Molecular cloning and further characterization of a probable plant vacuolar sorting receptor

A Sugarcane Smut Fungus Effector Hijacks Plant Vacuolar Sorting Receptor-Mediated Trafficking to Evade Host Immune Detection

The smut fungus Sporisorium scitamineum is a major pathogen in sugarcane, causing significant agricultural losses worldwide. However, the molecular mechanisms by which its effectors facilitate infection and evade host immunity remain largely unclear. In this study, we identified the sugarcane vacuolar sorting receptor 1 gene (ScVSR1), whose expression negatively correlate with several putative S. scitamineum effector genes in a co-expression network. Overexpression of ScVSR1 in Arabidopsis thaliana reduced resistance to a fungal powdery mildew pathogen, indicating the negative role of ScVSR1 in plant defence. Among the co-expressed S. scitamineum effectors, SsPE15, a secreted cerato-platanin-like protein (CPP), physically interacts with ScVSR1 and is sorted into the prevacuolar compartment (PVC) by interacting with ScVSR1 in plant cells. Deletion of SsPE15 in S. scitamineum enhanced fungal virulence, suggesting that SsPE15 acts as an immune elicitor. Furthermore, the C-terminal domain of the SsPE15, containing the VSR sorting signal, was found to facilitate vesicular location. Notably, fusing this C-terminal domain to the bacterial effector AvrRpt2 significantly reduced AvrRpt2-triggered programmed cell death in Arabidopsis, a process partially dependent on AtVSR1 and AtVSR2. These findings reveal an immune evasion strategy by which S. scitamineum effector SsPE15 hijacks the host's vesicular trafficking system to avoid immune detection.

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.

Functional implication of the homotrimeric multidomain vacuolar sorting receptor 1 (VSR1) from Arabidopsis thaliana

The vacuolar sorting receptors (VSRs) are specific to plants and are responsible for sorting and transporting particular proteins from the trans-Golgi network to the vacuole. This process is critically important for various cellular functions, including storing nutrients during seed development. Despite many years of intense studies on VSRs, a complete relation between function and structure has not yet been revealed. Here, we present the crystal structure of the entire luminal region of glycosylated VSR1 from Arabidopsis thaliana (AtVSR1) for the first time. The structure provides insights into the tertiary and quaternary structures of VSR1, which are composed of an N-terminal protease-associated (PA) domain, a unique central region, and one epidermal growth factor (EGF)-like domain followed by two disordered EGF-like domains. The structure of VSR1 exhibits unique characteristics, the significance of which is yet to be fully understood.

Characterization of intracellular membrane structures derived from a massive expansion of endoplasmic reticulum (ER) membrane due to synthetic ER-membrane-resident polyproteins

The endoplasmic reticulum (ER) is a dynamic organelle that is amenable to major restructuring. Introduction of recombinant ER-membrane-resident proteins that form homo oligomers is a known method of inducing ER proliferation: interaction of the proteins with each other alters the local structure of the ER network, leading to the formation large aggregations of expanded ER, sometimes leading to the formation of organized smooth endoplasmic reticulum (OSER). However, these membrane structures formed by ER proliferation are poorly characterized and this hampers their potential development for plant synthetic biology. Here, we characterize a range of ER-derived membranous compartments in tobacco and show how the nature of the polyproteins introduced into the ER membrane affect the morphology of the final compartment. We show that a cytosol-facing oligomerization domain is an essential component for compartment formation. Using fluorescence recovery after photobleaching, we demonstrate that although the compartment retains a connection to the ER, a diffusional barrier exists to both the ER and the cytosol associated with the compartment. Using quantitative image analysis, we also show that the presence of the compartment does not disrupt the rest of the ER network. Moreover, we demonstrate that it is possible to recruit a heterologous, bacterial enzyme to the compartment, and for the enzyme to accumulate to high levels. Finally, transgenic Arabidopsis constitutively expressing the compartment-forming polyproteins grew and developed normally under standard conditions.

The seven rice vacuolar sorting receptors localize to prevacuolar compartments

Vacuolar sorting is critically important in plants as it regulates the mobilization of proteins and plays a major role in important agricultural traits like yield and seed protein content. Vacuolar sorting receptors (VSRs) are integral membrane proteins that mediate protein trafficking from the Golgi apparatus to the vacuole via the intermediate membrane-bound prevacuolar compartment (PVC)/multivesicular body (MVB). VSR proteins, such as an 80 kD (BP-80) from pea, also serve as markers for PVC/MVB. Dissecting VSR-mediated protein trafficking pathways may provide ways to enhance agronomic traits and crop yield. Green fluorescence protein (GFP) fusions with the seven Arabidopsis (Arabidopsis thaliana) VSRs were previously shown to localize to PVCs in transgenic tobacco BY-2 cells. The Rice (Oryza sativa) genome contains seven VSRs (OsVSR1-7), but little is known about their subcellular localizations. Here we studied the subcellular localization of OsVSR1-7 b y using a reporter approach, in which GFP-OsVSR1-7 fusions containing the transmembrane domain (TMD) and cytoplasmic tail (CT) of individual OsVSR were expressed in the protoplasts of rice, transgenic tobacco BY-2 cells and transgenic rice plants. Immunofluorescent labelling studies and confocal laser scanning microscope observation demonstrated that the seven OsVSRs are localized to PVCs and form ring-like structures upon wortmannin treatment. Therefore, we have verified the subcellular localization of OsVSR1-7 in this study. The OsVSRs tagged with GFP can serve as PVCs/MVBs markers in rice for the future studies.

Nanobody-based VSR7 tracing shows clathrin-dependent TGN to Golgi recycling

Receptor-mediated transport of soluble proteins is nature's key to empowering eukaryotic cells to access a plethora of macromolecules, either by direct accumulation or as products from resulting biochemical pathways. The transport efficiency of these mechanisms results from the receptor's capability to capture, transport, and release ligands on the one hand and the cycling ability that allows for performing multiple rounds of ligand transport on the other. However, the plant VACUOLAR SORTING RECEPTOR (VSR) protein family is diverse, and their ligand-specificity and bidirectional trafficking routes and transport mechanisms remain highly controversial. Here we employ nanobody-epitope interaction-based molecular tools to assess the function of the VSR 7 in vivo. We demonstrate the specificity of the VSR7 for sequence-specific vacuolar sorting signals, and we trace its anterograde transport and retrograde recycling route. VSR7 localizes at the cis-Golgi apparatus at steady state conditions and transports ligands downstream to release them in the trans-Golgi network/early endosome (TGN/EE) before undergoing clathrin-dependent recycling from the TGN/EE back to the cis-Golgi.

Recent advances in plant endomembrane research and new microscopical techniques

The endomembrane system consists of various membrane-bound organelles including the endoplasmic reticulum (ER), Golgi apparatus, trans-Golgi network (TGN), endosomes, and the lysosome/vacuole. Membrane trafficking between distinct compartments is mainly achieved by vesicular transport. As the endomembrane compartments and the machineries regulating the membrane trafficking are largely conserved across all eukaryotes, our current knowledge on organelle biogenesis and endomembrane trafficking in plants has mainly been shaped by corresponding studies in mammals and yeast. However, unique perspectives have emerged from plant cell biology research through the characterization of plant-specific regulators as well as the development and application of the state-of-the-art microscopical techniques. In this review, we summarize our current knowledge on the plant endomembrane system, with a focus on several distinct pathways: ER-to-Golgi transport, protein sorting at the TGN, endosomal sorting on multivesicular bodies, vacuolar trafficking/vacuole biogenesis, and the autophagy pathway. We also give an update on advanced imaging techniques for the plant cell biology research.

Glycosylphosphatidylinositol-anchoring is required for the proper transport and extensive glycosylation of a classical arabinogalactan protein precursor in tobacco BY-2 cells

Many precursors of plant arabinogalactan proteins (AGPs) contain a C-terminal glycosylphosphatidylinositol (GPI)-anchoring signal. Using NtAGP1, a classical tobacco AGP, as a model, and green fluorescent protein (GFP) and sweet potato sporamin (SPO) as tags, we analyzed the localization and modification of AGP and its mutant without GPI-anchoring signal (AGPΔC) in tobacco BY-2 cells. The NtAGP1 fusion proteins migrated as large smear on SDS-polyacrylamide gel, and these proteins also localized preferentially to the plasma membrane. In contrast, fusions of AGPΔC with GFP and SPO yielded several forms: The largest were secreted, whereas others were recovered in the endomembrane organelles, including vacuoles. Comparison of the glycan structures of the microsomal SPO-AGP and the secreted SPO-AGPΔC using antibodies against the glycan epitopes of AGP indicated that the glycan structures of these proteins are different. These observations indicate that GPI-anchoring is required for the proper transport and glycosylation of the AGP precursor.

Retrograde transport in plants: Circular economy in the endomembrane system

The study of endomembrane trafficking is crucial for understanding how cells and whole organisms function. Moreover, there is a special interest in investigating endomembrane trafficking in plants, given its role in transport and accumulation of seed storage proteins and in secretion of cell wall material, arguably the two most essential commodities obtained from crops. The mechanisms of anterograde transport in the biosynthetic and endocytic pathways of plants have been thoroughly discussed in recent reviews, but, comparatively, retrograde trafficking pathways have received less attention. Retrograde trafficking is essential to recover membranes, retrieve proteins that have escaped from their intended localization, maintain homeostasis in maturing compartments, and recycle trafficking machinery for its reuse in anterograde transport reactions. Here, we review the current understanding on retrograde trafficking pathways in the endomembrane system of plants, discussing their integration with anterograde transport routes, describing conserved and plant-specific retrieval mechanisms at play, highlighting contentious issues and identifying open questions for future research.

Transcriptome and DNA methylome provide insights into the molecular regulation of drought stress in sea buckthorn

Sea buckthorn is a typical drought-resistant tree species. However, there is a general lack of understanding of the pattern of DNA methylation linked with sea buckthorn responses to drought, and its relationship with drought tolerance mechanisms. In this study, we performed whole-transcriptome RNA sequencing and methylome sequencing in response to drought stress to explore differentially expressed mRNAs, miRNAs, lncRNAs and circRNAs in sea buckthorn leaves. Based on predicted DE pairs, we constructed a competitive endogenous RNA network, which revealed potential transcriptional regulatory roles in response to drought stress. The results of methylome sequencing revealed that the DNA methylation level was increased in sea buckthorn leaves under drought stress. We identified 13,405 differentially methylated regions between CK and TR. We found one DMR-associated DEG (Vacuolar-sorting receptor 6) involved in the ABA accumulation pathway. In addition, two DNA methyltransferases (HrMET1 and HrDRM1) were closely associated with drought-induced hypermethylation in sea buckthorn. Together, we firstly conducted a comprehensive transcriptomic and epigenetic analysis of sea buckthorn under drought stress, providing a resource for further study of the potential functions of genes, miRNAs, lncRNAs, circRNAs and DNA methyltransferases.

Structural insights into how vacuolar sorting receptors recognize the sorting determinants of seed storage proteins

Seeds such as rice and soybean are major food staples in the human diet. During seed development, storage proteins are deposited in a specialized organelle called the protein storage vacuole and are mobilized to provide nutrients during germination. Storage proteins are transported as cargoes via specific protein–protein interactions with the vacuolar sorting receptors. Supported by structural and mutagenesis studies, our work provides insights into how the sequence-specific information, or the vacuolar sorting determinant, on the storage proteins is recognized by the vacuolar sorting receptors for their targeting to the vacuoles. Insights gained into the rules of receptor–cargo recognition will be useful in engineering recombinant proteins for biotechnological applications of the protein storage vacuoles in seeds.

In Arabidopsis, vacuolar sorting receptor isoform 1 (VSR1) sorts 12S globulins to the protein storage vacuoles during seed development. Vacuolar sorting is mediated by specific protein–protein interactions between VSR1 and the vacuolar sorting determinant located at the C terminus (ctVSD) on the cargo proteins. Here, we determined the crystal structure of the protease-associated domain of VSR1 (VSR1-PA) in complex with the C-terminal pentapeptide (₄₆₈RVAAA₄₇₂) of cruciferin 1, an isoform of 12S globulins. The ₄₆₈RVA₄₇₀ motif forms a parallel β-sheet with the switch III residues (₁₂₇TMD₁₂₉) of VSR1-PA, and the ₄₇₁AA₄₇₂ motif docks to a cradle formed by the cargo-binding loop (₉₅RGDCYF₁₀₀), making a hydrophobic interaction with Tyr99. The C-terminal carboxyl group of the ctVSD is recognized by forming salt bridges with Arg95. The C-terminal sequences of cruciferin 1 and vicilin-like storage protein 22 were sufficient to redirect the secretory red fluorescent protein (spRFP) to the vacuoles in Arabidopsis protoplasts. Adding a proline residue to the C terminus of the ctVSD and R95M substitution of VSR1 disrupted receptor–cargo interactions in vitro and led to increased secretion of spRFP in Arabidopsis protoplasts. How VSR1-PA recognizes ctVSDs of other storage proteins was modeled. The last three residues of ctVSD prefer hydrophobic residues because they form a hydrophobic cluster with Tyr99 of VSR1-PA. Due to charge–charge interactions, conserved acidic residues, Asp129 and Glu132, around the cargo-binding site should prefer basic residues over acidic ones in the ctVSD. The structural insights gained may be useful in targeting recombinant proteins to the protein storage vacuoles in seeds.

SbCASP4 improves salt exclusion by enhancing the root apoplastic barrier

SbCASP4 improves the salt tolerance of sweet sorghum [Sorghum bicolor (L.) Mocnch] by enhancing the root apoplastic barrier and blocking the transport of sodium ions to the shoot. Sweet sorghum [Sorghum bicolor (L.) Mocnch] is a C4 crop with high biomass and tolerance to abiotic stresses such as salt, drought, and waterlogging. Sweet sorghum is widely used in bioenergy production, as a forage crop, and in liquors and beer. Root salt exclusion has been reported to underlie the salt tolerance of sweet sorghum. The Casparian strip has a key role in root salt exclusion, and the membrane domain protein (CASP) family participates in Casparian strip aggregation. However, the function and the regulatory mechanisms of SbCASP in response to salt stress in sweet sorghum are unclear. In the current study, we cloned SbCASP4 and determined that it is induced by salt stress and expressed in the endodermis cells of sweet sorghum. Histochemical staining and physiological indicators showed that heterologous expression of SbCASP4 significantly increased the tolerance to salt stress in transgenic Arabidopsis thaliana. Compared with wild type and casp5 mutants, under 50 mM NaCl treatment, SbCASP4-expression lines had the less leaf Na⁺, lower PI accumulation in stele, smaller oxidative damage and higher salinity threshold, longer root length and higher expression levels of the genes related to Casparian strip formation.

Organelle Visualization With Multicolored Fluorescent Markers in Bamboo

Bamboo is an important model plant to study the molecular mechanisms of rapid shoot growth and flowering once in a lifetime. However, bamboo research about protein functional characterization is largely lagged behind, mainly due to the lack of gene transformation platforms. In this study, a protoplast transient gene expression system in moso bamboo has been first established. Using this reliable and efficient system, we have generated a set of multicolored fluorescent markers based on the targeting sequences from endogenous proteins, which have been validated by their comparative localization with Arabidopsis organelle markers, in a combination with pharmaceutical treatments. Moreover, we further demonstrated the power of this multicolor marker set for rapid, combinatorial analysis of the subcellular localization of uncharacterized proteins, which may play potential functions in moso bamboo flowering and fast growth of shoots. Finally, this protoplast transient gene expression system has been elucidated for functional analysis in protein-protein interaction by fluorescence resonance energy transfer (FRET) and co-immunoprecipitation analysis. Taken together, in combination with the set of moso bamboo organelle markers, the protoplast transient gene expression system could be used for subcellular localization and functional study of unknown proteins in bamboo and will definitely promote rapid progress in diverse areas of research in bamboo plants.

MTV proteins unveil ER- and microtubule-associated compartments in the plant vacuolar trafficking pathway

The factors and mechanisms involved in vacuolar transport in plants, and in particular those directing vesicles to their target endomembrane compartment, remain largely unknown. To identify components of the vacuolar trafficking machinery, we searched for Arabidopsis modified transport to the vacuole (mtv) mutants that abnormally secrete the synthetic vacuolar cargo VAC2. We report here on the identification of 17 mtv mutations, corresponding to mutant alleles of MTV2/VSR4, MTV3/PTEN2A MTV7/EREL1, MTV8/ARFC1, MTV9/PUF2, MTV10/VPS3, MTV11/VPS15, MTV12/GRV2, MTV14/GFS10, MTV15/BET11, MTV16/VPS51, MTV17/VPS54, and MTV18/VSR1 Eight of the MTV proteins localize at the interface between the trans-Golgi network (TGN) and the multivesicular bodies (MVBs), supporting that the trafficking step between these compartments is essential for segregating vacuolar proteins from those destined for secretion. Importantly, the GARP tethering complex subunits MTV16/VPS51 and MTV17/VPS54 were found at endoplasmic reticulum (ER)- and microtubule-associated compartments (EMACs). Moreover, MTV16/VPS51 interacts with the motor domain of kinesins, suggesting that, in addition to tethering vesicles, the GARP complex may regulate the motors that transport them. Our findings unveil a previously uncharacterized compartment of the plant vacuolar trafficking pathway and support a role for microtubules and kinesins in GARP-dependent transport of soluble vacuolar cargo in plants.

AtSec62 is critical for plant development and is involved in ER-phagy in Arabidopsis thaliana

The endoplasmic reticulum (ER) is the major site for protein folding in eukaryotic cells. ER homeostasis is essential for the development of an organism, whereby the unfolded protein response (UPR) within the ER is precisely regulated. ER-phagy is a newly identified selective autophagic pathway for removal of misfolded or unfolded proteins within the ER in mammalian cells. Sec62, a component of the translocon complex, was recently characterized as an ER-phagy receptor during the ER stress recovery phase in mammals. In this study, we demonstrated that the Arabidopsis Sec62 (AtSec62) is required for plant development and might function as an ER-phagy receptor in plants. We showed that AtSec62 is an ER-localized membrane protein with three transmembrane domains (TMDs) with its C-terminus facing to the ER lumen. AtSec62 is required for plant development because atsec62 mutants display impaired vegetative growth, abnormal pollen and decreased fertility. atsec62 mutants are sensitive towards tunicamycin (TM)-induced ER stress, whereas overexpression of AtSec62 subsequently enhances stress tolerance during the ER stress recovery phase. Moreover, YFP-AtSec62 colocalizes with the autophagosome marker mCh-Atg8e in ring-like structures upon ER stress induction. Taken together, these data provide evidence for the pivotal roles of AtSec62 in plant development and ER-phagy.

A Diverse Membrane Interaction Network for Plant Multivesicular Bodies: Roles in Proteins Vacuolar Delivery and Unconventional Secretion

Vesicle trafficking between the membrane-bound organelles in plant cells plays crucial roles in the precise transportation of various materials, and thus supports cell proliferation and cellular polarization. Conventionally, plant prevacuolar compartments (PVCs), identified as multivesicular bodies (MVBs), play important roles in both the secretory pathway as intermediate compartments and the endocytic pathway as late endosomes. In recent years, the PVC/MVBs have been proposed to play important roles in both protein vacuolar delivery and unconventional secretion, but several important questions on the new regulators and environmental cues that coordinate the PVC/MVB-organelle membrane interactions and their biological significances remain. In this review, we first summarize the identity and nature of the plant PVC/MVBs, and then we present an update on our current understanding on the interaction of PVC/MVBs with other organelles in the plant endomembrane system with focus on the vacuole, autophagosome, and plasma membrane (PM) in plant development and stress responses. Finally, we raise some open questions and present future perspectives in the study of PVC/MVB-organelle interactions and associated biological functions.

Plant Vacuoles

Plant vacuoles are multifunctional organelles. On the one hand, most vegetative tissues develop lytic vacuoles that have a role in degradation. On the other hand, seed cells have two types of storage vacuoles: protein storage vacuoles (PSVs) in endosperm and embryonic cells and metabolite storage vacuoles in seed coats. Vacuolar proteins and metabolites are synthesized on the endoplasmic reticulum and then transported to the vacuoles via Golgi-dependent and Golgi-independent pathways. Proprotein precursors delivered to the vacuoles are converted into their respective mature forms by vacuolar processing enzyme, which also regulates various kinds of programmed cell death in plants. We summarize two types of vacuolar membrane dynamics that occur during defense responses: vacuolar membrane collapse to attack viral pathogens and fusion of vacuolar and plasma membranes to attack bacterial pathogens. We also describe the chemical defense against herbivores brought about by the presence of PSVs in the idioblast myrosin cell.

Nanobody-triggered lockdown of VSRs reveals ligand reloading in the Golgi

Protein degradation in lytic compartments is crucial for eukaryotic cells. At the heart of this process, vacuolar sorting receptors (VSRs) bind soluble hydrolases in the secretory pathway and release them into the vacuolar route. Sorting efficiency is suggested to result from receptor recycling. However, how and to where plant VSRs recycle remains controversial. Here we present a nanobody-epitope interaction-based protein labeling and tracking approach to dissect their anterograde and retrograde transport routes in vivo. We simultaneously employ two different nanobody-epitope pairs: one for the location-specific post-translational fluorescence labeling of receptors and the other pair to trigger their compartment-specific lockdown via an endocytosed dual-epitope linker protein. We demonstrate VSR recycling from the TGN/EE, thereby identifying the cis-Golgi as the recycling target and show that recycled VSRs reload ligands. This is evidence that bidirectional VSR-mediated sorting of vacuolar proteins exists and occurs between the Golgi and the TGN/EE.

Nanobody-triggered lockdown of VSRs reveals ligand reloading in the Golgi

Protein degradation in lytic compartments is crucial for eukaryotic cells. At the heart of this process, vacuolar sorting receptors (VSRs) bind soluble hydrolases in the secretory pathway and release them into the vacuolar route. Sorting efficiency is suggested to result from receptor recycling. However, how and to where plant VSRs recycle remains controversial. Here we present a nanobody-epitope interaction-based protein labeling and tracking approach to dissect their anterograde and retrograde transport routes in vivo. We simultaneously employ two different nanobody-epitope pairs: one for the location-specific post-translational fluorescence labeling of receptors and the other pair to trigger their compartment-specific lockdown via an endocytosed dual-epitope linker protein. We demonstrate VSR recycling from the TGN/EE, thereby identifying the cis-Golgi as the recycling target and show that recycled VSRs reload ligands. This is evidence that bidirectional VSR-mediated sorting of vacuolar proteins exists and occurs between the Golgi and the TGN/EE.

Signal motif-dependent ER export of the Qc-SNARE BET12 interacts with MEMB12 and affects PR1 trafficking in Arabidopsis

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) are well-known for their role in controlling membrane fusion, the final, but crucial step, in vesicular transport in eukaryotes. SNARE proteins contribute to various biological processes including pathogen defense and channel activity regulation, as well as plant growth and development. Precise targeting of SNARE proteins to destined compartments is a prerequisite for their proper functioning. However, the underlying mechanism(s) for SNARE targeting in plants remains obscure. Here, we investigate the targeting mechanism of the Arabidopsis thaliana Qc-SNARE BET12, which is involved in protein trafficking in the early secretory pathway. Two distinct signal motifs that are required for efficient BET12 ER export were identified. Pulldown assays and in vivo imaging implicated that both the COPI and COPII pathways were required for BET12 targeting. Further studies using an ER-export-defective form of BET12 revealed that the Golgi-localized Qb-SNARE MEMB12, a negative regulator of pathogenesis-related protein 1 (PR1; At2g14610) secretion, was its interacting partner. Ectopic expression of BET12 caused no inhibition in the general ER-Golgi anterograde transport but caused intracellular accumulation of PR1, suggesting that BET12 has a regulatory role in PR1 trafficking in A. thaliana.

Visualization of RMRs (Receptor Membrane RING-H2) Dimerization in Nicotiana benthamiana Leaves Using a Bimolecular Fluorescence Complementation (BiFC) Assay

The bimolecular fluorescent complementation (BiFC) is a fluorescent complementation method largely used to investigate protein-protein interaction in living cells. This method is based on the ability of two nonfluorescent fragments to assemble forming a native fluorescent reporter with the same spectral properties of the native reporter. Such fragments are fused to putative protein partners that in case of interaction will bring the two halves in close proximity, allowing for the reconstitution of an active fluorescent reporter. The BiFC has been used to investigate protein-protein interaction in a number of different organisms, including plants. In plant cells, many essential pathways of protein trafficking and subcellular localization necessitate the intervention of several protein units organized in multisubunit complexes. It is well known that vacuolar sorting of many secretory soluble proteins require the intervention of specific transmembrane cargo receptors able to interact forming dimers. In this chapter we describe a BiFC method for the efficient visualization of RMR (Receptor Membrane RING-H2) type 2 dimerization in agro-infiltrated Nicotiana benthamiana leaves. Furthermore, this relatively simple method represents an optimal strategy to test protein-protein interaction using any other putative protein partners of interest in plant cells.

RMR (Receptor Membrane RING-H2) type 1 and 2 show different promoter activities and subcellular localizations in Arabidopsis thaliana

Soluble vacuolar proteins reach their compartments of final accumulation through the binding with specific transmembrane cargo receptors. In Arabidopsis thaliana two different families of receptors have been characterized. The AtVSRs (Vacuolar Sorting Receptor), which are known to be involved in the protein sorting to lytic vacuoles (LV), and the AtRMRs (Receptor Membrane RING-H2), for which there is less evidence for a role in the traffic to the protein storage vacuole (PSV). In this study we investigated the localization and tissue expression of two RMRs (AtRMR1 and 2) in their species of origin, A. thaliana. Our experiments using leaf protoplasts and transgenic plants supported previous results of subcellular localization in Nicotiana benthamiana that visualized AtRMR1 and 2 in the cisternae of endoplasmic reticulum (ER) and in the trans-Golgi network (TGN), respectively. The promoter activities of AtRMR1 and AtRMR2 detected in transgenic A. thaliana lines suggest that the expression of these two receptors only partially overlap in some organs and tissues. These results suggest that AtRMR1 and 2 are not functionally redundant, but could also interact and participate in the same cellular process in tissues with an overlapping expression.

A Secretion System for Cargo Protein Identification of Vacuolar Sorting Receptors

Vacuolar sorting receptors (VSRs) are type I integral membrane family proteins in plant cells that can sort cargo proteins at the late Golgi or trans-Golgi network (TGN) for vacuolar transport via the prevacuolar compartment (PVC). However, little is known about VSR cargo proteins in plants. Here, we describe a new method for the identification of VSR cargos, which is based on the premise that the expressed N-terminus of VSRs will be secreted into the culture media along with their corresponding cargo proteins. The protocol described here should be applicable to all VSRs and should be also useful for other receptor cargo identification and protein-protein interaction in vivo.

Bicarbonate Induced Redox Proteome Changes in Arabidopsis Suspension Cells

Climate change as a result of increasing atmospheric CO₂ affects plant growth and productivity. CO₂ is not only a carbon donor for photosynthesis but also an environmental signal that can perturb cellular redox homeostasis and lead to modifications of redox-sensitive proteins. Although redox regulation of protein functions has emerged as an important mechanism in several biological processes, protein redox modifications and how they function in plant CO₂ response remain unclear. Here a new iodoTMTRAQ proteomics technology was employed to analyze changes in protein redox modifications in Arabidopsis thaliana suspension cells in response to bicarbonate (mimic of elevated CO₂) in a time-course study. A total of 47 potential redox-regulated proteins were identified with functions in carbohydrate and energy metabolism, transport, ROS scavenging, cell structure modulation and protein turnover. This inventory of previously unknown redox responsive proteins in Arabidopsis bicarbonate responses lays a foundation for future research toward understanding the molecular mechanisms underlying plant CO₂ responses.

Dimerization of the Vacuolar Receptors AtRMR1 and -2 from Arabidopsis thaliana Contributes to Their Localization in the trans-Golgi Network

In Arabidopsis thaliana, different types of vacuolar receptors were discovered. The AtVSR (Vacuolar Sorting Receptor) receptors are well known to be involved in the traffic to lytic vacuole (LV), while few evidences demonstrate the involvement of the receptors from AtRMR family (Receptor Membrane RING-H2) in the traffic to the protein storage vacuole (PSV). In this study we focused on the localization of two members of AtRMR family, AtRMR1 and -2, and on the possible interaction between these two receptors in the plant secretory pathway. Our experiments with agroinfiltrated Nicotiana benthamiana leaves demonstrated that AtRMR1 was localized in the endoplasmic reticulum (ER), while AtRMR2 was targeted to the trans-Golgi network (TGN) due to the presence of a cytosolic 23-amino acid sequence linker. The fusion of this linker to an equivalent position in AtRMR1 targeted this receptor to the TGN, instead of the ER. By using a Bimolecular Fluorescent Complementation (BiFC) technique and experiments of co-localization, we demonstrated that AtRMR2 can make homodimers, and can also interact with AtRMR1 forming heterodimers that locate to the TGN. Such interaction studies strongly suggest that the transmembrane domain and the few amino acids surrounding it, including the sequence linker, are essential for dimerization. These results suggest a new model of AtRMR trafficking and dimerization in the plant secretory pathway.

Receptor-mediated sorting of soluble vacuolar proteins: myths, facts, and a new model

To prevent their being released to the cell exterior, acid hydrolases are recognized by receptors at some point in the secretory pathway and diverted towards the lytic compartment of the cell (lysosome or vacuole). In animal cells, the receptor is called the mannosyl 6-phosphate receptor (MPR) and it binds hydrolase ligands in the trans-Golgi network (TGN). These ligands are then sequestered into clathrin-coated vesicles (CCVs) because of motifs in the cytosolic tail of the MPR which interact first with monomeric adaptors (Golgi-localized, Gamma-ear-containing, ARF-binding proteins, GGAs) and then with tetrameric (adaptin) adaptor complexes. The CCVs then fuse with an early endosome, whose more acidic lumen causes the ligands to dissociate. The MPRs are then recycled back to the TGN via retromer-coated carriers. Plants have vacuolar sorting receptors (VSRs) which were originally identified in CCVs isolated from pea (Pisum sativum L.) cotyledons. It was therefore assumed that VSRs would have an analogous function in plants to MPRs in animals. Although this dogma has enjoyed wide support over the last 20 years there are many inconsistencies. Recently, results have been published which are quite contrary to it. It now emerges that VSRs and their ligands can interact very early in the secretory pathway, and dissociate in the TGN, which, in contrast to its mammalian counterpart, has a pH of 5.5. Multivesicular endosomes in plants lack proton pump complexes and consequently have an almost neutral internal pH, which discounts them as organelles of pH-dependent receptor-ligand dissociation. These data force a critical re-evaluation of the role of CCVs at the TGN, especially considering that vacuolar cargo ligands have never been identified in them. We propose that one population of TGN-derived CCVs participate in retrograde transport of VSRs from the TGN. We also present a new model to explain how secretory and vacuolar cargo proteins are effectively separated after entering the late Golgi/TGN compartments.

Biogenesis of Plant Prevacuolar Multivesicular Bodies

Plant prevacuolar compartments (PVCs), or multivesicular bodies (MVBs), are single membrane-bound organelles that play important roles in mediating protein trafficking to vacuoles in the secretory pathway. PVC/MVB also serves as a late endosome in the endocytic pathway in plants. Since the plant PVC was identified as an MVB more than 10 years ago, great progress has been made toward the understanding of PVC/MVB function and biogenesis in plants. In this review, we first summarize previous research into the identification and characterization of plant PVCs/MVBs, and then highlight recent advances on the mechanisms underlying intraluminal vesicle formation and maturation of plant PVCs/MVBs. In addition, we discuss the possible crosstalk that appears to occur between PVCs/MVBs and autophagosomes during autophagy in plants. Finally, we list some open questions and present future perspectives in this field.

Receptor-mediated sorting of soluble vacuolar proteins ends at the trans-Golgi network/early endosome

The sorting of soluble proteins for degradation in the vacuole is of vital importance in plant cells, and relies on the activity of vacuolar sorting receptors (VSRs). In the plant endomembrane system, VSRs bind vacuole-targeted proteins and facilitate their transport to the vacuole. Where exactly these interactions take place has remained controversial, however. Here, we examine the potential for VSR-ligand interactions in all compartments of the vacuolar transport system in tobacco mesophyll protoplasts. To do this, we developed compartment-specific VSR sensors that assemble as a result of a nanobody-epitope interaction, and monitored the degree of ligand binding by analysing Förster resonance energy transfer using fluorescence lifetime imaging microscopy (FRET-FLIM). We show that VSRs bind ligands in the endoplasmic reticulum (ER) and in the Golgi, but not in the trans-Golgi network/early endosome (TGN/EE) or multivesicular late endosomes, suggesting that the post-TGN/EE trafficking of ligands towards the vacuole is VSR independent. We verify this by showing that non-VSR-ligands are also delivered to the vacuole from the TGN/EE after endocytic uptake. We conclude that VSRs are required for the transport of ligands from the ER and the Golgi to the TGN/EE, and suggest that the onward transport to the vacuole occurs by default.

pH Regulation by NHX-Type Antiporters Is Required for Receptor-Mediated Protein Trafficking to the Vacuole in Arabidopsis

Protein trafficking requires proper ion and pH homeostasis of the endomembrane system. The NHX-type Na(+)/H(+) antiporters NHX5 and NHX6 localize to the Golgi, trans-Golgi network, and prevacuolar compartments and are required for growth and trafficking to the vacuole. In the nhx5 nhx6 T-DNA insertional knockouts, the precursors of the 2S albumin and 12S globulin storage proteins accumulated and were missorted to the apoplast. Immunoelectron microscopy revealed the presence of vesicle clusters containing storage protein precursors and vacuolar sorting receptors (VSRs). Isolation and identification of complexes of VSRs with unprocessed 12S globulin by 2D blue-native PAGE/SDS-PAGE indicated that the nhx5 nhx6 knockouts showed compromised receptor-cargo association. In vivo interaction studies using bimolecular fluorescence complementation between VSR2;1, aleurain, and 12S globulin suggested that nhx5 nhx6 knockouts showed a significant reduction of VSR binding to both cargoes. In vivo pH measurements indicated that the lumens of VSR compartments containing aleurain, as well as the trans-Golgi network and prevacuolar compartments, were significantly more acidic in nhx5 nhx6 knockouts. This work demonstrates the importance of NHX5 and NHX6 in maintaining endomembrane luminal pH and supports the notion that proper vacuolar trafficking and proteolytic processing of storage proteins require endomembrane pH homeostasis.

N-linked glycosylation of AtVSR1 is important for vacuolar protein sorting in Arabidopsis

Vacuolar sorting receptors (VSRs) in Arabidopsis mediate the sorting of soluble proteins to vacuoles in the secretory pathway. The VSRs are post-translationally modified by the attachment of N-glycans, but the functional significance of such a modification remains unknown. Here we have studied the role(s) of glycosylation in the stability, trafficking and vacuolar protein transport of AtVSR1 in Arabidopsis protoplasts. AtVSR1 harbors three complex-type N-glycans, which are located in the N-terminal 'PA domain', the central region and the C-terminal epidermal growth factor repeat domain, respectively. We have demonstrated that: (i) the N-glycans do not affect the targeting of AtVSR1 to pre-vacuolar compartments (PVCs) and its vacuolar degradation; and (ii) N-glycosylation alters the binding affinity of AtVSR1 to cargo proteins and affects the transport of cargo into the vacuole. Hence, N-glycosylation of AtVSR1 plays a critical role in its function as a VSR in plants.

How vacuolar sorting receptor proteins interact with their cargo proteins: crystal structures of apo and cargo-bound forms of the protease-associated domain from an Arabidopsis vacuolar sorting receptor

In plant cells, soluble proteins are directed to vacuoles because they contain vacuolar sorting determinants (VSDs) that are recognized by vacuolar sorting receptors (VSR). To understand how a VSR recognizes its cargo, we present the crystal structures of the protease-associated domain of VSR isoform 1 from Arabidopsis thaliana (VSR1PA) alone and complexed with a cognate peptide containing the barley (Hordeum vulgare) aleurain VSD sequence of 1ADSNPIRPVT10. The crystal structures show that VSR1PA binds the sequence, Ala-Asp-Ser, preceding the NPIR motif. A conserved cargo binding loop, with a consensus sequence of 95RGxCxF100, forms a cradle that accommodates the cargo-peptide. In particular, Arg-95 forms a hydrogen bond to the Ser-3 position of the VSD, and the essential role of Arg-95 and Ser-3 in receptor-cargo interaction was supported by a mutagenesis study. Cargo binding induces conformational changes that are propagated from the cargo binding loop to the C terminus via conserved residues in switch I-IV regions. The resulting 180° swivel motion of the C-terminal tail is stabilized by a hydrogen bond between Glu-24 and His-181. A mutagenesis study showed that these two residues are essential for cargo interaction and trafficking. Based on our structural and functional studies, we present a model of how VSRs recognize their cargos.

Vacuolar Sorting Receptor-Mediated Trafficking of Soluble Vacuolar Proteins in Plant Cells

Vacuoles are one of the most prominent organelles in plant cells, and they play various important roles, such as degradation of waste materials, storage of ions and metabolites, and maintaining turgor. During the past two decades, numerous advances have been made in understanding how proteins are specifically delivered to the vacuole. One of the most crucial steps in this process is specific sorting of soluble vacuolar proteins. Vacuolar sorting receptors (VSRs), which are type I membrane proteins, are involved in the sorting and packaging of soluble vacuolar proteins into transport vesicles with the help of various accessory proteins. To date, large amounts of data have led to the development of two different models describing VSR-mediated vacuolar trafficking that are radically different in multiple ways, particularly regarding the location of cargo binding to, and release from, the VSR and the types of carriers utilized. In this review, we summarize current literature aimed at elucidating VSR-mediated vacuolar trafficking and compare the two models with respect to the sorting signals of vacuolar proteins, as well as the molecular machinery involved in VSR-mediated vacuolar trafficking and its action mechanisms.

Receptor-mediated transport of vacuolar proteins: a critical analysis and a new model

In this article we challenge the widely accepted view that receptors for soluble vacuolar proteins (VSRs) bind to their ligands at the trans-Golgi network (TGN) and transport this cargo via clathrin-coated vesicles (CCV) to a multivesicular prevacuolar compartment. This notion, which we term the "classical model" for vacuolar protein sorting, further assumes that low pH in the prevacuolar compartment causes VSR-ligand dissociation, resulting in a retromer-mediated retrieval of the VSRs to the TGN. We have carefully evaluated the literature with respect to morphology and function of the compartments involved, localization of key components of the sorting machinery, and conclude that there is little direct evidence in its favour. Firstly, unlike mammalian cells where the sorting receptor for lysosomal hydrolases recognizes its ligand in the TGN, the available data suggests that in plants VSRs interact with vacuolar cargo ligands already in the endoplasmic reticulum. Secondly, the evidence supporting the packaging of VSR-ligand complexes into CCV at the TGN is not conclusive. Thirdly, the prevacuolar compartment appears to have a pH unsuitable for VSR-ligand dissociation and lacks the retromer core and the sorting nexins needed for VSR recycling. We present an alternative model for protein sorting in the TGN that draws attention to the much overlooked role of Ca(2+) in VSR-ligand interactions and which may possibly also be a factor in the sequestration of secretory proteins.

In vivo intracellular pH measurements in tobacco and Arabidopsis reveal an unexpected pH gradient in the endomembrane system

The pH homeostasis of endomembranes is essential for cellular functions. In order to provide direct pH measurements in the endomembrane system lumen, we targeted genetically encoded ratiometric pH sensors to the cytosol, the endoplasmic reticulum, and the trans-Golgi, or the compartments labeled by the vacuolar sorting receptor (VSR), which includes the trans-Golgi network and prevacuoles. Using noninvasive live-cell imaging to measure pH, we show that a gradual acidification from the endoplasmic reticulum to the lytic vacuole exists, in both tobacco (Nicotiana tabacum) epidermal (ΔpH -1.5) and Arabidopsis thaliana root cells (ΔpH -2.1). The average pH in VSR compartments was intermediate between that of the trans-Golgi and the vacuole. Combining pH measurements with in vivo colocalization experiments, we found that the trans-Golgi network had an acidic pH of 6.1, while the prevacuole and late prevacuole were both more alkaline, with pH of 6.6 and 7.1, respectively. We also showed that endosomal pH, and subsequently vacuolar trafficking of soluble proteins, requires both vacuolar-type H(+) ATPase-dependent acidification as well as proton efflux mediated at least by the activity of endosomal sodium/proton NHX-type antiporters.

Organelle pH in the Arabidopsis endomembrane system

The pH of intracellular compartments is essential for the viability of cells. Despite its relevance, little is known about the pH of these compartments. To measure pH in vivo, we have first generated two pH sensors by combining the improved-solubility feature of solubility-modified green fluorescent protein (GFP) (smGFP) with the pH-sensing capability of the pHluorins and codon optimized for expression in Arabidopsis. PEpHluorin (plant-solubility-modified ecliptic pHluorin) gradually loses fluorescence as pH is lowered with fluorescence vanishing at pH 6.2 and PRpHluorin (plant-solubility-modified ratiomatric pHluorin), a dual-excitation sensor, allowing for precise measurements. Compartment-specific sensors were generated by further fusing specific sorting signals to PEpHluorin and PRpHluorin. Our results show that the pH of cytosol and nucleus is similar (pH 7.3 and 7.2), while peroxisomes, mitochondrial matrix, and plastidial stroma have alkaline pH. Compartments of the secretory pathway reveal a gradual acidification, spanning from pH 7.1 in the endoplasmic reticulum (ER) to pH 5.2 in the vacuole. Surprisingly, pH in the trans-Golgi network (TGN) and multivesicular body (MVB) is, with pH 6.3 and 6.2, quite similar. The inhibition of vacuolar-type H(+)-ATPase (V-ATPase) with concanamycin A (ConcA) caused drastic increase in pH in TGN and vacuole. Overall, the PEpHluorin and PRpHluorin are excellent pH sensors for visualization and quantification of pH in vivo, respectively.

An in vivo expression system for the identification of cargo proteins of vacuolar sorting receptors in Arabidopsis culture cells

Vacuolar sorting receptors (VSRs) are type I integral membrane family proteins that in plant cells are thought to recognize cargo proteins at the late Golgi or trans-Golgi network (TGN) for vacuolar transport via the pre-vacuolar compartment (PVC). However, little is known about VSR cargo proteins in plants. Here we developed and tested an in vivo expression system for the identification of VSR cargos which is based on the premise that the expressed N-terminus of VSRs will be secreted into the culture medium along with their corresponding cargo proteins. Indeed, transgenic Arabidopsis culture cell lines expressing VSR N-terminal binding domains (VSRNTs) were shown to secrete truncated VSRs (BP80NT, AtVSR1NT and AtVSR4NT) with attached cargo molecules into the culture medium. Putative cargo proteins were identified through mass spectrometry. Several identified cargo proteins were confirmed by localization studies and interaction analysis with VSRs. The screening strategy described here should be applicable to all VSRs and will help identify and study cargo proteins for individual VSR proteins. This method should be useful for both cargo identification and protein-protein interaction in vivo.

Comparison of membrane targeting strategies for the accumulation of the human immunodeficiency virus p24 protein in transgenic tobacco

Membrane anchorage was tested as a strategy to accumulate recombinant proteins in transgenic plants. Transmembrane domains of different lengths and topology were fused to the cytosolic HIV antigen p24, to promote endoplasmic reticulum (ER) residence or traffic to distal compartments of the secretory pathway in transgenic tobacco. Fusions to a domain of the maize seed storage protein γ-zein were also expressed, as a reference strategy that leads to very high stability via the formation of large polymers in the ER lumen. Although all the membrane anchored constructs were less stable compared to the zein fusions, residence at the ER membrane either as a type I fusion (where the p24 sequence is luminal) or a tail-anchored fusion (where the p24 sequence is cytosolic) resulted in much higher stability than delivery to the plasma membrane or intermediate traffic compartments. Delivery to the tonoplast was never observed. The inclusion of a thrombin cleavage site allowed for the quantitative in vitro recovery of p24 from all constructs. These results point to the ER as suitable compartment for the accumulation of membrane-anchored recombinant proteins in plants.

Vacuolar degradation of two integral plasma membrane proteins, AtLRR84A and OsSCAMP1, is cargo ubiquitination-independent and prevacuolar compartment-mediated in plant cells

In plant cells, how integral plasma membrane (PM) proteins are degraded in a cargo ubiquitination-independent manner remains elusive. Here, we studied the degradative pathway of two plant PM proteins: AtLRR84A, a type I integral membrane protein belonging to the leucine-rich repeat receptor-like kinase protein family, and OsSCAMP1 (rice secretory carrier membrane protein 1), a tetraspan transmembrane protein located on the PM and trans-Golgi network (TGN) or early endosome (EE). Using wortmannin and ARA7(Q69L) mutant that could enlarge the multivesicular body (MVB) or prevacuolar compartment (PVC) as tools, we demonstrated that, when expressed as green fluorescent protein (GFP) fusions in tobacco BY-2 or Arabidopsis protoplasts, both AtLRR84A and OsSCAMP1 were degraded in the lytic vacuole via the internal vesicles of MVB/PVC in a cargo ubiquitination-independent manner. Such MVB/PVC-mediated vacuolar degradation of PM proteins was further supported by immunocytochemical electron microscopy (immunoEM) study showing the labeling of the fusions on the internal vesicles of the PVC/MVB. Thus, cargo ubiquitination-independent and PVC-mediated degradation of PM proteins in the vacuole is functionally operated in plant cells.

The Golgi-localized Arabidopsis endomembrane protein12 contains both endoplasmic reticulum export and Golgi retention signals at its C terminus

Endomembrane proteins (EMPs), belonging to the evolutionarily conserved transmembrane nine superfamily in yeast and mammalian cells, are characterized by the presence of a large lumenal N terminus, nine transmembrane domains, and a short cytoplasmic tail. The Arabidopsis thaliana genome contains 12 EMP members (EMP1 to EMP12), but little is known about their protein subcellular localization and function. Here, we studied the subcellular localization and targeting mechanism of EMP12 in Arabidopsis and demonstrated that (1) both endogenous EMP12 (detected by EMP12 antibodies) and green fluorescent protein (GFP)-EMP12 fusion localized to the Golgi apparatus in transgenic Arabidopsis plants; (2) GFP fusion at the C terminus of EMP12 caused mislocalization of EMP12-GFP to reach post-Golgi compartments and vacuoles for degradation in Arabidopsis cells; (3) the EMP12 cytoplasmic tail contained dual sorting signals (i.e., an endoplasmic reticulum export motif and a Golgi retention signal that interacted with COPII and COPI subunits, respectively); and (4) the Golgi retention motif of EMP12 retained several post-Golgi membrane proteins within the Golgi apparatus in gain-of-function analysis. These sorting signals are highly conserved in all plant EMP isoforms and, thus, likely represent a general mechanism for EMP targeting in plant cells.

Mechanisms and concepts paving the way towards a complete transport cycle of plant vacuolar sorting receptors

Delivery of proteins to the lytic vacuole in plants is a complex cascade of selective interactions that specifically excludes residents of the endoplasmic reticulum and secreted proteins. Vacuolar transport must be highly efficient to avoid mistargeting of hydrolytic enzymes to locations where they could be harmful. While plant vacuolar sorting signals have been well described for two decades, it is only during the last 5 years that a critical mass of data was gathered that begins to reveal how vacuolar sorting receptors (VSRs) may complete a full transport cycle. Yet, the field is far from reaching a consensus regarding the organelles that could be involved in vacuolar sorting, their potential biogenesis, and the ultimate recycling of membranes and protein machinery that maintain this pathway. This review will highlight the important landmarks in our understanding of VSR function and compare recent transport models that have been proposed so that an emerging picture of plant vacuolar sorting mechanisms can be drawn.

Secretory carrier membrane proteins

Secretory carrier membrane proteins (SCAMPs) are a family of integral membrane proteins that play roles in mediating exocytosis in animal cells. However, relatively little is known about the subcellular localization, trafficking, and function of SCAMPs in plants. Several recent studies in plant cells indicate that plant SCAMPs share many similarities with their mammalian homologs although there are differences. In this review, we will first summarize and compare animal and plant SCAMPs in terms of their subcellular localization, trafficking, and possible functions. We will then present a phylogenetic analysis of plant and animal SCAMPs. Finally, we will present expression analysis on selective Arabidopsis SCAMPs in the hope of pointing to directions for functional characterization of plant SCAMPs in the future.

Structure and function of endosomes in plant cells

Endosomes are a heterogeneous collection of organelles that function in the sorting and delivery of internalized material from the cell surface and the transport of materials from the Golgi to the lysosome or vacuole. Plant endosomes have some unique features, with an organization distinct from that of yeast or animal cells. Two clearly defined endosomal compartments have been studied in plant cells, the trans-Golgi network (equivalent to the early endosome) and the multivesicular body (equivalent to the late endosome), with additional endosome types (recycling endosome, late prevacuolar compartment) also a possibility. A model has been proposed in which the trans-Golgi network matures into a multivesicular body, which then fuses with the vacuole to release its cargo. In addition to basic trafficking functions, endosomes in plant cells are known to function in maintenance of cell polarity by polar localization of hormone transporters and in signaling pathways after internalization of ligand-bound receptors. These signaling functions are exemplified by the BRI1 brassinosteroid hormone receptor and by receptors for pathogen elicitors that activate defense responses. After endocytosis of these receptors from the plasma membrane, endosomes act as a signaling platform, thus playing an essential role in plant growth, development and defense responses. Here we describe the key features of plant endosomes and their differences from those of other organisms and discuss the role of these organelles in cell polarity and signaling pathways.

A fluorescent reporter protein containing AtRMR1 domains is targeted to the storage and central vacuoles in Arabidopsis thaliana and tobacco leaf cells

To develop a new strategy to target recombinant proteins to the vacuolar storage system in transgenic plants, the ability of the transmembrane and cytosolic domains of Arabidopsis receptor homology-transmembrane-RING H2-1 (AtRMR1) was evaluated. A secreted version of RFP (secRFP) and a fusion of it to the transmembrane and cytosolic domains of AtRMR1 (RFP-TMCT) were produced and studied both in transient and stable expression assays. Transient expression in leaves of Nicotiana tabacum showed that secRFP is secreted to the apoplast while its fusion to TMCT of AtRMR1 is sufficient to prevent secretion of the reporter. In tobacco leaves, RFP-TMCT reporter showed an endoplasmic reticulum pattern in early expression stages while in late expression stages, it was found in the vacuolar lumen. For the first time, the role of TM and CT domains of AtRMR1 in stable expression in Arabidopsis thaliana is presented; the fusion of TMCT to secRFP is sufficient to sort RFP to the lumen of the central vacuoles in leaves and roots and to the lumen of PSV in cotyledons of mature embryos. In addition, biochemical studies performed in extract from transgenic plants showed that RFP-TMCT is an integral membrane protein. Full-length RFP-TMCT was also found in the vacuolar lumen, suggesting internalization into destination vacuole. Not colocalization of RFP-TMCT with tonoplast and plasma membrane markers were observed. This membrane vacuolar determinant sorting signal could be used for future application in molecular pharming as an alternative means to sort proteins of interest to vacuoles.

Plant defensin AhPDF1.1 is not secreted in leaves but it accumulates in intracellular compartments

• Apart from their antifungal role, plant defensins have recently been shown to be involved in abiotic stress tolerance or in inhibition of root growth when added in plant culture medium. We studied the subcellular localization of these proteins, which may account for these different roles. • Stable and transient expression of AhPDF1.1::GFP (green fluorescent protein) fusion proteins were analysed in yeast and plants. Functional tests established that the GFP tag did not alter the action of the defensin. Subcellular localization of AhPDF1.1 was characterized: by imaging AhPDF1.1::GFP together with organelle markers; and by immunolabelling AhPDF1.1 in Arabidopsis halleri and Arabidopsis thaliana leaves using a polyclonal serum. • All our independent approaches demonstrated that AhPDF1.1 is retained in intracellular compartments on the way to the lytic vacuole, instead of being addressed to the apoplasm. • These findings challenge the commonly accepted idea of secretion of defensins. The subcellular localization highlighted in this study could partly explain the dual role of plant defensins on plant cells and is of major importance to unravel the mechanisms of action of these proteins at the cellular level.

The rice RMR1 associates with a distinct prevacuolar compartment for the protein storage vacuole pathway

Transport of vacuolar proteins from Golgi apparatus or trans-Golgi network (TGN) to vacuoles is a receptor-mediated process via an intermediate membrane-bound prevacuolar compartment (PVC) in plant cells. Both vacuolar sorting receptor (VSR) and receptor homology region-transmembrane domain-RING-H2 (RMR) proteins have been shown to function in transporting storage proteins to protein storage vacuole (PSV), but little is known about the nature of the PVC for the PSV pathway. Here, we use the rice RMR1 (OsRMR1) as a probe to study the PSV pathway in plants. Immunogold electron microscopy (EM) with specific OsRMR1 antibodies showed that OsRMR1 proteins were found in the Golgi apparatus, TGN, and a distinct organelle with characteristics of PVC in both rice culture cells and developing rice seeds, as well as the protein body type II (PBII) or PSV in developing rice seeds. This organelle, also found in both tobacco BY-2 and Arabidopsis suspension cultured cells, is morphologically distinct from the VSR-positive multivesicular lytic PVC or multivesicular body (MVB) and thus represent a PVC for the PSV pathway that we name storage PVC (sPVC). Further in vivo and in vitro interaction studies using truncated OsRMR1 proteins secreted into the culture media of transgenic BY-2 suspension cells demonstrated that OsRMR1 functions as a sorting receptor in transporting vicilin-like storage proteins.

Vacuolar sorting receptor (VSR) proteins reach the plasma membrane in germinating pollen tubes

Vacuolar sorting receptors (VSRs) are type I integral membrane proteins that mediate the vacuolar transport of soluble cargo proteins via prevacuolar compartments (PVCs) in plants. Confocal immunofluorescent and immunogold Electron Microscope (EM) studies have localized VSRs to PVCs or multivesicular bodies (MVBs) and trans-Golgi network (TGN) in various plant cell types, including suspension culture cells, root cells, developing and germinating seeds. Here, we provide evidence that VSRs reach plasma membrane (PM) in growing pollen tubes. Both immunofluorescent and immunogold EM studies with specific VSR antibodies show that, in addition to the previously demonstrated PVC/MVB localization, VSRs also localize to PM in lily and tobacco pollen tubes prepared from chemical fixation or high-pressure freezing/frozen substitution. Such a PM localization suggests an additional role of VSR proteins in mediating protein transport to PM and endocytosis in growing pollen tubes. Using a high-speed Spinning Disc Confocal Microscope, the possible fusion between VSR-positive PVC organelles and the PM was also observed in living tobacco pollen tubes transiently expressing the PVC reporter GFP-VSR. In contrast, the lack of a prominent PM localization of GFP-VSR in living pollen tubes may be due to the highly dynamic situation of vesicular transport in this fast-growing cell type.

Multivesicular bodies mature from the trans-Golgi network/early endosome in Arabidopsis

The plant trans-Golgi network/early endosome (TGN/EE) is a major hub for secretory and endocytic trafficking with complex molecular mechanisms controlling sorting and transport of cargo. Vacuolar transport from the TGN/EE to multivesicular bodies/late endosomes (MVBs/LEs) is assumed to occur via clathrin-coated vesicles, although direct proof for their participation is missing. Here, we present evidence that post-TGN transport toward lytic vacuoles occurs independently of clathrin and that MVBs/LEs are derived from the TGN/EE through maturation. We show that the V-ATPase inhibitor concanamycin A significantly reduces the number of MVBs and causes TGN and MVB markers to colocalize in Arabidopsis thaliana roots. Ultrastructural analysis reveals the formation of MVBs from the TGN/EE and their fusion with the vacuole. The localization of the ESCRT components VPS28, VPS22, and VPS2 at the TGN/EE and MVBs/LEs indicates that the formation of intraluminal vesicles starts already at the TGN/EE. Accordingly, a dominant-negative mutant of VPS2 causes TGN and MVB markers to colocalize and blocks vacuolar transport. RNA interference-mediated knockdown of the annexin ANNAT3 also yields the same phenotype. Together, these data indicate that MVBs originate from the TGN/EE in a process that requires the action of ESCRT for the formation of intraluminal vesicles and annexins for the final step of releasing MVBs as a transport carrier to the vacuole.