Understanding Plant Cellulose Synthases through a Comprehensive Investigation of the Cellulose Synthase Family Sequences

Banana MaNAC1 activates secondary cell wall cellulose biosynthesis to enhance chilling resistance in fruit

Chilling injury has a negative impact on the quantity and quality of crops, especially subtropical and tropical plants. The plant cell wall is not only the main source of biomass production, but also the first barrier to various stresses. Therefore, improving the understanding of the alterations in cell wall architecture is of great significance for both biomass production and stress adaptation. Herein, we demonstrated that the cell wall principal component cellulose accumulated during chilling stress, which was caused by the activation of MaCESA proteins. The sequence-multiple comparisons show that a cold-inducible NAC transcriptional factor MaNAC1, a homologue of Secondary Wall NAC transcription factors, has high sequence similarity with Arabidopsis SND3. An increase in cell wall thickness and cellulosic glucan content was observed in MaNAC1-overexpressing Arabidopsis lines, indicating that MaNAC1 participates in cellulose biosynthesis. Over-expression of MaNAC1 in Arabidopsis mutant snd3 restored the defective secondary growth of thinner cell walls and increased cellulosic glucan content. Furthermore, the activation of MaCESA7 and MaCESA6B cellulose biosynthesis genes can be directly induced by MaNAC1 through binding to SNBE motifs within their promoters, leading to enhanced cellulose content during low-temperature stress. Ultimately, tomato fruit showed greater cold resistance in MaNAC1 overexpression lines with thickened cell walls and increased cellulosic glucan content. Our findings revealed that MaNAC1 performs a vital role as a positive modulator in modulating cell wall cellulose metabolism within banana fruit under chilling stress.

Comprehensive Time-Course Transcriptome Reveals the Crucial Biological Pathways Involved in the Seasonal Branch Growth in Siberian Elm (Ulmus pumila)

Timber, the most prevalent organic material on this planet, is the result of a secondary xylem emerging from vascular cambium. Yet, the intricate processes governing its seasonal generation are largely a mystery. To better understand the cyclic growth of vascular tissues in elm, we undertook an extensive study examining the anatomy, physiology, and genetic expressions in Ulmus pumila. We chose three robust 15-year-old elm trees for our study. The cultivars used in this study were collected from the Inner Mongolia Autonomous Region in China and nurtured in the tree farm of Shandong Normal University. Monthly samples of 2-year-old elm branches were taken from the tree from February to September. Marked seasonal shifts in elm branch vascular tissues were observed by phenotypic observation: In February, the cambium of the branch emerged from dormancy, spurring growth. By May, elms began generating secondary xylem, or latewood, recognized by its tiny pores and dense cell structure. From June to August, there was a marked increase in the thickness of the secondary xylem. Transcriptome sequencing provides a potential molecular mechanism for the thickening of elm branches and their response to stress. In February, the tree enhanced its genetic responses to cold and drought stress. The amplified expression of CDKB, CYCB, WOX4, and ARF5 in the months of February and March reinforced their essential role in the development of the vascular cambium in elm. Starting in May, the elm deployed carbohydrates as a carbon resource to synthesize the abundant cellulose and lignin necessary for the formation of the secondary wall. Major genes participating in cellulose (SUC and CESA homologs), xylan (UGD, UXS, IRX9, IRX10, and IRX14), and lignin (PAL, C4H, 4CL, HCT, C3H, COMT, and CAD) biosynthetic pathways for secondary wall formation were up-regulated by May or/and June. In conclusion, our findings provided a foundation for an in-depth exploration of the molecular processes dictating the seasonal growth of elm timber.

Biochemical and SSR based molecular characterization of elite rice varieties for straw lignocellulose

BACKGROUND

Lignocellulosic biomass from rice straw possesses enormous potential in generating bioenergy thereby reducing the dependence of human on non-renewable fuel sources. Developing rice varieties of such calibre necessitates biochemical characterization as well as assessing the presence of genetic diversity among the rice genotypes with respect to cellulose content.

METHODS AND RESULTS

Forty-three elite rice genotypes were selected for biochemical characterization and SSR marker-based genetic fingerprinting. For genotyping, 13 cellulose synthase specific polymorphic markers were used. The diversity analysis was performed using TASSEL 5.0 and GenAlE × 6.51b2, software program. Of the 43 rice varieties, CR-Dhan-601, CR-Dhan-1014, Mahanadi, Jagabandhu, Gouri, Samanta and Chandrama were found to possess desirable lignocellulosic composition with respect to harnessing green fuels. The marker OsCESA-1.3 expressed the highest PIC (0.640), while the marker OsCESA-6.3 of lowest PIC (0.128). A moderate average estimate (0.367) of PIC was observed under current set of genotypes and marker system. The dendrogram analysis grouped the rice genotypes into two principal clusters i.e., cluster I and II. Cluster-II is monogenetic, while cluster-I is having 42 genotypes.

CONCLUSIONS

The moderate level of both PIC and H average estimates indicate the narrow genetic bases of the germplasms. The varieties falling under different clusters possessing desirable lignocellulosic composition can be used in a hybridization programme to develop bioenergy efficient varieties. The promising varietal combinations that can be used as parents for developing bioenergy efficient genotypes are Kanchan / Gobinda, Mahanadi / Ramachandi, Mahanadi / Rambha, Mahanadi / Manika, Rambha / Manika, Rambha / Indravati and CR-Dhan-601 / Manika as they offer an advantage of higher cellulose accumulation. This study helped in identification of suitable dual purpose rice varieties for biofuel production without compromising food security.

Physiological and Full-Length Transcriptome Analyses Reveal the Dwarfing Regulation in Trifoliate Orange (Poncirus trifoliata L.)

Dwarfing rootstocks are capable of high-density planting and are therefore urgently needed in the modern citrus cultivation system. However, little is known about the physiological relevance and molecular basis underlying citrus height. This study aimed to comprehensively analyze phytohormone, carbohydrate, and associated transcriptome changes in the stem of two weak growth rootstocks ('TO' and 'FD') relative to the vigorous 'CC' rootstock. The phenotypic observation revealed that the plant height, plant weight, and internode length were reduced in dwarfing rootstocks. Moreover, the contents of indole-3-acetic acid (IAA), trans-zeatin (tZ), and abscisic acid (ABA), were higher in TO and FD rootstocks, whereas the gibberellin 3 (GA3) content was higher in the CC rootstocks. The carbohydrate contents, including sucrose, fructose, glucose, starch, and lignin significantly decreased in both the TO and FD rootstocks. The full-length transcriptome analysis revealed a potential mechanism regulating dwarfing phenotype that was mainly related to the phytohormone signaling transduction, sugar and starch degradation, lignin synthesis, and cellulose and hemicellulose degradation processes. In addition, many transcription factors (TFs), long non-coding RNAs (lncRNAs), and alternative splicing (AS) events were identified, which might act as important contributors to control the stem elongation and development in the weak growth rootstocks. These findings might deepen the understanding of the complex mechanisms of the stem development responsible for citrus dwarfing and provide a series of candidate genes for the application in breeding new rootstocks with intensive dwarfing.

Essential amino acids in the Plant-Conserved and Class-Specific Regions of cellulose synthases

The Plant-Conserved Region (P-CR) and the Class-Specific Region (CSR) are two plant-unique sequences in the catalytic core of cellulose synthases (CESAs) for which specific functions have not been established. Here, we used site-directed mutagenesis to replace amino acids and motifs within these sequences predicted to be essential for assembly and function of CESAs. We developed an in vivo method to determine the ability of mutated CesA1 transgenes to complement an Arabidopsis (Arabidopsis thaliana) temperature-sensitive root-swelling1 (rsw1) mutant. Replacement of a Cys residue in the CSR, which blocks dimerization in vitro, rendered the AtCesA1 transgene unable to complement the rsw1 mutation. Examination of the CSR sequences from 33 diverse angiosperm species showed domains of high-sequence conservation in a class-specific manner but with variation in the degrees of disorder, indicating a nonredundant role of the CSR structures in different CESA isoform classes. The Cys residue essential for dimerization was not always located in domains of intrinsic disorder. Expression of AtCesA1 transgene constructs, in which Pro417 and Arg453 were substituted for Ala or Lys in the coiled-coil of the P-CR, were also unable to complement the rsw1 mutation. Despite an expected role for Arg457 in trimerization of CESA proteins, AtCesA1 transgenes with Arg457Ala mutations were able to fully restore the wild-type phenotype in rsw1. Our data support that Cys662 within the CSR and Pro417 and Arg453 within the P-CR of Arabidopsis CESA1 are essential residues for functional synthase complex formation, but our data do not support a specific role for Arg457 in trimerization in native CESA complexes.

Cellulose synthesis in land plants

All plant cells are surrounded by a cell wall that provides cohesion, protection, and a means of directional growth to plants. Cellulose microfibrils contribute the main biomechanical scaffold for most of these walls. The biosynthesis of cellulose, which typically is the most prominent constituent of the cell wall and therefore Earth's most abundant biopolymer, is finely attuned to developmental and environmental cues. Our understanding of the machinery that catalyzes and regulates cellulose biosynthesis has substantially improved due to recent technological advances in, for example, structural biology and microscopy. Here, we provide a comprehensive overview of the structure, function, and regulation of the cellulose synthesis machinery and its regulatory interactors. We aim to highlight important knowledge gaps in the field, and outline emerging approaches that promise a means to close those gaps.

Cellulose synthesis complexes are homo-oligomeric and hetero-oligomeric in Physcomitrium patens

The common ancestor of seed plants and mosses contained homo-oligomeric cellulose synthesis complexes (CSCs) composed of identical subunits encoded by a single CELLULOSE SYNTHASE (CESA) gene. Seed plants use different CESA isoforms for primary and secondary cell wall deposition. Both primary and secondary CESAs form hetero-oligomeric CSCs that assemble and function in planta only when all the required isoforms are present. The moss Physcomitrium (Physcomitrella) patens has seven CESA genes that can be grouped into two functionally and phylogenetically distinct classes. Previously, we showed that PpCESA3 and/or PpCESA8 (class A) together with PpCESA6 and/or PpCESA7 (class B) form obligate hetero-oligomeric complexes required for normal secondary cell wall deposition. Here, we show that gametophore morphogenesis requires a member of class A, PpCESA5, and is sustained in the absence of other PpCESA isoforms. PpCESA5 also differs from the other class A PpCESAs as it is able to self-interact and does not co-immunoprecipitate with other PpCESA isoforms. These results are consistent with the hypothesis that homo-oligomeric CSCs containing only PpCESA5 subunits synthesize cellulose required for gametophore morphogenesis. Analysis of mutant phenotypes also revealed that, like secondary cell wall deposition, normal protonemal tip growth requires class B isoforms (PpCESA4 or PpCESA10), along with a class A partner (PpCESA3, PpCESA5, or PpCESA8). Thus, P. patens contains both homo-oligomeric and hetero-oligomeric CSCs.

Small but Mighty: An Update on Small Molecule Plant Cellulose Biosynthesis Inhibitors

Cellulose is one of the most abundant biopolymers on Earth. It provides mechanical support to growing plant cells and important raw materials for paper, textiles and biofuel feedstocks. Cellulose biosynthesis inhibitors (CBIs) are invaluable tools for studying cellulose biosynthesis and can be important herbicides for controlling weed growth. Here, we review CBIs with particular focus on the most widely used CBIs and recently discovered CBIs. We discuss the effects of these CBIs on plant growth and development and plant cell biology and summarize what is known about the mode of action of these different CBIs.

Comparative Analysis of Herbaceous and Woody Cell Wall Digestibility by Pathogenic Fungi

Fungal pathogens have evolved combinations of plant cell-wall-degrading enzymes (PCWDEs) to deconstruct host plant cell walls (PCWs). An understanding of this process is hoped to create a basis for improving plant biomass conversion efficiency into sustainable biofuels and bioproducts. Here, an approach integrating enzyme activity assay, biomass pretreatment, field emission scanning electron microscopy (FESEM), and genomic analysis of PCWDEs were applied to examine digestibility or degradability of selected woody and herbaceous biomass by pathogenic fungi. Preferred hydrolysis of apple tree branch, rapeseed straw, or wheat straw were observed by the apple-tree-specific pathogen Valsa mali, the rapeseed pathogen Sclerotinia sclerotiorum, and the wheat pathogen Rhizoctonia cerealis, respectively. Delignification by peracetic acid (PAA) pretreatment increased PCW digestibility, and the increase was generally more profound with non-host than host PCW substrates. Hemicellulase pretreatment slightly reduced or had no effect on hemicellulose content in the PCW substrates tested; however, the pretreatment significantly changed hydrolytic preferences of the selected pathogens, indicating a role of hemicellulose branching in PCW digestibility. Cellulose organization appears to also impact digestibility of host PCWs, as reflected by differences in cellulose microfibril organization in woody and herbaceous PCWs and variation in cellulose-binding domain organization in cellulases of pathogenic fungi, which is known to influence enzyme access to cellulose. Taken together, this study highlighted the importance of chemical structure of both hemicelluloses and cellulose in host PCW digestibility by fungal pathogens.

The Functionally Characterization of Putative Genes Involved in the Formation of Mannose in the Aplanospore Cell Wall of Haematococcus pluvialis (Volvocales, Chlorophyta)

Unicellular volvocalean green algal Haematococcus pluvialis, known as astaxanthin rich microalgae, transforms into aplanospore stage from the flagellate stage when exposed to the stress environments. However, the mechanism of the formation of aplanospore cell wall, which hinders the extraction of astaxanthin and the genetic manipulation is still unclear. In this study, the cell wall components under salicylic acid and high light stresses were explored, and cellulose was considered the main component in the flagellates, which changed gradually into mannose in the aplanospore stages. During the period, the genes related to the cellulose and mannose metabolisms were identified based on the RNA-seq data, which presented a similar expression pattern. The positive correlations were observed among these studied genes by Pearson Correlation (PC) analysis, indicating the coordination between pathways of cellulose and mannose metabolism. The study firstly explored the formation mechanism of aplanospore cell wall, which might be of scientific significance in the study of H. pluvialis.

Posttranscriptional regulation of cellulose synthase genes by small RNAs derived from cellulose synthase antisense transcripts

Transcriptional regulatory mechanisms governing plant cell wall biosynthesis are incomplete. Expression programs that activate wall biosynthesis are well understood, but mechanisms that control the attenuation of gene expression networks remain elusive. Previous work has shown that small RNAs (sRNAs) derived from the HvCESA6 (Hordeum vulgare, Hv) antisense transcripts are naturally produced and are capable of regulating aspects of wall biosynthesis. Here, we further test the hypothesis that CESA-derived sRNAs generated from CESA antisense transcripts are involved in the regulation of cellulose and broader cell wall biosynthesis. Antisense transcripts were detected for some but not all members of the CESA gene family in both barley and Brachypodium distachyon. Phylogenetic analysis indicates that antisense transcripts are detected for most primary cell wall CESA genes, suggesting a possible role in the transition from primary to secondary cell wall biosynthesis. Focusing on one antisense transcript, HvCESA1 shows dynamic expression throughout development, is correlated with corresponding sRNAs over the same period and is anticorrelated with HvCESA1 mRNA expression. To assess the broader impacts of CESA-derived sRNAs on the regulation of cell wall biosynthesis, transcript profiling was performed on barley tissues overexpressing CESA-derived sRNAs. Together, the data support the hypothesis that CESA antisense transcripts function through an RNA-induced silencing mechanism, to degrade cis transcripts, and may also trigger trans-acting silencing on related genes to alter the expression of cell wall gene networks.

Intragenic complementation at the Lotus japonicus CELLULOSE SYNTHASE-LIKE D1 locus rescues root hair defects

Root hair cells form the primary interface of plants with the soil environment, playing key roles in nutrient uptake and plant defense. In legumes, they are typically the first cells to become infected by nitrogen-fixing soil bacteria during root nodule symbiosis. Here, we report a role for the CELLULOSE SYNTHASE-LIKE D1 (CSLD1) gene in root hair development in the legume species Lotus japonicus. CSLD1 belongs to the cellulose synthase protein family that includes cellulose synthases and cellulose synthase-like proteins, the latter thought to be involved in the biosynthesis of hemicellulose. We describe 11 Ljcsld1 mutant alleles that impose either short (Ljcsld1-1) or variable (Ljcsld1-2 to 11) root hair length phenotypes. Examination of Ljcsld1-1 and one variable-length root hair mutant, Ljcsld1-6, revealed increased root hair cell wall thickness, which in Ljcsld1-1 was significantly more pronounced and also associated with a strong defect in root nodule symbiosis. Lotus japonicus plants heterozygous for Ljcsld1-1 exhibited intermediate root hair lengths, suggesting incomplete dominance. Intragenic complementation was observed between alleles with mutations in different CSLD1 domains, suggesting CSLD1 function is modular and that the protein may operate as a homodimer or multimer during root hair development.

Changes in secondary metabolites and fiber quality of cotton (Gossypium hirsutum) seed under consecutive water stress and in silico analysis of cellulose synthase and xyloglucan endotransglucosylase

Global warming has led to severe drought conditions. The selection of plant varieties that can withstand drought and produce increased yields are of utmost importance. In the current study, secondary metabolites, seed trait and fiber characteristic of cottonseeds (Gossypium hirsutum) exposed to double and third water stress exposure was investigated. Total phenol and tannin content in W₁S₃₃ increased significantly after third water stress exposure. Accumulation of wax was enhanced in seeds of W₃S₃₃ and W₃S₃₄ that were subjected to third water stress. Fiber quality parameters decreased when cottonseeds were rainfed. High irrigation resulted in fragile and delicate fiber. Seeds grown under 66% FC irrigation saved water and produced seeds that had the potential of producing high quality fibers. In silico analysis was performed on cellulose synthase A (CesA) and xyloglucan endotransglycosylase (XET) enzymes present in Gossypium hirsutum. The intracellular locations of the CesA and XET1 enzymes are the plasma membrane and cell wall, respectively. Proline is conserved in the C-terminal of the CesA enzyme and plays an important role in enzyme functionality. This study provides a better understanding as to the mechanisms by which the plant can tolerate and combat water stress conditions as well as reduce water consumption. In order to grow cotton seeds with desirable morphometric characteristics and optimal fibers under water stress exposure and in dry areas, it is better to use seeds that are irrigated under optimal irrigation conditions, ie 66% FC.

Phenotypic effects of changes in the FTVTxK region of an Arabidopsis secondary wall cellulose synthase compared with results from analogous mutations in other isoforms

Understanding protein structure and function relationships in cellulose synthase (CesA), including divergent isomers, is an important goal. Here, we report results from mutant complementation assays that tested the ability of sequence variants of AtCesA7, a secondary wall CesA of Arabidopsis thaliana, to rescue the collapsed vessels, short stems, and low cellulose content of the irx3-1 AtCesA7 null mutant. We tested a catalytic null mutation and seven missense or small domain changes in and near the AtCesA7 FTVTSK motif, which lies near the catalytic domain and may, analogously to bacterial CesA, exist within a substrate "gating loop." A low-to-high gradient of rescue occurred, and even inactive AtCesA7 had a small positive effect on stem cellulose content but not stem elongation. Overall, secondary wall cellulose content and stem length were moderately correlated, but the results were consistent with threshold amounts of cellulose supporting particular developmental processes. Vibrational sum frequency generation microscopy allowed tissue-specific analysis of cellulose content in stem xylem and interfascicular fibers, revealing subtle differences between selected genotypes that correlated with the extent of rescue of the collapsing xylem phenotype. Similar tests on PpCesA5 from the moss Physcomitrium (formerly Physcomitrella) patens helped us to synergize the AtCesA7 results with prior results on AtCesA1 and PpCesA5. The cumulative results show that the FTVTxK region is important for the function of an angiosperm secondary wall CesA as well as widely divergent primary wall CesAs, while differences in complementation results between isomers may reflect functional differences that can be explored in further work.

Cross species multi-omics reveals cell wall sequestration and elevated global transcript abundance as mechanisms of boron tolerance in plants

Boron toxicity is a world-wide problem for crops, yet we have a limited understanding of the genetic responses and adaptive mechanisms to this stress in plants. We employed a cross-species comparison between boron stress-sensitive Arabidopsis thaliana and its boron stress-tolerant extremophyte relative Schrenkiella parvula, and a multi-omics approach integrating genomics, transcriptomics, metabolomics and ionomics to assess plant responses and adaptations to boron stress. Schrenkiella parvula maintains lower concentrations of total boron and free boric acid than Arabidopsis when grown with excess boron. Schrenkiella parvula excludes excess boron more efficiently than Arabidopsis, which we propose is partly driven by SpBOR5, a boron transporter that we functionally characterize in this study. Both species use cell walls as a partial sink for excess boron. When accumulated in the cytoplasm, excess boron appears to interrupt RNA metabolism. The extremophyte S. parvula facilitates critical cellular processes while maintaining the pool of ribose-containing compounds that can bind with boric acid. The S. parvula transcriptome is pre-adapted to boron toxicity. It exhibits substantial overlaps with the Arabidopsis boron-stress responsive transcriptome. Cell wall sequestration and increases in global transcript levels under excess boron conditions emerge as key mechanisms for sustaining plant growth under boron toxicity.

The molecular basis of plant cellulose synthase complex organisation and assembly

The material properties of cellulose are heavily influenced by the organisation of β-1,4-glucan chains into a microfibril. It is likely that the structure of this microfibril is determined by the spatial arrangement of catalytic cellulose synthase (CESA) proteins within the cellulose synthase complex (CSC). In land plants, CESA proteins form a large complex composed of a hexamer of trimeric lobes termed the rosette. Each rosette synthesises a single microfibril likely composed of 18 glucan chains. In this review, the biochemical events leading to plant CESA protein assembly into the rosette are explored. The protein interfaces responsible for CESA trimerization are formed by regions that define rosette-forming CESA proteins. As a consequence, these regions are absent from the ancestral bacterial cellulose synthases (BcsAs) that do not form rosettes. CSC assembly occurs within the context of the endomembrane system, however the site of CESA assembly into trimers and rosettes is not determined. Both the N-Terminal Domain and Class Specific Region of CESA proteins are intrinsically disordered and contain all of the identified phosphorylation sites, making both regions candidates as sites for protein-protein interactions and inter-lobe interface formation. We propose a sequential assembly model, whereby CESA proteins form stable trimers shortly after native folding, followed by sequential recruitment of lobes into a rosette, possibly assisted by Golgi-localised STELLO proteins. A comprehensive understanding of CESA assembly into the CSC will enable directed engineering of CESA protein spatial arrangements, allowing changes in cellulose crystal packing that alter its material properties.

The N-terminal zinc finger of CELLULOSE SYNTHASE6 is critical in defining its functional properties by determining the level of homodimerization in Arabidopsis

Primary cell wall cellulose is synthesized by the cellulose synthase complex (CSC) containing CELLULOSE SYNTHASE1 (CESA1), CESA3 and one of four CESA6-like proteins in Arabidopsis. It has been proposed that the CESA6-like proteins occupy the same position in the CSC, but their underlying selection mechanism remains unclear. We produced a chimeric CESA5 by replacing its N-terminal zinc finger with its CESA6 counterpart to investigate the consequences for its homodimerization, a crucial step in forming higher-order structures during assembly of the CSC. We found that the mutant phenotypes of prc1-1, a cesa6 null mutant, were rescued by the chimeric CESA5, and became comparable to the wild type (WT) and prc1-1 complemented by WT CESA6 in regard to plant growth, cellulose content, cellulose microfibril organization, CSC dynamics and subcellular localization. Bimolecular fluorescence complementation assays were employed to evaluate pairwise interactions between the N-terminal regions of CESA1, CESA3, CESA5, CESA6 and the chimeric CESA5. We verified that the chimeric CESA5 explicitly interacted with all the other CESA partners, comparable to CESA6, whereas interaction between CESA5 with itself was significantly weaker than that of all other CESA pairs. Our findings suggest that the homodimerization of CESA6 through its N-terminal zinc finger is critical in defining its functional properties, and possibly determines its intrinsic roles in facilitating higher-order structures in CSCs.

The Cytoskeleton and Its Role in Determining Cellulose Microfibril Angle in Secondary Cell Walls of Woody Tree Species

Recent advances in our understanding of the molecular control of secondary cell wall (SCW) formation have shed light on molecular mechanisms that underpin domestication traits related to wood formation. One such trait is the cellulose microfibril angle (MFA), an important wood quality determinant that varies along tree developmental phases and in response to gravitational stimulus. The cytoskeleton, mainly composed of microtubules and actin filaments, collectively contribute to plant growth and development by participating in several cellular processes, including cellulose deposition. Studies in Arabidopsis have significantly aided our understanding of the roles of microtubules in xylem cell development during which correct SCW deposition and patterning are essential to provide structural support and allow for water transport. In contrast, studies relating to SCW formation in xylary elements performed in woody trees remain elusive. In combination, the data reviewed here suggest that the cytoskeleton plays important roles in determining the exact sites of cellulose deposition, overall SCW patterning and more specifically, the alignment and orientation of cellulose microfibrils. By relating the reviewed evidence to the process of wood formation, we present a model of microtubule participation in determining MFA in woody trees forming reaction wood (RW).

Biochemical and physiological flexibility accompanies reduced cellulose biosynthesis in Brachypodium cesa1 S830N

Here, we present a study into the mechanisms of primary cell wall cellulose formation in grasses, using the model cereal grass Brachypodium distachyon. The exon found adjacent to the BdCESA1 glycosyltransferase QXXRW motif was targeted using Targeting Induced Local Lesions in Genomes (TILLING) and sequencing candidate amplicons in multiple parallel reactions (SCAMPRing) leading to the identification of the Bdcesa1 ^S830N allele. Plants carrying this missense mutation exhibited a significant reduction in crystalline cellulose content in tissues that rely on the primary cell wall for biomechanical support. However, Bdcesa1 ^S830N plants failed to exhibit the predicted reduction in plant height. In a mechanism unavailable to eudicotyledons, B. distachyon plants homozygous for the Bdcesa1 ^S830N allele appear to overcome the loss of internode expansion anatomically by increasing the number of nodes along the stem. Stem biomechanics were resultantly compromised in Bdcesa1 ^S830N . The Bdcesa1 ^S830N missense mutation did not interfere with BdCESA1 gene expression. However, molecular dynamic simulations of the CELLULOSE SYNTHASE A (CESA) structure with modelled membrane interactions illustrated that Bdcesa1 ^S830N exhibited structural changes in the translated gene product responsible for reduced cellulose biosynthesis. Molecular dynamic simulations showed that substituting S830N resulted in a stabilizing shift in the flexibility of the class specific region arm of the core catalytic domain of CESA, revealing the importance of this motion to protein function.

Convergent evolution of hetero-oligomeric cellulose synthesis complexes in mosses and seed plants

In seed plants, cellulose is synthesized by rosette-shaped cellulose synthesis complexes (CSCs) that are obligate hetero-oligomeric, comprising three non-interchangeable cellulose synthase (CESA) isoforms. The moss Physcomitrella patens has rosette CSCs and seven CESAs, but its common ancestor with seed plants had rosette CSCs and a single CESA gene. Therefore, if P. patens CSCs are hetero-oligomeric, then CSCs of this type evolved convergently in mosses and seed plants. Previous gene knockout and promoter swap experiments showed that PpCESAs from class A (PpCESA3 and PpCESA8) and class B (PpCESA6 and PpCESA7) have non-redundant functions in secondary cell wall cellulose deposition in leaf midribs, whereas the two members of each class are redundant. Based on these observations, we proposed the hypothesis that the secondary class A and class B PpCESAs associate to form hetero-oligomeric CSCs. Here we show that transcription of secondary class A PpCESAs is reduced when secondary class B PpCESAs are knocked out and vice versa, as expected for genes encoding isoforms that occupy distinct positions within the same CSC. The class A and class B isoforms co-accumulate in developing gametophores and co-immunoprecipitate, suggesting that they interact to form a complex in planta. Finally, secondary PpCESAs interact with each other, whereas three of four fail to self-interact when expressed in two different heterologous systems. These results are consistent with the hypothesis that obligate hetero-oligomeric CSCs evolved independently in mosses and seed plants and we propose the constructive neutral evolution hypothesis as a plausible explanation for convergent evolution of hetero-oligomeric CSCs.

Knockdown of Dinoflagellate Cellulose Synthase CesA1 Resulted in Malformed Intracellular Cellulosic Thecal Plates and Severely Impeded Cyst-to-Swarmer Transition

Cellulose synthesis (CS) is conducted by membrane-bound cellulose synthase complexes (CSCs), containing cellulose synthases (CesA), that are either arranged in hexagonal structures in higher plants or in linear arrays in most microbial organisms, including dinoflagellates. Dinoflagellates are a major phytoplankton group having linear-type CSCs and internal cellulosic thecal plates (CTPs) in large cortical vesicles. Immunological study suggested CesA1p were cortically localized to the periphery of CTPs. During cyst-to-swarmer transition (T_C–S), synchronized peaks of CesA1 transcription, CesA1p expression, CS and CTP formation occurred in respective order, over 12–16 h, strategically allowing the study of CS regulation and CTP biogenesis. CesA1-knockdown resulted in 40% reduction in CesA1p level and time required for swarmer cells reappearance. CTPs were severely malformed with reduced cellulose content. As CTPs are deposited in internal organelle, the present study demonstrated dinoflagellate CesA1 ortholog was adapted for non-surface deposition; this is different to paradigm of other CesAps which require plasmamembrane for cellulose fiber deposition. This pioneer gene-knockdown study demonstrated the requirement of a gene for dinoflagellate cell wall remodeling and proper T_C–S, which are prominent in dinoflagellate life-cycles.

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

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

Characterization of Cellulose Synthase A (CESA) Gene Family in Eudicots

Cellulose synthase A (CESA) is a key enzyme involved in the complex process of plant cell wall biosynthesis, and it remains a productive subject for research. We employed systems biology approaches to explore structural diversity of eudicot CESAs by exon-intron organization, mode of duplication, synteny, and splice site analyses. Using a combined phylogenetics and comparative genomics approach coupled with co-expression networks we reconciled the evolution of cellulose synthase gene family in eudicots and found that the basic forms of CESA proteins are retained in angiosperms. Duplications have played an important role in expansion of CESA gene family members in eudicots. Co-expression networks showed that primary and secondary cell wall modules are duplicated in eudicots. We also identified 230 simple sequence repeat markers in 103 eudicot CESAs. The 13 identified conserved motifs in eudicots will provide a basis for gene identification and functional characterization in other plants. Furthermore, we characterized (in silico) eudicot CESAs against senescence and found that expression levels of CESAs decreased during leaf senescence.

Cellulose synthase complexes display distinct dynamic behaviors during xylem transdifferentiation

In plants, plasma membrane-embedded CELLULOSE SYNTHASE (CESA) enzyme complexes deposit cellulose polymers into the developing cell wall. Cellulose synthesis requires two different sets of CESA complexes that are active during cell expansion and secondary cell wall thickening, respectively. Hence, developing xylem cells, which first undergo cell expansion and subsequently deposit thick secondary walls, need to completely reorganize their CESA complexes from primary wall- to secondary wall-specific CESAs. Using live-cell imaging, we analyzed the principles underlying this remodeling. At the onset of secondary wall synthesis, the primary wall CESAs ceased to be delivered to the plasma membrane and were gradually removed from both the plasma membrane and the Golgi. For a brief transition period, both primary wall- and secondary wall-specific CESAs coexisted in banded domains of the plasma membrane where secondary wall synthesis is concentrated. During this transition, primary and secondary wall CESAs displayed discrete dynamic behaviors and sensitivities to the inhibitor isoxaben. As secondary wall-specific CESAs were delivered and inserted into the plasma membrane, the primary wall CESAs became concentrated in prevacuolar compartments and lytic vacuoles. This adjustment in localization between the two CESAs was accompanied by concurrent decreased primary wall CESA and increased secondary wall CESA protein abundance. Our data reveal distinct and dynamic subcellular trafficking patterns that underpin the remodeling of the cellulose biosynthetic machinery, resulting in the removal and degradation of the primary wall CESA complex with concurrent production and recycling of the secondary wall CESAs.

Revised Phylogeny of the Cellulose Synthase Gene Superfamily: Insights into Cell Wall Evolution

Cell walls are crucial for the integrity and function of all land plants and are of central importance in human health, livestock production, and as a source of renewable bioenergy. Many enzymes that mediate the biosynthesis of cell wall polysaccharides are encoded by members of the large cellulose synthase (CesA) gene superfamily. Here, we analyzed 29 sequenced genomes and 17 transcriptomes to revise the phylogeny of the CesA gene superfamily in angiosperms. Our results identify ancestral gene clusters that predate the monocot-eudicot divergence and reveal several novel evolutionary observations, including the expansion of the Poaceae-specific cellulose synthase-like CslF family to the graminids and restiids and the characterization of a previously unreported eudicot lineage, CslM, that forms a reciprocally monophyletic eudicot-monocot grouping with the CslJ clade. The CslM lineage is widely distributed in eudicots, and the CslJ clade, which was thought previously to be restricted to the Poales, is widely distributed in monocots. Our analyses show that some members of the CslJ lineage, but not the newly identified CslM genes, are capable of directing (1,3;1,4)-β-glucan biosynthesis, which, contrary to current dogma, is not restricted to Poaceae.

Domain swaps of Arabidopsis secondary wall cellulose synthases to elucidate their class specificity

Cellulose microfibrils are synthesized by membrane-embedded cellulose synthesis complexes (CSCs), currently modeled as hexamers of cellulose synthase (CESA) trimers. The three paralogous CESAs involved in secondary cell wall (SCW) cellulose biosynthesis in Arabidopsis (CESA4, CESA7, CESA8) are similar, but nonredundant, with all three isoforms required for assembly and function of the CSC. The molecular basis of protein-protein recognition among the isoforms is not well understood. To investigate the locations of the interfaces that are responsible for isoform recognition, we swapped three domains between the Arabidopsis CESAs required for SCW synthesis (CESA4, CESA7, and CESA8): N-terminus, central domain containing the catalytic core, and C-terminus. Chimeric genes with all pairwise permutations of the domains were tested for in vivo functionality within knockout mutant backgrounds of cesa4, cesa7, and cesa8. Immunoblotting with isoform-specific antibodies confirmed the anticipated protein expression in transgenic plants. The percent recovery of stem height and crystalline cellulose content was assayed, as compared to wild type, the mutant background lines, and other controls. Retention of the native central domain was sufficient for CESA8 chimeras to function, with neither its N-terminal nor C-terminal domains required. The C-terminal domain is required for class-specific function of CESA4 and CESA7, and CESA7 also requires its own N-terminus. Across all isoforms, the results indicate that the central domain, as well as the N- and C-terminal regions, contributes to class-specific function variously in Arabidopsis CESA4, CESA7, and CESA8.

Cellulose synthase 'class specific regions' are intrinsically disordered and functionally undifferentiated

Cellulose synthases (CESAs) are glycosyltransferases that catalyze formation of cellulose microfibrils in plant cell walls. Seed plant CESA isoforms cluster in six phylogenetic clades, whose non-interchangeable members play distinct roles within cellulose synthesis complexes (CSCs). A 'class specific region' (CSR), with higher sequence similarity within versus between functional CESA classes, has been suggested to contribute to specific activities or interactions of different isoforms. We investigated CESA isoform specificity in the moss, Physcomitrella patens (Hedw.) B. S. G. to gain evolutionary insights into CESA structure/function relationships. Like seed plants, P. patens has oligomeric rosette-type CSCs, but the PpCESAs diverged independently and form a separate CESA clade. We showed that P. patens has two functionally distinct CESAs classes, based on the ability to complement the gametophore-negative phenotype of a ppcesa5 knockout line. Thus, non-interchangeable CESA classes evolved separately in mosses and seed plants. However, testing of chimeric moss CESA genes for complementation demonstrated that functional class-specificity is not determined by the CSR. Sequence analysis and computational modeling showed that the CSR is intrinsically disordered and contains predicted molecular recognition features, consistent with a possible role in CESA oligomerization and explaining the evolution of class-specific sequences without selection for class-specific function.

The cell biology of secondary cell wall biosynthesis

BACKGROUND

Secondary cell walls (SCWs) form the architecture of terrestrial plant biomass. They reinforce tracheary elements and strengthen fibres to permit upright growth and the formation of forest canopies. The cells that synthesize a strong, thick SCW around their protoplast must undergo a dramatic commitment to cellulose, hemicellulose and lignin production.

SCOPE

This review puts SCW biosynthesis in a cellular context, with the aim of integrating molecular biology and biochemistry with plant cell biology. While SCWs are deposited in diverse tissue and cellular contexts including in sclerenchyma (fibres and sclereids), phloem (fibres) and xylem (tracheids, fibres and vessels), the focus of this review reflects the fact that protoxylem tracheary elements have proven to be the most amenable experimental system in which to study the cell biology of SCWs.

CONCLUSIONS

SCW biosynthesis requires the co-ordination of plasma membrane cellulose synthases, hemicellulose production in the Golgi and lignin polymer deposition in the apoplast. At the plasma membrane where the SCW is deposited under the guidance of cortical microtubules, there is a high density of SCW cellulose synthase complexes producing cellulose microfibrils consisting of 18-24 glucan chains. These microfibrils are extruded into a cell wall matrix rich in SCW-specific hemicelluloses, typically xylan and mannan. The biosynthesis of eudicot SCW glucuronoxylan is taken as an example to illustrate the emerging importance of protein-protein complexes in the Golgi. From the trans-Golgi, trafficking of vesicles carrying hemicelluloses, cellulose synthases and oxidative enzymes is crucial for exocytosis of SCW components at the microtubule-rich cell membrane domains, producing characteristic SCW patterns. The final step of SCW biosynthesis is lignification, with monolignols secreted by the lignifying cell and, in some cases, by neighbouring cells as well. Oxidative enzymes such as laccases and peroxidases, embedded in the polysaccharide cell wall matrix, determine where lignin is deposited.

Exploiting CELLULOSE SYNTHASE (CESA) Class Specificity to Probe Cellulose Microfibril Biosynthesis

Cellulose microfibrils are the basic units of cellulose in plants. The structure of these microfibrils is at least partly determined by the structure of the cellulose synthase complex. In higher plants, this complex is composed of 18 to 24 catalytic subunits known as CELLULOSE SYNTHASE A (CESA) proteins. Three different classes of CESA proteins are required for cellulose synthesis and for secondary cell wall cellulose biosynthesis these classes are represented by CESA4, CESA7, and CESA8. To probe the relationship between CESA proteins and microfibril structure, we created mutant cesa proteins that lack catalytic activity but retain sufficient structural integrity to allow assembly of the cellulose synthase complex. Using a series of Arabidopsis (Arabidopsis thaliana) mutants and genetic backgrounds, we found consistent differences in the ability of these mutant cesa proteins to complement the cellulose-deficient phenotype of the cesa null mutants. The best complementation was observed with catalytically inactive cesa4, while the equivalent mutation in cesa8 exhibited significantly lower levels of complementation. Using a variety of biophysical techniques, including solid-state nuclear magnetic resonance and Fourier transform infrared microscopy, to study these mutant plants, we found evidence for changes in cellulose microfibril structure, but these changes largely correlated with cellulose content and reflected differences in the relative proportions of primary and secondary cell walls. Our results suggest that individual CESA classes have similar roles in determining cellulose microfibril structure, and it is likely that the different effects of mutating members of different CESA classes are the consequence of their different catalytic activity and their influence on the overall rate of cellulose synthesis.

Grass cell walls have a role in the inherent tolerance of grasses to the cellulose biosynthesis inhibitor isoxaben

BACKGROUND

Cellulose biosynthesis inhibitors (CBIs) are pre-emergence herbicides that inhibit anisotropic cell expansion resulting in a severely swollen and stunted growth phenotype. Resistance to group 21 CBIs, such as isoxaben, is conferred by missense mutations in CELLOSE SYNTHASE A (CesA) genes required for primary cell wall synthesis, concluding that this is their in vivo target.

RESULTS

Herein, we show that grasses exhibit tolerance to group 21 CBIs and explore the mechanism of tolerance to isoxaben in the grass Brachypodium distachyon (L.). Comparative genomics failed to identify synonymous point mutations that have been found to confer isoxaben resistance in the dicot Arabidopsis thaliana (L.). Brachypodium did not metabolize ¹⁴ C-isoxaben. We next explored the role of grass-specific non-cellulosic cell wall components, specifically the hemicellulose polysaccharide mix linkage glucans (MLG), as a potential tolerance mechanism by compensating for the loss of cellulose during cell elongation. A partial-transcriptional knockdown T-DNA insertion was found in a key MLG synthesis gene, Cellulose synthase-like F6 (CslF6) and this mutant was found to be 2.1 times more sensitive to isoxaben than wild-type plants.

CONCLUSION

These data suggest that the composition and compensatory response of grass cell walls may be a factor in conferring tolerance to group 21 CBIs. © 2017 Society of Chemical Industry.

Genome and transcriptome-wide analyses of cellulose synthase gene superfamily in soybean

The plant cellulose synthase gene superfamily belongs to the category of type-2 glycosyltransferases, and is involved in cellulose and hemicellulose biosynthesis. These enzymes are vital for maintaining cell-wall structural integrity throughout plant life. Here, we identified 78 putative cellulose synthases (CS) in the soybean genome. Phylogenetic analysis against 40 reference Arabidopsis CS genes clustered soybean CSs into seven major groups (CESA, CSL A, B, C, D, E and G), located on 19 chromosomes (except chromosome 18). Soybean CS expansion occurred in 66 duplication events. Additionally, we identified 95 simple sequence repeat makers related to 44 CSs. We next performed digital expression analysis using publically available datasets to understand potential CS functions in soybean. We found that CSs were highly expressed during soybean seed development, a pattern confirmed with an Affymatrix soybean IVT array and validated with RNA-seq profiles. Within CS groups, CESAs had higher relative expression than CSLs. Soybean CS models were designed based on maximum average RPKM values. Gene co-expression networks were developed to explore which CSs could work together in soybean. Finally, RT-PCR analysis confirmed the expression of 15 selected CSs during all four seed developmental stages.

Cellulosic fibres of flax recruit both primary and secondary cell wall cellulose synthases during deposition of thick tertiary cell walls and in the course of graviresponse

Cellulose synthesising complex consists of cellulose synthase (CESA) subunits encoded by a multigene family; different sets of CESA genes are known to be expressed during primary and secondary cell wall formation. We examined the expression of LusCESAs in flax (Linum usitatissimum L.) cellulosic fibres at various stages of development and in the course of graviresponse by means of RNA-Seq and quantitative PCR. Transcripts for both primary and secondary cell wall-related CESAs were abundant in fibres depositing highly cellulosic tertiary cell walls. Gravistimulation of flax plants temporally increased the abundance of CESA transcripts, specifically in phloem fibres located at the pulling stem side. Construction of coexpression networks for LusCESAs revealed that both primary and secondary cell wall-related CESAs were involved in the joint coexpression group in fibres depositing tertiary cell walls, as distinct from other tissues, where these genes were within separate groups. The obtained data suggest that fibres depositing tertiary cell walls have a specific mechanism of cellulose biosynthesis and a specific way of its regulation.

Functional Analysis of Cellulose Synthase (CESA) Protein Class Specificity

The cellulose synthase complex (CSC) exhibits a 6-fold symmetry and is known as a "rosette." Each CSC is believed to contain between 18 and 24 CESA proteins that each synthesize an individual glucan chain. These chains form the microfibrils that confer the remarkable structural properties of cellulose. At least three different classes of CESA proteins are essential to form the CSC However, while organization of the CSC determines microfibril structure, how individual CESA proteins are organized within the CSC remains unclear. Parts of the plant CESA proteins map sufficiently well onto the bacterial CESA (BcsA) structure, indicating that they are likely to share a common catalytic mechanism. However, plant CESA proteins are much larger than the bacterial BcsA protein, prompting the suggestion that these plant-specific regions are important for interactions between CESA proteins and for conferring CESA class specificity. In this study, we have undertaken a comprehensive analysis of well-defined regions of secondary cell wall CESA proteins, with the aim of defining what distinguishes different CESA proteins and hence what determines the specificity of each CESA class. Our results demonstrate that CESA class specificity extends throughout the protein and not just in the highly variable regions. Furthermore, we find that different CESA isoforms vary greatly in their levels of site specificity and this is likely to be determined by the constraints imposed by their position within the CSC rather than their primary structure.

The Emerging Role of Protein Phosphorylation as a Critical Regulatory Mechanism Controlling Cellulose Biosynthesis

Plant cell walls are extracellular matrices that surround plant cells and critically influence basic cellular processes, such as cell division and expansion. Cellulose is a major constituent of plant cell walls, and this paracrystalline polysaccharide is synthesized at the plasma membrane by a large protein complex known as the cellulose synthase complex (CSC). Recent efforts have identified numerous protein components of the CSC, but relatively little is known about regulation of cellulose biosynthesis. Numerous phosphoproteomic surveys have identified phosphorylation events in CSC associated proteins, suggesting that protein phosphorylation may represent an important regulatory control of CSC activity. In this review, we discuss the composition and dynamics of the CSC in vivo, the catalog of CSC phosphorylation sites that have been identified, the function of experimentally examined phosphorylation events, and potential kinases responsible for these phosphorylation events. Additionally, we discuss future directions in cellulose synthase kinase identification and functional analyses of CSC phosphorylation sites.

Functional analyses of cellulose synthase genes in flax (Linum usitatissimum) by virus-induced gene silencing

Flax (Linum usitatissimum) bast fibres are located in the stem cortex where they play an important role in mechanical support. They contain high amounts of cellulose and so are used for linen textiles and in the composite industry. In this study, we screened the annotated flax genome and identified 14 distinct cellulose synthase (CESA) genes using orthologous sequences previously identified. Transcriptomics of 'primary cell wall' and 'secondary cell wall' flax CESA genes showed that some were preferentially expressed in different organs and stem tissues providing clues as to their biological role(s) in planta. The development for the first time in flax of a virus-induced gene silencing (VIGS) approach was used to functionally evaluate the biological role of different CESA genes in stem tissues. Quantification of transcript accumulation showed that in many cases, silencing not only affected targeted CESA clades, but also had an impact on other CESA genes. Whatever the targeted clade, inactivation by VIGS affected plant growth. In contrast, only clade 1- and clade 6-targeted plants showed modifications in outer-stem tissue organization and secondary cell wall formation. In these plants, bast fibre number and structure were severely impacted, suggesting that the targeted genes may play an important role in the establishment of the fibre cell wall. Our results provide new fundamental information about cellulose biosynthesis in flax that should facilitate future plant improvement/engineering.

Establishing a Role for Bacterial Cellulose in Environmental Interactions: Lessons Learned from Diverse Biofilm-Producing Proteobacteria

Bacterial cellulose (BC) serves as a molecular glue to facilitate intra- and inter-domain interactions in nature. Biosynthesis of BC-containing biofilms occurs in a variety of Proteobacteria that inhabit diverse ecological niches. The enzymatic and regulatory systems responsible for the polymerization, exportation, and regulation of BC are equally as diverse. Though the magnitude and environmental consequences of BC production are species-specific, the common role of BC-containing biofilms is to establish close contact with a preferred host to facilitate efficient host-bacteria interactions. Universally, BC aids in attachment, adherence, and subsequent colonization of a substrate. Bi-directional interactions influence host physiology, bacterial physiology, and regulation of BC biosynthesis, primarily through modulation of intracellular bis-(3'→5')-cyclic diguanylate (c-di-GMP) levels. Depending on the circumstance, BC producers exhibit a pathogenic or symbiotic relationship with plant, animal, or fungal hosts. Rhizobiaceae species colonize plant roots, Pseudomonadaceae inhabit the phyllosphere, Acetobacteriaceae associate with sugar-loving insects and inhabit the carposphere, Enterobacteriaceae use fresh produce as vehicles to infect animal hosts, and Vibrionaceae, particularly Aliivibrio fischeri, colonize the light organ of squid. This review will highlight the diversity of the biosynthesis and regulation of BC in nature by discussing various examples of Proteobacteria that use BC-containing biofilms to facilitate host-bacteria interactions. Through discussion of current data we will establish new directions for the elucidation of BC biosynthesis, its regulation and its ecophysiological roles.

Unidirectional movement of cellulose synthase complexes in Arabidopsis seed coat epidermal cells deposit cellulose involved in mucilage extrusion, adherence, and ray formation

Cellulose synthase5 (CESA5) synthesizes cellulose necessary for seed mucilage adherence to seed coat epidermal cells of Arabidopsis (Arabidopsis thaliana). The involvement of additional CESA proteins in this process and details concerning the manner in which cellulose is deposited in the mucilage pocket are unknown. Here, we show that both CESA3 and CESA10 are highly expressed in this cell type at the time of mucilage synthesis and localize to the plasma membrane adjacent to the mucilage pocket. The isoxaben resistant1-1 and isoxaben resistant1-2 mutants affecting CESA3 show defects consistent with altered mucilage cellulose biosynthesis. CESA3 can interact with CESA5 in vitro, and green fluorescent protein-tagged CESA5, CESA3, and CESA10 proteins move in a linear, unidirectional fashion around the cytoplasmic column of the cell, parallel with the surface of the seed, in a pattern similar to that of cortical microtubules. Consistent with this movement, cytological evidence suggests that the mucilage is coiled around the columella and unwinds during mucilage extrusion to form a linear ray. Mutations in CESA5 and CESA3 affect the speed of mucilage extrusion and mucilage adherence. These findings imply that cellulose fibrils are synthesized in an ordered helical array around the columella, providing a distinct structure to the mucilage that is important for both mucilage extrusion and adherence.

Plant cellulose synthesis: CESA proteins crossing kingdoms

Cellulose is a biopolymer of considerable economic importance. It is synthesised by the cellulose synthase complex (CSC) in species ranging from bacteria to higher plants. Enormous progress in our understanding of bacterial cellulose synthesis has come with the recent publication of both the crystal structure and biochemical characterisation of a purified complex able to synthesis cellulose in vitro. A model structure of a plant CESA protein suggests considerable similarity between the bacterial and plant cellulose synthesis. In this review article we will cover current knowledge of how plant CESA proteins synthesise cellulose. In particular the focus will be on the lessons learned from the recent work on the catalytic mechanism and the implications that new data on cellulose structure has for the assembly of CESA proteins into the large complex that synthesis plant cellulose microfibrils.

DNL1, encodes cellulose synthase-like D4, is a major QTL for plant height and leaf width in rice (Oryza sativa L.)

To better understand the genetic of rice agronomic traits, we selected two different rice germplasms in phenotypes, Xian80 and Suyunuo, to construct genetic population for QTL analysis. A total of 25 QTLs for six traits were found in a 175 F2 population. Major QTLs, qPH12,qLW12.2, qLL12 and qGW12.1, explaining 50.00%, 57.08%, 15.41% and 22.51% phenotypic variation for plant height, leaf width, leaf length and grain width, respectively, were located on the same interval of chromosome 12 flanking SSR markers RM519 and RM1103. In consideration of the great effects on plant height and leaf width, the locus was named DNL1 (Dwarf and Narrowed Leaf 1). Using a segregating population derived from F2 heterozygous individuals, a total of 1363 dwarfism and narrowed-leaf individuals was selected for screening recombinants. By high-resolution linkage analysis in 141 recombination events, DNL1 was narrowed to a 62.39kb region of InDel markers ID12M28 and HF43. The results of ORF analysis in target region and nucleotide sequence alignment indicated that DNL1 encodes cellulose synthase-like D4 protein, and a single nucleotide substitution (C2488T) in dnl1 result in decrease in plant height and leaf width. Bioinformatical analysis demonstrated that a conserved role for OsCSLD4 in the regulation of plant growth and development. Expression analysis for OsCSLDs showed OsCSLD4 highly expressed in roots, while other CSLD members had comparatively lower expression levels. However, no clear evidence about CSLD4/DNL1 expression was associated with its function.

The Arabidopsis cellulose synthase complex: a proposed hexamer of CESA trimers in an equimolar stoichiometry

Cellulose is the most abundant renewable polymer on Earth and a major component of the plant cell wall. In vascular plants, cellulose synthesis is catalyzed by a large, plasma membrane-localized cellulose synthase complex (CSC), visualized as a hexameric rosette structure. Three unique cellulose synthase (CESA) isoforms are required for CSC assembly and function. However, elucidation of either the number or stoichiometry of CESAs within the CSC has remained elusive. In this study, we show a 1:1:1 stoichiometry between the three Arabidopsis thaliana secondary cell wall isozymes: CESA4, CESA7, and CESA8. This ratio was determined utilizing a simple but elegant method of quantitative immunoblotting using isoform-specific antibodies and (35)S-labeled protein standards for each CESA. Additionally, the observed equimolar stoichiometry was found to be fixed along the axis of the stem, which represents a developmental gradient. Our results complement recent spectroscopic analyses pointing toward an 18-chain cellulose microfibril. Taken together, we propose that the CSC is composed of a hexamer of catalytically active CESA trimers, with each CESA in equimolar amounts. This finding is a crucial advance in understanding how CESAs integrate to form higher order complexes, which is a key determinate of cellulose microfibril and cell wall properties.

KORRIGAN1 interacts specifically with integral components of the cellulose synthase machinery

Cellulose is synthesized by the so called rosette protein complex and the catalytic subunits of this complex are the cellulose synthases (CESAs). It is thought that the rosette complexes in the primary and secondary cell walls each contains at least three different non-redundant cellulose synthases. In addition to the CESA proteins, cellulose biosynthesis almost certainly requires the action of other proteins, although few have been identified and little is known about the biochemical role of those that have been identified. One of these proteins is KORRIGAN (KOR1). Mutant analysis of this protein in Arabidopsis thaliana showed altered cellulose content in both the primary and secondary cell wall. KOR1 is thought to be required for cellulose synthesis acting as a cellulase at the plasma membrane–cell wall interface. KOR1 has recently been shown to interact with the primary cellulose synthase rosette complex however direct interaction with that of the secondary cell wall has never been demonstrated. Using various methods, both in vitro and in planta, it was shown that KOR1 interacts specifically with only two of the secondary CESA proteins. The KOR1 protein domain(s) involved in the interaction with the CESA proteins were also identified by analyzing the interaction of truncated forms of KOR1 with CESA proteins. The KOR1 transmembrane domain has shown to be required for the interaction between KOR1 and the different CESAs, as well as for higher oligomer formation of KOR1.

Alfalfa Cellulose synthase gene expression under abiotic stress: a Hitchhiker's guide to RT-qPCR normalization

Abiotic stress represents a serious threat affecting both plant fitness and productivity. One of the promptest responses that plants trigger following abiotic stress is the differential expression of key genes, which enable to face the adverse conditions. It is accepted and shown that the cell wall senses and broadcasts the stress signal to the interior of the cell, by triggering a cascade of reactions leading to resistance. Therefore the study of wall-related genes is particularly relevant to understand the metabolic remodeling triggered by plants in response to exogenous stresses. Despite the agricultural and economical relevance of alfalfa (Medicago sativa L.), no study, to our knowledge, has addressed specifically the wall-related gene expression changes in response to exogenous stresses in this important crop, by monitoring the dynamics of wall biosynthetic gene expression. We here identify and analyze the expression profiles of nine cellulose synthases, together with other wall-related genes, in stems of alfalfa plants subjected to different abiotic stresses (cold, heat, salt stress) at various time points (e.g. 0, 24, 72 and 96 h). We identify 2 main responses for specific groups of genes, i.e. a salt/heat-induced and a cold/heat-repressed group of genes. Prior to this analysis we identified appropriate reference genes for expression analyses in alfalfa, by evaluating the stability of 10 candidates across different tissues (namely leaves, stems, roots), under the different abiotic stresses and time points chosen. The results obtained confirm an active role played by the cell wall in response to exogenous stimuli and constitute a step forward in delineating the complex pathways regulating the response of plants to abiotic stresses.

Cellulose synthases: new insights from crystallography and modeling

Detailed information about the structure and biochemical mechanisms of cellulose synthase (CelS) proteins remained elusive until a complex containing the catalytic subunit (BcsA) of CelS from Rhodobacter sphaeroides was crystalized. Additionally, a 3D structure of most of the cytosolic domain of a plant CelS (GhCESA1 from cotton, Gossypium hirsutum) was produced by computational modeling. This predicted structure contributes to our understanding of how plant CelS proteins may be similar and different as compared with BcsA. In this review, we highlight how these structures impact our understanding of the synthesis of cellulose and other extracellular polysaccharides. We show how the structures can be used to generate hypotheses for experiments testing mechanisms of glucan synthesis and translocation in plant CelS.

Cellulose synthesis and its regulation

Cellulose, the most abundant biopolymer synthesized on land, is made of linear chains of ß (1-4) linked D-glucose. As a major structural component of the cell wall, cellulose is important not only for industrial use but also for plant growth and development. Cellulose microfibrils are tethered by other cell wall polysaccharides such as hemicellulose, pectin, and lignin. In higher plants, cellulose is synthesized by plasma membrane-localized rosette cellulose synthase complexes. Despite the recent advances using a combination of molecular genetics, live cell imaging, and spectroscopic tools, many aspects of the cellulose synthesis remain a mystery. In this chapter, we highlight recent research progress towards understanding the mechanism of cellulose synthesis in Arabidopsis.

Perturbation of Brachypodium distachyon CELLULOSE SYNTHASE A4 or 7 results in abnormal cell walls

BACKGROUND

Cellulose is an integral component of the plant cell wall and accounts for approximately forty percent of total plant biomass but understanding its mechanism of synthesis remains elusive. CELLULOSE SYNTHASE A (CESA) proteins function as catalytic subunits of a rosette-shaped complex that synthesizes cellulose at the plasma membrane. Arabidopsis thaliana and rice (Oryza sativa) secondary wall CESA loss-of-function mutants have weak stems and irregular or thin cell walls.

RESULTS

Here, we identify candidates for secondary wall CESAs in Brachypodium distachyon as having similar amino acid sequence and expression to those characterized in A. thaliana, namely CESA4/7/8. To functionally characterize BdCESA4 and BdCESA7, we generated loss-of-function mutants using artificial microRNA constructs, specifically targeting each gene driven by a maize (Zea mays) ubiquitin promoter. Presence of the transgenes reduced BdCESA4 and BdCESA7 transcript abundance, as well as stem area, cell wall thickness of xylem and fibers, and the amount of crystalline cellulose in the cell wall.

CONCLUSION

These results suggest BdCESA4 and BdCESA7 play a key role in B. distachyon secondary cell wall biosynthesis.