Arabidopsis TRAPPII is functionally linked to Rab-A, but not Rab-D in polar protein trafficking in trans-Golgi network

Regulation of adaptive growth decisions via phosphorylation of the TRAPPII complex in Arabidopsis

Plants often adapt to adverse or stress conditions via differential growth. The trans-Golgi network (TGN) has been implicated in stress responses, but it is not clear in what capacity it mediates adaptive growth decisions. In this study, we assess the role of the TGN in stress responses by exploring the previously identified interactome of the Transport Protein Particle II (TRAPPII) complex required for TGN structure and function. We identified physical and genetic interactions between AtTRAPPII and shaggy-like kinases (GSK3/AtSKs) and provided in vitro and in vivo evidence that the TRAPPII phosphostatus mediates adaptive responses to abiotic cues. AtSKs are multifunctional kinases that integrate a broad range of signals. Similarly, the AtTRAPPII interactome is vast and considerably enriched in signaling components. An AtSK-TRAPPII interaction would integrate all levels of cellular organization and instruct the TGN, a central and highly discriminate cellular hub, as to how to mobilize and allocate resources to optimize growth and survival under limiting or adverse conditions.

Regulation of adaptive growth decisions via phosphorylation of the TRAPPII complex in Arabidopsis

Plants often adapt to adverse or stress conditions via differential growth. The trans-Golgi Network (TGN) has been implicated in stress responses, but it is not clear in what capacity it mediates adaptive growth decisions. In this study, we assess the role of the TGN in stress responses by exploring the interactome of the Transport Protein Particle II (TRAPPII) complex, required for TGN structure and function. We identified physical and genetic interactions between TRAPPII and shaggy-like kinases (GSK3/AtSKs). Kinase assays and pharmacological inhibition provided in vitro and in vivo evidence that AtSKs target the TRAPPII-specific subunit AtTRS120/TRAPPC9. GSK3/AtSK phosphorylation sites in AtTRS120/TRAPPC9 were mutated, and the resulting AtTRS120 phosphovariants subjected to a variety of single and multiple stress conditions in planta . The non-phosphorylatable TRS120 mutant exhibited enhanced adaptation to multiple stress conditions and to osmotic stress whereas the phosphomimetic version was less resilient. Higher order inducible trappii atsk mutants had a synthetically enhanced defect in root gravitropism. Our results suggest that the TRAPPII phosphostatus mediates adaptive responses to abiotic cues. AtSKs are multifunctional kinases that integrate a broad range of signals. Similarly, the TRAPPII interactome is vast and considerably enriched in signaling components. An AtSK-TRAPPII interaction would integrate all levels of cellular organization and instruct the TGN, a central and highly discriminate cellular hub, as to how to mobilize and allocate resources to optimize growth and survival under limiting or adverse conditions.

TRIPP Is a Plant-Specific Component of the Arabidopsis TRAPPII Membrane Trafficking Complex with Important Roles in Plant Development

How the membrane trafficking system spatially organizes intracellular activities and intercellular signaling networks in plants is not well understood. Transport Protein Particle (TRAPP) complexes play key roles in the selective delivery of membrane vesicles to various subcellular compartments in yeast and animals but remain to be fully characterized in plants. Here, we investigated TRAPP complexes in Arabidopsis (Arabidopsis thaliana) using immunoprecipitation followed by quantitative mass spectrometry analysis of AtTRS33, a conserved core component of all TRAPP complexes. We identified 14 AtTRS33-interacting proteins, including homologs of all 13 TRAPP components in mammals and a protein that has homologs only in multicellular photosynthetic organisms and is thus named TRAPP-Interacting Plant Protein (TRIPP). TRIPP specifically associates with the TRAPPII complex through binary interactions with two TRAPPII-specific subunits. TRIPP colocalized with a subset of TRS33 compartments and trans-Golgi network markers in a TRS33-dependent manner. Loss-of-function tripp mutants exhibited dwarfism, sterility, partial photomorphogenesis in the dark, reduced polarity of the auxin transporter PIN2, incomplete cross wall formation, and altered localization of a TRAPPII-specific component. Therefore, TRIPP is a plant-specific component of the TRAPPII complex with important functions in trafficking, plant growth, and development.

The Small GTPase Superfamily in Plants: A Conserved Regulatory Module with Novel Functions

Small GTP-binding proteins represent a highly conserved signaling module in eukaryotes that regulates diverse cellular processes such as signal transduction, cytoskeletal organization and cell polarity, cell proliferation and differentiation, intracellular membrane trafficking and transport vesicle formation, and nucleocytoplasmic transport. These proteins function as molecular switches that cycle between active and inactive states, and this cycle is linked to GTP binding and hydrolysis. In this review, the roles of the plant complement of small GTP-binding proteins in these cellular processes are described, as well as accessory proteins that control their activity, and current understanding of the functions of individual members of these families in plants-with a focus on the model organism Arabidopsis-is presented. Some potential novel roles of these GTPases in plants, relative to their established roles in yeast and/or animal systems, are also discussed.

An essential role for Arabidopsis Trs33 in cell growth and organization in plant apical meristems

Trafficking protein particle (TRAPP) complexes subunit gene AtTrs33 plays an important role in keeping apical meristematic activity and dominance in Arabidopsis. TRAPP complexes, composed of multimeric subunits, are guanine-nucleotide exchange factors for certain Rab GTPases and are believed to be involved in the regulation of membrane trafficking, but the cases in Arabidopsis are largely unknown. Trs33, recently proposed to be a component of TRAPP IV, is non-essential in yeast cells. A single copy of Trs33 gene, AtTrs33, was identified in Arabidopsis. GUS activity assay indicated that AtTrs33 was ubiquitously expressed. Based on a T-DNA insertion line, we found that loss-of-function of AtTrs33 is lethal for apical growth. Knock-down or knock-in of AtTrs33 affects apical meristematic growth and fertility, which indicates that AtTrs33 plays an important role in keeping apical meristematic activity and dominance in Arabidopsis. Analysis of auxin responses and PIN1/2 localization indicate that impaired apical meristematic activity and dominance were caused by altered auxin responses through non-polarized PIN1 localization. The present study reported that AtTrs33 plays an essential role in Arabidopsis cell growth and organization, which is different with its homologue in yeast. These findings provide new insights into the functional divergence of TRAPP subunits.

AtTRAPPC11 is involved in TRAPPIII mediated control of post-Golgi protein trafficking

The plant trans-Golgi Network/Early Endosome (TGN/EE), as an organizer of vesicle trafficking, fulfills a crucial role for plant development and adaptation. Because it coordinates the transport of cell material along different routes, it is expected that a number of TGN/EE associated factors function in the rapid organization of post-Golgi trafficking to ensure that proteins reach their destination. The roles of Transport Protein Particle (TRAPP) complexes in the regulation of plant post-Golgi trafficking start to emerge. We previously demonstrated that the plant TRAPPIII complex is involved in maintenance of TGN organization and function and has a role in endocytic trafficking mediated by the SYP61 TGN/EE compartment. Here we show that attrappc11 mutants display accumulation of the plasma membrane resident proteins CESA6, BRI1 and PIP1;4 in aberrant intracellular compartments. This adds further insights into the functions of TRAPPIII as a regulators of post-Golgi/endosomal traffic.

Interactions between Transport Protein Particle (TRAPP) complexes and Rab GTPases in Arabidopsis

Transport Protein Particle II (TRAPPII) is essential for exocytosis, endocytosis, protein sorting and cytokinesis. In spite of a considerable understanding of its biological role, little information is known about Arabidopsis TRAPPII complex topology and molecular function. In this study, independent proteomic approaches initiated with TRAPP components or Rab-A GTPase variants converge on the TRAPPII complex. We show that the Arabidopsis genome encodes the full complement of 13 TRAPPC subunits, including four previously unidentified components. A dimerization model is proposed to account for binary interactions between TRAPPII subunits. Preferential binding to dominant negative (GDP-bound) versus wild-type or constitutively active (GTP-bound) RAB-A2a variants discriminates between TRAPPII and TRAPPIII subunits and shows that Arabidopsis complexes differ from yeast but resemble metazoan TRAPP complexes. Analyzes of Rab-A mutant variants in trappii backgrounds provide genetic evidence that TRAPPII functions upstream of RAB-A2a, allowing us to propose that TRAPPII is likely to behave as a guanine nucleotide exchange factor (GEF) for the RAB-A2a GTPase. GEFs catalyze exchange of GDP for GTP; the GTP-bound, activated, Rab then recruits a diverse local network of Rab effectors to specify membrane identity in subsequent vesicle fusion events. Understanding GEF-Rab interactions will be crucial to unravel the co-ordination of plant membrane traffic.

AtTRAPPC11/ROG2: A Role for TRAPPs in Maintenance of the Plant Trans-Golgi Network/Early Endosome Organization and Function

The dynamic trans-Golgi network/early endosome (TGN/EE) facilitates cargo sorting and trafficking and plays a vital role in plant development and environmental response. Transport protein particles (TRAPPs) are multi-protein complexes acting as guanine nucleotide exchange factors and possibly as tethers, regulating intracellular trafficking. TRAPPs are essential in all eukaryotic cells and are implicated in a number of human diseases. It has been proposed that they also play crucial roles in plants; however, our current knowledge about the structure and function of plant TRAPPs is very limited. Here, we identified and characterized AtTRAPPC11/RESPONSE TO OLIGOGALACTURONIDE2 (AtTRAPPC11/ROG2), a TGN/EE-associated, evolutionarily conserved TRAPP protein in Arabidopsis (Arabidopsis thaliana). AtTRAPPC11/ROG2 regulates TGN integrity, as evidenced by altered TGN/EE association of several residents, including SYNTAXIN OF PLANTS61, and altered vesicle morphology in attrappc11/rog2 mutants. Furthermore, endocytic traffic and brefeldin A body formation are perturbed in attrappc11/rog2, suggesting a role for AtTRAPPC11/ROG2 in regulation of endosomal function. Proteomic analysis showed that AtTRAPPC11/ROG2 defines a hitherto uncharacterized TRAPPIII complex in plants. In addition, attrappc11/rog2 mutants are hypersensitive to salinity, indicating an undescribed role of TRAPPs in stress responses. Overall, our study illustrates the plasticity of the endomembrane system through TRAPP protein functions and opens new avenues to explore this dynamic network.

Independent yet overlapping pathways ensure the robustness and responsiveness of trans-Golgi network functions in Arabidopsis

The trans-Golgi-network (TGN) has essential housekeeping functions in secretion, endocytosis and protein sorting, but also more specialized functions in plant development. How the robustness of basal TGN function is ensured while specialized functions are differentially regulated is poorly understood. Here, we investigate two key regulators of TGN structure and function, ECHIDNA and the Transport Protein Particle II (TRAPPII) tethering complex. An analysis of physical, network and genetic interactions suggests that two network communities are implicated in TGN function and that ECHIDNA and TRAPPII belong to distinct yet overlapping pathways. Whereas ECHIDNA and TRAPPII colocalized at the TGN in interphase cells, their localization diverged in dividing cells. Moreover, ECHIDNA and TRAPPII localization patterns were mutually independent. TGN structure, endocytosis and sorting decisions were differentially impacted in echidna and trappii mutants. Our analyses point to a partitioning of specialized TGN functions, with ECHIDNA being required for cell elongation and TRAPPII for cytokinesis. Two independent pathways able to compensate for each other might contribute to the robustness of TGN housekeeping functions and to the responsiveness and fine tuning of its specialized functions.

AtBET5 is essential for exine pattern formation and apical meristem organization in Arabidopsis

BET5 is a component of trafficking protein particle (TRAPP) which has been studied extensively in non-plant organisms where they are involved in membrane trafficking within Golgi and between Golgi and early endosomes. Recent analysis of TRAPP in different classes of organisms indicates that TRAPP function might exhibit differences among organisms. A single copy of the BET5 gene named AtBET5 was found in the Arabidopsis genome based on sequence similarity. Developmental phenotype and the underlying mechanisms have been characterized upon transcriptional knock-down lines generated by both T-DNA insertion and RNAi. Pollen grains of the T-DNA insertional line present reduced fertility and pilate exine instead of tectate exine. Perturbation of the AtBET5 expression by RNAi leads to apical meristematic organization defects and reduced fertility as well. The reduced fertility was due to the pollination barrier caused by an altered composition and structure of pollen walls. Auxin response in root tip cells is altered and there is a severe disruption in polar localization of PIN1-GFP, but to a less extent of PIN2-GFP in the root tips, which causes the apical meristematic organization defects and might also be responsible for the secretion of sporopollenin precursor or polar targeting of sporopollenin precursor transporters.

Myosin XI localizes at the mitotic spindle and along the cell plate during plant cell division in Physcomitrella patens

Cell division is a fundamental biological process that has been extensively investigated in different systems. Similar to most eukaryotic cells, plant cells assemble a mitotic spindle to separate replicated chromosomes. In contrast, to complete cell division, plant cells assemble a phragmoplast, which is composed of aligned microtubules and actin filaments. This structure helps transport vesicles containing new cell wall material, which then fuse to form the cell plate; the cell plate will expand to create the new dividing cell wall. Because vesicles are known to be transported by myosin motors during interphase, we hypothesized this could also be the case during cell division and we investigated the localization of the plant homologue of myosin V - myosin XI, in cell division. In this work, we used the protonemal cells of the moss Physcomitrella patens as a model, because of its simple cellular morphology and ease to generate transgenic cell lines expressing fluorescent tagged proteins. Using a fluorescent protein fusion of myosin XI, we found that, during mitosis, this molecule appears to associate with the kinetochores immediately after nuclear envelope breakdown. Following metaphase, myosin XI stays associated with the spindle's midzone during the rest of mitosis, and when the phragmoplast is formed, it concentrates at the cell plate. Using an actin polymerization inhibitor, latrunculin B, we found that the association of myosin XI with the mitotic spindle and the phragmoplast are only partially dependent on the presence of filamentous actin. We also showed that myosin XI on the spindle partially overlaps with a v-SNARE vesicle marker but is not co-localized with the endoplasmic reticulum and a RabA vesicle marker. These observations suggest an actin-dependent and an actin-independent behavior of myosin XI during cell division, and provide novel insights to our understanding of the function of myosin XI during plant cell division.

Multisubunit tethering complexes in higher plants

Tethering complexes mediate the initial, specific contact between donor and acceptor membranes. This review focuses on the modularity and function of multisubunit tethering complexes (MTCs) in higher plants. One emphasis is on molecular interactions of plant MTCs. Here, a number of insights have been gained concerning interactions between different tethering complexes, and between tethers and microtubule-associated proteins. The roles of tethering complexes in abiotic stress responses appear indirect, but in the context of biotic stress responses it has been suggested that some tethers are direct targets of pathogen effectors or virulence factors. In light of the central roles tethering complexes play in plant development, an emerging concept is that tethers may be co-opted for plant adaptive responses.

SCD1 and SCD2 Form a Complex That Functions with the Exocyst and RabE1 in Exocytosis and Cytokinesis

Although exocytosis is critical for the proper trafficking of materials to the plasma membrane, relatively little is known about the mechanistic details of post-Golgi trafficking in plants. Here, we demonstrate that the DENN (Differentially Expressed in Normal and Neoplastic cells) domain protein STOMATAL CYTOKINESIS DEFECTIVE1 (SCD1) and SCD2 form a previously unknown protein complex, the SCD complex, that functionally interacts with subunits of the exocyst complex and the RabE1 family of GTPases in Arabidopsis thaliana Consistent with a role in post-Golgi trafficking, scd1 and scd2 mutants display defects in exocytosis and recycling of PIN2-GFP. Perturbation of exocytosis using the small molecule Endosidin2 results in growth inhibition and PIN2-GFP trafficking defects in scd1 and scd2 mutants. In addition to the exocyst, the SCD complex binds in a nucleotide state-specific manner with Sec4p/Rab8-related RabE1 GTPases and overexpression of wild-type RabE1 rescues scd1 temperature-sensitive mutants. Furthermore, SCD1 colocalizes with the exocyst subunit, SEC15B, and RabE1 at the cell plate and in distinct punctae at or near the plasma membrane. Our findings reveal a mechanism for plant exocytosis, through the identification and characterization of a protein interaction network that includes the SCD complex, RabE1, and the exocyst.

Cell cycle-regulated PLEIADE/AtMAP65-3 links membrane and microtubule dynamics during plant cytokinesis

Cytokinesis, the partitioning of the cytoplasm following nuclear division, requires extensive coordination between cell cycle cues, membrane trafficking and microtubule dynamics. Plant cytokinesis occurs within a transient membrane compartment known as the cell plate, to which vesicles are delivered by a plant-specific microtubule array, the phragmoplast. While membrane proteins required for cytokinesis are known, how these are coordinated with microtubule dynamics and regulated by cell cycle cues remains unclear. Here, we document physical and genetic interactions between Transport Protein Particle II (TRAPPII) tethering factors and microtubule-associated proteins of the PLEIADE/AtMAP65 family. These interactions do not specifically affect the recruitment of either TRAPPII or MAP65 proteins to the cell plate or midzone. Rather, and based on single versus double mutant phenotypes, it appears that they are required to coordinate cytokinesis with the nuclear division cycle. As MAP65 family members are known to be targets of cell cycle-regulated kinases, our results provide a conceptual framework for how membrane and microtubule dynamics may be coordinated with each other and with the nuclear cycle during plant cytokinesis.

Tethering Complexes in the Arabidopsis Endomembrane System

Targeting of endomembrane transport containers is of the utmost importance for proper land plant growth and development. Given the immobility of plant cells, localized membrane vesicle secretion and recycling are amongst the main processes guiding proper cell, tissue and whole plant morphogenesis. Cell wall biogenesis and modification are dependent on vectorial membrane traffic, not only during normal development, but also in stress responses and in plant defense against pathogens and/or symbiosis. It is surprising how little we know about these processes in plants, from small GTPase regulation to the tethering complexes that act as their effectors. Tethering factors are single proteins or protein complexes mediating first contact between the target membrane and arriving membrane vesicles. In this review we focus on the tethering complexes of the best-studied plant model—Arabidopsis thaliana. Genome-based predictions indicate the presence of all major tethering complexes in plants that are known from a hypothetical last eukaryotic common ancestor (LECA). The evolutionary multiplication of paralogs of plant tethering complex subunits has produced the massively expanded EXO70 family, indicating a subfunctionalization of the terminal exocytosis machinery in land plants. Interpretation of loss of function (LOF) mutant phenotypes has to consider that related, yet clearly functionally-specific complexes often share some common core subunits. It is therefore impossible to conclude with clarity which version of the complex is responsible for the phenotypic deviations observed. Experimental interest in the analysis of plant tethering complexes is growing and we hope to contribute with this review by attracting even more attention to this fascinating field of plant cell biology.

Journey to the cell surface--the central role of the trans-Golgi network in plants

The secretion of proteins, lipids, and carbohydrates to the cell surface is essential for plant development and adaptation. Secreted substances synthesized at the endoplasmic reticulum pass through the Golgi apparatus and trans-Golgi network (TGN) en route to the plasma membrane via the conventional secretion pathway. The TGN is morphologically and functionally distinct from the Golgi apparatus. The TGN is located at the crossroads of many trafficking pathways and regulates a range of crucial processes including secretion to the cell surface, transport to the vacuole, and the reception of endocytic cargo. This review outlines the TGN's central role in cargo secretion, showing that its behavior is more complex and controlled than the bulk-flow hypothesis suggests. Its formation, structure, and maintenance are discussed along with the formation and release of secretory vesicles.

Plant cytokinesis is orchestrated by the sequential action of the TRAPPII and exocyst tethering complexes

Plant cytokinesis is initiated in a transient membrane compartment, the cell plate, and completed by a process of maturation during which the cell plate becomes a cross wall. How the transition from juvenile to adult stages occurs is poorly understood. In this study, we monitor the Arabidopsis transport protein particle II (TRAPPII) and exocyst tethering complexes throughout cytokinesis. We show that their appearance is predominantly sequential, with brief overlap at the onset and end of cytokinesis. The TRAPPII complex is required for cell plate biogenesis, and the exocyst is required for cell plate maturation. The TRAPPII complex sorts plasma membrane proteins, including exocyst subunits, at the cell plate throughout cytokinesis. We show that the two tethering complexes physically interact and propose that their coordinated action may orchestrate not only plant but also animal cytokinesis.

VAN4 encodes a putative TRS120 that is required for normal cell growth and vein development in Arabidopsis

Leaf venation develops complex patterns in angiosperms, but the mechanism underlying this process is largely unknown. To elucidate the molecular mechanisms governing vein pattern formation, we previously isolated vascular network defective (van) mutants that displayed venation discontinuities. Here, we report the phenotypic analysis of van4 mutants, and we identify and characterize the VAN4 gene. Detailed phenotypic analysis shows that van4 mutants are defective in procambium cell differentiation and subsequent vascular cell differentiation. Reduced shoot and root cell growth is observed in van4 mutants, suggesting that VAN4 function is important for cell growth and the establishment of venation continuity. Consistent with these phenotypes, the VAN4 gene is strongly expressed in vascular and meristematic cells. VAN4 encodes a putative TRS120, which is a known guanine nucleotide exchange factor (GEF) for Rab GTPase involved in regulating vesicle transport, and a known tethering factor that determines the specificity of membrane fusion. VAN4 protein localizes at the trans-Golgi network/early endosome (TGN/EE). Aberrant recycling of the auxin efflux carrier PIN proteins is observed in van4 mutants. These results suggest that VAN4-mediated exocytosis at the TGN plays important roles in plant vascular development and cell growth in shoot and root. Our identification of VAN4 as a putative TRS120 shows that Rab GTPases are crucial (in addition to ARF GTPases) for continuous vascular development, and provides further evidence for the importance of vesicle transport in leaf vascular formation.

ECHIDNA-mediated post-Golgi trafficking of auxin carriers for differential cell elongation

The plant hormone indole-acetic acid (auxin) is essential for many aspects of plant development. Auxin-mediated growth regulation typically involves the establishment of an auxin concentration gradient mediated by polarly localized auxin transporters. The localization of auxin carriers and their amount at the plasma membrane are controlled by membrane trafficking processes such as secretion, endocytosis, and recycling. In contrast to endocytosis or recycling, how the secretory pathway mediates the localization of auxin carriers is not well understood. In this study we have used the differential cell elongation process during apical hook development to elucidate the mechanisms underlying the post-Golgi trafficking of auxin carriers in Arabidopsis. We show that differential cell elongation during apical hook development is defective in Arabidopsis mutant echidna (ech). ECH protein is required for the trans-Golgi network (TGN)-mediated trafficking of the auxin influx carrier AUX1 to the plasma membrane. In contrast, ech mutation only marginally perturbs the trafficking of the highly related auxin influx carrier LIKE-AUX1-3 or the auxin efflux carrier PIN-FORMED-3, both also involved in hook development. Electron tomography reveals that the trafficking defects in ech mutant are associated with the perturbation of secretory vesicle genesis from the TGN. Our results identify differential mechanisms for the post-Golgi trafficking of de novo-synthesized auxin carriers to plasma membrane from the TGN and reveal how trafficking of auxin influx carriers mediates the control of differential cell elongation in apical hook development.

Rab-A1c GTPase defines a population of the trans-Golgi network that is sensitive to endosidin1 during cytokinesis in Arabidopsis

In plant cells, Rab-A proteins have been implicated to play important roles in membrane trafficking from the trans-Golgi network (TGN) to the plasma membrane/cell wall and to the newly formed cell plate in cytokinesis. But how different Rab-A proteins may work in the TGN is not well studied. We show here that RAB-A1c defines a population of TGN that is partially overlapped with the VHA-a1 marked-TGN. Interestingly, the morphology of RAB-A1c defined-TGN is sensitive to endosidin 1 (ES1), but not to wortmannin. In mitotic cells, RAB-A1c is relocated to the cell plate. We revealed that this process could be interrupted by ES1, but not by wortmannin. In addition, root growth and cytokinesis in root mitotic cells of rab-a1a/b/c triple mutant seedlings are hypersensitive to lower concentrations of ES1. ES1 is known to selectively block the transport of several plasma membrane auxin transporters, including PIN2 and AUX1 at the TGN. Together with the known facts that members of Rab-A1 proteins are involved in auxin-mediated responses in root growth and that mutations in TRAPPII, a protein complex that acts upstream of RAB-A1c, also selectively impair the transport of PIN2 and AUX1 at the TGN, we propose that the Rab-A1-mediated trafficking pathways around the TGN, but not Rab-A1s directly, are the target of ES1.

Cytoskeletal and membrane dynamics during higher plant cytokinesis

Following mitosis, cytoplasm, organelles and genetic material are partitioned into daughter cells through the process of cytokinesis. In somatic cells of higher plants, two cytoskeletal arrays, the preprophase band and the phragmoplast, facilitate the positioning and de novo assembly of the plant-specific cytokinetic organelle, the cell plate, which develops across the division plane and fuses with the parental plasma membrane to yield distinct new cells. The coordination of cytoskeletal and membrane dynamics required to initiate, assemble and shape the cell plate as it grows toward the mother cell cortex is dependent upon a large array of proteins, including molecular motors, membrane tethering, fusion and restructuring factors and biosynthetic, structural and regulatory elements. This review focuses on the temporal and molecular requirements of cytokinesis in somatic cells of higher plants gleaned from recent studies using cell biology, genetics, pharmacology and biochemistry.

Functional analysis of small Rab GTPases in cytokinesis in Arabidopsis thaliana

Rab proteins are key regulators of membrane transport in eukaryotes. Recent evidence from different species supports the notion that some Rab proteins are crucial for cytokinesis, a pivotal procedure for successful cell division. As a family of monomeric small GTPases of the Ras superfamily, the function of Rab proteins is modulated by guanine nucleotide binding and hydrolysis. To investigate the function of Rab proteins, creating dominant negative or constitutively active mutant forms of a Rab protein is a widely used approach. To study cytokinesis in plant cells, using fluorescent dye to highlight the cell shape and the nuclei, and to monitor the formation of the newly formed cell plate in mitotic cells, is easy and useful. In this chapter, we describe detailed methods for (1) generating transgenic plants expressing dominant negative or constitutively active form of RAB-A1c; (2) fluorescent staining of cell shape, cell wall, and nuclei of mitotic root tip cells; (3) fluorescent staining of newly formed cell plate; and (4) detecting fluorescent signals using Confocal Laser Scanning Microscopy in the genetic model plant species Arabidopsis thaliana.

Untangling the evolution of Rab G proteins: implications of a comprehensive genomic analysis

BACKGROUND

Membrane-bound organelles are a defining feature of eukaryotic cells, and play a central role in most of their fundamental processes. The Rab G proteins are the single largest family of proteins that participate in the traffic between organelles, with 66 Rabs encoded in the human genome. Rabs direct the organelle-specific recruitment of vesicle tethering factors, motor proteins, and regulators of membrane traffic. Each organelle or vesicle class is typically associated with one or more Rab, with the Rabs present in a particular cell reflecting that cell's complement of organelles and trafficking routes.

RESULTS

Through iterative use of hidden Markov models and tree building, we classified Rabs across the eukaryotic kingdom to provide the most comprehensive view of Rab evolution obtained to date. A strikingly large repertoire of at least 20 Rabs appears to have been present in the last eukaryotic common ancestor (LECA), consistent with the 'complexity early' view of eukaryotic evolution. We were able to place these Rabs into six supergroups, giving a deep view into eukaryotic prehistory.

CONCLUSIONS

Tracing the fate of the LECA Rabs revealed extensive losses with many extant eukaryotes having fewer Rabs, and none having the full complement. We found that other Rabs have expanded and diversified, including a large expansion at the dawn of metazoans, which could be followed to provide an account of the evolutionary history of all human Rabs. Some Rab changes could be correlated with differences in cellular organization, and the relative lack of variation in other families of membrane-traffic proteins suggests that it is the changes in Rabs that primarily underlies the variation in organelles between species and cell types.

Hijack it, change it: how do plant viruses utilize the host secretory pathway for efficient viral replication and spread?

The secretory pathway of eukaryotic cells has an elaborated set of endomembrane compartments involved in the synthesis, modification, and sorting of proteins and lipids. The secretory pathway in plant cells shares many features with that in other eukaryotic cells but also has distinct characteristics important for fundamental cell and developmental processes and for proper immune responses. Recently, there has been evidence that the remodeling of this pathway, and often the formation of viral-induced organelles, play an important role in viral replication and spread. The modification of the host secretory pathway seems to be a common feature among most single-stranded positive ss(+)RNA and even some DNA viruses. In this review, we will present the recent advances in the understanding of the organization and dynamics of the plant secretory pathway and the molecular regulation of membrane trafficking in the pathway. We will also discuss how different plant viruses may interact with the host secretory pathway for their efficient replication and spread, with a focus on tobacco mosaic virus and turnip mosaic virus.