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

Localization of vacuolar transport receptors and cargo proteins in the Golgi apparatus of developing Arabidopsis embryos

Using immunogold electron microscopy, we have investigated the relative distribution of two types of vacuolar sorting receptors (VSR) and two different types of lumenal cargo proteins, which are potential ligands for these receptors in the secretory pathway of developing Arabidopsis embryos. Interestingly, both cargo proteins are deposited in the protein storage vacuole, which is the only vacuole present during the bent-cotyledon stage of embryo development. Cruciferin and aleurain do not share the same pattern of distribution in the Golgi apparatus. Cruciferin is mainly detected in the cis and medial cisternae, especially at the rims where storage proteins aggregate into dense vesicles (DVs). Aleurain is found throughout the Golgi stack, particularly in the trans cisternae and trans Golgi network where clathrin-coated vesicles (CCVs) are formed. Nevertheless, aleurain was detected in both DV and CCV. VSR-At1, a VSR that recognizes N-terminal vacuolar sorting determinants (VSDs) of the NPIR type, localizes mainly to the trans Golgi and is hardly detectable in DV. Receptor homology-transmembrane-RING H2 domain (RMR), a VSR that recognizes C-terminal VSDs, has a distribution that is very similar to that of cruciferin and is found in DV. Our results do not support a role for VSR-At1 in storage protein sorting, instead RMR proteins because of their distribution similar to that of cruciferin in the Golgi apparatus and their presence in DV are more likely candidates. Aleurain, which has an NPIR motif and seems to be primarily sorted via VSR-At1 into CCV, also possesses putative hydrophobic sorting determinants at its C-terminus that could allow the additional incorporation of this protein into DV.

Functional utrastructure of Genlisea (Lentibulariaceae) digestive hairs

BACKGROUND AND AIMS

Digestive structures of carnivorous plants produce external digestive enzymes, and play the main role in absorption. In Lentibulariaceae, the ultrastructure of digestive hairs has been examined in some detail in Pinguicula and Utricularia, but the sessile digestive hairs of Genlisea have received very little attention so far. The aim of this study was to fill this gap by expanding their morphological, anatomical and histochemical characterization.

METHODS

Several imaging techniques were used, including light, confocal and electron microscopy, to reveal the structure and function of the secretory hairs of Genlisea traps. This report demonstrates the application of cryo-SEM for fast imaging of whole, physically fixed plant secretory structures.

KEY RESULTS AND CONCLUSION

The concentration of digestive hairs along vascular bundles in subgenus Genlisea is a primitive feature, indicating its basal position within the genus. Digestive hairs of Genlisea consist of three compartments with different ultrastructure and function. In subgenus Tayloria the terminal hair cells are transfer cells, but not in species of subgenus Genlisea. A digestive pool of viscous fluid occurs in Genlisea traps. In spite of their similar architecture, the digestive-absorptive hairs of Lentibulariaceae feature differences in morphology and ultrastructure.

Intra-vacuolar reserves of membranes during stomatal closure: the possible role of guard cell vacuoles estimated by 3-D reconstruction

Stomatal apertures are regulated by morphological changes in guard cells which have been associated with guard cell vacuolar structures. To investigate the contribution of guard cell vacuoles to stomatal movement, we examined the dynamics of vacuolar membrane structures in guard cells and evaluated the changes in vacuolar volumes and surface areas during stomatal movement. Using a transgenic Arabidopsis line expressing green fluorescent protein (GFP)-AtVAM3, we have found that the guard cell vacuolar structures became complicated during stomatal closure with the appearance of numerous intra-vacuolar membrane structures. A three-dimensional (3-D) reconstruction using our originally developed software, REANT (reconstructor and analyzer of 3-D structure), and photobleaching analysis revealed the continuity of the vacuolar structures, even when they appeared to be compartmented in confocal images of closed stomata. Furthermore, calculations of the surface area by REANT revealed an increase in vacuolar surface area during stomatal closure but a decrease in the surface area of the guard cells. Movement of a vital staining dye, FM4-64, to the vacuolar membrane was accelerated during ABA-induced stomatal closure in Vicia faba. These results suggest that the guard cell vacuoles store some portion of the excess membrane materials produced during stomatal closure as intra-vacuolar structures.

A role for the AtMTP11 gene of Arabidopsis in manganese transport and tolerance

The Arabidopsis AtMTP family of genes encode proteins of the cation diffusion facilitator (CDF) family, with several members having roles in metal tolerances. Four of the 11 proteins in the family form a distinct cluster on a phylogenetic tree and are closely related to ShMTP8, a CDF identified in the tropical legume Stylosanthes hamata that is implicated in the transport of Mn(2+) into the vacuole as a tolerance mechanism. Of these four genes, AtMTP11 was the most highly expressed member of the Arabidopsis subgroup. When AtMTP11 was expressed in Saccharomyces cerevisiae, it conferred Mn(2+) tolerance and transported Mn(2+) by a proton-antiport mechanism. A mutant of Arabidopsis with a disrupted AtMTP11 gene (mtp11) was found to have increased sensitivity to Mn(2+) but not to Cu(2+) or Zn(2+). At a non-toxic but sufficient Mn(2+) supply (basal), the mutant accumulated more Mn(2+) than the wild type, but did not show any obvious deleterious effects on growth. When grown with Mn(2+) supplies that ranged from basal to toxic, the mutant accumulated Mn(2+) concentrations in shoots similar to those in wild-type plants, despite showing symptoms of Mn(2+) toxicity. AtMTP11 fused to green fluorescent protein co-localized with a reporter specific for pre-vacuolar compartments. These findings provide evidence for Mn(2+)-specific transport activity by AtMTP11, and implicate the pre-vacuolar compartments in both Mn(2+) tolerance and Mn(2+) homeostasis mechanisms of Arabidopsis.

Enigmatic brefeldin a

The fungal macrocyclic lactone brefeldin A (BFA) has been a useful tool in studying protein trafficking in the secretory and endocytic pathways in plant cells. The development of various GFP-tagged organelle markers expressed in transgenic plant cells has allowed dynamic study of organelles in response to BFA in living cells. Several organelles including the endoplasmic reticulum (ER), the Golgi apparatus and endosomal compartment have been shown to have visible morphological changes in response to BFA treatment, resulting in the formation of BFA-induced aggregated compartments or ER-Golgi hybrids in various plant cells. Using transgenic tobacco BY-2 cells expressing membrane-anchored yellow fluorescent protein (YFP) reporters marking Golgi apparatus or prevacuolar compartment (PVC), we have recently demonstrated that Golgi and PVC organelles have different sensitivity to BFA, where BFA at recoverable high concentrations (50 to 100 microg/ml) also induced PVC or multivesicular body (MVB) to form aggregates in plant cells. We have thus extended the BFA action to plant PVCs/MVBs, which will serve as a useful tool for studying PVC-mediated protein sorting and PVC biogenesis.

EpsinR2 interacts with clathrin, adaptor protein-3, AtVTI12, and phosphatidylinositol-3-phosphate. Implications for EpsinR2 function in protein trafficking in plant cells

Members of the epsin family of proteins (epsins) are characterized by the presence of an epsin N-terminal homology (ENTH) domain. Epsins have been implicated in various protein-trafficking pathways in animal and yeast (Saccharomyces cerevisiae) cells. Plant cells also contain multiple epsin-related proteins. In Arabidopsis (Arabidopsis thaliana), EPSIN1 is involved in vacuolar trafficking of soluble proteins. In this study, we investigated the role of Arabidopsis EpsinR2 in protein trafficking in plant cells. EpsinR2 contains a highly conserved ENTH domain but a fairly divergent C-terminal sequence. We found that the N-terminal ENTH domain specifically binds to phosphatidylinositol-3-P in vitro and has a critical role in the targeting of EpsinR2. Upon transient expression in protoplasts, hemagglutinin epitope-tagged EpsinR2 was translocated primarily to a novel cellular compartment, while a minor portion localized to the Golgi complex. Protein-binding experiments showed that EpsinR2 interacts with clathrin, AtVTI12, and the Arabidopsis homologs of adaptor protein-3 delta-adaptin and adaptor protein-2 alpha-adaptin. Localization experiments revealed that hemagglutinin epitope-tagged EpsinR2 colocalizes primarily with delta-adaptin and partially colocalizes with clathrin and AtVTI12. Based on these findings, we propose that EpsinR2 plays an important role in protein trafficking through interactions with delta-adaptin, AtVTI12, clathrin, and phosphatidylinositol-3-P.

Dual location of a family of proteinase inhibitors within the stigmas of Nicotiana alata

Reproductive and storage tissues of many plants produce large amounts of serine proteinase inhibitors (PIs). The ornamental tobacco, Nicotiana alata, produces a series of 6 kDa chymotrypsin and trypsin inhibitors that accumulate to up to 30% of soluble protein in the stigma. These inhibitors are derived by proteolytic processing of two closely related multidomain precursor proteins. Using immunogold electron microscopy, we find that the stigmatic PIs accumulate in both the central vacuole and in the extracellular mucilage. Labelling with antibodies specific for the C-terminal vacuolar targeting peptide (VTS) of each precursor confirms earlier biochemical data showing that the VTS is removed during passage through the secretory pathway. We have isolated and characterised the extracellular population of PIs, which are largely identical to PIs isolated from whole stigmas and are functional inhibitors of serine proteases. Subcellular fractionation of immature stigmas reveals that a sub-population of the PI precursor protein is proteolytically processed within the endoplasmic reticulum. This proteolysis results in the removal of the vacuolar sorting information, causing secretion of this PI population. We propose a novel mechanism whereby a single gene product may be simultaneously trafficked to two separate compartments mediated by proteolysis early in the secretory pathway.

Protein mobilization in germinating mung bean seeds involves vacuolar sorting receptors and multivesicular bodies

Plants accumulate and store proteins in protein storage vacuoles (PSVs) during seed development and maturation. Upon seed germination, these storage proteins are mobilized to provide nutrients for seedling growth. However, little is known about the molecular mechanisms of protein degradation during seed germination. Here we test the hypothesis that vacuolar sorting receptor (VSR) proteins play a role in mediating protein degradation in germinating seeds. We demonstrate that both VSR proteins and hydrolytic enzymes are synthesized de novo during mung bean (Vigna radiata) seed germination. Immunogold electron microscopy with VSR antibodies demonstrate that VSRs mainly locate to the peripheral membrane of multivesicular bodies (MVBs), presumably as recycling receptors in day 1 germinating seeds, but become internalized to the MVB lumen, presumably for degradation at day 3 germination. Chemical cross-linking and immunoprecipitation with VSR antibodies have identified the cysteine protease aleurain as a specific VSR-interacting protein in germinating seeds. Further confocal immunofluorescence and immunogold electron microscopy studies demonstrate that VSR and aleurain colocalize to MVBs as well as PSVs in germinating seeds. Thus, MVBs in germinating seeds exercise dual functions: as a storage compartment for proteases that are physically separated from PSVs in the mature seed and as an intermediate compartment for VSR-mediated delivery of proteases from the Golgi apparatus to the PSV for protein degradation during seed germination.

The Arabidopsis AAA ATPase SKD1 is involved in multivesicular endosome function and interacts with its positive regulator LYST-INTERACTING PROTEIN5

In yeast and mammals, the AAA ATPase Vps4p/SKD1 (for Vacuolar protein sorting 4/SUPPRESSOR OF K(+) TRANSPORT GROWTH DEFECT1) is required for the endosomal sorting of secretory and endocytic cargo. We identified a VPS4/SKD1 homolog in Arabidopsis thaliana, which localizes to the cytoplasm and to multivesicular endosomes. In addition, green fluorescent protein-SKD1 colocalizes on multivesicular bodies with fluorescent fusion protein endosomal Rab GTPases, such as ARA6/RabF1, RHA1/RabF2a, and ARA7/RabF2b, and with the endocytic marker FM4-64. The expression of SKD1(E232Q), an ATPase-deficient version of SKD1, induces alterations in the endosomal system of tobacco (Nicotiana tabacum) Bright Yellow 2 cells and ultimately leads to cell death. The inducible expression of SKD1(E232Q) in Arabidopsis resulted in enlarged endosomes with a reduced number of internal vesicles. In a yeast two-hybrid screen using Arabidopsis SKD1 as bait, we isolated a putative homolog of mammalian LYST-INTERACTING PROTEIN5 (LIP5)/SKD1 BINDING PROTEIN1 and yeast Vta1p (for Vps twenty associated 1 protein). Arabidopsis LIP5 acts as a positive regulator of SKD1 by increasing fourfold to fivefold its in vitro ATPase activity. We isolated a knockout homozygous Arabidopsis mutant line with a T-DNA insertion in LIP5. lip5 plants are viable and show no phenotypic alterations under normal growth conditions, suggesting that basal SKD1 ATPase activity is sufficient for plant development and growth.

Clathrin is involved in organization of mitotic spindle and phragmoplast as well as in endocytosis in tobacco cell cultures

We previously identified a 175 kDa polypeptide in Lilium longiflorum germinating pollen using a monoclonal antibody raised against myosin II heavy chain from Physarum polycephalum. In the present study, the equivalent polypeptide was also found in cultured tobacco BY-2 cells. Analysis of the amino acid sequences revealed that the 175 kDa polypeptide is clathrin heavy chain and not myosin heavy chain. After staining of BY-2 cells, punctate clathrin signals were distributed throughout the cytoplasm at interphase. During mitosis and cytokinesis, clathrin began to accumulate in the spindle and the phragmoplast and then was intensely concentrated in the cell plate. Expression of the C-terminal region of clathrin heavy chain, in which light chain binding and trimerization domains reside, induced the suppression of endocytosis and the formation of an aberrant spindle, phragmoplast, and cell plate, the likely cause of the observed multinucleate cells. These data strongly suggest that clathrin is intimately involved in the formation of the spindle and phragmoplast, as well as in endocytosis.

The Arabidopsis elch mutant reveals functions of an ESCRT component in cytokinesis

Recently, an alternative route to the proteasomal protein-degradation pathway was discovered that specifically targets transmembrane proteins marked with a single ubiquitin to the endosomal multivesicular body (MVB) and, subsequently, to the vacuole (yeast) or lysosome (animals), where they are degraded by proteases. Vps23p/TSG101 is a key component of the ESCRT I-III machinery in yeast and animals that recognizes mono-ubiquitylated proteins and sorts them into the MVB. Here, we report that the Arabidopsis ELCH (ELC) gene encodes a Vps23p/TSG101 homolog, and that homologs of all known ESCRT I-III components are present in the Arabidopsis genome. As with its animal and yeast counterparts, ELC binds ubiquitin and localizes to endosomes. Gel-filtration experiments indicate that ELC is a component of a high-molecular-weight complex. Yeast two-hybrid and immunoprecipitation assays showed that ELC interacts with Arabidopsis homologs of the ESCRT I complex. The elc mutant shows multiple nuclei in various cell types, indicating a role in cytokinesis. Double-mutant analysis with kaktus shows that increased ploidy levels do not influence the cytokinesis effect of elc mutants, suggesting that ELC is only important during the first endoreduplication cycle. Double mutants with tubulin folding cofactor a mutants show a synergistic phenotype, suggesting that ELC regulates cytokinesis through the microtubule cytoskeleton.

Transient expression of fluorescent fusion proteins in protoplasts of suspension cultured cells

Transient expression of fluorescent fusion proteins in plant cells has dramatically facilitated our study of newly identified genes and proteins. This protocol details an in vivo transient expression system to study the subcellular localization and dynamic associations of plant proteins using protoplasts freshly prepared from Arabidopsis or tobacco BY-2 suspension cultured cells. The method relies on the transformation of DNA constructs into protoplasts via electroporation. The whole protocol is comprised of three major stages: protoplast generation and purification, transformation of DNA into protoplasts via electroporation and incubation of protoplasts for protein analysis. Similar to stably transformed cell lines, transformed protoplasts are compatible with protein localization studies, pharmaceutical drug treatment and western blot analysis. This protocol can be completed within 11-24 h from protoplast production to protein detection.

Do plant cells secrete exosomes derived from multivesicular bodies?

Multivesicular bodies (MVBs) are spherical endosomal organelles containing small vesicles formed by inward budding of the limiting membrane into the endosomal lumen. In mammalian red cells and cells of immune system, MVBs fuse with the plasma membrane in an exocytic manner, leading to release their contents including internal vesicles into the extracellular space. These released vesicles are termed exosomes. Transmission electron microscopy studies have shown that paramural vesicles situated between the plasma membrane and the cell wall occur in various cell wall-associated processes and are similar to exosomes both in location and in morphology. Our recent studies have revealed that MVBs and paramural vesicles proliferate when cell wall appositions are rapidly deposited beneath fungal penetration attempts or during plugging of plasmodesmata between hypersensitive cells and their intact neighboring cells. This indicates a potential secretion of exosome-like vesicles into the extracellular space by fusion of MVBs with the plasma membrane. This MVB-mediated secretion pathway was proposed on the basis of pioneer studies of MVBs and paramural vesicles in plants some forty years ago. Here, we recall the attention to the occurrence of MVB-mediated secretion of exosomes in plants.

Rice SCAMP1 defines clathrin-coated, trans-golgi-located tubular-vesicular structures as an early endosome in tobacco BY-2 cells

We recently identified multivesicular bodies (MVBs) as prevacuolar compartments (PVCs) in the secretory and endocytic pathways to the lytic vacuole in tobacco (Nicotiana tabacum) BY-2 cells. Secretory carrier membrane proteins (SCAMPs) are post-Golgi, integral membrane proteins mediating endocytosis in animal cells. To define the endocytic pathway in plants, we cloned the rice (Oryza sativa) homolog of animal SCAMP1 and generated transgenic tobacco BY-2 cells expressing yellow fluorescent protein (YFP)-SCAMP1 or SCAMP1-YFP fusions. Confocal immunofluorescence and immunogold electron microscopy studies demonstrated that YFP-SCAMP1 fusions and native SCAMP1 localize to the plasma membrane and mobile structures in the cytoplasm of transgenic BY-2 cells. Drug treatments and confocal immunofluorescence studies demonstrated that the punctate cytosolic organelles labeled by YFP-SCAMP1 or SCAMP1 were distinct from the Golgi apparatus and PVCs. SCAMP1-labeled organelles may represent an early endosome because the internalized endocytic markers FM4-64 and AM4-64 reached these organelles before PVCs. In addition, wortmannin caused the redistribution of SCAMP1 from the early endosomes to PVCs, probably as a result of fusions between the two compartments. Immunogold electron microscopy with high-pressure frozen/freeze-substituted samples identified the SCAMP1-positive organelles as tubular-vesicular structures at the trans-Golgi with clathrin coats. These early endosomal compartments resemble the previously described partially coated reticulum and trans-Golgi network in plant cells.

Arabidopsis KAM2/GRV2 is required for proper endosome formation and functions in vacuolar sorting and determination of the embryo growth axis

We isolated an Arabidopsis thaliana mutant, katamari2 (kam2), that has a defect in the organization of endomembranes. This mutant had deformed endosomes and formed abnormally large aggregates with various organelles. Map-based cloning revealed that kam2 is allelic to gravitropism defective 2 (grv2). The KAM2/GRV2 gene encodes a homolog of a DnaJ domain-containing RECEPTOR-MEDIATED ENDOCYTOSIS-8, which is considered to play a vital role in the endocytotic pathway from the plasma membrane to lysosomes in animal cells. Immunofluorescent staining showed that KAM2/GRV2 protein localizes on punctate structures, which did not merge with any markers for Golgi, trans-Golgi network, endosomes, or prevacuolar compartments. KAM2/GRV2, which does not have a predicted transmembrane domain, was peripherally associated with the membrane surface of uncharacterized compartments. KAM2/GRV2 was expressed at the early to middle stages of seed maturation. We found kam2 mis-sorted seed storage proteins by secreting them from cells, indicating that KAM2/GRV2 is involved in the transport of the proteins into protein storage vacuoles. kam2 had another defect in embryogenesis. Half of the developing kam2-1 cotyledons grew into the opposite space of the seeds before the walking stick-shaped embryo stage. Our findings suggest that KAM2/GRV2 is required for proper formation of the endosomes involving protein trafficking to the vacuoles and determination of growth axis of the embryo.

The type IV secretion system of Sinorhizobium meliloti strain 1021 is required for conjugation but not for intracellular symbiosis

The type IV secretion system (T4SS) of the plant intracellular symbiont Sinorhizobium meliloti 1021 is required for conjugal transfer of DNA. However, it is not required for host invasion and persistence, unlike the T4SSs of closely related mammalian intracellular pathogens. A comparison of the requirement for a bacterial T4SS in plant versus animal host invasion suggests an important difference in the intracellular niches occupied by these bacteria.

Vesicular trafficking, cytoskeleton and signalling in root hairs and pollen tubes

Root hairs and pollen tubes show strictly polar cell expansion called tip growth. Recent studies of tip growth in root hairs and pollen tubes have revealed that small GTPases of the Rab, Arf and Rho/Rac families, along with their regulatory proteins, are essential for spatio-temporal regulation of vesicular trafficking, cytoskeleton organization and signalling. ROP/RAC GTPases are involved in a multiplicity of functions including the regulation of cytoskeleton organization, calcium signalling and endocytosis in pollen tubes and root hairs. One of the most exciting recent discoveries is the preferential localization of vesicles of the trans-Golgi network (TGN), defined by specific RAB GTPases, in the apical "clear zone" and the definition of TGN as a bona fide organelle involved in both polarized secretion and endocytosis. The TGN is thought to serve the function of an early endosome in plants because it is involved in early endocytosis and rapid vesicular recycling of the plasma membrane in root epidermal cells.

Endocytosis in signalling and development

After a long period of neglect, endocytosis in plants is finally coming of age. The constitutive recycling of plasma membrane proteins has been well established in the past few years, and recent studies report the ligand-induced endocytosis of receptors and other plasma membrane proteins. Signalling by ligand-bound receptors from endosomes has not, however, been demonstrated in plants. Although novel markers have been used to map endocytic pathways, the functional compartmentalisation of endosomes is still controversial. It is thus not clear where and how cargo proteins such as receptors are sorted towards either recycling to the plasma membrane or targeting to the vacuole for degradation.

The ins and outs of plant cell walls

New findings reveal that many membrane proteins undergo regulated trafficking between intracellular compartments and the plasma membrane. This also appears to be a common regulatory mechanism in the control of cell wall metabolism.

Trafficking of the human transferrin receptor in plant cells: effects of tyrphostin A23 and brefeldin A

Plant cells possess much of the molecular machinery necessary for receptor-mediated endocytosis (RME), but this process still awaits detailed characterization. In order to identify a reliable and well-characterized marker to investigate RME in plant cells, we have expressed the human transferrin receptor (hTfR) in Arabidopsis protoplasts. We have found that hTfR is mainly found in endosomal (Ara7- and FM4-64-positive) compartments, but also at the plasma membrane, where it mediates binding and internalization of its natural ligand transferrin (Tfn). Cell surface expression of hTfR increases upon treatment with tyrphostin A23, which inhibits the interaction between the YTRF endocytosis signal in the hTfR cytosolic tail and the mu2-subunit of the AP2 complex. Indeed, tyrphostin A23 inhibits Tfn internalization and redistributes most of hTfR to the plasma membrane, suggesting that the endocytosis signal of hTfR is functional in Arabidopsis protoplasts. Co-immunoprecipitation experiments show that hTfR is able to interact with a mu-adaptin subunit from Arabidopsis cytosol, a process that is blocked by tyrphostin A23. In contrast, treatment with brefeldin A, which inhibits recycling from endosomes back to the plasma membrane in plant cells, leads to the accumulation of Tfn and hTfR in larger patches inside the cell, reminiscent of BFA compartments. Therefore, hTfR has the same trafficking properties in Arabidopsis protoplasts as in animal cells, and cycles between the plasma membrane and endosomal compartments. The specific inhibition of Tfn/hTfR internalization and recycling by tyrphostin A23 and BFA, respectively, thus provide valuable molecular tools to characterize RME and the recycling pathway in plant cells.

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.

Localization of green fluorescent protein fusions with the seven Arabidopsis vacuolar sorting receptors to prevacuolar compartments in tobacco BY-2 cells

We have previously demonstrated that vacuolar sorting receptor (VSR) proteins are concentrated on prevacuolar compartments (PVCs) in plant cells. PVCs in tobacco (Nicotiana tabacum) BY-2 cells are multivesicular bodies (MVBs) as defined by VSR proteins and the BP-80 reporter, where the transmembrane domain (TMD) and cytoplasmic tail (CT) sequences of BP-80 are sufficient and specific for correct targeting of the reporter to PVCs. The genome of Arabidopsis (Arabidopsis thaliana) contains seven VSR proteins, but little is known about their individual subcellular localization and function. Here, we study the subcellular localization of the seven Arabidopsis VSR proteins (AtVSR1-7) based on the previously proven hypothesis that the TMD and CT sequences correctly target individual VSR to its final destination in transgenic tobacco BY-2 cells. Toward this goal, we have generated seven chimeric constructs containing signal peptide (sp) linked to green fluorescent protein (GFP) and TMD/CT sequences (sp-GFP-TMD/CT) of the seven individual AtVSR. Transgenic tobacco BY-2 cell lines expressing these seven sp-GFP-TMD-CT fusions all exhibited typical punctate signals colocalizing with VSR proteins by confocal immunofluorescence. In addition, wortmannin caused the GFP-marked prevacuolar organelles to form small vacuoles, and VSR antibodies labeled these enlarged MVBs in transgenic BY-2 cells. Wortmannin also caused VSR-marked PVCs to vacuolate in other cell types, including Arabidopsis, rice (Oryza sativa), pea (Pisum sativum), and mung bean (Vigna radiata). Therefore, the seven AtVSRs are localized to MVBs in tobacco BY-2 cells, and wortmannin-induced vacuolation of PVCs is a general response in plants.

The proteolytic processing of seed storage proteins in Arabidopsis embryo cells starts in the multivesicular bodies

We have investigated the transport of storage proteins, their processing proteases, and the Vacuolar Sorting Receptor-1/Epidermal Growth Factor Receptor-Like Protein1 (VSR-1/ATELP1) receptor during the formation of protein storage vacuoles in Arabidopsis thaliana embryos by means of high-pressure freezing/freeze substitution, electron tomography, immunolabeling techniques, and subcellular fractionation. The storage proteins and their processing proteases are segregated from each other within the Golgi cisternae and packaged into separate vesicles. The storage protein-containing vesicles but not the processing enzyme-containing vesicles carry the VSR-1/ATELP1 receptor. Both types of secretory vesicles appear to fuse into a type of prevacuolar multivesicular body (MVB). We have also determined that the proteolytic processing of the 2S albumins starts in the MVBs. We hypothesize that the compartmentalized processing of storage proteins in the MVBs may allow for the sequential activation of processing proteases as the MVB lumen gradually acidifies.

Overexpression of the Arabidopsis syntaxin PEP12/SYP21 inhibits transport from the prevacuolar compartment to the lytic vacuole in vivo

Golgi-mediated transport to the lytic vacuole involves passage through the prevacuolar compartment (PVC), but little is known about how vacuolar proteins exit the PVC. We show that this last step is inhibited by overexpression of Arabidopsis thaliana syntaxin PEP12/SYP21, causing an accumulation of soluble and membrane cargo and the plant vacuolar sorting receptor BP80 in the PVC. Anterograde transport proceeds normally from the endoplasmic reticulum to the Golgi and the PVC, although export from the PVC appears to be compromised, affecting both anterograde membrane flow to the vacuole and the recycling route of BP80 to the Golgi. However, Golgi-mediated transport of soluble and membrane cargo toward the plasma membrane is not affected, but a soluble BP80 ligand is partially mis-sorted to the culture medium. We also observe clustering of individual PVC bodies that move together and possibly fuse with each other, forming enlarged compartments. We conclude that PEP12/SYP21 overexpression specifically inhibits export from the PVC without affecting the Golgi complex or compromising the secretory branch of the endomembrane system. The results provide a functional in vivo assay that confirms PEP12/SYP21 involvement in vacuolar sorting and indicates that excess of this syntaxin in the PVC can be detrimental for further transport from this organelle.

Arabidopsis EPSIN1 plays an important role in vacuolar trafficking of soluble cargo proteins in plant cells via interactions with clathrin, AP-1, VTI11, and VSR1

Epsin and related proteins play important roles in various steps of protein trafficking in animal and yeast cells. Many epsin homologs have been identified in plant cells from analysis of genome sequences. However, their roles have not been elucidated. Here, we investigate the expression, localization, and biological role in protein trafficking of an epsin homolog, Arabidopsis thaliana EPSIN1, which is expressed in most tissues we examined. In the cell, one pool of EPSIN1 is associated with actin filaments, producing a network pattern, and a second pool localizes primarily to the Golgi complex with a minor portion to the prevacuolar compartment, producing a punctate staining pattern. Protein pull-down and coimmunoprecipitation experiments reveal that Arabidopsis EPSIN1 interacts with clathrin, VTI11, gamma-adaptin-related protein (gamma-ADR), and vacuolar sorting receptor1 (VSR1). In addition, EPSIN1 colocalizes with clathrin and VTI11. The epsin1 mutant, which has a T-DNA insertion in EPSIN1, displays a defect in the vacuolar trafficking of sporamin:green fluorescent protein (GFP), but not in the secretion of invertase:GFP into the medium. Stably expressed HA:EPSIN1 complements this trafficking defect. Based on these data, we propose that EPSIN1 plays an important role in the vacuolar trafficking of soluble proteins at the trans-Golgi network via its interaction with gamma-ADR, VTI11, VSR1, and clathrin.

Multivesicular bodies participate in a cell wall-associated defence response in barley leaves attacked by the pathogenic powdery mildew fungus

Localized cell wall modification and accumulation of antimicrobial compounds beneath sites of fungal attack are common mechanisms for plant resistance to fungal penetration. In barley (Hordeum vulgare) leaves, light-microscopically visible vesicle-like bodies (VLBs) containing H(2)O(2) or phenolics frequently accumulate around cell wall appositions (syn. papillae), in which the penetration attempt of the biotrophic powdery mildew fungus Blumeria graminis f. sp. hordei (Bgh) is halted. By ultrastructural analyses, we demonstrated that the Bgh-induced VLBs represent different structures. VLBs intensively stained by H(2)O(2)-reactive dyes were actually small papillae instead of cytoplasmic vesicles. Other VLBs were identified as osmiophilic bodies or multivesicular compartments, designated paramural bodies (PMBs) and multivesicular bodies (MVBs). MVBs seemingly followed two distinct pathways: either they were engulfed by the tonoplast for degradation in the vacuole or they fused with the plasma membrane to release their internal vesicles into the paramural space and hence could be the origin of PMBs. MVBs and PMBs appeared to be multicomponent kits possibly containing building blocks to be readily assembled into papilla and antimicrobial compounds to be discharged against fungal penetration. Finally, we propose that released paramural vesicles might be similar to exosomes in animal cells.

Fluid phase endocytic uptake of artificial nano-spheres and fluorescent quantum dots by sycamore cultured cells: evidence for the distribution of solutes to different intracellular compartments

Fluid phase endocytic uptake of external solutes in plant cells was further substantiated using artificial polystyrene nano-spheres (40 nm) and CdSe/ZnS quantum dots (20 nm). Both types of artificial nano-particles were taken up by sycamore-cultured cells. However, whereas polystyrene nano-spheres were delivered to the central vacuole, CdSe/ZnS nano-dots were sequestered into cytoplasmic vesicular structures. Using dextran-Texas Red (m.w. 3,000; d-TR) as additional marker, confocal micrographs confirmed the distinct topographic distribution of CdSe/ZnS quantum dots within the cell. Initially, d-TR and CdSe/ZnS quantum dots colocalized within cytoplasmic vesicles. After 18 h incubation, d-TR was distinctly localized in the vacuole whereas CdSe/ZnS quantum dots remained sequestered in cytoplasmic membranous compartments. The data provide a first evidence for the rapid distribution of solutes taken up by endocytosis to distinct intracellular compartments.

Targeting of the plant vacuolar sorting receptor BP80 is dependent on multiple sorting signals in the cytosolic tail

Although signals for vacuolar sorting of soluble proteins are well described, we have yet to learn how the plant vacuolar sorting receptor BP80 reaches its correct destination and recycles. To shed light on receptor targeting, we used an in vivo competition assay in which a truncated receptor (green fluorescent protein-BP80) specifically competes with sorting machinery and causes hypersecretion of BP80-ligands from tobacco (Nicotiana tabacum) leaf protoplasts. We show that both the transmembrane domain and the cytosolic tail of BP80 contain information necessary for efficient progress to the prevacuolar compartment (PVC). Furthermore, the tail must be exposed on the correct membrane surface to compete with sorting machinery. Mutational analysis of conserved residues revealed that multiple sequence motifs are necessary for competition, one of which is a typical Tyr-based motif (YXXPhi). Substitution of Tyr-612 for Ala causes partial retention in the Golgi apparatus, mistargeting to the plasma membrane (PM), and slower progress to the PVC. A role in Golgi-to-PVC transport was confirmed by generating the corresponding mutation on full-length BP80. The mutant receptor was partially mistargeted to the PM and induced the secretion of a coexpressed BP80-ligand. Further mutants indicate that the cytosolic tail is likely to contain other information besides the YXXPhi motif, possibly for endoplasmic reticulum export, endocytosis from the PM, and PVC-to-Golgi recycling.

Fine Structural Analysis of Brefeldin A-Induced Compartment Formation After High-Pressure Freeze Fixation of Maize Root Epidermis: Compound Exocytosis Resembling Cell Plate Formation during Cytokinesis

Formation of large perinuclear brefeldin A (BFA)-induced compartments is a characteristic feature of root apex cells, but it does not occur in shoot apex cells. BFA-induced compartments have been studied mostly using low resolution fluorescence microscopy techniques. Here, we have employed a high-resolution ultrastructural method based on ultra rapid freeze fixation of samples in order to study the formation of BFA-induced compartments in intact maize root epidermis cells in detail. This approach reveals five novel findings. Firstly, plant TGN/PGN elements are not tubular networks, as generally assumed, but rather vesicular compartments. Secondly, TGN/PGN vesicles interact with one another extensively via stalk-like connections and even fuse together via bridge-like structures. Thirdly, BFA-induced compartments are formed via extensive homotypic fusions of the TGN/PGN vesicles. Fourthly, multivesicular bodies (MVBs) are present within the BFA-induced compartments. Fifthly, mitochondria and small vacuoles accummulate abundantly around the large perinuclear BFA-induced compartments.

Functional characterization of ice plant SKD1, an AAA-type ATPase associated with the endoplasmic reticulum-Golgi network, and its role in adaptation to salt stress

A salt-induced gene mcSKD1 (suppressor of K+ transport growth defect) able to facilitate K+ uptake has previously been identified from the halophyte ice plant (Mesembryanthemum crystallinum). The sequence of mcSKD1 is homologous to vacuolar protein sorting 4, an ATPase associated with a variety of cellular activities-type ATPase that participates in the sorting of vacuolar proteins into multivesicular bodies in yeast (Saccharomyces cerevisiae). Recombinant mcSKD1 exhibited ATP hydrolytic activities in vitro with a half-maximal rate at an ATP concentration of 1.25 mm. Point mutations on active site residues abolished its ATPase activity. ADP is both a product and a strong inhibitor of the reaction. ADP-binding form of mcSDK1 greatly reduced its catalytic activity. The mcSKD1 protein accumulated ubiquitously in both vegetative and reproductive parts of plants. Highest accumulation was observed in cells actively engaging in the secretory processes, such as bladder cells of leaf epidermis. Membrane fractionation and double-labeling immunofluorescence showed the predominant localization of mcSKD1 in the endoplasmic reticulum-Golgi network. Immunoelectron microscopy identified the formation of mcSKD1 proteins into small aggregates in the cytosol and associated with membrane continuum within the endomembrane compartments. These results indicated that this ATPase participates in the endoplasmic reticulum-Golgi mediated protein sorting machinery for both housekeeping function and compartmentalization of excess Na+ under high salinity.

Plant retromer, localized to the prevacuolar compartment and microvesicles in Arabidopsis, may interact with vacuolar sorting receptors

Receptors for acid hydrolases destined for the lytic compartment in yeast and mammalian cells are retrieved from intermediate, endosomal organelles with the help of a pentameric protein complex called the retromer. We cloned the Arabidopsis thaliana homologs of the three yeast proteins (Vps35, Vps29, and Vps26) constituting the larger subunit of retromer and prepared antisera against them. With these antibodies, we demonstrated the presence of a retromer-like protein complex in salt extracts prepared from Arabidopsis microsomes. This complex is associated with membranes that coequilibrate with prevacuolar compartment markers and with high-density sedimenting membranes. Immunogold negative staining identified these membranes as 90-nm-diameter coated microvesicles. Confocal laser scanning immunofluorescence studies performed on tobacco (Nicotiana tabacum) BY-2 cells revealed high degrees of colabeling between all three retromer antisera and the prevacuolar compartment (PVC) markers PEP12 and vacuolar sorting receptor VSR(At-1). The presence of plant retromer at the surface of multivesicular bodies was also demonstrated by immunogold labeling of sections obtained from high-pressure frozen/freeze-substituted specimens. Treatment of BY-2 cells with wortmannin led to swelling of the PVC and a separation of the VPS35 and VSR signals. Preliminary data suggesting that retromer interacts with the cytosolic domain of a VSR were obtained by immunoprecipitation experiments performed on detergent-solubilized microsomes with Vps35 antibodies.

Multiple vacuolar sorting determinants exist in soybean 11S globulin

The sorting determinants of glycinin, a soybean (Glycine max) 11S globulin, which mediates protein targeting to the protein storage vacuole (PSV), were investigated in maturing soybean cotyledons by transient expression assays. A C-terminal stretch of 10 amino acids of A1aB1b, a glycinin group I subunit, was sufficient to direct green fluorescent protein (GFP) to the PSV. This peptide may correspond to a C-terminal vacuolar sorting determinant (ctVSD). Because functional inhibition of this putative ctVSD of A1aB1b did not block PSV sorting of A1aB1b, we used the three-dimensional structure of A1aB1b to identify candidates for a sequence-specific determinant (ssVSD). We found that the sequence downstream of disordered region 4 could direct GFP to the PSV and that Ile-297 is critical for sorting. However, functional inhibition of the ctVSD, combined with the Ile297Gly mutation, did not abolish the vacuolar sorting of A1aB1b, suggesting that A1aB1b has a third sorting determinant in addition to ctVSD and ssVSD. A glycinin group II subunit, A3B4, lacked a ctVSD but contained a VSD reminiscent of an ssVSD and an additional sorting determinant. We also demonstrate, by expression of dominant negative mutants of small GTPases and drug treatment experiments, that the trafficking of A1aB1b is COPII vesicle-dependent and wortmannin- and brefeldin A-sensitive.

Mapping the Arabidopsis organelle proteome

A challenging task in the study of the secretory pathway is the identification and localization of new proteins to increase our understanding of the functions of different organelles. Previous proteomic studies of the endomembrane system have been hindered by contaminating proteins, making it impossible to assign proteins to organelles. Here we have used the localization of organelle proteins by the isotope tagging technique in conjunction with isotope tags for relative and absolute quantitation and 2D liquid chromatography for the simultaneous assignment of proteins to multiple subcellular compartments. With this approach, the density gradient distributions of 689 proteins from Arabidopsis thaliana were determined, enabling confident and simultaneous localization of 527 proteins to the endoplasmic reticulum, Golgi apparatus, vacuolar membrane, plasma membrane, or mitochondria and plastids. This parallel analysis of endomembrane components has enabled protein steady-state distributions to be determined. Consequently, genuine organelle residents have been distinguished from contaminating proteins and proteins in transit through the secretory pathway.

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.

Protein transport in the plant secretory pathwayThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

The study of the plant secretory pathway is a relatively new field, developing rapidly over the last 30 years. Many exciting discoveries have already been made in this area, but as old questions are answered new ones become apparent. Our understanding of the functions and mechanisms of the plant secretory pathway is constantly expanding, in part because of the development of new technologies, mainly in bioimaging. The increasing accessibility of these new tools in combination with more established methods provides an ideal way to increase knowledge of the secretory pathway in plants. In this review we discuss recent developments in understanding protein transport between organelles in the plant secretory pathway.

Illuminating subcellular structures and dynamics in plants: a fluorescent protein toolboxThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

The discovery and development of multicoloured fluorescent proteins has led to the exciting possibility of observing a remarkable array of subcellular structures and dynamics in living cells. This minireview highlights a number of the more common fluorescent protein probes in plants and is a testimonial to the fact that the plant cell has not lagged behind during the live-imaging revolution and is ready for even more in-depth exploration.

The role of mRNA and protein sorting in seed storage protein synthesis, transport, and deposition

Rice synthesizes and accumulates high levels of 2 distinct classes of seed storage proteins and sorts them to separate intracellular compartments, making it an ideal model system for studying the mechanisms of storage protein synthesis, transport, and deposition. In rice, RNA localization dictates the initial site of storage protein synthesis on specific subdomains of the cortical endoplasmic reticulum (ER), and there is a direct relation between the RNA localization site and the final destination of the encoded protein within the endomembrane system. Current data support the existence of 3 parallel RNA localization pathways leading from the nucleus to the actively synthesizing cortical ER. Additional pathways may exist for the synthesis of cytoplasmic and nuclear-encoded proteins targeted to organelles, the latter located in a stratified arrangement in developing endosperm cells. The study of rice mutants, which accumulate unprocessed glutelin precursors, indicates that these multiple pathways prevent nonproductive interactions between different classes of storage proteins that would otherwise disrupt protein sorting. Indeed, it appears that the prevention of disruptive interactions between different classes of storage proteins plays a key role in their biosynthesis in rice. In addition to highlighting the unique features of the plant endomembrane system and describing the relation between RNA and protein localization, this minireview will attempt to address a number of questions raised by recent studies on these processes.

Ligand-induced endocytosis of the pattern recognition receptor FLS2 in Arabidopsis

Pattern-recognition receptors (PRRs) trigger innate immune responses in animals and plants. One such PRR is the flagellin receptor FLS2 in Arabidopsis. Here, we demonstrate that a functional fusion of FLS2 to the green fluorescent protein (GFP) resides in cell membranes of most tissues. Stimulation with the flagellin epitope flg22 induces its transfer into intracellular mobile vesicles, followed by degradation. FLS2 internalization depends on cytoskeleton and proteasome functions, and receptor activation. A variant FLS2 mutated in Thr 867, a potential phosphorylation site, binds flg22 normally, but is impaired in flg22 responses and FLS2 endocytosis. We propose that plant cells regulate pathogen-associated molecular pattern (PAMP)-mediated PRR activities by subcellular compartmentalization.

Exploring the ESCRTing machinery in eukaryotes

The profile of protein sorting into multivesicular bodies (MVBs) has risen recently with the identification of three heteromeric complexes known as ESCRT-I,-II,-III (Endosomal Sorting Complex Required for Transport). Genetic analyses in yeast have identified up to 15 soluble class E VPS (vacuolar protein sorting) proteins that have been assigned to the ESCRT machinery and function in cargo recognition and sorting, complex assembly, vesicle formation and dissociation. Despite their functional importance in yeast and mammalian cells, little is known about their presence and function in other organisms including plants. We have made use of the fully sequenced genomes of Arabidopsis thaliana and Oryza sativa, Drosophila melanogaster and Caenorhabditis elegans to explore the identity, structural characteristics and phylogenetic relationships of proteins assigned to the ESCRT machinery.

Cell cycle-dependent changes in Golgi stacks, vacuoles, clathrin-coated vesicles and multivesicular bodies in meristematic cells of Arabidopsis thaliana: a quantitative and spatial analysis

Cytokinesis in plants involves both the formation of a new wall and the partitioning of organelles between the daughter cells. To characterize the cellular changes that accompany the latter process, we have quantitatively analyzed the cell cycle-dependent changes in cell architecture of shoot apical meristem cells of Arabidopsis thaliana. For this analysis, the cells were preserved by high-pressure freezing and freeze-substitution techniques, and their Golgi stacks, multivesicular bodies, vacuoles and clathrin-coated vesicles (CCVs) characterized by means of serial thin section reconstructions, stereology and electron tomography techniques. Interphase cells possess approximately 35 Golgi stacks, and this number doubles during G2 immediately prior to mitosis. At the onset of cytokinesis, the stacks concentrate around the periphery of the growing cell plate, but do not orient towards the cell plate. Interphase cells contain approximately 18 multivesicular bodies, most of which are located close to a Golgi stack. During late cytokinesis, the appearance of a second group of cell plate-associated multivesicular bodies coincides with the onset of CCV formation at the cell plate. During this period a 4x increase in CCVs is paralleled by a doubling in number and a 4x increase in multivesicular bodies volume. The vacuole system also undergoes major changes in organization, size, and volume, with the most notable change seen during early telophase cytokinesis. In particular, the vacuoles form sausage-like tubular compartments with a 50% reduced surface area and an 80% reduced volume compared to prometaphase cells. We postulate that this transient reduction in vacuole volume during early telophase provides a means for increasing the volume of the cytosol to accommodate the forming phragmoplast microtubule array and associated cell plate-forming structures.

Golgi-mediated vacuolar sorting of the endoplasmic reticulum chaperone BiP may play an active role in quality control within the secretory pathway

Quality control in the endoplasmic reticulum (ER) prevents the arrival of incorrectly or incompletely folded proteins at their final destinations and targets permanently misfolded proteins for degradation. Such proteins have a high affinity for the ER chaperone BiP and are finally degraded via retrograde translocation from the ER lumen back to the cytosol. This ER-associated protein degradation (ERAD) is currently thought to constitute the main disposal route, but there is growing evidence for a vacuolar role in quality control. We show that BiP is transported to the vacuole in a wortmannin-sensitive manner in tobacco (Nicotiana tabacum) and that it could play an active role in this second disposal route. ER export of BiP occurs via COPII-dependent transport to the Golgi apparatus, where it competes with other HDEL receptor ligands. When HDEL-mediated retrieval from the Golgi fails, BiP is transported to the lytic vacuole via multivesicular bodies, which represent the plant prevacuolar compartment. We also demonstrate that a subset of BiP-ligand complexes is destined to the vacuole and differs from those likely to be disposed of via the ERAD pathway. Vacuolar disposal could act in addition to ERAD to maximize the efficiency of quality control in the secretory pathway.

Endocytosis of Cell Surface Material Mediates Cell Plate Formation during Plant Cytokinesis

Dividing plant cells perform a remarkable task of building a new cell wall within the cytoplasm in a few minutes. A long-standing paradigm claims that this primordial cell wall, known as the cell plate, is generated by delivery of newly synthesized material from Golgi apparatus-originated secretory vesicles. Here, we show that, in diverse plant species, cell surface material, including plasma membrane proteins, cell wall components, and exogenously applied endocytic tracers, is rapidly delivered to the forming cell plate. Importantly, this occurs even when de novo protein synthesis is blocked. In addition, cytokinesis-specific syntaxin KNOLLE as well as plasma membrane (PM) resident proteins localize to endosomes that fuse to initiate the cell plate. The rate of endocytosis is strongly enhanced during cell plate formation, and its genetic or pharmacological inhibition leads to cytokinesis defects. Our results reveal that endocytic delivery of cell surface material significantly contributes to cell plate formation during plant cytokinesis.

Multivesicular compartments proliferate in susceptible and resistant MLA12-barley leaves in response to infection by the biotrophic powdery mildew fungus

There is growing evidence that multivesicular bodies and cell wall-associated paramural bodies participate in the enhanced vesicle trafficking induced by pathogen attack. Here, we performed transmission electron microscopy in combination with cytochemical localization of H2O2 to investigate multivesicular compartments during establishment of compatible interaction in susceptible barley (Hordeum vulgare) and during hypersensitive response in resistant MLA12-barley infected by the barley powdery mildew fungus (Blumeria graminis f. sp. hordei). Multivesicular bodies, intravacuolar vesicle aggregates and paramural bodies proliferated in the penetrated epidermal cell during development of the fungal haustorium. These vesicular structures also proliferated at the periphery of intact cells, which were adjacent to the hypersensitive dying cells and deposited cell wall appositions associated with H2O2 accumulation. All plasmodesmata between intact cells and hypersensitive cells were constricted or blocked by cell wall appositions. These results suggest that multivesicular compartments participate in secretion of building blocks for cell wall appositions not only to arrest fungal penetration but also to contain hypersensitive cell death through blocking plasmodesmata. They may also participate in internalization of damaged membranes, deleterious materials, nutrients, elicitors and elicitor receptors.

Plant Prevacuolar/Endosomal Compartments

Multivesicular endosomes or prevacuolar compartments (PVCs) are membrane-bound organelles that play an important role in mediating protein traffic in the secretory and endocytic pathways of eukaryotic cells. PVCs function as an intermediate compartment for sorting proteins from the Golgi apparatus to vacuoles, sending missorted proteins back to the Golgi from the PVC, and receiving proteins from plasma membrane in the endocytic pathway. PVCs have been identified as multivesicular bodies in mammalian cells and yeast and more recently in plant cells. Whereas much is known about PVC-mediated protein trafficking and PVC biogenesis in mammalian cells and yeast, relatively little is known about the molecular mechanism of plant PVCs. In this review, we summarize and discuss our understanding of the plant PVC and compare it with its counterparts in yeast and mammalian cells.

Effects of brefeldin A on pollen germination and tube growth. Antagonistic effects on endocytosis and secretion

We assessed the effects of brefeldin A (BFA) on pollen tube development in Picea meyeri using fluorescent marker FM4-64 as a membrane-inserted endocytic/recycling marker, together with ultrastructural studies and Fourier transform infrared analysis of cell walls. BFA inhibited pollen germination and pollen tube growth, causing morphological changes in a dose-dependent manner, and pollen tube tip growth recovered after transferring into BFA-free medium. FM4-64 labeling showed typical bright apical staining in normally growing P. meyeri pollen tubes; this apical staining pattern differed from the V-formation pattern found in angiosperm pollen tubes. Confocal microscopy revealed that exocytosis was greatly inhibited in the presence of BFA. In contrast, the overall uptake of FM4-64 dye was about 2-fold that in the control after BFA (5 microg mL(-1)) treatment, revealing that BFA stimulated endocytosis in a manner opposite to the induced changes in exocytosis. Transmission electron microscopic observation showed that the number of secretory vesicles at the apical zone dramatically decreased, together with the disappearance of paramural bodies, while the number of vacuoles and other larger organelles increased. An acid phosphatase assay confirmed that the addition of BFA significantly inhibited secretory pathways. Importantly, Fourier transform infrared microspectroscopy documented significant changes in the cell wall composition of pollen tubes growing in the presence of BFA. These results suggest that enhanced endocytosis, together with inhibited secretion, is responsible for the retarded growth of pollen tubes induced by BFA.

Pathways for protein transport to seed storage vacuoles

Plant vacuoles have multiple functions: they can act both as digestive organelles and as receptacles for storage proteins. Different types of vacuoles can coexist in the same cell, which adds complexity to the process of targeting to these compartments. A fuller understanding of this process is of evident value when endeavouring to exploit the plant secretory pathway for heterologous protein production. Positive sorting signals are required in order to sort proteins to vacuoles, and these have been split into three groups: ctVSS [C-terminal VSS (vacuolar sorting signals)], ssVSS (sequence-specific VSS) and physical structure VSS. The current working model posits that soluble proteins are delivered from the Golgi apparatus to the lytic vacuoles in clathrin-coated vesicles by virtue of their ssVSS, or to the storage vacuole [PSV (protein-storage vacuole)] in dense vesicles in a manner dependent on ctVSS or physical structure VSS. Although targeting to LV appears to be receptor-mediated, no such receptor has been identified for the recruitment of proteins to the PSV. We have studied the vacuolar targeting of two castor bean (Ricinus communis L.) storage proteins, proricin and pro 2 S albumin, in their native endosperm and in the heterologous system of tobacco protoplasts. We have found that both these proteins contain bona fide ssVSS and bind to sorting receptors in vitro in a similarly sequence-specific manner. The apparent similarities to lytic VSS and possible implications with respect to the working model for transport to storage vacuoles are discussed.

Cell wall pectins and xyloglucans are internalized into dividing root cells and accumulate within cell plates during cytokinesis

Recently, we have reported that cell wall pectins are internalized into apical meristem root cells. In cells exposed to the fungal metabolite brefeldin A, all secretory pathways were inhibited, while endocytic pathways remained intact, resulting in accumulation of internalized cell wall pectins within brefeldin A-induced compartments. Here we report that, in addition to the already published cell wall epitopes, rhamnogalacturonan I and xyloglucans also undergo large-scale internalization into dividing root cells. Interestingly, multilamellar endosomes were identified as compartments internalizing arabinan cell wall pectins reactive to the 6D7 antibody, while large vacuole-like endosomes internalized homogalacturonans reactive to the 2F4 antibody. As all endosomes belong topographically to the exocellular space, cell wall pectins deposited in these "cell wall islands", enclosed by the plasma-membrane-derived membrane, are ideally suited to act as temporary stores for rapid formation of cell wall and generation of new plasma membrane. In accordance with this notion, we report that all cell wall pectins and xyloglucans that internalize into endosomes are highly enriched within cytokinetic cell plates and accumulate within brefeldin A compartments. On the other hand, only small amounts of the pectins reactive to the JIM7 antibody, which are produced in the Golgi apparatus, localize to cell plates and they do not accumulate within brefeldin A compartments. In conclusion, meristematic root cells have developed pathways for internalization and recycling of cell wall molecules which are relevant for plant-specific cytokinesis.

Endocytosis and vesicle trafficking during tip growth of root hairs

The directional elongation of root hairs, "tip growth", depends on the coordinated and highly regulated trafficking of vesicles which fill the tip cytoplasm and are active in secretion of cell wall material. So far, little is known about the dynamics of endocytosis in living root hairs. We analyzed the motile behaviour of vesicles in the apical region of living root hairs of Arabidopsis thaliana and of Triticum aestivum by live cell microscopy. For direct observation of endocytosis and of the fate of endocytic vesicles, we used the fluorescent endocytosis marker dyes FM 1-43 and FM 4-64. Rapid endocytosis was detected mainly in the tip, where it caused a bright fluorescence of the apical cytoplasm. The internalized membranes proceeded through highly dynamic putative early endosomes in the clear zone to larger endosomal compartments in the subapical region that are excluded from the clear zone. The internalized cargo ended up in the dynamic vacuole by fusion of large endosomal compartments with the tonoplast. Before export to these lytic compartments, putative early endosomes remained in the apical zone, where they most probably recycled to the plasma membrane and back into the cytoplasm for more than 30 min. Endoplasmic reticulum was not involved in trafficking pathways of endosomes. Actin cytoskeleton was needed for the endocytosis itself, as well as for further membrane trafficking. The actin-depolymerizing drug latrunculin B modified the dynamic properties of vesicles and endosomes; they became immobilized and aggregated in the tip. Treatment with brefeldin A inhibited membrane trafficking and caused the disappearance of FM-containing vesicles and putative early endosomes from the clear zone; labelled structures accumulated in motile brefeldin A-induced compartments. These large endocytic compartments redispersed upon removal of the drug. Our results hence prove that endocytosis occurs in growing root hairs. We show the localization of endocytosis in the tip and indicate specific endomembrane compartments and their recycling.