Plant cell walls

The plant cell wall in the feeding sites of cyst nematodes

Plant parasitic cyst nematodes (genera Heterodera and Globodera) are serious pests for many crops. They enter the host roots as migratory second stage juveniles (J2) and migrate intracellularly toward the vascular cylinder using their stylet and a set of cell wall degrading enzymes produced in the pharyngeal glands. They select an initial syncytial cell (ISC) within the vascular cylinder or inner cortex layers to induce the formation of a multicellular feeding site called a syncytium, which is the only source of nutrients for the parasite during its entire life. A syncytium can consist of more than hundred cells whose protoplasts are fused together through local cell wall dissolutions. While the nematode produces a cocktail of cell wall degrading and modifying enzymes during migration through the root, the cell wall degradations occurring during syncytium development are due to the plants own cell wall modifying and degrading proteins. The outer syncytial cell wall thickens to withstand the increasing osmotic pressure inside the syncytium. Furthermore, pronounced cell wall ingrowths can be formed on the outer syncytial wall at the interface with xylem vessels. They increase the surface of the symplast-apoplast interface, thus enhancing nutrient uptake into the syncytium. Processes of cell wall degradation, synthesis and modification in the syncytium are facilitated by a variety of plant proteins and enzymes including expansins, glucanases, pectate lyases and cellulose synthases, which are produced inside the syncytium or in cells surrounding the syncytium.

Super-resolution imaging with Pontamine Fast Scarlet 4BS enables direct visualization of cellulose orientation and cell connection architecture in onion epidermis cells

BACKGROUND

In plants, a complex cell wall protects cells and defines their shape. Cellulose fibrils form a multilayered network inside the cell-wall matrix that plays a direct role in controlling cell expansion. Resolving the structure of this network will allow us to comprehend the relationship of cellulose fibril orientation and growth.The fluorescent dye Pontamine Fast Scarlet 4BS (PFS) was shown to stain cellulose with high specificity and could be used to visualize cellulose bundles in cell walls of Arabidopsis root epidermal cells with confocal microscopy. The resolution limit of confocal microscopy of some 200 nm in xy and 550 nm in z for green light, restricts the direct visualization of cellulose to relatively large bundles, whereas the structure of cellulose microfibrils with their diameter below 10 nm remains unresolved. Over the last decade, several so-called super-resolution microscopy approaches have been developed; in this paper we explore the potential of such approaches for the direct visualization of cellulose.

RESULTS

To ensure optimal imaging we determined the spectral properties of PFS-stained tissue. PFS was found not to affect cell viability in the onion bulb scale epidermis. We present the first super-resolution images of cellulose bundles in the plant cell wall produced by direct stochastic optical reconstruction microscopy (dSTORM) in combination with total internal reflection fluorescence (TIRF) microscopy. Since TIRF limits observation to the cell surface, we tested as alternatives 3D-structured illumination microscopy (3D-SIM) and confocal microscopy, combined with image deconvolution. Both methods offer lower resolution than STORM, but enable 3D imaging. While 3D-SIM produced strong artifacts, deconvolution gave good results. The resolution was improved over conventional confocal microscopy and the approach could be used to demonstrate differences in fibril orientation in different layers of the cell wall as well as particular cellulose fortifications around plasmodesmata.

CONCLUSIONS

Super-resolution light microscopy of PFS-stained cellulose fibrils is possible and the increased resolution over conventional approaches makes it a valuable tool for the investigation of the cell-wall structure. This is one step in method developments that will close the gap to more invasive techniques, such as atomic force and electron microscopy.

Biomass digestibility is predominantly affected by three factors of wall polymer features distinctive in wheat accessions and rice mutants

BACKGROUND

Wheat and rice are important food crops with enormous biomass residues for biofuels. However, lignocellulosic recalcitrance becomes a crucial factor on biomass process. Plant cell walls greatly determine biomass recalcitrance, thus it is essential to identify their key factors on lignocellulose saccharification. Despite it has been reported about cell wall factors on biomass digestions, little is known in wheat and rice. In this study, we analyzed nine typical pairs of wheat and rice samples that exhibited distinct cell wall compositions, and identified three major factors of wall polymer features that affected biomass digestibility.

RESULTS

Based on cell wall compositions, ten wheat accessions and three rice mutants were classified into three distinct groups each with three typical pairs. In terms of group I that displayed single wall polymer alternations in wheat, we found that three wall polymer levels (cellulose, hemicelluloses and lignin) each had a negative effect on biomass digestibility at similar rates under pretreatments of NaOH and H2SO4 with three concentrations. However, analysis of six pairs of wheat and rice samples in groups II and III that each exhibited a similar cell wall composition, indicated that three wall polymer levels were not the major factors on biomass saccharification. Furthermore, in-depth detection of the wall polymer features distinctive in rice mutants, demonstrated that biomass digestibility was remarkably affected either negatively by cellulose crystallinity (CrI) of raw biomass materials, or positively by both Ara substitution degree of non-KOH-extractable hemicelluloses (reverse Xyl/Ara) and p-coumaryl alcohol relative proportion of KOH-extractable lignin (H/G). Correlation analysis indicated that Ara substitution degree and H/G ratio negatively affected cellulose crystallinity for high biomass enzymatic digestion. It was also suggested to determine whether Ara and H monomer have an interlinking with cellulose chains in the future.

CONCLUSIONS

Using nine typical pairs of wheat and rice samples having distinct cell wall compositions and wide biomass saccharification, Ara substitution degree and monolignin H proportion have been revealed to be the dominant factors positively determining biomass digestibility upon various chemical pretreatments. The results demonstrated the potential of genetic modification of plant cell walls for high biomass saccharification in bioenergy crops.

Heterogeneity and glycan masking of cell wall microstructures in the stems of Miscanthus x giganteus, and its parents M. sinensis and M. sacchariflorus

Plant cell walls, being repositories of fixed carbon, are important sources of biomass and renewable energy. Miscanthus species are fast growing grasses with a high biomass yield and they have been identified as potential bioenergy crops. Miscanthus x giganteus is the sterile hybrid between M. sinensis and M. sacchariflorus, with a faster and taller growth than its parents. In this study, the occurrence of cell wall polysaccharides in stems of Miscanthus species has been determined using fluorescence imaging with sets of cell wall directed monoclonal antibodies. Heteroxylan and mixed linkage-glucan (MLG) epitopes are abundant in stem cell walls of Miscanthus species, but their distributions are different in relation to the interfascicular parenchyma and these epitopes also display different developmental dynamics. Detection of pectic homogalacturonan (HG) epitopes was often restricted to intercellular spaces of parenchyma regions and, notably, the high methyl ester LM20 HG epitope was specifically abundant in the pith parenchyma cell walls of M. x giganteus. Some cell wall probes cannot access their target glycan epitopes because of masking by other polysaccharides. In the case of Miscanthus stems, masking of xyloglucan by heteroxylan and masking of pectic galactan by heteroxylan and MLG was detected in certain cell wall regions. Knowledge of tissue level heterogeneity of polysaccharide distributions and molecular architectures in Miscanthus cell wall structures will be important for both understanding growth mechanisms and also for the development of potential strategies for the efficient deconstruction of Miscanthus biomass.

Arabidopsis GLASSY HAIR genes promote trichome papillae development

Specialized plant cells form cell walls with distinct composition and properties pertinent to their function. Leaf trichomes in Arabidopsis form thick cell walls that support the upright growth of these large cells and, curiously, have strong light-reflective properties. To understand the process of trichome cell-wall maturation and the molecular origins of this optical property, mutants affected in trichome light reflection were isolated and characterized. It was found that GLASSY HAIR (GLH) genes are required for the formation of surface papillae structures at late stages of trichome development. Trichomes in these mutants appeared transparent due to unobstructed light transmission. Genetic analysis of the isolated mutants revealed seven different gene loci. Two--TRICHOME BIREFRINGENCE (TBR) and NOK (Noeck)--have been reported previously to have the glassy trichome mutant phenotype. The other five glh mutants were analysed for cell-wall-related phenotypes. A significant reduction was found in cellulose content in glh2 and glh4 mutant trichomes. In addition to the glassy trichome phenotype, the glh6 mutants showed defects in leaf cuticular wax, and glh6 was found to represent a new allele of the eceriferum 10 (cer10) mutation. Trichomes of the glh1 and glh3 mutants did not show any other phenotypes beside reduced papillae formation. These data suggest that the GLH1 and GLH3 genes may have specific functions in trichome papillae formation, whereas GLH2, GLH4, and GLH6 genes are also involved in deposition of other cell-wall components.

YUCCA8 and YUCCA9 overexpression reveals a link between auxin signaling and lignification through the induction of ethylene biosynthesis

Auxin is associated with the regulation of virtually every aspect of plant growth and development. Many previous genetic and biochemical studies revealed that, among the proposed routes for the production of auxin, the so-called indole-3-pyruvic acid (IPA) pathway is the main source for indole-3-acetic acid (IAA) in plants. The IPA pathway involves the action of 2 classes of enzymes, tryptophan-pyruvate aminotransferases (TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1(TAA1)/TRYPTOPHAN AMINOTRANSFERASE RELATED (TAR)) and flavin monooxygenases (YUCCA). Both enzyme classes appear to be encoded by small gene families in Arabidopsis consisting of 5 and 11 members, respectively. We recently showed that it is possible to induce transcript accumulation of 2 YUCCA genes, YUC8 and YUC9, by methyl jasmonate treatment. Both gene products were demonstrated to contribute to auxin biosynthesis in planta. (1) Here we report that the overexpression of YUC8 as well as YUC9 led to strong lignification of plant aerial tissues. Furthermore, new evidence indicates that this abnormally strong secondary growth is linked to increased levels of ethylene production.

Mapping and candidate genes associated with saccharification yield in sorghum

Sorghum (Sorghum bicolor (L.) Moench) is a high-yielding, stress tolerant energy crop for lignocellulosic-based biofuel production. Saccharification is a process by which hydrolytic enzymes break down lignocellulosic materials to fermentable sugars for biofuel production, and mapping and identifying genes underlying saccharification yield is an important first step to genetically improve the plant for higher biofuel productivity. In this study, we used the ICRISAT sorghum mini core germplasm collection and 14 739 single nucleotide polymorphism markers to map saccharification yield. Seven marker loci were associated with saccharification yield and five of these loci were syntenic with regions in the maize genome that contain quantitative trait loci underlying saccharification yield and cell wall component traits. Candidate genes from the seven loci were identified but must be validated, with the most promising candidates being β-tubulin, which determines the orientation of cellulose microfibrils in plant secondary cell walls, and NST1, a master transcription factor controlling secondary cell wall biosynthesis in fibers. Other candidate genes underlying the different saccharification loci included genes that play a role in vascular development and suberin deposition in plants. The identified loci and candidate genes provide information into the factors controlling saccharification yield and may facilitate increasing biofuel production in sorghum.

Contribution of proteomics in the identification of novel proteins associated with plant growth

The epidermis is not only the interphase between the plant and the environment but also a growth-limiting tissue. Understanding the initiation and regulation of its expansion growth is essential for addressing the need for more food and fuel. We used mass spectrometry to identify proteins from auxin (indole-3-acetic acid; IAA)-induced rapidly growing corn (Zea mays) coleoptiles to find possible candidates controlling this growth as well as the underlying cell wall and cuticle biosynthesis. Excised sections were incubated for 4 h in the absence or presence of IAA, protein extracted, and analyzed using LC-ESI-MS/MS. Of 86 proteins identified, 15 showed a predicted association with cell wall/cuticle biosynthesis or trafficking machinery; four identifications revealed novel proteins of unknown function. In parallel, real-time PCR indicated that the steady-state mRNA levels of genes with a known or predicted role in cell-wall biosynthesis increase upon treatment with auxin. Importantly, genes encoding two of the hypothetical proteins also show higher levels of mRNA; additionally, their gene expression is down-regulated as coleoptile growth ceases and up-regulated in expanding leaves. This suggests a major role of those novel proteins in the regulation of processes related to cell and organ expansion and thus plant growth.

A comparative genome analysis of PME and PMEI families reveals the evolution of pectin metabolism in plant cell walls

Pectins are fundamental polysaccharides in the plant primary cell wall. Pectins are synthesized and secreted to cell walls as highly methyl-esterified polymers and then demethyl-esterified by pectin methylesterases (PMEs), which are spatially regulated by pectin methylesterase inhibitors (PMEIs). Although PME and PMEI genes are pivotal in plant cell wall formation, few studies have focused on the evolutionary patterns of the PME and PMEI gene families. In this study, the gene origin, evolution, and expression diversity of these two families were systematically analyzed using 11 representative species, including algae, bryophytes, lycophytes and flowering land plants. The results show that 1) for the two subfamilies (PME and proPME) of PME, the origin of the PME subfamily is consistent with the appearance of pectins in early charophyte cell walls, 2) Whole genome duplication (WGD) and tandem duplication contribute to the expansion of proPME and PMEI families in land plants, 3) Evidence of selection pressure shows that the proPME and PMEI families have rapidly evolved, particularly the PMEI family in vascular plants, and 4) Comparative expression profile analysis of the two families indicates that the eudicot Arabidopsis and monocot rice have different expression patterns. In addition, the gene structure and sequence analyses show that the origin of the PMEI domain may be derived from the neofunctionalization of the pro domain after WGD. This study will advance the evolutionary understanding of the PME and PMEI families and plant cell wall development.

Organization of the ER-Golgi interface for membrane traffic control

Coat protein complex I (COPI) and COPII are required for bidirectional membrane trafficking between the endoplasmic reticulum (ER) and the Golgi. While these core coat machineries and other transport factors are highly conserved across species, high-resolution imaging studies indicate that the organization of the ER-Golgi interface is varied in eukaryotic cells. Regulation of COPII assembly, in some cases to manage distinct cellular cargo, is emerging as one important component in determining this structure. Comparison of the ER-Golgi interface across different systems, particularly mammalian and plant cells, reveals fundamental elements and distinct organization of this interface. A better understanding of how these interfaces are regulated to meet varying cellular secretory demands should provide key insights into the mechanisms that control efficient trafficking of proteins and lipids through the secretory pathway.

Developmental distribution of the plasma membrane-enriched proteome in the maize primary root growth zone

Within the growth zone of the maize primary root, there are well-defined patterns of spatial and temporal organization of cell division and elongation. However, the processes underlying this organization remain poorly understood. To gain additional insights into the differences amongst the defined regions, we performed a proteomic analysis focusing on fractions enriched for plasma membrane (PM) proteins. The PM is the interface between the plant cell and the apoplast and/or extracellular space. As such, it is a key structure involved in the exchange of nutrients and other molecules as well as in the integration of signals that regulate growth and development. Despite the important functions of PM-localized proteins in mediating these processes, a full understanding of dynamic changes in PM proteomes is often impeded by low relative concentrations relative to total proteins. Using a relatively simple strategy of treating microsomal fractions with Brij-58 detergent to enrich for PM proteins, we compared the developmental distribution of proteins within the root growth zone which revealed a number of previously known as well as novel proteins with interesting patterns of abundance. For instance, the quantitative proteomic analysis detected a gradient of PM aquaporin proteins similar to that previously reported using immunoblot analyses, confirming the veracity of this strategy. Cellulose synthases increased in abundance with increasing distance from the root apex, consistent with expected locations of cell wall deposition. The similar distribution pattern for Brittle-stalk-2-like protein implicates that this protein may also have cell wall related functions. These results show that the simplified PM enrichment method previously demonstrated in Arabidopsis can be successfully applied to completely unrelated plant tissues and provide insights into differences in the PM proteome throughout growth and development zones of the maize primary root.

Phyllotaxis

The precise arrangement of plant organs, also called phyllotaxis, has fascinated scientists from multiple disciplines. Whereas early work focused on morphological observations of phyllotaxis, recent findings have started to reveal the mechanisms behind this process, showing how molecular regulation and biochemical gradients interact with physical components to generate such precise patterns of growth. Here, I review new insights into the regulation of phyllotactic patterning and provide an overview of the various factors that can drive these robust growth patterns.

Endomembrane and Golgi traffic in plant cells

Leaf epidermal cells make ideal specimens for the investigation of the plant secretory pathway in that it is relatively easy to tag with fluorescent proteins and visualize in vivo the various organelles of the pathway. A number of techniques can be employed to identify and study proteins within the endomembrane organelles and to study their dynamics and interactions. Here, we discuss the most commonly used approaches to express proteins within arabidopsis and tobacco leaves, the use of mutant screens to identify trafficking proteins, and the use of two in vivo techniques, Fluorescence recovery after photobleaching and Förster resonance energy transfer, to study protein dynamics in plant cells.

Systems analysis of shoot apical meristem growth and development: integrating hormonal and mechanical signaling

The shoot apical meristem (SAM) is a small population of stem cells that continuously generates organs and tissues. This review covers our current understanding of organ initiation by the SAM in Arabidopsis thaliana. Meristem function and maintenance involves two major hormones, cytokinins and auxins. Cytokinins appear to play a major role in meristem maintenance and in controlling meristematic properties, such as cell proliferation. Self-organizing transport processes, which are still only partially understood, lead to the patterned accumulation of auxin at particular positions, where organs will grow out. A major downstream target of auxin-mediated growth regulation is the cell wall, which is a determinant for both growth rates and growth distribution, but feedbacks with metabolism and the synthetic capacity of the cytoplasm are crucial as well. Recent work has also pointed at a potential role of mechanical signals in growth coordination, but the precise mechanisms at work remain to be elucidated.

Radiometric and spectrophotometric in vitro assays of glycosyltransferases involved in plant cell wall carbohydrate biosynthesis

Most of the glycosyltransferases (GTs) that catalyze the formation of plant cell wall carbohydrates remain to be biochemically characterized. This can be achieved only if specific assays are available for these enzymes. Here we present a protocol for in vitro assays of processive and nonprocessive membrane-bound GTs. The assays are either based on the use of radioactive nucleotide sugars (NDP sugars; e.g., UDP-[U-(14)C]glucose) and the quantification of the radiolabeled monosaccharides incorporated into soluble or insoluble carbohydrates, or on the coupling of the GT reaction with that of pyruvate kinase (PK) and the oxidation of NADH by lactate dehydrogenase (LDH). The radiometric assays are more suitable for exploratory work on poorly characterized enzymes, whereas the spectrophotometric assays require the availability of highly enriched GTs. Both assays can be performed within 1 d, depending on the number of fractions to be assayed or reaction mixtures to be tested.

Xyloglucan xylosyltransferases XXT1, XXT2, and XXT5 and the glucan synthase CSLC4 form Golgi-localized multiprotein complexes

Xyloglucan is the major hemicellulosic polysaccharide in the primary cell walls of most vascular dicotyledonous plants and has important structural and physiological functions in plant growth and development. In Arabidopsis (Arabidopsis thaliana), the 1,4-β-glucan synthase, Cellulose Synthase-Like C4 (CSLC4), and three xylosyltransferases, XXT1, XXT2, and XXT5, act in the Golgi to form the xylosylated glucan backbone during xyloglucan biosynthesis. However, the functional organization of these enzymes in the Golgi membrane is currently unknown. In this study, we used bimolecular fluorescence complementation and in vitro pull-down assays to investigate the supramolecular organization of the CSLC4, XXT1, XXT2, and XXT5 proteins in Arabidopsis protoplasts. Quantification of bimolecular fluorescence complementation fluorescence by flow cytometry allowed us to perform competition assays that demonstrated the high probability of protein-protein complex formation in vivo and revealed differences in the abilities of these proteins to form multiprotein complexes. Results of in vitro pull-down assays using recombinant proteins confirmed that the physical interactions among XXTs occur through their catalytic domains. Additionally, coimmunoprecipitation of XXT2YFP and XXT5HA proteins from Arabidopsis protoplasts indicated that while the formation of the XXT2-XXT2 homocomplex involves disulfide bonds, the formation of the XXT2-XXT5 heterocomplex does not involve covalent interactions. The combined data allow us to propose that the proteins involved in xyloglucan biosynthesis function in a multiprotein complex composed of at least two homocomplexes, CSLC4-CSLC4 and XXT2-XXT2, and three heterocomplexes, XXT2-XXT5, XXT1-XXT2, and XXT5-CSLC4.

Cellulose degradation by Sulfolobus solfataricus requires a cell-anchored endo-β-1-4-glucanase

A sequence encoding a putative extracellular endoglucanase (sso1354) was identified in the complete genome sequence of Sulfolobus solfataricus. The encoded protein shares signature motifs with members of glycoside hydrolases family 12. After an unsuccessful first attempt at cloning the full-length coding sequences in Escherichia coli, an active but unstable recombinant enzyme lacking a 27-residue N-terminal sequence was generated. This 27-amino-acid sequence shows significant similarity with corresponding regions in the sugar binding proteins AraS, GlcS, and TreS of S. solfataricus that are responsible for anchoring them to the plasma membrane. A strategy based on an effective vector/host genetic system for Sulfolobus and on expression control by the promoter of the S. solfataricus gene which encodes the glucose binding protein allowed production of the enzyme in sufficient quantities for study. In fact, the enzyme expressed in S. solfataricus was stable and highly thermoresistant and showed optimal activity at low pH and high temperature. The protein was detected mainly in the plasma membrane fraction, confirming the structural similarity to the sugar binding proteins. The results of the protein expression in the two different hosts showed that the SSO1354 enzyme is endowed with an endo-β-1-4-glucanase activity and specifically hydrolyzes cellulose. Moreover, it also shows significant but distinguishable specificity toward several other sugar polymers, such as lichenan, xylan, debranched arabinan, pachyman, and curdlan.

Illuminating the wall: using click chemistry to image pectins in Arabidopsis cell walls

Plant cell walls are the most abundant biomaterials on Earth and serve a multitude of purposes in human society. These complex extracellular matrices are mainly composed of polysaccharides, including cellulose, hemicelluloses, and pectins, which cannot be cytologically examined using conventional techniques. Click chemistry, which exploits a bio-orthogonal cycloaddition reaction between alkynyl and azido groups, has proven to be useful for the metabolic incorporation and detection of modified sugars in polysaccharides in animals, fungi, and bacteria, but its use to interrogate the biosynthesis or dynamics of plant cell walls has not been previously reported. Recently, we found that an alkynylated analog of fucose can be metabolically incorporated into Arabidopsis thaliana cell walls and click labeled with fluorescent probes, facilitating imaging of cell wall carbohydrates. Despite the presence of fucose in several classes of wall polysaccharides, fucose-alkyne was primarily incorporated into rhamnogalacturonan-I, a type of pectin. Using timecourse and pulse-labeling experiments, we observed the dynamics of pectin delivery and reorganization in expanding cell walls. The use of click chemistry to investigate plant cell wall architecture should help bridge the gap between biochemical characterization of isolated cell wall components and an understanding of how those components interact in intact cell walls.

Immunological approaches to plant cell wall and biomass characterization: immunolocalization of glycan epitopes

Plant cell walls are dynamic structures that show changes in composition and configuration depending on the developmental stage, biotic, and abiotic factors. Therefore, it is necessary to have tools for visualizing the components of the cell wall in situ at any stage. Here, we describe how specific monoclonal antibodies can be used to examine the distribution of plant cell wall glycan epitopes at the whole plant, tissue, cell, and subcellular levels. Understanding the basic cell wall structure is essential for devising efficient strategies to convert cell walls to fermentable sugars for ethanol production.

Immunological approaches to plant cell wall and biomass characterization: Glycome Profiling

The native complexity of plant cell walls makes research on them challenging. Hence, it is advantageous to have a diversity of tools that can be used to analyze and characterize plant cell walls. In this chapter, we describe one of two immunological approaches that can be employed for screening of plant cell wall/biomass materials from diverse plants and tissues. This approach, Glycome Profiling, lends itself well to moderate to high-throughput screening of plant cell wall/biomass samples. Glycome Profiling is being further optimized to reduce the amount of sample required for the analysis, and to improve the sensitivity and throughput of the assay. We are optimistic that Glycome Profiling will prove to be a broadly applicable experimental approach that will find increasing application to a wide variety of studies on plant cell wall/biomass samples.

Post-synthetic modification of plant cell walls by expression of microbial hydrolases in the apoplast

The systematic creation of defined cell wall modifications in the model plant Arabidopsis thaliana by expression of microbial hydrolases with known specific activities is a promising approach to examine the impacts of cell wall composition and structure on both plant fitness and cell wall recalcitrance. Moreover, this approach allows the direct evaluation in living plants of hydrolase specificity, which can differ from in vitro specificity. To express genes encoding microbial hydrolases in A. thaliana, and target the hydrolases to the apoplast compartment, we constructed an expression cassette composed of the Cauliflower Mosaic Virus 35S RNA promoter, the A. thaliana β-expansin signal peptide, and the fluorescent marker protein YFP. Using this construct we successfully introduced into Colombia-0 plants three Aspergillus nidulans hydrolases, β-xylosidase/α-arabinosidase, feruloyl esterase, acetylxylan esterase, and a Xanthomonas oryzae putative a-L: -arabinofuranosidase. Fusion with YFP permitted quick and easy screening of transformants, detection of apoplastic localization, and protein size confirmation. Compared to wild-type Col-0, all transgenic lines showed a significant increase in the corresponding hydrolytic activity in the apoplast and changes in cell wall composition. Examination of hydrolytic activity in the transgenic plants also showed, for the first time, that the X. oryzae gene indeed encoded an enzyme with α-L: -arabinofuranosidase activity. None of the transgenic plants showed a visible phenotype; however, the induced compositional changes increased the degradability of biomass from plants expressing feruloyl esterase and β-xylosidase/α-arabinosidase. Our results demonstrate the viability of creating a set of transgenic A. thaliana plants with modified cell walls to use as a toolset for investigation of how cell wall composition contributes to recalcitrance and affects plant fitness.

QUASIMODO 3 (QUA3) is a putative homogalacturonan methyltransferase regulating cell wall biosynthesis in Arabidopsis suspension-cultured cells

Pectins are complex polysaccharides that are essential components of the plant cell wall. In this study, a novel putative Arabidopsis S-adenosyl-L-methionine (SAM)-dependent methyltransferase, termed QUASIMODO 3 (QUA3, At4g00740), has been characterized and it was demonstrated that it is a Golgi-localized, type II integral membrane protein that functions in methylesterification of the pectin homogalacturonan (HG). Although transgenic Arabidopsis seedlings with overexpression, or knock-down, of QUA3 do not show altered phenotypes or changes in pectin methylation, this enzyme is highly expressed and abundant in Arabidopsis suspension-cultured cells. In contrast, in cells subjected to QUA3 RNA interference (RNAi) knock-down there is less pectin methylation as well as altered composition and assembly of cell wall polysaccharides. Taken together, these observations point to a Golgi-localized QUA3 playing an essential role in controlling pectin methylation and cell wall biosynthesis in Arabidopsis suspension cell cultures.

Cell polarity: PIN it down!

How do plants create and maintain cell polarity? Recent studies reveal a plant-specific mechanism, which links the static cellulose-based extracellular matrix to the dynamic localization of PIN auxin carrier proteins.