The ins and outs of plant cell walls

Vaccine candidate designed against carcinoembryonic antigen-related cell adhesion molecules using immunoinformatics tools

Carcinoembryonic antigen-related cell adhesion (CEACAM) molecules belong to a family of membrane glycoproteins that mediate intercellular interactions influencing cellular growth, immune cell activation, apoptosis, and tumor suppression. Several family members (CEACAM1, CEACAM5, and CEACAM6) are highly expressed in cancers, and they share a conserved N-terminal domain that serves as an attractive target for cancer immunotherapy. A multi-epitope vaccine candidate against this conserved domain has been developed using immunoinformatics tools. Specifically, several epitopes predicted to interact with MHC class I and II molecules were linked together with appropriate linkers. The tertiary structure of the vaccine is generated by homology and ab initio modeling. Molecular docking of epitopes to MHC structures have revealed that the lowest energy conformations are the epitopes bound to the antigen-binding groove of the MHC molecules. Subsequent molecular dynamics simulation has confirmed the stability of the binding conformations in solution. The predicted vaccine has relatively high antigenicity and low allergenicity, suggesting that it is an ideal candidate for further refinement and development.Communicated by Ramaswamy H. Sarma.

Regulatory roles of phosphoinositides in membrane trafficking and their potential impact on cell-wall synthesis and re-modelling

BACKGROUND

Plant cell walls are complex matrices of carbohydrates and proteins that control cell morphology and provide protection and rigidity for the plant body. The construction and maintenance of this intricate system involves the delivery and recycling of its components through a precise balance of endomembrane trafficking, which is controlled by a plethora of cell signalling factors. Phosphoinositides (PIs) are one class of signalling molecules with diverse roles in vesicle trafficking and cytoskeleton structure across different kingdoms. Therefore, PIs may also play an important role in the assembly of plant cell walls.

SCOPE

The eukaryotic PI pathway is an intricate network of different lipids, which appear to be divided in different pools that can partake in vesicle trafficking or signalling. Most of our current understanding of how PIs function in cell metabolism comes from yeast and mammalian systems; however, in recent years significant progress has been made towards a better understanding of the plant PI system. This review examines the current state of knowledge of how PIs regulate vesicle trafficking and their potential influence on plant cell-wall architecture. It considers first how PIs are formed in plants and then examines their role in the control of vesicle trafficking. Interactions between PIs and the actin cytoskeleton and small GTPases are also discussed. Future challenges for research are suggested.

Rice brittleness mutants: a way to open the 'black box' of monocot cell wall biosynthesis

Rice is a model organism for studying the mechanism of cell wall biosynthesis and remolding in Gramineae. Mechanical strength is an important agronomy trait of rice (Oryza sativa L.) plants that affects crop lodging and grain yield. As a prominent physical property of cell walls, mechanical strength reflects upon the structure of different wall polymers and how they interact. Studies on the mechanisms that regulate the mechanical strength therefore consequently results in uncovering the genes functioning in cell wall biosynthesis and remodeling. Our group focuses on the study of isolation of brittle culm (bc) mutants and characterization of their corresponding genes. To date, several bc mutants have been reported. The identified genes have covered several pathways of cell wall biosynthesis, revealing many secrets of monocot cell wall biosynthesis. Here, we review the progress achieved in this research field and also highlight the perspectives in expectancy. All of those lend new insights into mechanisms of cell wall formation and are helpful for harnessing the waste rice straws for biofuel production.

Protein trafficking to the cell wall occurs through mechanisms distinguishable from default sorting in tobacco

The secretory pathway in plants involves sustained traffic to the cell wall, as matrix components, polysaccharides and proteins reach the cell wall through the endomembrane system. We studied the secretion pattern of cell-wall proteins in tobacco protoplasts and leaf epidermal cells using fluorescent forms of a pectin methylesterase inhibitor protein (PMEI1) and a polygalacturonase inhibitor protein (PGIP2). The two most representative protein fusions, secGFP-PMEI1 and PGIP2-GFP, reached the cell wall by passing through ER and Golgi stacks but using distinct mechanisms. secGFP-PMEI1 was linked to a glycosylphosphatidylinositol (GPI) anchor and stably accumulated in the cell wall, regulating the activity of the endogenous pectin methylesterases (PMEs) that are constitutively present in this compartment. A mannosamine-induced non-GPI-anchored form of PMEI1 as well as a form (PMEI1-GFP) that was unable to bind membranes failed to reach the cell wall, and accumulated in the Golgi stacks. In contrast, PGIP2-GFP moved as a soluble cargo protein along the secretory pathway, but was not stably retained in the cell wall, due to internalization to an endosomal compartment and eventually the vacuole. Stable localization of PGIP2 in the wall was observed only in the presence of a specific fungal endopolygalacturonase ligand in the cell wall. Both secGFP-PMEI1 and PGIP2-GFP sorting were distinguishable from that of a secreted GFP, suggesting that rigorous and more complex controls than the simple mechanism of bulk flow are the basis of cell-wall growth and differentiation.

Membrane trafficking mediated by OsDRP2B is specific for cellulose biosynthesis

Increasing evidence has revealed that membrane trafficking is highly associated with cell wall metabolism. Factors involved in vesicle delivery, e.g. cytoskeleton and motor proteins, have showed regulatory effects on cell wall structure and components. However, little is known about the involvement of other trafficking components in distribution of cell wall-related compartments. Dynamins are important proteins functioning in membrane tubulation and vesiculation. Recently, we have reported characterization of the rice dynamin-related protein 2B (OsDRP2B). Mutation in OsDRP2B causes a significant reduction in cellulose content. Its association with the trans-Golgi network (TGN) and clathrin-coated vesicles and the reduced CESA4 abundance at the bc3 plasma membrane suggest that BC3/OsDRP2B is involved in the transport of essential elements for cellulose synthesis. Here, we provide additional evidence for BC3 subcellular localization via observing OsDRP2B-GFP in living root hairs of transgenic plants. Uronic acid and fractional composition analyses further confirm that the amount of arabinoxylan and other noncellulosic polysaccharides is increased in bc3. However, three putative xylan synthesis genes are down-regulated in mutant plant revealed by real-time PCR analysis. These results imply that compartments delivered by OsDRP2B are specifically responsible for cellulose biosynthesis.

The rice dynamin-related protein DRP2B mediates membrane trafficking, and thereby plays a critical role in secondary cell wall cellulose biosynthesis

Membrane trafficking between the plasma membrane (PM) and intracellular compartments is an important process that regulates the deposition and metabolism of cell wall polysaccharides. Dynamin-related proteins (DRPs), which function in membrane tubulation and vesiculation are closely associated with cell wall biogenesis. However, the molecular mechanisms by which DRPs participate in cell wall formation are poorly understood. Here, we report the functional characterization of Brittle Culm3 (BC3), a gene encoding OsDRP2B. Consistent with the expression of BC3 in mechanical tissues, the bc3 mutation reduces mechanical strength, which results from decreased cellulose content and altered secondary wall structure. OsDRP2B, one of three members of the DRP2 subfamily in rice (Oryza sativa L.), was identified as an authentic membrane-associated dynamin via in vitro biochemical analyses. Subcellular localization of fluorescence-tagged OsDRP2B and several compartment markers in protoplast cells showed that this protein not only lies at the PM and the clathrin-mediated vesicles, but also is targeted to the trans-Golgi network (TGN). An FM4-64 uptake assay in transgenic plants that express green fluorescent protein-tagged OsDRP2B verified its involvement in an endocytic pathway. BC3 mutation and overexpression altered the abundance of cellulose synthase catalytic subunit 4 (OsCESA4) in the PM and in the endomembrane systems. All of these findings lead us to conclude that OsDRP2B participates in the endocytic pathway, probably as well as in post-Golgi membrane trafficking. Mutation of OsDRP2B disturbs the membrane trafficking that is essential for normal cellulose biosynthesis of the secondary cell wall, thereby leading to inferior mechanical properties in rice plants.

H-gene-mediated resistance to Hessian fly exhibits features of penetration resistance to fungi

Features shared by host-specific phytophagous insects and biotrophic plant pathogens include gene-for-gene interactions and the ability to induce susceptibility in plants. The Hessian fly shows both. To protect against Hessian fly, grasses have H genes. Avirulent larvae die on H-gene-containing resistant plants but the cause of death is not known. Imaging techniques were used to examine epidermal cells at larval attack sites, comparing four resistant wheat genotypes (H6, H9, H13, and H26) to a susceptible genotype. Present in both resistant and susceptible plants attacked by larvae were small holes in the tangential cell wall, with the size of the holes (0.1 microm in diameter) matching that of the larval mandible. Absent from attacked resistant plants were signs of induced susceptibility, including nutritive tissue and ruptured cell walls. Present in attacked resistant plants were signs of induced resistance, including cell death and fortification of the cell wall. Both presumably limit larval access to food, because the larva feeds on the leaf surface by sucking up liquids released from ruptured cells. Resistance was associated with several subcellular responses, including elaboration of the endoplasmic reticulum-Golgi complex and associated vesicles. Similar responses are observed in plant resistance to fungi, suggesting that "vesicle-associated penetration resistance" also functions against insects.

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

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

Improved accumulation of ajmalicine and tetrahydroalstonine in Catharanthus cells expressing an ABC transporter

The biosynthetic pathway of monoterpenoid indole alkaloids in Catharanthus roseus is located throughout various membranes at both the cellular and intercellular levels. ATP-binding cassette (ABC) transporters are known to export vincristine and vinblastine from human cancer cells. It has recently been shown that ABC transporters are also involved in the transport of various monoterpenoid alkaloids in Catharanthus roseus cells. Over-expression of an ABC transporter in this plant might therefore affect the regulation of the alkaloid biosynthetic pathway. CjMDR1, an ABC transporter gene originally isolated from Coptis japonica, was expressed in Catharanthus roseus cell cultures. Cells showing a positive PCR signal of the transgene in both cDNA and genomic DNA samples were subject to transport studies using selected substrates. Unexpectedly, transport of the isoquinoline alkaloid berberine, the main substrate of CjMDR1 transporter in Coptis japonica, was not affected as compared with control and wild-type Catharanthus cells. On the other hand, the endogenous alkaloids ajmalicine and tetrahydroalstonine were accumulated significantly more in Catharanthus roseus cells expressing CjMDR1 in comparison with control lines after feeding these alkaloids.

The power and control of gravitropic movements in plants: a biomechanical and systems biology view

The study of gravitropic movements in plants has enjoyed a long history of research going back to the pioneering works of the 19th century and the famous book entitled 'The power of movement in plants' by Charles and Francis Darwin. Over the last few decades, the emphasis has shifted towards the cellular and molecular biology of gravisensing and the onset of auxin gradients across the organs. However, our understanding of plant movement cannot be completed before quantifying spatio-temporal changes in curvature and how they are produced through the motor process of active bending and controlled by gravisensing. This review sets out to show how combining approaches borrowed from continuum mechanics (kinematic imaging, structural modelling) with approaches from physiology and modern molecular biology has made it possible to generate integrative biomechanical models of the processes involved in gravitropism at several levels. The physiological and biomechanical bases are reviewed and two of the most complete integrative models of the gravireaction organ available are then compared, highlighting how the comparison between movements driven by differential growth and movements driven by reaction wood formation in woody organs has provided highly informative key insights. The advantages of these models as tools for analysing genetic control through quantitative process-based phenotyping as well as for identifying target traits for ecological studies are discussed. It is argued that such models are tools for a systems biology approach to gravitropic movement that has the potential to resolve at least some of the research questions raised 150 years ago.

Inducible expression of Pisum sativum xyloglucan fucosyltransferase in the pea root cap meristem, and effects of antisense mRNA expression on root cap cell wall structural integrity

Mitosis and cell wall synthesis in the legume root cap meristem can be induced and synchronized by the nondestructive removal of border cells from the cap periphery. Newly synthesized cells can be examined microscopically as they differentiate progressively during cap development, and ultimately detach as a new population of border cells. This system was used to demonstrate that Pisum sativum L. fucosyl transferase (PsFut1) mRNA expression is strongly expressed in root meristematic tissues, and is induced >2-fold during a 5-h period when mitosis in the root cap meristem is increased. Expression of PsFut1 antisense mRNA in pea hairy roots under the control of the CaMV35S promoter, which exhibits meristem localized expression in pea root caps, resulted in a 50-60% reduction in meristem localized endogenous PsFut1 mRNA expression measured using whole mount in situ hybridization. Changes in gross levels of cell wall fucosylated xyloglucan were not detected, but altered surface localization patterns were detected using whole mount immunolocalization with CCRC-M1, an antibody that recognizes fucosylated xyloglucan. Emerging hairy roots expressing antisense PsFut1 mRNA appeared normal macroscopically but scanning electron microscopy of tissues with altered CCRC-M1 localization patterns revealed wrinkled, collapsed cell surfaces. As individual border cells separated from the cap periphery, cell death occurred in correlation with extrusion of cellular contents through breaks in the wall.

Sad3 and sad4 are required for saponin biosynthesis and root development in oat

Avenacins are antimicrobial triterpene glycosides that are produced by oat (Avena) roots. These compounds confer broad-spectrum resistance to soil pathogens. Avenacin A-1, the major avenacin produced by oats, is strongly UV fluorescent and accumulates in root epidermal cells. We previously defined nine loci required for avenacin synthesis, eight of which are clustered. Mutants affected at seven of these (including Saponin-deficient1 [Sad1], the gene for the first committed enzyme in the pathway) have normal root morphology but reduced root fluorescence. In this study, we focus on mutations at the other two loci, Sad3 (also within the gene cluster) and Sad4 (unlinked), which result in stunted root growth, membrane trafficking defects in the root epidermis, and root hair deficiency. While sad3 and sad4 mutants both accumulate the same intermediate, monodeglucosyl avenacin A-1, the effect on avenacin A-1 glucosylation in sad4 mutants is only partial. sad1/sad1 sad3/sad3 and sad1/sad1 sad4/sad4 double mutants have normal root morphology, implying that the accumulation of incompletely glucosylated avenacin A-1 disrupts membrane trafficking and causes degeneration of the epidermis, with consequential effects on root hair formation. Various lines of evidence indicate that these effects are dosage-dependent. The significance of these data for the evolution and maintenance of the avenacin gene cluster is discussed.

Sustainable liquid biofuels from biomass: the writing's on the walls

Domination of the global biosphere by human beings is unprecedented in the history of the planet, and our impact is such that substantive changes in ecosystems, and the global environment as a whole, are now becoming apparent. Our activity drives the steady increase in global temperature observed in recent decades. The realization of the adverse effects of greenhouse gas emissions on the environment, together with declining petroleum reserves, has ensured that the quest for sustainable and environmentally benign sources of energy for our industrial economies and consumer societies has become urgent in recent years. Consequently, there is renewed interest in the production and use of fuels from plants. The 'first-generation' biofuels made from starch and sugar appear unsustainable because of the potential stress that their production places on food commodities. Second-generation biofuels, produced from cheap and abundant plant biomass, are seen as the most attractive solution to this problem, but a number of technical hurdles must be overcome before their potential is realized. This review will focus on the underpinning research necessary to enable the cost-effective production of liquid fuels from plant biomass, with a particular focus on aspects related to plant cell walls and their bioconversion.

Identification of woolliness response genes in peach fruit after post-harvest treatments

Woolliness is a physiological disorder of peaches and nectarines that becomes apparent when fruit are ripened after prolonged periods of cold storage. This disorder is of commercial importance since shipping of peaches to distant markets and storage before selling require low temperature. However, knowledge about the molecular basis of peach woolliness is still incomplete. To address this issue, a nylon macroarray containing 847 non-redundant expressed sequence tags (ESTs) from a ripe peach fruit cDNA library was developed and used. Gene expression changes of peach fruit (Prunus persica cv. O'Henry) ripened for 7 d at 21 degrees C (juicy fruit) were compared with those of fruit stored for 15 d at 4 degrees C and then ripened for 7 d at 21 degrees C (woolly fruit). A total of 106 genes were found to be differentially expressed between juicy and woolly fruit. Data analysis indicated that the activity of most of these genes (>90%) was repressed in the woolly fruit. In cold-stored peaches (cv. O'Henry), the expression level of selected genes (cobra, endopolygalacturonase, cinnamoyl-CoA-reductase, and rab11) was lower than in the juicy fruit, and it remained low in woolly peaches after ripening, a pattern that was conserved in woolly fruit from two other commercial cultivars (cv. Flamekist and cv. Elegant Lady). In addition, the results of this study indicate that molecular changes during fruit woolliness involve changes in the expression of genes associated with cell wall metabolism and endomembrane trafficking. Overall, the results reported here provide an initial characterization of the transcriptome activity of peach fruit under different post-harvest treatments.

The role of Rab GTPases in cell wall metabolism

The synthesis and modification of the cell wall must involve the production of new cell wall polymers and enzymes. Their targeted secretion to the apoplast is one of many potential control points. Since Rab GTPases have been strongly implicated in the regulation of vesicle trafficking, a review of their involvement in cell wall metabolism should throw light on this possibility. Cell wall polymer biosynthesis occurs mainly in the Golgi apparatus, except for cellulose and callose, which are made at the plasma membrane by an enzyme complex that cycles through the endomembrane system and which may be regulated by this cycling. Several systems, including the growth of root hairs and pollen tubes, cell wall softening in fruit, and the development of root nodules, are now being dissected. In these systems, secretion of wall polymers and modifying enzymes has been documented, and Rab GTPases are highly expressed. Reverse genetic experiments have been used to interfere with these GTPases and this is revealing their importance in regulation of trafficking to the wall. The role of the RabA (or Rab11) GTPases is particularly exciting in this respect.

Regulation of cell wall biosynthesis

Plant cell walls differ in their amount and composition among various cell types and even in different microdomains of the wall of a given cell. Plants must have evolved regulatory mechanisms controlling biosynthesis, targeted secretion, and assembly of wall components to achieve the heterogeneity in cell walls. A number of factors, including hormones, the cytoskeleton, glycosylphosphatidylinositol-anchored proteins, phosphoinositides, and sugar nucleotide supply, have been implicated in the regulation of cell wall biosynthesis or deposition. In the past two years, there have been important discoveries in transcriptional regulation of secondary wall biosynthesis. Several transcription factors in the NAC and MYB families have been shown to be the key switches for activation of secondary wall biosynthesis. These studies suggest a transcriptional network comprised of a hierarchy of transcription factors is involved in regulating secondary wall biosynthesis. Further investigation and integration of the regulatory players participating in the making of cell walls will certainly lead to our understanding of how wall amounts and composition are controlled in a given cell type. This may eventually allow custom design of plant cell walls on the basis of our needs.

Coated vesicles in plant cells

Coated vesicles represent vital transport intermediates in all eukaryotic cells. While the basic mechanisms of membrane exchange are conserved through the kingdoms, the unique topology of the plant endomembrane system is mirrored by several differences in the genesis, function and regulation of coated vesicles. Efforts to unravel the complex network of proteins underlying the behaviour of these vesicles have recently benefited from the application in planta of several molecular tools used in mammalian systems, as well as from advances in imaging technology and the ongoing analysis of the Arabidopsis genome. In this review, we provide an overview of the roles of coated vesicles in plant cells and highlight salient new developments in the field.