Mechanism of mixed-linkage glucan biosynthesis by barley cellulose synthase-like CslF6 (1,3;1,4)-β-glucan synthase

Physiological and molecular investigations on a high-yielding variety and near-isogenic line of rice under continuous phosphorus stress reveal major regulatory function of Pup1 QTL

Phosphorous (P) plays crucial roles in cellular functioning including respiration, photosynthesis, and membranes. P deficiency in the soil causes stunted growth, smaller/erect leaves, lesser tillers, and a considerable decrease in yield. To decipher the functions of Pup1 QTL and delineate the potential effects of continuous P stress on plant growth, yield/quality, physio-biochemical and molecular analyses of rice [Pusa-44 (P deficiency sensitive) and a near-isogenic line-23 (NIL-23), (harbouring Pup1 QTL, tolerant genotype)] were hydroponically grown under P continuous stress [deficiency (4 ppm) or extravagance (≥32 ppm)] till maturity. Decrease in the number of tillers and panicles under stress led to poor agronomic performance of rice. P concentration in roots, leaves, and seeds raised significantly with increasing concentration of P in hydroponic culture. Higher P concentration in the medium led to elevated phytate concentration in seeds; however, it was comparatively more in seeds of the tolerant (NIL-23) genotype. Comparative transcriptome analysis indicated differential expression of genes for P transporters and those implicated in P mobilization/homeostasis, carbohydrate/lipid metabolism, etc. on P deficiency. Moreover, the regulatory function of Pup1 in reprograming the gene expression involved in chromatin assembly, histone/DNA methylation, cell wall organization, etc. was detected in the panicle of tolerant genotype on P deficiency. This study confirms a major regulatory function of Pup1 and outlines the potential effects of excessive P on plant development, productivity, and quality of seeds. These findings would be useful in improving P uptake/use efficiency in rice and prudent/sustainable usage of phosphatic fertilizers.

Multiprotein Complexes of Plant Glycosyltransferases Involved in Their Function and Trafficking

Plant cells utilize protein oligomerization for their functions in numerous important cellular processes. Protein-protein interactions are necessary to stabilize, optimize, and activate enzymes, as well as localize proteins to specific organelles and membranes. Glycosyltransferases-enzymes that attach sugars to polysaccharides, proteins, lipids, and RNA-across multiple plant biosynthetic processes have been demonstrated to interact with one another. The mechanisms behind these interactions are still unknown, but recent research has highlighted extensive examples of protein-protein interactions, specifically in the plant cell wall hemicellulose and pectin biosynthesis that takes place in the Golgi apparatus. In this review, we will discuss what is known so far about the interactions among Golgi-localized glycosyltransferases that are important for their functioning, trafficking, as well as structural aspects.

β-Glucan content increase in Waxy-mutated barley is closely associated with positive stress responses and is regulated by ASR1

Mixed-linkage (1,3; 1,4)-β-D-glucan (MLG) impacts the food and industrial end-uses of barley, but the molecular mechanism of variations in MLG content remains unclear. MLG content usually increases in Waxy-mutated barley. This study applied transcriptomic, proteomic, and metabolomic analyses to Waxy-mutated recombinant inbred lines with higher MLG content and wild-type lines with lower MLG content, and identified candidate genes and pathways regulating MLG content through combining preliminary gene function analysis. MLG biosynthesis differed significantly during late grain development in the Waxy-mutated and wild-type barley lines. The MLG increase was closely associated with strongly active sugar and starch metabolism and stress-responsive plant hormones, particularly abscisic acid (ABA) signaling process. Stress-responsive transcript factors ILR3, BTF3, RGGA, and PR13 protein bind to CslF6, which is critical for barley MLG biosynthesis, and the stress-responsive gene ASR1 also had a positive effect on MLG increase. Waxy mutation enhances barley stress responses by activating ABA- or other stress-responsive plant hormones signaling processes, which facilitates MLG biosynthesis. This study provides a new approach for elucidating the variations in MLG content of barley grains.

Is the CslF6 gene involved in the accumulation of (1,3;1,4)-β-D-glucan in wheats, their wild relatives and their hybrids?

Mixed linkage (1,3;1,4)-β-d-glucan (MLG) is a well-recognized bioactive carbohydrate and dietary fibre with expanding applications in food industry. The MLG are small components of the cell wall of vegetative tissues of cereals synthetized by members of the Cellulose Synthase-Like genes (Csl). Within the family, the CslF6 has been the major contributor in wheat. It is of significant health and economic benefits to enhance MLG content in wheat, a staple grain with naturally low MLG levels. This study investigated the role of CslF6 gene in MLG synthesis and analysed total MLG contents, cell wall monosaccharide, glycosidic linkage composition, and profile of major comprising oligosaccharides of MLG in various wheat genotypes, their wild relatives (Aegilops caudata and Dasypyrum villosum), and hybrids between them. We observed a relationship between CslF6 gene expression and MLG accumulation across the different wheat lines. While Aegilops caudata and Dasypyrum villosum exhibited higher MLG content than other genotypes, hybrid breeding led to an increase in MLG content by 24.4% in durum wheat and 43.3% in T. aestivum. Variations in the ratios of major oligosaccharides released from MLG by lichenase treatment and in the compositions of cell wall monosaccharides and glycosidic linkages were also found. This study demonstrates that HPAEC-PAD and GC-MS-based glycomics are invaluable tools to assist breeders in selecting high MLG lines.

Cell walls, a comparative view of the composition of cell surfaces of plants, algae and microorganisms

While evolutionary studies indicate that the most ancient groups of organisms on Earth likely descended from a common wall-less ancestor, contemporary organisms lacking a carbohydrate-rich cell surface are exceedingly rare. By developing a cell wall to cover the plasma membrane, cells were able to withstand higher osmotic pressures, colonise new habitats and develop complex multicellular structures. This way, the cells of plants, algae and microorganisms are covered by a cell wall, which can generally be defined as a highly complex structure whose main framework is usually composed of carbohydrates. Rather than static structures, they are highly dynamic and serve a multitude of functions that modulate vital cellular processes, such as growth and interactions with neighbouring cells or the surrounding environment. Thus, despite its vital importance for many groups of life, it is striking that there are few comprehensive documents comparing the cell wall composition of these groups. Thus, the aim of this review was to compare the cell walls of plants with those of algae and microorganisms, paying particular attention to their polysaccharide components. It should be highlighted that, despite the important differences in composition, we have also found numerous common aspects and functionalities.

Genome-Wide Analysis of CSL Family Genes Involved in Petiole Elongation, Floral Petalization, and Response to Salinity Stress in Nelumbo nucifera

Lotus (Nelumbo nucifera), a perennial aquatic plant, endures various environmental stresses. Its diverse ornamental traits make it an ideal model for studying multigene family functional differentiation and abiotic stress responses. The cellulose synthase-like (CSL) gene family includes multiple subfamilies and holds potentially pivotal roles in plant growth, development, and stress responses. Thus, understanding this family is essential for uncovering the attributes of ancient dicotyledonous lotus species and offering new genetic resources for targeted genetic improvement. Herein, we conducted a genome-wide NnCSL gene identification study, integrating tissue-specific expression analysis, RNA-seq, and qRT-PCR validation. We identified candidate NnCSL genes linked to petiole elongation, floral petalization, salinity stress responses, and potential co-expressed TFs. 22 NnCSL genes were categorized into six subfamilies: NnCSLA, NnCSLB, NnCSLC, NnCSLD, NnCSLE, and NnCSLG. Promoter regions contain numerous cis-acting elements related to growth, development, stress responses, and hormone regulation. Nineteen NnCSL genes showed specific differential expression in LPA (large plant architecture) versus SPA (small plant architecture): petioles, petalized carpels (CP) and normal carpels (C), and petalized stamens (SP) and normal stamens (S). Notably, most NnCSLC, NnCSLA, and NnCSLB subfamily genes play diverse roles in various aspects of lotus growth and development, while NnCSLE and NnCSLG are specifically involved in carpel petalization and petiole elongation, respectively. Additionally, 11 candidate NnCSL genes responsive to salinity stress were identified, generally exhibiting antagonistic effects on growth and developmental processes. These findings provide an important theoretical foundation and novel insights for the functional study of NnCSL genes in growth, development, and stress resistance in lotus.

Advancing plant cell wall modelling: Atomistic insights into cellulose, disordered cellulose, and hemicelluloses - A review

The complexity of plant cell walls on different hierarchical levels still impedes the detailed understanding of biosynthetic pathways, interferes with processing in industry and finally limits applicability of cellulose materials. While there exist many challenges to readily accessing these hierarchies at (sub-) angström resolution, the development of advanced computational methods has the potential to unravel important questions in this field. Here, we summarize the contributions of molecular dynamics simulations in advancing the understanding of the physico-chemical properties of natural fibres. We aim to present a comprehensive view of the advancements and insights gained from molecular dynamics simulations in the field of carbohydrate polymers research. The review holds immense value as a vital reference for researchers seeking to undertake atomistic simulations of plant cell wall constituents. Its significance extends beyond the realm of molecular modeling and chemistry, as it offers a pathway to develop a more profound comprehension of plant cell wall chemistry, interactions, and behavior. By delving into these fundamental aspects, the review provides invaluable insights into future perspectives for exploration. Researchers within the molecular modeling and carbohydrates community can greatly benefit from this resource, enabling them to make significant strides in unraveling the intricacies of plant cell wall dynamics.

Carbohydrate-active enzymes involved in rice cell wall metabolism

Plant cell walls are complex, multifunctional structures, built up of polysaccharides and proteins. The configuration and abundance of cell wall constituents determine cellular elongation and plant growth. The emphasis of this review is on rice, a staple crop with economic importance, serving as model for grasses/cereals. Recent advancements have contributed to a better understanding of the grass/cereal cell wall. This review brings together current knowledge of the organization and metabolism of the rice cell wall, and addresses gaps in the information regarding the cell wall and enzymes involved. Several cell wall fractions, including cellulose, mixed-linkage glucans, and glucuronoarabinoxylans, are well understood in rice and other grasses/grains. Conversely, there are still open questions and missing links in relation to xyloglucans, glucomannans, pectin, lignin, and arabinogalactan proteins. There is still a large and untapped potential to identify carbohydrate-active enzymes (CAZymes), to characterize their activity, and to elucidate their involvement in the metabolism of the mentioned cell wall fractions. This review highlights the involvement of carbohydrate-active enzymes in rice cell wall metabolism, providing an update of current understanding with the aim of demarcating research areas with potential for further investigations.

Renewable hemicellulose-based materials for value-added applications

The importance of renewable resources and environmentally friendly materials has grown globally in recent time. Hemicellulose is renewable lignocellulosic materials that have been the subject of substantial valorisation research. Due to its distinctive benefits, including its wide availability, low cost, renewability, biodegradability, simplicity of chemical modification, etc., it has attracted increasing interest in a number of value-added fields. In this review, a systematic summarizes of the structure, extraction method, and characterization technique for hemicellulose-based materials was carried out. Also, their most current developments in a variety of value-added adsorbents, biomedical, energy-related, 3D-printed materials, sensors, food packaging applications were discussed. Additionally, the most recent challenges and prospects of hemicellulose-based materials are emphasized and examined in-depth. It is anticipated that in the near future, persistent scientific efforts will enable the renewable hemicellulose-based products to achieve practical applications.

Potential Roles of the GRF Transcription Factors in Sorghum Internodes during Post-Reproductive Stages

Growth-regulating factor (GRF) is a plant-specific family of transcription factors crucial for meristem development and plant growth. Sorghum (Sorghum bicolor L. Moench) is a cereal species widely used for food, feed and fuel. While sorghum stems are important biomass components, the regulation of stem development and the carbohydrate composition of the stem tissues remain largely unknown. Here, we identified 11 SbGRF-encoding genes and found the SbGRF expansion driven by whole-genome duplication events. By comparative analyses of GRFs between rice and sorghum, we demonstrated the divergence of whole-genome duplication (WGD)-derived OsGRFs and SbGRFs. A comparison of SbGRFs' expression profiles supports that the WGD-duplicated OsGRFs and SbGRFs experienced distinct evolutionary trajectories, possibly leading to diverged functions. RNA-seq analysis of the internode tissues identified several SbGRFs involved in internode elongation, maturation and cell wall metabolism. We constructed co-expression networks with the RNA-seq data of sorghum internodes. Network analysis discovered that SbGRF1, 5 and 7 could be involved in the down-regulation of the biosynthesis of cell wall components, while SbGRF4, 6, 8 and 9 could be associated with the regulation of cell wall loosening, reassembly and/or starch biosynthesis. In summary, our genome-wide analysis of SbGRFs reveals the distinct evolutionary trajectories of WGD-derived SbGRF pairs. Importantly, expression analyses highlight previously unknown functions of several SbGRFs in internode elongation, maturation and the potential involvement in the metabolism of the cell wall and starch during post-anthesis stages.

β-glucan: a potent adjuvant in immunotherapy for digestive tract tumors

The immunotherapy for gastrointestinal tumors, as a significant research direction in the field of oncology treatment in recent years, has garnered extensive attention due to its potential therapeutic efficacy and promising clinical application prospects. Recent advances in immunotherapy notwithstanding, challenges persist, such as side effects, the complexity of the tumor immune microenvironment, variable patient responses, and drug resistance. Consequently, there is a pressing need to explore novel adjunctive therapeutic modalities. β-glucan, an immunomodulatory agent, has exhibited promising anti-tumor efficacy in preclinical studies involving colorectal cancer, pancreatic cancer, and gastric cancer, while also mitigating the adverse reactions associated with chemotherapy and enhancing patients' quality of life. However, further clinical and fundamental research is warranted to comprehensively evaluate its therapeutic potential and underlying biological mechanisms. In the future, β-glucan holds promise as an adjunctive treatment for gastrointestinal tumors, potentially bringing significant benefits to patients.

Glycosyl transferase GT2 genes mediate the biosynthesis of an unusual (1,3;1,4)-β-glucan exopolysaccharide in the bacterium Sarcina ventriculi

Linear, unbranched (1,3;1,4)-β-glucans (mixed-linkage glucans or MLGs) are commonly found in the cell walls of grasses, but have also been detected in basal land plants, algae, fungi and bacteria. Here we show that two family GT2 glycosyltransferases from the Gram-positive bacterium Sarcina ventriculi are capable of synthesizing MLGs. Immunotransmission electron microscopy demonstrates that MLG is secreted as an exopolysaccharide, where it may play a role in organizing individual cells into packets that are characteristic of Sarcina species. Heterologous expression of these two genes shows that they are capable of producing MLGs in planta, including an MLG that is chemically identical to the MLG secreted from S. ventriculi cells but which has regularly spaced (1,3)-β-linkages in a structure not reported previously for MLGs. The tandemly arranged, paralogous pair of genes are designated SvBmlgs1 and SvBmlgs2. The data indicate that MLG synthases have evolved different enzymic mechanisms for the incorporation of (1,3)-β- and (1,4)-β-glucosyl residues into a single polysaccharide chain. Amino acid variants associated with the evolutionary switch from (1,4)-β-glucan (cellulose) to MLG synthesis have been identified in the active site regions of the enzymes. The presence of MLG synthesis in bacteria could prove valuable for large-scale production of MLG for medical, food and beverage applications.

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.

Genome-wide identification and functional analysis of the cellulose synthase-like gene superfamily in common oat (Avena sativa L.)

Hemicelluloses constitute approximately one-third of the plant cell wall and can be used as a dietary fiber and food additive, and as raw materials for biofuels. Although genes involved in hemicelluloses synthesis have been investigated in some model plants, no comprehensive analysis has been conducted in common oat at present. In this study, we identified and systematically analyzed the cellulose synthase-like gene (Csl) family members in common oat and investigated them using various bioinformatics tools. The results showed that there are 76 members of the oat Csl gene family distributed on 17 chromosomes, and phylogenetic analysis indicated that the 76 Csl genes belong to the CslA, CslC, CslD, CslE, CslF, CslH, and CslJ subfamilies. A total of 14 classes of cis-acting elements were identified in the promoter regions, including hormone response, light response, cell development, and defense stress elements. The collinearity analysis identified 28 pairs of segmentally duplicated genes, most of which were found on chromosomes 2D and 6A. Expression pattern analysis showed that oat Csl genes display strong tissue-specific expression; of the 76 Csl genes, 33 were significantly up-regulated in stems and 30 were up-regulated in immature seeds. The expression of most members of the AsCsl gene family is repressed by abiotic stress, while the expression of some members is up-regulated by light. Immunoelectron microscopy shows that the product of AsCsl61, a member of CslF subfamily, mediates (1,3; 1,4)-β-D-glucan synthesis in transgenic Arabidopsis. These findings provide a fundamental understanding of the structural, functional, and evolutionary features of the oat Csl genes and may contribute to our general understanding of hemicellulose biosynthesis. Moreover, this information will be helpful in designing experiments for genetic manipulation of mixed-linkage glucan (MLG) synthesis with the goal of quality improvement in oat.

Enzymes in 3D: Synthesis, remodelling, and hydrolysis of cell wall (1,3;1,4)-β-glucans

Recent breakthroughs in structural biology have provided valuable new insights into enzymes involved in plant cell wall metabolism. More specifically, the molecular mechanism of synthesis of (1,3;1,4)-β-glucans, which are widespread in cell walls of commercially important cereals and grasses, has been the topic of debate and intense research activity for decades. However, an inability to purify these integral membrane enzymes or apply transgenic approaches without interpretative problems associated with pleiotropic effects has presented barriers to attempts to define their synthetic mechanisms. Following the demonstration that some members of the CslF sub-family of GT2 family enzymes mediate (1,3;1,4)-β-glucan synthesis, the expression of the corresponding genes in a heterologous system that is free of background complications has now been achieved. Biochemical analyses of the (1,3;1,4)-β-glucan synthesized in vitro, combined with 3-dimensional (3D) cryogenic-electron microscopy and AlphaFold protein structure predictions, have demonstrated how a single CslF6 enzyme, without exogenous primers, can incorporate both (1,3)- and (1,4)-β-linkages into the nascent polysaccharide chain. Similarly, 3D structures of xyloglucan endo-transglycosylases and (1,3;1,4)-β-glucan endo- and exohydrolases have allowed the mechanisms of (1,3;1,4)-β-glucan modification and degradation to be defined. X-ray crystallography and multi-scale modeling of a broad specificity GH3 β-glucan exohydrolase recently revealed a previously unknown and remarkable molecular mechanism with reactant trajectories through which a polysaccharide exohydrolase can act with a processive action pattern. The availability of high-quality protein 3D structural predictions should prove invaluable for defining structures, dynamics, and functions of other enzymes involved in plant cell wall metabolism in the immediate future.

Cell- and development-specific degradation controls the levels of mixed-linkage glucan in sorghum leaves

Mixed-linkage glucan (MLG) is a component of the cell wall (CW) of grasses and is composed of glucose monomers linked by β-1,3 and β-1,4 bonds. MLG is believed to have several biological functions, such as the mobilizable storage of carbohydrates and structural support of the CW. The extracellular levels of MLG are largely controlled by rates of synthesis mediated by cellulose synthase-like (CSL) enzymes, and turnover by lichenases. Economically important crops like sorghum accumulate MLG to variable levels during development. While in sorghum, like other grasses, there is one major MLG synthase (CSLF6), the identity of lichenases is yet unknown. To fill this gap, we identified three sorghum lichenases (SbLCH1-3) and characterized them in leaves in relation to the expression of SbCSLF6, and the abundance of MLG and starch. We established that SbLCH1-3 are secreted to the apoplast, consistent with a role of degrading MLG extracellularly. Furthermore, while SbCSLF6 expression was associated with cell development, the SbLCH genes exhibited distinct development-, cell-type-specific and diel-regulated expression. Therefore, our study identifies three functional sorghum MLG lichenases and highlights that MLG accumulation in sorghum leaves is likely controlled by the activity of lichenases that tune MLG levels, possibly to suit distinct cell and developmental needs in planta. These findings have important implications for improving the growth, yield, and composition of sorghum as a feedstock.

The Gram-positive bacterium Romboutsia ilealis harbors a polysaccharide synthase that can produce (1,3;1,4)-β-D-glucans

(1,3;1,4)-β-D-Glucans are widely distributed in the cell walls of grasses (family Poaceae) and closely related families, as well as some other vascular plants. Additionally, they have been found in other organisms, including fungi, lichens, brown algae, charophycean green algae, and the bacterium Sinorhizobium meliloti. Only three members of the Cellulose Synthase-Like (CSL) genes in the families CSLF, CSLH, and CSLJ are implicated in (1,3;1,4)-β-D-glucan biosynthesis in grasses. Little is known about the enzymes responsible for synthesizing (1,3;1,4)-β-D-glucans outside the grasses. In the present study, we report the presence of (1,3;1,4)-β-D-glucans in the exopolysaccharides of the Gram-positive bacterium Romboutsia ilealis CRIB^T. We also report that RiGT2 is the candidate gene of R. ilealis that encodes (1,3;1,4)-β-D-glucan synthase. RiGT2 has conserved glycosyltransferase family 2 (GT2) motifs, including D, D, D, QXXRW, and a C-terminal PilZ domain that resembles the C-terminal domain of bacteria cellulose synthase, BcsA. Using a direct gain-of-function approach, we insert RiGT2 into Saccharomyces cerevisiae, and (1,3;1,4)-β-D-glucans are produced with structures similar to those of the (1,3;1,4)-β-D-glucans of the lichen Cetraria islandica. Phylogenetic analysis reveals that putative (1,3;1,4)-β-D-glucan synthase candidate genes in several other bacterial species support the finding of (1,3;1,4)-β-D-glucans in these species.

Mixed-Linkage Glucan Is the Main Carbohydrate Source and Starch Is an Alternative Source during Brachypodium Grain Germination

Seeds of the model grass Brachypodium distachyon are unusual because they contain very little starch and high levels of mixed-linkage glucan (MLG) accumulated in thick cell walls. It was suggested that MLG might supplement starch as a storage carbohydrate and may be mobilised during germination. In this work, we observed massive degradation of MLG during germination in both endosperm and nucellar epidermis. The enzymes responsible for the MLG degradation were identified in germinated grains and characterized using heterologous expression. By using mutants targeting MLG biosynthesis genes, we showed that the expression level of genes coding for MLG and starch-degrading enzymes was modified in the germinated grains of knocked-out cslf6 mutants depleted in MLG but with higher starch content. Our results suggest a substrate-dependent regulation of the storage sugars during germination. These overall results demonstrated the function of MLG as the main carbohydrate source during germination of Brachypodium grain. More astonishingly, cslf6 Brachypodium mutants are able to adapt their metabolism to the lack of MLG by modifying the energy source for germination and the expression of genes dedicated for its use.

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.