Two xyloglucan xylosyltransferases catalyze the addition of multiple xylosyl residues to cellohexaose

Xyloglucan deficiency leads to a reduction in turgor pressure and changes in cell wall properties, affecting early seedling establishment

Xyloglucan is believed to play a significant role in cell wall mechanics of dicot plants. Surprisingly, Arabidopsis plants defective in xyloglucan biosynthesis exhibit nearly normal growth and development. We investigated a mutant line, cslc-Δ5, lacking activity in all five Arabidopsis cellulose synthase like-C (CSLC) genes responsible for xyloglucan backbone biosynthesis. We observed that this xyloglucan-deficient line exhibited reduced cellulose crystallinity and increased pectin levels, suggesting the existence of feedback mechanisms that regulate wall composition to compensate for the absence of xyloglucan. These alterations in cell wall composition in the xyloglucan-absent plants were further linked to a decrease in cell wall elastic modulus and rupture stress, as observed through atomic force microscopy (AFM) and extensometer-based techniques. This raised questions about how plants with such modified cell wall properties can maintain normal growth. Our investigation revealed two key factors contributing to this phenomenon. First, measurements of turgor pressure, a primary driver of plant growth, revealed that cslc-Δ5 plants have reduced turgor, preventing the compromised walls from bursting while still allowing growth to occur. Second, we discovered the conservation of elastic asymmetry (ratio of axial to transverse wall elasticity) in the mutant, suggesting an additional mechanism contributing to the maintenance of normal growth. This novel feedback mechanism between cell wall composition and mechanical properties, coupled with turgor pressure regulation, plays a central role in the control of plant growth and is critical for seedling establishment in a mechanically challenging environment by affecting shoot emergence and root penetration.

The plant cell wall-dynamic, strong, and adaptable-is a natural shapeshifter

Mythology is replete with good and evil shapeshifters, who, by definition, display great adaptability and assume many different forms-with several even turning themselves into trees. Cell walls certainly fit this definition as they can undergo subtle or dramatic changes in structure, assume many shapes, and perform many functions. In this review, we cover the evolution of knowledge of the structures, biosynthesis, and functions of the 5 major cell wall polymer types that range from deceptively simple to fiendishly complex. Along the way, we recognize some of the colorful historical figures who shaped cell wall research over the past 100 years. The shapeshifter analogy emerges more clearly as we examine the evolving proposals for how cell walls are constructed to allow growth while remaining strong, the complex signaling involved in maintaining cell wall integrity and defense against disease, and the ways cell walls adapt as they progress from birth, through growth to maturation, and in the end, often function long after cell death. We predict the next century of progress will include deciphering cell type-specific wall polymers; regulation at all levels of polymer production, crosslinks, and architecture; and how walls respond to developmental and environmental signals to drive plant success in diverse environments.

A core of cell wall proteins functions in wall integrity responses in Arabidopsis thaliana

Cell walls surround all plant cells, and their composition and structure are tightly regulated to maintain cellular and organismal homeostasis. In response to wall damage, the cell wall integrity (CWI) system is engaged to ameliorate effects on plant growth. Despite the central role CWI plays in plant development, our current understanding of how this system functions at the molecular level is limited. Here, we investigated the transcriptomes of etiolated seedlings of mutants of Arabidopsis thaliana with defects in three major wall polysaccharides, pectin (quasimodo2), cellulose (cellulose synthase3 je5), and xyloglucan (xyloglucan xylosyltransferase1 and 2), to probe whether changes in the expression of cell wall-related genes occur and are similar or different when specific wall components are reduced or missing. Many changes occurred in the transcriptomes of pectin- and cellulose-deficient plants, but fewer changes occurred in the transcriptomes of xyloglucan-deficient plants. We hypothesize that this might be because pectins interact with other wall components and/or integrity sensors, whereas cellulose forms a major load-bearing component of the wall; defects in either appear to trigger the expression of structural proteins to maintain wall cohesion in the absence of a major polysaccharide. This core set of genes functioning in CWI in plants represents an attractive target for future genetic engineering of robust and resilient cell walls.

Engineering of substrate specificity in a plant cell-wall modifying enzyme through alterations of carboxyl-terminal amino acid residues

Structural determinants of substrate recognition remain inadequately defined in broad specific cell-wall modifying enzymes, termed xyloglucan xyloglucosyl transferases (XETs). Here, we investigate the Tropaeolum majus seed TmXET6.3 isoform, a member of the GH16_20 subfamily of the GH16 network. This enzyme recognises xyloglucan (XG)-derived donors and acceptors, and a wide spectrum of other chiefly saccharide substrates, although it lacks the activity with homogalacturonan (pectin) fragments. We focus on defining the functionality of carboxyl-terminal residues in TmXET6.3, which extend acceptor binding regions in the GH16_20 subfamily but are absent in the related GH16_21 subfamily. Site-directed mutagenesis using double to quintuple mutants in the carboxyl-terminal region - substitutions emulated on barley XETs recognising the XG/penta-galacturonide acceptor substrate pair - demonstrated that this activity could be gained in TmXET6.3. We demonstrate the roles of semi-conserved Arg238 and Lys237 residues, introducing a net positive charge in the carboxyl-terminal region (which complements a negative charge of the acidic penta-galacturonide) for the transfer of xyloglucan fragments. Experimental data, supported by molecular modelling of TmXET6.3 with the XG oligosaccharide donor and penta-galacturonide acceptor substrates, indicated that they could be accommodated in the active site. Our findings support the conclusion on the significance of positively charged residues at the carboxyl terminus of TmXET6.3 and suggest that a broad specificity could be engineered via modifications of an acceptor binding site. The definition of substrate specificity in XETs should prove invaluable for defining the structure, dynamics, and function of plant cell walls, and their metabolism; these data could be applicable in various biotechnologies.

Galactosylation of xyloglucan is essential for the stabilization of the actin cytoskeleton and endomembrane system through the proper assembly of cell walls

Xyloglucan, a major hemicellulose, interacts with cellulose and pectin to assemble primary cell walls in plants. Loss of the xyloglucan galactosyltransferase MURUS3 (MUR3) leads to the deficiency of galactosylated xyloglucan and perturbs plant growth. However, it is unclear whether defects in xyloglucan galactosylation influence the synthesis of other wall polysaccharides, cell wall integrity, cytoskeleton behaviour, and endomembrane homeostasis. Here, we found that in mur3-7 etiolated seedlings cellulose was reduced, CELLULOSE SYNTHASE (CESA) genes were down-regulated, the density and mobility of cellulose synthase complexes (CSCs) were decreased, and cellulose microfibrils become discontinuous. Pectin, rhamnogalacturonan II (RGII), and boron contents were reduced in mur3-7 plants, and B-RGII cross-linking was abnormal. Wall porosity and thickness were significantly increased in mur3-7 seedlings. Endomembrane aggregation was also apparent in the mur3-7 mutant. Furthermore, mutant seedlings and their actin filaments were more sensitive to Latrunculin A (LatA) treatment. However, all defects in mur3-7 mutants were substantially restored by exogenous boric acid application. Our study reveals the importance of MUR3-mediated xyloglucan galactosylation for cell wall structural assembly and homeostasis, which is required for the stabilization of the actin cytoskeleton and the endomembrane system.

The Arabidopsis xylosyltransferases, XXT3, XXT4, and XXT5, are essential to complete the fully xylosylated glucan backbone XXXG-type structure of xyloglucans

Although most xyloglucans (XyGs) biosynthesis enzymes have been identified, the molecular mechanism that defines XyG branching patterns is unclear. Four out of five XyG xylosyltransferases (XXT1, XXT2, XXT4, and XXT5) are known to add the xylosyl residue from UDP-xylose onto a glucan backbone chain; however, the function of XXT3 has yet to be demonstrated. Single xxt3 and triple xxt3xxt4xxt5 mutant Arabidopsis (Arabidopsis thaliana) plants were generated using CRISPR-Cas9 technology to determine the specific function of XXT3. Combined biochemical, bioinformatic, and morphological data conclusively established for the first time that XXT3, together with XXT4 and XXT5, adds xylosyl residue specifically at the third glucose in the glucan chain to synthesize XXXG-type XyGs. We propose that the specificity of XXT3, XXT4, and XXT5 is directed toward the prior synthesis of the acceptor substrate by the other two enzymes, XXT1 and XXT2. We also conclude that XXT5 plays a dominant role in the synthesis of XXXG-type XyGs, while XXT3 and XXT4 complementarily contribute their activities in a tissue-specific manner. The newly generated xxt3xxt4xxt5 mutant produces only XXGG-type XyGs, which further helps to understand the impact of structurally deficient polysaccharides on plant cell wall organization, growth, and development.

Open questions in plant cell wall synthesis

Plant cells are surrounded by strong yet flexible polysaccharide-based cell walls that support the cell while also allowing growth by cell expansion. Plant cell wall research has advanced tremendously in recent years. Sequenced genomes of many model and crop plants have facilitated cataloging and characterization of many enzymes involved in cell wall synthesis. Structural information has been generated for several important cell wall synthesizing enzymes. Important tools have been developed including antibodies raised against a variety of cell wall polysaccharides and glycoproteins, collections of enzyme clones and synthetic glycan arrays for characterizing enzymes, herbicides that specifically affect cell wall synthesis, live-cell imaging probes to track cell wall synthesis, and an inducible secondary cell wall synthesis system. Despite these advances, and often because of the new information they provide, many open questions about plant cell wall polysaccharide synthesis persist. This article highlights some of the key questions that remain open, reviews the data supporting different hypotheses that address these questions, and discusses technological developments that may answer these questions in the future.

Reconsidering the function of the xyloglucan endotransglucosylase/hydrolase family

Plants possess an outer cell layer called the cell wall. This matrix comprises various molecules, such as polysaccharides and proteins, and serves a wide array of physiologically important functions. This structure is not static but rather flexible in response to the environment. One of the factors responsible for this plasticity is the xyloglucan endotransglucosylase/hydrolase (XTH) family, which cleaves and reconnects xyloglucan molecules. Since xyloglucan molecules have been hypothesised to tether cellulose microfibrils forming the main load-bearing network in the primary cell wall, XTHs have been thought to play a central role in cell wall loosening for plant cell expansion. However, multiple lines of recent evidence have questioned this classic model. Nevertheless, reverse genetic analyses have proven the biological importance of XTHs; therefore, a major challenge at present is to reconsider the role of XTHs in planta. Recent advances in analytical techniques have allowed for gathering rich information on the structure of the primary cell wall. Thus, the integration of accumulated knowledge in current XTH studies may offer a turning point for unveiling the precise functions of XTHs. In the present review, we redefine the biological function of the XTH family based on the recent architectural model of the cell wall. We highlight three key findings regarding this enzyme family: (1) XTHs are not strictly required for cell wall loosening during plant cell expansion but play vital roles in response to specific biotic or abiotic stresses; (2) in addition to their transglycosylase activity, the hydrolase activity of XTHs is involved in physiological benefits; and (3) XTHs can recognise a wide range of polysaccharides other than xyloglucans.

Using CRISPR-Cas9 Technology to Eliminate Xyloglucan in Tobacco Cell Walls and Change the Uptake and Translocation of Inorganic Arsenic

Xyloglucan is a quantitatively major polysaccharide in the primary cell walls of flowering plants and has been reported to affect plants' ability to tolerate toxic elements. However, it is not known if altering the amounts of xyloglucan in the wall influences the uptake and translocation of inorganic arsenic (As). Here, we identified two Nicotiana tabacum genes that encode xyloglucan-specific xylosyltransferases (XXT), which we named NtXXT1 and NtXXT2. We used CRISPR-Cas9 technology to generate ntxxt1, ntxxt2, and ntxxt1/2 mutant tobacco plants to determine if preventing xyloglucan synthesis affects plant growth and their ability to accumulate As. We show that NtXXT1 and NtXXT2 are required for xyloglucan biosynthesis because no discernible amounts of xyloglucan were present in the cell walls of the ntxxt1/2 double mutant. The tobacco double mutant (ntxxt1/2) and the corresponding Arabidopsis mutant (atxxt1/2) do not have severe growth defects but do have a short root hair phenotype and a slow growth rate. This phenotype is rescued by overexpressing NtXXT1 or NtXXT2 in atxxt1/2. Growing ntxxt mutants in the presence of AsIII or AsV showed that the absence of cell wall xyloglucan affects the accumulation and translocation of As. Most notably, root retention of As increased substantially and the amounts of As translocated to the shoots decreased in ntxxt1/2. Our results suggest that xyloglucan-deficient plants provide a strategy for the phytoremediation of As contaminated soils.

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.

Xyloglucan Biosynthesis: From Genes to Proteins and Their Functions

The plant's recalcitrant cell wall is composed of numerous polysaccharides, including cellulose, hemicellulose, and pectin. The most abundant hemicellulose in dicot cell walls is xyloglucan, which consists of a β-(1- > 4) glucan backbone with α-(1- > 6) xylosylation producing an XXGG or XXXG pattern. Xylose residues of xyloglucan are branched further with different patterns of arabinose, fucose, galactose, and acetylation that varies between species. Although xyloglucan research in other species lag behind Arabidopsis thaliana, significant advances have been made into the agriculturally relevant species Oryza sativa and Solanum lycopersicum, which can be considered model organisms for XXGG type xyloglucan. In this review, we will present what is currently known about xyloglucan biosynthesis in A. thaliana, O. sativa, and S. lycopersicum and discuss the recent advances in the characterization of the glycosyltransferases involved in this complex process and their organization in the Golgi.

Xyloglucan Xylosyltransferase 1 Displays Promiscuity Toward Donor Substrates During in Vitro Reactions

Glycosyltransferases (GTs) are a large family of enzymes that add sugars to a broad range of acceptor substrates, including polysaccharides, proteins and lipids, by utilizing a wide variety of donor substrates in the form of activated sugars. Individual GTs have generally been considered to exhibit a high level of substrate specificity, but this has not been thoroughly investigated across the extremely large set of GTs. Here we investigate xyloglucan xylosyltransferase 1 (XXT1), a GT involved in the synthesis of the plant cell wall polysaccharide, xyloglucan. Xyloglucan has a glucan backbone, with initial side chain substitutions exclusively composed of xylose from uridine diphosphate (UDP)-xylose. While this conserved substitution pattern suggests a high substrate specificity for XXT1, our in vitro kinetic studies elucidate a more complex set of behavior. Kinetic studies demonstrate comparable kcat values for reactions with UDP-xylose and UDP-glucose, while reactions with UDP-arabinose and UDP-galactose are over 10-fold slower. Using kcat/KM as a measure of efficiency, UDP-xylose is 8-fold more efficient as a substrate than the next best alternative, UDP-glucose. To the best of our knowledge, we are the first to demonstrate that not all plant XXTs are highly substrate specific and some do show significant promiscuity in their in vitro reactions. Kinetic parameters alone likely do not explain the high substrate selectivity in planta, suggesting that there are additional control mechanisms operating during polysaccharide biosynthesis. Improved understanding of substrate specificity of the GTs will aid in protein engineering, development of diagnostic tools, and understanding of biological systems.

Whole Exome Sequencing-Based Identification of a Novel Gene Involved in Root Hair Development in Barley (Hordeum vulgare L.)

Root hairs play a crucial role in anchoring plants in soil, interaction with microorganisms and nutrient uptake from the rhizosphere. In contrast to Arabidopsis, there is a limited knowledge of root hair morphogenesis in monocots, including barley (Hordeum vulgare L.). We have isolated barley mutant rhp1.e with an abnormal root hair phenotype after chemical mutagenesis of spring cultivar ‘Sebastian’. The development of root hairs was initiated in the mutant but inhibited at the very early stage of tip growth. The length of root hairs reached only 3% of the length of parent cultivar. Using a whole exome sequencing (WES) approach, we identified G1674A mutation in the HORVU1Hr1G077230 gene, located on chromosome 1HL and encoding a cellulose synthase-like C1 protein (HvCSLC1) that might be involved in the xyloglucan (XyG) synthesis in root hairs. The identified mutation led to the retention of the second intron and premature termination of the HvCSLC1 protein. The mutation co-segregated with the abnormal root hair phenotype in the F₂ progeny of rhp1.e mutant and its wild-type parent. Additionally, different substitutions in HORVU1Hr1G077230 were found in four other allelic mutants with the same root hair phenotype. Here, we discuss the putative role of HvCSLC1 protein in root hair tube elongation in barley.

A Single Xyloglucan Xylosyltransferase Is Sufficient for Generation of the XXXG Xylosylation Pattern of Xyloglucan

Xyloglucan is the most abundant hemicellulose in the primary cell walls of dicots. Dicot xyloglucan is the XXXG type consisting of repeating units of three consecutive xylosylated Glc residues followed by one unsubstituted Glc. Its xylosylation is catalyzed by xyloglucan 6-xylosyltransferases (XXTs) and there exist five XXTs (AtXXT1-5) in Arabidopsis. While AtXXT1 and AtXXT2 have been shown to add the first two Xyl residues in the XXXG repeat, which XXTs are responsible for the addition of the third Xyl residue remains elusive although AtXXT5 was a proposed candidate. In this report, we generated recombinant proteins of all five Arabidopsis XXTs and one rice XXT (OsXXT1) in the mammalian HEK293 cells and investigated their ability to sequentially xylosylate Glc residues to generate the XXXG xylosylation pattern. We found that like AtXXT1/2, AtXXT4 and OsXXT1 could efficiently xylosylate the cellohexaose (G6) acceptor to produce mono- and di-xylosylated G6, whereas AtXXT5 was only barely capable of adding one Xyl onto G6. When AtXXT1-catalyzed products were used as acceptors, AtXXT1/2/4 and OsXXT1, but not AtXXT5, were able to xylosylate additional Glc residues to generate tri- and tetra-xylosylated G6. Further characterization of the tri- and tetra-xylosylated G6 revealed that they had the sequence of GXXXGG and GXXXXG with three and four consecutive xylosylated Glc residues, respectively. In addition, we have found that although tri-xylosylation occurred on G6, cello-oligomers with a degree of polymerization of 3 to 5 could only be mono- and di-xylosylated. Together, these results indicate that each of AtXXT1/2/4 and OsXXT1 is capable of sequentially adding Xyl onto three contiguous Glc residues to generate the XXXG xylosylation pattern and these findings provide new insight into the biochemical mechanism underlying xyloglucan biosynthesis.

Efficient Cas9 multiplex editing using unspaced sgRNA arrays engineering in a Potato virus X vector

Systems based on the clustered, regularly interspaced, short palindromic repeat (CRISPR) and CRISPR-associated proteins (Cas) have revolutionized genome editing in many organisms, including plants. Most CRISPR-Cas strategies in plants rely on genetic transformation using Agrobacterium tumefaciens to supply the gene editing reagents, such as Cas nucleases or the synthetic guide RNA (sgRNA). While Cas nucleases are constant elements in editing approaches, sgRNAs are target-specific and a screening process is usually required to identify those most effective. Plant virus-derived vectors are an alternative for the fast and efficient delivery of sgRNAs into adult plants, due to the virus capacity for genome amplification and systemic movement, a strategy known as virus-induced genome editing. We engineered Potato virus X (PVX) to build a vector that easily expresses multiple sgRNAs in adult solanaceous plants. Using the PVX-based vector, Nicotiana benthamiana genes were efficiently targeted, producing nearly 80% indels in a transformed line that constitutively expresses Streptococcus pyogenes Cas9. Interestingly, results showed that the PVX vector allows expression of arrays of unspaced sgRNAs, achieving highly efficient multiplex editing in a few days in adult plant tissues. Moreover, virus-free edited progeny can be obtained from plants regenerated from infected tissues or infected plant seeds, which exhibit a high rate of heritable biallelic mutations. In conclusion, this new PVX vector allows easy, fast and efficient expression of sgRNA arrays for multiplex CRISPR-Cas genome editing and will be a useful tool for functional gene analysis and precision breeding across diverse plant species, particularly in Solanaceae crops.

Subcellular coordination of plant cell wall synthesis

Organelles of the plant cell cooperate to synthesize and secrete a strong yet flexible polysaccharide-based extracellular matrix: the cell wall. Cell wall composition varies among plant species, across cell types within a plant, within different regions of a single cell wall, and in response to intrinsic or extrinsic signals. This diversity in cell wall makeup is underpinned by common cellular mechanisms for cell wall production. Cellulose synthase complexes function at the plasma membrane and deposit their product into the cell wall. Matrix polysaccharides are synthesized by a multitude of glycosyltransferases in hundreds of mobile Golgi stacks, and an extensive set of vesicle trafficking proteins govern secretion to the cell wall. In this review, we discuss the different subcellular locations at which cell wall synthesis occurs, review the molecular mechanisms that control cell wall biosynthesis, and examine how these are regulated in response to different perturbations to maintain cell wall homeostasis.

Polysaccharide Biosynthesis: Glycosyltransferases and Their Complexes

Glycosyltransferases (GTs) are enzymes that catalyze reactions attaching an activated sugar to an acceptor substrate, which may be a polysaccharide, peptide, lipid, or small molecule. In the past decade, notable progress has been made in revealing and cloning genes encoding polysaccharide-synthesizing GTs. However, the vast majority of GTs remain structurally and functionally uncharacterized. The mechanism by which they are organized in the Golgi membrane, where they synthesize complex, highly branched polysaccharide structures with high efficiency and fidelity, is also mostly unknown. This review will focus on current knowledge about plant polysaccharide-synthesizing GTs, specifically focusing on protein-protein interactions and the formation of multiprotein complexes.

Magnetic Field Induced Changes in the Shoot and Root Proteome of Barley (Hordeum vulgare L.)

The geomagnetic field (GMF) has been present since the beginning of plant evolution. Recently, some researchers have focused their efforts on employing magnetic fields (MFs) higher than GMF to improve the seed germination, growth, and harvest of agriculturally important crop plants, as the use of MFs is an inexpensive and environment-friendly technique. In this study, we have employed different treatments of MF at 7 mT (milliTesla) at different time points of exposure, including 1, 3, and 6 h. The extended exposure was followed by five consecutive days at 6 h per day in barley seeds. The results showed a positive impact of MF on growth characteristics for 5-day-old seedlings, including seed germination rate, root and shoot length, and biomass weight. Furthermore, ~5 days of delay of flowering in pre-treated plants was also observed. We used a shotgun proteomics approach to identify changes in the protein signatures of root and shoot tissues under MF effects. In total, we have identified 2,896 proteins. Thirty-eight proteins in the shoot and 15 proteins in the root showed significant changes under the MF effect. Proteins involved in primary metabolic pathways were increased in contrast to proteins with a metal ion binding function, proteins that contain iron ions in their structure, and proteins involved in electron transfer chain, which were all decreased significantly in the treated tissues. The upregulated proteins' overall biological processes included carbohydrate metabolic process, oxidation-reduction process, and cell redox homeostasis, while down-regulated processes included translation and protein refolding. In general, shoot response was more affected by MF effect than root tissue, leading to the identification of 41 shoot specific proteins. This study provides an initial insight into the proteome regulation response to MF during barley's seedling stage.

The synthesis of xyloglucan, an abundant plant cell wall polysaccharide, requires CSLC function

Plant cells have a polysaccharide-based wall that maintains their structural and functional integrity and determines their shape. Reorganization of wall components is required to allow growth and differentiation. One matrix polysaccharide that is postulated to play an important role in this reorganization is xyloglucan (XyG). While the structure of XyG is well understood, its biosynthesis is not. Through genetic studies with Arabidopsis CSLC genes, we demonstrate that they are responsible for the synthesis of the XyG glucan backbone. A quintuple cslc mutant is able to grow and develop normally but lacks detectable XyG. These results raise important questions regarding cell wall structure and its reorganization during growth. The series of cslc mutants will be valuable tools for investigating these questions.

Xyloglucan (XyG) is an abundant component of the primary cell walls of most plants. While the structure of XyG has been well studied, much remains to be learned about its biosynthesis. Here we employed reverse genetics to investigate the role of Arabidopsis cellulose synthase like-C (CSLC) proteins in XyG biosynthesis. We found that single mutants containing a T-DNA in each of the five Arabidopsis CSLC genes had normal levels of XyG. However, higher-order cslc mutants had significantly reduced XyG levels, and a mutant with disruptions in all five CSLC genes had no detectable XyG. The higher-order mutants grew with mild tissue-specific phenotypes. Despite the apparent lack of XyG, the cslc quintuple mutant did not display significant alteration of gene expression at the whole-genome level, excluding transcriptional compensation. The quintuple mutant could be complemented by each of the five CSLC genes, supporting the conclusion that each of them encodes a XyG glucan synthase. Phylogenetic analyses indicated that the CSLC genes are widespread in the plant kingdom and evolved from an ancient family. These results establish the role of the CSLC genes in XyG biosynthesis, and the mutants described here provide valuable tools with which to study both the molecular details of XyG biosynthesis and the role of XyG in plant cell wall structure and function.

Elongating maize root: zone-specific combinations of polysaccharides from type I and type II primary cell walls

The dynamics of cell wall polysaccharides may modulate the cell wall mechanics and thus control the expansion growth of plant cells. The unique composition of type II primary cell wall characteristic of grasses suggests that they employ specific mechanisms for cell enlargement. We characterized the transcriptomes in five zones along maize root, clustered the expression of genes for numerous glycosyltransferases and performed extensive immunohistochemical analysis to relate the changes in cell wall polysaccharides to critical stages of cell development in Poaceae. Specific patterns of cell wall formation differentiate the initiation, realization and cessation of elongation growth. Cell walls of meristem and early elongation zone represent a mixture of type I and type II specific polysaccharides. Xyloglucans and homogalacturonans are synthesized there actively together with mixed-linkage glucans and glucuronoarabinoxylans. Rhamnogalacturonans-I with the side-chains of branched 1,4-galactan and arabinan persisted in cell walls throughout the development. Thus, the machinery to generate the type I primary cell wall constituents is completely established and operates. The expression of glycosyltransferases responsible for mixed-linkage glucan and glucuronoarabinoxylan synthesis peaks at active or late elongation. These findings widen the number of jigsaw pieces which should be put together to solve the puzzle of grass cell growth.

A Group of O-Acetyltransferases Catalyze Xyloglucan Backbone Acetylation and Can Alter Xyloglucan Xylosylation Pattern and Plant Growth When Expressed in Arabidopsis

Xyloglucan is a major hemicellulose in plant cell walls and exists in two distinct types, XXXG and XXGG. While the XXXG-type xyloglucan from dicot species only contains O-acetyl groups on side-chain galactose (Gal) residues, the XXGG-type xyloglucan from Poaceae (grasses) and Solanaceae bears O-acetyl groups on backbone glucosyl (Glc) residues. Although O-acetyltransferases responsible for xyloglucan Gal acetylation have been characterized, the biochemical mechanism underlying xyloglucan backbone acetylation remains to be elucidated. In this study, we showed that recombinant proteins of a group of DUF231 members from rice and tomato were capable of transferring acetyl groups onto O-6 of Glc residues in cello-oligomer acceptors, indicating that they are xyloglucan backbone 6-O-acetyltransferases (XyBATs). We further demonstrated that XyBAT-acetylated cellohexaose oligomers could be readily xylosylated by AtXXT1 (Arabidopsis xyloglucan xylosyltransferase 1) to generate acetylated, xylosylated cello-oligomers, whereas AtXXT1-xylosylated cellohexaose oligomers were much less effectively acetylated by XyBATs. Heterologous expression of a rice XyBAT in Arabidopsis led to a severe reduction in cell expansion and plant growth and a drastic alteration in xyloglucan xylosylation pattern with the formation of acetylated XXGG-type units, including XGG, XGGG, XXGG, XXGG,XXGGG and XXGGG (G denotes acetylated Glc). In addition, recombinant proteins of two Arabidopsis XyBAT homologs also exhibited O-acetyltransferase activity toward cellohexaose, suggesting their possible role in mediating xyloglucan backbone acetylation in vivo. Our findings provide new insights into the biochemical mechanism underlying xyloglucan backbone acetylation and indicate the importance of maintaining the regular xyloglucan xylosylation pattern in cell wall function.

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.

Identification of two functional xyloglucan galactosyltransferase homologs BrMUR3 and BoMUR3 in brassicaceous vegetables

Xyloglucan (XyG) is the predominant hemicellulose in the primary cell walls of most dicotyledonous plants. Current models of these walls predict that XyG interacts with cellulose microfibrils to provide the wall with the rigidity and strength necessary to maintain cell integrity. Remodeling of this network is required to allow cell elongation and plant growth. In this study, homologs of Arabidopsis thaliana MURUS3 (MUR3), which encodes a XyG-specific galactosyltransferase, were obtained from Brassica rapa (BrMUR3) to Brassica oleracea (BoMUR3). Genetic complementation showed that BrMUR3 and BoMUR3 rescue the phenotypic defects of the mur3-3 mutant. Xyloglucan subunit composition analysis provided evidence that BrMUR3 and BoMUR3 encode a galactosyltransferase, which transfers a galactose residue onto XyG chains. The detection of XXFG and XLFG XyG subunits (restoration of fucosylated side chains) in mur3-3 mutants overexpressing BrMUR3 or BoMUR3 show that MUR3 from Brassica to Arabidopsis are comparable as they add Gal to the third xylosyl residue of the XXXG subunit. Our results provide additional information for functional dissection and evolutionary analysis of MUR3 genes derived from brassicaceous species.

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.

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.

Xyloglucan O-acetyltransferases from Arabidopsis thaliana and Populus trichocarpa catalyze acetylation of fucosylated galactose residues on xyloglucan side chains

AXY4/XGOAT1, AXY4L/XGOAT2 and PtrXGOATs are O-acetyltransferases acetylating fucosylated galactose residues on xyloglucan and AXY9 does not directly catalyze O-acetylation of xyloglucan but exhibits weak acetylesterase activity. Xyloglucan is a major hemicellulose that cross-links cellulose in the primary walls of dicot plants and the galactose (Gal) residues on its side chains can be mono- and di-O-acetylated. In Arabidopsis thaliana, mutations of three AXY (altered xyloglucan) genes, AXY4, AXY4L and AXY9, have previously been shown to cause a reduction in xyloglucan acetylation, but their biochemical functions remain to be investigated. In this report, we demonstrated that recombinant proteins of AXY4/XGOAT1 (xyloglucan O-acetyltransferase1), AXY4L/XGOAT2 and their close homologs from Populus trichocarpa, PtrXGOATs, displayed O-acetyltransferase activities transferring acetyl groups from acetyl CoA onto xyloglucan oligomers. Structural analysis of XGOAT-catalyzed reaction products revealed that XGOATs mediated predominantly 6-O-monoacetylation and a much lesser degree of 3-O and 4-O-monoacetylation and 4,6-di-O-acetylation of Gal residues on xyloglucan side chains. XGOATs appeared to preferentially acetylate fucosylated Gal residues with little activity toward non-fucosylated Gal residues. Mutations of the conserved amino acid residues in the GDS and DXXH motifs in AXY4/XGOAT1 resulted in a drastic reduction in its ability to transfer acetyl groups onto xyloglucan oligomers. In addition, although recombinant AXY9 was unable to transfer acetyl groups from acetyl CoA onto xyloglucan oligomers, it was catalytically active as demonstrated by its weak acetylesterase activity that was also exhibited by AXY4/XGOAT1 and AXY4L/XGOAT2. Furthermore, we showed that the AXY8 fucosidase was able to hydrolyze fucosyl residues from both non-acetylated and acetylated xyloglucan oligomers. These findings provide biochemical evidence that AXY4/XGOAT1, AXY4L/XGOAT2 and PtrXGOATs are xyloglucan O-acetyltransferases catalyzing acetyl transfer onto fucosylated Gal residues on xyloglucan side chains and the defucosylation of these acetylated side chains by apoplastic AXY8 generates side chains with acetylated, non-fucosylated Gal residues.

Structure of xyloglucan xylosyltransferase 1 reveals simple steric rules that define biological patterns of xyloglucan polymers

The plant cell wall is primarily a polysaccharide mesh of the most abundant biopolymers on earth. Although one of the richest sources of biorenewable materials, the biosynthesis of the plant polysaccharides is poorly understood. Structures of many essential plant glycosyltransferases are unknown and suitable substrates are often unavailable for in vitro analysis. The dearth of such information impedes the development of plants better suited for industrial applications. Presented here are structures of Arabidopsis xyloglucan xylosyltransferase 1 (XXT1) without ligands and in complexes with UDP and cellohexaose. XXT1 initiates side-chain extensions from a linear glucan polymer by transferring the xylosyl group from UDP-xylose during xyloglucan biosynthesis. XXT1, a homodimer and member of the GT-A fold family of glycosyltransferases, binds UDP analogously to other GT-A fold enzymes. Structures here and the properties of mutant XXT1s are consistent with a SNi-like catalytic mechanism. Distinct from other systems is the recognition of cellohexaose by way of an extended cleft. The XXT1 dimer alone cannot produce xylosylation patterns observed for native xyloglucans because of steric constraints imposed by the acceptor binding cleft. Homology modeling of XXT2 and XXT5, the other two xylosyltransferases involved in xyloglucan biosynthesis, reveals a structurally altered cleft in XXT5 that could accommodate a partially xylosylated glucan chain produced by XXT1 and/or XXT2. An assembly of the three XXTs can produce the xylosylation patterns of native xyloglucans, suggesting the involvement of an organized multienzyme complex in the xyloglucan biosynthesis.

Automated glycan assembly of galactosylated xyloglucan oligosaccharides and their recognition by plant cell wall glycan-directed antibodies

We report the automated glycan assembly of oligosaccharides related to the plant cell wall hemicellulosic polysaccharide xyloglucan. The synthesis of galactosylated xyloglucan oligosaccharides was enabled by introducing p-methoxybenzyl (PMB) as a temporary protecting group for automated glycan assembly. The generated oligosaccharides were printed as microarrays, and the binding of a collection of xyloglucan-directed monoclonal antibodies (mAbs) to the oligosaccharides was assessed. We also demonstrated that the printed glycans can be further enzymatically modified while appended to the microarray surface by Arabidopsis thaliana xyloglucan xylosyltransferase 2 (AtXXT2).

Three UDP-xylose transporters participate in xylan biosynthesis by conveying cytosolic UDP-xylose into the Golgi lumen in Arabidopsis

UDP-xylose (UDP-Xyl) is synthesized by UDP-glucuronic acid decarboxylases, also termed UDP-Xyl synthases (UXSs). The Arabidopsis genome encodes six UXSs, which fall into two groups based upon their subcellular location: the Golgi lumen and the cytosol. The latter group appears to play an important role in xylan biosynthesis. Cytosolic UDP-Xyl is transported into the Golgi lumen by three UDP-Xyl transporters (UXT1, 2, and 3). However, while single mutants affected in the UDP-Xyl transporter 1 (UXT1) showed a substantial reduction in cell wall xylose content, a double mutant affected in UXT2 and UXT3 had no obvious effect on cell wall xylose deposition. This prompted us to further investigate redundancy among the members of the UXT family. Multiple uxt mutants were generated, including a triple mutant, which exhibited collapsed vessels and reduced cell wall thickness in interfascicular fiber cells. Monosaccharide composition, molecular weight, nuclear magnetic resonance, and immunolabeling studies demonstrated that both xylan biosynthesis (content) and fine structure were significantly affected in the uxt triple mutant, leading to phenotypes resembling those of the irx mutants. Pollination was also impaired in the uxt triple mutant, likely due to reduced filament growth and anther dehiscence caused by alterations in the composition of the cell walls. Moreover, analysis of the nucleotide sugar composition of the uxt mutants indicated that nucleotide sugar interconversion is influenced by the cytosolic UDP-Xyl pool within the cell. Taken together, our results underpin the physiological roles of the UXT family in xylan biosynthesis and provide novel insights into the nucleotide sugar metabolism and trafficking in plants.

Analyzing Xyloglucan Endotransglycosylases by Incorporating Synthetic Oligosaccharides into Plant Cell Walls

The plant cell wall is a cellular exoskeleton consisting predominantly of a complex polysaccharide network that defines the shape of cells. During growth, this network can be loosened through the action of xyloglucan endotransglycosylases (XETs), glycoside hydrolases that "cut and paste" xyloglucan polysaccharides through a transglycosylation process. We have analyzed cohorts of XETs in different plant species to evaluate the substrate specificities of xyloglucan acceptors by using a set of synthetic oligosaccharides obtained by automated glycan assembly. The ability of XETs to incorporate the oligosaccharides into polysaccharides printed as microarrays and into stem sections of Arabidopsis thaliana, beans, and peas was assessed. We found that single xylose substitutions are sufficient for transfer, and xylosylation of the terminal glucose residue is not required by XETs, independent of plant species. To obtain information on the potential xylosylation pattern of the natural acceptor of XETs, that is, the nonreducing end of xyloglucan, we further tested the activity of xyloglucan xylosyl transferase (XXT) 2 on the synthetic xyloglucan oligosaccharides. These data shed light on inconsistencies between previous studies towards determining the acceptor substrate specificities of XETs and have important implications for further understanding plant cell wall polysaccharide synthesis and remodeling.

DGE-seq analysis of MUR3-related Arabidopsis mutants provides insight into how dysfunctional xyloglucan affects cell elongation

Our previous study of the Arabidopsis mur3-3 mutant and mutant plants in which the mur3-3 phenotypes are suppressed (xxt2mur3-3, xxt5mur3-3, xxt1xxt2mur3-3 and 35Spro:XLT2:mur3-3) showed that hypocotyl cell elongation is decreased in plants that synthesize galactose-deficient xyloglucan. To obtain genome-wide insight into the transcriptome changes and regulatory networks that may be involved in this decreased elongation, we performed digital gene expression analyses of the etiolated hypocotyls of wild type (WT), mur3-3 and the four suppressor lines. Numerous differentially expressed genes (DEGs) were detected in comparisons between WT and mur3-3 (1423), xxt2mur3-3 and mur3-3 (675), xxt5mur3-3 and mur3-3 (1272), xxt1xxt2mur3-3 and mur3-3 (1197) and 35Spro:XLT2:mur3-3 vs mur3-3 (121). 550 overlapped DEGs were detected among WT vs mur3-3, xxt2mur3-3 vs mur3-3, xxt5mur3-3 vs mur3-3, and xxt1xxt2mur3-3 vs mur3-3 comparisons. These DEGs include 46 cell wall-related genes, 24 transcription factors, 6 hormone-related genes, 9 protein kinase genes and 9 aquaporin genes. The expression of all of the 550 overlapped genes is restored to near wild-type levels in the four mur3-3 suppressor lines. qRT-PCR of fifteen of these 550 genes showed that their expression levels are consistent with the digital gene expression data. Overexpression of some of these genes (XTH4, XTH30, PME3, EXPA11, MYB88, ROT3, AT5G37790, WAG2 and TIP2;3) that are down-regulated in mur3-3 partially rescued the short hypocotyl phenotype but not the aerial phenotype of mur3-3, indicating that different mechanisms exist between hypocotyl cell elongation and leaf cell elongation.

Components of the SNARE-containing regulon are co-regulated in root cells undergoing defense

The term regulon has been coined in the genetic model plant Arabidopsis thaliana, denoting a structural and physiological defense apparatus defined genetically through the identification of the penetration (pen) mutants. The regulon is composed partially by the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) syntaxin PEN1. PEN1 has homology to a Saccharomyces cerevisae gene that regulates a Secretion (Sec) protein, Suppressor of Sec 1 (Sso1p). The regulon is also composed of the β-glucosidase (PEN2) and an ATP binding cassette (ABC) transporter (PEN3). While important in inhibiting pathogen infection, limited observations have been made regarding the transcriptional regulation of regulon genes until now. Experiments made using the model agricultural Glycine max (soybean) have identified co-regulated gene expression of regulon components. The results explain the observation of hundreds of genes expressed specifically in the root cells undergoing the natural process of defense. Data regarding additional G. max genes functioning within the context of the regulon are presented here, including Sec 14, Sec 4 and Sec 23. Other examined G. max homologs of membrane fusion genes include an endosomal bromo domain-containing protein1 (Bro1), syntaxin6 (SYP6), SYP131, SYP71, SYP8, Bet1, coatomer epsilon (ϵ-COP), a coatomer zeta (ζ-COP) paralog and an ER to Golgi component (ERGIC) protein. Furthermore, the effectiveness of biochemical pathways that would function within the context of the regulon ave been examined, including xyloglucan xylosyltransferase (XXT), reticuline oxidase (RO) and galactinol synthase (GS). The experiments have unveiled the importance of the regulon during defense in the root and show how the deposition of callose relates to the process.

Interconnections between cell wall polymers, wall mechanics, and cortical microtubules: Teasing out causes and consequences

In plants, cell wall components including cellulose, hemicelluloses, and pectins interact with each other to form complex extracellular network structures that control cell growth and maintain cell shape. However, it is still not clear exactly how different wall polymers interact, how the conformations and interactions of cell wall polymers relate to wall mechanics, and how these factors impinge on intracellular structures such as the cortical microtubule cytoskeleton. Here, based on studies of Arabidopsis thaliana xxt1 xxt2 mutants, which lack detectable xyloglucan in their walls and display aberrant wall mechanics, altered cellulose patterning and biosynthesis, and reduced cortical microtubule stability, we discuss the potential relationships between cell wall biosynthesis, wall mechanics, and cytoskeletal dynamics in an effort to better understand their roles in controlling plant growth and morphogenesis.

Expression, purification and biochemical characterization of AtFUT1, a xyloglucan-specific fucosyltransferase from Arabidopsis thaliana

Efforts to identify genes and characterize enzymes involved in the biosynthesis of plant cell wall polysaccharides have yet to produce and purify to homogeneity an active plant cell wall synthesizing enzyme suitable for structural studies. In Arabidopsis, the last step of xyloglucan (XG) biosynthesis is catalyzed by fucosyltransferase 1 (AtFUT1), which transfers l-fucose from GDP-β-l-fucose to a specific galactose on the XG core. Here, we describe the production of a soluble form of AtFUT1 (HisΔ68-AtFUT1) and its purification to milligram quantities. An active form of AtFUT1 was produced in an insect cell culture medium, using a large-scale expression system, and purified in a two-step protocol. Characterization of purified HisΔ68-AtFUT1 revealed that the enzyme behaves as a non-covalent homodimer in solution. A bioluminescent transferase assay confirmed HisΔ68-AtFUT1 activity on its substrates, namely GDP-fucose and tamarind XG, with calculated Km values of 42 μM and 0.31 μM, respectively. Moreover, the length of the XG-derived acceptor quantitatively affected fucosyltransferase activity in a size-dependent manner. The affinity of HisΔ68-AtFUT1 for tamarind XG and GDP was determined using isothermal titration calorimetry (ITC). Interestingly, ITC data suggest that HisΔ68-AtFUT1 undergoes conformational changes in the presence of its first co-substrate (XG or GDP), which then confers greater affinity for the second co-substrate. The procedure described in this study can potentially be transferred to other enzymes involved in plant cell wall synthesis.

Role of UDP-Glucuronic Acid Decarboxylase in Xylan Biosynthesis in Arabidopsis

UDP-xylose (UDP-Xyl) is the Xyl donor used in the synthesis of major plant cell-wall polysaccharides such as xylan (as a backbone-chain monosaccharide) and xyloglucan (as a branching monosaccharide). The biosynthesis of UDP-Xyl from UDP-glucuronic acid (UDP-GlcA) is irreversibly catalyzed by UDP-glucuronic acid decarboxylase (UXS). Until now, little has been known about the physiological roles of UXS in plants. Here, we report that AtUXS1, AtUXS2, and AtUXS4 are located in the Golgi apparatus whereas AtUXS3, AtUXS5, and AtUXS6 are located in the cytosol. Although all six single AtUXS T-DNA mutants and the uxs1 usx2 uxs4 triple mutant show no obvious phenotype, the uxs3 uxs5 uxs6 triple mutant has an irregular xylem phenotype. Monosaccharide analysis showed that Xyl levels decreased in uxs3 uxs5 uxs6 and linkage analysis confirmed that the xylan content in uxs3 xus5 uxs6 declined, indicating that UDP-Xyl from cytosol AtUXS participates in xylan synthesis. Gel-permeation chromatography showed that the molecular weight of non-cellulosic polysaccharides in the triple mutants, mainly composed of xylans, is lower than that in the wild type, suggesting an effect on the elongation of the xylan backbone. Upon saccharification treatment stems of the uxs3 uxs5 uxs6 triple mutants released monosaccharides with a higher efficiency than those of the wild type. Taken together, our results indicate that the cytosol UXS plays a more important role than the Golgi-localized UXS in xylan biosynthesis.

Truncations of xyloglucan xylosyltransferase 2 provide insights into the roles of the N- and C-terminus

Xyloglucan is the most abundant hemicellulose in the primary cell wall of dicotyledonous plants. In Arabidopsis, three xyloglucan xylosyltransferases, XXT1, XXT2, and XXT5, participate in xylosylation of the xyloglucan backbone. Despite the importance of these enzymes, there is a lack of information on their structure and the critical residues required for substrate binding and transferase activity. In this study, the roles of different domains of XX2 in protein expression and catalytic activity were investigated by constructing a series of N- and C-terminal truncations. XXT2 with an N-terminal truncation of 31 amino acids after the predicted transmembrane domain showed the highest protein expression, but truncations of more than 31 residues decreased protein expression and catalytic activity. XXT2 constructs with C-terminal truncations showed increased protein expression but decreased activity, particularly for truncations of 44 or more amino acids. Site-directed mutagenesis was also used to investigate six positively charged residues near the C-terminus and found that four of the mutants showed decreased enzymatic activity. We conclude that the N- and C-termini of XXT2 have important roles in protein folding and enzymatic activity: the stem region (particularly the N-terminus of the catalytic domain) is critical for protein folding and the C-terminus is essential for enzymatic activity but not for protein folding.

The galactoside 2-α-L-fucosyltransferase FUT1 from Arabidopsis thaliana: crystallization and experimental MAD phasing

The plant cell wall is a complex network of polysaccharides made up of cellulose, hemicelluloses and pectins. Xyloglucan (XyG), which is the main hemicellulosic component of dicotyledonous plants, has attracted much attention for its role in plant development and for its many industrial applications. The XyG-specific fucosyltransferase (FUT1) adds a fucose residue from GDP-fucose to the 2-O position of the terminal galactosyl residues on XyG side chains. Recombinant FUT1 from Arabidopsis thaliana was crystallized in two different crystal forms, with the best diffracting crystals (up to 1.95 Å resolution) belonging to the monoclinic space group P21, with unit-cell parameters a = 87.6, b = 84.5, c = 150.3 Å, β = 96.3°. Ab initio phases were determined using a two-wavelength anomalous dispersion experiment on a tantalum bromide-derivatized crystal with data collected at the rising and descending inflection points of the Ta white line. An interpretable electron-density map was obtained after elaborate density modification. Model completion and structural analysis are currently under way.

Biosynthesis of the Plant Cell Wall Matrix Polysaccharide Xyloglucan

Xyloglucan (XyG) is a matrix polysaccharide that is present in the cell walls of all land plants. It consists of a β-1,4-linked glucan backbone that is further substituted with xylosyl residues. These xylosyl residues can be further substituted with other glycosyl and nonglycosyl substituents that vary depending on the plant family and specific tissue. Advances in plant mutant isolation and characterization, functional genomics, and DNA sequencing have led to the identification of nearly all transferases and synthases necessary to synthesize XyG. Thus, in terms of the molecular mechanisms of plant cell wall polysaccharide biosynthesis, XyG is the most well understood. However, much remains to be learned about the molecular mechanisms of polysaccharide assembly and the regulation of these processes. Knowledge of the XyG biosynthetic machinery allows the XyG structure to be tailored in planta to ascertain the functions of this polysaccharide and its substituents in plant growth and interactions with the environment.

Expression of heterologous xyloglucan xylosyltransferases in Arabidopsis to investigate their role in determining xyloglucan xylosylation substitution patterns

Putative XyG xylosyltransferases from Tropaeolum majus (nasturtium) and Solanum lycopersicum (tomato) homologous to characterized Arabidopsis genes were identified and shown to functionally complement Arabidopsis mutants lacking xyloglucan demonstrating they represent xyloglucan xylosyltransferases. Xyloglucan is a major hemicellulose in the plant cell wall and is important for the structural organization of the wall. The fine structure of xyloglucan can vary dependent on plant species and tissue type. Most vascular seed-bearing plants including Arabidopsis thaliana and nasturtium (Tropaeolum majus) have a xyloglucan structure, in which three out of four backbone glucosyl-residues are substituted with xylosyl-residues. In contrast, the xyloglucan found in plants of the Solanaceae family, which includes tomato (Solanum lycopersicum), is typically less xylosylated with only two of the four backbone glucosyl-residues substituted with xylosyl-residues. To investigate the genetics of xyloglucan xylosylation, candidate xyloglucan xylosyltransferase genes (XXTs) homologous to known A. thaliana XXTs were cloned from nasturtium and tomato. These candidate XXTs were expressed in the A. thaliana xxt1/2 double and xxt1/2/5 triple mutant, whose walls lack detectable xyloglucan. Expression of the orthologs of XXT5 resulted in no detectable xyloglucan in the transgenic A. thaliana plants, consistent with a lack of xyloglucan in the A. thaliana xxt1/2 double mutant. However, transformation of both the tomato and nasturtium orthologs of AtXXT1 and AtXXT2 resulted in the production of xyloglucan with a xylosylation pattern similar to wild type A. thaliana indicating that both SlXXT2 and TmXXT2 likely have xylosyltransferase activity. As the expression of the SlXXT2 did not result in xyloglucan with a decreased xylosylation frequency found in tomato, this gene is not responsible for the unique xylosylation pattern found in the solanaceous plants.

Protein-protein interactions among xyloglucan-synthesizing enzymes and formation of Golgi-localized multiprotein complexes

Arabidopsis thaliana xyloglucan has an XXXG structure, with branches of xylosyl residues, β-D-galacosyl-(1,2)-α-d-xylosyl motifs and fucosylated β-D-galactosyl-(1,2)-α-D-xylosyl motifs. Most of the enzymes involved in xyloglucan biosynthesis in Arabidopsis have been identified, including the glucan synthase CSLC4 (cellulose synthase-like C4), three xylosyltransferases (XXT1, XXT2 and XXT5), two galactosyltransferases (MUR3 and XLT2) and the fucosyltransferase FUT1. The XXTs and CSLC4 form homo- and heterocomplexes and were proposed to co-localize in the same complex, but the organization of the other xyloglucan-synthesizing enzymes remains unclear. Here we investigate whether the glycosyltransferases MUR3, XLT2 and FUT1 interact with the XXT-CSLC4 complexes in the Arabidopsis Golgi. We used co-immunoprecipitation and bimolecular fluorescence complementation, with signal quantification by flow cytometry, to demonstrate that CSLC4 interacts with MUR3, XLT2 and FUT1. FUT1 forms homocomplexes and interacts with MUR3, XLT2, XXT2 and XXT5. XLT2 interacts with XXT2 and XXT5, but MUR3 does not. Co-immunoprecipitation assays showed that FUT1 forms a homocomplex through disulfide bonds, and formation of the heterocomplexes does not involve covalent interactions. In vitro pull-down assays indicated that interactions in the FUT1-MUR3 and FUT1-XXT2 complexes occur through the protein catalytic domains. We propose that enzymes involved in xyloglucan biosynthesis are functionally organized in multiprotein complexes localized in the Golgi.

A reversible Renilla luciferase protein complementation assay for rapid identification of protein-protein interactions reveals the existence of an interaction network involved in xyloglucan biosynthesis in the plant Golgi apparatus

A growing body of evidence suggests that protein-protein interactions (PPIs) occur amongst glycosyltransferases (GTs) required for plant glycan biosynthesis (e.g. cell wall polysaccharides and N-glycans) in the Golgi apparatus, and may control the functions of these enzymes. However, identification of PPIs in the endomembrane system in a relatively fast and simple fashion is technically challenging, hampering the progress in understanding the functional coordination of the enzymes in Golgi glycan biosynthesis. To solve the challenges, we adapted and streamlined a reversible Renilla luciferase protein complementation assay (Rluc-PCA), originally reported for use in human cells, for transient expression in Nicotiana benthamiana. We tested Rluc-PCA and successfully identified luminescence complementation amongst Golgi-localizing GTs known to form a heterodimer (GAUT1 and GAUT7) and those which homooligomerize (ARAD1). In contrast, no interaction was shown between negative controls (e.g. GAUT7, ARAD1, IRX9). Rluc-PCA was used to investigate PPIs amongst Golgi-localizing GTs involved in biosynthesis of hemicelluloses. Although no PPI was identified among six GTs involved in xylan biosynthesis, Rluc-PCA confirmed three previously proposed interactions and identified seven novel PPIs amongst GTs involved in xyloglucan biosynthesis. Notably, three of the novel PPIs were confirmed by a yeast-based split-ubiquitin assay. Finally, Gateway-enabled expression vectors were generated, allowing rapid construction of fusion proteins to the Rluc reporters and epitope tags. Our results show that Rluc-PCA coupled with transient expression in N. benthamiana is a fast and versatile method suitable for analysis of PPIs between Golgi resident proteins in an easy and mid-throughput fashion in planta.

Structural Diversity and Function of Xyloglucan Sidechain Substituents

Xyloglucan (XyG) is a hemicellulose found in the cell walls of all land plants including early-divergent groups such as liverworts, hornworts and mosses. The basic structure of XyG, a xylosylated glucan, is similar in all of these plants but additional substituents can vary depending on plant family, tissue, and developmental stage. A comprehensive list of known XyG sidechain substituents is assembled including their occurrence within plant families, thereby providing insight into the evolutionary origin of the various sidechains. Recent advances in DNA sequencing have enabled comparative genomics approaches for the identification of XyG biosynthetic enzymes in Arabidopsis thaliana as well as in non-model plant species. Characterization of these biosynthetic genes not only allows the determination of their substrate specificity but also provides insights into the function of the various substituents in plant growth and development.

Mutation in xyloglucan 6-xylosytransferase results in abnormal root hair development in Oryza sativa

Root hairs are important for nutrient uptake, anchorage, and plant-microbe interactions. From a population of rice (Oryza sativa) mutagenized by ethyl methanesulfonate (EMS), a short root hair2 (srh2) mutant was identified. In hydroponic culture, srh2 seedlings were significantly reduced in root hair length. Bubble-like extrusions and irregular epidermal cells were observed at the tips of srh2 root hairs when grown under acidic conditions, suggesting the possible reduction of the tensile strength of the cell wall in this mutant. Map-based cloning identified a mutation in the gene encoding xyloglucan (XyG) 6-xylosyltransferase (OsXXT1). OsXXT1 displays more than 70% amino acid sequence identity with the previously characterized Arabidopsis thaliana XYG XYLOSYL TRANSFERASE 1 (AtXXT1) and XYG XYLOSYL TRANSFERASE 2 (AtXXT2), which catalyse the transfer of xylose onto β-1,4-glucan chains. Furthermore, expression of the full-length coding sequence of OsXXT1 could complement the root hair defect, and slow growth and XyG synthesis in the Arabidopsis xxt1 xxt2 double mutant. Transgenic plants expressing the β-glucuronidase (GUS) reporter under the control of the OsXXT1 promoter displayed GUS expression in multiple tissues, most prominently in root epidermal cells. These results demonstrate the importance of OsXXT1 in maintaining cell wall structure and tensile strength in rice, a typical grass species that contains relatively low XyG content in cell walls.

Family 34 glycosyltransferase (GT34) genes and proteins in Pinus radiata (radiata pine) and Pinus taeda (loblolly pine)

Using a functional genomics approach, four candidate genes (PtGT34A, PtGT34B, PtGT34C and PtGT34D) were identified in Pinus taeda. These genes encode CAZy family GT34 glycosyltransferases that are involved in the synthesis of cell-wall xyloglucans and heteromannans. The full-length coding sequences of three orthologs (PrGT34A, B and C) were isolated from a xylem-specific cDNA library from the closely related Pinus radiata. PrGT34B is the ortholog of XXT1 and XXT2, the two main xyloglucan (1→6)-α-xylosyltransferases in Arabidopsis thaliana. PrGT34C is the ortholog of XXT5 in A. thaliana, which is also involved in the xylosylation of xyloglucans. PrGT34A is an ortholog of a galactosyltransferase from fenugreek (Trigonella foenum-graecum) that is involved in galactomannan synthesis. Truncated coding sequences of the genes were cloned into plasmid vectors and expressed in a Sf9 insect cell-culture system. The heterologous proteins were purified, and in vitro assays showed that, when incubated with UDP-xylose and cellotetraose, cellopentaose or cellohexaose, PrGT34B showed xylosyltransferase activity, and, when incubated with UDP-galactose and the same cello-oligosaccharides, PrGT34B showed some galactosyltransferase activity. The ratio of xylosyltransferase to galactosyltransferase activity was 434:1. Hydrolysis of the galactosyltransferase reaction products using galactosidases showed the linkages formed were α-linkages. Analysis of the products of PrGT34B by MALDI-TOF MS showed that up to three xylosyl residues were transferred from UDP-xylose to cellohexaose. The heterologous proteins PrGT34A and PrGT34C showed no detectable enzymatic activity.

Regulatory specialization of xyloglucan (XG) and glucuronoarabinoxylan (GAX) in pericarp cell walls during fruit ripening in tomato (Solanum lycopersicum)

Disassembly of cell wall polysaccharides by various cell wall hydrolases during fruit softening causes structural changes in hemicellulose and pectin that affect the physical properties and softening of tomato fruit. In a previous study, we showed that the changes in pectin during tomato fruit ripening were unique in each fruit tissue. In this study, to clarify the changes in hemicellulose in tissues during tomato fruit ripening, we focused on glucuronoarabinoxylan (GAX) and xyloglucan (XG). GAX was detected only in the skin and inner epidermis of the pericarp using LM11 antibodies, whereas a large increase in XG was detected in all fruit tissues using LM15 antibodies. The activity of hemicellulose degradation enzymes, such as β-xylosidase and α-arabinofuranosidase, decreased gradually during fruit ripening, although the tomato fruits continued to soften. In contrast, GAX and XG biosynthesis-related genes were expressed in all tomato fruit tissues even during ripening, indicating that XG was synthesized throughout the fruit and that GAX may be synthesized only in the vascular bundles and the inner epidermis. Our results suggest that changes in the cell wall architecture and tissue-specific distribution of XG and GAX might be required for the regulation of fruit softening and the maintenance of fruit shape.

Hemicellulose biosynthesis

One major component of plant cell walls is a diverse group of polysaccharides, the hemicelluloses. Hemicelluloses constitute roughly one-third of the wall biomass and encompass the heteromannans, xyloglucan, heteroxylans, and mixed-linkage glucan. The fine structure of these polysaccharides, particularly their substitution, varies depending on the plant species and tissue type. The hemicelluloses are used in numerous industrial applications such as food additives as well as in medicinal applications. Their abundance in lignocellulosic feedstocks should not be overlooked, if the utilization of this renewable resource for fuels and other commodity chemicals becomes a reality. Fortunately, our understanding of the biosynthesis of the various hemicelluloses in the plant has increased enormously in recent years mainly through genetic approaches. Taking advantage of this knowledge has led to plant mutants with altered hemicellulosic structures demonstrating the importance of the hemicelluloses in plant growth and development. However, while we are on a solid trajectory in identifying all necessary genes/proteins involved in hemicellulose biosynthesis, future research is required to combine these single components and assemble them to gain a holistic mechanistic understanding of the biosynthesis of this important class of plant cell wall polysaccharides.