Organization of cellulose synthase complexes involved in primary cell wall synthesis in Arabidopsis thaliana

The missing link: do cortical microtubules define plasma membrane nanodomains that modulate cellulose biosynthesis?

Cellulose production is a crucial aspect of plant growth and development. It is functionally linked to cortical microtubules, which self-organize into highly ordered arrays often situated in close proximity to plasma membrane-bound cellulose synthase complexes (CSCs). Although most models put forward to explain the microtubule-cellulose relationship have considered mechanisms by which cortical microtubule arrays influence the orientation of cellulose microfibrils, little attention has been paid to how microtubules affect the physicochemical properties of cellulose. A recent study using the model system Arabidopsis, however, indicates that microtubules can modulate the crystalline and amorphous content of cellulose microfibrils. Microtubules are required during rapid growth for reducing crystalline content, which is predicted to increase the degree to which cellulose is tethered by hemicellulosic polysaccharides. Such tethering is, in turn, critical for maintaining unidirectional cell expansion. In this article, we hypothesize that cortical microtubules influence the crystalline content of cellulose either by controlling plasma membrane fluidity or by modulating the deposition of noncellulosic wall components in the vicinity of the CSCs. We discuss the current limitations of imaging technology to address these hypotheses and identify the image acquisition and processing strategies that will integrate live imaging with super resolution three-dimensional information.

The FEI2-SOS5 pathway and CELLULOSE SYNTHASE 5 are required for cellulose biosynthesis in the Arabidopsis seed coat and affect pectin mucilage structure

A common adaptation in angiosperms is the deposition of hydrophilic mucilage into the apoplast of seed coat epidermal cells during the course of their differentiation. Upon imbibition, seed mucilage, composed mainly of carbohydrates (i.e. pectins, hemicelluloses and glycans) expands rapidly, encapsulating the seed and aiding in seed dispersal and germination. The FEI1/FEI2 receptor-like kinases and the SOS5 extracellular GPI-anchored protein were previously shown to act on a pathway regulating cellulose biosynthesis during Arabidopsis root elongation. In the highlighted study, we demonstrated that FEI2 and SOS5 regulate the production of the cellulosic rays deposited across the inner adherent-layer of seed mucilage. Mutations in either fei2 or sos5 disrupted the formation of rays, which was associated with an increase in the soluble, outer layer of pectin mucilage and accompanied by a reduction in the inner adherent-layer. Mutations in CELLULOSE SYNTHASE 5 also led to reduced rays and mal-partitioning of the pectic component of seed mucilage, further establishing a structural role for cellulose in seed mucilage. Here, we show that FEI2 expressed from a CaMV 35S promoter complemented both root and seed mucilage defects of the fei1 fei2 double mutant. In contrast, expression of FEI1 from a 35S promoter complemented the root, but not the seed phenotype of the fei1 fei2 double mutant, suggesting that unlike in the root, FEI2 plays a unique and non-redundant role in the regulation of cellulose synthesis in seed mucilage. Altogether, these data suggest a novel role for cellulose in anchoring the pectic component of seed mucilage to the seed surface and indicate that the FEI2 protein has a function distinct from that of FEI1, despite the high sequence similarity of these RLKs.

Chitinase-like1/pom-pom1 and its homolog CTL2 are glucan-interacting proteins important for cellulose biosynthesis in Arabidopsis

Plant cells are encased by a cellulose-containing wall that is essential for plant morphogenesis. Cellulose consists of β-1,4-linked glucan chains assembled into paracrystalline microfibrils that are synthesized by plasma membrane-located cellulose synthase (CESA) complexes. Associations with hemicelluloses are important for microfibril spacing and for maintaining cell wall tensile strength. Several components associated with cellulose synthesis have been identified; however, the biological functions for many of them remain elusive. We show that the chitinase-like (CTL) proteins, CTL1/POM1 and CTL2, are functionally equivalent, affect cellulose biosynthesis, and are likely to play a key role in establishing interactions between cellulose microfibrils and hemicelluloses. CTL1/POM1 coincided with CESAs in the endomembrane system and was secreted to the apoplast. The movement of CESAs was compromised in ctl1/pom1 mutant seedlings, and the cellulose content and xyloglucan structures were altered. X-ray analysis revealed reduced crystalline cellulose content in ctl1 ctl2 double mutants, suggesting that the CTLs cooperatively affect assembly of the glucan chains, which may affect interactions between hemicelluloses and cellulose. Consistent with this hypothesis, both CTLs bound glucan-based polymers in vitro. We propose that the apoplastic CTLs regulate cellulose assembly and interaction with hemicelluloses via binding to emerging cellulose microfibrils.

The translation elongation factor eEF-1Bβ1 is involved in cell wall biosynthesis and plant development in Arabidopsis thaliana

The eukaryotic translation elongation factor eEF-1Bβ1 (EF1Bβ) is a guanine nucleotide exchange factor that plays an important role in translation elongation. In this study, we show that the EF1Bβ protein is localized in the plasma membrane and cytoplasm, and that the transcripts should be expressed in most tissue types in seedlings. Sectioning of the inflorescence stem revealed that EF1Bβ predominantly localizes to the xylem vessels and in the interfascicular cambium. EF1Bβ gene silencing in efβ caused a dwarf phenotype with 38% and 20% reduction in total lignin and crystalline cellulose, respectively. This loss-of-function mutant also had a lower S/G lignin monomer ratio relative to wild type plants, but no changes were detected in a gain-of-function mutant transformed with the EF1Bβ gene. Histochemical analysis showed a reduced vascular apparatus, including smaller xylem vessels in the inflorescence stem of the loss-of-function mutant. Over-expression of EF1Bβ in an eli1 mutant background restored a WT phenotype and abolished ectopic lignin deposition as well as cell expansion defects in the mutant. Taken together, these data strongly suggest a role for EF1Bβ in plant development and cell wall formation in Arabidopsis.

Divergence and Redundancy in CSLD2 and CSLD3 Function During Arabidopsis Thaliana Root Hair and Female Gametophyte Development

The Arabidopsis cellulose synthase-like D (CSLD) 2 and 3 genes are known to function in root hair development. Here, we show that these genes also play a role in female gametophyte development because csld2 csld3 double mutants were observed to have low seed set that could be traced to defects in female transmission efficiency. Cell biological studies of csld2 csld3 ovules showed synergid cell degeneration during megagametogenesis and reduced pollen tube penetration during fertilization. Although CSLD2 and CSLD3 function redundantly in female gametophyte development, detailed analyses of root hair phenotypes of progeny from genetic crosses between csld2 and csld3, suggest that CSLD3 might play a more prominent role than CSLD2 in root hair development. Phylogenetic and gene duplication studies of CSLD2 and CSLD3 homologs in Arabidopsis lyrata, Populus, Medicago, maize, and Physcomitrella were further performed to investigate the course of evolution for these genes. Our analyses indicate that the ancestor of land plants possibly contained two copies of CSLD genes, one of which developed into the CSLD5 lineage in flowering plants, and the other formed the CSLD1/2/3/4 clade. In addition, CSLD2 and CSLD3 likely originated from a recent genome-wide duplication event explaining their redundancy. Moreover, sliding-window dN/dS analysis showed that most of the coding regions of CSLD2 and CSLD3 have been under strong purifying selection pressure. However, the region that encodes the N-terminus of CSLD3 has been under relatively relaxed selection pressure as indicated by its high dN/dS value, suggesting that CSLD3 might have gained additional functions through more frequent non-synonymous sequence changes at the N-terminus, which could partly explain the more prominent role of CSLD3 during root hair development compared to CSLD2.

Fluorescent tags to explore cell wall structure and dynamics

Plant cell walls are highly dynamic and heterogeneous structures, which vary between cell types, growth stages but also between microdomains within a single cell wall. In this review, we summarize the imaging techniques using fluorescent tags that are currently being used and which should in the coming years revolutionize our understanding of the dynamics of cell wall architecture and the cellular processes involved in the synthesis of cell wall components.

Co-expression of cell-wall related genes: new tools and insights

Global transcript analyses based on publicly available microarray dataset have revealed that genes with similar function tend to be transcriptionally coordinated. Indeed, many genes involved in the formation of cellulose, hemicelluloses, and lignin have been identified using co-expression approaches in Arabidopsis. To facilitate these transcript analyses, several web-based tools have been developed that allow researchers to investigate co-expression relationships of their gene(s) of interest. In addition, several tools now also provide the possibility of comparative transcriptional analyses across species, which potentially increases the predictive power. In this short review, we describe recent developments and updates of plant-related co-expression tools, and summarize studies that have successfully used expression profiling in cell wall research. Finally, we illustrate the value of comparative co-expression relationships across species using genes involved in lignin biosynthesis.

Golgi-mediated synthesis and secretion of matrix polysaccharides of the primary cell wall of higher plants

The Golgi apparatus of eukaryotic cells is known for its central role in the processing, sorting, and transport of proteins to intra- and extra-cellular compartments. In plants, it has the additional task of assembling and exporting the non-cellulosic polysaccharides of the cell wall matrix including pectin and hemicelluloses, which are important for plant development and protection. In this review, we focus on the biosynthesis of complex polysaccharides of the primary cell wall of eudicotyledonous plants. We present and discuss the compartmental organization of the Golgi stacks with regards to complex polysaccharide assembly and secretion using immuno-electron microscopy and specific antibodies recognizing various sugar epitopes. We also discuss the significance of the recently identified Golgi-localized glycosyltransferases responsible for the biosynthesis of xyloglucan (XyG) and pectin.

POM-POM2/cellulose synthase interacting1 is essential for the functional association of cellulose synthase and microtubules in Arabidopsis

In plants, regulation of cellulose synthesis is fundamental for morphogenesis and plant growth. Cellulose is synthesized at the plasma membrane, and the orientation of synthesis is guided by cortical microtubules; however, the guiding mechanism is currently unknown. We show that the conditional root elongation pom2 mutants are impaired in cell elongation, fertility, and microtubule-related functions. Map-based cloning of the POM-POM2 locus revealed that it is allelic to CELLULOSE SYNTHASE INTERACTING1 (CSI1). Fluorescently tagged POM2/CSI1s associated with both plasma membrane-located cellulose synthases (CESAs) and post-Golgi CESA-containing compartments. Interestingly, while CESA insertions coincided with cortical microtubules in the pom2/csi1 mutants, the microtubule-defined movement of the CESAs was significantly reduced in the mutant. We propose that POM2/CSI1 provides a scaffold between the CESAs and cortical microtubules that guide cellulose synthesis.

Cellulose synthase complexes: composition and regulation

Live cell imaging has greatly advanced our knowledge on the molecular mechanism by which cellulose is deposited. Both the actin and microtubule cytoskeleton are involved in assuring the proper distribution, organization, and dynamics of cellulose synthase complexes (CSCs). This review is an update on the most recent progress on the characterization of the composition, regulation, and trafficking of CSCs. With the newly identified cellulose synthase interactive protein 1 (CSI1) on hand, we begin to unveil the mystery of an intimate relationship between cellulose microfibrils and microtubules.

Small molecule probes for plant cell wall polysaccharide imaging

Plant cell walls are composed of interlinked polymer networks consisting of cellulose, hemicelluloses, pectins, proteins, and lignin. The ordered deposition of these components is a dynamic process that critically affects the development and differentiation of plant cells. However, our understanding of cell wall synthesis and remodeling, as well as the diverse cell wall architectures that result from these processes, has been limited by a lack of suitable chemical probes that are compatible with live-cell imaging. In this review, we summarize the currently available molecular toolbox of probes for cell wall polysaccharide imaging in plants, with particular emphasis on recent advances in small molecule-based fluorescent probes. We also discuss the potential for further development of small molecule probes for the analysis of cell wall architecture and dynamics.

Cellulose synthase interactive protein 1 (CSI1) links microtubules and cellulose synthase complexes

Cellulose synthase (CESA) complexes can be observed by live-cell imaging to move with trajectories that parallel the underlying cortical microtubules. Here we report that CESA interactive protein 1 (CSI1) is a microtubule-associated protein that bridges CESA complexes and cortical microtubules. Simultaneous in vivo imaging of CSI1, CESA complexes, and microtubules demonstrates that the association of CESA complexes and cortical microtubules is dependent on CSI1. CSI1 directly binds to microtubules as demonstrated by in vitro microtubule-binding assay.

Cellulose synthesis via the FEI2 RLK/SOS5 pathway and cellulose synthase 5 is required for the structure of seed coat mucilage in Arabidopsis

The seeds of Arabidopsis thaliana and many other plants are surrounded by a pectinaceous mucilage that aids in seed hydration and germination. Mucilage is synthesized during seed development within maternally derived seed coat mucilage secretory cells (MSCs), and is released to surround the seed upon imbibition. The FEI1/FEI2 receptor-like kinases and the SOS5 extracellular GPI-anchored protein were shown previously to act on a pathway that regulates the synthesis of cellulose in Arabidopsis roots. Here, we demonstrate that both FEI2 and SOS5 also play a role in the synthesis of seed mucilage. Disruption of FEI2 or SOS5 leads to a reduction in the rays of cellulose observed across the seed mucilage inner layer, which alters the structure of the mucilage in response to hydration. Mutations in CESA5, which disrupts an isoform of cellulose synthase involved in primary cell wall synthesis, result in a similar seed mucilage phenotype. The data indicate that CESA5-derived cellulose plays an important role in the synthesis and structure of seed coat mucilage and that the FEI2/SOS5 pathway plays a role in the regulation of cellulose synthesis in MSCs. Moreover, these results establish a novel structural role for cellulose in anchoring the pectic component of seed coat mucilage to the seed surface.

Identification of candidate genes in Arabidopsis and Populus cell wall biosynthesis using text-mining, co-expression network analysis and comparative genomics

Populus is an important bioenergy crop for bioethanol production. A greater understanding of cell wall biosynthesis processes is critical in reducing biomass recalcitrance, a major hindrance in efficient generation of biofuels from lignocellulosic biomass. Here, we report the identification of candidate cell wall biosynthesis genes through the development and application of a novel bioinformatics pipeline. As a first step, via text-mining of PubMed publications, we obtained 121 Arabidopsis genes that had the experimental evidence supporting their involvement in cell wall biosynthesis or remodeling. The 121 genes were then used as bait genes to query an Arabidopsis co-expression database, and additional genes were identified as neighbors of the bait genes in the network, increasing the number of genes to 548. The 548 Arabidopsis genes were then used to re-query the Arabidopsis co-expression database and re-construct a network that captured additional network neighbors, expanding to a total of 694 genes. The 694 Arabidopsis genes were computationally divided into 22 clusters. Queries of the Populus genome using the Arabidopsis genes revealed 817 Populus orthologs. Functional analysis of gene ontology and tissue-specific gene expression indicated that these Arabidopsis and Populus genes are high likelihood candidates for functional characterization in relation to cell wall biosynthesis.

Cellulose synthesis in two secondary cell wall processes in a single cell type

Plant cells have a rigid cell wall that constrains internal turgor pressure yet extends in a regulated and organized manner to allow the cell to acquire shape. The primary load-bearing macromolecule of a plant cell wall is cellulose, which forms crystalline microfibrils that are organized with respect to a cell's function and shape requirements. A primary cell wall is deposited during expansion whereas secondary cell wall is synthesized post expansion during differentiation. A complex form of asymmetrical cellular differentiation occurs in Arabidopsis seed coat epidermal cells, where we have recently shown that two secondary cell wall processes occur that utilize different cellulose synthase (CESA) proteins. One process is to produce pectinaceous mucilage that expands upon hydration and the other is a radial wall thickening that reinforced the epidermal cell structure. Our data illustrate polarized specialization of CESA5 in facilitating mucilage attachment to the parent seed and CESA2, CESA5 and CESA9 in radial cell wall thickening and formation of the columella. Herein, we present a model for the complexity of cellulose biosynthesis in this highly differentiated cell type with further evidence supporting each cellulosic secondary cell wall process.

Phytochrome regulation of cellulose synthesis in Arabidopsis

Plant development is highly plastic and dependent on light quantity and quality monitored by specific photoreceptors. Although we have a detailed knowledge of light signaling pathways, little is known about downstream targets involved in growth control. Cell size and shape are in part controlled by cellulose microfibrils extruded from large cellulose synthase complexes (CSCs) that migrate in the plasma membrane along cortical microtubules. Here we show a role for the red/far-red light photoreceptor PHYTOCHROME B (PHYB) in the regulation of cellulose synthesis in the growing Arabidopsis hypocotyl. In this organ, CSCs contains three distinct cellulose synthase (CESA) isoform classes: nonredundant CESA1 and CESA3 and a third class represented by partially redundant CESA2, CESA5, and CESA6. Interestingly, in the dark, depending on which CESA subunits occupy the third position, CSC velocity is more or less inhibited through an interaction with microtubules. Activation of PHYB overrules this inhibition. The analysis of cesa5 mutants shows a role for phosphorylation in the control of CSC velocity. These results, combined with the cesa5 mutant phenotype, suggest that cellulose synthesis is fine tuned through the regulated interaction of CSCs with microtubules and that PHYB signaling impinges on this process to maintain cell wall strength and growth in changing environments.

Subfunctionalization of cellulose synthases in seed coat epidermal cells mediates secondary radial wall synthesis and mucilage attachment

Arabidopsis (Arabidopsis thaliana) epidermal seed coat cells follow a complex developmental program where, following fertilization, cells of the ovule outer integument differentiate into a unique cell type. Two hallmarks of these cells are the production of a doughnut-shaped apoplastic pocket filled with pectinaceous mucilage and the columella, a thick secondary cell wall. Cellulose is thought to be a key component of both these secondary cell wall processes. Here, we investigated the role of cellulose synthase (CESA) subunits CESA2, CESA5, and CESA9 in the seed coat epidermis. We characterized the roles of these CESA proteins in the seed coat by analyzing cell wall composition and morphology in cesa mutant lines. Mutations in any one of these three genes resulted in lower cellulose content, a loss of cell shape uniformity, and reduced radial wall integrity. In addition, we found that attachment of the mucilage halo to the parent seed following extrusion is maintained by cellulose-based connections requiring CESA5. Hence, we show that cellulose fulfills an adhesion role between the extracellular mucilage matrix and the parent cell in seed coat epidermal cells. We propose that mucilage remains attached to the seed coat through interactions between components in the seed mucilage and cellulose. Our data suggest that CESA2 and CESA9 serve in radial wall reinforcement, as does CESA5, but CESA5 also functions in mucilage biosynthesis. These data suggest unique roles for different CESA subunits in one cell type and illustrate a complex role for cellulose biosynthesis in plant developmental biology.

Enhancing Arabidopsis leaf growth by engineering the BRASSINOSTEROID INSENSITIVE1 receptor kinase

The BRASSINOSTEROID INSENSITIVE1 (BRI1) receptor kinase has recently been shown to possess tyrosine kinase activity, and preventing autophosphorylation of the tyrosine-831 regulatory site by site-directed mutagenesis enhances shoot growth. In this study, we characterized the increased leaf growth of Arabidopsis (Arabidopsis thaliana) plants expressing BRI1(Y831F)-Flag compared with BRI1-Flag (both driven by the native promoter and expressed in the bri1-5 weak allele background) and provide insights into the possible mechanisms involved. On average, relative leaf growth rate was increased 16% in the Y831F plants (in the bri1-5 background), and the gain of function of the Y831F-directed mutant was dominant in the wild-type background. Leaves were larger as a result of increased cell numbers and had substantially increased vascularization. Transcriptome analysis indicated that genes associated with brassinolide biosynthesis, secondary cell wall biosynthesis and vascular development, and regulation of growth were altered in expression and may contribute to the observed changes in leaf architecture and whole plant growth. Analysis of gas exchange and chlorophyll fluorescence indicated that Y831F mutant plants had higher rates of photosynthesis, and metabolite analysis documented enhanced accumulation of starch, sucrose, and several amino acids, most prominently glycine and proline. These results demonstrate that mutation of BRI1 can enhance photosynthesis and leaf growth/vascularization and may suggest new approaches to increase whole plant carbon assimilation and growth.

Unraveling tobacco BY-2 protein complexes with BN PAGE/LC-MS/MS and clustering methods

To understand physiological processes, insight into protein complexes is very important. Through a combination of blue native gel electrophoresis and LC-MS/MS, we were able to isolate protein complexes and identify their potential subunits from Nicotiana tabacum cv. Bright Yellow-2. For this purpose, a bioanalytical approach was used that works without a priori knowledge of the interacting proteins. Different clustering methods (e.g., k-means and hierarchical clustering) and a biclustering approach were evaluated according to their ability to group proteins by their migration profile and to correlate the proteins to a specific complex. The biclustering approach was identified as a very powerful tool for the exploration of protein complexes of whole cell lysates since it allows for the promiscuous nature of proteins. Furthermore, it searches for associations between proteins that co-occur frequently throughout the BN gel, which increases the confidence of the putative associations between co-migrating proteins. The statistical significance and biological relevance of the profile clusters were verified using functional gene ontology annotation. The proof of concept for identifying protein complexes by our BN PAGE/LC-MS/MS approach is provided through the analysis of known protein complexes. Both well characterized long-lived protein complexes as well as potential temporary sequential multi-enzyme complexes were characterized.

CESA5 is required for the synthesis of cellulose with a role in structuring the adherent mucilage of Arabidopsis seeds

Imbibed Arabidopsis (Arabidopsis thaliana) seeds are encapsulated by mucilage that is formed of hydrated polysaccharides released from seed coat epidermal cells. The mucilage is structured with water-soluble and adherent layers, with cellulose present uniquely in an inner domain of the latter. Using a reverse-genetic approach to identify the cellulose synthases (CESAs) that produce mucilage cellulose, cesa5 mutants were shown to be required for the correct formation of these layers. Expression of CESA5 in the seed coat was specific to epidermal cells and coincided with the accumulation of mucilage polysaccharides in their apoplast. Analysis of sugar composition showed that although total sugar composition or amounts were unchanged, their partition between layers was different in the mutant, with redistribution from adherent to water-soluble mucilage. The macromolecular characteristics of the water-soluble mucilage were also modified. In accordance with a role for CESA5 in mucilage cellulose synthesis, crystalline cellulose contents were reduced in mutant seeds and birefringent microfibrils were absent from adherent mucilage. Although the mucilage-modified5 mutant showed similar defects to cesa5 in the distribution of sugar components between water-soluble and adherent mucilage, labeling of residual adherent mucilage indicated that cesa5 contained less cellulose and less pectin methyl esterification. Together, the results demonstrate that CESA5 plays a major and essential role in cellulose production in seed mucilage, which is critical for the establishment of mucilage structured in layers and domains.

Brassinosteroids can regulate cellulose biosynthesis by controlling the expression of CESA genes in Arabidopsis

The phytohormones, brassinosteroids (BRs), play important roles in regulating cell elongation and cell size, and BR-related mutants in Arabidopsis display significant dwarf phenotypes. Cellulose is a biopolymer which has a major contribution to cell wall formation during cell expansion and elongation. However, whether BRs regulate cellulose synthesis, and if so, what the underlying mechanism of cell elongation induced by BRs is, is unknown. The content of cellulose and the expression levels of the cellulose synthase genes (CESAs) was measured in BR-related mutants and their wild-type counterpart. The chromatin immunoprecipitation (CHIP) experiments and genetic analysis were used to demonstrate that BRs regulate CESA genes. It was found here that the BR-deficient or BR-perceptional mutants contain less cellulose than the wild type. The expression of CESA genes, especially those related to primary cell wall synthesis, was reduced in det2-1 and bri1-301, and was only inducible by BRs in the BR-deficient mutant det2-1. CHIP experiments show that the BR-activated transcription factor BES1 can associate with upstream elements of most CESA genes particularly those related with the primary cell wall. Furthermore, over-expression of the BR receptor BRI1 in CESA1, 3, and 6 mutants can only partially rescue the dwarf phenotypes. Our findings provide potential insights into the mechanism that BRs regulate cellulose synthesis to accomplish the cell elongation process in plant development.

The use of FTIR spectroscopy to monitor modifications in plant cell wall architecture caused by cellulose biosynthesis inhibitors

Fourier Transform InfraRed (FTIR) spectroscopy is a powerful and rapid technique for analysing cell wall components and putative cross-links, which is able to non-destructively recognize polymers and functional groups and provide abundant information about their in muro organization. FTIR spectroscopy has been reported to be a useful tool for monitoring cell wall changes occurring in muro as a result of various factors, such as growth and development processes, mutations or biotic and abiotic stresses. This mini-review examines the use of FTIR spectroscopy in conjunction with multivariate analyses to monitor cell wall changes related to (1) the exposure of diverse plant materials to cellulose biosynthesis inhibitors (CBIs), and (2) the habituation/dehabituation of plant cell cultures to this kind of herbicides. The spectra analyses show differences not only regarding the inhibitor, but also regarding how long cells have been growing in its presence.

A role for CSLD3 during cell-wall synthesis in apical plasma membranes of tip-growing root-hair cells

In plants, cell shape is defined by the cell wall, and changes in cell shape and size are dictated by modification of existing cell walls and deposition of newly synthesized cell-wall material. In root hairs, expansion occurs by a process called tip growth, which is shared by root hairs, pollen tubes and fungal hyphae. We show that cellulose-like polysaccharides are present in root-hair tips, and de novo synthesis of these polysaccharides is required for tip growth. We also find that eYFP-CSLD3 proteins, but not CESA cellulose synthases, localize to a polarized plasma-membrane domain in root hairs. Using biochemical methods and genetic complementation of a csld3 mutant with a chimaeric CSLD3 protein containing a CESA6 catalytic domain, we provide evidence that CSLD3 represents a distinct (1→4)-β-glucan synthase activity in apical plasma membranes during tip growth in root-hair cells.

The auxin signalling network translates dynamic input into robust patterning at the shoot apex

We provide a comprehensive expression map of the different genes (TIR1/AFBs, ARFs and Aux/IAAs) involved in the signalling pathway regulating gene transcription in response to auxin in the shoot apical meristem (SAM).

We demonstrate a relatively simple structure of this pathway using a high-throughput yeast two-hybrid approach to obtain the Aux/IAA-ARF full interactome.

The topology of the signalling network was used to construct a model for auxin signalling and to predict a role for the spatial regulation of auxin signalling in patterning of the SAM.

We used a new sensor to monitor the input in the auxin signalling pathway and to confirm the model prediction, thus demonstrating that auxin signalling is essential to create robust patterns at the SAM.

The plant hormone auxin is a key morphogenetic signal involved in the control of cell identity throughout development. A striking example of auxin action is at the shoot apical meristem (SAM), a population of stem cells generating the aerial parts of the plant. Organ positioning and patterning depends on local accumulations of auxin in the SAM, generated by polar transport of auxin (Vernoux et al, 2010). However, it is still unclear how auxin is distributed at cell resolution in tissues and how the hormone is sensed in space and time during development. A complex ensemble of 29 Aux/IAAs and 23 ARFs is central to the regulation of gene transcription in response to auxin (for review, see Leyser, 2006; Guilfoyle and Hagen, 2007; Chapman and Estelle, 2009). Protein–protein interactions govern the properties of this transduction pathway (Del Bianco and Kepinski, 2011). Limited interaction studies suggest that, in the absence of auxin, the Aux/IAA repressors form heterodimers with the ARF transcription factors, preventing them from regulating target genes. In the presence of auxin, the Aux/IAA proteins are targeted to the proteasome by an SCF E3 ubiquitin ligase complex (Chapman and Estelle, 2009; Leyser, 2006). In this process, auxin promotes the interaction between Aux/IAA proteins and the TIR1 F-box of the SCF complex (or its AFB homologues) that acts as an auxin co-receptor (Dharmasiri et al, 2005a, 2005b; Kepinski and Leyser, 2005; Tan et al, 2007). The auxin-induced degradation of Aux/IAAs would then release ARFs to regulate transcription of their target genes. This includes activation of most of the Aux/IAA genes themselves, thus establishing a negative feedback loop (Guilfoyle and Hagen, 2007). Although this general scenario provides a framework for understanding gene regulation by auxin, the underlying protein–protein network remains to be fully characterized.

In this paper, we combined experimental and theoretical analyses to understand how this pathway contributes to sensing auxin in space and time (Figure 1). We first analysed the expression patterns of the ARFs, Aux/IAAs and TIR1/AFBs genes in the SAM. Our results demonstrate a general tendency for most of the 25 ARFs and Aux/IAAs detected in the SAM: a differential expression with low levels at the centre of the meristem (where the stem cells are located) and high levels at the periphery of the meristem (where organ initiation takes place). We also observed a similar differential expression for TIR1/AFB co-receptors. To understand the functional significance of the distribution of ARFs and Aux/IAAs in the SAM, we next investigated the global structure of the Aux/IAA-ARF network using a high-throughput yeast two-hybrid approach and uncover a rather simple topology that relies on three basic generic features: (i) Aux/IAA proteins interact with themselves, (ii) Aux/IAA proteins interact with ARF activators and (iii) ARF repressors have no or very limited interactions with other proteins in the network.

The results of our interaction analysis suggest a model for the Aux/IAA-ARF signalling pathway in the SAM, where transcriptional activation by ARF activators would be negatively regulated by two independent systems, one involving the ARF repressors, the other the Aux/IAAs. The presence of auxin would remove the inhibitory action of Aux/IAAs, but leave the ARF repressors to compete with ARF activators for promoter-binding sites. To explore the regulatory properties of this signalling network, we developed a mathematical model to describe the transcriptional output as a function of the signalling input that is the combinatorial effect of auxin concentration and of its perception. We then used the model and a simplified view of the meristem (where the same population of Aux/IAAs and ARFs exhibit a low expression at the centre and a high expression in the peripheral zone) for investigating the role of auxin signalling in SAM function. We show that in the model, for a given ARF activator-to-repressor ratio, the gene induction capacity increases with the absolute levels of ARF proteins. We thus predict that the differential expression of the ARFs generates differences in auxin sensitivities between the centre (low sensitivity) and the periphery (high sensitivity), and that the expression of TIR1/AFB participates to this regulation (prediction 1). We also use the model to analyse the transcriptional response to rapidly changing auxin concentrations. By simulating situations equivalent either to the centre or the periphery of our simplified representation of the SAM, we predict that the signalling pathway buffers its response to the auxin input via the balance between ARF activators and repressors, in turn generated by their differential spatial distributions (prediction 2).

To test the predictions from the model experimentally, we needed to assess both the input (auxin level and/or perception) and the output (target gene induction) of the signalling cascade. For measuring the transcriptional output, the widely used DR5 reporter is perfectly adapted (Figure 5) (Ulmasov et al, 1997; Sabatini et al, 1999; Benkova et al, 2003; Heisler et al, 2005). For assaying pathway input, we designed DII-VENUS, a novel auxin signalling sensor that comprises a constitutively expressed fusion of the auxin-binding domain (termed domain II or DII) (Dreher et al, 2006; Tan et al, 2007) of an IAA to a fast-maturating variant of YFP, VENUS (Figure 5). The degradation patterns from DII-VENUS indicate a high auxin signalling input both in flower primordia and at the centre of the SAM. This is in contrast to the organ-specific expression pattern of DR5::VENUS (Figure 5). These results indicate that the signalling pathway limits gene activation in response to auxin at the meristem centre and confirm the differential sensitivity to auxin between the centre and the periphery (prediction 1). We further confirmed the buffering capacities of the signalling pathway (prediction 2) by carrying out live imaging experiments to monitor DII-VENUS and DR5::VENUS expression in real time (Figure 5). This analysis reveals the presence of important temporal variations of DII-VENUS fluorescence, while DR5::VENUS does not show such global variations. Our approach thus provides evidence that the Aux/IAA-ARF pathway has a key role in patterning in the SAM, alongside the auxin transport system. Our results illustrate how the tight spatio-temporal regulation of both the distribution of a morphogenetic signal and the activity of the downstream signalling pathway provides robustness to a dynamic developmental process.

A comprehensive expression and interaction map of auxin signalling factors in the Arabidopsis shoot apical meristem is constructed and used to derive a mathematical model of auxin signalling, from which key predictions are experimentally confirmed.

The plant hormone auxin is thought to provide positional information for patterning during development. It is still unclear, however, precisely how auxin is distributed across tissues and how the hormone is sensed in space and time. The control of gene expression in response to auxin involves a complex network of over 50 potentially interacting transcriptional activators and repressors, the auxin response factors (ARFs) and Aux/IAAs. Here, we perform a large-scale analysis of the Aux/IAA-ARF pathway in the shoot apex of Arabidopsis, where dynamic auxin-based patterning controls organogenesis. A comprehensive expression map and full interactome uncovered an unexpectedly simple distribution and structure of this pathway in the shoot apex. A mathematical model of the Aux/IAA-ARF network predicted a strong buffering capacity along with spatial differences in auxin sensitivity. We then tested and confirmed these predictions using a novel auxin signalling sensor that reports input into the signalling pathway, in conjunction with the published DR5 transcriptional output reporter. Our results provide evidence that the auxin signalling network is essential to create robust patterns at the shoot apex.

Differential regulation of cellulose orientation at the inner and outer face of epidermal cells in the Arabidopsis hypocotyl

It is generally believed that cell elongation is regulated by cortical microtubules, which guide the movement of cellulose synthase complexes as they secrete cellulose microfibrils into the periplasmic space. Transversely oriented microtubules are predicted to direct the deposition of a parallel array of microfibrils, thus generating a mechanically anisotropic cell wall that will favor elongation and prevent radial swelling. Thus far, support for this model has been most convincingly demonstrated in filamentous algae. We found that in etiolated Arabidopsis thaliana hypocotyls, microtubules and cellulose synthase trajectories are transversely oriented on the outer surface of the epidermis for only a short period during growth and that anisotropic growth continues after this transverse organization is lost. Our data support previous findings that the outer epidermal wall is polylamellate in structure, with little or no anisotropy. By contrast, we observed perfectly transverse microtubules and microfibrils at the inner face of the epidermis during all stages of cell expansion. Experimental perturbation of cortical microtubule organization preferentially at the inner face led to increased radial swelling. Our study highlights the previously underestimated complexity of cortical microtubule organization in the shoot epidermis and underscores a role for the inner tissues in the regulation of growth anisotropy.

Next generation functional proteomics in non-model plants: A survey on techniques and applications for the analysis of protein complexes and post-translational modifications

The congruent development of computational technology, bioinformatics and analytical instrumentation makes proteomics ready for the next leap. Present-day state of the art proteomics grew from a descriptive method towards a full stake holder in systems biology. High throughput and genome wide studies are now made at the functional level. These include quantitative aspects, functional aspects with respect to protein interactions as well as post translational modifications and advanced computational methods that aid in predicting protein function and mapping these functionalities across the species border. In this review an overview is given of the current status of these aspects in plant studies with special attention to non-genomic model plants.

Mechanisms for shaping, orienting, positioning and patterning plant secondary cell walls

Xylem vessels are cells that develop a specifically ornamented secondary cell wall to ensure their vascular function, conferring both structural strength and impermeability. Further plasticity is given to these vascular cells by a range of different patterns described by their secondary cell walls that-as for the growth of all plant organs-are developmentally regulated. Microtubules and their associated proteins, named MAPs, are essential to define the shape, the orientation, the position and the overall pattern of these secondary cell walls. Key actors in this process are the land-plant specific MAP70 proteins which not only allow the secondary cell wall to be positioned at the cell cortex but also determine the overall pattern described by xylem vessel secondary cell walls. 

Cortical microtubules optimize cell-wall crystallinity to drive unidirectional growth in Arabidopsis

The shape of plants depends on cellulose, a biopolymer that self-assembles into crystalline, inextensible microfibrils (CMFs) upon synthesis at the plasma membrane by multi-enzyme cellulose synthase complexes (CSCs). CSCs are displaced in directions predicted by underlying parallel arrays of cortical microtubules, but CMFs remain transverse in cells that have lost the ability to expand unidirectionally as a result of disrupted microtubules. These conflicting findings suggest that microtubules are important for some physico-chemical property of cellulose that maintains wall integrity. Using X-ray diffraction, we demonstrate that abundant microtubules enable a decrease in the degree of wall crystallinity during rapid growth at high temperatures. Reduced microtubule polymer mass in the mor1-1 mutant at high temperatures is associated with failure of crystallinity to decrease and a loss of unidirectional expansion. Promotion of microtubule bundling by over-expressing the RIC1 microtubule-associated protein reduced the degree of crystallinity. Using live-cell imaging, we detected an increase in the proportion of CSCs that track in microtubule-free domains in mor1-1, and an increase in the CSC velocity. These results suggest that microtubule domains affect glucan chain crystallization during unidirectional cell expansion. Microtubule disruption had no obvious effect on the orientation of CMFs in dark-grown hypocotyl cells. CMFs at the outer face of the hypocotyl epidermal cells had highly variable orientation, in contrast to the transverse CMFs on the radial and inner periclinal walls. This suggests that the outer epidermal mechanical properties are relatively isotropic, and that axial expansion is largely dependent on the inner tissue layers.

Cell wall integrity controls root elongation via a general 1-aminocyclopropane-1-carboxylic acid-dependent, ethylene-independent pathway

Cell expansion in plants requires cell wall biosynthesis and rearrangement. During periods of rapid elongation, such as during the growth of etiolated hypocotyls and primary root tips, cells respond dramatically to perturbation of either of these processes. There is growing evidence that this response is initiated by a cell wall integrity-sensing mechanism and dedicated signaling pathway rather than being an inevitable consequence of lost structural integrity. However, the existence of such a pathway in root tissue and its function in a broader developmental context have remained largely unknown. Here, we show that various types of cell wall stress rapidly reduce primary root elongation in Arabidopsis (Arabidopsis thaliana). This response depended on the biosynthesis of 1-aminocyclopropane-1-carboxylic acid (ACC). In agreement with the established ethylene signaling pathway in roots, auxin signaling and superoxide production are required downstream of ACC to reduce elongation. However, this cell wall stress response unexpectedly does not depend on the perception of ethylene. We show that the short-term effect of ACC on roots is partially independent of its conversion to ethylene or ethylene signaling and that this ACC-dependent pathway is also responsible for the rapid reduction of root elongation in response to pathogen-associated molecular patterns. This acute response to internal and external stress thus represents a novel, noncanonical signaling function of ACC.

Root hair-specific disruption of cellulose and xyloglucan in AtCSLD3 mutants, and factors affecting the post-rupture resumption of mutant root hair growth

The glycosyl transferase encoded by the cellulose synthase-like gene CSLD3/KJK/RHD7 (At3g03050) is required for cell wall integrity during root hair formation in Arabidopsis thaliana but it remains unclear whether it contributes to the synthesis of cellulose or hemicellulose. We identified two new alleles, root hair-defective (rhd) 7-1 and rhd7-4, which affect the C-terminal end of the encoded protein. Like root hairs in the previously characterized kjk-2 putative null mutant, rhd7-1 and rhd7-4 hairs rupture before tip growth but, depending on the growth medium and temperature, hairs are able to survive rupture and initiate tip growth, indicating that these alleles retain some function. At 21°C, the rhd7 tip-growing root hairs continued to rupture but at 5ºC, rupture was inhibited, resulting in long, wild type-like root hairs. At both temperatures, the expression of another root hair-specific CSLD gene, CSLD2, was increased in the rhd7-4 mutant but reduced in the kjk-2 mutant, suggesting that CSLD2 expression is CSLD3-dependent, and that CSLD2 could partially compensate for CSLD3 defects to prevent rupture at 5°C. Using a fluorescent brightener (FB 28) to detect cell wall (1 → 4)-β-glucans (primarily cellulose) and CCRC-M1 antibody to detect fucosylated xyloglucans revealed a patchy distribution of both in the mutant root hair cell walls. Cell wall thickness varied, and immunogold electron microscopy indicated that xyloglucan distribution was altered throughout the root hair cell walls. These cell wall defects indicate that CSLD3 is required for the normal organization of both cellulose and xyloglucan in root hair cell walls.

Arabidopsis genes IRREGULAR XYLEM (IRX15) and IRX15L encode DUF579-containing proteins that are essential for normal xylan deposition in the secondary cell wall

There are 10 genes in the Arabidopsis genome that contain a domain described in the Pfam database as domain of unknown function 579 (DUF579). Although DUF579 is widely distributed in eukaryotic species, there is no direct experimental evidence to assign a function to it. Five of the 10 Arabidopsis DUF579 family members are co-expressed with marker genes for secondary cell wall formation. Plants in which two closely related members of the DUF579 family have been disrupted by T-DNA insertions contain less xylose in the secondary cell wall as a result of decreased xylan content, and exhibit mildly distorted xylem vessels. Consequently we have named these genes IRREGULAR XYLEM 15 (IRX15) and IRX15L. These mutant plants exhibit many features of previously described xylan synthesis mutants, such as the replacement of glucuronic acid side chains with methylglucuronic acid side chains. By contrast, immunostaining of xylan and transmission electron microscopy (TEM) reveals that the walls of these irx15 irx15l double mutants are disorganized, compared with the wild type or other previously described xylan mutants, and exhibit dramatic increases in the quantity of sugar released in cell wall digestibility assays. Furthermore, localization studies using fluorescent fusion proteins label both the Golgi and also an unknown intracellular compartment. These data are consistent with irx15 and irx15l defining a new class of genes involved in xylan biosynthesis. How these genes function during xylan biosynthesis and deposition is discussed.

Characterization of the plasma membrane proteins and receptor-like kinases associated with secondary vascular differentiation in poplar

The constituents of plasma membrane proteins, particularly the integral membrane proteins, are closely associated with the differentiation of plant cells. Secondary vascular differentiation, which gives rise to the increase in plant stem diameter, is the key process by which the volume of the plant body grows. However, little is known about the plasma membrane proteins that specifically function in the vascular differentiation process. Proteomic analysis of the membrane proteins in poplar differentiating secondary vascular tissues led to the identification 226 integral proteins in differentiating xylem and phloem tissues. A majority of the integral proteins identified were receptors (55 proteins), transporters (34 proteins), cell wall formation related (27 proteins) or intracellular trafficking (17 proteins) proteins. Gene expression analysis in developing vascular cells further demonstrated that cambium differentiation involves the expression of a group of receptor kinases which mediate an array of signaling pathways during secondary vascular differentiation. This paper provides an outline of the protein composition of the plasma membrane in differentiating secondary vascular tissues and sheds light on the role of receptor kinases during secondary vascular development.

Cellulose synthases and synthesis in Arabidopsis

Plant cell walls are complex structures composed of high-molecular-weight polysaccharides, proteins, and lignins. Among the wall polysaccharides, cellulose, a hydrogen-bonded β-1,4-linked glucan microfibril, is the main load-bearing wall component and a key precursor for industrial applications. Cellulose is synthesized by large multi-meric cellulose synthase (CesA) complexes, tracking along cortical microtubules at the plasma membrane. The only known components of these complexes are the cellulose synthase proteins. Recent studies have identified tentative interaction partners for the CesAs and shown that the migratory patterns of the CesA complexes depend on phosphorylation status. These advances may become good platforms for expanding our knowledge about cellulose synthesis in the near future. In addition, our current understanding of cellulose chain polymerization in the context of the CesA complex is discussed.

A putative Arabidopsis thaliana glycosyltransferase, At4g01220, which is closely related to three plant cell wall-specific xylosyltransferases, is differentially expressed spatially and temporally

Plant cell wall polysaccharides are amongst the most complex, heterogeneous and abundant bio-molecules on earth. This makes the biosynthetic enzymes, namely the glycosyltransferases and polysaccharide synthases, important research targets in plant science and biotechnology. As an initial step to characterize At4g01220, a putative Arabidopsis thaliana encoding glycosyltransferases in CAZy GT-family-77 that is similar to three known xylosyltransferases involved in the biosynthesis of the pectic polysaccharide, rhamnogalacturonan II, we conducted an expression analysis. In transgenic Arabidopsis thaliana plants containing a fusion between the At4g01220 promoter and the gusA reporter gene we found the expression to be spatially and developmentally regulated. Analysis of Nicotiana benthamiana transfected with the At2g01220::YFP fusion protein revealed that the fusion protein resided in a Brefeldin A-sensitive compartment consistent with a sub-cellular location in the Golgi apparatus. In addition, in silico expression analysis from the Genevestigator database revealed that At4g01220 was up-regulated upon treatment with isoxaben, an inhibitor of cellulose synthesis, which, together with a co-expression analysis that identified a number of plant cell wall co-related biosynthetic genes, suggests involvement in cell wall biosynthesis with pectin being a prime candidate. The data presented provide insights into the expression, sub-cellular location and regulation of At4g01220 under various conditions and may help elucidate its specific function.

AUXIN RESPONSE FACTOR8 regulates Arabidopsis petal growth by interacting with the bHLH transcription factor BIGPETALp

Plant organ growth and final size are determined by coordinated cell proliferation and expansion. The BIGPETALp (BPEp) basic helix-loop-helix (bHLH) transcription factor was shown to limit Arabidopsis thaliana petal growth by influencing cell expansion. We demonstrate here that BPEp interacts with AUXIN RESPONSE FACTOR8 (ARF8) to affect petal growth. This interaction is mediated through the BPEp C-terminal domain (SD(BPEp)) and the C-terminal domain of ARF8. Site-directed mutagenesis identified an amino acid consensus motif in SD(BPEp) that is critical for mediating BPEp-ARF8 interaction. This motif shares sequence similarity with motif III of ARF and AUXIN/INDOLE-3-ACETIC ACID proteins. Petals of arf8 mutants are significantly larger than those of the wild type due to increased cell number and increased cell expansion. bpe arf8 double mutant analyses show that during early petal development stages, ARF8 and BPEp work synergistically to limit mitotic growth. During late stages, ARF8 and BPEp interact to limit cell expansion. The alterations in cell division and cell expansion observed in arf8 and/or bpe mutants are associated with a change in expression of early auxin-responsive genes. The data provide evidence of an interaction between an ARF and a bHLH transcription factor and of its biological significance in regulating petal growth, with local auxin levels likely influencing such a biological function.

Distribution of callose synthase, cellulose synthase, and sucrose synthase in tobacco pollen tube is controlled in dissimilar ways by actin filaments and microtubules

Callose and cellulose are fundamental components of the cell wall of pollen tubes and are probably synthesized by distinct enzymes, callose synthase and cellulose synthase, respectively. We examined the distribution of callose synthase and cellulose synthase in tobacco (Nicotiana tabacum) pollen tubes in relation to the dynamics of actin filaments, microtubules, and the endomembrane system using specific antibodies to highly conserved peptide sequences. The role of the cytoskeleton and membrane flow was investigated using specific inhibitors (latrunculin B, 2,3-butanedione monoxime, taxol, oryzalin, and brefeldin A). Both enzymes are associated with the plasma membrane, but cellulose synthase is present along the entire length of pollen tubes (with a higher concentration at the apex) while callose synthase is located in the apex and in distal regions. In longer pollen tubes, callose synthase accumulates consistently around callose plugs, indicating its involvement in plug synthesis. Actin filaments and endomembrane dynamics are critical for the distribution of callose synthase and cellulose synthase, showing that enzymes are transported through Golgi bodies and/or vesicles moving along actin filaments. Conversely, microtubules appear to be critical in the positioning of callose synthase in distal regions and around callose plugs. In contrast, cellulose synthases are only partially coaligned with cortical microtubules and unrelated to callose plugs. Callose synthase also comigrates with tubulin by Blue Native-polyacrylamide gel electrophoresis. Membrane sucrose synthase, which expectedly provides UDP-glucose to callose synthase and cellulose synthase, binds to actin filaments depending on sucrose concentration; its distribution is dependent on the actin cytoskeleton and the endomembrane system but not on microtubules.

Microtubules and CESA tracks at the inner epidermal wall align independently of those on the outer wall of light-grown Arabidopsis hypocotyls

Microtubules are classically described as being transverse, which is perpendicular to the direction of cell elongation. However, fixation studies have indicated that microtubules can be variably aligned across the epidermis of elongating shoots. In addition, microtubules are reported to have different orientations on inner and outer epidermal surfaces, undermining the idea of hoop-reinforcement. Here, long-term movies of Arabidopsis seedlings expressing GFP-TUA6 allowed microtubule alignment to be directly correlated with the rate of elongation within individual growing cells. We also investigated whether microtubule alignment at the inner or the outer epidermal wall better reflected the growth rate. Movies confirmed that transverse microtubules form on the inner wall throughout elongation, but orientation of microtubules is variable at the outer wall, where they tend to become transverse only during episodes of accelerated growth. Because this appears to contradict the concept that circumferential arrays of transverse microtubules or microfibrils are essential for cell elongation, we checked the organisation of cellulose synthase tracks using GFP-CESA3 and found a similar mismatch between trajectories on inner and outer epidermal surfaces. We conclude that microtubule alignment on the inner wall appears to be a more stable predictor of growth anisotropy, whereas outer-wall alignment is more sensitive to the elongation rate.

OsBC1L4 encodes a COBRA-like protein that affects cellulose synthesis in rice

Plant morphogenesis is highly dependent on the regulation of cell division and expansion. The organization of the cellulose microfibrils in the cell wall is a key determinant of cell expansion. Previously, a dwarf mutant with fewer tillers, Osbc1l4 (Oryza sativa brittle culm 1 like 4), was identified by screening a rice T-DNA insertion mutant library. It is reported here that OsBC1L4 encodes a COBRA-like protein that exhibits typical structural features of a glycosylphosphatidylinositol-anchor protein. The T-DNA insertion in OsBC1L4 results in abnormal cell expansion. A decrease in cellulose content but the increase in pectin and starch contents was identified in Osbc1l4 mutants by measuring the content of wall components. OsBC1L4 was expressed in all tissues/organs examined, with a low level in leaves. OsBC1L4 protein is mainly located in the cell wall and plasma membrane. Correlation analysis indicated that the expression of OsBC1L4 was highly correlated to that of several primary wall-forming cellulose synthase genes (CESAs). Moreover, the expression level of several cellulose-related genes is increased in Osbc1l4 mutants, which suggests that a feedback mechanism may exist during cellulose synthesis.

Male gametophyte defective 4 encodes a rhamnogalacturonan II xylosyltransferase and is important for growth of pollen tubes and roots in Arabidopsis

In flowering plants, the growth of pollen tubes is essential for the delivery of sperm to the egg cells. Although many factors (including cell-wall properties) are involved in this process, little is known about the underlying molecular mechanisms that regulate the growth of pollen tubes. We report here the characterization of an Arabidopsis mutant male gametophyte defective 4 (mgp4) that is severely defective in pollen tube growth. The mgp4 mutation also impairs root growth of pollen-rescued mgp4 mutant plants generated by expressing MGP4 cDNA under the control of a pollen grain/tube-specific promoter. The MGP4 gene encodes a putative xylosyltransferase and is expressed in many organs/tissues, including pollen tubes and roots. MGP4 protein expressed in Pichia pastoris exhibited xylosyltransferase activity and transferred d-xylose onto l-fucose. The pectic polysaccharide rhamnogalacturonan II (RG-II), isolated from 7-day-old pollen-rescued mutant seedlings, exhibited a 30% reduction in 2-O-methyl d-xylose residues. Furthermore, an exogenous supply of boric acid enhanced RG-II dimer formation and partially restored the root growth of the pollen-rescued mutant seedlings. Taken together, these results suggest that MGP4 plays important roles in pollen tube and root growth by acting as a xylosyltransferase involved in the biosynthesis of pectic RG-II.

Genetic engineering of energy crops: a strategy for biofuel production in China

Biomass utilization is increasingly considered as a practical way for sustainable energy supply and long-term environment care around the world. In concerns with food security in China, starch or sugar-based bioethanol and edible-oil-derived biodiesel are harshly restricted for large scale production. However, conversion of lignocellulosic residues from food crops is a potential alternative. Because of its recalcitrance, current biomass process is unacceptably expensive, but genetic breeding of energy crops is a promising solution. To meet the need, energy crops are defined with a high yield for both food and biofuel purposes. In this review, main grasses (rice, wheat, maize, sorghum and miscanthus) are evaluated for high biomass production, the principles are discussed on modification of plant cell walls that lead to efficient biomass degradation and conversion, and the related biotechnologies are proposed in terms of energy crop selection.

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

The development of cellulose as an organizing structure in the plant cell wall was a key event in both the initial colonization and the subsequent domination of the terrestrial ecosystem by vascular plants. A wealth of experimental data has demonstrated the complicated genetic interactions required to form the large synthetic complex that synthesizes cellulose. However, these results are lacking an extensive analysis of the evolution, specialization, and regulation of the proteins that compose this complex. Here we perform an in-depth analysis of the sequences in the cellulose synthase (CesA) family. We investigate the phylogeny of the CesA family, with emphasis on evolutionary specialization. We define specialized clades and identify the class-specific regions within the CesA sequence that may explain this specialization. We investigate changes in regulation of CesAs by looking at the conservation of proposed phosphorylation sites. We investigate the conservation of sites where mutations have been documented that impair CesA function, and compare these sites to those observed in the closest cellulose synthase-like (Csl) families to better understand what regions may separate the CesAs from other Csls. Finally we identify two positions with strong conservation of the aromatic trait, but lacking conservation of amino acid identity, which may represent residues important for positioning the sugar substrate for catalysis. These analyses provide useful tools for understanding characterized mutations and post-translational modifications, and for informing further experiments to probe CesA assembly, regulation, and function through site-directed mutagenesis or domain swapping experiments.

Large-scale co-expression approach to dissect secondary cell wall formation across plant species

Plant cell walls are complex composites largely consisting of carbohydrate-based polymers, and are generally divided into primary and secondary walls based on content and characteristics. Cellulose microfibrils constitute a major component of both primary and secondary cell walls and are synthesized at the plasma membrane by cellulose synthase (CESA) complexes. Several studies in Arabidopsis have demonstrated the power of co-expression analyses to identify new genes associated with secondary wall cellulose biosynthesis. However, across-species comparative co-expression analyses remain largely unexplored. Here, we compared co-expressed gene vicinity networks of primary and secondary wall CESAsin Arabidopsis, barley, rice, poplar, soybean, Medicago, and wheat, and identified gene families that are consistently co-regulated with cellulose biosynthesis. In addition to the expected polysaccharide acting enzymes, we also found many gene families associated with cytoskeleton, signaling, transcriptional regulation, oxidation, and protein degradation. Based on these analyses, we selected and biochemically analyzed T-DNA insertion lines corresponding to approximately twenty genes from gene families that re-occur in the co-expressed gene vicinity networks of secondary wall CESAs across the seven species. We developed a statistical pipeline using principal component analysis and optimal clustering based on silhouette width to analyze sugar profiles. One of the mutants, corresponding to a pinoresinol reductase gene, displayed disturbed xylem morphology and held lower levels of lignin molecules. We propose that this type of large-scale co-expression approach, coupled with statistical analysis of the cell wall contents, will be useful to facilitate rapid knowledge transfer across plant species.

Functional characterization of the HD-ZIP IV transcription factor OCL1 from maize

OCL1 (OUTER CELL LAYER1) encodes a maize HD-ZIP class IV transcription factor (TF) characterized by the presence of a homeo DNA-binding domain (HD), a dimerization leucine zipper domain (ZIP), and a steroidogenic acute regulatory protein (StAR)-related lipid transfer domain (START) involved in lipid transport in animals but the function of which is still unknown in plants. By combining yeast and plant trans-activation assays, the transcriptional activation domain of OCL1 was localized to 85 amino acids in the N-terminal part of the START domain. Full-length OCL1 devoid of this activation domain is unable to trans-activate a reporter gene under the control of a minimal promoter fused to six repeats of the L1 box, a cis-element present in target genes of HD-ZIP IV TFs in Arabidopsis. In addition, ectopic expression of OCL1 leads to pleiotropic phenotypic aberrations in transgenic maize plants, the most conspicuous one being a strong delay in flowering time which is correlated with the misexpression of molecular markers for floral transition such as ZMM4 (Zea Mays MADS-box4) or DLF1 (DELAYED FLOWERING1). As suggested by the interaction in planta between OCL1 and SWI3C1, a bona fide subunit of the SWI/SNF complex, OCL1 may modulate transcriptional activity of its target genes by interaction with a chromatin remodelling complex.

Conjugated polymer nanoparticles for effective siRNA delivery to tobacco BY-2 protoplasts

BACKGROUND

Post transcriptional gene silencing (PTGS) is a mechanism harnessed by plant biologists to knock down gene expression. siRNAs contribute to PTGS that are synthesized from mRNAs or viral RNAs and function to guide cellular endoribonucleases to target mRNAs for degradation. Plant biologists have employed electroporation to deliver artificial siRNAs to plant protoplasts to study gene expression mechanisms at the single cell level. One drawback of electroporation is the extensive loss of viable protoplasts that occurs as a result of the transfection technology.

RESULTS

We employed fluorescent conjugated polymer nanoparticles (CPNs) to deliver siRNAs and knockdown a target gene in plant protoplasts. CPNs are non toxic to protoplasts, having little impact on viability over a 72 h period. Microscopy and flow cytometry reveal that CPNs can penetrate protoplasts within 2 h of delivery. Cellular uptake of CPNs/siRNA complexes were easily monitored using epifluorescence microscopy. We also demonstrate that CPNs can deliver siRNAs targeting specific genes in the cellulose biosynthesis pathway (NtCesA-1a and NtCesA-1b).

CONCLUSIONS

While prior work showed that NtCesA-1 is a factor involved in cell wall synthesis in whole plants, we demonstrate that the same gene plays an essential role in cell wall regeneration in isolated protoplasts. Cell wall biosynthesis is central to cell elongation, plant growth and development. The experiments presented here shows that NtCesA is also a factor in cell viability. We show that CPNs are valuable vehicles for delivering siRNAs to plant protoplasts to study vital cellular pathways at the single cell level.

Expression profiling and integrative analysis of the CESA/CSL superfamily in rice

BACKGROUND

The cellulose synthase and cellulose synthase-like gene superfamily (CESA/CSL) is proposed to encode enzymes for cellulose and non-cellulosic matrix polysaccharide synthesis in plants. Although the rice (Oryza sativa L.) genome has been sequenced for a few years, the global expression profiling patterns and functions of the OsCESA/CSL superfamily remain largely unknown.

RESULTS

A total of 45 identified members of OsCESA/CSL were classified into two clusters based on phylogeny and motif constitution. Duplication events contributed largely to the expansion of this superfamily, with Cluster I and II mainly attributed to tandem and segmental duplication, respectively. With microarray data of 33 tissue samples covering the entire life cycle of rice, fairly high OsCESA gene expression and rather variable OsCSL expression were observed. While some members from each CSL family (A1, C9, D2, E1, F6 and H1) were expressed in all tissues examined, many of OsCSL genes were expressed in specific tissues (stamen and radicles). The expression pattern of OsCESA/CSL and OsBC1L which extensively co-expressed with OsCESA/CSL can be divided into three major groups with ten subgroups, each showing a distinct co-expression in tissues representing typically distinct cell wall constitutions. In particular, OsCESA1, -3 & -8 and OsCESA4, -7 & -9 were strongly co-expressed in tissues typical of primary and secondary cell walls, suggesting that they form as a cellulose synthase complex; these results are similar to the findings in Arabidopsis. OsCESA5/OsCESA6 is likely partially redundant with OsCESA3 for OsCESA complex organization in the specific tissues (plumule and radicle). Moreover, the phylogenetic comparison in rice, Arabidopsis and other species can provide clues for the prediction of orthologous gene expression patterns.

CONCLUSIONS

The study characterized the CESA/CSL of rice using an integrated approach comprised of phylogeny, transcriptional profiling and co-expression analyses. These investigations revealed very useful clues on the major roles of CESA/CSL, their potentially functional complement and their associations for appropriate cell wall synthesis in higher plants.

Analyses of sorting nexins reveal distinct retromer-subcomplex functions in development and protein sorting in Arabidopsis thaliana

Sorting nexins (SNXs) are conserved eukaryotic proteins that associate with three types of vacuolar protein sorting (VPS) proteins to form the retromer complex. How SNXs act in this complex and whether they might work independently of the retromer remains elusive. Here, we show by genetic and cell imaging approaches that the Arabidopsis thaliana SNX1 protein recruits SNX2 at the endosomal membrane, a process required for SNX1-SNX2 dimer activity. We report that, in contrast with the mammalian retromer, SNXs are dispensable for membrane binding and function of the retromer complex. We also show that VPS retromer components can work with or independently of SNXs in the trafficking of seed storage proteins, which reveals distinct functions for subcomplexes of the plant retromer. Finally, we provide compelling evidence that the combined loss of function of SNXs and VPS29 leads to embryo or seedling lethality, underlining the essential role of these proteins in development.

Cellulose synthase interacting protein: a new factor in cellulose synthesis

Cellulose is the most abundant biopolymer on earth. The great abundance of cellulose places it at the forefront as a primary source of biomass for renewable biofuels. However, the knowledge of how plant cells make cellulose remains very rudimentary. Cellulose microfibrils are synthesized at the plasma membrane by hexameric protein complexes, also known as cellulose synthase complexes. The only known components of cellulose synthase complexes are cellulose synthase (CESA) proteins until the recent identification of a novel component. CSI1, which encodes CESA interacting protein 1 (CSI1) in Arabidopsis. CSI1, as the first non-CESA proteins associated with cellulose synthase complexes, opens up many opportunities.