The plant Golgi apparatus: a factory for complex polysaccharides and glycoproteins

Subcellular compartmentalization in the biosynthesis and engineering of plant natural products

Plant natural products (PNPs) are specialized metabolites with diverse bioactivities. They are extensively used in the pharmaceutical, cosmeceutical and food industries. PNPs are synthesized in plant cells by enzymes that are distributed in different subcellular compartments with unique microenvironments, such as ions, co-factors and substrates. Plant metabolic engineering is an emerging and promising approach for the sustainable production of PNPs, for which the knowledge of the subcellular compartmentalization of their biosynthesis is instrumental. In this review we describe the state of the art on the role of subcellular compartments in the biosynthesis of major types of PNPs, including terpenoids, phenylpropanoids, alkaloids and glucosinolates, and highlight the efforts to target biosynthetic pathways to subcellular compartments in plants. In addition, we will discuss the challenges and strategies in the field of plant synthetic biology and subcellular engineering. We expect that newly developed methods and tools, together with the knowledge gained from the microbial chassis, will greatly advance plant metabolic engineering.

Mutations in type II Golgi-localized proton pyrophosphatase AVP2;1/VHP2;1 affect pectic polysaccharide rhamnogalacturonan-II and alter root growth under low boron condition in Arabidopsis thaliana

The essential plant nutrient boron is required for the crosslinking of the pectin polysaccharide, rhamnogalacturonan II (RG-II). The synthesis of the pectic polysaccharides takes place in the Golgi apparatus, acidified by proton pumps. AVP2;1/VHP2;1 is a type II proton pyrophosphatase localized in the Golgi apparatus, which possesses proton pumping activity coupled with pyrophosphate hydrolysis. Its activity and expression patterns have been previously revealed but its role in plants remains unknown. The aim of the present work therefore was to explore the physiological role of AVP2;1 in Arabidopsis thaliana. In the screening of mutants under low boron, a mutant carrying a missense mutation in AVP2;1 was isolated. This mutant showed increased primary root growth under low boron conditions but no significant difference under normal boron condition compared to wild type plants. T-DNA insertion caused similar growth, suggesting that reduced function of AVP2;1 was responsible. Root cell observation revealed an increase in meristematic zone length, cell number in meristem and length of matured cell in avp2;1 mutants compared to wild type under low boron. Calcium concentration was reduced in mutant root cell wall under low boron. RG-II specific sugars also tended to be decreased in mutant root cell wall under low and normal boron conditions. These results suggest that changes in cell wall component by mutations in AVP2;1 may possibly explain the increased root length of mutants under low boron. This supports the idea that AVP2;1 plays a role in pH homoeostasis in Golgi apparatus for pectin synthesis.

Function of Rice High-Affinity Potassium Transporters in Pollen Development and Fertility

Plant High-affinity K+ transporters/K+ uptake permeases/K+ transporters (HAK/KUP/KT) transporters have been predicted as membrane H+-K+ symporters in facilitating K+ uptake and distribution, while their role in seed production remains to be elucidated. In this study, we report that OsHAK26 is preferentially expressed in anthers and seed husks and located in the Golgi apparatus. Knockout of either OsHAK26 or plasma membrane located H+-K+ symporter gene OsHAK1 or OsHAK5 in both Nipponbare and Dongjin cultivars caused distorted anthers, reduced number and germination rate of pollen grains. Seed-setting rate assay by reciprocal cross-pollination between the mutants of oshak26, oshak1, oshak5 and their wild types confirmed that each HAK transporter is foremost for pollen viability, seed-setting and grain yield. Intriguingly, the pollens of oshak26 showed much thinner wall and were more vulnerable to desiccation than those of oshak1 or oshak5. In vitro assay revealed that the pollen germination rate of oshak5 was dramatically affected by external K+ concentration. The results suggest that the role of OsHAK26 in maintaining pollen development and fertility may relate to its proper cargo sorting for construction of pollen walls, while the role of OsHAK1 and OsHAK5 in maintaining seed production likely relates to their transcellular K+ transport activity.

The Golgi Apparatus and its Next-Door Neighbors

The Golgi apparatus represents a central compartment of membrane traffic. Its apparent architecture, however, differs considerably among species, from unstacked and scattered cisternae in the budding yeast Saccharomyces cerevisiae to beautiful ministacks in plants and further to gigantic ribbon structures typically seen in mammals. Considering the well-conserved functions of the Golgi, its fundamental structure must have been optimized despite seemingly different architectures. In addition to the core layers of cisternae, the Golgi is usually accompanied by next-door compartments on its cis and trans sides. The trans-Golgi network (TGN) can be now considered as a compartment independent from the Golgi stack. On the cis side, the intermediate compartment between the ER and the Golgi (ERGIC) has been known in mammalian cells, and its functional equivalent is now suggested for yeast and plant cells. High-resolution live imaging is extremely powerful for elucidating the dynamics of these compartments and has revealed amazing similarities in their behaviors, indicating common mechanisms conserved along the long course of evolution. From these new findings, I would like to propose reconsideration of compartments and suggest a new concept to describe their roles comprehensively around the Golgi and in the post-Golgi trafficking.

Effects of polygalacturonase overexpression on pectin distribution in the elongation zones of roots under aluminium stress

The roots of many plant species contain large amounts of pectin and it contributes to the formation of the rhizosphere. In the present study, the relationship between the root-tip pectin content and aluminium (Al) tolerance in wild-type (WT) and demethylesterified pectin degradation enzyme gene overexpressor (OsPG2-FOX) rice lines was compared. OsPG2-FOX rice showed reduced pectin content in roots, even under control conditions; Al treatment reduced root elongation and the pectin content in the root elongation zone. Wild-type rice showed more pectin accumulation in the root elongation zone after Al treatment. Relative to WT rice, OsPG2-FOX rice showed more Al accumulation in the root elongation zone. These results indicate that the amount of pectin influences Al tolerance and that the distribution of pectin in the root elongation zone inhibits Al accumulation in rice roots. Pectin accumulation in cell walls in the root elongation zone may play a role in protecting rice plants from the Al-induced inhibition of root elongation by regulating pectin distribution.

Transcriptome analysis reveals the mechanism of internode development affecting maize stalk strength

BACKGROUND

The stalk rind is one of the important factors affecting maize stalk strength that is closely related to stalk lodging. However, the mechanism of rind development in maize is still largely unknown.

RESULTS

In this study, we analyzed the mechanical, anatomical, and biochemical properties of the third basal internode in one maize non-stiff-stalk (NSS) line and two stiff-stalk (SS) lines. Compared with the NSS line, the two SS lines had a significantly higher rind penetrometer resistance, thicker rind, and higher dry matter, hemicellulose, cellulose, and lignin weights per unit length. RNA-seq analysis was used to compare transcriptomes of the third basal internode of the two SS lines and the NSS line at the ninth leaf and tasseling stages. Gene Ontology (GO) enrichment analysis revealed that genes involved in hydrolase activity (hydrolyzing O-glycosyl compounds) and cytoskeleton organization were significantly up-regulated in the two SS lines at the ninth leaf stage and that microtubule process-related genes were significantly up-regulated in the two SS lines at the tasseling stage. Moreover, the two SS lines had enhanced expression of cell wall metabolism-related genes at the tasseling stage.

CONCLUSIONS

The synthesis of cell wall polysaccharides and the cytoskeleton might play important roles in internode development. Our results can be applied for screening lodging-resistant inbred lines and breeding lodging-resistant cultivars in maize.

Rice Putative Pectin Methyltransferase Gene OsPMT10 Is Required for Maintaining the Cell Wall Properties of Pistil Transmitting Tissues via Pectin Modification

Precise directional control of pollen tube growth via mechanical guidance by pistil tissue is critical for the successful fertilization of flowering plants and requires active cell-to-cell communication and maintenance of softness in the transmitting tissue. However, the regulation of transmitting tissue softness as controlled by cell wall properties, especially pectin, has not been reported. Here we report that regulation of pectin methylesterification supports pollen elongation through pistil transmitting tissues in Oryza sativa. The rice pectin methylesterase gene OsPMT10 was strongly expressed in reproductive tissues, especially the pistil. The ospmt10 mutant did not have a significant effect on vegetative growth, but the fertility rate was reduced by approximately half. In the ospmt10 mutant, pollen tube elongation was observed in the transmitting tissue of the style, but approximately half of the pollen tubes did not extend all the way to the ovule. Tissue cross-sections of the upper ovary were prepared, and immunohistochemical staining using LM19 and LM20 showed that methylesterified pectin distribution was decreased in ospmt10 compared with the wild type. The decreased expression of methylesterified pectins in ospmt10 may have resulted in loss of fluidity in the apoplast space of the transmitting tissue, rendering it difficult for the pollen tube to elongate in the transmitting tissue and thereby preventing it from reaching the ovule.

Mechanisms of membrane traffic in plant cells

The organelles of the secretory pathway are characterized by specific organization and function but they communicate in different ways with intense functional crosstalk. The best known membrane-bound transport carriers are known as protein-coated vesicles. Other traffic mechanisms, despite the intense investigations, still show incongruences. The review intends to provide a general view of the mechanisms involved in membrane traffic. We evidence that organelles' biogenesis involves mechanisms that actively operate during the entire cell cycle and the persistent interconnections between the Endoplasmic reticulum (ER), Golgi apparatus, trans-Golgi network (TGN) and endosomes, the vacuolar complex and the plasma membrane (PM) may be seen as a very dynamic membrane network in which vesicular traffic is part of a general maturation process.

Plastid role in phytomelanin synthesis in Piptocarpha axillaris (Less.) Baker stems (Asteraceae, Vernonieae)

Phytomelanin is a brown to black pigment found in plant tissues, mainly in Asparagales and Asteraceae species. However, few studies deal with the processes of its synthesis, and there are still many questions to be answered regarding the organelles involved in this process and their functions, especially in vegetative organs. In a previous study with stems and leaves of 77 Vernonieae (Asteraceae) species, phytomelanin was demonstrated to always be associated with sclereids, which suggests the involvement of these cells in the pigment synthesis. Thus, we selected another species of tribe Vernonieae, Piptocarpha axillaris (Less.) Baker, which produces abundant phytomelanin secretion in stem tissues, to investigate which cells and organelles are involved in the synthesis and release of this pigment, as well as its distribution in the tissues. To achieve this goal, stems in different developmental phases were analyzed under light and transmission electron microscopy. Anatomical analysis showed that the polymerization of phytomelanin in P. axillaris starts at the second stem node, in the pith region, and occurs simultaneously with sclereid differentiation. The plastids of cells that will differentiate into sclereids actively participate in the phenolic material synthesis, following the "tannosome" and the "pearl necklace" models, giving rise to the main precursor of phytomelanin, which is then polymerized in the intercellular spaces during the sclerification process of sclereids. In stems with an established secondary structure, the pigment can be observed more frequently in the cortex, pericycle, primary phloem, secondary phloem, and pith.

Functional organisation of the endomembrane network in the digestive gland of the Venus flytrap: revisiting an old story with a new microscopy toolbox

Up-to-date imaging approaches were used to address the spatiotemporal organisation of the endomembrane system in secretory cells of Dionaea muscipula. Different 'slice and view' methodologies were performed on resin-embedded samples to finally achieve a 3D reconstruction of the cell architecture, using ultrastructural tomography, array tomography, serial block face-scanning electron microscopy (SBF-SEM), correlation, and volume rendering at the light microscopy level. Observations of cryo-fixed samples by high-pressure freezing revealed changes of the endomembrane system that occur after trap activation and prey digestion. They provide evidence for an original strategy that adapts the secretory machinery to a specific and unique case of stimulated exocytosis in plant cells. A first secretion peak is part of a rapid response to deliver digestive fluids to the cell surface, which delivers the needed stock of digestive materials 'on site'. The second peak of activity could then be associated with the reconstruction of the Golgi apparatus (GA), endoplasmic reticulum (ER) and vacuolar machinery, in order to prepare for a subsequent round of prey capture. Tubular continuum between ER and Golgi stacks observed on ZIO-impregnated tissues may correspond to an efficient transfer mechanism for lipids and/or proteins, especially for use in rapidly resetting the molecular GA machinery. The occurrence of one vacuolar continuum may permit continuous adjustment of cell homeostasy. The subcellular features of the secretory cells of Dionaea muscipula outline key innovations in the organisation of plant cell compartmentalisation that are used to cope with specific cell needs such as the full use of the GA as a protein factory, and the ability to create protein reservoirs in the periplasmic space. Shape-derived forces of the pleiomorphic vacuole may act as signals to accompany the sorting and entering flows of the cell.

Rice Putative Methyltransferase Gene OsPMT16 Is Required for Pistil Development Involving Pectin Modification

Pectin synthesis and modification are vital for plant development, although the underlying mechanisms are still not well understood. Furthermore, reports on the function of pectin in the pistil are limited. Herein, we report the functional characterization of the OsPMT16 gene, which encodes a putative pectin methyltransferase (PMT) in rice. The cell walls of rice leaves contain less pectin, and chemical analysis of pectin in the flower organ had not been previously performed. Therefore, in the present study, the amount of pectin in the reproductive tissues of rice was investigated. Of the reproductive tissues, the pistil was especially rich in pectin; thus, we focused on the pistil. OsPMT16 expression was confirmed in the pistil, and effects of pectin methylesterification regulation on the reproductive stage were investigated by studying the phenotype of the T-DNA insertion mutant. The ospmt16 mutant showed significantly reduced fertility. When the flowers were observed, tissue morphogenesis was abnormal in the pistil. Immunofluorescence staining by pectin-specific monoclonal antibodies of the pistil revealed that total pectin and esterified pectin were decreased among ospmt16 mutants. These results indicate that OsPMT16 contributes significantly to pistil development during reproductive growth.

Identification of the Virulence Factors of Candidatus Liberibacter asiaticus via Heterologous Expression in Nicotiana benthamiana using Tobacco Mosaic Virus

Huanglongbing (HLB), also known as citrus greening, is the most destructive disease of citrus worldwide. HLB is associated with the non-culturable bacterium, Candidatus Liberibacter asiaticus (CaLas) in the United States. The virulence mechanism of CaLas is largely unknown, partly because of the lack of a mutant library. In this study, Tobacco mosaic virus (TMV) and Nicotiana benthamiana (N. benthamiana) were used for large-scale screening of the virulence factors of CaLas. Agroinfiltration of 60 putative virulence factors in N. benthamiana led to the identification of four candidates that caused severe symptoms in N. benthamiana, such as growth inhibition and cell death. CLIBASIA_05150 and CLIBASIA_04065C (C-terminal of CLIBASIA_04065) could cause cell death in the infiltrated leaves at five days post infiltration. Two low-molecular-weight candidates, CLIBASIA_00470 and CLIBASIA_04025, could inhibit plant growth. By converting start codon to stop codon or frameshifting, the four genes lost their harmful effects to N. benthamiana. It indicated that the four virulence factors functioned at the protein level rather than at the RNA level. The subcellular localization of the four candidates was determined by confocal laser scanning microscope. CLIBASIA_05150 located in the Golgi apparatus; CLIBASIA_04065 located in the mitochondrion; CLIBASIA_00470 and CLIBASIA_04025 distributed in cells as free GFP. The host proteins interacting with the four virulence factors were identified by yeast two-hybrid. The host proteins interacting with CLIBASIA_00470 and CLIBASIA_04025 were overlapping. Based on the phenotypes, the subcellular localization and the host proteins identified by yeast two-hybrid, CLIBASIA_00470 and CLIBASIA_04025, functioned redundantly. The hypothesis of CaLas virulence was proposed. CaLas affects citrus development and suppresses citrus disease resistance, comprehensively, in a complicated manner. Ubiquitin-mediated protein degradation might play a vital role in CaLas virulence. Deep characterization of the interactions between the identified virulence factors and their prey will shed light on HLB. Eventually, it will help in developing HLB-resistant citrus and save the endangered citrus industry worldwide.

A Kinetic View of Membrane Traffic Pathways Can Transcend the Classical View of Golgi Compartments

A long-standing assumption is that the cisternae of the Golgi apparatus can be grouped into functionally distinct compartments, yet the molecular identities of those compartments have not been clearly described. The concept of a compartmentalized Golgi is challenged by the cisternal maturation model, which postulates that cisternae form de novo and then undergo progressive biochemical changes. Cisternal maturation can potentially be reconciled with Golgi compartmentation by defining compartments as discrete kinetic stages in the maturation process. These kinetic stages are distinguished by the traffic pathways that are operating. For example, a major transition occurs when a cisterna stops producing COPI vesicles and begins producing clathrin-coated vesicles. This transition separates one kinetic stage, the "early Golgi," from a subsequent kinetic stage, the "late Golgi" or "trans-Golgi network (TGN)." But multiple traffic pathways drive Golgi maturation, and the periods of operation for different traffic pathways can partially overlap, so there is no simple way to define a full set of Golgi compartments in terms of kinetic stages. Instead, we propose that the focus should be on the series of transitions experienced by a Golgi cisterna as various traffic pathways are switched on and off. These traffic pathways drive changes in resident transmembrane protein composition. Transitions in traffic pathways seem to be the fundamental, conserved determinants of Golgi organization. According to this view, the initial goal is to identify the relevant traffic pathways and place them on the kinetic map of Golgi maturation, and the ultimate goal is to elucidate the logic circuit that switches individual traffic pathways on and off as a cisterna matures.

Genomic comparison of two independent seagrass lineages reveals habitat-driven convergent evolution

Seagrasses are marine angiosperms that live fully submerged in the sea. They evolved from land plant ancestors, with multiple species representing at least three independent return-to-the-sea events. This raises the question of whether these marine angiosperms followed the same adaptation pathway to allow them to live and reproduce under the hostile marine conditions. To compare the basis of marine adaptation between seagrass lineages, we generated genomic data for Halophila ovalis and compared this with recently published genomes for two members of Zosteraceae, as well as genomes of five non-marine plant species (Arabidopsis, Oryza sativa, Phoenix dactylifera, Musa acuminata, and Spirodela polyrhiza). Halophila and Zosteraceae represent two independent seagrass lineages separated by around 30 million years. Genes that were lost or conserved in both lineages were identified. All three species lost genes associated with ethylene and terpenoid biosynthesis, and retained genes related to salinity adaptation, such as those for osmoregulation. In contrast, the loss of the NADH dehydrogenase-like complex is unique to H. ovalis. Through comparison of two independent return-to-the-sea events, this study further describes marine adaptation characteristics common to seagrass families, identifies species-specific gene loss, and provides molecular evidence for convergent evolution in seagrass lineages.

The plant secretory pathway seen through the lens of the cell wall

Secretion in plant cells is often studied by looking at well-characterised, evolutionarily conserved membrane proteins associated with particular endomembrane compartments. Studies using live cell microscopy and fluorescent proteins have illuminated the highly dynamic nature of trafficking, and electron microscopy studies have resolved the ultrastructure of many compartments. Biochemical and molecular analyses have further informed about the function of particular proteins and endomembrane compartments. In plants, there are over 40 cell types, each with highly specialised functions, and hence potential variations in cell biological processes and cell wall structure. As the primary function of secretion in plant cells is for the biosynthesis of cell wall polysaccharides and apoplastic transport complexes, it follows that utilising our knowledge of cell wall glycosyltransferases (GTs) and their polysaccharide products will inform us about secretion. Indeed, this knowledge has led to novel insights into the secretory pathway, including previously unseen post-TGN secretory compartments. Conversely, our knowledge of trafficking routes of secretion will inform us about polarised and localised deposition of cell walls and their constituent polysaccharides/glycoproteins. In this review, we look at what is known about cell wall biosynthesis and the secretory pathway and how the different approaches can be used in a complementary manner to study secretion and provide novel insights into these processes.

Overexpression of Golgi Protein CYP21-4s Improves Crop Productivity in Potato and Rice by Increasing the Abundance of Mannosidic Glycoproteins

CYP21-4 is a novel Golgi-localized cyclophilin protein involved in oxidative stress tolerance. Here, we generated transgenic plants overexpressing AtCYP21-4 and OsCYP21-4 in potato and rice, respectively. The stems and roots of AtCYP21-4-overexpressing potato plants were longer than those of wild-type (WT) plants, which resulted in heavier tubers. In vitro tuberization in the transgenic potato also resulted in significantly greater tuber number and weight, as well as a shorter time to microtuber formation. Similarly, OsCYP21-4-overexpressing transgenic rice plants had higher biomass and productivity with longer early-stage internodes than the WT and higher seed weight. Immunoblot analysis with CYP21-4 antibody showed that these productivity-enhancing phenotypes were associated with high CYP21-4s protein expression. Anatomically, transgenic potato stems exhibited higher lignin content in xylem cells and thicker leaves. In addition, relative content of mannosidic glycoproteins per unit of total protein was above 20% in transgenic potato tubers and rice grains. Based on these findings, we propose that CYP21-4s are involved in the growth and development of plant vegetative and storage tissues via their effects on glycoprotein abundance or glycan processing in the Golgi apparatus. Thus, increasing CYP21-4s expression in crops could represent an alternative way to increase crop productivity and yield.

Alterations of Golgi organization in Alzheimer's disease: A cause or a consequence?

The Golgi apparatus is a central organelle of the secretory pathway involved in the post-translational modification and sorting of lipids and proteins. In mammalian cells, the Golgi apparatus is composed of stacks of cisternae organized in polarized manner, which are interconnected by membrane tubules to constitute the Golgi ribbon, located in the proximity of the centrosome. Besides the processing and transport of cargo, the Golgi complex is actively involved in the regulation of mitotic entry, cytoskeleton organization and dynamics, calcium homeostasis, and apoptosis, representing a signalling platform for the control of several cellular functions, including signalling initiated by receptors located at the plasma membrane. Alterations of the conventional Golgi organization are associated to many disorders, such as cancer or different neurodegenerative diseases. In this review, we examine the functional implications of modifications of Golgi structure in neurodegenerative disorders, with a focus on the role of Golgi fragmentation in the development of Alzheimer's disease. The comprehension of the mechanism that induces Golgi fragmentation and of its downstream effects on neuronal function have the potential to contribute to the development of more effective therapies to treat or prevent some of these disorders.

Pollen and microsporangium development in Hovenia dulcis (Rhamnaceae): a different type of tapetal cell ultrastructure

Despite that there is some literature on pollen morphology of Rhamnaceae, studies addressing general aspects of the microsporogenesis, microgametogenesis, and anther development are rare. The aim of this paper is to describe the ultrastructure of pollen grain ontogeny with special attention to tapetum cytology in Hovenia dulcis. Anthers at different stages of development were processed for transmission and scanning electron microscopy, bright-field microscopy, and fluorescence microscopy. Different histochemical reactions were carried out. The ultrastructural changes observed during the development of the tapetal cells and pollen grains are described. Large vesicles containing carbohydrates occur in the tapetal cell cytoplasm during the early stages of pollen development. Its origin and composition are described and discussed. This is the first report on the ontogeny and ultrastructure of the pollen grain and related sporophytic structures of H. dulcis.

RNA-Seq-based transcriptome analysis of stem development and dwarfing regulation in Agapanthus praecox ssp. orientalis (Leighton) Leighton

Agapanthus praecox is a monocotyledonous ornamental bulb plant. Generally, the scape (inflorescence stem) length can develop more than 1m, however application 400 mg·L(-1) paclobutrazol can shorten the length beyond 70%. To get a deeper insight into its dwarfism mechanism, de novo RNA-Seq technology has been employed, for the first time, to describe the scape transcriptome of A. praecox. We got 71,258 assembled unigenes, and 45,597 unigenes obtained protein functional annotation. Take the above sequencing results as a reference gene set, using RNA-seq (quantification) technology analyzed gene expression profiles between the control and paclobutrazol-treated samples, and screened 2838 differentially expressed genes. GO, KEGG and MapMan pathway analyses indicated that these differentially expressed genes were significantly enriched in response to stimulus, hormonal signaling, carbohydrate metabolism, cell wall, cell size, and cell cycle related biological process. To validate the expression profiles obtained by RNA-Seq, real-time qPCR was performed on 24 genes selected from key significantly enriched pathways. Comprehensive analysis suggested that paclobutrazol blocks GA signal that can effectively inhibit scape elongation; the GA signal interact with other hormonal signals including auxin, ethylene, brassinosteroid and cytokinins, and trigger downstream signaling cascades leading to metabolism, cell wall biosynthesis, cell division and the cycle decreased obviously, and finally induced dwarfism trait. Furthermore, AP2/EREBP, bHLH, C2H2, ARR, WRKY and ARF family's transcription factors were involved in the regulation of scape development in A. praecox. This transcriptome dataset will serve as an important public information platform to accelerate research on the gene expression and functional genomics of Agapanthus.

Proteomic analysis of etiolated juvenile tetraploid Robinia pseudoacacia branches during different cutting periods

The propagation of hard-branch cuttings of tetraploid Robinia pseudoacacia (black locust) is restricted by the low rooting rate; however, etiolated juvenile tetraploid black locust branches result in a significantly higher rooting rate of cuttings compared with non-etiolated juvenile tetraploid branches. To identify proteins that influence the juvenile tetraploid branch rooting process, two-dimensional electrophoresis (2-DE) and matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectra (MALDI-TOF/TOF-MS) were used to analyze proteomic differences in the phloem of tetraploid R. pseudoacacia etiolated and non-etiolated juvenile branches during different cutting periods. A total of 58 protein spots differed in expression level, and 16 protein spots were only expressed in etiolated branches or non-etiolated ones. A total of 40 highly expressed protein spots were identified by mass spectrometry, 14 of which were accurately retrieved. They include nucleoglucoprotein metabolic proteins, signaling proteins, lignin synthesis proteins and phyllochlorin. These results help to reveal the mechanism of juvenile tetraploid R. pseudoacacia etiolated branch rooting and provide a valuable reference for the improvement of tetraploid R. pseudoacacia cutting techniques.

Cell wall, cytoskeleton, and cell expansion in higher plants

To accommodate two seemingly contradictory biological roles in plant physiology, providing both the rigid structural support of plant cells and the adjustable elasticity needed for cell expansion, the composition of the plant cell wall has evolved to become an intricate network of cellulosic, hemicellulosic, and pectic polysaccharides and protein. Due to its complexity, many aspects of the cell wall influence plant cell expansion, and many new and insightful observations and technologies are forthcoming. The biosynthesis of cell wall polymers and the roles of the variety of proteins involved in polysaccharide synthesis continue to be characterized. The interactions within the cell wall polymer network and the modification of these interactions provide insight into how the plant cell wall provides its dual function. The complex cell wall architecture is controlled and organized in part by the dynamic intracellular cytoskeleton and by diverse trafficking pathways of the cell wall polymers and cell wall-related machinery. Meanwhile, the cell wall is continually influenced by hormonal and integrity sensing stimuli that are perceived by the cell. These many processes cooperate to construct, maintain, and manipulate the intricate plant cell wall--an essential structure for the sustaining of the plant stature, growth, and life.

GO-PROMTO illuminates protein membrane topologies of glycan biosynthetic enzymes in the Golgi apparatus of living tissues

The Golgi apparatus is the main site of glycan biosynthesis in eukaryotes. Better understanding of the membrane topology of the proteins and enzymes involved can impart new mechanistic insights into these processes. Publically available bioinformatic tools provide highly variable predictions of membrane topologies for given proteins. Therefore we devised a non-invasive experimental method by which the membrane topologies of Golgi-resident proteins can be determined in the Golgi apparatus in living tissues. A Golgi marker was used to construct a series of reporters based on the principle of bimolecular fluorescence complementation. The reporters and proteins of interest were recombinantly fused to split halves of yellow fluorescent protein (YFP) and transiently co-expressed with the reporters in the Nicotiana benthamiana leaf tissue. Output signals were binary, showing either the presence or absence of fluorescence with signal morphologies characteristic of the Golgi apparatus and endoplasmic reticulum (ER). The method allows prompt and robust determinations of membrane topologies of Golgi-resident proteins and is termed GO-PROMTO (for GOlgi PROtein Membrane TOpology). We applied GO-PROMTO to examine the topologies of proteins involved in the biosynthesis of plant cell wall polysaccharides including xyloglucan and arabinan. The results suggest the existence of novel biosynthetic mechanisms involving transports of intermediates across Golgi membranes.

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.

Live-cell imaging of dual-labeled Golgi stacks in tobacco BY-2 cells reveals similar behaviors for different cisternae during movement and brefeldin A treatment

In plant cells, the Golgi apparatus consists of numerous stacks that, in turn, are composed of several flattened cisternae with a clear cis-to-trans polarity. During normal functioning within living cells, this unusual organelle displays a wide range of dynamic behaviors such as whole stack motility, constant membrane flux through the cisternae, and Golgi enzyme recycling through the ER. In order to further investigate various aspects of Golgi stack dynamics and integrity, we co-expressed pairs of established Golgi markers in tobacco BY-2 cells to distinguish sub-compartments of the Golgi during monensin treatments, movement, and brefeldin A (BFA)-induced disassembly. A combination of cis and trans markers revealed that Golgi stacks remain intact as they move through the cytoplasm. The Golgi stack orientation during these movements showed a slight preference for the cis side moving ahead, but trans cisternae were also found at the leading edge. During BFA treatments, the different sub-compartments of about half of the observed stacks fused with the ER sequentially; however, no consistent order could be detected. In contrast, the ionophore monensin resulted in swelling of trans cisternae while medial and particularly cis cisternae were mostly unaffected. Our results thus demonstrate a remarkable equivalence of the different cisternae with respect to movement and BFA-induced fusion with the ER. In addition, we propose that a combination of dual-label fluorescence microscopy and drug treatments can provide a simple alternative approach to the determination of protein localization to specific Golgi sub-compartments.

Sub-compartmental organization of Golgi-resident N-glycan processing enzymes in plants

In all eukaryotes, the Golgi apparatus is the main site of protein glycosylation. It is widely accepted that the glycosidases and glycosyltransferases involved in N-glycan processing are found concentrated within the Golgi stack where they provide their function. This means that enzymes catalyzing early steps in the processing pathway are located mainly at the cis-side, whereas late-acting enzymes mostly locate to the trans-side of the stacks, creating a non-uniform distribution along the cis-trans axis of the Golgi. There is compelling evidence that the information for their sorting to specific Golgi cisternae depends on signals encoded in the proteins themselves as well as on the trafficking machinery that recognizes these signals and it is believed that cisternal sub-compartmentalization is achieved and maintained by a combination of retention and retrieval mechanisms. Yet, the signals, mechanism(s), and molecular factors involved are still unknown. Here, we address recent findings and summarize the current understanding of this fundamental process in plant cell biology.

Differences in enzymic properties of five recombinant xyloglucan endotransglucosylase/hydrolase (XTH) proteins of Arabidopsis thaliana

Xyloglucan endotransglucosylase/hydrolases (XTHs) are cell wall enzymes that are able to graft xyloglucan chains to oligosaccharides or to other available xyloglucan chains and/or to hydrolyse xyloglucan chains. As they are involved in the modification of the load-bearing cell-wall components, they are believed to be very important in the regulation of growth and development. Given the large number (33) of XTH genes in Arabidopsis and the overlapping expression patterns, specific enzymic properties may be expected. Five predominantly root-expressed Arabidopsis thaliana XTHs belonging to subgroup I/II were analysed here. These represent two sets of closely related genes: AtXTH12 and 13 on the one hand (trichoblast-enriched) and AtXTH17, 18, and 19 on the other (expressed in nearly all cell types in the root). They were all recombinantly produced in the yeast Pichia pastoris and partially purified by ammonium sulphate precipitation before they were subsequently all subjected to a series of identical in vitro tests. The kinetic properties of purified AtXTH13 were investigated in greater detail to rule out interference with the assays by contaminating yeast proteins. All five proteins were found to exhibit only the endotransglucosylase (XET; EC 2.4.1.207) activity towards xyloglucan and non-detectable endohydrolytic (XEH; EC 3.2.1.151) activity. Their endotransglucosylase activity was preferentially directed towards xyloglucan and, in some cases, water-soluble cellulose acetate, rather than to mixed-linkage β-glucan. Isoforms differed in optimum pH (5.0-7.5), in temperature dependence and in acceptor substrate preferences.

A clade in the QUASIMODO2 family evolved with vascular plants and supports a role for cell wall composition in adaptation to environmental changes

The evolution of plant vascular tissue is tightly linked to the evolution of specialised cell walls. Mutations in the QUASIMODO2 (QUA2) gene from Arabidopsis thaliana were previously shown to result in cell adhesion defects due to reduced levels of the cell wall component homogalacturonic acid. In this study, we provide additional information about the role of QUA2 and its closest paralogues, QUASIMODO2 LIKE1 (QUL1) and QUL2. Within the extensive QUA2 family, our phylogenetic analysis shows that these three genes form a clade that evolved with vascular plants. Consistent with a possible role of this clade in vasculature development, QUA2 is highly expressed in the vascular tissue of embryos and inflorescence stems and overexpression of QUA2 resulted in temperature-sensitive xylem collapse. Moreover, in-depth characterisation of qua2 qul1 qul2 triple mutant and 35S::QUA2 overexpression plants revealed contrasting temperature-dependent stem development with dramatic effects on stem width. Taken together, our results suggest that the QUA2-specific clade contributed to the evolution of vasculature and illustrate the important role that modification of cell wall composition plays in the adaptation to changing environmental conditions, including changes in temperature.

Arginine/lysine residues in the cytoplasmic tail promote ER export of plant glycosylation enzymes

Plant N-glycan processing enzymes are arranged along the early secretory pathway, forming an assembly line to facilitate the step-by-step modification of oligosaccharides on glycoproteins. Thus, these enzymes provide excellent tools to study signals and mechanisms, promoting their localization and retention in the endoplasmic reticulum (ER) and Golgi apparatus. Herein, we focused on a detailed investigation of amino acid sequence motifs present in their short cytoplasmic tails in respect to ER export. Using site-directed mutagenesis, we determined that single arginine/lysine residues within the cytoplasmic tail are sufficient to promote rapid Golgi targeting of Golgi-resident N-acetylglucosaminyltransferase I (GnTI) and alpha-mannosidase II (GMII). Furthermore, we reveal that an intact ER export motif is essential for proper in vivo function of GnTI. Coexpression studies with Sar1p provided evidence for COPII-dependent transport of GnTI to the Golgi. Our data provide evidence that efficient ER export of Golgi-resident plant N-glycan processing enzymes occurs through a selective mechanism based on recognition of single basic amino acids present in their cytoplasmic tails.

OCCURRENCE AND CHARACTERIZATION OF ARABINOGALACTAN-LIKE PROTEINS AND HEMICELLULOSES IN MICRASTERIAS (STREPTOPHYTA)(1)

The cell wall of the green alga Micrasterias denticulata Bréb. ex Ralfs (Desmidiaceae, Zygnematophyceae, Streptophyta) was investigated to obtain information on the composition of component polysaccharides and proteoglycans to allow comparison with higher plants and to understand cell wall functions during development. Various epitopes currently assigned to arabinogalactan-proteins (AGPs) of higher plants could be detected in Micrasterias by immuno TEM and immunofluorescence methods, but the walls did not bind the β-d-glycosyl-Yariv (β-GlcY) reagent. Secretory vesicles and the primary wall were labeled by antibodies against AGPs (JIM8, JIM13, JIM14). Dot and Western blot experiments indicated a proteoglycan nature of the epitopes recognized, which consisted of galactose and xylose as major sugars by high performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD). Epitopes of alkali-soluble polysaccharides assigned to noncellulosic polysaccharides in higher plants could be detected and located in the wall during its formation. The polyclonal anti-xyloglucan (anti-XG) antibody labeled primary and secondary wall of Micrasterias, whereas the monoclonal antibody CCRC-M1, directed against the fucose/galactose side chain of xyloglucan (XyG), did not recognize any structures. Labeling by anti-XG antibody at the trans-sites of the dictyosomes and at wall material containing vesicles indicated that secretion of the epitopes occurred similar to higher plants. The presence of (1→3, 1→4)-β-glucan (mixed linked glucan) in the secondary cell wall but not in the primary cell wall of Micrasterias could be demonstrated by an antibody recognizing this glucan type, whereas (1→3)-β-glucan (callose) could not be detected. The analytical results revealed that alkali-soluble polysaccharides in the secondary wall of Micrasterias consist mostly of (1→3, 1→4)-β-d-glucan.

PpRab1, a Rab GTPase from maritime pine is differentially expressed during embryogenesis

Rab-related small GTP-binding proteins are known to be involved in the regulation of the vesicular transport system in eukaryotic cells. We report the characterization of a previously isolated full-length cDNA PpRab1 from Pinus pinaster. Amino acid sequence analysis revealed the presence of G1-G5 conserved domains of the GTPase Ras superfamily and a double cysteine motif in the C-terminal, characteristic of Rab proteins. The PpRab1 protein shows high sequence similarity to several Rab1 GTP-binding proteins in plants. Phylogenetic analysis showed that, within the Ras superfamily, PpRab1 is more closely related to the Rab family and within this, PpRab1 protein was found to cluster with Arabidopsis subfamily AtRABE, whose members are known to regulate ER-to-Golgi membrane trafficking steps. PpRab1 transcripts were expressed at constitutively high levels for the initial stages of zygotic embryo development, and then their relative abundance decreased as embryo matures. The PpRab1 transcript is not embryo-specific as it was found in roots, cotyledons and hypocotyls. An increase in PpRab1 expression level was observed when seeds are germinated and collected at successive time points of development. In situ RT-PCR analysis revealed an expression signal in early zygotic embryos. In view of the proposed roles of Rab1 GTP-binding protein, the possible function of the protein encoded by PpRab1 in embryogenesis is discussed.

The TUMOROUS SHOOT DEVELOPMENT2 gene of Arabidopsis encoding a putative methyltransferase is required for cell adhesion and co-ordinated plant development

Mutations in the TUMOROUS SHOOT DEVELOPMENT2 (TSD2) gene reduce cell adhesion, and in strongly affected individuals cause non-coordinated shoot development that leads to disorganized tumor-like growth in vitro. tsd2 mutants showed increased activity of axial meristems, reduced root growth and enhanced de-etiolation. The expression domains of the shoot meristem marker genes KNAT1 and KNAT2 were enlarged in the mutant background. Soil-grown tsd2 mutants were dwarfed, but overall showed morphology similar to that of the wild-type (WT). The TSD2 gene was identified by map-based cloning. It encodes a novel 684 amino acid polypeptide containing a single membrane-spanning domain in the N-terminal part and S-adenosyl-l-methionine binding and methyltransferase domains in the C-terminal part. Expression of a TSD2:GUS reporter gene was detected mainly in meristems and young tissues. A green fluorescent protein-tagged TSD2 protein localized to the Golgi apparatus. The cell-adhesion defects indicated altered pectin properties, and we hypothesize that TSD2 acts as a pectin methyltransferase. However, analyses of the cell-wall composition revealed no significant differences of the monosaccharide composition, the uronic acid content and the overall degree of pectin methylesterification between tsd2 and WT. The findings support a function of TSD2 as a methyltransferase, with an essential role in cell adhesion and coordinated plant development.

Identification and characterization of a host reversibly glycosylated peptide that interacts with the Tomato leaf curl virus V1 protein

Monopartite geminiviruses of the genus Begomovirus have two virion-sense genes, V1 and V2. V2 encodes the viral coat protein, but the function of V1 is largely unknown, although some studies suggest that it may play a role in cell-to-cell movement. Yeast two-hybrid technology was used to identify possible host binding partners of V1 from Tomato leaf curl virus (TLCV) to better understand its function. A protein closely related to a family of plant reversibly glycosylated peptides, designated SlUPTG1, was found to interact with V1 in yeast and in vitro. SlUPTG1 may function endogenously in the synthesis of cell wall polysaccharides, since a bacterially expressed form of the protein acted as an autocatalytic glycosyltransferase in vitro, a SlUPTG1:GFP fusion protein localized to the cell wall, and expression of SlUPTG1 appeared to be highest in actively dividing tissues. However, expression of SlUPTG1 in a transient TLCV replication assay increased the accumulation of viral DNA, suggesting that this host factor also plays a role in viral infection. Together, these data provide new insight into the role of V1 in TLCV infection and reveal another host pathway which geminiviruses may manipulate to achieve an efficient infection.

The reb1-1 mutation of Arabidopsis. Effect on the structure and localization of galactose-containing cell wall polysaccharides

The Arabidopsis (Arabidopsis thaliana) root epidermal bulger1-1 (reb1-1) mutant (allelic to root hair defective1 [rhd1]) is characterized by a reduced root elongation rate and by bulging of trichoblast cells. The REB1/RHD1 gene belongs to a family of UDP-D-Glucose 4-epimerases involved in the synthesis of D-Galactose (Gal). Our previous study showed that certain arabinogalactan protein epitopes were not expressed in bulging trichoblasts of the mutant. In this study, using a combination of microscopical and biochemical methods, we have investigated the occurrence and the structure of three major Gal-containing polysaccharides, namely, xyloglucan (XyG), rhamnogalacturonan (RG)-I, and RG-II in the mutant root cell walls. Our immunocytochemical data show that swollen trichoblasts were not stained with the monoclonal antibody CCRC-M1 specific for alpha-L-Fucp-(1-->2)-beta-D-Galp side chains of XyG, whereas they were stained with anti-XyG antibodies specific for XyG backbone. In addition, analysis of a hemicellulosic fraction from roots demonstrates the presence of two structurally different XyGs in reb1-1. One is structurally similar to wild-type XyG and the other is devoid of fuco-galactosylated side chains and has the characteristic of being insoluble. Similar to anti-XyG antibodies, anti-bupleuran 2IIC, a polyclonal antibody specific for galactosyl epitopes associated with pectins, stained all root epidermal cells of both wild type and reb1-1. Similarly, anti-RG-II antibodies also stained swollen trichoblasts in the mutant. In addition, structural analysis of pectic polymers revealed no change in the galactosylation of RG-I and RG-II isolated from reb1-1 root cells. These findings demonstrate that the reb1-1 mutation affects XyG structure, but not that of pectic polysaccharides, thus lending support to the hypothesis that biosynthesis of Gal as well as galactosylation of complex polysaccharides is regulated at the polymer level.

Loss of pectin is an early event during infection of cocoyam roots by Pythium myriotylum

Cocoyam (Xanthosoma sagittifolium) is an important tuber crop in most tropical zones of Africa and America. In Cameroon, its cultivation is hampered by a soil-borne fungus Pythium myriotylum which is responsible for root rot disease. The mechanism of root colonisation by the fungus has yet to be elucidated. In this study, using microscopical and immunocytochemical methods, we provide a new evidence regarding the mode of action of the fungus and we describe the reaction of the plant to the early stages of fungal invasion. We show that the fungal attack begins with the colonisation of the peripheral and epidermal cells of the root apex. These cells are rapidly lost upon infection, while cortical and stele cells are not. Labelling with the cationic gold, which binds to negatively charged wall polymers such as pectins, is absent in cortical cells and in the interfacial zone of the infected roots while it is abundant in the cell walls of stele cells. A similar pattern of labelling is also found when using the anti-pectin monoclonal antibody JIM5, but not with anti-xyloglucan antibodies. This suggests that early during infection, the fungus causes a significant loss of pectin probably via degradation by hydrolytic enzymes that diffuse and act away from the site of attack. Additional support for pectin loss is the demonstration, via sugar analysis, that a significant decrease in galacturonic acid content occurred in infected root cell walls. In addition, we demonstrate that one of the early reactions of X. sagittifolium to the fungal invasion is the formation of wall appositions that are rich in callose and cellulose.

Differential expression profiles of growth-related genes in the elongation zone of maize primary roots

Growth in the apical elongation zone of plant roots is central to the development of functional root systems. Rates of root segmental elongation change from accelerating to decelerating as cell development proceeds from newly formed to fully elongated status. One of the primary variables regulating these changes in elongation rates is the extensibility of the elongating cell walls. To help decipher the complex molecular mechanisms involved in spatially variable root growth, we performed a gene identification study along primary root tips of maize (Zea mays) seedlings using suppression subtractive hybridization (SSH) and candidate gene approaches. Using SSH we isolated 150 non-redundant cDNA clones representing root growth-related genes (RGGs) that were preferentially expressed in the elongation zone. Differential expression patterns were revealed by Northern blot analysis for 41 of the identified genes and several candidate genes. Many of the genes have not been previously reported to be involved in root growth processes in maize. Genes were classified into groups based on the predicted function of the encoded proteins: cell wall metabolism, cytoskeleton, general metabolism, signaling and unknown. In-situ hybridization performed for two selected genes, confirmed the spatial distribution of expression shown by Northern blots and revealed subtle differences in tissue localization. Interestingly, spatial profiles of expression for some cell wall related genes appeared to correlate with the profile of accelerating root elongation and changed appropriately under growth-inhibitory water deficit.

The irregular xylem 2 mutant is an allele of korrigan that affects the secondary cell wall of Arabidopsis thaliana

The irregular xylem 2 (irx2) mutant of Arabidopsis thaliana exhibits a cellulose deficiency in the secondary cell wall, which is brought about by a point mutation in the KORRIGAN (KOR) beta,1-4 endoglucanase (beta,1-4 EGase) gene. Measurement of the total crystalline cellulose in the inflorescence stem indicates that the irx2 mutant contains approximately 30% of the level present in the wild type (WT). Fourier-Transform Infra Red (FTIR) analysis, however, indicates that there is no decrease in cellulose in primary cell walls of the cortical and epidermal cells of the stem. KOR expression is correlated with cellulose synthesis and is highly expressed in cells synthesising a secondary cell wall. Co-precipitation experiments, using either an epitope-tagged form of KOR or IRX3 (AtCesA7), suggest that KOR is not an integral part of the cellulose synthase complex. These data are supported by immunolocalisation of KOR that suggests that KOR does not localise to sites of secondary cell wall deposition in the developing xylem. The defect in irx2 plant is consistent with a role for KOR in the later stages of secondary cell wall formation, suggesting a role in processing of the growing microfibrils or release of the cellulose synthase complex.

Sub-cellular trafficking of phytochemicals explored using auto-fluorescent compounds in maize cells

BACKGROUND

Little is known regarding the trafficking mechanisms of small molecules within plant cells. It remains to be established whether phytochemicals are transported by pathways similar to those used by proteins, or whether the expansion of metabolic pathways in plants was associated with the evolution of novel trafficking pathways. In this paper, we exploited the induction of green and yellow auto-fluorescent compounds in maize cultured cells by the P1 transcription factor to investigate their targeting to the cell wall and vacuole, respectively.

RESULTS

We investigated the accumulation and sub-cellular localization of the green and yellow auto-fluorescent compounds in maize BMS cells expressing the P1 transcription factor from an estradiol inducible promoter. We established that the yellow fluorescent compounds accumulate inside the vacuole in YFBs that resemble AVIs. The green fluorescent compounds accumulate initially in the cytoplasm in large spherical GFBs. Cells accumulating GFBs also contain electron-dense structures that accumulate initially in the ER and which later appear to fuse with the plasma membrane. Structures resembling the GFBs were also observed in the periplasmic space of plasmolized cells. Ultimately, the green fluorescence accumulates in the cell wall, in a process that is insensitive to the Golgi-disturbing agents BFA and monensin.

CONCLUSIONS

Our results suggest the presence of at least two distinct trafficking pathways, one to the cell wall and the other to the vacuole, for different auto-fluorescent compounds induced by the same transcription factor in maize BMS cells. These compartments represent two of the major sites of accumulation of phenolic compounds characteristic of maize cells. The secretion of the green auto-fluorescent compounds occurs by a pathway that does not involve the TGN, suggesting that it is different from the secretion of most proteins, polysaccharides or epicuticular waxes. The yellow auto-fluorescent compounds accumulate in a vacuolar compartment, in structures that resemble the AVIs present in many cells accumulating anthocyanins. Together, our studies suggest that the accumulation of auto-fluorescent compounds can provide a powerful tool to dissect the trafficking of phytochemicals, knowledge necessary for the efficient engineering of plant metabolism.

Expression and characterization of an anti-(hepatitis B surface antigen) glycosylated mouse antibody in transgenic tobacco (Nicotiana tabacum) plants and its use in the immunopurification of its target antigen

Transgenic plants expressing recombinant immunoglobulins have arisen as an alternative technology for the large-scale production of antibodies useful in therapeutics and in industrial processes. In the present paper we report the expression in transgenic tobacco ( Nicotiana tabacum ) of an anti-HBsAg [anti-(hepatitis B virus surface antigen)] mouse IgG1 mAb (monoclonal antibody), currently used for the industrial purification of the recombinant vaccine antigen. Using the sweet potato sporamin signal peptide, a KDEL (Lys-Asp-Glu-Leu) ER (endoplasmic reticulum) anchorage domain, and a heavy- and light-chain gene tandem construction, we generated F1 plants in which the expression of the antibody accounted for 0.5% of the total soluble proteins. The 'plantibody' (functional IgG antibody produced in plants) was easily purified by Protein A-Sepharose chromatography with a yield of approximately 35 microg/g of fresh leaf material, and its glycosylation indicated that, irrespective of the KDEL signal, the molecule is modified in both the ER and Golgi. Finally, a successful comparison of the plantibody with the ascites-derived mAb in the immunoaffinity purification of the vaccine recombinant HBsAg was performed. Taken as a whole, our results show that the large-scale production of this antibody of industrial relevance in transgenic tobacco is feasible.

An improved chemical fixation method suitable for immunogold localization of green fluorescent protein in the Golgi apparatus of tobacco Bright Yellow (BY-2) cells

In plant systems, the green fluorescent protein (GFP) is increasingly used as a marker to study dynamics of the secretory apparatus using fluorescence microscopy. The purpose of this study was to immunogold localize the GFP, at the electron microscopic level, in a line of tobacco BY-2-cultured cells, expressing a GFP-tagged Golgi glycosyltransferase. To this end we have developed a simple, one-step chemical fixation method that allow good structural preservation and specific labeling with anti-GFP antibodies. Using this method, we have been able to show that an N-glycan GFP-tagged xylosyltransferase is specifically associated with Golgi stacks of BY-2 transformed cells and is preferentially located in medial cisternae. As an alternative to cryofixation methods, such as high-pressure freezing, which requires specialized and expensive equipment not available in most laboratories, this method offers researchers the opportunity to investigate GFP-tagged proteins of the endomembrane system in tobacco BY-2 cells.

Grapevine fanleaf virus replication occurs on endoplasmic reticulum-derived membranes

Infection by Grapevine fanleaf nepovirus (GFLV), a bipartite RNA virus of positive polarity belonging to the Comoviridae family, causes extensive cytopathic modifications of the host endomembrane system that eventually culminate in the formation of a perinuclear "viral compartment." We identified by immunoconfocal microscopy this compartment as the site of virus replication since it contained the RNA1-encoded proteins necessary for replication, newly synthesized viral RNA, and double-stranded replicative forms. In addition, by using transgenic T-BY2 protoplasts expressing green fluorescent protein in the endoplasmic reticulum (ER) or in the Golgi apparatus (GA), we could directly show that GFLV replication induced a depletion of the cortical ER, together with a condensation and redistribution of ER-derived membranes, to generate the viral compartment. Brefeldin A, a drug known to inhibit vesicle trafficking between the GA and the ER, was found to inhibit GFLV replication. Cerulenin, a drug inhibiting de novo synthesis of phospholipids, also inhibited GFLV replication. These observations imply that GFLV replication depends both on ER-derived membrane recruitment and on de novo lipid synthesis. In contrast to proteins involved in viral replication, the 2B movement protein and, to a lesser extent, the 2C coat protein were not confined to the viral compartment but were transported toward the cell periphery, a finding consistent with their role in cell-to-cell movement of virus particles.

Glucosylation activity and complex formation of two classes of reversibly glycosylated polypeptides

Reversibly glycosylated polypeptides (RGPs) have been implicated in polysaccharide biosynthesis. In plants, these proteins may function, for example, in cell wall synthesis and/or in synthesis of starch. We have isolated wheat (Triticum aestivum) and rice (Oryza sativa) Rgp cDNA clones to study the function of RGPs. Sequence comparisons showed the existence of two classes of RGP proteins, designated RGP1 and RGP2. Glucosylation activity of RGP1 and RGP2 from wheat and rice was studied. After separate expression of Rgp1 and Rgp2 in Escherichia coli or yeast (Saccharomyces cerevisiae), only RGP1 showed self-glucosylation. In Superose 12 fractions from wheat endosperm extract, a polypeptide with a molecular mass of about 40 kD is glucosylated by UDP-glucose. Transgenic tobacco (Nicotiana tabacum) plants, overexpressing either wheat Rgp1 or Rgp2, were generated. Subsequent glucosylation assays revealed that in RGP1-containing tobacco extracts as well as in RGP2-containing tobacco extracts UDP-glucose is incorporated, indicating that an RGP2-containing complex is active. Gel filtration experiments with wheat endosperm extracts and extracts from transgenic tobacco plants, overexpressing either wheat Rgp1 or Rgp2, showed the presence of RGP1 and RGP2 in high-molecular mass complexes. Yeast two-hybrid studies indicated that RGP1 and RGP2 form homo- and heterodimers. Screening of a cDNA library using the yeast two-hybrid system and purification of the complex by an antibody affinity column did not reveal the presence of other proteins in the RGP complexes. Taken together, these results suggest the presence of active RGP1 and RGP2 homo- and heteromultimers in wheat endosperm.

Isolation and characterization of mutants defective in seed coat mucilage secretory cell development in Arabidopsis

In Arabidopsis, fertilization induces the epidermal cells of the outer ovule integument to differentiate into a specialized seed coat cell type producing extracellular pectinaceous mucilage and a volcano-shaped secondary cell wall. Differentiation involves a regulated series of cytological events including growth, cytoplasmic rearrangement, mucilage synthesis, and secondary cell wall production. We have tested the potential of Arabidopsis seed coat epidermal cells as a model system for the genetic analysis of these processes. A screen for mutants defective in seed mucilage identified five novel genes (MUCILAGE-MODIFIED [MUM]1-5). The seed coat development of these mutants, and that of three previously identified ones (TRANSPARENT TESTA GLABRA1, GLABRA2, and APETALA2) were characterized. Our results show that the genes identified define several events in seed coat differentiation. Although APETALA2 is needed for differentiation of both outer layers of the seed coat, TRANSPARENT TESTA GLABRA1, GLABRA2, and MUM4 are required for complete mucilage synthesis and cytoplasmic rearrangement. MUM3 and MUM5 may be involved in the regulation of mucilage composition, whereas MUM1 and MUM2 appear to play novel roles in post-synthesis cell wall modifications necessary for mucilage extrusion.

A murine monoclonal antibody produced in transgenic plants with plant-specific glycans is not immunogenic in mice

Previous studies have shown that the production of recombinant antibodies in plants is highly efficient and presents numerous therapeutic applications. It is, however, known that plant glycoproteins display different glycosylation patterns to those exhibited by mammalian glycoproteins. Thus, it is important to know if these plant recombinant antibodies could induce undesirable immune responses in mammals; and to date no report has documented the potential immunogenicity of parenterally administered plant recombinant antibodies in animals. In order to answer this question, mice were immunised subcutaneously with a recombinant mouse monoclonal antibody produced in tobacco plants, together with alum as adjuvant. Two control groups were immunised in the same way with either the original murine monoclonal antibody or horseradish peroxidase (a plant glycoprotein). Analyses by direct immunoassay, competition immunoassay and real-time surface plasmon resonance, showed undetectable levels of antibody directed against both the protein and the glycan part of the plant recombinant antibody. These results have a direct relevance for the application of plant recombinant proteins as therapeutic agents and vaccines in humans.

Differentiation of mucilage secretory cells of the Arabidopsis seed coat

In some plant species, including Arabidopsis, fertilization induces the epidermal cells of the outer ovule integument to differentiate into a specialized seed coat cell type with a unique morphology and containing large quantities of polysaccharide mucilage (pectin). Such seed coat mucilage cells are necessary for neither viability nor germination under normal laboratory conditions. Thus, the Arabidopsis seed coat offers a unique system with which to use genetics to identify genes controlling cell morphogenesis and complex polysaccharide biosynthesis and secretion. As a first step in the application of this system, we have used microscopy to investigate the structure and differentiation of Arabidopsis seed coat mucilage cells, including cell morphogenesis and the synthesis, secretion, and extrusion of mucilage. During seed coat development in Arabidopsis, the epidermal cells of the outer ovule integument grow and differentiate into cells that produce large quantities of mucilage between the primary cell wall and plasma membrane. Concurrent with mucilage production, the cytoplasm is shaped into a column in the center of the cell. Following mucilage secretion the cytoplasmic column is surrounded by a secondary cell wall to form a structure known as the columella. Thus, differentiation of the seed coat mucilage cells involves a highly regulated series of events including growth, morphogenesis, mucilage biosynthesis and secretion, and secondary cell wall synthesis.

Xyloglucan fucosyltransferase, an enzyme involved in plant cell wall biosynthesis

Cell walls are crucial for development, signal transduction, and disease resistance in plants. Cell walls are made of cellulose, hemicelluloses, and pectins. Xyloglucan (XG), the principal load-bearing hemicellulose of dicotyledonous plants, has a terminal fucosyl residue. A 60-kilodalton fucosyltransferase (FTase) that adds this residue was purified from pea epicotyls. Peptide sequence information from the pea FTase allowed the cloning of a homologous gene, AtFT1, from Arabidopsis. Antibodies raised against recombinant AtFTase immunoprecipitate FTase enzyme activity from solubilized Arabidopsis membrane proteins, and AtFT1 expressed in mammalian COS cells results in the presence of XG FTase activity in these cells.