Cellulose synthesis in higher plants

Characterization of cellulose synthase complexes in Populus xylem differentiation

*It is generally hypothesized that the synthesis of cellulose in higher plants is mediated by cellulose synthase complexes (CSCs) localized on the plasma membrane. However, CSCs have not been investigated thoroughly through their isolation. The availability of ample Populus tissue allowed Populus CSCs to be isolated and characterized in association with xylem differentiation. *The methods used here included co-immunoprecipitation, proteomic analysis, laser microdissection, immunolocalization and others. *Western blot analysis of the immunoprecipitated CSCs led to the identification of at least two types of CSC in the membrane protein of Populus xylem tissue. Proteomic analysis further revealed that the two types of CSC were assembled from different cellulose synthase proteins. Immunolocalization confirmed that both types of CSC were involved in secondary cell wall formation. In addition, a number of noncellulose synthase proteins were also identified in association with CSC precipitation. *The results indicate that two types of CSC participate in secondary wall formation in Populus, suggesting a new mechanism of cellulose formation involved in the thickening of wood cell walls. This study also suggests that the CSC machinery may be aided by other proteins in addition to cellulose synthase proteins.

Analysis of the cellulose synthase genes associated with primary cell wall synthesis in Bambusa oldhamii

The synthesis of cell wall polysaccharides is highly active in rapidly growing bamboo shoots. We cloned a set of BoCesA cDNAs that encode cellulose synthase from bamboo (Bambusa oldhamii) and investigated the expression patterns of the BoCesA2, BoCesA5, BoCesA6 and BoCesA7 genes. The four BoCesA genes were differentially expressed in the different parts of growing bamboo shoots, in various organs, and in multiple shoots that were cultured in vitro. They were down-regulated by alpha-naphthaleneacetic acid and differentially affected by thidiazuron in the multiple shoots. In situ RT-PCR analyses demonstrated that BoCesA2, BoCesA5, BoCesA6, and BoCesA7 mRNAs were present throughout the base and the internode regions of the etiolated shoots that emerged from pseudorhizomes, and in the internode regions of the juvenile branch shoots that emerged from nodes of mature bamboo culms; however, the expression of the four genes in the lignified internode of the branch shoot was predominantly detected in the center of the vascular bundles. Our results for cDNA cloning, expression analyses, and phylogenetic analysis suggest that the 10 BoCesA genes cloned from the etiolated bamboo shoots participate in cellulose synthesis in the primary cell walls of the growing bamboo, and that at least three additional BoCesA genes involved in cellulose synthesis in the secondary walls may be present in the bamboo genome. The expressions of BoCesA genes may be under fine control in response to the various developmental stages and physiological conditions of bamboo.

Identification of a cellulose synthase-associated protein required for cellulose biosynthesis

Cellulose synthase-interactive protein 1 (CSI1) was identified in a two-hybrid screen for proteins that interact with cellulose synthase (CESA) isoforms involved in primary plant cell wall synthesis. CSI1 encodes a 2,150-amino acid protein that contains 10 predicted Armadillo repeats and a C2 domain. Mutations in CSI1 cause defective cell elongation in hypocotyls and roots and reduce cellulose content. CSI1 is associated with CESA complexes, and csi1 mutants affect the distribution and movement of CESA complexes in the plasma membrane.

An NAC transcription factor orchestrates multiple features of cell wall development in Medicago truncatula

To identify genes controlling secondary cell wall biosynthesis in the model legume Medicago truncatula, we screened a Tnt1 retrotransposon insertion mutant population for plants with altered patterns of lignin autofluorescence. From more than 9000 R1 plants screened, four independent lines were identified with a total lack of lignin in the interfascicular region. The mutants also showed loss of lignin in phloem fibers, reduced lignin in vascular elements, failure in anther dehiscence and absence of phenolic autofluorescence in stomatal guard cell walls. Microarray and PCR analyses confirmed that the mutations were caused by the insertion of Tnt1 in a gene annotated as encoding a NAM (no apical meristem)-like protein (here designated Medicago truncatula NAC SECONDARY WALL THICKENING PROMOTING FACTOR 1, MtNST1). MtNST1 is the only family member in Medicago, but has three homologs (AtNST1-AtNST3) in Arabidopsis thaliana, which function in different combinations to control cell wall composition in stems and anthers. Loss of MtNST1 function resulted in reduced lignin content, associated with reduced expression of most lignin biosynthetic genes, and a smaller reduction in cell wall polysaccharide content, associated with reduced expression of putative cellulose and hemicellulose biosynthetic genes. Acid pre-treatment and cellulase digestion released significantly more sugars from cell walls of nst1 mutants compared with the wild type. We discuss the implications of these findings for the development of alfalfa (Medicago sativa) as a dedicated bioenergy crop.

A cellulose synthase-like protein is required for osmotic stress tolerance in Arabidopsis

Osmotic stress imposed by soil salinity and drought stress significantly affects plant growth and development, but osmotic stress sensing and tolerance mechanisms are not well understood. Forward genetic screens using a root-bending assay have previously identified salt overly sensitive (sos) mutants of Arabidopsis that fall into five loci, SOS1 to SOS5. These loci are required for the regulation of ion homeostasis or cell expansion under salt stress, but do not play a major role in plant tolerance to the osmotic stress component of soil salinity or drought. Here we report an additional sos mutant, sos6-1, which defines a locus essential for osmotic stress tolerance. sos6-1 plants are hypersensitive to salt stress and osmotic stress imposed by mannitol or polyethylene glycol in culture media or by water deficit in the soil. SOS6 encodes a cellulose synthase-like protein, AtCSLD5. Only modest differences in cell wall chemical composition could be detected, but we found that sos6-1 mutant plants accumulate high levels of reactive oxygen species (ROS) under osmotic stress and are hypersensitive to the oxidative stress reagent methyl viologen. The results suggest that SOS6/AtCSLD5 is not required for normal plant growth and development but has a critical role in osmotic stress tolerance and this function likely involves its regulation of ROS under stress.

Exploring genomes for glycosyltransferases

Glycosyltransferases are one of the largest and most diverse enzyme groups in Nature. They catalyse the synthesis of glycosidic linkages by the transfer of a sugar residue from a donor to an acceptor substrate. These enzymes have been classified into families on the basis of amino acid sequence similarity that are kept updated in the Carbohydrate Active enZyme database (CAZy, ). The repertoire of glycosyltransferases in genomes is believed to determine the diversity of cellular glycan structures, and current estimates suggest that for most genomes about 1% of the coding regions are glycosyltransferases. However, plants tend to have far more glycosyltransferase genes than any other organism sequenced to date, and this can be explained by the highly complex polysaccharide network that form the cell wall and also by the numerous glycosylated secondary metabolites. In recent years, various bioinformatics strategies have been used to search bacterial and plant genomes for new glycosyltransferase genes. These are based on the use of remote homology detection methods that act at the 1D, 2D, and 3D level. The combined use of methods such as profile Hidden Markov Model (HMM) and fold recognition appears to be appropriate for this class of enzyme. Chemometric tools are also particularly well suited for obtaining an overview of multivariate data and revealing hidden latent information when dealing with large and highly complex datasets.

Analysis of the Arabidopsis IRX9/IRX9-L and IRX14/IRX14-L pairs of glycosyltransferase genes reveals critical contributions to biosynthesis of the hemicellulose glucuronoxylan

The hemicellulose glucuronoxylan (GX) is a major component of plant secondary cell walls. However, our understanding of GX synthesis remains limited. Here, we identify and analyze two new genes from Arabidopsis (Arabidopsis thaliana), IRREGULAR XYLEM9-LIKE (IRX9-L) and IRX14-LIKE (IRX14-L) that encode glycosyltransferase family 43 members proposed to function during xylan backbone elongation. We place IRX9-L and IRX14-L in a genetic framework with six previously described glycosyltransferase genes (IRX9, IRX10, IRX10-L, IRX14, FRAGILE FIBER8 [FRA8], and FRA8 HOMOLOG [F8H]) and investigate their function in GX synthesis. Double-mutant analysis identifies IRX9-L and IRX14-L as functional homologs of IRX9 and IRX14, respectively. Characterization of irx9 irx10 irx14 fra8 and irx9-L irx10-L irx14-L f8h quadruple mutants allows definition of a set of genes comprising IRX9, IRX10, IRX14, and FRA8 that perform the main role in GX synthesis during vegetative development. The IRX9-L, IRX10-L, IRX14-L, and F8H genes are able to partially substitute for their respective homologs and normally perform a minor function. The irx14 irx14-L double mutant virtually lacks xylan, whereas irx9 irx9-L and fra8 f8h double mutants form lowered amounts of GX displaying a greatly reduced degree of backbone polymerization. Our findings reveal two distinct sets of four genes each differentially contributing to GX biosynthesis.

Transcriptome analysis of secondary-wall-enriched seed coat tissues of canola (Brassica napus L.)

The seed coat of Brassica napus (canola, oilseed rape) is derived from ovule integuments and contains a layer of palisade cells, which have thick secondary walls. Because cellulosic walls and other indigestible components of the seed coat contribute negatively to the value of oilseeds, efforts are underway to alter seed development. To facilitate these efforts, and to better understand the biology of seed coats, we used a 90,000 element microarray to identify genes whose transcripts were expressed in developing seed coats of B. napus. After dissecting seed coats into three layers, and comparing transcript expression in the middle fraction (which contained the palisade-enriched tissue and bulk of inner integument) to transcript expression in developing hypocotyls, we identified 674 genes whose transcripts were more abundant in the middle fraction of the seed coat. Among these were well-characterized markers of seed coat identity and many genes associated with metabolism of cell wall polysaccharides, flavonoids and various cell wall proteins and transcription factors. Conversely, we identified 1,203 genes whose transcripts were more abundant in the hypocotyl tissue as compared to seed coat, including xylem-specific markers, such as XCP1 and XCP2. We validated 21 of the differentially expressed transcripts using quantitative RT-PCR. The results define a set of transcripts that are highly enriched in the developing seed coat of B. napus.

Arabidopsis - a powerful model system for plant cell wall research

Plant cell walls are composites of various carbohydrates, proteins and other compounds. Cell walls provide plants with strength and protection, and also represent the most abundant source of renewable biomass. Despite the importance of plant cell walls, comparatively little is known about the identities of genes and functions of proteins involved in their biosynthesis. The model plant Arabidopsis and the availability of its genome sequence have been invaluable for the identification and functional characterization of genes encoding enzymes involved in plant cell-wall biosynthesis. This review covers recent progress in the identification and characterization of genes encoding proteins involved in the biosynthesis of Arabidopsis cell-wall polysaccharides and arabinogalactan proteins. These studies have improved our understanding of both the mechanisms of cell-wall biosynthesis and the functions of various cell-wall polymers, and have highlighted areas where further research is needed.

Chitinase-like protein CTL1 plays a role in altering root system architecture in response to multiple environmental conditions

Plant root architecture is highly responsive to changes in nutrient availability. However, the molecular mechanisms governing the adaptability of root systems to changing environmental conditions is poorly understood. A screen for abnormal root architecture responses to high nitrate in the growth medium was carried out for a population of ethyl methanesulfonate-mutagenized Arabidopsis (Arabidopsis thaliana). The growth and root architecture of the arm (for anion altered root morphology) mutant described here was similar to wild-type plants when grown on low to moderate nitrate concentrations, but on high nitrate, arm exhibited reduced primary root elongation, radial swelling, increased numbers of lateral roots, and increased root hair density when compared to the wild-type control. High concentrations of chloride and sucrose induced the same phenotype. In contrast, hypocotyl elongation in the dark was decreased independently of nitrate availability. Positional cloning identified a point mutation in the AtCTL1 gene that encodes a chitinase-related protein, although molecular and biochemical analysis showed that this protein does not possess chitinase enzymatic activity. CTL1 appears to play two roles in plant growth and development based on the constitutive effect of the arm mutation on primary root growth and its conditional impact on root architecture. We hypothesize that CTL1 plays a role in determining cell wall rigidity and that the activity is differentially regulated by pathways that are triggered by environmental conditions. Moreover, we show that mutants of some subunits of the cellulose synthase complex phenocopy the conditional effect on root architecture under nonpermissive conditions, suggesting they are also differentially regulated in response to a changing environment.

Real-time imaging of cellulose reorientation during cell wall expansion in Arabidopsis roots

Cellulose forms the major load-bearing network of the plant cell wall, which simultaneously protects the cell and directs its growth. Although the process of cellulose synthesis has been observed, little is known about the behavior of cellulose in the wall after synthesis. Using Pontamine Fast Scarlet 4B, a dye that fluoresces preferentially in the presence of cellulose and has excitation and emission wavelengths suitable for confocal microscopy, we imaged the architecture and dynamics of cellulose in the cell walls of expanding root cells. We found that cellulose exists in Arabidopsis (Arabidopsis thaliana) cell walls in large fibrillar bundles that vary in orientation. During anisotropic wall expansion in wild-type plants, we observed that these cellulose bundles rotate in a transverse to longitudinal direction. We also found that cellulose organization is significantly altered in mutants lacking either a cellulose synthase subunit or two xyloglucan xylosyltransferase isoforms. Our results support a model in which cellulose is deposited transversely to accommodate longitudinal cell expansion and reoriented during expansion to generate a cell wall that is fortified against strain from any direction.

Phylogenetically distinct cellulose synthase genes support secondary wall thickening in arabidopsis shoot trichomes and cotton fiber

Abstract Through exploring potential analogies between cotton seed trichomes (or cotton fiber) and arabidopsis shoot trichomes we discovered that CesAs from either the primary or secondary wall phylogenetic clades can support secondary wall thickening. CesA genes that typically support primary wall synthesis, AtCesA1,2,3,5, and 6, underpin expansion and secondary wall thickening of arabidopsis shoot trichomes. In contrast, apparent orthologs of CesA genes that support secondary wall synthesis in arabidopsis xylem, AtCesA4,7, and 8, are up-regulated for cotton fiber secondary wall deposition. These conclusions arose from: (a) analyzing the expression of CesA genes in arabidopsis shoot trichomes; (b) observing birefringent secondary walls in arabidopsis shoot trichomes with mutations in AtCesA4, 7, or 8; (c) assaying up-regulated genes during different stages of cotton fiber development; and (d) comparing genes that were co-expressed with primary or secondary wall CesAs in arabidopsis with genes up-regulated in arabidopsis trichomes, arabidopsis secondary xylem, or cotton fiber during primary or secondary wall deposition. Cumulatively, the data show that: (a) the xylem of arabidopsis provides the best model for secondary wall cellulose synthesis in cotton fiber; and (b) CesA genes within a "cell wall toolbox" are used in diverse ways for the construction of particular specialized cell walls.

Sucrose synthase is an integral component of the cellulose synthesis machinery

Cellulose synthesis in plants is believed to be carried out by the plasma membrane-associated rosette structure which can be observed by electron microscopy. Despite decade-long speculation, it had not been demonstrated whether the rosette is the site of catalytic activity of cellulose synthesis. To determine the relationship between this structure and cellulose synthesis, we successfully isolated detergent-insoluble rosettes from the plasma membrane of bean epicotyls. However, the purified rosettes did not possess cellulose synthesis activity in vitro. Conversely, detergent-soluble granular particles of approximately 9.5-10 nm diameter were also isolated and exhibited UDP-glucose binding activity and possessed beta-1,4-glucan (cellulose) synthesis activity in vitro. The particle, referred to as the catalytic unit of cellulose synthesis, was enriched with a 78 kDa polypeptide which was verified as sucrose synthase like by mass spectrometry and immunoblotting. The catalytic units were able to bind to the rosettes and retained the cellulose synthesis activity in the presence of UDP-glucose or sucrose plus UDP when supplemented with magnesium. The incorporation of the catalytic unit into the rosette structure was confirmed by immunogold labeling with anti-sucrose synthase antibodies under an electron microscope. Our results suggest that the plasma membrane-associated rosette anchors the catalytic unit of cellulose synthesis to form the functional cellulose synthesis machinery.

On the robustness of the geometrical model for cell wall deposition

All plant cells are provided with the necessary rigidity to withstand the turgor by an exterior cell wall. This wall is composed of long crystalline cellulose microfibrils embedded in a matrix of other polysaccharides. The cellulose microfibrils are deposited by mobile membrane bound protein complexes in remarkably ordered lamellar textures. The mechanism by which these ordered textures arise, however, is still under debate. The geometrical model for cell wall deposition proposed by Emons and Mulder (Proc. Natl. Acad. Sci. 95, 7215-7219, 1998) provides a detailed approach to the case of cell wall deposition in non-growing cells, where there is no evidence for the direct influence of other cellular components such as microtubules. The model successfully reproduces even the so-called helicoidal wall; the most intricate texture observed. However, a number of simplifying assumptions were made in the original calculations. The present work addresses the issue of the robustness of the model to relaxation of these assumptions, by considering whether the helicoidal solutions survive when three aspects of the model are varied. These are: (i) the shape of the insertion domain, (ii) the distribution of lifetimes of individual CSCs, and (iii) fluctuations and overcrowding. Although details of the solutions do change, we find that in all cases the overall character of the helicoidal solutions is preserved.

Rice cellulose synthase-like D4 is essential for normal cell-wall biosynthesis and plant growth

Cellulose synthase-like (CSL) proteins of glycosyltransferase family 2 (GT2) are believed to be involved in the biosynthesis of cell-wall polymers. The CSL D sub-family (CSLD) is common to all plants, but the functions of CSLDs remain to be elucidated. We report here an in-depth characterization of a narrow leaf and dwarf1 (nd1) rice mutant that shows significant reduction in plant growth due to retarded cell division. Map-based cloning revealed that ND1 encodes OsCSLD4, one of five members of the CSLD sub-family in rice. OsCSLD4 is mainly expressed in tissues undergoing rapid growth. Expression of OsCSLD4 fluorescently tagged at the C- or N-terminus in rice protoplast cells or Nicotiana benthamiana leaves showed that the protein is located in the endoplasmic reticulum or Golgi vesicles. Golgi localization was verified using phenotype-rescued transgenic plants expressing OsCSLD4-GUS under the control of its own promoter. Two phenotype-altered tissues, culms and root tips, were used to investigate the specific wall defects. Immunological studies and monosaccharide compositional and glycosyl linkage analyses explored several wall compositional effects caused by disruption of OsCSLD4, including alterations in the structure of arabinoxylan and the content of cellulose and homogalacturonan, which are distinct in the monocot grass species Oryza sativa (rice). The inconsistent alterations in the two tissues and the observable structural defects in primary walls indicate that OsCSLD4 plays important roles in cell-wall formation and plant growth.

STD-NMR studies suggest that two acceptor substrates for GlfT2, a bifunctional galactofuranosyltransferase required for the biosynthesis of Mycobacterium tuberculosis arabinogalactan, compete for the same binding site

The mycobacterial cell wall is a complex architecture, which has, as its major structural component, a lipidated polysaccharide covalently bound to peptidoglycan. This structure, termed the mycolyl-arabinogalactan-peptidoglycan complex, possesses a core galactan moiety composed of approximately 30 galactofuranosyl (Galf) resides attached via alternating beta-(1-->6) and beta-(1-->5) linkages. Recent studies have shown that the entire galactan is synthesized by the action of only two bifunctional galactofuranosyltransferases, GlfT1 and GlfT2. We report here saturation-transfer difference (STD) NMR spectroscopy studies with GlfT2 using two trisaccharide acceptor substrates, beta-D-Galf-(1-->6)-beta-D-Galf-(1-->5)-beta-D-Galf-O(CH(2))(7)CH(3) (2) and beta-D-Galf-(1-->5)-beta-D-Galf-(1-->6)-beta-D-Galf-O(CH(2))(7)CH(3) (3), as well as the donor substrate for the enzyme, UDP-Galf. Competition STD-NMR titration experiments and saturation transfer double difference (STDD) experiments with 2 and 3 were undertaken to explore the bifunctionality of this enzyme, in particular to answer whether one or two active sites are responsible for the formation of both beta-(1-->5)- and beta-(1-->6)-Galf linkages. It was demonstrated that 2 and 3 bind competitively at the same site; this suggests that GlfT2 has one active site pocket capable of catalyzing both beta-(1-->5) and beta-(1-->6) galactofuranosyl transfer reactions. The addition of UDP-Galf to GlfT2 in the presence of either 2 or 3 generated a tetrasaccharide product; this indicates that the enzyme was catalytically active under the conditions at which the STD-NMR experiments were carried out.

Ectopic expression of MYB46 identifies transcriptional regulatory genes involved in secondary wall biosynthesis in Arabidopsis

MYB46 functions as a transcriptional switch that turns on the genes necessary for secondary wall biosynthesis. Elucidating the transcriptional regulatory network immediately downstream of MYB46 is crucial to our understanding of the molecular and biochemical processes involved in the biosynthesis and deposition of secondary walls in plants. To gain insights into MYB46-mediated transcriptional regulation, we first established an inducible secondary wall thickening system in Arabidopsis by expressing MYB46 under the control of dexamethasone-inducible promoter. Then, we used an ATH1 GeneChip microarray and Illumina digital gene expression system to obtain a series of transcriptome profiles with regard to the induction of secondary wall development. These analyses allowed us to identify a group of transcription factors whose expression coincided with or preceded the induction of secondary wall biosynthetic genes. A transient transcriptional activation assay was used to confirm the hierarchical relationships among the transcription factors in the network. The in vivo assay showed that MYB46 transcriptionally activates downstream target transcription factors, three of which (AtC3H14, MYB52 and MYB63) were shown to be able to activate secondary wall biosynthesis genes. AtC3H14 activated the transcription of all of the secondary wall biosynthesis genes tested, suggesting that AtC3H14 may be another master regulator of secondary wall biosynthesis. The transcription factors identified here may include direct activators of secondary wall biosynthesis genes. The present study discovered novel hierarchical relationships among the transcription factors involved in the transcriptional regulation of secondary wall biosynthesis, and generated several testable hypotheses.

A missense mutation in the transmembrane domain of CESA4 affects protein abundance in the plasma membrane and results in abnormal cell wall biosynthesis in rice

Cellulose synthase (CESA) is a critical catalytic subunit of the cellulose synthase complex responsible for glucan chain elongation. Our knowledge about how CESA functions is still very limited. Here, we report the functional characterization of a rice mutant, brittle culm11, that shows growth retardation and dramatically reduced plant strength. Map-based cloning revealed that all the mutant phenotypes result from a missense mutation in OsCESA4 (G858R), a highly conserved residue at the end of the fifth transmembrane domain. The aberrant secondary cell wall of the mutant plants is attributed to significantly reduced cellulose content, abnormal secondary wall structure of sclerenchyma cells, and overall altered wall composition, as detected by chemical analyses and immunochemical staining. Importantly, we have found that this point mutation decreases the abundance of OsCESA4 in the plasma membrane, probably due to a defect in the process of CESA complex secretion. The data from our biochemical, genetic, and pharmacological analyses indicate that this residue is critical for maintaining the normal level of CESA proteins in the plasma membrane.

Selective modification of rice (Oryza sativa) gene expression by rice stripe virus infection

Rice stripe disease, caused by rice stripe virus (RSV), is one of the major virus diseases in east Asia. Rice plants infected with RSV usually show symptoms such as chlorosis, weakness, necrosis in newly emerged leaves and stunting. To reveal rice cellular systems influenced by RSV infection, temporal changes in the transcriptome of RSV-infected plants were monitored by a customized rice oligoarray system. The transcriptome changes in RSV-infected plants indicated that protein-synthesis machineries and energy production in the mitochondrion were activated by RSV infection, whereas energy production in the chloroplast and synthesis of cell-structure components were suppressed. The transcription of genes related to host-defence systems under hormone signals and those for gene silencing were not activated at the early infection phase. Together with concurrent observation of virus concentration and symptom development, such transcriptome changes in RSV-infected plants suggest that different sets of various host genes are regulated depending on the development of disease symptoms and the accumulation of RSV.

Plant cell wall matrix polysaccharide biosynthesis

The wall of an expanding plant cell consists primarily of cellulose microfibrils embedded in a matrix of hemicellulosic and pectic polysaccharides along with small amounts of structural and enzymatic proteins. Matrix polysaccharides are synthesized in the Golgi and exported to the cell wall by exocytosis, where they intercalate among cellulose microfibrils, which are made at the plasma membrane and directly deposited into the cell wall. Involvement of Golgi glucan synthesis in auxin-induced cell expansion has long been recognized; however, only recently have the genes corresponding to glucan synthases been identified. Biochemical purification was unsuccessful because of the labile nature and very low abundance of these enzymes. Mutational genetics also proved fruitless. Expression of candidate genes identified through gene expression profiling or comparative genomics in heterologous systems followed by functional characterization has been relatively successful. Several genes from the cellulose synthase-like (Csl) family have been found to be involved in the synthesis of various hemicellulosic glycans. The usefulness of this approach, however, is limited to those enzymes that probably do not form complexes consisting of unrelated proteins. Nonconventional approaches will continue to incrementally unravel the mechanisms of Golgi polysaccharide biosynthesis.

The CELLULOSE-SYNTHASE LIKE C (CSLC) family of barley includes members that are integral membrane proteins targeted to the plasma membrane

The CELLULOSE SYNTHASE-LIKE C (CSLC) family is an ancient lineage within the CELLULOSE SYNTHASE/CELLULOSE SYNTHASE-LIKE (CESA/CSL) polysaccharide synthase superfamily that is thought to have arisen before the divergence of mosses and vascular plants. As studies in the flowering plant Arabidopsis have suggested synthesis of the (1,4)-beta-glucan backbone of xyloglucan (XyG), a wall polysaccharide that tethers adjacent cellulose microfibrils to each other, as a probable function for the CSLCs, CSLC function was investigated in barley (Hordeum vulgare L.), a species with low amounts of XyG in its walls. Four barley CSLC genes were identified (designated HvCSLC1-4). Phylogenetic analysis reveals three well supported clades of CSLCs in flowering plants, with barley having representatives in two of these clades. The four barley CSLCs were expressed in various tissues, with in situ PCR detecting transcripts in all cell types of the coleoptile and root, including cells with primary and secondary cell walls. Co-expression analysis showed that HvCSLC3 was coordinately expressed with putative XyG xylosyltransferase genes. Both immuno-EM and membrane fractionation showed that HvCSLC2 was located in the plasma membrane of barley suspension-cultured cells and was not in internal membranes such as endoplasmic reticulum or Golgi apparatus. Based on our current knowledge of the sub-cellular locations of polysaccharide synthesis, we conclude that the CSLC family probably contains more than one type of polysaccharide synthase.

Purification of an active, oligomeric chitin synthase complex from the midgut of the tobacco hornworm

Chitin formation depends on the activity of a family II glycosyltransferase known as chitin synthase, whose biochemical and structural properties are largely unknown. Previously, we have demonstrated that the chitin portion of the peritrophic matrix in the midgut of the tobacco hornworm, Manduca sexta, is produced by chitin synthase 2 (CHS-2), one of two isoenzymes encoded by the Chs-1 and Chs-2 genes (also named Chs-A and Chs-B), and that CHS-2 is located at the apical tips of the brush border microvilli. Here we report the purification of the chitin synthase from the Manduca midgut as monitored by its activity and immuno-reactivity with antibodies to the chitin synthase. After gel permeation chromatography, the final step of the developed purification protocol, the active enzyme eluted in a fraction corresponding to a molecular mass between 440 and 670 kDa. Native PAGE revealed a single, immuno-reactive band of about 520 kDa, thrice the molecular mass of the chitin synthase monomer. SDS-PAGE and immunoblotting indicated finally that an active, oligomeric complex of the chitin synthase was purified. In summary, the chitin synthase from the midgut of Manduca may prove to be a good model for investigating the enzymes' mode of action.

A tethering mechanism for length control in a processive carbohydrate polymerization

Carbohydrate polymers are the most abundant organic substances on earth. Their degrees of polymerization range from tens to thousands of units, yet polymerases generate the relevant lengths without the aid of a template. To gain insight into template-independent length control, we investigated how the mycobacterial galactofuranosyl-transferase GlfT2 mediates formation of the galactan, a polymer of galactofuranose residues that is an integral part of the cell wall. We show that isolated recombinant GlfT2 can catalyze the synthesis of polymers with degrees of polymerization that are commensurate with values observed in mycobacteria, indicating that length control by GlfT2 is intrinsic. Investigations using synthetic substrates reveal that GlfT2 is processive. The data indicate that GlfT2 controls length by using a substrate tether, which is distal from the site of elongation. The strength of interaction of that tether with the polymerase influences the length of the resultant polymer. Thus, our data identify a mechanism for length control by a template-independent polymerase.

Solutions for dissolution--engineering cell walls for deconstruction

Photosynthetic carbon capture by terrestrial plants represents a major sink for atmospheric CO(2), ultimately terminating in the synthesis of a secondary plant cell wall-a complex matrix of polysaccharides intricately linked to lignin. The production and co-ordinated deposition of this lignocellulosic composite confers both protective and structural properties to the plant cell. The inherent properties of this complex cell wall also represent a major obstacle for its effective industrial utilization, as operationally effective mechanisms for the removal of lignin and the consequential release of carbohydrate constituents remain elusive. Perturbing plants by mis-regulating key genes/enzymes integral to major cell wall pathways can provide both rich insights into cell wall development and architecture, and concurrently provide significant opportunities for improved lignocellulose utilization.

An update on the nomenclature for the cellulose synthase genes in Populus

Cellulose synthase (CesA) is a central catalyst in the generation of the plant cell wall biomass and is, therefore, the focus of intense research. Characterization of individual CesA genes from Populus species has led to the publication of several different naming conventions for CesA gene family members in this model tree. To help reduce the resulting confusion, we propose here a new phylogeny-based CesA nomenclature that aligns the Populus CesA gene family with the established Arabidopsis thaliana CesA family structure.

Cell wall biosynthesis and the molecular mechanism of plant enlargement

Recently discovered reactions allow the green alga Chara corallina (Klien ex. Willd., em. R.D.W.) to grow well without the benefit of xyloglucan or rhamnogalactan II in its cell wall. Growth rates are controlled by polygalacturonic acid (pectate) bound with calcium in the primary wall, and the reactions remove calcium from these bonds when new pectate is supplied. The removal appears to occur preferentially in bonds distorted by wall tension produced by the turgor pressure (P). The loss of calcium accelerates irreversible wall extension if P is above a critical level. The new pectate (now calcium pectate) then binds to the wall and decelerates wall extension, depositing new wall material on and within the old wall. Together, these reactions create a non-enzymatic but stoichiometric link between wall growth and wall deposition. In green plants, pectate is one of the most conserved components of the primary wall, and it is therefore proposed that the acceleration-deceleration-wall deposition reactions are of wide occurrence likely to underlie growth in virtually all green plants. C. corallina is one of the closest relatives of the progenitors of terrestrial plants, and this review focuses on the pectate reactions and how they may fit existing theories of plant growth.

Combination of several bioinformatics approaches for the identification of new putative glycosyltransferases in Arabidopsis

Approximately 450 glycosyltransferase (GT) sequences have been already identified in the Arabidopsis genome that organize into 40 sequence-based families, but a vast majority of these gene products remain biochemically uncharacterized open reading frames. Given the complexity of the cell wall carbohydrate network, it can be inferred that some of the biosynthetic genes have not yet been identified by classical bioinformatics approaches. With the objective to identify new plant GT genes, we designed a bioinformatic strategy that is based on the use of several remote homology detection methods that act at the 1D, 2D, and 3D level. Together, these methods led to the identification of more than 150 candidate protein sequences. Among them, 20 are considered as putative glycosyltransferases that should further be investigated since known GT signatures were clearly identified.

Pausing of Golgi bodies on microtubules regulates secretion of cellulose synthase complexes in Arabidopsis

Plant growth and organ formation depend on the oriented deposition of load-bearing cellulose microfibrils in the cell wall. Cellulose is synthesized by plasma membrane-bound complexes containing cellulose synthase proteins (CESAs). Here, we establish a role for the cytoskeleton in intracellular trafficking of cellulose synthase complexes (CSCs) through the in vivo study of the green fluorescent protein (GFP)-CESA3 fusion protein in Arabidopsis thaliana hypocotyls. GFP-CESA3 localizes to the plasma membrane, Golgi apparatus, a compartment identified by the VHA-a1 marker, and, surprisingly, a novel microtubule-associated cellulose synthase compartment (MASC) whose formation and movement depend on the dynamic cortical microtubule array. Osmotic stress or treatment with the cellulose synthesis inhibitor CGA 325'615 induces internalization of CSCs in MASCs, mimicking the intracellular distribution of CSCs in nongrowing cells. Our results indicate that cellulose synthesis is coordinated with growth status and regulated in part through CSC internalization. We find that CSC insertion in the plasma membrane is regulated by pauses of the Golgi apparatus along cortical microtubules. Our data support a model in which cortical microtubules not only guide the trajectories of CSCs in the plasma membrane, but also regulate the insertion and internalization of CSCs, thus allowing dynamic remodeling of CSC secretion during cell expansion and differentiation.

The temperature-sensitive brush mutant of the legume Lotus japonicus reveals a link between root development and nodule infection by rhizobia

The brush mutant of Lotus japonicus exhibits a temperature-dependent impairment in nodule, root, and shoot development. At 26 degrees C, brush formed fewer nodules, most of which were not colonized by rhizobia bacteria. Primary root growth was retarded and the anatomy of the brush root apical meristem revealed distorted cellular organization and reduced cell expansion. Reciprocal grafting of brush with wild-type plants indicated that this genotype only affected the root and that the shoot phenotype was a secondary effect. The root and nodulation phenotype cosegregated as a single Mendelian trait and the BRUSH gene could be mapped to the short arm of chromosome 2. At 18 degrees C, the brush root anatomy was rescued and similar to the wild type, and primary root length, number of infection threads, and nodule formation were partially rescued. Superficially, the brush root phenotype resembled the ethylene-related thick short root syndrome. However, treatment with ethylene inhibitor did not recover the observed phenotypes, although brush primary roots were slightly longer. The defects of brush in root architecture and infection thread development, together with intact nodule architecture and complete absence of symptoms from shoots, suggest that BRUSH affects cellular differentiation in a tissue-dependent way.

In vivo interference with AtTCP20 function induces severe plant growth alterations and deregulates the expression of many genes important for development

AtTCP20 is a transcription factor belonging to the Arabidopsis (Arabidopsis thaliana) TCP-P subfamily, characterized by its capacity to bind to site II motifs (TGGGCY). Our aim was to understand the role of AtTCP20 in plant development. The expression pattern of a translational fusion of Prom(TCP20):CDS20GUSGFP suggested a function for AtTCP20 in several plant organs and stages of development. The role of AtTCP20 was challenged in planta by inducing expression of AtTCP20 proteins fused with either a transcriptional activator domain (VP16) or a repressor domain (EAR). Expression of both modified proteins led to severe developmental phenotypes. In-depth analysis suggested that AtTCP20 may participate in the regulation of cell expansion, cell division, and cell differentiation. Gene expression profiling in roots and hypocotyls revealed that 252 genes were down-regulated in both organs after induction of the AtTCP20EAR repressor gene. Site II motifs (TGGGCY) were underrepresented in their promoters. Conversely, GG(A/T)CCC sequences related to binding sites identified for TCP proteins in rice (Oryza sativa) were overrepresented, and a TCP20 fusion protein was shown to bind to these sequences in vitro. Gene ontology indicated that many targeted genes were involved in cell wall biogenesis and modification during expansion and also encoded numerous transcription factors controlling plant development. Our results are consistent with the previous proposal that AtTCP20 is involved in cell division and growth coordination. Moreover, they further suggest that AtTCP20 also contributes to cell expansion control and indicate a different involvement of this protein in plant morphogenesis depending on the organ and the developmental stage.

BC10, a DUF266-containing and Golgi-located type II membrane protein, is required for cell-wall biosynthesis in rice (Oryza sativa L.)

Glycosyltransferases (GTs) are one of the largest enzyme groups required for the synthesis of complex wall polysaccharides and glycoproteins in plants. However, due to the limited number of related mutants that have observable phenotypes, the biological function(s) of most GTs in cell-wall biosynthesis and assembly have remained elusive. We report here the isolation and in-depth characterization of a brittle rice mutant, brittle culm 10 (bc10). bc10 plants show pleiotropic phenotypes, including brittleness of the plant body and retarded growth. The BC10 gene was cloned through a map-based approach, and encodes a Golgi-located type II membrane protein that contains a domain designated as 'domain of unknown function 266' (DUF266) and represents a multiple gene family in rice. BC10 has low sequence similarity with the domain to a core 2 beta-1,6-N-acetylglucosaminyltransferase (C2GnT), and its in vitro enzymatic activity suggests that it functions as a glycosyltransferase. Monosaccharide analysis of total and fractioned wall residues revealed that bc10 showed impaired cellulose biosynthesis. Immunolocalization and isolation of arabinogalactan proteins (AGPs) in the wild-type and bc10 showed that the level of AGPs in the mutant is significantly affected. BC10 is mainly expressed in the developing sclerenchyma and vascular bundle cells, and its deficiency causes a reduction in the levels of cellulose and AGPs, leading to inferior mechanical properties.

Cellulosic biofuels

The development of sustainable, low-carbon, liquid fuels from cellulosic biomass will require advances in many areas of science and engineering. This review describes the major topics of enquiry concerning cellulosic biofuels with an emphasis on those areas of research and development that include research problems of interest to plant biologists.

Dependence of stress resistance on a spore coat heteropolysaccharide in Dictyostelium

In Dictyostelium, sporulation occurs synchronously as prespore cells approach the apex of the aerial stalk during culmination. Each prespore cell becomes surrounded by its own coat comprised of a core of crystalline cellulose and a branched heteropolysaccharide sandwiched between heterogeneous cysteine-rich glycoproteins. The function of the heteropolysaccharide, which consists of galactose and N-acetylgalactosamine, is unknown. Two glycosyltransferase-like genes encoding multifunctional proteins, each with predicted features of a heteropolysaccharide synthase, were identified in the Dictyostelium discoideum genome. pgtB and pgtC transcripts were modestly upregulated during early development, and pgtB was further intensely upregulated at the time of heteropolysaccharide accumulation. Disruption of either gene reduced synthase-like activity and blocked heteropolysaccharide formation, based on loss of cytological labeling with a lectin and absence of component sugars after acid hydrolysis. Cell mixing experiments showed that heteropolysaccharide expression is spore cell autonomous, suggesting a physical association with other coat molecules during assembly. Mutant coats expressed reduced levels of crystalline cellulose based on chemical analysis after acid degradation, and cellulose was heterogeneously affected based on flow cytometry and electron microscopy. Mutant coats also contained elevated levels of selected coat proteins but not others and were sensitive to shear. Mutant spores were unusually susceptible to hypertonic collapse and damage by detergent or hypertonic stress. Thus, the heteropolysaccharide is essential for spore integrity, which can be explained by a role in the formation of crystalline cellulose and regulation of the protein content of the coat.

Plant cell walls throughout evolution: towards a molecular understanding of their design principles

Throughout their life, plants typically remain in one location utilizing sunlight for the synthesis of carbohydrates, which serve as their sole source of energy as well as building blocks of a protective extracellular matrix, called the cell wall. During the course of evolution, plants have repeatedly adapted to their respective niche, which is reflected in the changes of their body plan and the specific design of cell walls. Cell walls not only changed throughout evolution but also are constantly remodelled and reconstructed during the development of an individual plant, and in response to environmental stress or pathogen attacks. Carbohydrate-rich cell walls display complex designs, which together with the presence of phenolic polymers constitutes a barrier for microbes, fungi, and animals. Throughout evolution microbes have co-evolved strategies for efficient breakdown of cell walls. Our current understanding of cell walls and their evolutionary changes are limited as our knowledge is mainly derived from biochemical and genetic studies, complemented by a few targeted yet very informative imaging studies. Comprehensive plant cell wall models will aid in the re-design of plant cell walls for the purpose of commercially viable lignocellulosic biofuel production as well as for the timber, textile, and paper industries. Such knowledge will also be of great interest in the context of agriculture and to plant biologists in general. It is expected that detailed plant cell wall models will require integrated correlative multimodal, multiscale imaging and modelling approaches, which are currently underway.

Laying down the bricks: logistic aspects of cell wall biosynthesis

Plant cell wall polysaccharides are synthesised at the plasma membrane and in the Golgi apparatus. Current research efforts mainly try to address how these molecules are synthesised or modified. However, it is clear that polysaccharide synthesis in the two compartments needs to be carried out in a coordinated fashion, and that carbohydrates and proteins that are delivered from the Golgi to the cell surface have to undergo a range of modifications. Consequently, there appears to be a need for a fine-tuned system that coalesces signals from the wall, synthesis of carbohydrate-based molecules and vesicle shuttling. Several recent papers have scratched the surface for an initial understanding of these linked processes. For example, the impairment of the proton pumping activity in the trans-Golgi network, which is part of the cell's trafficking system, results in growth defects, changes in Golgi stack morphology and cellulose deficiency. An increased understanding of how cell wall synthesis is coordinated with the secretory machinery may facilitate avenues for modulating cell wall contents and therefore overall plant biomass.

Integrating genes and phenotype: a wheat-Arabidopsis-rice glycosyltransferase database for candidate gene analyses

Glycosyltransferases (GTs) constitute a very large multi-gene superfamily, containing several thousand members identified in sequenced organisms especially in plants. GTs are key enzymes involved in various biological processes such as cell wall formation, storage polysaccharides biosynthesis, and glycosylation of various metabolites. GTs have been identified in rice (Oryza sativa) and Arabidopsis thaliana, but their precise function has been demonstrated biochemically for only a few. In this work we have established a repertoire of virtually all the wheat (Triticum aestivum) GT sequences, using the large publicly available banks of expressed sequences. Based on sequence similarity with Arabidopsis and rice GTs compiled in the carbohydrate active enzyme database (CAZY), we have identified and classified these wheat sequences. The results were used to feed a searchable database available on the web ( http://wwwappli.nantes.inra.fr:8180/GTIDB ) that can be used for initiating an exhaustive candidate gene survey in wheat applied to a particular biological process. This is illustrated through the identification of GT families which are expressed during cell wall formation in wheat grain maturation.

Two leucine-rich repeat receptor kinases mediate signaling, linking cell wall biosynthesis and ACC synthase in Arabidopsis

The plant cell wall is a dynamic structure that changes in response to developmental and environmental cues through poorly understood signaling pathways. We identified two Leu-rich repeat receptor-like kinases in Arabidopsis thaliana that play a role in regulating cell wall function. Mutations in these FEI1 and FEI2 genes (named for the Chinese word for fat) disrupt anisotropic expansion and the synthesis of cell wall polymers and act additively with inhibitors or mutations disrupting cellulose biosynthesis. While FEI1 is an active protein kinase, a kinase-inactive version of FEI1 was able to fully complement the fei1 fei2 mutant. The expansion defect in fei1 fei2 roots was suppressed by inhibition of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, an enzyme that converts Ado-Met to ACC in ethylene biosynthesis, but not by disruption of the ethylene response pathway. Furthermore, the FEI proteins interact directly with ACC synthase. These results suggest that the FEI proteins define a novel signaling pathway that regulates cell wall function, likely via an ACC-mediated signal.