The N-myristoylated Rab-GTPase m-Rabmc is involved in post-Golgi trafficking events to the lytic vacuole in plant cells

Development of a Golgi-targeted superoxide anion fluorescent probe for elucidating protein GOLPH3 function in myocardial ischemia-reperfusion injury

Superoxide anion (O₂^•-) is an important reactive oxygen species (ROS) and participates in various physiological and pathological processes in the organism. The O₂^•- burst induced by ischemia-reperfusion (I/R) is associated with cardiovascular disease and promotes the cell apoptosis. In this work, a turn-on type Golgi-targeting fluorescent probe Gol-Cou-O₂^•- was rationally designed for sensitive and selective detection of O₂^•-. The minimum detection limit concentration for O₂^•- was about 3.9 × 10^-7 M in aqueous solution. Gol-Cou-O₂^•- showed excellent capacity of detecting exogenous and endogenous O₂^•- in living cells and zebrafish, and was also used to capture the up-regulated O₂^•- level during the duration of I/R process in cardiomyocytes. Golgi Phosphoprotein 3 (GOLPH3) is a potential Golgi stress marker protein and plays a key role in cells apoptosis during I/R. The fluorescence imaging and flow cytometry assay results indicated that silencing GOLPH3 through siRNA could give rise to the down-regulated O₂^•- level and alleviation of apoptosis in I/R myocardial cells. Thus, development of Gol-Cou-O₂^•- provides a diagnostic tool for myocardial oxidative stress injury and distinct insights on roles of GOLPH3 in myocardial I/R injury.

Acidic dileucine motifs in the cylindrical inclusion protein of turnip mosaic virus are crucial for endosomal targeting and viral replication

Previously we reported that the multifunctional cylindrical inclusion (CI) protein of turnip mosaic virus (TuMV) is targeted to endosomes through the interaction with the medium subunit of adaptor protein complex 2 (AP2β), which is essential for viral infection. Although several functionally important regions in the CI have been identified, little is known about the determinant(s) for endosomal trafficking. The CI protein contains seven conserved acidic dileucine motifs [(D/E)XXXL(L/I)] typical of endocytic sorting signals recognized by AP2β. Here, we selected five motifs for further study and identified that they all were located in the regions of CI interacting with AP2β. Coimmunoprecipitation assays revealed that alanine substitutions in the each of these acidic dileucine motifs decreased binding with AP2β. Moreover, these CI mutants also showed decreased accumulation of punctate bodies, which enter endocytic-tracking styryl-stained endosomes. The mutations were then introduced into a full-length infectious clone of TuMV, and each mutant had reduced viral replication and systemic infection. The data suggest that the acidic dileucine motifs in CI are indispensable for interacting with AP2β for efficient viral replication. This study provides new insights into the role of endocytic sorting motifs in the intracellular movement of viral proteins for replication.

Turnip mosaic virus co-opts the vacuolar sorting receptor VSR4 to promote viral genome replication in plants by targeting viral replication vesicles to the endosome

Accumulated experimental evidence has shown that viruses recruit the host intracellular machinery to establish infection. It has recently been shown that the potyvirus Turnip mosaic virus (TuMV) transits through the late endosome (LE) for viral genome replication, but it is still largely unknown how the viral replication vesicles labelled by the TuMV membrane protein 6K2 target LE. To further understand the underlying mechanism, we studied the involvement of the vacuolar sorting receptor (VSR) family proteins from Arabidopsis in this process. We now report the identification of VSR4 as a new host factor required for TuMV infection. VSR4 interacted specifically with TuMV 6K2 and was required for targeting of 6K2 to enlarged LE. Following overexpression of VSR4 or its recycling-defective mutant that accumulates in the early endosome (EE), 6K2 did not employ the conventional VSR-mediated EE to LE pathway, but targeted enlarged LE directly from cis-Golgi and viral replication was enhanced. In addition, VSR4 can be N-glycosylated and this is required for its stability and for monitoring 6K2 trafficking to enlarged LE. A non-glycosylated VSR4 mutant enhanced the dissociation of 6K2 from cis-Golgi, leading to the formation of punctate bodies that targeted enlarged LE and to more robust viral replication than with glycosylated VSR4. Finally, TuMV hijacks N-glycosylated VSR4 and protects VSR4 from degradation via the autophagy pathway to assist infection. Taken together, our results have identified a host factor VSR4 required for viral replication vesicles to target endosomes for optimal viral infection and shed new light on the role of N-glycosylation of a host factor in regulating viral infection.

A key feature of the replication of positive-strand RNA viruses is the rearrangement of the host endomembrane system to produce a membranous replication organelle. Recent reports suggest that the late endosome (LE) serves as a replication site for the potyvirus Turnip mosaic virus (TuMV), but the mechanism(s) by which TuMV replication vesicles target LE are far from being fully elucidated. Identification of the host factors involved in this transport process could lead to new strategies to combat TuMV infection. In this report, we provide evidence that TuMV replication depends on functional vesicle transport from cis-Golgi to the enlarged LE pathway that is mediated by a specific VSR family member, VSR4, from Arabidopsis. Knock out of VSR4 impaired the targeting of TuMV replication vesicles to enlarged LE and suppressed viral infection, and this process depends on the specific interaction between VSR4 and the viral replication vesicle-forming protein 6K2. We also showed that N-glycosylation of VSR4 modulates the targeting of TuMV replication vesicles to enlarged LE and enhances viral infection, thus contributing to our understanding of how TuMV manipulates host factors in order to establish optimal infection. These results may have implications for the role of VSR in other positive-strand RNA viruses.

Straw Competition and Wheat Root Endophytism of Trichoderma gamsii T6085 as Useful Traits in the Biological Control of Fusarium Head Blight

Trichoderma gamsii T6085 has been investigated for many years as a beneficial isolate for use in the biocontrol of Fusarium head blight (FHB) of wheat caused primarily by Fusarium graminearum. Previous work focused on application of T6085 to wheat spikes at anthesis, whereas application to soil before or at sowing has received limited attention. In the present study, the competitive ability of T6085 on plant residues against F. graminearum was investigated. Results showed a significant reduction of wheat straw colonization by the pathogen and of the development of perithecia, not only when T6085 was applied alone but also in the presence of a F. oxysporum isolate (7121), well known as a natural competitor on wheat plant residues. T6085 was able to endophytically colonize wheat roots, resulting in internal colonization of the radical cortex area, without reaching the vascular system, as confirmed by confocal microscopy. This intimate interaction with the plant resulted in a significant increase of the expression of the plant defense-related genes PAL1 and PR1. Taken together, competitive ability, endophytic behavior, and host resistance induction represent three important traits that can be of great use in the application of T6085 against FHB not only on spikes at anthesis but potentially also in soil before or at sowing.

To Lead or to Follow: Contribution of the Plant Vacuole to Cell Growth

Cell division and cell elongation are fundamental processes for growth. In contrast to animal cells, plant cells are surrounded by rigid walls and therefore loosening of the wall is required during elongation. On the other hand, vacuole size has been shown to correlate with cell size and inhibition of vacuolar expansion limits cell growth. However, the specific role of the vacuole during cell elongation is still not fully resolved. Especially the question whether the vacuole is the leading unit during cellular growth or just passively expands upon water uptake remains to be answered. Here, we review recent findings about the contribution of the vacuole to cell elongation. In addition, we also discuss the connection between cell wall status and vacuolar morphology. In particular, we focus on the question whether vacuolar size is dictated by cell size or vice versa and share our personnel view about the sequential steps during cell elongation.

Integration of two RAB5 groups during endosomal transport in plants

RAB5 is a key regulator of endosomal functions in eukaryotic cells. Plants possess two different RAB5 groups, canonical and plant-unique types, which act via unknown counteracting mechanisms. Here, we identified an effector molecule of the plant-unique RAB5 in Arabidopsis thaliana, ARA6, which we designated PLANT-UNIQUE RAB5 EFFECTOR 2 (PUF2). Preferential colocalization with canonical RAB5 on endosomes and genetic interaction analysis indicated that PUF2 coordinates vacuolar transport with canonical RAB5, although PUF2 was identified as an effector of ARA6. Competitive binding of PUF2 with GTP-bound ARA6 and GDP-bound canonical RAB5, together interacting with the shared activating factor VPS9a, showed that ARA6 negatively regulates canonical RAB5-mediated vacuolar transport by titrating PUF2 and VPS9a. These results suggest a unique and unprecedented function for a RAB effector involving the integration of two RAB groups to orchestrate endosomal trafficking in plant cells.

Integration of two RAB5 groups during endosomal transport in plants

RAB5 is a key regulator of endosomal functions in eukaryotic cells. Plants possess two different RAB5 groups, canonical and plant-unique types, which act via unknown counteracting mechanisms. Here, we identified an effector molecule of the plant-unique RAB5 in Arabidopsis thaliana, ARA6, which we designated PLANT-UNIQUE RAB5 EFFECTOR 2 (PUF2). Preferential colocalization with canonical RAB5 on endosomes and genetic interaction analysis indicated that PUF2 coordinates vacuolar transport with canonical RAB5, although PUF2 was identified as an effector of ARA6. Competitive binding of PUF2 with GTP-bound ARA6 and GDP-bound canonical RAB5, together interacting with the shared activating factor VPS9a, showed that ARA6 negatively regulates canonical RAB5-mediated vacuolar transport by titrating PUF2 and VPS9a. These results suggest a unique and unprecedented function for a RAB effector involving the integration of two RAB groups to orchestrate endosomal trafficking in plant cells.

Living cells often contain compartments that pass proteins, fats and other biological molecules to one another via a process called membrane trafficking. Endosomes are one of the key platforms of membrane trafficking. These structures accumulate molecules from the outside of the cell, sort them, and then redirect them back to the cell surface or send them to other compartments within the cell where they can be broken down. Proteins known as RAB5s regulate many of the activities of endosomes. Some are found in a wide range of organisms, including animals, fungi, and plants, and are referred to as the “canonical” RAB5 group. Another group of RAB5 proteins are unique to land plants and some green algae.

The existence of two RAB5 groups (i.e. canonical and plant-unique) is a distinctive feature of plant cells. In 2011, researchers showed that a plant-unique RAB5 could interfere with and counteract the activities of a canonical RAB5. However, it remained ambiguous how these proteins could do this.

To resolve this question, Ito et al. – who include several researchers from the 2011 study – set out to find proteins that interact with a plant-unique RAB5 from Arabidopsis thaliana. The experiments identified one partner of a plant-unique RAB5, which was named PUF2. Unexpectedly, further experiments revealed that PUF2 also regulates canonical RAB5. PUF2 was found on the surface of the endosome together with RAB5s and a protein that activates RAB5s. Notably, PUF2 also interacted with the activating factor and the inactive form of canonical RAB5.

Based on these findings, Ito et al. propose that PUF2 acts as a landmark to bring inactive canonical RAB5 close to its activating factor, which helps to activate canonical RAB5. They suggest that the plant-unique RAB5 also competitively binds to the landmark and blocks the canonical RAB5.

Membrane trafficking is a universal system for all living organisms, yet the system seems to be customized among different organisms. These new findings provide further evidence that land plants have evolved a unique mechanism to regulate the activities of their endosomes. The next step is to reconstruct how this system evolved and unravel its relevance to the evolution of plant-specific traits.

5-Azacytidine mediated reactivation of silenced transgenes in potato (Solanum tuberosum) at the whole plant level

Transient 5-azacytidine treatment of leaf explants from potato plants with transcriptionally silenced transgenes allows de novo regeneration of plants with restored transgene expression at the whole plant level. Transgenes introduced into plant genomes frequently become silenced either at the transcriptional or the posttranscriptional level. Transcriptional silencing is usually associated with DNA methylation in the promoter region. Treatments with inhibitors of maintenance DNA methylation were previously shown to allow reactivation of transcriptionally silenced transgenes in single cells or tissues, but not at the whole plant level. Here we analyzed the effect of DNA methylation inhibitor 5-azacytidine (AzaC) on the expression of two silenced reporter genes encoding green fluorescent protein (GFP) and neomycin phosphotransferase (NPTII) in potato plants. Whereas no obvious reactivation was observed in AzaC-treated stem cuttings, transient treatment of leaf segments with 10 μM AzaC and subsequent de novo regeneration of shoots on the selective medium with kanamycin resulted in the production of whole plants with clearly reactivated expression of previously silenced transgenes. Reactivation of nptII expression was accompanied by a decrease in cytosine methylation in the promoter region of the gene. Using the plants with reactivated GFP expression, we found that re-silencing of this transgene can be accidentally triggered by de novo regeneration. Thus, testing the incidence of transgene silencing during de novo regeneration could be a suitable procedure for negative selection of transgenic lines (insertion events) which have an inclination to be silenced. Based on our analysis of non-specific inhibitory effects of AzaC on growth of potato shoots in vitro, we estimated that AzaC half-life in the culture media is approximately 2 days.

Distribution of RAB5-positive multivesicular endosomes and the trans-Golgi network in root meristematic cells of Arabidopsis thaliana

In plant cells, the trans-Golgi network (TGN) is known to act as the early endocytic compartment, whereas RAB5-localizing multivesicular endosomes (MVEs) act as the later compartment. Land plants and certain green algal species possess plant-unique RAB5 homologs (ARA6/RABF1 in Arabidopsis thaliana) in addition to the orthologs of animal RAB5 (RHA1/RABF2a and ARA7/RABF2b in A. thaliana), and these two RAB5 members reside in substantially overlapping but different subpopulations of MVEs. Several studies indicate that the TGN and MVEs are closely related; however, the distribution of the two RAB5 groups in relation to the TGN remains elusive. Here, we quantitatively showed that ARA6 and ARA7 are closely associated with the TGN, and the subpopulation of ARA6 and ARA7 overlaps with the TGN in the root epidermal cells of A. thaliana.

Functional analyses of the plant-specific C-terminal region of VPS9a: the activating factor for RAB5 in Arabidopsis thaliana

Recent studies demonstrated that endosomal transport played important roles in various plant functions. The RAB GTPase regulates the tethering and fusion steps of vesicle trafficking to target membranes in each trafficking pathway by acting as a molecular switch. RAB GTPase activation is catalyzed by specific guanine nucleotide exchange factors (GEFs) that promote the exchange of GDP on the RAB GTPase with GTP. RAB5 is a key regulator of endosomal trafficking and is uniquely diversified in plants; the plant-unique RAB5 group ARA6 was acquired in addition to conventional RAB5 during evolution. In Arabidopsis thaliana, conventional RAB5, ARA7 and RHA1 regulate the endosomal/vacuolar trafficking pathways, whereas ARA6 acts in the pathway from the endosome to the plasma membrane. Despite their distinct functions, all RAB5 members are activated by the common GEF VACUOLAR PROTEIN SORTING 9a (VPS9a). VPS9a consists of an N-terminal conserved domain and C-terminal region (CTR) with no similarity to known functional domains. In this study, we investigated the function of the CTR by generating truncated versions of VPS9a and found that it was specifically responsible for ARA6 regulation; moreover, the CTR was required for the oligomerization and correct localization of VPS9a. The oligomerization of VPS9a was mediated by a distinctive region consisting of 36 amino acids in the CTR that was conserved in plant RAB5 GEFs. Thus the VPS9a CTR plays an important role in the regulation of the two RAB5 groups in plants.

Guanine nucleotide exchange factor 2 for Rab5 proteins coordinated with GLUP6/GEF regulates the intracellular transport of the proglutelin from the Golgi apparatus to the protein storage vacuole in rice endosperm

Rice glutelin polypeptides are initially synthesized on the endoplasmic reticulum (ER) membrane as a proglutelin, which are then transported to the protein storage vacuole (PSV) via the Golgi apparatus. Rab5 and its cognate activator guanine nucleotide exchange factor (GEF) are essential for the intracellular transport of proglutelin from the Golgi apparatus to the PSV. Results from previous studies showed that the double recessive type of glup4/rab5a and glup6/gef mutant accumulated much higher amounts of proglutelin than either parent line. The present study demonstrates that the double recessive type of glup4/rab5a and glup6/gef mutant showed not only elevated proglutelin levels and much larger paramural bodies but also reduced the number and size of PSVs, indicating a synergistic mutation effect. These observations led us to the hypothesis that other isoforms of Rab5 and GEF also participate in the intracellular transport of rice glutelin. A database search identified a novel guanine nucleotide exchange factor, Rab5-GEF2. Like GLUP6/GEF, Rab5-GEF2 was capable of activating Rab5a and two other Rab5 isoforms in in vitro GTP/GDP exchange assays. GEF proteins consist of the helical bundle (HB) domain at the N-terminus, Vps9 domain, and a C-terminal region. By the deletion analysis of GEFs, the HB domain was found essential for the activation of Rab5 proteins.

The Plant-Specific RAB5 GTPase ARA6 is Required for Starch and Sugar Homeostasis in Arabidopsis thaliana

Endosomal trafficking plays integral roles in various eukaryotic cell activities. In animal cells, a member of the RAB GTPase family, RAB5, is a key regulator of various endosomal functions. In addition to orthologs of animal RAB5, plants harbor the plant-specific RAB5 group, the ARA6 group, which is conserved in land plant lineages. In Arabidopsis thaliana, ARA6 and conventional RAB5 act in distinct endosomal trafficking pathways; ARA6 mediates trafficking from endosomes to the plasma membrane, whereas conventional RAB5 acts in endocytic and vacuolar trafficking pathways. ARA6 is also required for normal salt and osmotic stress tolerance, although the functional link between ARA6 and stress tolerance remains unclear. In this study, we investigated ARA6 function in stress tolerance by monitoring broad-scale changes in gene expression in the ara6 mutant. A comparison of the expression profiles between wild-type and ara6-1 plants revealed that the expression of the Qua-Quine Starch (QQS) gene was significantly affected by the ara6-1 mutation. QQS is involved in starch homeostasis, consistent with the starch content decreasing in the ara6 mutants to approximately 60% of that of the wild-type plant. In contrast, the free and total glucose content increased in the ara6 mutants. Moreover, the proliferation of Pseudomonas syringae pv. tomato DC3000 was repressed in ara6 mutants, which could be attributed to the elevated sugar content. These results suggest that ARA6 is responsible for starch and sugar homeostasis, most probably through the function of QQS.

BEACH-domain proteins act together in a cascade to mediate vacuolar protein trafficking and disease resistance in Arabidopsis

Membrane trafficking to the protein storage vacuole (PSV) is a specialized process in seed plants. However, this trafficking mechanism to PSV is poorly understood. Here, we show that three types of Beige and Chediak-Higashi (BEACH)-domain proteins contribute to both vacuolar protein transport and effector-triggered immunity (ETI). We screened a green fluorescent seed (GFS) library of Arabidopsis mutants with defects in vesicle trafficking and isolated two allelic mutants gfs3 and gfs12 with a defect in seed protein transport to PSV. The gene responsible for the mutant phenotype was found to encode a putative protein belonging to group D of BEACH-domain proteins, which possess kinase domains. Disruption of other BEACH-encoding loci in the gfs12 mutant showed that BEACH homologs acted in a cascading manner for PSV trafficking. The epistatic genetic interactions observed among BEACH homologs were also found in the ETI responses of the gfs12 and gfs12 bchb-1 mutants, which showed elevated avirulent bacterial growth. The GFS12 kinase domain interacted specifically with the pleckstrin homology domain of BchC1. These results suggest that a cascade of multiple BEACH-domain proteins contributes to vacuolar protein transport and plant defense.

Membrane trafficking pathways and their roles in plant-microbe interactions

Membrane trafficking functions in the delivery of proteins that are newly synthesized in the endoplasmic reticulum (ER) to their final destinations, such as the plasma membrane (PM) and the vacuole, and in the internalization of extracellular components or PM-associated proteins for recycling or degradative regulation. These trafficking pathways play pivotal roles in the rapid responses to environmental stimuli such as challenges by microorganisms. In this review, we provide an overview of the current knowledge of plant membrane trafficking and its roles in plant-microbe interactions. Although there is little information regarding the mechanism of pathogenic modulation of plant membrane trafficking thus far, recent research has identified many membrane trafficking factors as possible targets of microbial modulation.

Molecular and biochemical analysis of the first ARA6 homologue, a RAB5 GTPase, from green algae

RAB5 GTPases are important regulators of endosomal membrane traffic in yeast, plants, and animals. A specific subgroup of this family, the ARA6 group, has been described in land plants including bryophytes, lycophytes, and flowering plants. Here, we report on the isolation of an ARA6 homologue in a green alga. CaARA6 (CaRABF1) from Chara australis, a member of the Characeae that is a close relative of land plants, encodes a polypeptide of 237 aa with a calculated molecular mass of 25.4 kDa, which is highly similar to ARA6 members from Arabidopsis thaliana and other land plants and has GTPase activity. When expressed in Nicotiana benthamiana leaf epidermal cells, fluorescently tagged CaARA6 labelled organelles with diameters between 0.2 and 1.2 µm, which co-localized with fluorescently tagged AtARA6 known to be present on multivesicular endosomes. Mutations in the membrane-anchoring and GTP-binding sites altered the localization of CaARA6 comparable to that of A. thaliana ARA6 (RABF1). In characean internodal cells, confocal immunofluorescence and immunogold electron microscopy with antibodies against AtARA6 and CaARA6 revealed ARA6 epitopes not only at multivesicular endosomes but also at the plasma membrane, including convoluted domains (charasomes), and at the trans-Golgi network. Our findings demonstrate that ARA6-like proteins have a more ancient origin than previously thought. They indicate further that ARA6-like proteins could have different functions in spite of the high similarity between characean algae and flowering plants.

RABA members act in distinct steps of subcellular trafficking of the FLAGELLIN SENSING2 receptor

Cell surface proteins play critical roles in the perception of environmental stimuli at the plasma membrane (PM) and ensuing signal transduction. Intracellular localization of such proteins must be strictly regulated, which requires elaborate integration of exocytic and endocytic trafficking pathways. Subcellular localization of Arabidopsis thaliana FLAGELLIN SENSING2 (FLS2), a receptor that recognizes bacterial flagellin, also depends on membrane trafficking. However, our understanding about the mechanisms involved is still limited. In this study, we visualized ligand-induced endocytosis of FLS2 using green fluorescent protein (GFP)-tagged FLS2 expressed in Nicotiana benthamiana. Upon treatment with the flg22 peptide, internalized FLS2-GFP from the PM was transported to a compartment with properties intermediate between the trans-Golgi network (TGN) and the multivesicular endosome. This compartment gradually discarded the TGN characteristics as it continued along the trafficking pathway. We further found that FLS2 endocytosis involves distinct RABA/RAB11 subgroups at different steps. Moreover, we demonstrated that transport of de novo-synthesized FLS2 to the PM also involves a distinct RABA/RAB11 subgroup. Our results demonstrate the complex regulatory system for properly localizing FLS2 and functional differentiation in RABA members in endo- and exocytosis.

Evidence for sequential action of Rab5 and Rab7 GTPases in prevacuolar organelle partitioning

GTPases of the Rab5 and Rab7 families were shown to control vacuolar sorting but their specific subcellular localization is controversial in plants. Here, we show that both the canonical as well as the plant-specific Rab5 reside at the newly discovered 'late prevacuolar compartment' (LPVC) while Rab7 partitions to the vacuolar membrane when expressed at low levels. Higher expression levels of wild-type Rab5 GTPases but not Rab7 lead to dose-dependent inhibition of biosynthetic vacuolar transport. In the case of Ara6, this included aberrant co-localization with markers for earlier post-Golgi compartments including the trans-Golgi network. However, nucleotide-free mutants of all three GTPases (Rha1, Ara6 and Rab7) cause stronger dose-dependent inhibition of vacuolar sorting. In addition, nucleotide-free Rha1 led to a later maturation defect and co-localization of markers for the prevacuolar compartment (PVC) and the LPVC. The corresponding Rab7 mutant strongly inhibited vacuolar delivery without merging of PVC and LPVC markers. Evidence for functional differentiation of the Rab5 family members is underlined by the fact that mutant Rha1 expression can be suppressed by increasing wild-type Rha1 levels while mutant Ara6 specifically titrates the nucleotide exchange factor Vps9. A model describing the sequential action of Rab5 and Rab7 GTPases is presented in the light of the current observations.

Hybrid proline-rich proteins: novel players in plant cell elongation?

BACKGROUND AND AIMS

Hybrid proline-rich proteins (HyPRPs) represent a large family of putative cell-wall proteins characterized by the presence of a variable N-terminal domain and a conserved C-terminal domain that is related to non-specific lipid transfer proteins. The function of HyPRPs remains unclear, but their widespread occurrence and abundant expression patterns indicate that they may be involved in a basic cellular process.

METHODS

To elucidate the cellular function of HyPRPs, we modulated the expression of three HyPRP genes in tobacco (Nicotiana tabacum) BY-2 cell lines and in potato (Solanum tuberosum) plants.

KEY RESULTS

In BY-2 lines, over-expression of the three HyPRP genes with different types of N-terminal domains resulted in similar phenotypic changes, namely increased cell elongation, both in suspension culture and on solid media where the over-expression resulted in enhanced calli size. The over-expressing cells showed increased plasmolysis in a hypertonic mannitol solution and accelerated rate of protoplast release, suggesting loosening of the cell walls. In contrast to BY-2 lines, no phenotypic changes were observed in potato plants over-expressing the same or analogous HyPRP genes, presumably due to more complex compensatory mechanisms in planta.

CONCLUSIONS

Based on the results from BY-2 lines, we propose that HyPRPs, more specifically their C-terminal domains, represent a novel group of proteins involved in cell expansion.

Endosomal trafficking pathway regulated by ARA6, a RAB5 GTPase unique to plants

Lineage-specific expansion, followed by functional diversification of key components that act in membrane trafficking, is thought to contribute to lineage-specific diversification of organelles and membrane trafficking pathways. Indeed, recent comparative genomic studies have indicated that specific expansion of RAB and SNARE molecules occurred independently in various eukaryotic lineages over evolutionary history. However, experimental verification of this notion is difficult, because detailed functional analyses of RAB and SNARE proteins uniquely acquired by specific lineages are essential to understanding how new membrane trafficking pathways may have evolved. Recently, we found that a plant-specific RAB GTPase, ARA6, and a plant-unique R-SNARE, VAMP727, mediate a trafficking pathway from endosomes to the plasma membrane in Arabidopsis thaliana. Although a similar endosomal trafficking pathway was also reported in animals, the molecular machineries acting in these trafficking systems differ between animals and plants. Thus, trafficking pathways from endosomes to the plasma membrane appear to have been acquired independently in animal and plant systems. We further demonstrated that the ARA6-mediated trafficking pathway is required for the proper salt-stress response of A. thaliana. These results indicate that acquisition of a new membrane trafficking pathway may be associated with maximization of the fitness of each organism in a lineage-specific manner.

The small GTPase Rab5a is essential for intracellular transport of proglutelin from the Golgi apparatus to the protein storage vacuole and endosomal membrane organization in developing rice endosperm

Rice (Oryza sativa) glutelins are synthesized on the endoplasmic reticulum as larger precursors, which are then transported via the Golgi to the protein storage vacuole (PSV), where they are processed into acidic and basic subunits. Three independent glutelin precursor mutant4 (glup4) rice lines, which accumulated elevated levels of proglutelin over the wild type, were identified as loss-of-function mutants of Rab5a, the small GTPase involved in vesicular membrane transport. In addition to the plasma membrane, Rab5a colocalizes with glutelins on the Golgi apparatus, Golgi-derived dense vesicles, and the PSV, suggesting that Rab5a participates in the transport of the proglutelin from the Golgi to the PSV. This spatial distribution pattern was dramatically altered in the glup4 mutants. Numerous smaller protein bodies containing glutelin and α-globulin were evident, and the proteins were secreted extracellularly. Moreover, all three independent glup4 allelic lines displayed the novel appearance of a large dilated, structurally complex paramural body containing proglutelins, α-globulins, membrane biomarkers for the Golgi apparatus, prevacuolar compartment, PSV, and the endoplasmic reticulum luminal chaperones BiP and protein disulfide isomerase as well as β-glucan. These results indicate that the formation of the paramural bodies in glup4 endosperm was due to a significant disruption of endocytosis and membrane vesicular transport by Rab5a loss of function. Overall, Rab5a is required not only for the intracellular transport of proglutelins from the Golgi to the PSV in rice endosperm but also in the maintenance of the general structural organization of the endomembrane system in developing rice seeds.

Vacuolar transport in tobacco leaf epidermis cells involves a single route for soluble cargo and multiple routes for membrane cargo

We tested if different classes of vacuolar cargo reach the vacuole via distinct mechanisms by interference at multiple steps along the transport route. We show that nucleotide-free mutants of low molecular weight GTPases, including Rab11, the Rab5 members Rha1 and Ara6, and the tonoplast-resident Rab7, caused induced secretion of both lytic and storage vacuolar cargo. In situ analysis in leaf epidermis cells indicates a sequential action of Rab11, Rab5, and Rab7 GTPases. Compared with Rab5 members, mutant Rab11 mediates an early transport defect interfering with the arrival of cargo at prevacuoles, while mutant Rab7 inhibits the final delivery to the vacuole and increases cargo levels in prevacuoles. In contrast with soluble cargo, membrane cargo may follow different routes. Tonoplast targeting of an α-TIP chimera was impaired by nucleotide-free Rha1, Ara6, and Rab7 similar to soluble cargo. By contrast, the tail-anchored tonoplast SNARE Vam3 shares only the Rab7-mediated vacuolar deposition step. The most marked difference was observed for the calcineurin binding protein CBL6, which was insensitive to all Rab mutants tested. Unlike soluble cargo, α-TIP and Vam3, CBL6 transport to the vacuole was COPII independent. The results indicate that soluble vacuolar proteins follow a single route to vacuoles, while membrane spanning proteins may use at least three different transport mechanisms.

Live cell imaging of paxillin in rolling neutrophils by dual-color quantitative dynamic footprinting

OBJECTIVE

Neutrophil recruitment to sites of inflammation involves P-selectin-dependent rolling. qDF is a useful tool to visualize the topography of the neutrophil footprint as it interacts with the substrate. However, elucidating the role of specific proteins in addition to topography requires simultaneous visualization of two fluorochromes.

METHODS

To validate DqDF, mouse neutrophils were labeled with the membrane dyes DiO and DiI and perfused into microchannels coated with P-selectin-Fc. Footprints of rolling neutrophils were recorded as two separate images, one for each fluorochrome. To assess the localization of the cytoskeletal protein paxillin, we applied DqDF to DiO-stained neutrophils of mice expressing an mCherry-paxillin fusion protein.

RESULTS

The footprint topographies obtained from DiO and DiI in the plasma membrane were identical. The z-coordinates of the microvilli tips obtained with the two fluorochromes in the footprint were also identical. Paxillin was found to be localized to some, but not all ridges in the neutrophil footprint.

CONCLUSIONS

Our data suggest that the spectral properties of the fluorochrome do not affect the results. DqDF will be useful for simultaneous visualization of two fluorochromes in the footprint of rolling cells.

Vacuolar Ca(2+) uptake

Calcium transporters that mediate the removal of Ca(2+) from the cytosol and into internal stores provide a critical role in regulating Ca(2+) signals following stimulus induction and in preventing calcium toxicity. The vacuole is a major calcium store in many organisms, particularly plants and fungi. Two main pathways facilitate the accumulation of Ca(2+) into vacuoles, Ca(2+)-ATPases and Ca(2+)/H(+) exchangers. Here I review the biochemical and regulatory features of these transporters that have been characterised in yeast and plants. These Ca(2+) transport mechanisms are compared with those being identified from other vacuolated organisms including algae and protozoa. Studies suggest that Ca(2+) uptake into vacuoles and other related acidic Ca(2+) stores occurs by conserved mechanisms which developed early in evolution.

Cell-specific vacuolar calcium storage mediated by CAX1 regulates apoplastic calcium concentration, gas exchange, and plant productivity in Arabidopsis

The physiological role and mechanism of nutrient storage within vacuoles of specific cell types is poorly understood. Transcript profiles from Arabidopsis thaliana leaf cells differing in calcium concentration ([Ca], epidermis <10 mM versus mesophyll >60 mM) were compared using a microarray screen and single-cell quantitative PCR. Three tonoplast-localized Ca(2+) transporters, CAX1 (Ca(2+)/H(+)-antiporter), ACA4, and ACA11 (Ca(2+)-ATPases), were identified as preferentially expressed in Ca-rich mesophyll. Analysis of respective loss-of-function mutants demonstrated that only a mutant that lacked expression of both CAX1 and CAX3, a gene ectopically expressed in leaves upon knockout of CAX1, had reduced mesophyll [Ca]. Reduced capacity for mesophyll Ca accumulation resulted in reduced cell wall extensibility, stomatal aperture, transpiration, CO(2) assimilation, and leaf growth rate; increased transcript abundance of other Ca(2+) transporter genes; altered expression of cell wall-modifying proteins, including members of the pectinmethylesterase, expansin, cellulose synthase, and polygalacturonase families; and higher pectin concentrations and thicker cell walls. We demonstrate that these phenotypes result from altered apoplastic free [Ca(2+)], which is threefold greater in cax1/cax3 than in wild-type plants. We establish CAX1 as a key regulator of apoplastic [Ca(2+)] through compartmentation into mesophyll vacuoles, a mechanism essential for optimal plant function and productivity.

Membrane traffic and fusion at post-Golgi compartments

Complete sequencing of the Arabidopsis genome a decade ago has facilitated the functional analysis of various biological processes including membrane traffic by which many proteins are delivered to their sites of action and turnover. In particular, membrane traffic between post-Golgi compartments plays an important role in cell signaling, taking care of receptor-ligand interaction and inactivation, which requires secretion, endocytosis, and recycling or targeting to the vacuole for degradation. Here, we discuss recent studies that address the identity of post-Golgi compartments, the machinery involved in traffic and fusion or functionally characterized cargo proteins that are delivered to or pass through post-Golgi compartments. We also provide an outlook on future challenges in this area of research.

The solution structure of a plant calmodulin and the CaM-binding domain of the vacuolar calcium-ATPase BCA1 reveals a new binding and activation mechanism

The type IIb class of plant Ca(2+)-ATPases contains a unique N-terminal extension that encompasses a calmodulin (CaM) binding domain and an auto-inhibitory domain. Binding of Ca(2+)-CaM to this region can release auto-inhibition and activates the calcium pump. Using multidimensional NMR spectroscopy, we have determined the solution structure of the complex of a plant CaM isoform with the CaM-binding domain of the well characterized Ca(2+)-ATPase BCA1 from cauliflower. The complex has a rather elongated structure in which the two lobes of CaM do not contact each other. The anchor residues Trp-23 and Ile-40 form a 1-8-18 interaction motif. Binding of Ca(2+)-CaM gives rise to the induction of two helical parts in this unique target peptide. The two helical portions are connected by a highly positively charged bend region, which represents a relatively fixed angle and positions the two lobes of CaM in an orientation that has not been seen before in any complex structure of calmodulin. The behavior of the complex was further characterized by heteronuclear NMR dynamics measurements of the isotope-labeled protein and peptide. These data suggest a unique calcium-driven activation mechanism for BCA1 and other plant Ca(2+)-ATPases that may also explain the action of calcium-CaM on some other target enzymes. Moreover, CaM activation of plant Ca(2+)-ATPases seems to occur in an organelle-specific manner.

A recycling-defective vacuolar sorting receptor reveals an intermediate compartment situated between prevacuoles and vacuoles in tobacco

Plant vacuolar sorting receptors (VSRs) display cytosolic Tyr motifs (YMPL) for clathrin-mediated anterograde transport to the prevacuolar compartment. Here, we show that the same motif is also required for VSR recycling. A Y612A point mutation in Arabidopsis thaliana VSR2 leads to a quantitative shift in VSR2 steady state levels from the prevacuolar compartment to the trans-Golgi network when expressed in Nicotiana tabacum. By contrast, the L615A mutant VSR2 leaks strongly to vacuoles and accumulates in a previously undiscovered compartment. The latter is shown to be distinct from the Golgi stacks, the trans-Golgi network, and the prevacuolar compartment but is characterized by high concentrations of soluble vacuolar cargo and the rab5 GTPase Rha1(RabF2a). The results suggest that the prevacuolar compartment matures by gradual receptor depletion, leading to the formation of a late prevacuolar compartment situated between the prevacuolar compartment and the vacuole.

Successive silencing of tandem reporter genes in potato (Solanum tuberosum) over 5 years of vegetative propagation

BACKGROUND AND AIMS

Transgenic plants represent an excellent tool for experimental plant biology and are an important component of modern agriculture. Fully understanding the stability of transgene expression is critical in this regard. Most changes in transgene expression occur soon after transformation and thus unwanted lines can be discarded easily; however, transgenes can be silenced long after their integration.

METHODS

To study the long-term changes in transgene expression in potato (Solanum tuberosum), the activity of two reporter genes, encoding green fluorescent protein (GFP) and neomycin phosphotransferase (NPTII), was monitored in a set of 17 transgenic lines over 5 years of vegetative propagation in vitro.

KEY RESULTS

A decrease in transgene expression was observed mainly in lines with higher initial GFP expression and a greater number of T-DNA insertions. Complete silencing of the reporter genes was observed in four lines (nearly 25 %), all of which successively silenced the two reporter genes, indicating an interconnection between their silencing. The loss of GFP fluorescence always preceded the loss of kanamycin resistance. Treatment with the demethylation drug 5-azacytidine indicated that silencing of the NPTII gene, but probably not of GFP, occurred directly at the transcriptional level. Successive silencing of the two reporter genes was also reproduced in lines with reactivated expression of previously silenced transgenes.

CONCLUSIONS

We suggest a hypothetical mechanism involving the successive silencing of the two reporter genes that involves the switch of GFP silencing from the post-transcriptional to transcriptional level and subsequent spreading of methylation to the NPTII gene.

Exploring plant endomembrane dynamics using the photoconvertible protein Kaede

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

Mint essential oil can induce or inhibit potato sprouting by differential alteration of apical meristem

Sprouting of potatoes during storage, due to tuber dormancy release, is associated with weight loss and softening. Sprout-preventing chemicals, such as chlorpropham (CIPC), can negatively impact the environment and human health. Monthly thermal fogging with mint (Mentha spicata L.) essential oil (MEO) inhibited sprouting in eight potato cultivars during large-volume 6-month storage: the tubers remained firm with 38% lower weight loss after 140 days of storage. The sprout-inhibitory action may be nullified: treated tubers washed with water resumed sprouting within days, with reduced apical dominance. MEO application caused local necrosis of the bud meristem, and a few weeks later, axillary bud (AX) growth was induced in the same sprouting eye. MEO components analysis showed that 73% of its content is the monoterpene R-carvone. Tubers treated with synthetic R-carvone in equivalent dose, 4.5 microl l(-1), showed an inhibitory effect similar to that of MEO. Surprisingly, 0.5 microl l(-1) of MEO or synthetic R-carvone catalyzed AX sprouting in the tuber. To the best of our knowledge, this is the first report of an essential oil vapor inducing early sprouting of potato tubers. R-carvone caused visible damage to the meristem membrane at sprout-inhibiting, but not sprout-inducing doses, suggesting different underlying mechanisms. After 5 days' exposure to R-carvone, its derivatives transcarveol and neo-dihydrocarveol were found in buds of tubers treated with the inhibitory dose, suggesting biodegradation. These experiments demonstrate the potential of MEO vapor as an environmentally friendly alternative to CIPC in stored potatoes and as a research tool for the control of sprouting in plants.

Medicago N2-fixing symbiosomes acquire the endocytic identity marker Rab7 but delay the acquisition of vacuolar identity

Rhizobium bacteria form N(2)-fixing organelles, called symbiosomes, inside the cells of legume root nodules. The bacteria are generally thought to enter the cells via an endocytosis-like process. To examine this, we studied the identity of symbiosomes in relation to the endocytic pathway. We show that in Medicago truncatula, the small GTPases Rab5 and Rab7 are endosomal membrane identity markers, marking different (partly overlapping) endosome populations. Although symbiosome formation is considered to be an endocytosis-like process, symbiosomes do not acquire Rab5 at any stage during their development, nor do they accept the trans-Golgi network identity marker SYP4, presumed to mark early endosomes in plants. By contrast, the endosomal marker Rab7 does occur on symbiosomes from an early stage of development when they have stopped dividing up to the senescence stage. However, the symbiosomes do not acquire vacuolar SNAREs (SYP22 and VTI11) until the onset of their senescence. By contrast, symbiosomes acquire the plasma membrane SNARE SYP132 from the start of symbiosome formation throughout their development. Therefore, symbiosomes appear to be locked in a unique SYP132- and Rab7-positive endosome stage and the delay in acquiring (lytic) vacuolar identity (e.g., vacuolar SNAREs) most likely ensures their survival and maintenance as individual units.

Post-Golgi traffic in plants

Secretory and endocytic traffic through the post-Golgi endomembrane system regulates the abundance of plasma-membrane proteins such as receptors, transporters and ion channels, modulating the ability of a cell to communicate with its neighbours and to adapt to a changing environment. The major post-Golgi compartments are numerous and appear to be similar to their counterparts in animals. However, endosomes are rather ill defined morphologically but seem to be involved in specific trafficking pathways. Many plasma-membrane proteins cycle constitutively via endosomal compartments. The trans-Golgi network (TGN) appears to be an early endosome where secretory and endocytic traffic meet. Endocytosed proteins that are to be degraded are targeted to the vacuole via the multivesiculate prevacuolar compartment (PVC) whereas cycling proteins pass through recycling endosomes. The trafficking machinery involves the same classes of proteins as in other eukaryotes. However, there are modifications that match the specifics of post-Golgi traffic in plants. Although plants lack epithelia, some plasma-membrane proteins are located on specific faces of the cell which reflects polarized traffic and influences the physiological performance of the tissue. Plants also differentiate highly polarized tip-growing cells in which post-Golgi traffic is adapted to very high rates of targeted exocytosis, endocytosis and recycling.

Protein isoprenylation: the fat of the matter

Protein isoprenylation refers to the covalent attachment of a 15-carbon farnesyl or 20-carbon geranylgeranyl moiety to a cysteine residue at or near the carboxyl terminus. This post-translational lipid modification, which mediates protein-membrane and protein-protein interactions, is necessary for normal control of abscisic acid and auxin signaling, meristem development, and other fundamental processes. Recent studies have also revealed roles for protein isoprenylation in cytokinin biosynthesis and innate immunity. Most isoprenylated proteins are further modified by carboxyl terminal proteolysis and methylation and, collectively, these modifications are necessary for the targeting and function of isoprenylated proteins.

Unique mechanism of plant endocytic/vacuolar transport pathways

The post-Golgi traffic network in plant cells is highly complex, which is correlated with the large number of genes related to this function. RABs and SNAREs are key regulators of tethering and fusion of transport vesicles to target membranes, and the numbers of these regulators have also expanded in plant lineages. In addition to this increase in the net number of genes, plants also seem to have evolved new gene families tailored to fulfill plant-unique functions. In this article, we summarize recent progress in studies on plant-unique RABs and SNAREs functioning in post-Golgi trafficking, with a special focus on the endocytic pathway.

The functions of Rab GTPases in plant membrane traffic

Rab GTPases are important determinants of membrane identity and membrane targeting. Higher plants have evolved a unique set of Rab GTPases that presumably reflects the specific demands of plant cell trafficking. In recent years, significant progress has been made in identifying Rab GTPases involved in endosome organisation, cytokinesis and in post-Golgi traffic to the plasma membrane and vacuoles. These include members of the Rab-F1, Rab-F2, Rab-A1, Rab-A2 and Rab-A4 subclasses. Some important regulators or effectors have also been identified for Rab-F, Rab-A1 and Rab-A4 proteins. However, uncertainties remain about the trafficking pathways that connect the compartments in the trans-Golgi/prevacuolar/endosomal system and there is still little or no insight into the functions of several major subclasses within the Rab GTPase family.

Intermediate organelles of the plant secretory pathway: identity and function

The secretory pathway of eukaryotic cells comprises a network of organelles that connects three large membranes, the plasma membrane, the vacuole and the endoplasmic reticulum. The Golgi apparatus and the various post-Golgi organelles that control vacuolar sorting, secretion and endocytosis can be regarded as intermediate organelles of the endocytic and biosynthetic routes. Many processes in the secretory pathway have evolved differently in plants and cannot be studied using yeast or mammalian cells as models. The best characterized organelles are the Golgi apparatus and the prevacuolar compartment, but recent work has shed light on the role of the trans Golgi network, which has to be regarded as a separate organelle in plants. In this study, we wish to highlight recent findings regarding the late secretory pathway and its crosstalk with the early secretory pathway as well as the endocytic route in plants. Recently published findings and suggested models are discussed within the context of known features of the equivalent pathway in other eukaryotes.

Spatial quantitative analysis of fluorescently labeled nuclear structures: problems, methods, pitfalls

The vast majority of microscopic data in biology of the cell nucleus is currently collected using fluorescence microscopy, and most of these data are subsequently subjected to quantitative analysis. The analysis process unites a number of steps, from image acquisition to statistics, and at each of these steps decisions must be made that may crucially affect the conclusions of the whole study. This often presents a really serious problem because the researcher is typically a biologist, while the decisions to be taken require expertise in the fields of physics, computer image analysis, and statistics. The researcher has to choose between multiple options for data collection, numerous programs for preprocessing and processing of images, and a number of statistical approaches. Written for biologists, this article discusses some of the typical problems and errors that should be avoided. The article was prepared by a team uniting expertise in biology, microscopy, image analysis, and statistics. It considers the options a researcher has at the stages of data acquisition (choice of the microscope and acquisition settings), preprocessing (filtering, intensity normalization, deconvolution), image processing (radial distribution, clustering, co-localization, shape and orientation of objects), and statistical analysis.

Endosomal functions in plants

Plant endosomes are highly dynamic organelles that are involved in the constitutive recycling of plasma membrane cargo and the trafficking of polarized plasma membrane proteins such as auxin carriers. In addition, recent studies have shown that surface receptors such as the plant defense-related FLS2 receptor and the brassinosteroid receptor BRI1 appear to signal from endosomes upon ligand binding and internalization. In yeast and mammals, endosomes are also known to recycle vacuolar cargo receptors back to the trans Golgi network and sort membrane proteins for degradation in the vacuole/lysosome. Some of these sorting mechanisms are mediated by the retromer and endosomal sorting complex required for transport (ESCRT) complexes. Plants contain orthologs of all major retromer and ESCRT complex subunits, but they have also evolved variations in endosomal functions connected to plant-specific features such as the diversity of vacuolar transport pathways. This review focuses on recent studies in plants dealing with the regulation of endosomal recycling functions, architecture and formation of multivesicular bodies, ligand-mediated endocytosis and receptor signaling from endosomes as well as novel endosomal markers and the function of endosomes in the transport and processing of soluble vacuolar proteins.

Evidence for a sorting endosome in Arabidopsis root cells

In eukaryotic cells, the endocytic and secretory pathways are key players in several physiological processes. These pathways are largely inter-connected in animal and yeast cells through organelles named sorting endosomes. Sorting endosomes are multi-vesicular compartments that redirect proteins towards various destinations, such as the lysosomes or vacuoles for degradation, the trans-Golgi network for retrograde transport and the plasma membrane for recycling. In contrast, cross-talk between the endocytic and secretory pathways has not been clearly established in plants, especially in terms of cargo protein trafficking. Here we show by co-localization analyses that endosomes labelled with the AtSORTING NEXIN1 (AtSNX1) protein overlap with the pre-vacuolar compartment in Arabidopsis root cells. In addition, alteration of the routing functions of AtSNX1 endosomes by drug treatments leads to mis-routing of endocytic and secretory cargo proteins. Based on these results, we propose that the AtSNX1 endosomal compartment represents a sorting endosome in root cells, and that this specialized organelle is conserved throughout eukaryotes.

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

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

The role of Rab GTPases in cell wall metabolism

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

VPS9a, the common activator for two distinct types of Rab5 GTPases, is essential for the development of Arabidopsis thaliana

Rab5, a subfamily of Rab GTPases, regulates a variety of endosomal functions as a molecular switch. Arabidopsis thaliana has two different types of Rab5-member GTPases: conventional type, ARA7 and RHA1, and a plant-specific type, ARA6. We found that only one guanine nucleotide exchange factor (GEF), named VPS9a, can activate all Rab5 members to GTP-bound forms in vitro in spite of their diverged structures. In the vps9a-1 mutant, whose GEF activity is completely lost, embryogenesis was arrested at the torpedo stage. Green fluorescent protein (GFP)-ARA7 and ARA6-GFP were diffused in cytosol like GDP-fixed mutants of Rab5 in vps9a-1, indicating that both types of GTPase are regulated by VPS9a. In the leaky vps9a-2 mutant, elongation of the primary root was severely affected. Overexpression of the GTP-fixed form of ARA7 suppressed the vps9a-2 mutation, but overexpression of ARA6 had no apparent effects. These results indicate that the two types of plant Rab5 members are functionally differentiated, even though they are regulated by the same activator, VPS9a.

Post-Golgi protein traffic in the plant secretory pathway

The Golgi apparatus in plants is organized as a multitude of individual stacks that are motile in the cytoplasm and in close association with the endoplasmic reticulum (ER) (Boevink et al. in Plant J 15:441-447, 1998). These stacks operate as a sorting centre for cargo molecules, providing modification and redirection to other organelles as appropriate. In the post-Golgi direction, these include vacuole and plasma membrane, and specialized transport routes to each are required to prevent mislocalization. Recent evidence in plant cells points to the existence of post-Golgi organelles that function as intermediate stations for efficient protein traffic, as well as to the influence of small GTPases such as Rabs and ARFs on post-Golgi trafficking. This review focuses on the latest findings on post-Golgi trafficking routes and on the involvement of GTPases and their effectors on the trafficking of proteins in the plant secretory pathway.

Identification of a calmodulin-regulated autoinhibited Ca2+-ATPase (ACA11) that is localized to vacuole membranes in Arabidopsis

In plant cells, the vacuole functions as a major calcium store. Although a calmodulin-regulated Ca2+-ATPase (ACA4) is known to be present in prevacuolar compartments, the presence of an ACA-type Ca2+-ATPase in the mature vacuole of a plant cell has not been verified. Here we provide evidence that ACA11 localizes to the vacuole membrane. ACA11 tagged with GFP was expressed in stable transgenic plants, and visualized in root cells and protoplasts by confocal microscopy. A Ca2+-ATPase function for ACA11 was confirmed by complementation of yeast mutants. A calmodulin binding domain was identified within the first 37 residues of the N-terminal autoinhibitory region.

A guided tour into subcellular colocalization analysis in light microscopy

It is generally accepted that the functional compartmentalization of eukaryotic cells is reflected by the differential occurrence of proteins in their compartments. The location and physiological function of a protein are closely related; local information of a protein is thus crucial to understanding its role in biological processes. The visualization of proteins residing on intracellular structures by fluorescence microscopy has become a routine approach in cell biology and is increasingly used to assess their colocalization with well-characterized markers. However, image-analysis methods for colocalization studies are a field of contention and enigma. We have therefore undertaken to review the most currently used colocalization analysis methods, introducing the basic optical concepts important for image acquisition and subsequent analysis. We provide a summary of practical tips for image acquisition and treatment that should precede proper colocalization analysis. Furthermore, we discuss the application and feasibility of colocalization tools for various biological colocalization situations and discuss their respective strengths and weaknesses. We have created a novel toolbox for subcellular colocalization analysis under ImageJ, named JACoP, that integrates current global statistic methods and a novel object-based approach.

Divergent functions of VTI12 and VTI11 in trafficking to storage and lytic vacuoles in Arabidopsis

The protein storage vacuole (PSV) is a plant-specific organelle that accumulates reserve proteins, one of the main agricultural products obtained from crops. Despite the importance of this process, the cellular machinery required for transport and accumulation of storage proteins remains largely unknown. Interfering with transport to PSVs has been shown to result in secretion of cargo. Therefore, secretion of a suitable marker could be used as an assay to identify mutants in this pathway. CLV3, a negative regulator of shoot stem cell proliferation, is an extracellular ligand that is rendered inactive when targeted to vacuoles. We devised an assay where trafficking mutants secrete engineered vacuolar CLV3 and show reduced meristems, a phenotype easily detected by visual inspection of plants. We tested this scheme in plants expressing VAC2, a fusion of CLV3 to the vacuolar sorting signal from the storage protein barley lectin. In this way, we determined that trafficking of VAC2 requires the SNARE VTI12 but not its close homologue, the conditionally redundant VTI11 protein. Furthermore, a vti12 mutant is specifically altered in transport of storage proteins, whereas a vti11 mutant is affected in transport of a lytic vacuole marker. These results demonstrate the specialization of VTI12 and VTI11 in mediating trafficking to storage and lytic vacuoles, respectively. Moreover, they validate the VAC2 secretion assay as a simple method to isolate genes that mediate trafficking to the PSV.

The ATP binding cassette transporter AtMRP5 modulates anion and calcium channel activities in Arabidopsis guard cells

Stomatal guard cells control CO(2) uptake and water loss between plants and the atmosphere. Stomatal closure in response to the drought stress hormone, abscisic acid (ABA), results from anion and K(+) release from guard cells. Previous studies have shown that cytosolic Ca(2+) elevation and ABA activate S-type anion channels in the plasma membrane of guard cells, leading to stomatal closure. However, membrane-bound regulators of abscisic acid signaling and guard cell anion channels remain unknown. Here we show that the ATP binding cassette (ABC) protein AtMRP5 is localized to the plasma membrane. Mutation in the AtMRP5 ABC protein impairs abscisic acid and cytosolic Ca(2+) activation of slow (S-type) anion channels in the plasma membrane of guard cells. Interestingly, atmrp5 insertion mutant guard cells also show impairment in abscisic acid activation of Ca(2+)-permeable channel currents in the plasma membrane of guard cells. These data provide evidence that the AtMRP5 ABC transporter is a central regulator of guard cell ion channel during abscisic acid and Ca(2+) signal transduction in guard cells.

Seeking a way out: export of proteins from the plant endoplasmic reticulum

The functionality of the secretory pathway relies on the efficient transfer of cargo molecules from their site of synthesis in the endoplasmic reticulum (ER) to successive compartments within the pathway. Although transport mechanisms of secretory proteins have been studied in detail in various non-plant systems, it is only recently that our knowledge of secretory routes in plants has expanded dramatically. This review focuses on exciting new findings concerning the exit mechanisms of cargo proteins from the plant ER and the role of ER export sites in this process.

Fluorescent proteins as markers in the plant secretory pathway

The use of fluorescent proteins and live cell imaging has greatly increased our knowledge of cell biology in recent years. Not only can these technologies be used to study protein trafficking under different conditions, but they have also been of use in elucidating the relationships between different organelles in a noninvasive manner. The use of multiple different fluorochromes allows the observation of interactions between organelles and between proteins, making this one of the fastest-developing and exciting fields at this time. In this review, we discuss the multitude of fluorescent markers that have been generated to study the plant secretory pathway. Although these markers have been used to solve many mysteries in this field, some areas that require further discussion remain.