Multisubunit tethering complexes in higher plants

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.

Dysfunction of GmVPS8a causes compact plant architecture in soybean

Vacuolar Protein Sorting 8 (Vps8) protein is a specific subunit of the class C core vacuole/endosome tethering (CORVET) complex that plays a key role in endosomal trafficking in yeast (Saccharomyces cerevisiae). However, its functions remain largely unclear in plant vegetative growth. Here, we identified a soybean (Glycine max) T4219 mutant characterized with compact plant architecture. Map-based cloning targeted to a candidate gene GmVPS8a (Glyma.07g049700) and further found that two nucleotides deletion in the first exon of GmVPS8a causes a premature termination of the encoded protein in the T4219 mutant. Its functions were validated by CRISPR/Cas9-engineered mutation in the GmVPS8a gene that recapitulated the T4219 mutant phenotypes. Furthermore, NbVPS8a-silenced tobacco (Nicotiana benthamiana) plants exhibited similar phenotypes to the T4219 mutant, suggesting its conserved roles in plant growth. The GmVPS8a is widely expressed in multiple organs and its protein interacts with GmAra6a and GmRab5a. Combined analysis of transcriptomic and proteomic data revealed that dysfunction of GmVPS8a mainly affects pathways on auxin signal transduction, sugar transport and metabolism, and lipid metabolism. Collectively, our work reveals the function of GmVPS8a in plant architecture, which may extend a new way for genetic improvement of ideal plant-architecture breeding in soybean and other crops.

Rab GTPases, tethers, and SNAREs work together to regulate Arabidopsis cell plate formation

Cell plates are transient structures formed by the fusion of vesicles at the center of the dividing plane; furthermore, these are precursors to new cell walls and are essential for cytokinesis. Cell plate formation requires a highly coordinated process of cytoskeletal rearrangement, vesicle accumulation and fusion, and membrane maturation. Tethering factors have been shown to interact with the Ras superfamily of small GTP binding proteins (Rab GTPases) and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), which are essential for cell plate formation during cytokinesis and are fundamental for maintaining normal plant growth and development. In Arabidopsis thaliana, members of the Rab GTPases, tethers, and SNAREs are localized in cell plates, and mutations in the genes encoding these proteins result in typical cytokinesis-defective phenotypes, such as the formation of abnormal cell plates, multinucleated cells, and incomplete cell walls. This review highlights recent findings on vesicle trafficking during cell plate formation mediated by Rab GTPases, tethers, and SNAREs.

RAB GTPases and SNAREs at the trans-Golgi network in plants

Membrane traffic is a fundamental cellular system to exchange proteins and membrane lipids among single membrane-bound organelles or between an organelle and the plasma membrane in order to keep integrity of the endomembrane system. RAB GTPases and SNARE proteins, the key regulators of membrane traffic, are conserved broadly among eukaryotic species. However, genome-wide analyses showed that organization of RABs and SNAREs that regulate the post-Golgi transport pathways is greatly diversified in plants compared to other model eukaryotes. Furthermore, some organelles acquired unique properties in plant lineages. Like in other eukaryotic systems, the trans-Golgi network of plants coordinates secretion and vacuolar transport; however, uniquely in plants, it also acts as a platform for endocytic transport and recycling. In this review, we focus on RAB GTPases and SNAREs that function at the TGN, and summarize how these regulators perform to control different transport pathways at the plant TGN. We also highlight the current knowledge of RABs and SNAREs' role in regulation of plant development and plant responses to environmental stimuli.

MAG2 and MAL Regulate Vesicle Trafficking and Auxin Homeostasis With Functional Redundancy

Auxin is a central phytohormone and controls almost all aspects of plant development and stress response. Auxin homeostasis is coordinately regulated by biosynthesis, catabolism, transport, conjugation, and deposition. Endoplasmic reticulum (ER)-localized MAIGO2 (MAG2) complex mediates tethering of arriving vesicles to the ER membrane, and it is crucial for ER export trafficking. Despite important regulatory roles of MAG2 in vesicle trafficking, the mag2 mutant had mild developmental abnormalities. MAG2 has one homolog protein, MAG2-Like (MAL), and the mal-1 mutant also had slight developmental phenotypes. In order to investigate MAG2 and MAL regulatory function in plant development, we generated the mag2-1 mal-1 double mutant. As expected, the double mutant exhibited serious developmental defects and more alteration in stress response compared with single mutants and wild type. Proteomic analysis revealed that signaling, metabolism, and stress response in mag2-1 mal-1 were affected, especially membrane trafficking and auxin biosynthesis, signaling, and transport. Biochemical and cell biological analysis indicated that the mag2-1 mal-1 double mutant had more serious defects in vesicle transport than the mag2-1 and mal-1 single mutants. The auxin distribution and abundance of auxin transporters were altered significantly in the mag2-1 and mal-1 single mutants and mag2-1 mal-1 double mutant. Our findings suggest that MAG2 and MAL regulate plant development and auxin homeostasis by controlling membrane trafficking, with functional redundancy.

Molecular mechanisms of endomembrane trafficking in plants

Endomembrane trafficking is essential for all eukaryotic cells. The best-characterized membrane trafficking organelles include the endoplasmic reticulum (ER), Golgi apparatus, early and recycling endosomes, multivesicular body, or late endosome, lysosome/vacuole, and plasma membrane. Although historically plants have given rise to cell biology, our understanding of membrane trafficking has mainly been shaped by the much more studied mammalian and yeast models. Whereas organelles and major protein families that regulate endomembrane trafficking are largely conserved across all eukaryotes, exciting variations are emerging from advances in plant cell biology research. In this review, we summarize the current state of knowledge on plant endomembrane trafficking, with a focus on four distinct trafficking pathways: ER-to-Golgi transport, endocytosis, trans-Golgi network-to-vacuole transport, and autophagy. We acknowledge the conservation and commonalities in the trafficking machinery across species, with emphasis on diversity and plant-specific features. Understanding the function of organelles and the trafficking machinery currently nonexistent in well-known model organisms will provide great opportunities to acquire new insights into the fundamental cellular process of membrane trafficking.

Revising the Role of Cortical Cytoskeleton during Secretion: Actin and Myosin XI Function in Vesicle Tethering

In plants, secretion of cell wall components and membrane proteins plays a fundamental role in growth and development as well as survival in diverse environments. Exocytosis, as the last step of the secretory trafficking pathway, is a highly ordered and precisely controlled process involving tethering, docking, and fusion of vesicles at the plasma membrane (PM) for cargo delivery. Although the exocytic process and machinery are well characterized in yeast and animal models, the molecular players and specific molecular events that underpin late stages of exocytosis in plant cells remain largely unknown. Here, by using the delivery of functional, fluorescent-tagged cellulose synthase (CESA) complexes (CSCs) to the PM as a model system for secretion, as well as single-particle tracking in living cells, we describe a quantitative approach for measuring the frequency of vesicle tethering events. Genetic and pharmacological inhibition of cytoskeletal function, reveal that the initial vesicle tethering step of exocytosis is dependent on actin and myosin XI. In contrast, treatments with the microtubule inhibitor, oryzalin, did not significantly affect vesicle tethering or fusion during CSC exocytosis but caused a minor increase in transient or aborted tethering events. With data from this new quantitative approach and improved spatiotemporal resolution of single particle events during secretion, we generate a revised model for the role of the cortical cytoskeleton in CSC trafficking.

Arabidopsis myosin XIK interacts with the exocyst complex to facilitate vesicle tethering during exocytosis

Myosin motors are essential players in secretory vesicle trafficking and exocytosis in yeast and mammalian cells; however, similar roles in plants remain a matter for debate, at least for diffusely growing cells. Here, we demonstrate that Arabidopsis (Arabidopsis thaliana) myosin XIK, via its globular tail domain (GTD), participates in the vesicle tethering step of exocytosis through direct interactions with the exocyst complex. Specifically, myosin XIK GTD bound directly to several exocyst subunits in vitro and functional fluorescently tagged XIK colocalized with multiple exocyst subunits at plasma membrane (PM)-associated stationary foci. Moreover, genetic and pharmacological inhibition of myosin XI activity reduced the rate of appearance and lifetime of stationary exocyst complexes at the PM. By tracking single exocytosis events of cellulose synthase (CESA) complexes with high spatiotemporal resolution imaging and pair-wise colocalization of myosin XIK, exocyst subunits, and CESA6, we demonstrated that XIK associates with secretory vesicles earlier than exocyst and is required for the efficient localization and normal dynamic behavior of exocyst complex at the PM tethering site. This study reveals an important functional role for myosin XI in secretion and provides insights about the dynamic regulation of exocytosis in plants.

Molecular Mechanisms Underlying the Acclimation of Chlamydomonas reinhardtii Against Nitric Oxide Stress

The acclimation mechanism of Chlamydomonas reinhardtii to nitric oxide (NO) was studied by exposure to S-nitroso-N-acetylpenicillamine (SNAP), a NO donor. Treatment with 0.1 or 0.3 mM SNAP transiently inhibited photosynthesis within 1 h, followed by a recovery, while 1.0 mM SNAP treatment caused irreversible photosynthesis inhibition and mortality. The SNAP effects are avoided in the presence of the NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-l-oxyl-3-oxide (cPTIO). RNA-seq, qPCR, and biochemical analyses were conducted to decode the metabolic shifts under NO stress by exposure to 0.3 mM SNAP in the presence or absence of 0.4 mM cPTIO. These findings revealed that the acclimation to NO stress comprises a temporally orchestrated implementation of metabolic processes: (1). modulation of NADPH oxidase (respiratory burst oxidase-like 2, RBOL2) and ROS signaling pathways for downstream mechanism regulation, (2). trigger of NO scavenging elements to reduce NO level; (3). prevention of photo-oxidative risk through photosynthesis inhibition and antioxidant defense system induction; (4). acclimation to nitrogen and sulfur shortage; (5). attenuation of transcriptional and translational activity together with degradation of damaged proteins through protein trafficking machinery (ubiquitin, SNARE, and autophagy) and molecular chaperone system for dynamic regulation of protein homeostasis. In addition, the expression of the gene encoding NADPH oxidase, RBOL2, showed a transient increase while that of RBOL1 was slightly decreased after NO challenge. It reflects that NADPH oxidase, a regulator in ROS-mediated signaling pathway, may be involved in the responses of Chlamydomonas to NO stress. In conclusion, our findings provide insight into the molecular events underlying acclimation mechanisms in Chlamydomonas to NO stress.

Phytophthora infestans RXLR effector AVR1 disturbs the growth of Physcomitrium patens without affecting Sec5 localization

Plant pathogens often exploit a whole range of effectors to facilitate infection. The RXLR effector AVR1 produced by the oomycete plant pathogen Phytophthora infestans suppresses host defense by targeting Sec5. Sec5 is a subunit of the exocyst, a protein complex that is important for mediating polarized exocytosis during plant development and defense against pathogens. The mechanism by which AVR1 manipulates Sec5 functioning is unknown. In this study, we analyzed the effect of AVR1 on Sec5 localization and functioning in the moss Physcomitrium patens. P. patens has four Sec5 homologs. Two (PpSec5b and PpSec5d) were found to interact with AVR1 in yeast-two-hybrid assays while none of the four showed a positive interaction with AVR1ΔT, a truncated version of AVR1. In P. patens lines carrying β-estradiol inducible AVR1 or AVR1ΔT transgenes, expression of AVR1 or AVR1ΔT caused defects in the development of caulonemal protonema cells and abnormal morphology of chloronema cells. Similar phenotypes were observed in Sec5- or Sec6-silenced P. patens lines, suggesting that both AVR1 and AVR1ΔT affect exocyst functioning in P. patens. With respect to Sec5 localization we found no differences between β-estradiol-treated and untreated transgenic AVR1 lines. Sec5 localizes at the plasma membrane in growing caulonema cells, also during pathogen attack, and its subcellular localization is the same, with or without AVR1 in the vicinity.

The Protein Phosphatase PP2A Plays Multiple Roles in Plant Development by Regulation of Vesicle Traffic-Facts and Questions

The protein phosphatase PP2A is essential for the control of integrated eukaryotic cell functioning. Several cellular and developmental events, e.g., plant growth regulator (PGR) mediated signaling pathways are regulated by reversible phosphorylation of vesicle traffic proteins. Reviewing present knowledge on the relevant role of PP2A is timely. We discuss three aspects: (1) PP2A regulates microtubule-mediated vesicle delivery during cell plate assembly. PP2A dephosphorylates members of the microtubule associated protein family MAP65, promoting their binding to microtubules. Regulation of phosphatase activity leads to changes in microtubule organization, which affects vesicle traffic towards cell plate and vesicle fusion to build the new cell wall between dividing cells. (2) PP2A-mediated inhibition of target of rapamycin complex (TORC) dependent signaling pathways contributes to autophagy and this has possible connections to the brassinosteroid signaling pathway. (3) Transcytosis of vesicles transporting PIN auxin efflux carriers. PP2A regulates vesicle localization and recycling of PINs related to GNOM (a GTP-GDP exchange factor) mediated pathways. The proper intracellular traffic of PINs is essential for auxin distribution in the plant body, thus in whole plant development. Overall, PP2A has essential roles in membrane interactions of plant cell and it is crucial for plant development and stress responses.

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.

Update on plant cytokinesis: rule and divide

Many decisions made during plant development depend on the placement of the cytokinetic wall. Cytokinesis involves the biogenesis of the cell plate that progresses centrifugally and until the fusion of the cell plate with the parental cell wall. The phragmoplast facilitates the growth of the cell plate and directs it's insertion at the cell cortex by a mechanism known as phragmoplast guidance. Communication between the phragmoplast and its destination, the cortical division zone, however, is not well understood. The preprophase band predicts the site of cell plate fusion, seemingly controlling the site of the cortical division zone establishment, but recent results suggest the role of this cytoskeletal array to be rather subtle. This is indirectly supported by certain types of phragmoplast-driven cell division in mosses and algae, which lack preprophase bands. In this review article, we summarize recent insight concerning phragmoplast expansion and guidance.

De novo transcriptome and proteome analysis of Dictyophora indusiata fruiting bodies provides insights into the changes during morphological development

De novo transcriptome assembly and shotgun proteome analysis of Dictyophora indusiata fruiting bodies were performed. A total of 19,704 unigenes were sequenced, and 4380 proteins were identified. Annotation and functional analysis of the identified proteins were significantly enriched in small molecule synthetic and metabolic processes, protein modification regulation (phosphorylation and ubiquitination), and vesicle transport. Furthermore, quantitative developmental transcriptome analysis was performed between the peach-shaped and mature fruiting bodies, and the results revealed that the metabolism and transport activities were upregulated in the mature stage, while protein translation was downregulated; this regulation is likely the main reason for the significant changes in the nutrients of fruiting bodies. Furthermore, the cell wall stress-dependent MAPK sub-pathway was activated in the mature stage, and fungal cell wall degradation-related genes were upregulated, which could promote reconstruction of the cell wall and might play a key role in the morphological development of D. indusiata fruiting bodies.

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.

Plant cell division - defining and finding the sweet spot for cell plate insertion

The plant microtubules form unique arrays using acentrosomal microtubule nucleation pathways, yet utilizing evolutionary conserved centrosomal proteins. In cytokinesis, a multi-component cytoskeletal apparatus, the phragmoplast mediates the biosynthesis of the new cell plate by dynamic centrifugal expansion, a process that demands exquisite coordination of microtubule turnover and endomembrane trafficking. At the same time, the phragmoplast is guided to meet with the parental wall at a cortical site that is predefined before mitotic entry and transiently marked by the preprophase band of microtubules. The cortical division zone maintains positional information of the selected division plane for the entire duration of cell division and for the guidance of the phragmoplast during cytokinesis. Its establishment is an essential requirement for normal plant organogenesis, due to the confinement of cells by rigid cell walls.

Phragmoplast expansion: the four-stroke engine that powers plant cytokinesis

The phragmoplast is a plant-specific secretory module that partitions daughter cells during cytokinesis by constructing a cell plate from membranes and oligosaccharides. The cell plate is typically a long structure, which requires the phragmoplast to expand to complete cytokinesis. The phragmoplast expands by coordinating microtubule dynamics with membrane trafficking. Each step in phragmoplast expansion involves the establishment of anti-parallel microtubule overlaps that are enriched with the protein MAP65, which recruits cytokinetic vesicles through interaction with the tethering factor, TRAPPII. Cell plate assembly triggers dissolution of the anti-parallel overlaps and stabilization of microtubule plus ends through association with the cell plate assembly machinery. This opinion article discusses processes that drive phragmoplast expansion as well as highlights key questions that remain for better understanding its role in plant cell division.

Plant TGN in the stress response: a compartmentalized overview

The cellular responses to abiotic and biotic stress rely on the regulation of vesicle trafficking to ensure the correct localization of proteins specialized in sensing stress stimuli and effecting the response. Several studies have implicated the plant trans-Golgi network (TGN)-mediated trafficking in different types of biotic and abiotic stress responses; however, the underlying molecular mechanisms are poorly understood. Further, the identity, specialization and stress-relevant cargo transported by the TGN subcompartments involved in stress responses await more in depth characterization. This review presents TGN trafficking players implicated in stress and discusses potential avenues to understand the role of this dynamic network under such extreme circumstances.

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.

Role of the BUB3 protein in phragmoplast microtubule reorganization during cytokinesis

The evolutionarily conserved WD40 protein budding uninhibited by benzimidazole 3 (BUB3) is known for its function in spindle assembly checkpoint control. In the model plant Arabidopsis thaliana, nearly identical BUB3;1 and BUB3;2 proteins decorated the phragmoplast midline through interaction with the microtubule-associated protein MAP65-3 during cytokinesis. BUB3;1 and BUB3;2 interacted with the carboxy-terminal segment of MAP65-3 (but not MAP65-1), which harbours its microtubule-binding domain for its post-mitotic localization. Reciprocally, BUB3;1 and BUB3;2 also regulated MAP65-3 localization in the phragmoplast by enhancing its microtubule association. In the bub3;1 bub3;2 double mutant, MAP65-3 localization was often dissipated away from the phragmoplast midline and abolished upon treatment of low doses of the cytokinesis inhibitory drug caffeine that were tolerated by the control plant. The phragmoplast microtubule array exhibited uncoordinated expansion pattern in the double mutant cells as the phragmoplast edge reached the parental plasma membrane at different times in different areas. Upon caffeine treatment, phragmoplast expansion was halted as if the microtubule array was frozen. As a result, cytokinesis was abolished due to failed cell plate assembly. Our findings have uncovered a novel function of the plant BUB3 in MAP65-3-dependent microtubule reorganization during cytokinesis.

Post-Golgi Trafficking and Transport of Cell Wall Components

The cell wall, a complex macromolecular composite structure surrounding and protecting plant cells, is essential for development, signal transduction, and disease resistance. This structure is also integral to cell expansion, as its tensile resistance is the primary balancing mechanism against internal turgor pressure. Throughout these processes, the biosynthesis, transport, deposition, and assembly of cell wall polymers are tightly regulated. The plant endomembrane system facilitates transport of polysaccharides, polysaccharide biosynthetic and modifying enzymes and glycoproteins through vesicle trafficking pathways. Although a number of enzymes involved in cell wall biosynthesis have been identified, comparatively little is known about the transport of cell wall polysaccharides and glycoproteins by the endomembrane system. This review summarizes our current understanding of trafficking of cell wall components during cell growth and cell division. Emerging technologies, such as vesicle glycomics, are also discussed as promising avenues to gain insights into the trafficking of structural polysaccharides to the apoplast.