A century journey of organelles research in the plant endomembrane system

We are entering an exciting century in the study of the plant organelles in the endomembrane system. Over the past century, especially within the past 50 years, tremendous advancements have been made in the complex plant cell to generate a much clearer and informative picture of plant organelles, including the molecular/morphological features, dynamic/spatial behavior, and physiological functions. Importantly, all these discoveries and achievements in the identification and characterization of organelles in the endomembrane system would not have been possible without: (1) the innovations and timely applications of various state-of-art cell biology tools and technologies for organelle biology research; (2) the continuous efforts in developing and characterizing new organelle markers by the plant biology community; and (3) the landmark studies on the identification and characterization of the elusive organelles. While molecular aspects and results for individual organelles have been extensively reviewed, the development of the techniques for organelle research in plant cell biology is less appreciated. As one of the ASPB Centennial Reviews on "organelle biology," here we aim to take a journey across a century of organelle biology research in plants by highlighting the important tools (or landmark technologies) and key scientists that contributed to visualize organelles. We then highlight the landmark studies leading to the identification and characterization of individual organelles in the plant endomembrane systems.

Fluid-phase and membrane markers reveal spatio-temporal dynamics of membrane traffic and repair in the green alga Chara australis

We investigated the mechanisms and the spatio-temporal dynamics of fluid-phase and membrane internalization in the green alga Chara australis using fluorescent hydrazides markers alone, or in conjunction with styryl dyes. Using live-cell imaging, immunofluorescence and inhibitor studies we revealed that both fluid-phase and membrane dyes were actively taken up into the cytoplasm by clathrin-mediated endocytosis and stained various classes of endosomes including brefeldin A- and wortmannin-sensitive organelles (trans-Golgi network and multivesicular bodies). Uptake of fluorescent hydrazides was poorly sensitive to cytochalasin D, suggesting that actin plays a minor role in constitutive endocytosis in Chara internodal cells. Sequential pulse-labelling experiments revealed novel aspects of the temporal progression of endosomes in Chara internodal cells. The internalized fluid-phase marker distributed to early compartments within 10 min from dye exposure and after about 30 min, it was found almost exclusively in late endocytic compartments. Notably, fluid cargo consecutively internalized at time intervals of more than 1h, was not targeted to the same vesicular structures, but was sorted into distinct late compartments. We further found that fluorescent hydrazide dyes distributed not only to rapidly recycling endosomes but also to long-lived compartments that participated in plasma membrane repair after local laser injury. Our approach highlights the benefits of combining different fluid-phase markers in conjunction with membrane dyes in simultaneous and sequential application modus for investigating vesicle traffic, especially in organisms, which are still refractory to genetic transformation like characean algae.

Inhibition of endosomal trafficking by brefeldin A interferes with long-distance interaction between chloroplasts and plasma membrane transporters

The huge internodal cells of the characean green algae are a convenient model to study long-range interactions between organelles via cytoplasmic streaming. It has been shown previously that photometabolites and reactive oxygen species released by illuminated chloroplasts are transmitted to remote shaded regions where they interfere with photosynthetic electron transport and the differential activity of plasma membrane transporters, and recent findings indicated the involvement of organelle trafficking pathways. In the present study, we applied pulse amplitude-modulated microscopy and pH-sensitive electrodes to study the effect of brefeldin A (BFA), an inhibitor of vesicle trafficking, on long-distance interactions in Chara australis internodal cells. These data were compared with BFA-induced changes in organelle number, size and distribution using fluorescent dyes and confocal laser scanning microscopy. We found that BFA completely and immediately inhibited endocytosis in internodal cells and induced the aggregation of organelles into BFA compartments within 30-120 min of treatment. The comparison with the physiological data suggests that the early response, the arrest of endocytosis, is related to the attenuation of differences in surface pH, whereas the longer lasting formation of BFA compartments is probably responsible for the acceleration of the cyclosis-mediated interaction between chloroplasts. These data indicate that intracellular turnover of membrane material might be important for the circulation of electric currents between functionally distinct regions in illuminated characean internodes and that translational movement of metabolites is delayed by transient binding of the transported substances to organelles.

N-Linked Glycosylation Modulates Golgi-Independent Vacuolar Sorting Mediated by the Plant Specific Insert

In plant cells, the conventional route to the vacuole involves the endoplasmic reticulum, the Golgi and the prevacuolar compartment. However, over the years, unconventional sorting to the vacuole, bypassing the Golgi, has been described, which is the case of the Plant-Specific Insert (PSI) of the aspartic proteinase cardosin A. Interestingly, this Golgi-bypass ability is not a characteristic shared by all PSIs, since two related PSIs showed to have different sensitivity to ER-to-Golgi blockage. Given the high sequence similarity between the PSI domains, we sought to depict the differences in terms of post-translational modifications. In fact, one feature that draws our attention is that one is N-glycosylated and the other one is not. Using site-directed mutagenesis to obtain mutated versions of the two PSIs, with and without the glycosylation motif, we observed that altering the glycosylation pattern interferes with the trafficking of the protein as the non-glycosylated PSI-B, unlike its native glycosylated form, is able to bypass ER-to-Golgi blockage and accumulate in the vacuole. This is also true when the PSI domain is analyzed in the context of the full-length cardosin. Regardless of opening exciting research gaps, the results obtained so far need a more comprehensive study of the mechanisms behind this unconventional direct sorting to the vacuole.

Subcellular Localization Screening of Colletotrichum higginsianum Effector Candidates Identifies Fungal Proteins Targeted to Plant Peroxisomes, Golgi Bodies, and Microtubules

The genome of the hemibiotrophic anthracnose fungus, Colletotrichum higginsianum, encodes a large inventory of putative secreted effector proteins that are sequentially expressed at different stages of plant infection, namely appressorium-mediated penetration, biotrophy and necrotrophy. However, the destinations to which these proteins are addressed inside plant cells are unknown. In the present study, we selected 61 putative effector genes that are highly induced in appressoria and/or biotrophic hyphae. We then used Agrobacterium-mediated transformation to transiently express them as N-terminal fusions with fluorescent proteins in cells of Nicotiana benthamiana for imaging by confocal microscopy. Plant compartments labeled by the fusion proteins in N. benthamiana were validated by co-localization with specific organelle markers, by transient expression of the proteins in the true host plant, Arabidopsis thaliana, and by transmission electron microscopy-immunogold labeling. Among those proteins for which specific subcellular localizations could be verified, nine were imported into plant nuclei, three were imported into the matrix of peroxisomes, three decorated cortical microtubule arrays and one labeled Golgi stacks. Two peroxisome-targeted proteins harbored canonical C-terminal tripeptide signals for peroxisome import via the PTS1 (peroxisomal targeting signal 1) pathway, and we showed that these signals are essential for their peroxisome localization. Our findings provide valuable information about which host processes are potentially manipulated by this pathogen, and also reveal plant peroxisomes, microtubules, and Golgi as novel targets for fungal effectors.

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.

A pulse-chase strategy combining click-EdU and photoconvertible fluorescent reporter: tracking Golgi protein dynamics during the cell cycle

Imaging or quantifying protein synthesis in cellulo through a well-resolved analysis of the cell cycle (also defining G1 subcompartments) is a methodological challenge. Click chemistry is the method of choice to reveal the thymidine analogue 5-ethynyl-2'-deoxyuridine (EdU) and track proliferating nuclei undergoing DNA synthesis. However, the click reaction quenches fluorescent proteins. Our challenge was to reconcile these two tools. A robust protocol based on a high-resolution cytometric cell cycle analysis in tobacco (Nicotiana tabacum) BY2 cells expressing fluorescent Golgi markers has been established. This was broadly applicable to tissues, cell clusters, and other eukaryotic material, and compatible with Scale clearing. EdU was then used with the photoconvertible protein sialyl transferase (ST)-Kaede as a Golgi marker in a photoconversion pulse-chase cytometric configuration resolving, in addition, subcompartments of G1. Quantitative restoration of protein fluorescence was achieved by introducing acidic EDTA washes to strip the copper from these proteins which were then imaged at neutral pH. The rate of synthesis of this Golgi membrane marker was low during early G1, but in the second half of G1 (30% of cycle duration) much of the synthesis occurred. Marker synthesis then persisted during S and G2. These insights into Golgi biology are discussed in terms of the cell's ability to adapt exocytosis to cell growth needs.

ARF-GTPase activating protein mediates auxin influx carrier AUX1 early endosome trafficking to regulate auxin dependent plant development

Polar auxin transport (PAT) plays a critical role in the regulation of plant growth and development. Auxin influx carrier AUX1 is predominantly localized to the upper side of specific root cells in Arabidopsis. Overexpression of OsAGAP, an ARF-GTPase activating protein in rice, could induce the accumulation of AUX1. But the mechanism is poorly known. Here we reported that over-expression of ARF-GAP could reduce the thickness and bundling of microfilament (MF) which possibly could greatly interfere with the endocytosis of AUX1 early endosome; but not the exocytosis of AUX1 recycling endosome. Therefore, AFR-GAP over-expression suppressed-MF bundling is likely involved in regulating endocytosis of Auxin influx carrier AUX1 and in mediating auxin dependent plant development.    

Sphingolipids involvement in plant endomembrane differentiation: the BY2 case

Sphingolipids play an essential role in the functioning of the secretory pathway in eukaryotic organisms. Their importance in the functional organization of plant cells has not been studied in any detail before. The sphingolipid synthesis inhibitor fumonisin B1 (FB1), a mycotoxin acting as a specific inhibitor of ceramide synthase, was tested for its effects on cell growth, cell polarity, cell shape, cell cycle and on the ultrastructure of BY2 cells. We used cell lines expressing different GFP-tagged markers for plant cell compartments, as well as a Golgi marker fused to the photoconvertible protein Kaede. Light and electron microscopy, combined with flow cytometry, were applied to analyse the morphodynamics and architecture of compartments of the secretory pathway. The results indicate that FB1 treatment had severe effects on cell growth and cell shape, and induced a delay in cell division processes. The cell changes were accompanied by the formation of the endoplasmic reticulum (ER)-derived tubular aggregates (FB1-induced compartments), together with an inhibition of cargo transport from the ER to the Golgi apparatus. A change in polar localization of the auxin transporter PIN1 was also observed, but endocytic processes were little affected. Electron microscopy studies confirmed that molecular FB1 targets were distinct from brefeldin A (BFA) targets. We propose that the reported effects of inhibition of ceramide biosynthesis reflect the importance of sphingolipids during cell growth and establishment of cell polarity in higher plant cells, notably through their contribution to the functional organization of the ER or its differentiation into distinct compartments.

Sequential depletion and acquisition of proteins during Golgi stack disassembly and reformation

Herein, we report the stepwise transport of multiple plant Golgi membrane markers during disassembly of the Golgi apparatus in tobacco leaf epidermal cells in response to the induced expression of the GTP-locked Sar1p or Brefeldin A (BFA), and reassembly on BFA washout. The distribution of fluorescent Golgi-resident N-glycan processing enzymes and matrix proteins (golgins) with specific cis-trans-Golgi sub-locations was followed by confocal microscopy during disassembly and reassembly. The first event during Golgi disassembly was the loss of trans-Golgi enzymes and golgins from Golgi membranes, followed by a sequential redistribution of medial and cis-Golgi enzymes into the endoplasmic reticulum (ER), whilst golgins were relocated to the ER or cytoplasm. This event was confirmed by fractionation and immuno-blotting. The sequential redistribution of Golgi components in a trans-cis sequence may highlight a novel retrograde trafficking pathway between the trans-Golgi and the ER in plants. Release of Golgi markers from the ER upon BFA washout occurred in the opposite sequence, with cis-matrix proteins labelling Golgi-like structures before cis/medial enzymes. Trans-enzyme location was preceded by trans-matrix proteins being recruited back to Golgi membranes. Our results show that Golgi disassembly and reassembly occur in a highly ordered fashion in plants.

Exploring plant endomembrane dynamics using the photoconvertible protein Kaede

Photoactivatable and photoconvertible fluorescent proteins capable of pronounced light-induced spectral changes are a powerful addition to the fluorescent protein toolbox of the cell biologist. They permit specific tracking of one subcellular structure (organelle or cell subdomain) within a differentially labelled population. They also enable pulse-chase analysis of protein traffic. The Kaede gene codes for a tetrameric protein found in the stony coral Trachyphyllia geoffroyi, which emits green fluorescence that irreversibly shifts to red following radiation with UV or violet light. We report here the use of Kaede to explore the plant secretory pathway. Kaede versions of the Golgi marker sialyl-transferase (ST-Kaede) and of the vacuolar pathway marker cardosin A (cardA-Kaede) were engineered. Several optical devices enabling photoconversion and observation of Kaede using these two constructs were assessed to optimize Kaede-based imaging protocols. Photoconverted ST-Kaede red-labelled organelles can be followed within neighbouring populations of non-converted green Golgi stacks, by their gradual development of orange/yellow coloration from de novo synthesis of Golgi proteins (green). Results highlight some aspects on the dynamics of the plant Golgi. For plant bio-imaging, the photoconvertible Kaede offers a powerful tool to track the dynamic behaviour of designated subpopulations of Golgi within living cells, while visualizing the de novo formation of proteins and structures, such as a Golgi stack.

BFA effects are tissue and not just plant specific

Brefeldin A (BFA) is one of the most popular drugs used by researchers for studies on secretion and endocytosis because it interferes with specific vesicle coat proteins via action on a guanine nucleotide exchange factor. Due to its range of morphological effects on the Golgi apparatus in a variety of plant tissues, we believe that there is more to the BFA response than the primary molecular targets so far identified.

The POK/AtVPS52 protein localizes to several distinct post-Golgi compartments in sporophytic and gametophytic cells

The organization and dynamics of the plant endomembrane system require both universal and plant-specific molecules and compartments. The latter, despite the growing wealth of information, remains poorly understood. From the study of an Arabidopsis thaliana male gametophytic mutant, it was possible to isolate a gene named POKY POLLEN TUBE (POK) essential for pollen tube tip growth. The similarity between the predicted POK protein sequence and yeast Vps52p, a subunit from the GARP/VFT complex which is involved in the docking of vesicles from the prevacuolar compartment to the Golgi apparatus, suggested that the POK protein plays a role in plant membrane trafficking. Genetic analysis of Arabidopsis mutants affecting AtVPS53 or AtVPS54 genes which encode putative POK partners shows a transmission defect through the male gametophyte for all lines, which is similar to the pok mutant. Using a combination of biochemical approaches and specific antiserum it has been demonstrated that the POK protein is present in phylogenetically divergent plant species, associated with membranes and belongs to a high molecular weight complex. Combination of immunolocalization studies and pharmacological approaches in different plant cells revealed that the POK protein associates with Golgi and post-Golgi compartments. The role of POK in post-Golgi endomembrane trafficking and as a member of a putative plant GARP/VFT complex is discussed.

Brefeldin A action and recovery in Chlamydomonas are rapid and involve fusion and fission of Golgi cisternae

CHLAMYDOMONAS NOCTIGAMA has a non-motile Golgi apparatus consisting of several Golgi stacks adjacent to transitional ER. These domains are characterized by vesicle-budding profiles and the lack of ribosomes on the side of the ER proximal to the Golgi stacks. Immunogold labelling confirms the presence of COPI-proteins at the periphery of the Golgi stacks, and COPII-proteins at the ER-Golgi interface. After addition of BFA (10 microg/ml) a marked increase in the number of vesicular profiles lying between the ER and the Golgi stacks is seen. Serial sections of cells do not provide any evidence for the existence of tubular connections between the ER and the Golgi stacks, supporting the notion that COPI- but not COPII-vesicle production is affected by BFA. The fusion of COPII-vesicles at the CIS-Golgi apparatus apparently requires the presence of retrograde COPI-vesicles. After 15 min the cisternae of neighbouring Golgi stacks begin to fuse forming "mega-Golgis", which gradually curl before fragmenting into clusters of vesicles and tubules. These are surrounded by the transitional ER on which vesicle-budding profiles are still occasionally visible. Golgi remnants continue to survive for several hours and do not completely disappear. Washing out BFA leads to a very rapid reassembly of Golgi cisternae. At first, clusters of vesicles are seen adjacent to transitional ER, then "mini Golgis" are seen whose cisternae grow in length and number to produce "mega Golgis". These structures then divide by vertical fission to produce Golgi stacks of normal size and morphology roughly 60 min after drug wash-out.

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.

Dynamic response of prevacuolar compartments to brefeldin a in plant cells

Little is known about the dynamics and molecular components of plant prevacuolar compartments (PVCs) in the secretory pathway. Using transgenic tobacco (Nicotiana tabacum) Bright-Yellow-2 (BY-2) cells expressing membrane-anchored yellow fluorescent protein (YFP) reporters marking Golgi or PVCs, we have recently demonstrated that PVCs are mobile multivesicular bodies defined by vacuolar sorting receptor proteins. Here, we demonstrate that Golgi and PVCs have different sensitivity in response to brefeldin A (BFA) treatment in living tobacco BY-2 cells. BFA at low concentrations (5-10 microg mL(-1)) induced YFP-marked Golgi stacks to form both endoplasmic reticulum-Golgi hybrid structures and BFA-induced aggregates, but had little effect on YFP-marked PVCs in transgenic BY-2 cells at both confocal and immunogold electron microscopy levels. However, BFA at high concentrations (50-100 microg mL(-1)) caused both YFP-marked Golgi stacks and PVCs to form aggregates in a dose- and time-dependent manner. Normal Golgi or PVC signals can be recovered upon removal of BFA from the culture media. Confocal immunofluorescence and immunogold electron microscopy studies with specific organelle markers further demonstrate that the PVC aggregates are distinct, but physically associated, with Golgi aggregates in BFA-treated cells and that PVCs might lose their internal vesicle structures at high BFA concentration. In addition, vacuolar sorting receptor-marked PVCs in root-tip cells of tobacco, pea (Pisum sativum), mung bean (Vigna radiata), and Arabidopsis (Arabidopsis thaliana) upon BFA treatment are also induced to form similar aggregates. Thus, we have demonstrated that the effects of BFA are not limited to endoplasmic reticulum and Golgi, but extend to PVC in the endomembrane system, which might provide a quick tool for distinguishing Golgi from PVC for its identification and characterization, as well as a possible new tool in studying PVC-mediated protein traffic in plant cells.

The plasma membrane recycling pathway and cell polarity in plants: studies on PIN proteins

The PIN-FORMED (PIN) proteins are plasma-membrane-associated facilitators of auxin transport. They are often targeted to one side of the cell only through subcellular mechanisms that remain largely unknown. Here, we have studied the potential roles of the cytoskeleton and endomembrane system in the localisation of PIN proteins. Immunocytochemistry and image analysis on root cells from Arabidopsis thaliana and maize showed that 10-30% of the intracellular PIN proteins mapped to the Golgi network, but never to prevacuolar compartments. The remaining 70-90% were associated with yet to be identified structures. The maintenance of PIN proteins at the plasma membrane depends on a BFA-sensitive machinery, but not on microtubules and actin filaments.

The polar localisation of PIN proteins at the plasmamembrane was not reflected by any asymmetric distribution of cytoplasmic organelles. In addition, PIN proteins were inserted in a symmetrical manner at both sides of the cell plate during cytokinesis. Together, the data indicate that the localisation of PIN proteins is a postmitotic event, which depends on local characteristics of the plasma membrane and its direct environment. In this context, we present evidence that microtubule arrays might define essential positional information for PIN localisation. This information seems to require the presence of an intact cell wall.

An Arabidopsis endo-1,4-beta-D-glucanase involved in cellulose synthesis undergoes regulated intracellular cycling

The synthesis of cellulose microfibrils requires the presence of a membrane-bound endo-1,4-beta-D-glucanase, KORRIGAN1 (KOR1). Although the exact biochemical role of KOR1 in cellulose synthesis is unknown, we used the protein as a marker to explore the potential involvement of subcellular transport processes in cellulose synthesis. Using immunofluorescence and a green fluorescent protein (GFP)-KOR1 fusion that complemented the phenotype conferred by the kor1-1 mutant, we investigated the distribution of KOR1 in epidermal cells in the root meristem. KOR1 was localized in intracellular compartments corresponding to a heterogeneous population of organelles, which comprised the Golgi apparatus, FM4-64-labeled compartments referred to as early endosomes, and, in the case of GFP-KOR1, the tonoplast. Inhibition of cellulose synthesis by isoxaben promoted a net redistribution of GFP-KOR1 toward a homogeneous population of compartments, distinct from early endosomes, which were concentrated close to the plasma membrane facing the root surface. A redistribution of GFP-KOR1 away from early endosomes was also observed in the same cells at later stages of cell elongation. A subpopulation of GFP-KOR1-containing compartments followed trajectories along the plasma membrane, and this motility required intact microtubules. These observations demonstrate that the deposition of cellulose, like chitin synthesis in yeast, involves the regulated intracellular cycling of at least one enzyme required for its synthesis.

The plant Golgi apparatus--going with the flow

The plant Golgi apparatus is composed of many separate stacks of cisternae which are often associated with the endoplasmic reticulum and which in many cell types are motile. In this review, we discuss the latest data on the molecular regulation of Golgi function. The concept of the Golgi as a distinct organelle is challenged and the possibility of a continuum between the endoplasmic reticulum and Golgi is proposed.

Cell biology of the plant Golgi apparatus

The higher plant Golgi apparatus, comprising many individual stacks of membrane bounded cisternae, is one of the most enigmatic of the cytoplasmic organelles. Not only can the stacks receive material from the endoplasmic reticulum, process it and target it to the correct cellular destination, but they can also synthesise and export complex carbohydrates and lipids and most likely act as one end point of the endocytic pathway. In many cells such processing and sorting can take place while the stacks are moving within the cytoplasm and, remarkably, the organelle manages to retain its structural integrity. This review considers some of the latest data and views on transport both to and from the Golgi and the mechanisms by which such activity is regulated.

Identification of multivesicular bodies as prevacuolar compartments in Nicotiana tabacum BY-2 cells

Little is known about the dynamics and molecular components of plant prevacuolar compartments (PVCs). We have demonstrated recently that vacuolar sorting receptor (VSR) proteins are concentrated on PVCs. In this study, we generated transgenic Nicotiana tabacum (tobacco) BY-2 cell lines expressing two yellow fluorescent protein (YFP)-fusion reporters that mark PVC and Golgi organelles. Both transgenic cell lines exhibited typical punctate YFP signals corresponding to distinct PVC and Golgi organelles because the PVC reporter colocalized with VSR proteins, whereas the Golgi marker colocalized with mannosidase I in confocal immunofluorescence. Brefeldin A induced the YFP-labeled Golgi stacks but not the YFP-marked PVCs to form typical enlarged structures. By contrast, wortmannin caused YFP-labeled PVCs but not YFP-labeled Golgi stacks to vacuolate. VSR antibodies labeled multivesicular bodies (MVBs) on thin sections prepared from high-pressure frozen/freeze substituted samples, and the enlarged PVCs also were indentified as MVBs. MVBs were further purified from BY-2 cells and found to contain VSR proteins via immunogold negative staining. Similar to YFP-labeled Golgi stacks, YFP-labeled PVCs are mobile organelles in BY-2 cells. Thus, we have unequivocally identified MVBs as PVCs in N. tabacum BY-2 cells. Uptake studies with the styryl dye FM4-64 strongly indicate that PVCs also lie on the endocytic pathway of BY-2 cells.

Morphodynamics of the secretory pathway

A careful scrutiny of the dynamics of secretory compartments in the entire eukaryotic world reveals many common themes. The most fundamental theme is that the Golgi apparatus and related structures appear as compartments formed by the act of transporting cargo. The second common theme is the pivotal importance for endomembrane dynamics of shifting back and forth the equilibrium between full and perforated cisternae along the pathway. The third theme is the role of a continuous membrane flow in anterograde transfer of molecules from the endoplasmic reticulum through the Golgi apparatus. The last common theme is the self-regulatory balance between anatomical continuities and discontinuities of the endomembrane system. As this balance depends on secretory activity, it provides a source of morphological variability among cell types or, for a given cell type, according to environmental conditions. Beyond this first source of variability, it appears that divergent strategies pave the evolutionary routes in different eukaryotic kingdoms. These divergent strategies primarily affect the levels of stacking, of stabilization, and of clustering of the Golgi apparatus. They presumably underscore a trade-off between versatility and stability to adapt the secretory function to the degree of environmental variability. Nonequilibrium secretory structures would provide yeasts, and plants to a lesser extent, with the required versatility to cope with ever changing environments, by contrast to the stabler milieu intérieur of homeothermic animals.

The Arabidopsis GNOM ARF-GEF mediates endosomal recycling, auxin transport, and auxin-dependent plant growth

Exchange factors for ARF GTPases (ARF-GEFs) regulate vesicle trafficking in a variety of organisms. The Arabidopsis protein GNOM is a brefeldin A (BFA) sensitive ARF-GEF that is required for the proper polar localization of PIN1, a candidate transporter of the plant hormone auxin. Mutations in GNOM lead to developmental defects that resemble those caused by interfering with auxin transport. Both PIN1 localization and auxin transport are also sensitive to BFA. In this paper, we show that GNOM localizes to endosomes and is required for their structural integrity. We engineered a BFA-resistant version of GNOM. In plants harboring this fully functional GNOM variant, PIN1 localization and auxin transport are no longer sensitive to BFA, while trafficking of other proteins is still affected by the drug. Our results demonstrate that GNOM is required for the recycling of auxin transport components and suggest that ARF-GEFs regulate specific endosomal trafficking pathways.

Regulation of pollen tube growth by Rac-like GTPases

Plant Rac-like GTPases have been classified phylogenetically into two major groups-class I and class II. Several pollen-expressed class I Rac-like GTPases have been shown to be important regulators of polar pollen tube growth. The functional participation by some of the class I and all of the class II Arabidopsis Rac-like GTPases in pollen tube growth remains to be explored. It is shown that at least four members of the Arabidopsis Rac GTPase family are expressed in pollen, including a class II Rac, AtRac7. However, when over-expressed as fusion proteins with GFP, both pollen- and non-pollen-expressed AtRacs interfered with the normal pollen tube tip growth process. These observations suggest that these AtRacs share similar biochemical activities and may integrate into the pollen cellular machinery that regulates the polar tube growth process. Therefore, the functional contribution by individual Rac GTPase to the pollen tube growth process probably depends to a considerable extent on their expression characteristics in pollen. Among the Arabidopsis Racs, GFP-AtRac7 showed association with the cell membrane and Golgi bodies, a pattern distinct from all previously reported localization for other plant Racs. Over-expressing GFP-AtRac7 also induced the broadest spectrum of pollen tube growth defects, including pollen tubes that are bifurcated, with diverted growth trajectory or a ballooned tip. Transgenic plants with multiple copies of the chimeric Lat52-GFP-AtRac7 showed severely reduced seed set, probably many of these defective pollen tubes were arrested, or reduced in their growth rates that they did not arrive at the ovules while they were still receptive for fertilization. These observations substantiate the importance of Rac-like GTPases to sexual reproduction.

F-actin-dependent endocytosis of cell wall pectins in meristematic root cells. Insights from brefeldin A-induced compartments

Brefeldin A (BFA) inhibits exocytosis but allows endocytosis, making it a valuable agent to identify molecules that recycle at cell peripheries. In plants, formation of large intracellular compartments in response to BFA treatment is a unique feature of some, but not all, cells. Here, we have analyzed assembly and distribution of BFA compartments in development- and tissue-specific contexts of growing maize (Zea mays) root apices. Surprisingly, these unique compartments formed only in meristematic cells of the root body. On the other hand, BFA compartments were absent from secretory cells of root cap periphery, metaxylem cells, and most elongating cells, all of which are active in exocytosis. We report that cell wall pectin epitopes counting rhamnogalacturonan II dimers cross-linked by borate diol diester, partially esterified (up to 40%) homogalacturonan pectins, and (1-->4)-beta-D-galactan side chains of rhamnogalacturonan I were internalized into BFA compartments. In contrast, Golgi-derived secretory (esterified up to 80%) homogalacturonan pectins localized to the cytoplasm in control cells and did not accumulate within characteristic BFA compartments. Latrunculin B-mediated depolymerization of F-actin inhibited internalization and accumulation of cell wall pectins within intracellular BFA compartments. Importantly, cold treatment and protoplasting prevented internalization of wall pectins into root cells upon BFA treatment. These observations suggest that cell wall pectins of meristematic maize root cells undergo rapid endocytosis in an F-actin-dependent manner.

BP-80 and homologs are concentrated on post-Golgi, probable lytic prevacuolar compartments

Prevacuolar compartments (PVCs) are membrane-bound organelles that mediate protein traffic between Golgi and vacuoles in the plant secretory pathway. Here we identify and define organelles as the lytic prevacuolar compartments in pea and tobacco cells using confocal immunofluorescence. We use five different antibodies specific for a vacuolar sorting receptor (VSR) BP-80 and its homologs to detect the location of VSR proteins. In addition, we use well-established Golgi-markers to identify Golgi organelles. We further compare VSR-labeled organelles to Golgi organelles so that the relative proportion of VSR proteins in Golgi vs. PVCs can be quantitated. More than 90% of the BP-80-marked organelles are separate from Golgi organelles; thus, BP-80 and its homologs are predominantly concentrated on the lytic PVCs. Additionally, organelles marked by anti-AtPep12p (AtSYP21p) and anti-AtELP antibodies are also largely separate from Golgi apparatus, whereas VSR and AtPep12p (AtSYP21p) were largely colocalized. We have thus demonstrated in plant cells that VSR proteins are predominantly present in the lytic PVCs and have provided additional markers for defining plant PVCs using confocal immunofluorescence. Additionally, our approach will provide a rapid comparison between markers to quantitate protein distribution among various organelles.

Effects of brefeldin A and nordihydroguaiaretic acid on endomembrane dynamics and lipid synthesis in plant cells

Effects of brefeldin A (BFA) and nordihydroguaiaretic acid (NDGA) on endomembrane structures and lipid synthesis were compared in maize root cells and tobacco Bright Yellow-2 cells. Immunofluorescence and electron microscopy studies showed that NDGA altered the structure and distribution of the endoplasmic reticulum (ER) within 1 h but not of the Golgi apparatus whereas, as shown previously, BFA altered that organization of the Golgi apparatus and, only subsequently, of the ER. Biochemical studies revealed that both drugs and especially BFA led to a strong inhibition of the phytosterol biosynthetic pathway: BFA led to accumulation of sterol precursors. The importance of phytosterols in membrane architecture and membrane trafficking is discussed.

Rab2 GTPase regulates vesicle trafficking between the endoplasmic reticulum and the Golgi bodies and is important to pollen tube growth

Pollen tube elongation depends on the secretion of large amounts of membrane and cell wall materials at the pollen tube tip to sustain rapid growth. A large family of RAS-related small GTPases, Rabs or Ypts, is known to regulate both anterograde and retrograde trafficking of transport vesicles between different endomembrane compartments and the plasma membrane in mammalian and yeast cells. Studies on the functional roles of analogous plant proteins are emerging. We report here that a tobacco pollen-predominant Rab2, NtRab2, functions in the secretory pathway between the endoplasmic reticulum and the Golgi in elongating pollen tubes. Green fluorescent protein-NtRab2 fusion protein localized to the Golgi bodies in elongating pollen tubes. Dominant-negative mutations in NtRab2 proteins inhibited their Golgi localization, blocked the delivery of Golgi-resident as well as plasmalemma and secreted proteins to their normal locations, and inhibited pollen tube growth. On the other hand, when green fluorescent protein-NtRab2 was over-expressed in transiently transformed leaf protoplasts and epidermal cells, in which NtRab2 mRNA have not been observed to accumulate to detectable levels, these proteins did not target efficiently to Golgi bodies. Together, these observations indicate that NtRab2 is important for trafficking between the endoplasmic reticulum and the Golgi bodies in pollen tubes and may be specialized to optimally support the high secretory demands in these tip growth cells.

Redistribution of membrane proteins between the Golgi apparatus and endoplasmic reticulum in plants is reversible and not dependent on cytoskeletal networks

We have fused the signal anchor sequences of a rat sialyl transferase and a human galactosyl transferase along with the Arabidopsis homologue of the yeast HDEL receptor (AtERD2) to the jellyfish green fluorescent protein (GFP) and transiently expressed the chimeric genes in tobacco leaves. All constructs targeted the Golgi apparatus and co-expression with DsRed fusions along with immunolabelling of stably transformed BY2 cells indicated that the fusion proteins located all Golgi stacks. Exposure of tissue to brefeldin A (BFA) resulted in the reversible redistribution of ST-GFP into the endoplasmic reticulum. This effect occurred in the presence of a protein synthesis inhibitor and also in the absence of microtubules or actin filaments. Likewise, reformation of Golgi stacks on removal of BFA was not dependent on either protein synthesis or the cytoskeleton. These data suggest that ER to Golgi transport in the cell types observed does not require cytoskeletal-based mechanochemical motor systems. However, expression of an inhibitory mutant of Arabidopsis Rab 1b (AtRab1b(N121I) significantly slowed down the recovery of Golgi fluorescence in BFA treated cells indicating a role for Rab1 in regulating ER to Golgi anterograde transport.

Cell polarity signaling in Arabidopsis involves a BFA-sensitive auxin influx pathway

Coordination of cell and tissue polarity commonly involves directional signaling. In the Arabidopsis root epidermis, cell polarity is revealed by basal, root tip-oriented, hair outgrowth from hair-forming cells (trichoblasts). The plant hormone auxin displays polar movements and accumulates at maximum concentration in the root tip. The application of polar auxin transport inhibitors evokes changes in trichoblast polarity only at high concentrations and after long-term application. Thus, it remains open whether components of the auxin transport machinery mediate establishment of trichoblast polarity. Here we report that the presumptive auxin influx carrier AUX1 contributes to apical-basal hair cell polarity. AUX1 function is required for polarity changes induced by exogenous application of the auxin 2,4-D, a preferential influx carrier substrate. Similar to aux1 mutants, the vesicle trafficking inhibitor brefeldin A (BFA) interferes with polar hair initiation, and AUX1 function is required for BFA-mediated polarity changes. Consistently, BFA inhibits membrane trafficking of AUX1, trichoblast hyperpolarization induced by 2,4-D, and alters the distal auxin maximum. Our results identify AUX1 as one component of a novel BFA-sensitive auxin transport pathway polarizing cells toward a hormone maximum.

Golgi secretion is not required for marking the preprophase band site in cultured tobacco cells

The preprophase band predicts the future cell division site. However, the mechanism of how a transient preprophase band fulfils this function is unknown. We have investigated the possibility that Golgi secretion might be involved in marking the preprophase band site. Observations on living BY-2 cells labeled for microtubules and Golgi stacks indicated an increased Golgi stack frequency at the preprophase band site. However, inhibition of Golgi secretion by brefeldin A during preprophase band formation did not prevent accurate phragmoplast fusion, and subsequent cell plate formation, at the preprophase band site. The results show that Golgi secretion does not mark the preprophase band site and thus does not play an active role in determination of the cell division site.

Functional characterization of the KNOLLE-interacting t-SNARE AtSNAP33 and its role in plant cytokinesis

Cytokinesis requires membrane fusion during cleavage-furrow ingression in animals and cell plate formation in plants. In Arabidopsis, the Sec1 homologue KEULE (KEU) and the cytokinesis-specific syntaxin KNOLLE (KN) cooperate to promote vesicle fusion in the cell division plane. Here, we characterize AtSNAP33, an Arabidopsis homologue of the t-SNARE SNAP25, that was identified as a KN interactor in a yeast two-hybrid screen. AtSNAP33 is a ubiquitously expressed membrane-associated protein that accumulated at the plasma membrane and during cell division colocalized with KN at the forming cell plate. A T-DNA insertion in the AtSNAP33 gene caused loss of AtSNAP33 function, resulting in a lethal dwarf phenotype. atsnap33 plantlets gradually developed large necrotic lesions on cotyledons and rosette leaves, resembling pathogen-induced cellular responses, and eventually died before flowering. In addition, mutant seedlings displayed cytokinetic defects, and atsnap33 in combination with the cytokinesis mutant keu was embryo lethal. Analysis of the Arabidopsis genome revealed two further SNAP25-like proteins that also interacted with KN in the yeast two-hybrid assay. Our results suggest that AtSNAP33, the first SNAP25 homologue characterized in plants, is involved in diverse membrane fusion processes, including cell plate formation, and that AtSNAP33 function in cytokinesis may be replaced partially by other SNAP25 homologues.

Auxin transport inhibitors block PIN1 cycling and vesicle trafficking

Polar transport of the phytohormone auxin mediates various processes in plant growth and development, such as apical dominance, tropisms, vascular patterning and axis formation. This view is based largely on the effects of polar auxin transport inhibitors. These compounds disrupt auxin efflux from the cell but their mode of action is unknown. It is thought that polar auxin flux is caused by the asymmetric distribution of efflux carriers acting at the plasma membrane. The polar localization of efflux carrier candidate PIN1 supports this model. Here we show that the seemingly static localization of PIN1 results from rapid actin-dependent cycling between the plasma membrane and endosomal compartments. Auxin transport inhibitors block PIN1 cycling and inhibit trafficking of membrane proteins that are unrelated to auxin transport. Our data suggest that PIN1 cycling is of central importance for auxin transport and that auxin transport inhibitors affect efflux by generally interfering with membrane-trafficking processes. In support of our conclusion, the vesicle-trafficking inhibitor brefeldin A mimics physiological effects of auxin transport inhibitors.

Novel type Arabidopsis thaliana H(+)-PPase is localized to the Golgi apparatus

Vacuolar H(+)-PPase, a membrane bound proton-translocating pyrophosphatase found in various species including plants, some protozoan and prokaryotes, has been demonstrated to be localized to the vacuolar membrane in plants. Using a GUS reporter system and a green fluorescent protein (GFP) fusion protein, we investigated the tissue distribution and the subcellular localization, respectively, of a novel type H(+)-PPase encoded by AVP2/AVPL1 identified in the Arabidopsis thaliana genome. We showed that AVP2/AVPL1 is highly expressed at the trichome and the filament of stamen. Furthermore, the fluorescence of GFP-tagged AVP2/AVPL1 showed small dot-like structures that were observed throughout the cytoplasm of various Arabidopsis cells under a fluorescent microscope. The distribution of this dot-like fluorescent pattern was apparently affected by a treatment with brefeldin A. Moreover, we demonstrated that most dot-like fluorescent structures colocalized with a Golgi resident protein. These findings suggest that this novel type H(+)-PPase resides on the Golgi apparatus rather than the vacuolar membrane.

Brefeldin A affects growth, endoplasmic reticulum, Golgi bodies, tubular vacuole system, and secretory pathway in Pisolithus tinctorius

Brefeldin A (BFA) reduced radial growth in Pisolithus tinctorius at a concentration as low as 2 microM. Use of endoplasmic reticulum (ER)-Tracker dye, unconjugated BFA, and fluorescent BFA (BODIPY-BFA) allowed comparison of the effects of BFA on the endomembrane system of P. tinctorius at the light and electron microscope levels. Both ER-Tracker dye and BODIPY-BFA have been shown previously to label the ER. Unconjugated BFA and BODIPY-BFA modified the ER network and disrupted the tubular vacuole system in the tip region. The ultrastructure in freeze-substituted hyphae showed that BFA treatment resulted in (i) disruption of the Spitzenkörper, (ii) reduction in number of apical vesicles, (iii) redistribution and mild dilation of ER, and (iv) persistence and increased size and complexity of Golgi bodies. The effects of BFA on the ER were only partially reversible in the time period examined. We conclude that in P. tinctorius, BFA as the free metabolite or BODIPY-BFA affects the tubular vacuole system as well as anterograde membrane flow between the ER and the Golgi bodies and post-Golgi transport.

Stop-and-go movements of plant Golgi stacks are mediated by the acto-myosin system

The Golgi apparatus in plant cells consists of a large number of independent Golgi stack/trans-Golgi network/Golgi matrix units that appear to be randomly distributed throughout the cytoplasm. To study the dynamic behavior of these Golgi units in living plant cells, we have cloned a cDNA from soybean (Glycine max), GmMan1, encoding the resident Golgi protein alpha-1,2 mannosidase I. The predicted protein of approximately 65 kD shows similarity of general structure and sequence (45% identity) to class I animal and fungal alpha-1,2 mannosidases. Expression of a GmMan1::green fluorescent protein fusion construct in tobacco (Nicotiana tabacum) Bright Yellow 2 suspension-cultured cells revealed the presence of several hundred to thousands of fluorescent spots. Immuno-electron microscopy demonstrates that these spots correspond to individual Golgi stacks and that the fusion protein is largely confined to the cis-side of the stacks. In living cells, the stacks carry out stop-and-go movements, oscillating rapidly between directed movement and random "wiggling." Directed movement (maximal velocity 4.2 microm/s) is related to cytoplasmic streaming, occurs along straight trajectories, and is dependent upon intact actin microfilaments and myosin motors, since treatment with cytochalasin D or butanedione monoxime blocks the streaming motion. In contrast, microtubule-disrupting drugs appear to have a small but reproducible stimulatory effect on streaming behavior. We present a model that postulates that the stop-and-go motion of Golgi-trans-Golgi network units is regulated by "stop signals" produced by endoplasmic reticulum export sites and locally expanding cell wall domains to optimize endoplasmic reticulum to Golgi and Golgi to cell wall trafficking.

Coordinated polar localization of auxin efflux carrier PIN1 by GNOM ARF GEF

The plant hormone auxin is transported in a polar manner along the shoot-root axis, which requires efflux carriers such as PIN1. Asymmetric localization of PIN1 develops from a random distribution in Arabidopsis early embryogenesis. Coordinated polar localization of PIN1 is defective in gnom embryos. GNOM is a membrane-associated guanine-nucleotide exchange factor on ADP-ribosylation factor G protein (ARF GEF). Thus, GNOM-dependent vesicle trafficking may establish cell polarity, resulting in polar auxin transport.

Biosynthesis and immunolocalization of Lewis a-containing N-glycans in the plant cell

We recently demonstrated the presence of a new asparagine-linked complex glycan on plant glycoproteins that harbors the Lewis a (Lea), or Galbeta(1-3)[Fucalpha(1-4)]GlcNAc, epitope, which in mammalian cells plays an important role in cell-to-cell recognition. Here we show that the monoclonal antibody JIM 84, which is widely used as a Golgi marker in light and electron microscopy of plant cells, is specific for the Lea antigen. This antigen is present on glycoproteins of a number of flowering and non-flowering plants, but is less apparent in the Cruciferae, the family that includes Arabidopsis. Lea-containing oligosaccharides are found in the Golgi apparatus, and our immunocytochemical experiments suggest that it is synthesized in the trans-most part of the Golgi apparatus. Lea epitopes are abundantly present on extracellular glycoproteins, either soluble or membrane bound, but are never observed on vacuolar glycoproteins. Double-labeling experiments suggest that vacuolar glycoproteins do not bypass the late Golgi compartments where Lea is built, and that the absence of the Lea epitope from vacuolar glycoproteins is probably the result of its degradation by glycosidases en route to or after arrival in the vacuole.

Membrane trafficking in higher plant cells: GFP and antibodies, partners for probing the secretory pathway

Eukaryotic cells are characterised by the organised distribution of membrane bounded compartments in their cytoplasm. The endoplasmic reticulum (ER) and the Golgi apparatus (GA) are part of this endomembrane machinery. They are involved in protein flow, and are in charge of specific functions such as the assembly, sorting and transport of newly synthesised proteins, glycoproteins or polysaccharides to their final destination, where the macromolecules are recognised either for action, storage, deposition or degradation. The structural and functional relationship between the ER and GA in higher plants is still a matter of debate. Therefore, it was essential to develop probes that would specifically label proteins or glycoproteins of the endomembrane system in situ. Here we compare two complementary approaches to probe plant endomembranes; immunocytochemistry on fixed cells, and in vivo studies using the expression of GFP tagged chimeric proteins. The structural relationship between ER and GA as based on pharmacological approaches using the two systems is explored.

ATP-Dependent formation of phosphatidylserine-rich vesicles from the endoplasmic reticulum of leek cells

Leek (Allium porrum) plasma membrane is enriched in phosphatidylserine (PS) by the vesicular pathway, in a way similar to that already observed in animal cells (B. Sturbois-Balcerzak, D.J. Morre, O. Loreau, J.P. Noel, P. Moreau, C. Cassagne [1995] Plant Physiol Biochem 33: 625-637). In this paper we document the formation of PS-rich small vesicles from leek endoplasmic reticulum (ER) membranes upon addition of ATP and other factors. The omission of ATP or its replacement by ATPgamma-S prevents vesicle formation. These vesicles correspond to small structures (70-80 nm) and their phospholipid composition, characterized by a PS enrichment, is compatible with a role in PS transport. Moreover, the PS enrichment over phosphatidylinositol in the ER-derived vesicles is the first example, to our knowledge, of phospholipid sorting from the ER to ER-derived vesicles in plant cells.

Tomato spotted wilt virus particle morphogenesis in plant cells

A model for the maturation of tomato spotted wilt virus (TSWV) particles is proposed, mainly based on results with a protoplast infection system, in which the chronology of different maturation events could be determined. By using specific monoclonal and polyclonal antisera in immunofluorescence and electron microscopy, the site of TSWV particle morphogenesis was determined to be the Golgi system. The viral glycoproteins G1 and G2 accumulate in the Golgi prior to a process of wrapping, by which the viral nucleocapsids obtain a double membrane. In a later stage of the maturation, these doubly enveloped particles fuse to each other and to the endoplasmic reticulum to form singly enveloped particles clustered in membranes. Similarities and differences between the maturation of animal-infecting (bunya)viruses and plant-infecting tospoviruses are discussed.

The plant Golgi apparatus

The plant Golgi apparatus has an important role in protein glycosylation and sorting, but is also a major biosynthetic organelle that synthesises large quantities of cell wall polysaccharides. This is reflected in the organisation of the Golgi apparatus as numerous individual stacks of cisternae that are dispersed through the cell. Each stack is polarised: the shape of the cisternae and the staining of the membranes change in a cis to trans direction, and the cisternae on the trans side contain more polysaccharides. Numerous glycosyltransferases are required for the synthesis of the complex cell wall polysaccharides. Microscopy and biochemical fractionation studies suggest that these enzymes are compartmentalised within the stack. Although there is no obvious cis Golgi network, the trans-most cisterna or trans Golgi network often buds clathrin-coated and sometimes smooth dense vesicles as well. Here, vacuolar proteins are sorted from the secreted proteins and polysaccharides. This review highlights unique aspects of the organisation and function of the plant Golgi apparatus. Fundamentally similar processes probably underlie Golgi organisation in all organisms, and consideration of the plant Golgi specialisations can therefore be generally informative, as well as being of central importance to plant cell biology.

Stacks on tracks: the plant Golgi apparatus traffics on an actin/ER network

We have visualized the relationship between the endoplasmic reticulum (ER) and Golgi in leaf cells of Nicotiana clevelandii by expression of two Golgi proteins fused to green fluorescent protein (GFP). A fusion of the transmembrane domain (signal anchor sequence) of a rat sialyl transferase to GFP was targeted to the Golgi stacks. A second construct that expressed the Arabidopsis H/KDEL receptor homologue aERD2, fused to GFP, was targeted to both the Golgi apparatus and ER, allowing the relationship between these two organelles to be studied in living cells for the first time. The Golgi stacks were shown to move rapidly and extensively along the polygonal cortical ER network of leaf epidermal cells, without departing from the ER tubules. Co-localization of F-actin in the GFP-expressing cells revealed an underlying actin cytoskeleton that matched precisely the architecture of the ER network, while treatment of cells with the inhibitors cytochalasin D and N-ethylmaleimide revealed the dependency of Golgi movement on actin cables. These observations suggest that the leaf Golgi complex functions as a motile system of actin-directed stacks whose function is to pick up products from a relatively stationary ER system. Also, we demonstrate for the first time in vivo brefeldin A-induced retrograde transport of Golgi membrane protein to the ER.

Auxin deprivation induces synchronous Golgi differentiation in suspension-cultured tobacco BY-2 cells

To date, the lack of a method for inducing plant cells and their Golgi stacks to differentiate in a synchronous manner has made it difficult to characterize the nature and extent of Golgi retailoring in biochemical terms. Here we report that auxin deprivation can be used to induce a uniform population of suspension-cultured tobacco (Nicotiana tabacum cv BY-2) cells to differentiate synchronously during a 4-d period. Upon removal of auxin, the cells stop dividing, undergo elongation, and differentiate in a manner that mimics the formation of slime-secreting epidermal and peripheral root-cap cells. The morphological changes to the Golgi apparatus include a proportional increase in the number of trans-Golgi cisternae, a switch to larger-sized secretory vesicles that bud from the trans-Golgi cisternae, and an increase in osmium staining of the secretory products. Biochemical alterations include an increase in large, fucosylated, mucin-type glycoproteins, changes in the types of secreted arabinogalactan proteins, and an increase in the amounts and types of molecules containing the peripheral root-cap-cell-specific epitope JIM 13. Taken together, these findings support the hypothesis that auxin deprivation can be used to induce tobacco BY-2 cells to differentiate synchronously into mucilage-secreting cells.

The Arabidopsis KNOLLE protein is a cytokinesis-specific syntaxin

In higher plant cytokinesis, plasma membrane and cell wall originate by vesicle fusion in the plane of cell division. The Arabidopsis KNOLLE gene, which is required for cytokinesis, encodes a protein related to vesicle-docking syntaxins. We have raised specific rabbit antiserum against purified recombinant KNOLLE protein to show biochemically and by immunoelectron microscopy that KNOLLE protein is membrane associated. Using immunofluorescence microscopy, KNOLLE protein was found to be specifically expressed during mitosis and, unlike the plasma membrane H+-ATPase, to localize to the plane of division during cytokinesis. Arabidopsis dynamin-like protein ADL1 accumulates at the plane of cell plate formation in knolle mutant cells as in wild-type cells, suggesting that cytokinetic vesicle traffic is not affected. Furthermore, electron microscopic analysis indicates that vesicle fusion is impaired. KNOLLE protein was detected in mitotically dividing cells of various parts of the developing plant, including seedling root, inflorescence meristem, floral meristems and ovules, and the cellularizing endosperm, but not during cytokinesis after the male second meiotic division. Thus, KNOLLE is the first syntaxin-like protein that appears to be involved specifically in cytokinetic vesicle fusion.