and triterpenes. Although the differences in growth characteristics of C. songaricum between N. roborowskii and N. sibirica have been reported, the difference of the two hosts on growth and polysaccharides biosynthesis in C. songaricum as well as regulation mechanism are not limited. Here, the physiological characteristics and transcriptome of C. songaricum host in N. roborowskii (CR) and N. sibirica (CS) were conducted. The results showed that the fresh weight, soluble sugar content and antioxidant capacity on a per stem basis exhibited a 3.3-, 3.0- and 2.1-fold increase in CR compared to CS. A total of 16,921 differentially expressed genes (DEGs) were observed in CR versus CS, with 2573 characterized genes, 1725 up-regulated and 848 down-regulated. Based on biological functions, 50 DEGs were associated with polysaccharides and starch metabolism as well as their transport. The expression levels of the selected 37 genes were validated by qRT-PCR and almost consistent with their Reads Per kb per Million values. These findings would provide useful references for improving the yield and quality of C. songaricum.

Functional characterization of hydroxyproline-O-galactosyltransferases for Arabidopsis arabinogalactan-protein synthesis

BACKGROUND

Arabinogalactan-proteins (AGPs) are structurally complex hydroxyproline-rich cell wall glycoproteins ubiquitous in the plant kingdom. AGPs biosynthesis involves a series of post-translational modifications including the addition of type II arabinogalactans to non-contiguous Hyp residues. To date, eight Hyp-galactosyltransferases (Hyp-GALTs; GALT2-GALT9) belonging to CAZy GT31, are known to catalyze the addition of the first galactose residues to AGP protein backbones and enable subsequent AGP glycosylation. The extent of genetic redundancy, however, remains to be elucidated for the Hyp-GALT gene family.

RESULTS

To examine their gene redundancy and functions, we generated various multiple gene knock-outs, including a triple mutant (galt5 galt8 galt9), two quadruple mutants (galt2 galt5 galt7 galt8, galt2 galt5 galt7 galt9), and one quintuple mutant (galt2 galt5 galt7 galt8 galt9), and comprehensively examined their biochemical and physiological phenotypes. The key findings include: AGP precipitations with β-Yariv reagent showed that GALT2, GALT5, GALT7, GALT8 and GALT9 act redundantly with respect to AGP glycosylation in cauline and rosette leaves, while the activity of GALT7, GALT8 and GALT9 dominate in the stem, silique and flowers. Monosaccharide composition analysis showed that galactose was decreased in the silique and root AGPs of the Hyp-GALT mutants. TEM analysis of 25789 quintuple mutant stems indicated cell wall defects coincident with the observed developmental and growth impairment in these Hyp-GALT mutants. Correlated with expression patterns, galt2, galt5, galt7, galt8, and galt9 display equal additive effects on insensitivity to β-Yariv-induced growth inhibition, silique length, plant height, and pollen viability. Interestingly, galt7, galt8, and galt9 contributed more to primary root growth and root tip swelling under salt stress, whereas galt2 and galt5 played more important roles in seed morphology, germination defects and seed set. Pollen defects likely contributed to the reduced seed set in these mutants.

CONCLUSION

Additive and pleiotropic effects of GALT2, GALT5, GALT7, GALT8 and GALT9 on vegetative and reproductive growth phenotypes were teased apart via generation of different combinations of Hyp-GALT knock-out mutants. Taken together, the generation of higher order Hyp-GALT mutants demonstrate the functional importance of AG polysaccharides decorating the AGPs with respect to various aspects of plant growth and development.

GTDB: an integrated resource for glycosyltransferase sequences and annotations

Glycosyltransferases (GTs), a large class of carbohydrate-active enzymes, adds glycosyl moieties to various substrates to generate multiple bioactive compounds, including natural products with pharmaceutical or agrochemical values. Here, we first collected comprehensive information on GTs, including amino acid sequences, coding region sequences, available tertiary structures, protein classification families, catalytic reactions and metabolic pathways. Then, we developed sequence search and molecular docking processes for GTs, resulting in a GTs database (GTDB). In the present study, 520 179 GTs from approximately 21 647 species that involved in 394 kinds of different reactions were deposited in GTDB. GTDB has the following useful features: (i) text search is provided for retrieving the complete details of a query by combining multiple identifiers and data sources; (ii) a convenient browser allows users to browse data by different classifications and download data in batches; (iii) BLAST is offered for searching against pre-defined sequences, which can facilitate the annotation of the biological functions of query GTs; and lastly, (iv) GTdock using AutoDock Vina performs docking simulations of several GTs with the same single acceptor and displays the results based on 3Dmol.js allowing easy view of models.

Three β-Glucuronosyltransferase Genes Involved in Arabinogalactan Biosynthesis Function in Arabidopsis Growth and Development

Arabinogalactan proteins (AGPs) contain arabinogalactan (AG) polysaccharides that are biologically relevant to plant growth processes. Here, the biochemical and physiological roles of three Golgi localized β-glucuronosyltransferase genes (GLCAT14A, GLCAT14B and GLCAT14C) in Arabidopsis thaliana, responsible for the addition of glucuronic acid to AG chains, were further investigated using single, double and triple glcat14 mutant plants. These proteins were localized to the Golgi apparatus when transiently expressed in Nicotiana benthamiana. Sugar analysis of AGP extracts from Arabidopsis stem, leaf and siliques showed a consistent reduction in glucuronic acid in glcat14 mutants relative to wild type, with concomitant effects resulting in tissue-specific alterations, especially in arabinose and galactose sugars. Although we observed defects in trichome branching in glca14a/b and glca14a/b/c mutants, scanning electron microscope analysis/energy dispersive microanalysis (SEM/EDX) showed no difference in the calcium content of trichomes in these mutants relative to wild type. Immunoblot analyses of the stem and leaf showed a reduction in AGPs as detected with the LM2 antibody in glcat14a/b and glcat14a/b/c mutants relative to wild type. The current work exemplifies the possibility of conducting structure-function assessment of cell wall biosynthetic genes to identify their physiological roles in plant growth and development.

Coffee cell walls—composition, influence on cup quality and opportunities for coffee improvements

The coffee beverage is the second most consumed drink worldwide after water. In coffee beans, cell wall storage polysaccharides (CWSPs) represent around 50 per cent of the seed dry mass, mainly consisting of galactomannans and arabinogalactans. These highly abundant structural components largely influence the organoleptic properties of the coffee beverage, mainly due to the complex changes they undergo during the roasting process. From a nutritional point of view, coffee CWSPs are soluble dietary fibers shown to provide numerous health benefits in reducing the risk of human diseases. Due to their influence on coffee quality and their health-promoting benefits, CWSPs have been attracting significant research attention. The importance of cell walls to the coffee industry is not restricted to beans used for beverage production, as several coffee by-products also present high concentrations of cell wall components. These by-products include cherry husks, cherry pulps, parchment skin, silver skin, and spent coffee grounds, which are currently used or have the potential to be utilized either as food ingredients or additives, or for the generation of downstream products such as enzymes, pharmaceuticals, and bioethanol. In addition to their functions during plant development, cell walls also play a role in the plant’s resistance to stresses. Here, we review several aspects of coffee cell walls, including chemical composition, biosynthesis, their function in coffee’s responses to stresses, and their influence on coffee quality. We also propose some potential cell wall–related biotechnological strategies envisaged for coffee improvements.

Two β-glucuronosyltransferases involved in the biosynthesis of type II arabinogalactans function in mucilage polysaccharide matrix organization in Arabidopsis thaliana

BACKGROUND

Arabinogalactan-proteins (AGPs) are heavily glycosylated with type II arabinogalactan (AG) polysaccharides attached to hydroxyproline residues in their protein backbone. Type II AGs are necessary for plant growth and critically important for the establishment of normal cellular functions. Despite the importance of type II AGs in plant development, our understanding of the underlying role of these glycans/sugar residues in mucilage formation and seed coat epidermal cell development is poorly understood and far from complete. One such sugar residue is the glucuronic acid residues of AGPs that are transferred onto AGP glycans by the action of β-glucuronosyltransferase genes/enzymes.

RESULTS

Here, we have characterized two β-glucuronosyltransferase genes, GLCAT14A and GLCAT14C, that are involved in the transfer of β-glucuronic acid (GlcA) to type II AGs. Using a reverse genetics approach, we observed that glcat14a-1 mutants displayed subtle alterations in mucilage pectin homogalacturonan (HG) compared to wild type (WT), while glcat14a-1glcat14c-1 mutants displayed much more severe mucilage phenotypes, including loss of adherent mucilage and significant alterations in cellulose ray formation and seed coat morphology. Monosaccharide composition analysis showed significant alterations in the sugar amounts of glcat14a-1glcat14c-1 mutants relative to WT in the adherent and non-adherent seed mucilage. Also, a reduction in total mucilage content was observed in glcat14a-1glcat14c-1 mutants relative to WT. In addition, glcat14a-1glcat14c-1 mutants showed defects in pectin formation, calcium content and the degree of pectin methyl-esterification (DM) as well as reductions in crystalline cellulose content and seed size.

CONCLUSIONS

These results raise important questions regarding cell wall polymer interactions and organization during mucilage formation. We propose that the enzymatic activities of GLCAT14A and GLCAT14C play partially redundant roles and are required for the organization of the mucilage matrix and seed size in Arabidopsis thaliana. This work brings us a step closer towards identifying potential gene targets for engineering plant cell walls for industrial applications.

Biochemical and Functional Characterization of GALT8, an Arabidopsis GT31 β-(1,3)-Galactosyltransferase That Influences Seedling Development

Arabinogalactan-proteins (AGPs) are members of the hydroxyproline-rich glycoprotein (HRGP) superfamily, a group of highly diverse proteoglycans that are present in the cell wall, plasma membrane as well as secretions of almost all plants, with important roles in many developmental processes. The role of GALT8 (At1g22015), a Glycosyltransferase-31 (GT31) family member of the Carbohydrate-Active Enzyme database (CAZy), was examined by biochemical characterization and phenotypic analysis of a galt8 mutant line. To characterize its catalytic function, GALT8 was heterologously expressed in tobacco leaves and its enzymatic activity tested. GALT8 was shown to be a β-(1,3)-galactosyltransferase (GalT) that catalyzes the synthesis of a β-(1,3)-galactan, similar to the in vitro activity of KNS4/UPEX1 (At1g33430), a homologous GT31 member previously shown to have this activity. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) confirmed the products were of 2-6 degree of polymerisation (DP). Previous reporter studies showed that GALT8 is expressed in the central and synergid cells, from whence the micropylar endosperm originates after the fertilization of the central cell of the ovule. Homozygous mutants have multiple seedling phenotypes including significantly shorter hypocotyls and smaller leaf area compared to wild type (WT) that are attributable to defects in female gametophyte and/or endosperm development. KNS4/UPEX1 was shown to partially complement the galt8 mutant phenotypes in genetic complementation assays suggesting a similar but not identical role compared to GALT8 in β-(1,3)-galactan biosynthesis. Taken together, these data add further evidence of the important roles GT31 β-(1,3)-GalTs play in elaborating type II AGs that decorate AGPs and pectins, thereby imparting functional consequences on plant growth and development.

Cracking the "Sugar Code": A Snapshot of N- and O-Glycosylation Pathways and Functions in Plants Cells

Glycosylation is a fundamental co-translational and/or post-translational modification process where an attachment of sugars onto either proteins or lipids can alter their biological function, subcellular location and modulate the development and physiology of an organism. Glycosylation is not a template driven process and as such produces a vastly larger array of glycan structures through combinatorial use of enzymes and of repeated common scaffolds and as a consequence it provides a huge expansion of both the proteome and lipidome. While the essential role of N- and O-glycan modifications on mammalian glycoproteins is already well documented, we are just starting to decode their biological functions in plants. Although significant advances have been made in plant glycobiology in the last decades, there are still key challenges impeding progress in the field and, as such, holistic modern high throughput approaches may help to address these conceptual gaps. In this snapshot, we present an update of the most common O- and N-glycan structures present on plant glycoproteins as well as (1) the plant glycosyltransferases (GTs) and glycosyl hydrolases (GHs) responsible for their biosynthesis; (2) a summary of microorganism-derived GHs characterized to cleave specific glycosidic linkages; (3) a summary of the available tools ranging from monoclonal antibodies (mAbs), lectins to chemical probes for the detection of specific sugar moieties within these complex macromolecules; (4) selected examples of N- and O-glycoproteins as well as in their related GTs to illustrate the complexity on their mode of action in plant cell growth and stress responses processes, and finally (5) we present the carbohydrate microarray approach that could revolutionize the way in which unknown plant GTs and GHs are identified and their specificities characterized.

A Pipeline towards the Biochemical Characterization of the Arabidopsis GT14 Family

Glycosyltransferases (GTs) catalyze the synthesis of glycosidic linkages and are essential in the biosynthesis of glycans, glycoconjugates (glycolipids and glycoproteins), and glycosides. Plant genomes generally encode many more GTs than animal genomes due to the synthesis of a cell wall and a wide variety of glycosylated secondary metabolites. The Arabidopsis thaliana genome is predicted to encode over 573 GTs that are currently classified into 42 diverse families. The biochemical functions of most of these GTs are still unknown. In this study, we updated the JBEI Arabidopsis GT clone collection by cloning an additional 105 GT cDNAs, 508 in total (89%), into Gateway-compatible vectors for downstream characterization. We further established a functional analysis pipeline using transient expression in tobacco (Nicotiana benthamiana) followed by enzymatic assays, fractionation of enzymatic products by reversed-phase HPLC (RP-HPLC) and characterization by mass spectrometry (MS). Using the GT14 family as an exemplar, we outline a strategy for identifying effective substrates of GT enzymes. By addition of UDP-GlcA as donor and the synthetic acceptors galactose-nitrobenzodiazole (Gal-NBD), β-1,6-galactotetraose (β-1,6-Gal₄) and β-1,3-galactopentose (β-1,3-Gal₅) to microsomes expressing individual GT14 enzymes, we verified the β-glucuronosyltransferase (GlcAT) activity of three members of this family (AtGlcAT14A, B, and E). In addition, a new family member (AT4G27480, 248) was shown to possess significantly higher activity than other GT14 enzymes. Our data indicate a likely role in arabinogalactan-protein (AGP) biosynthesis for these GT14 members. Together, the updated Arabidopsis GT clone collection and the biochemical analysis pipeline present an efficient means to identify and characterize novel GT catalytic activities.

CRISPR-Cas9 multiplex genome editing of the hydroxyproline-O-galactosyltransferase gene family alters arabinogalactan-protein glycosylation and function in Arabidopsis

BACKGROUND

Arabinogalactan-proteins (AGPs) are a class of hydroxyproline-rich proteins (HRGPs) that are heavily glycosylated (> 90%) with type II arabinogalactans (AGs). AGPs are implicated in various plant growth and development processes including cell expansion, somatic embryogenesis, root and stem growth, salt tolerance, hormone signaling, male and female gametophyte development, and defense. To date, eight Hyp-O-galactosyltransferases (GALT2-6, HPGT1-3) have been identified; these enzymes are responsible for adding the first sugar, galactose, onto AGPs. Due to gene redundancy among the GALTs, single or double galt genetic knockout mutants are often not sufficient to fully reveal their biological functions.

RESULTS

Here, we report the successful application of CRISPR-Cas9 gene editing/multiplexing technology to generate higher-order knockout mutants of five members of the GALT gene family (GALT2-6). AGPs analysis of higher-order galt mutants (galt2 galt5, galt3 galt4 galt6, and galt2 galt3 galt4 galt5 gal6) demonstrated significantly less glycosylated AGPs in rosette leaves, stems, and siliques compared to the corresponding wild-type organs. Monosaccharide composition analysis of AGPs isolated from rosette leaves revealed significant decreases in arabinose and galactose in all the higher-order galt mutants. Phenotypic analyses revealed that mutation of two or more GALT genes was able to overcome the growth inhibitory effect of β-D-Gal-Yariv reagent, which specifically binds to β-1,3-galactan backbones on AGPs. In addition, the galt2 galt3 galt4 galt5 gal6 mutant exhibited reduced overall growth, impaired root growth, abnormal pollen, shorter siliques, and reduced seed set. Reciprocal crossing experiments demonstrated that galt2 galt3 galt4 galt5 gal6 mutants had defects in the female gametophyte which were responsible for reduced seed set.

CONCLUSIONS

Our CRISPR/Cas9 gene editing/multiplexing approach provides a simpler and faster way to generate higher-order mutants for functional characterization compared to conventional genetic crossing of T-DNA mutant lines. Higher-order galt mutants produced and characterized in this study provide insight into the relationship between sugar decorations and the various biological functions attributed to AGPs in plants.

The Spatio-Temporal Distribution of Cell Wall-Associated Glycoproteins During Wood Formation in Populus

Plant cell wall associated hydroxyproline-rich glycoproteins (HRGPs) are involved in several aspects of plant growth and development, including wood formation in trees. HRGPs such as arabinogalactan-proteins (AGPs), extensins (EXTs), and proline rich proteins (PRPs) are important for the development and architecture of plant cell walls. Analysis of publicly available gene expression data revealed that many HRGP encoding genes show tight spatio-temporal expression patterns in the developing wood of Populus that are indicative of specific functions during wood formation. Similar results were obtained for the expression of glycosyl transferases putatively involved in HRGP glycosylation. In situ immunolabelling of transverse wood sections using AGP and EXT antibodies revealed the cell type specificity of different epitopes. In mature wood AGP epitopes were located in xylem ray cell walls, whereas EXT epitopes were specifically observed between neighboring xylem vessels, and on the ray cell side of the vessel walls, likely in association with pits. Molecular mass and glycan analysis of AGPs and EXTs in phloem/cambium, developing xylem, and mature xylem revealed clear differences in glycan structures and size between the tissues. Separation of AGPs by agarose gel electrophoresis and staining with β-D-glucosyl Yariv confirmed the presence of different AGP populations in phloem/cambium and xylem. These results reveal the diverse changes in HRGP-related processes that occur during wood formation at the gene expression and HRGP glycan biosynthesis levels, and relate HRGPs and glycosylation processes to the developmental processes of wood formation.

Three Decades of Advances in Arabinogalactan-Protein Biosynthesis

Arabinogalactan-proteins (AGPs) are a large, complex, and highly diverse class of heavily glycosylated proteins that belong to the family of cell wall hydroxyproline-rich glycoproteins. Approximately 90% of the molecules consist of arabinogalactan polysaccharides, which are composed of arabinose and galactose as major sugars and minor sugars such as glucuronic acid, fucose, and rhamnose. About half of the AGP family members contain a glycosylphosphatidylinositol (GPI) lipid anchor, which allows for an association with the outer leaflet of the plasma membrane. The mysterious AGP family has captivated the attention of plant biologists for several decades. This diverse family of glycoproteins is widely distributed in the plant kingdom, including many algae, where they play fundamental roles in growth and development processes. The journey of AGP biosynthesis begins with the assembly of amino acids into peptide chains of proteins. An N-terminal signal peptide directs AGPs toward the endoplasmic reticulum, where proline hydroxylation occurs and a GPI anchor may be added. GPI-anchored AGPs, as well as unanchored AGPs, are then transferred to the Golgi apparatus, where extensive glycosylation occurs by the action of a variety glycosyltransferase enzymes. Following glycosylation, AGPs are transported by secretory vesicles to the cell wall or to the extracellular face of the plasma membrane (in the case of GPI-anchored AGPs). GPI-anchored proteins can be released from the plasma membrane into the cell wall by phospholipases. In this review, we present an overview of the accumulated knowledge on AGP biosynthesis over the past three decades. Particular emphasis is placed on the glycosylation of AGPs as the sugar moiety is essential to their function. Recent genetics and genomics approaches have significantly contributed to a broader knowledge of AGP biosynthesis. However, many questions remain to be elucidated in the decades ahead.

To gel or not to gel: differential expression of carrageenan-related genes between the gametophyte and tetasporophyte life cycle stages of the red alga Chondrus crispus

Chondrus crispus is a marine red alga with sulfated galactans, called carrageenans, in its extracellular matrix. Chondrus has a complex haplodiplontic life cycle, alternating between male and female gametophytes (n) and tetrasporophytes (2n). The Chondrus life cycle stages are isomorphic; however, a major phenotypic difference is that carrageenan composition varies significantly between the tetrasporophytes (mainly lambda-carrageenan) and the gametophytes (mainly kappa/iota-carrageenans). The disparity in carrageenan structures, which confer different chemical properties, strongly suggests differential regulation of carrageenan-active genes between the phases of the Chondrus life cycles. We used a combination of taxonomy, biochemistry and molecular biology to characterize the tetrasporophytes and male and female gametophytes from Chondrus individuals isolated from the rocky seashore off the northern coast of France. Transcriptomic analyses reveal differential gene expression of genes encoding several galactose-sulfurylases, carbohydrate-sulfotransferases, glycosyltransferases, and one family 16 glycoside hydrolase. Differential expression of carrageenan-related genes was found primarily between gametophytes and tetrasporophytes, but also between the male and female gametophytes. The differential expression of these multigenic genes provides a rare glimpse into cell wall biosynthesis in algae. Furthermore, it strongly supports that carrageenan metabolism holds an important role in the physiological differentiation between the isomorphic life cycle stages of Chondrus.

Functional Validation of Two Fungal Subfamilies in Carbohydrate Esterase Family 1 by Biochemical Characterization of Esterases From Uncharacterized Branches

The fungal members of Carbohydrate Esterase family 1 (CE1) from the CAZy database include both acetyl xylan esterases (AXEs) and feruloyl esterases (FAEs). AXEs and FAEs are essential auxiliary enzymes to unlock the full potential of feedstock. They are being used in many biotechnology applications including food and feed, pulp and paper, and biomass valorization. AXEs catalyze the hydrolysis of acetyl group from xylan, while FAEs release ferulic and other hydroxycinnamic acids from xylan and pectin. Previously, we reported a phylogenetic analysis for the fungal members of CE1, establishing five subfamilies (CE1_SF1–SF5). Currently, the characterized AXEs are in the subfamily CE1_SF1, whereas CE1_SF2 contains mainly characterized FAEs. These two subfamilies are more related to each other than to the other subfamilies and are predicted to have evolved from a common ancestor, but target substrates with a different molecular structure. In this study, four ascomycete enzymes from CE1_SF1 and SF2 were heterologously produced in Pichia pastoris and characterized with respect to their biochemical properties and substrate preference toward different model and plant biomass substrates. The selected enzymes from CE1_SF1 only exhibited AXE activity, whereas the one from CE1_SF2 possessed dual FAE/AXE activity. This dual activity enzyme also showed broad substrate specificity toward model substrates for FAE activity and efficiently released both acetic acid and ferulic acid (∼50%) from wheat arabinoxylan and wheat bran which was pre-treated with a commercial xylanase. These fungal AXEs and FAEs also showed promising biochemical properties, e.g., high stability over a wide pH range and retaining more than 80% of their residual activity at pH 6.0–9.0. These newly characterized fungal AXEs and FAEs from CE1 have high potential for biotechnological applications. In particular as an additional ingredient for enzyme cocktails to remove the ester-linked decorations which enables access for the backbone degrading enzymes. Among these novel enzymes, the dual FAE/AXE activity enzyme also supports the evolutionary relationship of CE1_SF1 and SF2.

The Rice BZ1 Locus Is Required for Glycosylation of Arabinogalactan Proteins and Galactolipid and Plays a Role in both Mechanical Strength and Leaf Color

BACKGROUND

The cell wall and chloroplast are two fundamental structures determining plant mechanical strength and grain yield. Therefore, understanding mechanisms that improve plants' ability to develop a robust cell wall and well-developed chloroplast is of utmost importance for agricultural activities.

RESULTS

In this study, we report the functional characterization of a novel rice mutant, brittle stem and zebra leaf (bz1), which displays altered cell wall composition and collapsed chloroplast membrane. Molecular and biochemical analysis revealed that BZ1 encodes a functional UDP-galactose/glucose epimerase (UGE) and is ubiquitously expressed with higher expression in stem and leaf tissues. Multiple techniques analyses, including immunoblots, immuno-gold, and cryogenic scanning electron microscopy, demonstrated a significantly impaired glycosylation of arabinogalactan proteins (AGPs) and disordered cellulose microfibril deposition in bz1. Lipid profiling assay showed that the amount of monogalactosyldiacylglycerols (MGDG), a major chloroplast membrane glycolipid, was significantly decreased in bz1. Taken together, these results strongly demonstrate that BZ1 participates in UDP-galactose supply for the sugar chains biosynthesis of AGPs and MGDG, which thereby, respectively, results in altered cell wall and abnormal chloroplast development. Due to inferior mechanical strength and reduced photosynthesis, bz1 plants displayed detrimental agronomic traits, whereas BZ1 overexpressing lines showed enhanced plant growth. Transcriptome analysis of stems and leaves further showed that numerous key genes involved in AGPs biosynthesis and photosynthesis metabolism were substantially suppressed in bz1.

CONCLUSIONS

Our finding identifies BZ1 as a dual-targeting UGE protein for glycosylation of AGPs and MGDG and suggests a strategy for breeding robust elite crops.

Golgi-localized exo-β1,3-galactosidases involved in cell expansion and root growth in Arabidopsis

Plant arabinogalactan proteins (AGPs) are a diverse group of cell surface- and wall-associated glycoproteins. Functionally important AGP glycans are synthesized in the Golgi apparatus, but the relationships among their glycosylation levels, processing, and functionalities are poorly understood. Here, we report the identification and functional characterization of two Golgi-localized exo-β-1,3-galactosidases from the glycosyl hydrolase 43 (GH43) family in Arabidopsis thaliana GH43 loss-of-function mutants exhibited root cell expansion defects in sugar-containing growth media. This root phenotype was associated with an increase in the extent of AGP cell wall association, as demonstrated by Yariv phenylglycoside dye quantification and comprehensive microarray polymer profiling of sequentially extracted cell walls. Characterization of recombinant GH43 variants revealed that the exo-β-1,3-galactosidase activity of GH43 enzymes is hindered by β-1,6 branches on β-1,3-galactans. In line with this steric hindrance, the recombinant GH43 variants did not release galactose from cell wall-extracted glycoproteins or AGP-rich gum arabic. These results indicate that the lack of exo-β-1,3-galactosidase activity alters cell wall extensibility in roots, a phenotype that could be explained by the involvement of galactosidases in AGP glycan biosynthesis.

Elucidating the roles of three β-glucuronosyltransferases (GLCATs) acting on arabinogalactan-proteins using a CRISPR-Cas9 multiplexing approach in Arabidopsis

BACKGROUND

Arabinogalactan-proteins (AGPs) are one of the most complex protein families in the plant kingdom and are present in the cell walls of all land plants. AGPs are implicated in diverse biological processes such as plant growth, development, reproduction, and stress responses. AGPs are extensively glycosylated by the addition of type II arabinogalactan (AG) polysaccharides to hydroxyproline residues in their protein cores. Glucuronic acid (GlcA) is the only negatively charged sugar added to AGPs and the functions of GlcA residues on AGPs remain to be elucidated.

RESULTS

Three members of the CAZy GT14 family (GLCAT14A-At5g39990, GLCAT14B-At5g15050, and GLCAT14C-At2g37585), which are responsible for transferring glucuronic acid (GlcA) to AGPs, were functionally characterized using a CRISPR/Cas9 gene editing approach in Arabidopsis. RNA seq and qRT-PCR data showed all three GLCAT genes were broadly expressed in different plant tissues, with GLCAT14A and GLCAT14B showing particularly high expression in the micropylar endosperm. Biochemical analysis of the AGPs from knock-out mutants of various glcat single, double, and triple mutants revealed that double and triple mutants generally had small increases of Ara and Gal and concomitant reductions of GlcA, particularly in the glcat14a glcat14b and glcat14a glcat14b glcat14c mutants. Moreover, AGPs isolated from all the glcat mutants displayed significant reductions in calcium binding compared to WT. Further phenotypic analyses found that the glcat14a glcat14b and glcat14a glcat14b glcat14c mutants exhibited significant delays in seed germination, reductions in root hair length, reductions in trichome branching, and accumulation of defective pollen grains. Additionally, both glcat14b glcat14c and glcat14a glcat14b glcat14c displayed significantly shorter siliques and reduced seed set. Finally, all higher-order mutants exhibited significant reductions in adherent seed coat mucilage.

CONCLUSIONS

This research provides genetic evidence that GLCAT14A-C function in the transfer of GlcA to AGPs, which in turn play a role in a variety of biochemical and physiological phenotypes including calcium binding by AGPs, seed germination, root hair growth, trichome branching, pollen development, silique development, seed set, and adherent seed coat mucilage accumulation.

Engineering of Bioenergy Crops: Dominant Genetic Approaches to Improve Polysaccharide Properties and Composition in Biomass

Large-scale, sustainable production of lignocellulosic bioenergy from biomass will depend on a variety of dedicated bioenergy crops. Despite their great genetic diversity, prospective bioenergy crops share many similarities in the polysaccharide composition of their cell walls, and the changes needed to optimize them for conversion are largely universal. Therefore, biomass modification strategies that do not depend on genetic background or require mutant varieties are extremely valuable. Due to their preferential fermentation and conversion by microorganisms downstream, the ideal bioenergy crop should contain a high proportion of C6-sugars in polysaccharides like cellulose, callose, galactan, and mixed-linkage glucans. In addition, the biomass should be reduced in inhibitors of fermentation like pentoses and acetate. Finally, the overall complexity of the plant cell wall should be modified to reduce its recalcitrance to enzymatic deconstruction in ways that do no compromise plant health or come at a yield penalty. This review will focus on progress in the use of a variety of genetically dominant strategies to reach these ideals. Due to the breadth and volume of research in the field of lignin bioengineering, this review will instead focus on approaches to improve polysaccharide component plant biomass. Carbohydrate content can be dramatically increased by transgenic overexpression of enzymes involved in cell wall polysaccharide biosynthesis. Additionally, the recalcitrance of the cell wall can be reduced via the overexpression of native or non-native carbohydrate active enzymes like glycosyl hydrolases or carbohydrate esterases. Some research in this area has focused on engineering plants that accumulate cell wall-degrading enzymes that are sequestered to organelles or only active at very high temperatures. The rationale being that, in order to avoid potential negative effects of cell wall modification during plant growth, the enzymes could be activated post-harvest, and post-maturation of the cell wall. A potentially significant limitation of this approach is that at harvest, the cell wall is heavily lignified, making the substrates for these enzymes inaccessible and their activity ineffective. Therefore, this review will only include research employing enzymes that are at least partially active under the ambient conditions of plant growth and cell wall development.

On the Potential Function of Type II Arabinogalactan O-Glycosylation in Regulating the Fate of Plant Secretory Proteins

In a plant-specific mode of protein glycosylation, various sugars and glycans are attached to hydroxyproline giving rise to a variety of diverse O-glycoproteins. The sub-family of arabinogalactan proteins is implicated in a multitude of biological functions, however, the mechanistic role of O-glycosylation on AGPs by type II arabinogalactans is largely elusive. Some models suggest roles of the O-glycans such as in ligand-receptor interactions and as localized calcium ion store. Structurally different but possibly analogous types of protein O-glycosylation exist in animal and yeast models and roles for O-glycans were suggested in determining the fate of O-glycoproteins by affecting intracellular sorting or proteolytic activation and degradation. At present, only few examples exist that describe how the fate of artificial and endogenous arabinogalactan proteins is affected by O-glycosylation with type II arabinogalactans. In addition to other roles, these glycans might act as a molecular determinant for cellular localization and protein lifetime of many endogenous proteins.

CRISPR/Cas9 Genome Editing Technology: A Valuable Tool for Understanding Plant Cell Wall Biosynthesis and Function

For the past 5 years, clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) technology has appeared in the molecular biology research spotlight. As a game-changing player in genome editing, CRISPR/Cas9 technology has revolutionized animal research, including medical research and human gene therapy as well as plant science research, particularly for crop improvement. One of the most common applications of CRISPR/Cas9 is to generate genetic knock-out mutants. Recently, several multiplex genome editing approaches utilizing CRISPR/Cas9 were developed and applied in various aspects of plant research. Here we summarize these approaches as they relate to plants, particularly with respect to understanding the biosynthesis and function of the plant cell wall. The plant cell wall is a polysaccharide-rich cell structure that is vital to plant cell formation, growth, and development. Humans are heavily dependent on the byproducts of the plant cell wall such as shelter, food, clothes, and fuel. Genes involved in the assembly of the plant cell wall are often highly redundant. To identify these redundant genes, higher-order knock-out mutants need to be generated, which is conventionally done by genetic crossing. Compared with genetic crossing, CRISPR/Cas9 multi-gene targeting can greatly shorten the process of higher-order mutant generation and screening, which is especially useful to characterize cell wall related genes in plant species that require longer growth time. Moreover, CRISPR/Cas9 makes it possible to knock out genes when null T-DNA mutants are not available or are genetically linked. Because of these advantages, CRISPR/Cas9 is becoming an ideal and indispensable tool to perform functional studies in plant cell wall research. In this review, we provide perspectives on how to design CRISPR/Cas9 to achieve efficient gene editing and multi-gene targeting in plants. We also discuss the recent development of the virus-based CRISPR/Cas9 system and the application of CRISPR/Cas9 to knock in genes. Lastly, we summarized current progress on using CRISPR/Cas9 for the characterization of plant cell wall-related genes.

Review: Plant cell wall biochemical omics: The high-throughput biochemistry for polysaccharide biosynthesis

Progress in the functional biochemical analysis of plant glycosyltransferases (GTs) has been slow because plant GTs are generally membrane proteins, operate as part of larger, multimeric complexes, and utilize a vast complexity of substrate acceptors. Therefore, the field would benefit from development of adequate high throughput expression as well as product detection and characterization techniques. Here we review current approaches to tackle such obstacles and suggest a new path forward: nucleic acid programmable protein arrays (NAPPA) with liquid sample desorption ionization (LS-DESI-MS) mass spectrometry. NAPPA utilizes in vitro transcription and translation to produce epitope-tagged fusion proteins from cloned GT cDNAs. LS-DESI is a soft ionization technique that allows rapid and sensitive MS-based product characterization in situ. Coupling both approaches provides the opportunity to examine individual GT functions as well as protein-protein interactions. Furthermore, advances in automated oligosaccharide synthesis and lipid nanodisc technology should allow testing of plant GT activity in presence of numerous substrate acceptors and lipid environments in a high throughput fashion. Thus, NAPPA-DESI-MS has great potential to make headway in biochemical characterization of the large number of plant GTs.

Poplar carbohydrate-active enzymes: whole-genome annotation and functional analyses based on RNA expression data

Carbohydrate-active enzymes (CAZymes) catalyze the formation and modification of glycoproteins, glycolipids, starch, secondary metabolites and cell wall biopolymers. They are key enzymes for the biosynthesis of food and renewable biomass. Woody biomass is particularly important for long-term carbon storage and as an abundant renewable natural resource for many industrial applications. This study presents a re-annotation of CAZyme genes in the current Populus trichocarpa genome assembly and in silico functional characterization, based on high-resolution RNA-Seq data sets. Altogether, 1914 CAZyme and expansin genes were annotated in 101 families. About 1797 of these genes were found expressed in at least one Populus organ. We identified genes involved in the biosynthesis of different cell wall polymers and their paralogs. Whereas similar families exist in poplar and Arabidopsis thaliana (with the exception of CBM13 found only in poplar), a few families had significantly different copy numbers between the two species. To identify the transcriptional coordination and functional relatedness within the CAZymes and other proteins, we performed co-expression network analysis of CAZymes in wood-forming tissues using the AspWood database (http://aspwood.popgenie.org/aspwood-v3.0/) for Populus tremula. This provided an overview of the transcriptional changes in CAZymes during the transition from primary to secondary wall formation, and the clustering of transcripts into potential regulons. Candidate enzymes involved in the biosynthesis of polysaccharides were identified along with many tissue-specific uncharacterized genes and transcription factors. These collections offer a rich source of targets for the modification of secondary cell wall biosynthesis and other developmental processes in woody plants.

Genome-wide association mapping in a diverse spring barley collection reveals the presence of QTL hotspots and candidate genes for root and shoot architecture traits at seedling stage

BACKGROUND

Adaptation to drought-prone environments requires robust root architecture. Genotypes with a more vigorous root system have the potential to better adapt to soils with limited moisture content. However, root architecture is complex at both, phenotypic and genetic level. Customized mapping panels in combination with efficient screenings methods can resolve the underlying genetic factors of root traits.

RESULTS

A mapping panel of 233 spring barley genotypes was evaluated for root and shoot architecture traits under non-stress and osmotic stress. A genome-wide association study elucidated 65 involved genomic regions. Among them were 34 root-specific loci, eleven hotspots with associations to up to eight traits and twelve stress-specific loci. A list of candidate genes was established based on educated guess. Selected genes were tested for associated polymorphisms. By this, 14 genes were identified as promising candidates, ten remained suggestive and 15 were rejected. The data support the important role of flowering time genes, including HvPpd-H1, HvCry2, HvCO4 and HvPRR73. Moreover, seven root-related genes, HERK2, HvARF04, HvEXPB1, PIN5, PIN7, PME5 and WOX5 are confirmed as promising candidates. For the QTL with the highest allelic effect for root thickness and plant biomass a homologue of the Arabidopsis Trx-m3 was revealed as the most promising candidate.

CONCLUSIONS

This study provides a catalogue of hotspots for seedling growth, root and stress-specific genomic regions along with candidate genes for future potential incorporation in breeding attempts for enhanced yield potential, particularly in drought-prone environments. Root architecture is under polygenic control. The co-localization of well-known major genes for barley development and flowering time with QTL hotspots highlights their importance for seedling growth. Association analysis revealed the involvement of HvPpd-H1 in the development of the root system. The co-localization of root QTL with HERK2, HvARF04, HvEXPB1, PIN5, PIN7, PME5 and WOX5 represents a starting point to explore the roles of these genes in barley. Accordingly, the genes HvHOX2, HsfA2b, HvHAK2, and Dhn9, known to be involved in abiotic stress response, were located within stress-specific QTL regions and await future validation.

Critical Review of Plant Cell Wall Matrix Polysaccharide Glycosyltransferase Activities Verified by Heterologous Protein Expression

The life cycle and development of plants requires the biosynthesis, deposition, and degradation of cell wall matrix polysaccharides. The structures of the diverse cell wall matrix polysaccharides influence commercially important properties of plant cells, including growth, biomass recalcitrance, organ abscission, and the shelf life of fruits. This review is a comprehensive summary of the matrix polysaccharide glycosyltransferase (GT) activities that have been verified using in vitro assays following heterologous GT protein expression. Plant cell wall (PCW) biosynthetic GTs are primarily integral transmembrane proteins localized to the endoplasmic reticulum and Golgi of the plant secretory system. The low abundance of these enzymes in plant tissues makes them particularly difficult to purify from native plant membranes in quantities sufficient for enzymatic characterization, which is essential to study the functions of the different GTs. Numerous activities in the synthesis of the major cell wall matrix glycans, including pectins, xylans, xyloglucan, mannans, mixed-linkage glucans (MLGs), and arabinogalactan components of AGP proteoglycans have been mapped to specific genes and multi-gene families. Cell wall GTs include those that synthesize the polymer backbones, those that elongate side branches with extended glycosyl chains, and those that add single monosaccharide linkages onto polysaccharide backbones and/or side branches. Three main strategies have been used to identify genes encoding GTs that synthesize cell wall linkages: analysis of membrane fractions enriched for cell wall biosynthetic activities, mutational genetics approaches investigating cell wall compositional phenotypes, and omics-directed identification of putative GTs from sequenced plant genomes. Here we compare the heterologous expression systems used to produce, purify, and study the enzyme activities of PCW GTs, with an emphasis on the eukaryotic systems Nicotiana benthamiana, Pichia pastoris, and human embryonic kidney (HEK293) cells. We discuss the enzymatic properties of GTs including kinetic rates, the chain lengths of polysaccharide products, acceptor oligosaccharide preferences, elongation mechanisms for the synthesis of long-chain polymers, and the formation of GT complexes. Future directions in the study of matrix polysaccharide biosynthesis are proposed.

A two-phase model for the non-processive biosynthesis of homogalacturonan polysaccharides by the GAUT1:GAUT7 complex

Homogalacturonan (HG) is a pectic glycan in the plant cell wall that contributes to plant growth and development and cell wall structure and function, and interacts with other glycans and proteoglycans in the wall. HG is synthesized by the galacturonosyltransferase (GAUT) gene family. Two members of this family, GAUT1 and GAUT7, form a heteromeric enzyme complex in Arabidopsis thaliana Here, we established a heterologous GAUT expression system in HEK293 cells and show that co-expression of recombinant GAUT1 with GAUT7 results in the production of a soluble GAUT1:GAUT7 complex that catalyzes elongation of HG products in vitro The reaction rates, progress curves, and product distributions exhibited major differences dependent upon small changes in the degree of polymerization (DP) of the oligosaccharide acceptor. GAUT1:GAUT7 displayed >45-fold increased catalytic efficiency with DP11 acceptors relative to DP7 acceptors. Although GAUT1:GAUT7 synthesized high-molecular-weight polymeric HG (>100 kDa) in a substrate concentration-dependent manner typical of distributive (nonprocessive) glycosyltransferases with DP11 acceptors, reactions primed with short-chain acceptors resulted in a bimodal product distribution of glycan products that has previously been reported as evidence for a processive model of GT elongation. As an alternative to the processive glycosyltransfer model, a two-phase distributive elongation model is proposed in which a slow phase, which includes the de novo initiation of HG and elongation of short-chain acceptors, is distinguished from a phase of rapid elongation of intermediate- and long-chain acceptors. Upon reaching a critical chain length of DP11, GAUT1:GAUT7 elongates HG to high-molecular-weight products.

Main Strategies of Plant Expression System Glycoengineering for Producing Humanized Recombinant Pharmaceutical Proteins

Most the pharmaceutical proteins are derived not from their natural sources, rather their recombinant analogs are synthesized in various expression systems. Plant expression systems, unlike mammalian cell cultures, combine simplicity and low cost of procaryotic systems and the ability for posttranslational modifications inherent in eucaryotes. More than 50% of all human proteins and more than 40% of the currently used pharmaceutical proteins are glycosylated, that is, they are glycoproteins, and their biological activity, pharmacodynamics, and immunogenicity depend on the correct glycosylation pattern. This review examines in detail the similarities and differences between N- and O-glycosylation in plant and mammalian cells, as well as the effect of plant glycans on the activity, pharmacokinetics, immunity, and intensity of biosynthesis of pharmaceutical proteins. The main current strategies of glycoengineering of plant expression systems aimed at obtaining fully humanized proteins for pharmaceutical application are summarized.

Arabinogalactan proteins and their sugar chains: functions in plant reproduction, research methods, and biosynthesis

The arabinogalactan protein (AGP) family is one of the most complex protein families and is ubiquitous in the plant kingdom. Moreover, it has been demonstrated to play various roles during plant reproduction. A typical AGP contains a hydroxyproline-rich core protein with high heterogeneity and varying numbers of polysaccharide side chains. However, the functions of the polysaccharide components (i.e. AG sugar chains) remain largely unknown due to the general difficulties associated with studying sugar chains in glycobiology. In recent years, methodological breakthroughs have resulted in substantial progress in AGP research. Here, we summarise the multiple roles of AGPs during plant gametophyte development and male-female communication, with a focus on recent advances. In addition, we discuss the analytical tools used in AGP research, and the biosynthesis and function of AG sugar chains. A comprehensive understanding of the AGP family will help clarify the mechanisms precisely controlling reproductive processes.

Identification and evolution of a plant cell wall specific glycoprotein glycosyl transferase, ExAD

Extensins are plant cell wall glycoproteins that act as scaffolds for the deposition of the main wall carbohydrate polymers, which are interlocked into the supramolecular wall structure through intra- and inter-molecular iso-di-tyrosine crosslinks within the extensin backbone. In the conserved canonical extensin repeat, Ser-Hyp₄, serine and the consecutive C4-hydroxyprolines (Hyps) are substituted with an α-galactose and 1–5 β- or α-linked arabinofuranoses (Arafs), respectively. These modifications are required for correct extended structure and function of the extensin network. Here, we identified a single Arabidopsis thaliana gene, At3g57630, in clade E of the inverting Glycosyltransferase family GT47 as a candidate for the transfer of Araf to Hyp-arabinofuranotriose (Hyp-β1,4Araf-β1,2Araf-β1,2Araf) side chains in an α-linkage, to yield Hyp-Araf₄ which is exclusively found in extensins. T-DNA knock-out mutants of At3g57630 showed a truncated root hair phenotype, as seen for mutants of all hitherto characterized extensin glycosylation enzymes; both root hair and glycan phenotypes were restored upon reintroduction of At3g57630. At3g57630 was named Extensin Arabinose Deficient transferase, ExAD, accordingly. The occurrence of ExAD orthologs within the Viridiplantae along with its’ product, Hyp-Araf₄, point to ExAD being an evolutionary hallmark of terrestrial plants and charophyte green algae.

Expanding the feruloyl esterase gene family of Aspergillus niger by characterization of a feruloyl esterase, FaeC

A feruloyl esterase (FAE) from Aspergillus niger N402, FaeC was heterologously produced in Pichia pastoris X-33 in a yield of 10mg/L. FaeC was most active at pH 7.0 and 50°C, and showed broad substrate specificity and catalyzed the hydrolysis of methyl 3,4-dimethoxycinnamate, ethyl ferulate, methyl ferulate, methyl p-coumarate, ethyl coumarate, methyl sinapate, and methyl caffeate. The enzyme released both ferulic acid and p-coumaric acid from wheat arabinoxylan and sugar beet pectin (up to 3mg/g polysaccharide), and acted synergistically with a commercial xylanase increasing the release of ferulic acid up to six-fold. The expression of faeC increased over time in the presence of feruloylated polysaccharides. Cinnamic, syringic, caffeic, vanillic and ferulic acid induced the expression of faeC. Overall expression of faeC was very low in all tested conditions, compared to two other A. niger FAE encoding genes, faeA and faeB. Our data showed that the fae genes responded differently towards the feruloylated polysaccharides and tested monomeric phenolic compounds suggesting that the corresponding FAE isoenzymes may target different substrates in a complementary manner. This may increase the efficiency of the degradation of diverse plant biomass.

The cotton β-galactosyltransferase 1 (GalT1) that galactosylates arabinogalactan proteins participates in controlling fiber development

Arabinogalactan proteins (AGPs) are highly glycosylated proteins that play pivotal roles in diverse developmental processes in plants. Type-II AG glycans, mostly O-linked to the hydroxyproline residues of the protein backbone, account for up to 95% w/w of the AGP, but their functions are still largely unclear. Cotton fibers are extremely elongated single-cell trichomes on the seed epidermis; however, little is known of the molecular basis governing the regulation of fiber cell development. Here, we characterized the role of a CAZy glycosyltransferase 31 (GT31) family member, GhGalT1, in cotton fiber development. The fiber length of the transgenic cotton overexpressing GhGalT1 was shorter than that of the wild type, whereas in the GhGalT1-silenced lines there was a notable increase in fiber length compared with wild type. The carbohydrate moieties of AGPs were altered in fibers of GhGalT1 transgenic cotton. The galactose: arabinose ratio of AG glycans was higher in GhGalT1 overexpression fibers, but was lower in GhGalT1-silenced lines, compared with that in the wild type. Overexpression of GhGalT1 upregulates transcript levels of a broad range of cell wall-related genes, especially the fasciclin-like AGP (FLA) backbone genes. An enzyme activity assay demonstrated that GhGalT1 is a β-1,3-galactosyltransferase (β-1,3-GalT) involved in biosynthesis of the β-1,3-galactan backbone of the type-II AG glycans of AGPs. We also show that GhGalT1 can form homo- and heterodimers with other cotton GT31 family members to facilitate AG glycan assembly of AGPs. Thus, our data demonstrate that GhGalT1 influences cotton fiber development via controlling the glycosylation of AGPs, especially FLAs.

Overexpression of a Domain of Unknown Function 266-containing protein results in high cellulose content, reduced recalcitrance, and enhanced plant growth in the bioenergy crop Populus

BACKGROUND

Domain of Unknown Function 266 (DUF266) is a plant-specific domain. DUF266-containing proteins (DUF266 proteins) have been categorized as 'not classified glycosyltransferases (GTnc)' due to amino acid similarity with GTs. However, little is known about the function of DUF266 proteins.

RESULTS

Phylogenetic analysis revealed that DUF266 proteins are only present in the land plants including moss and lycophyte. We report the functional characterization of one member of DUF266 proteins in Populus, PdDUF266A. PdDUF266A was ubiquitously expressed with high abundance in the xylem. In Populus transgenic plants overexpressing PdDUF266A (OXPdDUF266A), the glucose and cellulose contents were significantly higher, while the lignin content was lower than that in the wild type. Degree of polymerization of cellulose in OXPdDUF266A transgenic plants was also higher, whereas cellulose crystallinity index remained unchanged. Gene expression analysis indicated that cellulose biosynthesis-related genes such as CESA and SUSY were upregulated in mature leaf and xylem of OXPdDUF266A transgenic plants. Moreover, PdDUF266A overexpression resulted in an increase of biomass production. Their glucose contents and biomass phenotypes were further validated via heterologous expression of PdDUF266A in Arabidopsis. Results from saccharification treatment demonstrated that the rate of sugar release was increased by approximately 38% in the OXPdDUF266A transgenic plants.

CONCLUSIONS

These results suggest that the overexpression of PdDUF266A can increase cellulose content, reduce recalcitrance, and enhance biomass production, and that PdDUF266A is a promising target for genetic manipulation for biofuel production.

The plant secretory pathway seen through the lens of the cell wall

Secretion in plant cells is often studied by looking at well-characterised, evolutionarily conserved membrane proteins associated with particular endomembrane compartments. Studies using live cell microscopy and fluorescent proteins have illuminated the highly dynamic nature of trafficking, and electron microscopy studies have resolved the ultrastructure of many compartments. Biochemical and molecular analyses have further informed about the function of particular proteins and endomembrane compartments. In plants, there are over 40 cell types, each with highly specialised functions, and hence potential variations in cell biological processes and cell wall structure. As the primary function of secretion in plant cells is for the biosynthesis of cell wall polysaccharides and apoplastic transport complexes, it follows that utilising our knowledge of cell wall glycosyltransferases (GTs) and their polysaccharide products will inform us about secretion. Indeed, this knowledge has led to novel insights into the secretory pathway, including previously unseen post-TGN secretory compartments. Conversely, our knowledge of trafficking routes of secretion will inform us about polarised and localised deposition of cell walls and their constituent polysaccharides/glycoproteins. In this review, we look at what is known about cell wall biosynthesis and the secretory pathway and how the different approaches can be used in a complementary manner to study secretion and provide novel insights into these processes.

Global DNA methylation variations after short-term heat shock treatment in cultured microspores of Brassica napus cv. Topas

Heat stress can induce the cultured microspores into embryogenesis. In this study, whole genome bisulphite sequencing was employed to study global DNA methylation variations after short-term heat shock (STHS) treatments in cultured microspores of Brassica napus cv. Topas. Our results indicated that treatment on cultured Topas microspores at 32 °C for 6 h triggered DNA hypomethylation, particularly in the CG and CHG contexts. And the total number of T32 (Topas 32 °C for 6 h) vs. T0 (Topas 0 h) differentially methylated region-related genes (DRGs) was approximately two-fold higher than that of T18 (Topas 18 °C for 6 h) vs. T0 DRGs, which suggested that 32 °C might be a more intense external stimulus than 18 °C resulting in more changes in the DNA methylation status of cultured microspores. Additionally, 32 °C treatment for 6 h led to increased CHG differential methylations of transposons (DMTs), which were mainly constituted by overlaps between the hypomethylated differentially methylated regions (hypo-DMRs) and transposon elements (TEs). Further analysis demonstrated that the DRGs and their paralogs exhibited differential methylated/demethylated patterns. To summarize, the present study is the first methylome analysis of cultured microspores in response to STHS and may provide valuable information on the roles of DNA methylation in heat response.

Tumor-associated antigens: Tn antigen, sTn antigen, and T antigen

Glycosylation is one of the major posttranslational modifications of proteins. N-glycosylation (Asn-linked) and O-glycosylation (Ser/Thr-linked) are the two main forms. Abnormal O-glycosylation is frequently observed on the surface of tumor cells, and is associated with an adverse outcome and poor prognosis in patients with cancer. O-glycans (Tn, sTn, and T antigen) can be synthesized in the Golgi apparatus with the aid of several glycosyltransferases (such as T-synthase and ST6GalNAc-I) in a suitable environment. The unique molecular chaperone of T-synthase is Cosmc, which helps T-synthase to fold correctly in the endoplasmic reticulum. Dysregulation of these glycosyltransferases, molecular chaperones, or the environment is involved in the dysregulation of O-glycans. Tn, sTn, and T antigen neo- or over-expression occurs in many types of cancer including gastric, colon, breast, lung, esophageal, prostate, and endometrial cancer. This review discusses the major synthetic pathway of O-glycans and the mechanism by which Tn, sTn, and T antigens promote tumor metastasis.

Post-Synthetic Defucosylation of AGP by Aspergillus nidulans α-1,2-Fucosidase Expressed in Arabidopsis Apoplast Induces Compensatory Upregulation of α-1,2-Fucosyltransferases

Cell walls are essential components of plant cells which perform a variety of important functions for the different cell types, tissues and organs of a plant. Besides mechanical function providing cell shape, cell walls participate in intercellular communication, defense during plant-microbe interactions, and plant growth. The plant cell wall consists predominantly of polysaccharides with the addition of structural glycoproteins, phenolic esters, minerals, lignin, and associated enzymes. Alterations in the cell wall composition created through either changes in biosynthesis of specific constituents or their post-synthetic modifications in the apoplast compromise cell wall integrity and frequently induce plant compensatory responses as a result of these alterations. Here we report that post-synthetic removal of fucose residues specifically from arabinogalactan proteins in the Arabidopsis plant cell wall induces differential expression of fucosyltransferases and leads to the root and hypocotyl elongation changes. These results demonstrate that the post-synthetic modification of cell wall components presents a valuable approach to investigate the potential signaling pathways induced during plant responses to such modifications that usually occur during plant development and stress responses.

Glycosylation of arabinogalactan-proteins essential for development in Arabidopsis

Arabinogalactan-proteins (AGPs) are ubiquitous cell wall components present throughout the plant kingdom. They are extensively post translationally modified by conversion of proline to hydroxyproline (Hyp) and by addition of arabinogalactan (AG) polysaccharides to Hyp residues. Two small gene subfamilies within the CAZy GT31 family, referred to as Hyp-galactosyltransferases (Hyp-GALTs and HPGTs), encode enzymes that specifically add galactose to AGP protein backbones as revealed by heterologous expression of the genes coupled with an in vitro enzyme assay and by biochemical characterization of the genetic knock-out mutants. Biochemical analysis of galt2galt5 double and hpgt1hpgt2hpgt3 triple knockout mutants revealed significant reductions in both AGP-specific Hyp-GALT activity and β-Gal-Yariv precipitable AGPs. Further analysis of these mutants demonstrated both overlapping and distinct pleiotropic growth and development phenotypes, indicating the important contributions of the carbohydrate moieties toward AGP function. Current research indicates that all 8 Hyp-GALT/HPGT genes encode enzymes that catalyze the initial step for AGP glycosylation and that AGP glycans play essential roles in plant growth and development.

A DUF-246 family glycosyltransferase-like gene affects male fertility and the biosynthesis of pectic arabinogalactans

BACKGROUND

Pectins are a group of structurally complex plant cell wall polysaccharides whose biosynthesis and function remain poorly understood. The pectic polysaccharide rhamnogalacturonan-I (RG-I) has two types of arabinogalactan side chains, type-I and type-II arabinogalactans. To date few enzymes involved in the biosynthesis of pectin have been described. Here we report the identification of a highly conserved putative glycosyltransferase encoding gene, Pectic ArabinoGalactan synthesis-Related (PAGR), affecting the biosynthesis of RG-I arabinogalactans and critical for pollen tube growth.

RESULTS

T-DNA insertions in PAGR were identified in Arabidopsis thaliana and were found to segregate at a 1:1 ratio of heterozygotes to wild type. We were unable to isolate homozygous pagr mutants as pagr mutant alleles were not transmitted via pollen. In vitro pollen germination assays revealed reduced rates of pollen tube formation in pollen from pagr heterozygotes. To characterize a loss-of-function phenotype for PAGR, the Nicotiana benthamiana orthologs, NbPAGR-A and B, were transiently silenced using Virus Induced Gene Silencing. NbPAGR-silenced plants exhibited reduced internode and petiole expansion. Cell wall materials from NbPAGR-silenced plants had reduced galactose content compared to the control. Immunological and linkage analyses support that RG-I has reduced type-I arabinogalactan content and reduced branching of the RG-I backbone in NbPAGR-silenced plants. Arabidopsis lines overexpressing PAGR exhibit pleiotropic developmental phenotypes and the loss of apical dominance as well as an increase in RG-I type-II arabinogalactan content.

CONCLUSIONS

Together, results support a function for PAGR in the biosynthesis of RG-I arabinogalactans and illustrate the essential roles of these polysaccharides in vegetative and reproductive plant growth.

Extensin and Arabinogalactan-Protein Biosynthesis: Glycosyltransferases, Research Challenges, and Biosensors

Recent research, mostly in Arabidopsis thaliana, has led to the identification and characterization of the glycosyltransferases responsible for the biosynthesis of two of the most functionally important and abundant families of plant cell wall proteins, extensins, and arabinogalactan-proteins. Extensin glycosylation involves monogalactosylation of serine residues by O-α-serine galactosyltransferase and the addition of oligoarabinosides one to five arabinose units in length to contiguous hydroxyproline residues by a set of specific arabinosyltransferase enzymes, which includes hydroxyproline O-β-arabinosyltransferases, β-1,2-arabinosyltransferases, and at least one α-1,3-arabinosyltransferase. AGP glycosylation, however, is much more complex and involves the addition of large arabinogalactan polysaccharide chains to non-contiguous hydroxyproline residues. These arabinogalactan chains are composed of β-1,3-galactan backbones decorated with β-1,6-galactose side chains that are further modified with α-arabinose as well as other sugars, including β-(methyl)glucuronic acid, α-rhamnose, and α-fucose. Specific sets of hydroxyproline O-β-galactosyltransferases, β-1,3-galactosyltransferases, β-1,6-galactosyltransferases, α-arabinosyltransferases, β-glucuronosyltransferases, α-rhamnosyltransferases, and α-fucosyltransferases are responsible for the synthesis of these complex structures. This mini-review summarizes the EXT and AGP glycosyltransferases identified and characterized to date along with corresponding genetic mutant data, which addresses the functional importance of EXT and AGP glycosylation. In one case, genetic mutant data indicate that the carbohydrate moiety of arabinogalactan-proteins may serve as an extracellular biosensor or signal for normal cellular growth. Finally, future research challenges with respect to understanding the function of these enzymes more completely and discovering and characterizing additional glycosyltransferases responsible for extensin and arabinogalactan-protein biosynthesis are also discussed.

A small multigene hydroxyproline-O-galactosyltransferase family functions in arabinogalactan-protein glycosylation, growth and development in Arabidopsis

BACKGROUND

Arabinogalactan-proteins (AGPs) are ubiquitous components of cell walls throughout the plant kingdom and are extensively post translationally modified by conversion of proline to hydroxyproline (Hyp) and by addition of arabinogalactan polysaccharides (AG) to Hyp residues. AGPs are implicated to function in various aspects of plant growth and development, but the functional contributions of AGP glycans remain to be elucidated. Hyp glycosylation is initiated by the action of a set of Hyp-O-galactosyltransferase (Hyp-O-GALT) enzymes that remain to be fully characterized.

RESULTS

Three members of the GT31 family (GALT3-At3g06440, GALT4-At1g27120, and GALT6-At5g62620) were identified as Hyp-O-GALT genes by heterologous expression in tobacco leaf epidermal cells and examined along with two previously characterized Hyp-O-GALT genes, GALT2 and GALT5. Transcript profiling by real-time PCR of these five Hyp-O-GALTs revealed overlapping but distinct expression patterns. Transiently expressed GALT3, GALT4 and GALT6 fluorescent protein fusions were localized within Golgi vesicles. Biochemical analysis of knock-out mutants for the five Hyp-O-GALT genes revealed significant reductions in both AGP-specific Hyp-O-GALT activity and β-Gal-Yariv precipitable AGPs. Further phenotypic analysis of these mutants demonstrated reduced root hair growth, reduced seed coat mucilage, reduced seed set, and accelerated leaf senescence. The mutants also displayed several conditional phenotypes, including impaired root growth, and defective anisotropic growth of root tips under salt stress, as well as less sensitivity to the growth inhibitory effects of β-Gal-Yariv reagent in roots and pollen tubes.

CONCLUSIONS

This study provides evidence that all five Hyp-O-GALT genes encode enzymes that catalyze the initial steps of AGP galactosylation and that AGP glycans play essential roles in both vegetative and reproductive plant growth.

Xyloglucan, galactomannan, glucuronoxylan, and rhamnogalacturonan I do not have identical structures in soybean root and root hair cell walls

MAIN CONCLUSION

Chemical analyses and glycome profiling demonstrate differences in the structures of the xyloglucan, galactomannan, glucuronoxylan, and rhamnogalacturonan I isolated from soybean ( Glycine max ) roots and root hair cell walls. The root hair is a plant cell that extends only at its tip. All other root cells have the ability to grow in different directions (diffuse growth). Although both growth modes require controlled expansion of the cell wall, the types and structures of polysaccharides in the walls of diffuse and tip-growing cells from the same plant have not been determined. Soybean (Glycine max) is one of the few plants whose root hairs can be isolated in amounts sufficient for cell wall chemical characterization. Here, we describe the structural features of rhamnogalacturonan I, rhamnogalacturonan II, xyloglucan, glucomannan, and 4-O-methyl glucuronoxylan present in the cell walls of soybean root hairs and roots stripped of root hairs. Irrespective of cell type, rhamnogalacturonan II exists as a dimer that is cross-linked by a borate ester. Root hair rhamnogalacturonan I contains more neutral oligosaccharide side chains than its root counterpart. At least 90% of the glucuronic acid is 4-O-methylated in root glucuronoxylan. Only 50% of this glycose is 4-O-methylated in the root hair counterpart. Mono O-acetylated fucose-containing subunits account for at least 60% of the neutral xyloglucan from root and root hair walls. By contrast, a galacturonic acid-containing xyloglucan was detected only in root hair cell walls. Soybean homologs of the Arabidopsis xyloglucan-specific galacturonosyltransferase are highly expressed only in root hairs. A mannose-rich polysaccharide was also detected only in root hair cell walls. Our data demonstrate that the walls of tip-growing root hairs cells have structural features that distinguish them from the walls of other roots cells.

Two Hydroxyproline Galactosyltransferases, GALT5 and GALT2, Function in Arabinogalactan-Protein Glycosylation, Growth and Development in Arabidopsis

Hydroxyproline-O-galactosyltransferase (GALT) initiates O-glycosylation of arabinogalactan-proteins (AGPs). We previously characterized GALT2 (At4g21060), and now report on functional characterization of GALT5 (At1g74800). GALT5 was identified using heterologous expression in Pichia and an in vitro GALT assay. Product characterization showed GALT5 specifically adds galactose to hydroxyproline in AGP protein backbones. Functions of GALT2 and GALT5 were elucidated by phenotypic analysis of single and double mutant plants. Allelic galt5 and galt2 mutants, and particularly galt2 galt5 double mutants, demonstrated lower GALT activities and reductions in β-Yariv-precipitated AGPs compared to wild type. Mutant plants showed pleiotropic growth and development phenotypes (defects in root hair growth, root elongation, pollen tube growth, flowering time, leaf development, silique length, and inflorescence growth), which were most severe in the double mutants. Conditional mutant phenotypes were also observed, including salt-hypersensitive root growth and root tip swelling as well as reduced inhibition of pollen tube growth and root growth in response to β-Yariv reagent. These mutants also phenocopy mutants for an AGP, SOS5, and two cell wall receptor-like kinases, FEI1 and FEI2, which exist in a genetic signaling pathway. In summary, GALT5 and GALT2 function as redundant GALTs that control AGP O-glycosylation, which is essential for normal growth and development.