The role of the AP-1 adaptor complex in outgoing and incoming membrane traffic

The AP-1 adaptor complex is found in all eukaryotes, but it has been implicated in different pathways in different organisms. To look directly at AP-1 function, we generated stably transduced HeLa cells coexpressing tagged AP-1 and various tagged membrane proteins. Live cell imaging showed that AP-1 is recruited onto tubular carriers trafficking from the Golgi apparatus to the plasma membrane, as well as onto transferrin-containing early/recycling endosomes. Analysis of single AP-1 vesicles showed that they are a heterogeneous population, which starts to sequester cargo 30 min after exit from the ER. Vesicle capture showed that AP-1 vesicles contain transmembrane proteins found at the TGN and early/recycling endosomes, as well as lysosomal hydrolases, but very little of the anterograde adaptor GGA2. Together, our results support a model in which AP-1 retrieves proteins from post-Golgi compartments back to the TGN, analogous to COPI's role in the early secretory pathway. We propose that this is the function of AP-1 in all eukaryotes.

Arabidopsis HAPLESS13/AP-1µ is critical for pollen sac formation and tapetal function

The production of excess and viable pollen grains is critical for reproductive success of flowering plants. Pollen grains are produced within anthers, the male reproductive organ whose development involves precisely controlled cell differentiation, division, and intercellular communication. In Arabidopsis thaliana, specification of an archesporial cell (AC) at four corners of a developing anther, followed by programmed cell divisions, generates four pollen sacs, walled by four cell layers among which the tapetum is in close contact with developing microspores. Tapetum secretes callose-dissolving enzymes to release microspores at early stages and undergoes programmed cell death (PCD) to deliver nutrients and signals for microspore development at later stages. Except for transcription factors, plasma membrane (PM)-associated and secretory peptides have also been demonstrated to mediate anther development. Adaptor protein complexes (AP) recruit both cargos and coat proteins during vesicle trafficking. Arabidopsis AP-1µ/HAPLESS13 (HAP13) is a core component of AP-1 for protein sorting at the trans-Golgi network/early endosomes (TGN/EE). We report here that Arabidopsis HAP13 is critical for pollen sac formation and for sporophytic control of pollen production. Functional loss of HAP13 causes a reduction in pollen sac number. It also results in the dysfunction of tapetum such that secretory function of tapetum at early stages and PCD of tapetum at later stages are both compromised. We further show that the expression of SPL, the polar distribution of auxin maximum, as well as the asymmetric distribution of PIN1 are interfered in hap13 anthers, which in combination may lead to male sterility in hap13.

Adaptor protein complex interaction map in Arabidopsis identifies P34 as a common stability regulator

Adaptor protein (AP) complexes are evolutionarily conserved vesicle transport regulators that recruit coat proteins, membrane cargoes and coated vesicle accessory proteins. As in plants endocytic and post-Golgi trafficking intersect at the trans-Golgi network, unique mechanisms for sorting cargoes of overlapping vesicular routes are anticipated. The plant AP complexes are part of the sorting machinery, but despite some functional information, their cargoes, accessory proteins and regulation remain largely unknown. Here, by means of various proteomics approaches, we generated the overall interactome of the five AP and the TPLATE complexes in Arabidopsis thaliana. The interactome converged on a number of hub proteins, including the thus far unknown adaptin binding-like protein, designated P34. P34 interacted with the clathrin-associated AP complexes, controlled their stability and, subsequently, influenced clathrin-mediated endocytosis and various post-Golgi trafficking routes. Altogether, the AP interactome network offers substantial resources for further discoveries of unknown endomembrane trafficking regulators in plant cells.

Ice plant root plasma membrane aquaporins are regulated by clathrin-coated vesicles in response to salt stress

The regulation of root Plasma membrane (PM) Intrinsic Protein (PIP)-type aquaporins (AQPs) is potentially important for salinity tolerance. However, the molecular and cellular details underlying this process in halophytes remain unclear. Using free-flow electrophoresis and label-free proteomics, we report that the increased abundance of PIPs at the PM of the halophyte ice plant (Mesembryanthemum crystallinum L.) roots under salinity conditions is regulated by clathrin-coated vesicles (CCV). To understand this regulation, we analyzed several components of the M. crystallinum CCV complexes: clathrin light chain (McCLC) and subunits μ1 and μ2 of the adaptor protein (AP) complex (McAP1μ and McAP2μ). Co-localization analyses revealed the association between McPIP1;4 and McAP2μ and between McPIP2;1 and McAP1μ, observations corroborated by mbSUS assays, suggesting that AQP abundance at the PM is under the control of CCV. The ability of McPIP1;4 and McPIP2;1 to form homo- and hetero-oligomers was tested and confirmed, as well as their activity as water channels. Also, we found increased phosphorylation of McPIP2;1 only at the PM in response to salt stress. Our results indicate root PIPs from halophytes might be regulated through CCV trafficking and phosphorylation, impacting their localization, transport activity, and abundance under salinity conditions.

AP1/2β-mediated exocytosis of tapetum-specific transporters is required for pollen development in Arabidopsis thaliana

AP-1 and AP-2 adaptor protein (AP) complexes mediate clathrin-dependent trafficking at the trans-Golgi network (TGN) and the plasma membrane, respectively. Whereas AP-1 is required for trafficking to plasma membrane and vacuoles, AP-2 mediates endocytosis. These AP complexes consist of four subunits (adaptins): two large subunits (β1 and γ for AP-1 and β2 and α for AP-2), a medium subunit μ, and a small subunit σ. In general, adaptins are unique to each AP complex, with the exception of β subunits that are shared by AP-1 and AP-2 in some invertebrates. Here, we show that the two putative Arabidopsis thaliana AP1/2β adaptins co-assemble with both AP-1 and AP-2 subunits and regulate exocytosis and endocytosis in root cells, consistent with their dual localization at the TGN and plasma membrane. Deletion of both β adaptins is lethal in plants. We identified a critical role of β adaptins in pollen wall formation and reproduction, involving the regulation of membrane trafficking in the tapetum and pollen germination. In tapetal cells, β adaptins localize almost exclusively to the TGN and mediate exocytosis of the plasma membrane transporters such as ATP-binding cassette (ABC)G9 and ABCG16. This study highlights the essential role of AP1/2β adaptins in plants and their specialized roles in specific cell types.

The sorting of cargo proteins in the plant trans-Golgi network

Membrane trafficking contributes to distinct protein compositions of organelles and is essential for proper organellar maintenance and functions. The trans-Golgi network (TGN) acts as a sorting station where various cargo proteins are sorted and directed to post-Golgi compartments, such as the multivesicular body or pre-vacuolar compartment, vacuoles, and plasma membrane. The spatial and temporal segregation of cargo proteins within the TGN, which is mediated with different sets of regulators including small GTPases and cargo adaptors, is a fundamental process in the sorting machinery. Recent studies with powerful imaging technologies have suggested that the TGN possesses spatially distinct subdomains or zones for different trafficking pathways. In this review, we will summarize the spatially and dynamically characteristic features of the plant TGN and their relation to cargo protein trafficking.

AP1G2 Affects Mitotic Cycles of Female and Male Gametophytes in Arabidopsis

During sexual reproduction in flowering plants, haploid spores are formed from meiosis of spore mother cells. The spores then undergo mitosis, develop into female and male gametophytes, and give rise to seeds after fertilization. We identified a female sterile mutant ap1g2-4 from EMS mutagenesis, and analyses of two T-DNA insertion mutants, ap1g2-1 ^+/- and ap1g2-3 ^-/-, and detected a partial female and male sterility. The ap1g2 mutant gametophyte development was arrested at one nuclear stage. A complementation test using a genomic sequence of AP1G2 with its native promoter restored the function in the three ap1g2 mutant lines. Transcriptome profiling of ap1g2 ovules revealed that four genes encoding clathrin assembly proteins PICALM5A/B and PICALM9A/B, which were involved in endocytosis, were downregulated, which were confirmed to interact with AP1G2 through yeast two-hybrid assays and BIFC analysis. Our result also demonstrated that RALFL4-8-15-19-26 CML16 and several calcium-dependent protein kinases, including CPK14-16-17, were all downregulated in the ovules of ap1g2-1 ^+/-. Moreover, Ca²⁺ concentration was low in impaired gametophytes. Therefore, we proposed that through interaction with PICALM5A/B and PICALM9A/B, AP1G2 may mediate gametogenesis accompanied by Ca²⁺ signaling in Arabidopsis. Our findings revealed a crucial role of AP1G2 in female and male gametogenesis in Arabidopsis and enhanced our understanding of the molecular mechanisms underpinning sexual reproduction in flowering plants.

ADAPTOR PROTEIN-1 complex-mediated post-Golgi trafficking is critical for pollen wall development in Arabidopsis

Primexine deposition is essential for the formation of pollen wall patterns and is precisely regulated by the tapetum and microspores. While tapetum- and/or microspore-localized proteins are required for primexine biosynthesis, how their trafficking is established and controlled is poorly understood. In Arabidopsis thaliana, AP1σ1 and AP1σ2, two genes encoding the σ subunit of the trans-Golgi network/early endosome (TGN/EE)-localized ADAPTOR PROTEIN-1 complex (AP-1), are partially redundant for plant viability, and the loss of AP1σ1 function reduces male fertility due to defective primexine formation. Here, we investigated the role of AP-1 in pollen wall formation. The deposition of Acyl-CoA SYNTHETASE5 (ACOS5) and type III LIPID TRANSFER PROTEINs (LTPs) secreted from the anther tapetum, which are involved in exine formation, were impaired in ap1σ1 mutants. In addition, the microspore plasma membrane (PM) protein RUPTURED POLLEN GRAIN1 (RPG1), which regulates primexine deposition, accumulated abnormally at the TGN/EE in ap1σ1 mutants. We show that AP-1μ recognizes the YXXΦ motif of RPG1, thereby regulating its PM abundance through endocytic trafficking, and that loss of AP1σ1 decreases the levels of other AP-1 subunits at the TGN/EE. Our observations show that AP-1-mediated post-Golgi trafficking plays a vital role in pollen wall development by regulating protein transport in tapetal cells and microspores.

Plant cytokinesis and the construction of new cell wall

Cytokinesis in plants is fundamentally different from that in animals and fungi. In plant cells, a cell plate forms through the fusion of cytokinetic vesicles and then develops into the new cell wall, partitioning the cytoplasm of the dividing cell. The formation of the cell plate entails multiple stages that involve highly orchestrated vesicle accumulation, fusion, and membrane maturation, which occur concurrently with the timely deposition of polysaccharides such as callose, cellulose, and cross-linking glycans. This review summarizes the major stages in cytokinesis, endomembrane components involved in cell plate assembly and its transition to a new cell wall. An animation that can be widely used for educational purposes further summarizes the process.

Plant endosomes as protein sorting hubs

Endocytosis, secretion, and endosomal trafficking are key cellular processes that control the composition of the plasma membrane. Through the coordination of these trafficking pathways, cells can adjust the composition, localization, and turnover of proteins and lipids in response to developmental or environmental cues. Upon being incorporated into vesicles and internalized through endocytosis, plant plasma membrane proteins are delivered to the trans-Golgi network (TGN). At the TGN, plasma membrane proteins are recycled back to the plasma membrane or transferred to multivesicular endosomes (MVEs), where they are further sorted into intralumenal vesicles for degradation in the vacuole. Both types of plant endosomes, TGN and MVEs, act as sorting organelles for multiple endocytic, recycling, and secretory pathways. Molecular assemblies such as retromer, ESCRT (endosomal sorting complex required for transport) machinery, small GTPases, adaptor proteins, and SNAREs associate with specific domains of endosomal membranes to mediate different sorting and membrane-budding events. In this review, we discuss the mechanisms underlying the recognition and sorting of proteins at endosomes, membrane remodeling and budding, and their implications for cellular trafficking and physiological responses in plants.

An Update on Coat Protein Complexes for Vesicle Formation in Plant Post-Golgi Trafficking

Endomembrane trafficking is an evolutionarily conserved process for all eukaryotic organisms. It is a fundamental and essential process for the transportation of proteins, lipids, or cellular metabolites. The aforementioned cellular components are sorted across multiple membrane-bounded organelles. In plant cells, the endomembrane mainly consists of the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, trans-Golgi network or early endosome (TGN/EE), prevacuolar compartments or multivesicular bodies (PVCs/MVBs), and vacuole. Among them, Golgi apparatus and TGN represent two central sorting intermediates for cargo secretion and recycling from other compartments by anterograde or retrograde trafficking. Several protein sorting machineries have been identified to function in these pathways for cargo recognition and vesicle assembly. Exciting progress has been made in recent years to provide novel insights into the sorting complexes and also the underlying sorting mechanisms in plants. Here, we will highlight the recent findings for the adaptor protein (AP) complexes, retromer, and retriever complexes, and also their functions in the related coated vesicle formation in post-Golgi trafficking.

Role of adaptin protein complexes in intracellular trafficking and their impact on diseases

Adaptin proteins (APs) play a crucial role in intracellular cell trafficking. The 'classical' role of APs is carried out by AP1‒3, which bind to clathrin, cargo, and accessory proteins. Accordingly, AP1-3 are crucial for both vesicle formation and sorting. All APs consist of four subunits that are indispensable for their functions. In fact, based on studies using cells, model organism knockdown/knock-out, and human variants, each subunit plays crucial roles and contributes to the specificity of each AP. These studies also revealed that the sorting and intracellular trafficking function of AP can exert varying effects on pathology by controlling features such as cell development, signal transduction related to the apoptosis and proliferation pathways in cancer cells, organelle integrity, receptor presentation, and viral infection. Although the roles and functions of AP1‒3 are relatively well studied, the functions of the less abundant and more recently identified APs, AP4 and AP5, are still to be investigated. Further studies on these APs may enable a better understanding and targeting of specific diseases.APs known or suggested locations and functions.

Cross-talk between clathrin-dependent post-Golgi trafficking and clathrin-mediated endocytosis in Arabidopsis root cells

Coupling of post-Golgi and endocytic membrane transport ensures that the flow of materials to/from the plasma membrane (PM) is properly balanced. The mechanisms underlying the coordinated trafficking of PM proteins in plants, however, are not well understood. In plant cells, clathrin and its adaptor protein complexes, AP-2 and the TPLATE complex (TPC) at the PM, and AP-1 at the trans-Golgi network/early endosome (TGN/EE), function in clathrin-mediated endocytosis (CME) and post-Golgi trafficking. Here, we utilized mutants with defects in clathrin-dependent post-Golgi trafficking and CME, in combination with other cytological and pharmacological approaches, to further investigate the machinery behind the coordination of protein delivery and recycling to/from the TGN/EE and PM in Arabidopsis (Arabidopsis thaliana) root cells. In mutants with defective AP-2-/TPC-dependent CME, we determined that clathrin and AP-1 recruitment to the TGN/EE as well as exocytosis are significantly impaired. Likewise, defects in AP-1-dependent post-Golgi trafficking and pharmacological inhibition of exocytosis resulted in the reduced association of clathrin and AP-2/TPC subunits with the PM and a reduction in the internalization of cargoes via CME. Together, these results suggest that post-Golgi trafficking and CME are coupled via modulation of clathrin and adaptor protein complex recruitment to the TGN/EE and PM.

Motif-based endomembrane trafficking

Endomembrane trafficking, which allows proteins and lipids to flow between the different endomembrane compartments, largely occurs by vesicle-mediated transport. Transmembrane proteins intended for transport are concentrated into a vesicle or carrier by undulation of a donor membrane. This is followed by vesicle scission, uncoating, and finally, fusion at the target membrane. Three major trafficking pathways operate inside eukaryotic cells: anterograde, retrograde, and endocytic. Each pathway involves a unique set of machinery and coat proteins that pack the transmembrane proteins, along with their associated lipids, into specific carriers. Adaptor and coatomer complexes are major facilitators that function in anterograde transport and in endocytosis. These complexes recognize the transmembrane cargoes destined for transport and recruit the coat proteins that help form the carriers. These complexes use either linear motifs or posttranslational modifications to recognize the cargoes, which are then packaged and delivered along the trafficking pathways. In this review, we focus on the different trafficking complexes that share a common evolutionary branch in Arabidopsis (Arabidopsis thaliana), and we discuss up-to-date knowledge about the cargo recognition motifs they use.

Cargo sorting zones in the trans-Golgi network visualized by super-resolution confocal live imaging microscopy in plants

The trans-Golgi network (TGN) has been known as a key platform to sort and transport proteins to their final destinations in post-Golgi membrane trafficking. However, how the TGN sorts proteins with different destinies still remains elusive. Here, we examined 3D localization and 4D dynamics of TGN-localized proteins of Arabidopsis thaliana that are involved in either secretory or vacuolar trafficking from the TGN, by a multicolor high-speed and high-resolution spinning-disk confocal microscopy approach that we developed. We demonstrate that TGN-localized proteins exhibit spatially and temporally distinct distribution. VAMP721 (R-SNARE), AP (adaptor protein complex)-1, and clathrin which are involved in secretory trafficking compose an exclusive subregion, whereas VAMP727 (R-SNARE) and AP-4 involved in vacuolar trafficking compose another subregion on the same TGN. Based on these findings, we propose that the single TGN has at least two subregions, or "zones", responsible for distinct cargo sorting: the secretory-trafficking zone and the vacuolar-trafficking zone.

Targeting and signaling of Rho of plants guanosine triphosphatases require synergistic interaction between guanine nucleotide inhibitor and vesicular trafficking

Most eukaryotic cells are polarized. Common toolbox regulating cell polarization includes Rho guanosine triphosphatases (GTPases), in which spatiotemporal activation is regulated by a plethora of regulators. Rho of plants (ROPs) are the only Rho GTPases in plants. Although vesicular trafficking was hinted in the regulation of ROPs, it was unclear where vesicle-carried ROP starts, whether it is dynamically regulated, and which components participate in vesicle-mediated ROP targeting. In addition, although vesicle trafficking and guanine nucleotide inhibitor (GDI) pathways in Rho signaling have been extensively studied in yeast, it is unknown whether the two pathways interplay. Unclear are also cellular and developmental consequences of their interaction in multicellular organisms. Here, we show that the dynamic targeting of ROP through vesicles requires coat protein complex II and ADP-ribosylation factor 1-mediated post-Golgi trafficking. Trafficking of vesicle-carried ROPs between the plasma membrane and the trans-Golgi network is mediated through adaptor protein 1 and sterol-mediated endocytosis. Finally, we show that GDI and vesicle trafficking synergistically regulate cell polarization and ROP targeting, suggesting that the establishment and maintenance of cell polarity is regulated by an evolutionarily conserved mechanism.

EPSIN1 and MTV1 define functionally overlapping but molecularly distinct trans-Golgi network subdomains in Arabidopsis

The plant trans-Golgi network (TGN) is a central trafficking hub where secretory, vacuolar, recycling, and endocytic pathways merge. Among currently known molecular players involved in TGN transport, three different adaptor protein (AP) complexes promote vesicle generation at the TGN with different cargo specificity and destination. Yet, it remains unresolved how sorting into diverging vesicular routes is spatially organized. Here, we study the family of Arabidopsis thaliana Epsin-like proteins, which are accessory proteins to APs facilitating vesicle biogenesis. By comprehensive molecular, cellular, and genetic analysis of the EPSIN gene family, we identify EPSIN1 and MODIFIED TRANSPORT TO THE VACUOLE1 (MTV1) as its only TGN-associated members. Despite their large phylogenetic distance, they perform overlapping functions in vacuolar and secretory transport. By probing their relationship with AP complexes, we find that they define two molecularly independent pathways: While EPSIN1 associates with AP-1, MTV1 interacts with AP-4, whose function is required for MTV1 recruitment. Although both EPSIN1/AP-1 and MTV1/AP-4 pairs reside at the TGN, high-resolution microscopy reveals them as spatially separate entities. Our results strongly support the hypothesis of molecularly, functionally, and spatially distinct subdomains of the plant TGN and suggest that functional redundancy can be achieved through parallelization of molecularly distinct but functionally overlapping pathways.

AP3M harbors actin filament binding activity that is crucial for vacuole morphology and stomatal closure in Arabidopsis

Stomatal movement is essential for plant growth. This process is precisely regulated by various cellular activities in guard cells. F-actin dynamics and vacuole morphology are both involved in stomatal movement. The sorting of cargoes by clathrin adaptor protein (AP) complexes from the Golgi to the vacuole is critical for establishing a normal vacuole morphology. In this study, we demonstrate that the medium subunit of the AP3 complex (AP3M) binds to and severs actin filaments in vitro and that it participates in the sorting of cargoes (such as the sucrose exporter SUC4) to the tonoplast, and thereby regulates stomatal closure in Arabidopsis thaliana Defects in AP3 or SUC4 led to more rapid water loss and delayed stomatal closure, as well as hypersensitivity to drought stress. In ap3m mutants, the F-actin status was altered compared to the wild type, and the sorted cargoes failed to localize to the tonoplast. AP3M contains a previously unidentified F-actin binding domain that is conserved in AP3M homologs in both plants and animals. Mutations in the F-actin binding domain of AP3M abolished its F-actin binding activity in vitro, leading to an aberrant vacuole morphology and reduced levels of SUC4 on the tonoplast in guard cells. Our findings indicate that the F-actin binding activity of AP3M is required for the precise localization of AP3-dependent cargoes to the tonoplast and for the regulation of vacuole morphology in guard cells during stomatal closure.

The Arabidopsis receptor kinase STRUBBELIG undergoes clathrin-dependent endocytosis

Signaling mediated by cell surface receptor kinases is central to the coordination of growth patterns during organogenesis. Receptor kinase signaling is in part controlled through endocytosis and subcellular distribution of the respective receptor kinase. For the majority of plant cell surface receptors, the underlying trafficking mechanisms are not characterized. In Arabidopsis, tissue morphogenesis requires the atypical receptor kinase STRUBBELIG (SUB). Here, we studied the endocytic mechanism of SUB. Our data revealed that a functional SUB-enhanced green fluorescent protein (EGFP) fusion is ubiquitinated in vivo. We further showed that plasma membrane-bound SUB:EGFP becomes internalized in a clathrin-dependent fashion. We also found that SUB:EGFP associates with the trans-Golgi network and accumulates in multivesicular bodies and the vacuole. Co-immunoprecipitation experiments revealed that SUB:EGFP and clathrin are present within the same protein complex. Our genetic analysis showed that SUB and CLATHRIN HEAVY CHAIN (CHC) 2 regulate root hair patterning. By contrast, genetic reduction of CHC activity ameliorates the floral defects of sub mutants. Taken together, the data indicate that SUB undergoes clathrin-mediated endocytosis, that this process does not rely on stimulation of SUB signaling by an exogenous agent, and that SUB genetically interacts with clathrin-dependent pathways in a tissue-specific manner.

The AP-1 Complex is Required for Proper Mucilage Formation in Arabidopsis Seeds

The adaptor protein (AP) complexes play crucial roles in vesicle formation in post-Golgi trafficking. Land plants have five types of AP complexes (AP-1 to AP-5), each of which consists of two large subunits, one medium subunit and one small subunit. Here, we show that the Arabidopsis AP-1 complex mediates the polarized secretion and accumulation of a pectic polysaccharide called mucilage in seed coat cells. Previously, a loss-of-function mutant of AP1M2, the medium subunit of AP-1, has been shown to display deleterious growth defects because of defective cytokinesis. To investigate the function of AP-1 in interphase, we generated transgenic Arabidopsis plants expressing AP1M2-GFP (green fluorescent protein) under the control of the cytokinesis-specific KNOLLE (KN) promoter in the ap1m2 background. These transgenic plants, designated pKN lines, successfully rescued the cytokinesis defect and dwarf phenotype of ap1m2. pKN lines showed reduced mucilage extrusion from the seed coat. Furthermore, abnormal accumulation of mucilage was found in the vacuoles of the outermost integument cells of pKN lines. During seed development, the accumulation of AP1M2-GFP was greatly reduced in the integument cells of pKN lines. These results suggest that trans-Golgi network (TGN)-localized AP-1 is involved in the trafficking of mucilage components to the outer surface of seed coat cells. Our study highlights an evolutionarily conserved function of AP-1 in polarized sorting in eukaryotic cells.

Sorting of Arabidopsis NRAMP3 and NRAMP4 depends on adaptor protein complex AP4 and a dileucine-based motif

Adaptor protein complexes mediate cargo selection and vesicle trafficking to different cellular membranes in all eukaryotic cells. Information on the role of AP4 in plants is still limited. Here, we present the analyses of Arabidopsis thaliana mutants lacking different subunits of AP4. These mutants show abnormalities in their development and in protein sorting. We found that growth of roots and etiolated hypocotyls, as well as male fertility and trichome morphology are disturbed in ap4. Analyses of GFP-fusions transiently expressed in mesophyll protoplasts demonstrated that the tonoplast (TP) proteins MOT2, NRAMP3 and NRAMP4, but not INT1, are partially sorted to the plasma membrane (PM) in the absence of a functional AP4 complex. Moreover, alanine mutagenesis revealed that in wild-type plants, sorting of NRAMP3 and NRAMP4 to the TP requires an N-terminal dileucine-based motif. The NRAMP3 or NRAMP4 N-terminal domain containing the dileucine motif was sufficient to redirect the PM localized INT4 protein to the TP and to confer AP4-dependency on sorting of INT1. Our data show that correct sorting of NRAMP3 and NRAMP4 depends on both, an N-terminal dileucine-based motif as well as AP4.

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

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

Gene Expression Pattern and Protein Localization of Arabidopsis Phospholipase D Alpha 1 Revealed by Advanced Light-Sheet and Super-Resolution Microscopy

Phospholipase D alpha 1 (PLDα1, At3g15730) and its product phosphatidic acid (PA) are involved in a variety of cellular and physiological processes, such as cytoskeletal remodeling, regulation of stomatal closure and opening, as well as biotic and abiotic stress signaling. Here we aimed to study developmental expression patterns and subcellular localization of PLDα1 in Arabidopsis using advanced microscopy methods such as light-sheet fluorescence microscopy (LSFM) and structured illumination microscopy (SIM). We complemented two knockout pldα1 mutants with a YFP-tagged PLDα1 expressed under the PLDα1 native promoter in order to study developmental expression pattern and subcellular localization of PLDα1 in Arabidopsis thaliana under natural conditions. Imaging of tissue-specific and developmentally-regulated localization of YFP-tagged PLDα1 by LSFM in roots of growing seedlings showed accumulation of PLDα1-YFP in the root cap and the rhizodermis. Expression of PLDα1-YFP in the rhizodermis was considerably higher in trichoblasts before and during root hair formation and growth. Thus, PLDα1-YFP accumulated in emerging root hairs and in the tips of growing root hairs. PLDα1-YFP showed cytoplasmic subcellular localization in root cap cells and in cells of the root transition zone. In aerial parts of plants PLDα1-YFP was also localized in the cytoplasm showing enhanced accumulation in the cortical cytoplasmic layer of epidermal non-dividing cells of hypocotyls, leaves, and leaf petioles. However, in dividing cells of root apical meristem and leaf petiole epidermis PLDα1-YFP was enriched in mitotic spindles and phragmoplasts, as revealed by co-visualization with microtubules. Finally, super-resolution SIM imaging revealed association of PLDα1-YFP with both microtubules and clathrin-coated vesicles (CCVs) and pits (CCPs). In conclusion, this study shows the developmentally-controlled expression and subcellular localization of PLDα1 in dividing and non-dividing Arabidopsis cells.

Specificity of plant membrane trafficking - ARFs, regulators and coat proteins

Approximately one-third of all eukaryotic proteins are delivered to their destination by trafficking within the endomembrane system. Such cargo proteins are incorporated into forming membrane vesicles on donor compartments and delivered to acceptor compartments by vesicle fusion. How cargo proteins are sorted into forming vesicles is still largely unknown. Here we review the roles of small GTPases of the ARF/SAR1 family, their regulators designated ARF guanine-nucleotide exchange factors (ARF-GEFs) and ARF GTPase-activating proteins (ARF-GAPs) as well as coat protein complexes during membrane vesicle formation. Although conserved across eukaryotes, these four functional groups of proteins display plant-specific modifications in composition, structure and function.

Seeing the endomembrane system for the trees: Evolutionary analysis highlights the importance of plants as models for eukaryotic membrane-trafficking

Plant cells show many signs of a unique evolutionary history. This is seen in the system of intracellular organelles and vesicle transport pathways plants use to traffic molecular cargo. Bioinformatic and cell biological work in this area is beginning to tackle the question of how plant cells have evolved, and what this tells us about the evolution of other eukaryotes. Key protein families with membrane trafficking function, including Rabs, SNAREs, vesicle coat proteins, and ArfGAPs, show patterns of evolution that indicate both specialization and conservation in plants. These changes are accompanied by changes at the level of organelles and trafficking pathways between them. Major specializations include losses of several ancient Rabs, novel functions of many proteins, and apparent modification of trafficking in endocytosis and cytokinesis. Nevertheless, plants show extensive conservation of ancestral membrane trafficking genes, and conservation of their ancestral function in most duplicates. Moreover, plants have retained several ancient membrane trafficking genes lost in the evolution of animals and fungi. Considering this, plants such as Arabidopsis are highly valuable for investigating not only plant-specific aspects of membrane trafficking, but also general eukaryotic mechanisms.

Arabidopsis adaptor protein 1G is critical for pollen development

Pollen development is a pre-requisite for sexual reproduction of angiosperms, during which various cellular activities are involved. Pollen development accompanies dynamic remodeling of vacuoles through fission and fusion, disruption of which often compromises pollen viability. We previously reported that the Y subunit of adaptor protein 1 (AP1G) mediates synergid degeneration during pollen tube reception. Here, we demonstrate that AP1G is essential for pollen development. AP1G loss-of-function resulted in male gametophytic lethality due to defective pollen development. By ultrastructural analysis and fluorescence labeling, we demonstrate that AP1G loss-of-function compromised dynamic vacuolar remodeling during pollen development and impaired vacuolar acidification of pollen. Results presented here support a key role of vacuoles in gametophytic pollen development.

Update on adaptor protein-3 in Arabidopsis

Adaptor proteins (APs) mediate protein sorting within endomembrane compartments in eukaryotic cells. AP-3 is an ancient AP complex mediating vacuolar trafficking in different phyla. Only recently, a few tonoplast proteins have been identified as AP-3 cargos in Arabidopsis whereas the function of AP-3 was largely unexplored. Here, we summarize recent advances on AP-3 in Arabidopsis, pointing at the potential roles of AP-3 in plant development and cellular processes.

Dual role of the Toxoplasma gondii clathrin adaptor AP1 in the sorting of rhoptry and microneme proteins and in parasite division

Toxoplasma gondii possesses a highly polarized secretory system, which efficiently assembles de novo micronemes and rhoptries during parasite replication. These apical secretory organelles release their contents into host cells promoting parasite invasion and survival. Using a CreLox-based inducible knock-out strategy and the ddFKBP over-expression system, we unraveled novel functions of the clathrin adaptor complex TgAP1. First, our data indicate that AP1 in T. gondii likely functions as a conserved heterotetrameric complex composed of the four subunits γ, β, μ1, σ1 and interacts with known regulators of clathrin-mediated vesicular budding such as the unique ENTH-domain containing protein, which we named Epsin-like protein (TgEpsL). Disruption of the μ1 subunit resulted in the mis-sorting of microneme proteins at the level of the Trans-Golgi-Network (TGN). Furthermore, we demonstrated that TgAP1 regulates rhoptry biogenesis by activating rhoptry protein exit from the TGN, but also participates in the post-Golgi maturation process of preROP compartments into apically anchored club-shaped mature organelles. For this latter activity, our data indicate a specific functional relationship between TgAP1 and the Rab5A-positive endosome-like compartment. In addition, we unraveled an original role for TgAP1 in the regulation of parasite division. APμ1-depleted parasites undergo normal daughter cell budding and basal complex assembly but fail to segregate at the end of cytokinesis.

The phylum Apicomplexa comprises a large group of obligate intracellular parasites of wide human and agricultural significance. Most notable are Plasmodium, the causative agent of malaria, and Toxoplasma gondii, one of the most common human parasites, responsible for disease of the developing fetus and immune-compromised individuals. Apicomplexa are characterized by the presence of an apical complex consisting of secretory organelles named micronemes (MIC) and rhoptries (ROP). MIC and ROP proteins, released upon host cell recognition, are essential for host cell invasion and parasite survival. After invasion, these organelles are neo-synthesized at each parasite replication cycle. In our study, we demonstrate a crucial role for the T. gondii clathrin adaptor complex AP1 in the vesicular transport of neo-synthesized MIC and ROP proteins, thereby regulating mature apical organelle formation. In addition, we unravel an original role for TgAP1 in the late stages of the parasite division process during daughter cell segregation. Therefore, our study provides new insights into key regulatory mechanisms of the vesicular trafficking system essential for host invasion and intracellular survival of Toxoplasma gondii.

Resolving the homology-function relationship through comparative genomics of membrane-trafficking machinery and parasite cell biology

With advances in DNA sequencing technology, it is increasingly common and tractable to informatically look for genes of interest in the genomic databases of parasitic organisms and infer cellular states. Assignment of a putative gene function based on homology to functionally characterized genes in other organisms, though powerful, relies on the implicit assumption of functional homology, i.e. that orthology indicates conserved function. Eukaryotes reveal a dazzling array of cellular features and structural organization, suggesting a concomitant diversity in their underlying molecular machinery. Significantly, examples of novel functions for pre-existing or new paralogues are not uncommon. Do these examples undermine the basic assumption of functional homology, especially in parasitic protists, which are often highly derived? Here we examine the extent to which functional homology exists between organisms spanning the eukaryotic lineage. By comparing membrane trafficking proteins between parasitic protists and traditional model organisms, where direct functional evidence is available, we find that function is indeed largely conserved between orthologues, albeit with significant adaptation arising from the unique biological features within each lineage.

HAPLESS13-Mediated Trafficking of STRUBBELIG Is Critical for Ovule Development in Arabidopsis

Planar morphogenesis, a distinct feature of multicellular organisms, is crucial for the development of ovule, progenitor of seeds. Both receptor-like kinases (RLKs) such as STRUBBELIG (SUB) and auxin gradient mediated by PIN-FORMED1 (PIN1) play instructive roles in this process. Fine-tuned intercellular communications between different cell layers during ovule development demands dynamic membrane distribution of these cell-surface proteins, presumably through vesicle-mediated sorting. However, the way it's achieved and the trafficking routes involved are obscure. We report that HAPLESS13 (HAP13)-mediated trafficking of SUB is critical for ovule development. HAP13 encodes the μ subunit of adaptor protein 1 (AP1) that mediates protein sorting at the trans-Golgi network/early endosome (TGN/EE). The HAP13 mutant, hap13-1, is defective in outer integument growth, resulting in exposed nucellus accompanied with impaired pollen tube guidance and reception. SUB is mis-targeted in hap13-1. However, unlike that of PIN2, the distribution of PIN1 is independent of HAP13. Genetic interference of exocytic trafficking at the TGN/EE by specifically downregulating HAP13 phenocopied the defects of hap13-1 in SUB targeting and ovule development, supporting a key role of sporophytically expressed SUB in instructing female gametogenesis.

The tyrosine-sorting motif of the vacuolar sorting receptor VSR4 from Arabidopsis thaliana, which is involved in the interaction between VSR4 and AP1M2, μ1-adaptin type 2 of clathrin adaptor complex 1 subunits, participates in the post-Golgi sorting of VSR4

μ1-Adaptin of adaptor protein (AP) 1 complex, AP1M, is generally accepted to load cargo proteins into clathrin-coated vesicles (CCVs) at the trans-Golgi network through its binding to cargo-recognition sequences (CRSs). Plant vacuolar-sorting receptors (VSRs) function in sorting vacuolar proteins, which are reportedly mediated by CCV. We herein investigated the involvement of CRSs of Arabidopsis thaliana VSR4 in the sorting of VSR4. The results obtained showed the increased localization of VSR4 at the plasma membrane or vacuoles by mutations in CRSs including the tyrosine-sorting motif YMPL or acidic dileucine-like motif EIRAIM, respectively. Interaction analysis using the bimolecular fluorescence complementation (BiFC) system, V10-BiFC, which we developed, indicated an interaction between VSR4 and AP1M2, AP1M type 2, which was attenuated by a YMPL mutation, but not influenced by an EIRAIM mutation. These results demonstrated the significance of the recognition of YMPL in VSR4 by AP1M2 for the post-Golgi sorting of VSR4.

Endomembrane trafficking overarching cell plate formation

By contrast to other eukaryotic kingdoms, plant cytokinesis is an inside-out process. A coordinated action of cytoskeletal transitions and endomembrane trafficking events builds a novel membrane compartment, the cell plate. Deposition of cell wall polymers transforms the lumen of this membrane compartment into a new cross wall, physically separating the daughter cells. The characterization of tethering complexes acting at discrete phases during cell plate formation and upstream of vesicle fusion events, the presence of modulators directing secretion and recycling during cytokinesis, as well as the identification and temporal recruitment of the endocytic machinery, provides a starting point to dissect the transitions in endomembrane trafficking which shape this process. This review aims to integrate recent findings on endomembrane trafficking events which spatio-temporally act to construct the cell plate.

Plant cytokinesis-No ring, no constriction but centrifugal construction of the partitioning membrane

Plants have evolved a unique way of partitioning the cytoplasm of dividing cells: Instead of forming a contractile ring that constricts the plasma membrane, plant cells target membrane vesicles to the plane of division where the vesicles fuse with one another to form the partitioning membrane. Plant cytokinesis starts in the centre and progresses towards the periphery, culminating in the fusion of the partitioning membrane with the parental plasma membrane. This membrane dynamics is orchestrated by a specific cytoskeletal array named phragmoplast that originates from interzone spindle remnants. Here we review the properties of the process as well as molecules that play specific roles in that process.

Plant cytokinesis: a tale of membrane traffic and fusion

Cytokinesis separates the forming daughter cells. Higher plants have lost the ability to constrict the plasma membrane (PM) in the division plane. Instead, trans-Golgi network (TGN)-derived membrane vesicles are targeted to the centre of the division plane and generate, by homotypic fusion, the partitioning membrane named cell plate (CP). The CP expands in a centrifugal fashion until its margin fuses with the PM at the cortical division site. Mutant screens in Arabidopsis have identified a cytokinesis-specific syntaxin named KNOLLE and an interacting Sec1/Munc18 (SM) protein named KEULE both of which are required for vesicle fusion during cytokinesis. KNOLLE is only made during M-phase, targeted to the division plane and degraded in the vacuole at the end of cytokinesis. Here we address mechanisms of KNOLLE trafficking and interaction of KNOLLE with different soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE) partners and with SM-protein KEULE, ensuring membrane fusion in cytokinesis.

Polysaccharide deposition during cytokinesis: Challenges and future perspectives

De novo formation of a new cell wall partitions the cytoplasm of the dividing cell during plant cytokinesis. The development of the cell plate, a transient sheet-like structure, requires the accumulation of vesicles directed by the phragmoplast to the cell plate assembly matrix. Fusion and fission of the accumulated vesicles are accompanied by the deposition of polysaccharides and cell wall structural proteins; together, they are leading to the stabilization of the formed structure which after insertion into the parental wall lead to the maturation of the nascent cross wall. Callose is the most abundant polysaccharide during cell plate formation and during maturation is gradually replaced by cellulose. Matrix polysaccharides such as hemicellulose, and pectins presumably are present throughout all developmental stages, being delivered to the cell plate by secretory vesicles. The availability of novel chemical probes such as endosidin 7, which inhibits callose formation at the cell plate, has proved useful for dissecting the temporal accumulation of vesicles at the cell plate and establishing the critical role of callose during cytokinesis. The use of emerging approaches such as chemical genomics combined with live cell imaging; novel techniques of polysaccharide detection including tagged polysaccharide substrates, newly characterized polysaccharide antibodies and vesicle proteomics can be used to develop a comprehensive model of cell plate development.

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.

Roles of membrane trafficking in plant cell wall dynamics

The cell wall is one of the characteristic components of plant cells. The cell wall composition differs among cell types and is modified in response to various environmental conditions. To properly generate and modify the cell wall, many proteins are transported to the plasma membrane or extracellular space through membrane trafficking, which is one of the key protein transport mechanisms in eukaryotic cells. Given the diverse composition and functions of the cell wall in plants, the transport of the cell wall components and proteins that are involved in cell wall-related events could be specialized for each cell type, i.e., the machinery for cell wall biogenesis, modification, and maintenance could be transported via different trafficking pathways. In this review, we summarize the recent progress in the current understanding of the roles and mechanisms of membrane trafficking in plant cells and focus on the biogenesis and regulation of the cell wall.

Helping hands for budding prospects: ENTH/ANTH/VHS accessory proteins in endocytosis, vacuolar transport, and secretion

Coated vesicles provide a major mechanism for the transport of proteins through the endomembrane system of plants. Transport between the endoplasmic reticulum and the Golgi involves vesicles with COPI and COPII coats, whereas clathrin is the predominant coat in endocytosis and post-Golgi trafficking. Sorting of cargo, coat assembly, budding, and fission are all complex and tightly regulated processes that involve many proteins. The mechanisms and responsible factors are largely conserved in eukaryotes, and increasing organismal complexity tends to be associated with a greater numbers of individual family members. Among the key factors is the class of ENTH/ANTH/VHS domain-containing proteins, which link membrane subdomains, clathrin, and other adapter proteins involved in early steps of clathrin coated vesicle formation. More than 30 Arabidopsis thaliana proteins contain this domain, but their generally low sequence conservation has made functional classification difficult. Reports from the last two years have greatly expanded our knowledge of these proteins and suggest that ENTH/ANTH/VHS domain proteins are involved in various instances of clathrin-related endomembrane trafficking in plants. This review aims to summarize these new findings and discuss the broader context of clathrin-dependent plant vesicular transport.

Myrosin cell development is regulated by endocytosis machinery and PIN1 polarity in leaf primordia of Arabidopsis thaliana

Myrosin cells, which accumulate myrosinase to produce toxic compounds when they are ruptured by herbivores, form specifically along leaf veins in Arabidopsis thaliana. However, the mechanism underlying this pattern formation is unknown. Here, we show that myrosin cell development requires the endocytosis-mediated polar localization of the auxin-efflux carrier PIN1 in leaf primordia. Defects in the endocytic/vacuolar SNAREs (syp22 and syp22 vti11) enhanced myrosin cell development. The syp22 phenotype was rescued by expressing SYP22 under the control of the PIN1 promoter. Additionally, myrosin cell development was enhanced either by lacking the activator of endocytic/vacuolar RAB5 GTPase (VPS9A) or by PIN1 promoter-driven expression of a dominant-negative form of RAB5 GTPase (ARA7). By contrast, myrosin cell development was not affected by deficiencies of vacuolar trafficking factors, including the vacuolar sorting receptor VSR1 and the retromer components VPS29 and VPS35, suggesting that endocytic pathway rather than vacuolar trafficking pathway is important for myrosin cell development. The phosphomimic PIN1 variant (PIN1-Asp), which is unable to be polarized, caused myrosin cells to form not only along leaf vein but also in the intervein leaf area. We propose that Brassicales plants might arrange myrosin cells near vascular cells in order to protect the flux of nutrients and water via polar PIN1 localization.

The trafficking of the cellulose synthase complex in higher plants

BACKGROUND

Cellulose is an important constituent of plant cell walls in a biological context, and is also a material commonly utilized by mankind in the pulp and paper, timber, textile and biofuel industries. The biosynthesis of cellulose in higher plants is a function of the cellulose synthase complex (CSC). The CSC, a large transmembrane complex containing multiple cellulose synthase proteins, is believed to be assembled in the Golgi apparatus, but is thought only to synthesize cellulose when it is localized at the plasma membrane, where CSCs synthesize and extrude cellulose directly into the plant cell wall. Therefore, the delivery and endocytosis of CSCs to and from the plasma membrane are important aspects for the regulation of cellulose biosynthesis.

SCOPE

Recent progress in the visualization of CSC dynamics in living plant cells has begun to reveal some of the routes and factors involved in CSC trafficking. This review highlights the most recent major findings related to CSC trafficking, provides novel perspectives on how CSC trafficking can influence the cell wall, and proposes potential avenues for future exploration.

Trans-Golgi network localized small GTPase RabA1d is involved in cell plate formation and oscillatory root hair growth

BACKGROUND

Small Rab GTPases are important regulators of vesicular trafficking in plants. AtRabA1d, a member of the RabA1 subfamily of small GTPases, was previously found in the vesicle-rich apical dome of growing root hairs suggesting a role during tip growth; however, its specific intracellular localization and role in plants has not been well described.

RESULTS

The transient expression of 35S::GFP:RabA1d construct in Allium porrum and Nicotiana benthamiana revealed vesicular structures, which were further corroborated in stable transformed Arabidopsis thaliana plants. GFP-RabA1d colocalized with the trans-Golgi network marker mCherry-VTI12 and with early FM4-64-labeled endosomal compartments. Late endosomes and endoplasmic reticulum labeled with FYVE-DsRed and ER-DsRed, respectively, were devoid of GFP-RabA1d. The accumulation of GFP-RabA1d in the core of brefeldin A (BFA)-induced-compartments and the quantitative upregulation of RabA1d protein levels after BFA treatment confirmed the association of RabA1d with early endosomes/TGN and its role in vesicle trafficking. Light-sheet microscopy revealed involvement of RabA1d in root development. In root cells, GFP-RabA1d followed cell plate expansion consistently with cytokinesis-related vesicular trafficking and membrane recycling. GFP-RabA1d accumulated in disc-like structures of nascent cell plates, which progressively evolved to marginal ring-like structures of the growing cell plates. During root hair growth and development, GFP-RabA1d was enriched at root hair bulges and at the apical dome of vigorously elongating root hairs. Importantly, GFP-RabA1d signal intensity exhibited an oscillatory behavior in-phase with tip growth. Progressively, this tip localization dissapeared in mature root hairs suggesting a link between tip localization of RabA1d and root hair elongation. Our results support a RabA1d role in events that require vigorous membrane trafficking.

CONCLUSIONS

RabA1d is located in early endosomes/TGN and is involved in vesicle trafficking. RabA1d participates in both cell plate formation and root hair oscillatory tip growth. The specific GFP-RabA1d subcellular localization confirms a correlation between its specific spatio-temporal accumulation and local vesicle trafficking requirements during cell plate and root hair formation.

Polar delivery in plants; commonalities and differences to animal epithelial cells

Although plant and animal cells use a similar core mechanism to deliver proteins to the plasma membrane, their different lifestyle, body organization and specific cell structures resulted in the acquisition of regulatory mechanisms that vary in the two kingdoms. In particular, cell polarity regulators do not seem to be conserved, because genes encoding key components are absent in plant genomes. In plants, the broad knowledge on polarity derives from the study of auxin transporters, the PIN-FORMED proteins, in the model plant Arabidopsis thaliana. In animals, much information is provided from the study of polarity in epithelial cells that exhibit basolateral and luminal apical polarities, separated by tight junctions. In this review, we summarize the similarities and differences of the polarization mechanisms between plants and animals and survey the main genetic approaches that have been used to characterize new genes involved in polarity establishment in plants, including the frequently used forward and reverse genetics screens as well as a novel chemical genetics approach that is expected to overcome the limitation of classical genetics methods.

Delivery of endocytosed proteins to the cell-division plane requires change of pathway from recycling to secretion

Membrane trafficking is essential to fundamental processes in eukaryotic life, including cell growth and division. In plant cytokinesis, post-Golgi trafficking mediates a massive flow of vesicles that form the partitioning membrane but its regulation remains poorly understood. Here, we identify functionally redundant Arabidopsis ARF guanine-nucleotide exchange factors (ARF-GEFs) BIG1–BIG4 as regulators of post-Golgi trafficking, mediating late secretion from the trans-Golgi network but not recycling of endocytosed proteins to the plasma membrane, although the TGN also functions as an early endosome in plants. In contrast, BIG1-4 are absolutely required for trafficking of both endocytosed and newly synthesized proteins to the cell–division plane during cytokinesis, counteracting recycling to the plasma membrane. This change from recycling to secretory trafficking pathway mediated by ARF-GEFs confers specificity of cargo delivery to the division plane and might thus ensure that the partitioning membrane is completed on time in the absence of a cytokinesis-interphase checkpoint.

DOI:http://dx.doi.org/10.7554/eLife.02131.001

Cells are surrounded by a plasma membrane, and when a cell divides to create two new cells, it must grow a new membrane to keep the two new cells apart. Animal cells and plant cells tackle this challenge in different ways: in animal cells the new membrane grows inwards from the surface of the cell, whereas the new membrane grows outwards from the centre of the cell in plant cells.

The materials needed to make the plasma membrane are delivered in packages called vesicles: most of these materials arrive from a structure within the cell called the trans-Golgi network, but some materials are recycled from the existing plasma membrane. In plants the formation of the new cell membrane is orchestrated by scaffold-like structure that forms in the plant cell called the ‘phragmoplast’. It is widely thought that this structure guides the vesicles bringing materials from the trans-Golgi network, but the details of this process are not fully understood.

Now, Richter et al. have discovered four proteins, called BIG1 to BIG4, that control the formation of the new cell membrane in the flowering plant Arabidopsis thaliana, a species that is routinely studied by plant biologists. These four proteins belong to a larger family of proteins that control the trafficking of vesicles within a cell. Richter et al show that a plant cell can lose up to three of these four proteins and still divide, as the plant can still grow and develop as normal. Thus, BIG1 to BIG4 appear to perform essentially the same role in the plant.

Richter et al. also show that, when a plant cell is not dividing, these proteins are involved in controlling the delivery of new materials to surface membrane, and not the recycling of material. However, when a cell is dividing, these proteins switch to regulate both processes, but direct all the material to a new destination—the newly forming membrane, instead of the established surface membrane. Richter et al. suggest that this switch is important to stop any recycling to the plasma membrane that might move material away from the new membrane. The next challenge will be to identify the molecular signals and mechanisms that enable the proteins BIG1 to BIG4 to re-route the recycling of membrane material during cell division.

DOI:http://dx.doi.org/10.7554/eLife.02131.002

BEX1/ARF1A1C is required for BFA-sensitive recycling of PIN auxin transporters and auxin-mediated development in Arabidopsis

Correct positioning of membrane proteins is an essential process in eukaryotic organisms. The plant hormone auxin is distributed through intercellular transport and triggers various cellular responses. Auxin transporters of the PIN-FORMED (PIN) family localize asymmetrically at the plasma membrane (PM) and mediate the directional transport of auxin between cells. A fungal toxin, brefeldin A (BFA), inhibits a subset of guanine nucleotide exchange factors for ADP-ribosylation factor small GTPases (ARF GEFs) including GNOM, which plays a major role in localization of PIN1 predominantly to the basal side of the PM. The Arabidopsis genome encodes 19 ARF-related putative GTPases. However, ARF components involved in PIN1 localization have been genetically poorly defined. Using a fluorescence imaging-based forward genetic approach, we identified an Arabidopsis mutant, bfa-visualized exocytic trafficking defective1 (bex1), in which PM localization of PIN1-green fluorescent protein (GFP) as well as development is hypersensitive to BFA. We found that in bex1 a member of the ARF1 gene family, ARF1A1C, was mutated. ARF1A1C localizes to the trans-Golgi network/early endosome and Golgi apparatus, acts synergistically to BEN1/MIN7 ARF GEF and is important for PIN recycling to the PM. Consistent with the developmental importance of PIN proteins, functional interference with ARF1 resulted in an impaired auxin response gradient and various developmental defects including embryonic patterning defects and growth arrest. Our results show that ARF1A1C is essential for recycling of PIN auxin transporters and for various auxin-dependent developmental processes.

Cell wall, cytoskeleton, and cell expansion in higher plants

To accommodate two seemingly contradictory biological roles in plant physiology, providing both the rigid structural support of plant cells and the adjustable elasticity needed for cell expansion, the composition of the plant cell wall has evolved to become an intricate network of cellulosic, hemicellulosic, and pectic polysaccharides and protein. Due to its complexity, many aspects of the cell wall influence plant cell expansion, and many new and insightful observations and technologies are forthcoming. The biosynthesis of cell wall polymers and the roles of the variety of proteins involved in polysaccharide synthesis continue to be characterized. The interactions within the cell wall polymer network and the modification of these interactions provide insight into how the plant cell wall provides its dual function. The complex cell wall architecture is controlled and organized in part by the dynamic intracellular cytoskeleton and by diverse trafficking pathways of the cell wall polymers and cell wall-related machinery. Meanwhile, the cell wall is continually influenced by hormonal and integrity sensing stimuli that are perceived by the cell. These many processes cooperate to construct, maintain, and manipulate the intricate plant cell wall--an essential structure for the sustaining of the plant stature, growth, and life.

CONTINUOUS VASCULAR RING (COV1) is a trans-Golgi network-localized membrane protein required for Golgi morphology and vacuolar protein sorting

The trans-Golgi network (TGN) is a tubular-vesicular organelle that matures from the trans cisternae of the Golgi apparatus. In plants, the TGN functions as a central hub for three trafficking pathways: the secretory pathway, the vacuolar trafficking pathway and the endocytic pathway. Here, we describe a novel TGN-localized membrane protein, CONTINUOUS VASCULAR RING (COV1), that is crucial for TGN function in Arabidopsis. The COV1 gene was originally identified from the stem vascular patterning mutant of Arabidopsis thaliana. However, the molecular function of COV1 was not identified. Fluorescently tagged COV1 proteins co-localized with the TGN marker proteins, SYNTAXIN OF PLANTS 4 (SYP4) and vacuolar-type H(+)-ATPase subunit a1 (VHA-a1). Consistently, COV1-localized compartments were sensitive to concanamycin A, a specific inhibitor of VHA. Intriguingly, cov1 mutants exhibited abnormal Golgi morphologies, including a reduction in the number of Golgi cisternae and a reduced association between the TGN and the Golgi apparatus. A deficiency in COV1 also resulted in a defect in vacuolar protein sorting, which was characterized by the abnormal accumulation of storage protein precursors in seeds. Moreover, we found that the development of an idioblast, the myrosin cell, was abnormally increased in cov1 leaves. Our results demonstrate that the novel TGN-localized protein COV1 is required for Golgi morphology, vacuolar trafficking and myrosin cell development.

Adaptor proteins involved in polarized sorting

Polarized cells such as epithelial cells and neurons exhibit different plasma membrane domains with distinct protein compositions. Recent studies have shown that sorting of transmembrane proteins to the basolateral domain of epithelial cells and the somatodendritic domain of neurons is mediated by recognition of signals in the cytosolic domains of the proteins by adaptors. These adaptors are components of protein coats associated with the trans-Golgi network and/or recycling endosomes. The clathrin-associated adaptor protein 1 (AP-1) complex plays a preeminent role in this process, although other adaptors and coat proteins, such as AP-4, ARH, Numb, exomer, and retromer, have also been implicated.

Clathrin and post-Golgi trafficking: a very complicated issue

Clathrin-coated vesicles (CCVs) are formed at the plasma membrane and act as vectors for endocytosis. They also assemble at the trans-Golgi network (TGN), but their exact function at this organelle is unclear. Recent studies have examined the effects on vacuolar and secretory protein transport of knockout mutations of the adaptor protein 1 (AP1) μ-adaptin subunit AP1M, but these investigations do not clarify the situation. These mutations lead to the abrogation of multiple trafficking pathways at the TGN and cannot be used as evidence in favour of CCVs being agents for receptor-mediated export of vacuolar proteins out of the TGN. This transport process could just as easily occur through the maturation of the TGN into intermediate compartments that subsequently fuse with the vacuole.

Dynamin-related proteins in plant post-Golgi traffic

Membrane traffic between two organelles begins with the formation of transport vesicles from the donor organelle. Dynamin-related proteins (DRPs), which are large multidomain GTPases, play crucial roles in vesicle formation in post-Golgi traffic. Numerous in vivo and in vitro studies indicate that animal dynamins, which are members of DRP family, assemble into ring- or helix-shaped structures at the neck of a bud site on the donor membrane, where they constrict and sever the neck membrane in a GTP hydrolysis-dependent manner. While much is known about DRP-mediated trafficking in animal cells, little is known about it in plant cells. So far, two structurally distinct subfamilies of plant DRPs (DRP1 and DRP2) have been found to participate in various pathways of post-Golgi traffic. This review summarizes the structural and functional differences between these two DRP subfamilies, focusing on their molecular, cellular and developmental properties. We also discuss the molecular networks underlying the functional machinery centering on these two DRP subfamilies. Furthermore, we hope that this review will provide direction for future studies on the mechanisms of vesicle formation that are not only unique to plants but also common to eukaryotes.