Electron Tomography of RabA4b- and PI-4Kβ1-Labeled Trans Golgi Network Compartments in Arabidopsis

Regulatory roles of phosphoinositides in membrane trafficking and their potential impact on cell-wall synthesis and re-modelling

BACKGROUND

Plant cell walls are complex matrices of carbohydrates and proteins that control cell morphology and provide protection and rigidity for the plant body. The construction and maintenance of this intricate system involves the delivery and recycling of its components through a precise balance of endomembrane trafficking, which is controlled by a plethora of cell signalling factors. Phosphoinositides (PIs) are one class of signalling molecules with diverse roles in vesicle trafficking and cytoskeleton structure across different kingdoms. Therefore, PIs may also play an important role in the assembly of plant cell walls.

SCOPE

The eukaryotic PI pathway is an intricate network of different lipids, which appear to be divided in different pools that can partake in vesicle trafficking or signalling. Most of our current understanding of how PIs function in cell metabolism comes from yeast and mammalian systems; however, in recent years significant progress has been made towards a better understanding of the plant PI system. This review examines the current state of knowledge of how PIs regulate vesicle trafficking and their potential influence on plant cell-wall architecture. It considers first how PIs are formed in plants and then examines their role in the control of vesicle trafficking. Interactions between PIs and the actin cytoskeleton and small GTPases are also discussed. Future challenges for research are suggested.

Wide-range high-resolution transmission electron microscopy reveals morphological and distributional changes of endomembrane compartments during log to stationary transition of growth phase in tobacco BY-2 cells

Rapid growth of plant cells by cell division and expansion requires an endomembrane trafficking system. The endomembrane compartments, such as the Golgi stacks, endosome and vesicles, are important in the synthesis and trafficking of cell wall materials during cell elongation. However, changes in the morphology, distribution and number of these compartments during the different stages of cell proliferation and differentiation have not yet been clarified. In this study, we examined these changes at the ultrastructural level in tobacco Bright yellow 2 (BY-2) cells during the log and stationary phases of growth. We analyzed images of the BY-2 cells prepared by the high-pressure freezing/freeze substitution technique with the aid of an auto-acquisition transmission electron microscope system. We quantified the distribution of secretory and endosomal compartments in longitudinal sections of whole cells by using wide-range gigapixel-class images obtained by merging thousands of transmission electron micrographs. During the log phase, all Golgi stacks were composed of several thick cisternae. Approximately 20 vesicle clusters (VCs), including the trans-Golgi network and secretory vesicle cluster, were observed throughout the cell. In the stationary-phase cells, Golgi stacks were thin with small cisternae, and only a few VCs were observed. Nearly the same number of multivesicular body and small high-density vesicles were observed in both the stationary and log phases. Results from electron microscopy and live fluorescence imaging indicate that the morphology and distribution of secretory-related compartments dramatically change when cells transition from log to stationary phases of growth.

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

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

Retention mechanisms for ER and Golgi membrane proteins

Unless there are mechanisms to selectively retain membrane proteins in the endoplasmic reticulum (ER) or in the Golgi apparatus, they automatically proceed downstream to the plasma or vacuole membranes. Two types of coat protein complex I (COPI)-interacting motifs in the cytosolic tails of membrane proteins seem to facilitate membrane retention in the early secretory pathway of plants: a dilysine (KKXX) motif (which is typical of p24 proteins) for the ER and a KXE/D motif (which occurs in the Arabidopsis endomembrane protein EMP12) for the Golgi apparatus. The KXE/D motif is highly conserved in all eukaryotic EMPs and is additionally present in hundreds of other proteins of unknown subcellular localization and function. This novel signal may represent a new general mechanism for Golgi targeting and the retention of polytopic integral membrane proteins.

Constitutive salicylic acid accumulation in pi4kIIIβ1β2 Arabidopsis plants stunts rosette but not root growth

Phospholipids have recently been found to be integral elements of hormone signalling pathways. An Arabidopsis thaliana double mutant in two type III phosphatidylinositol-4-kinases (PI4Ks), pi4kIIIβ1β2, displays a stunted rosette growth. The causal link between PI4K activity and growth is unknown. Using microarray analysis, quantitative reverse transcription polymerase chain reaction (RT-qPCR) and multiple phytohormone analysis by LC-MS we investigated the mechanism responsible for the pi4kIIIβ1β2 phenotype. The pi4kIIIβ1β2 mutant accumulated a high concentration of salicylic acid (SA), constitutively expressed SA marker genes including PR-1, and was more resistant to Pseudomonas syringae. pi4kIIIβ1β2 was crossed with SA signalling mutants eds1 and npr1 and SA biosynthesis mutant sid2 and NahG. The dwarf phenotype of pi4kIIIβ1β2 rosettes was suppressed in all four triple mutants. Whereas eds1 pi4kIIIβ1β2, sid2 pi4kIIIβ1β2 and NahG pi4kIIIβ1β2 had similar amounts of SA as the wild-type (WT), npr1pi4kIIIβ1β2 had more SA than pi4kIIIβ1β2 despite being less dwarfed. This indicates that PI4KIIIβ1 and PI4KIIIβ2 are genetically upstream of EDS1 and need functional SA biosynthesis and perception through NPR1 to express the dwarf phenotype. The slow root growth phenotype of pi4kIIIβ1β2 was not suppressed in any of the triple mutants. The pi4kIIIβ1β2 mutations together cause constitutive activation of SA signalling that is responsible for the dwarf rosette phenotype but not for the short root phenotype.

Root hairs

Roots hairs are cylindrical extensions of root epidermal cells that are important for acquisition of nutrients, microbe interactions, and plant anchorage. The molecular mechanisms involved in the specification, differentiation, and physiology of root hairs in Arabidopsis are reviewed here. Root hair specification in Arabidopsis is determined by position-dependent signaling and molecular feedback loops causing differential accumulation of a WD-bHLH-Myb transcriptional complex. The initiation of root hairs is dependent on the RHD6 bHLH gene family and auxin to define the site of outgrowth. Root hair elongation relies on polarized cell expansion at the growing tip, which involves multiple integrated processes including cell secretion, endomembrane trafficking, cytoskeletal organization, and cell wall modifications. The study of root hair biology in Arabidopsis has provided a model cell type for insights into many aspects of plant development and cell biology.

Contribution of chitinase A's C-terminal vacuolar sorting determinant to the study of soluble protein compartmentation

Plant chitinases have been studied for their importance in the defense of crop plants from pathogen attacks and for their peculiar vacuolar sorting determinants. A peculiarity of the sequence of many family 19 chitinases is the presence of a C-terminal extension that seems to be important for their correct recognition by the vacuole sorting machinery. The 7 amino acids long C-terminal vacuolar sorting determinant (CtVSD) of tobacco chitinase A is necessary and sufficient for the transport to the vacuole. This VSD shares no homology with other CtVSDs such as the phaseolin’s tetrapeptide AFVY (AlaPheValTyr) and it is also sorted by different mechanisms. While a receptor for this signal has not yet been convincingly identified, the research using the chitinase CtVSD has been very informative, leading to the observation of phenomena otherwise difficult to observe such as the presence of separate vacuoles in differentiating cells and the existence of a Golgi-independent route to the vacuole. Thanks to these new insights in the endoplasmic reticulum (ER)-to-vacuole transport, GFPChi (Green Fluorescent Protein carrying the chitinase A CtVSD) and other markers based on chitinase signals will continue to help the investigation of vacuolar biogenesis in plants.

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.

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.

Identification of trans-golgi network proteins in Arabidopsis thaliana root tissue

Knowledge of protein subcellular localization assists in the elucidation of protein function and understanding of different biological mechanisms that occur at discrete subcellular niches. Organelle-centric proteomics enables localization of thousands of proteins simultaneously. Although such techniques have successfully allowed organelle protein catalogues to be achieved, they rely on the purification or significant enrichment of the organelle of interest, which is not achievable for many organelles. Incomplete separation of organelles leads to false discoveries, with erroneous assignments. Proteomics methods that measure the distribution patterns of specific organelle markers along density gradients are able to assign proteins of unknown localization based on comigration with known organelle markers, without the need for organelle purification. These methods are greatly enhanced when coupled to sophisticated computational tools. Here we apply and compare multiple approaches to establish a high-confidence data set of Arabidopsis root tissue trans-Golgi network (TGN) proteins. The method employed involves immunoisolations of the TGN, coupled to probability-based organelle proteomics techniques. Specifically, the technique known as LOPIT (localization of organelle protein by isotope tagging), couples density centrifugation with quantitative mass-spectometry-based proteomics using isobaric labeling and targeted methods with semisupervised machine learning methods. We demonstrate that while the immunoisolation method gives rise to a significant data set, the approach is unable to distinguish cargo proteins and persistent contaminants from full-time residents of the TGN. The LOPIT approach, however, returns information about many subcellular niches simultaneously and the steady-state location of proteins. Importantly, therefore, it is able to dissect proteins present in more than one organelle and cargo proteins en route to other cellular destinations from proteins whose steady-state location favors the TGN. Using this approach, we present a robust list of Arabidopsis TGN proteins.

ER and vacuoles: never been closer

The endoplasmic reticulum (ER) represents the gateway for intracellular trafficking of membrane proteins, soluble cargoes and lipids. In all eukaryotes, the best described mechanism of exiting the ER is via COPII-coated vesicles, which transport both membrane proteins and soluble cargoes to the cis-Golgi. The vacuole, together with the plasma membrane, is the most distal point of the secretory pathway, and many vacuolar proteins are transported from the ER through intermediate compartments. However, past results and recent findings demonstrate the presence of alternative transport routes from the ER towards the tonoplast, which are independent of Golgi- and post-Golgi trafficking. Moreover, the transport mechanism of the vacuolar proton pumps VHA-a3 and AVP1 challenges the current model of vacuole biogenesis, pointing to the endoplasmic reticulum for being the main membrane source for the biogenesis of the plant lytic compartment. This review gives an overview of the current knowledge on the transport routes towards the vacuole and discusses the possible mechanism of vacuole biogenesis in plants.

High-pressure freezing and low-temperature processing of plant tissue samples for electron microscopy

Use of electron tomography methods improves image resolution of transmission electron microscopy especially in the z-direction, enabling determination of complicated 3D structures of organelles and cytoskeleton arrays. The increase in resolution necessitates preservation of cellular structures close to the native states with minimum artifacts. High-pressure freezing (HPF) that immobilizes molecules in the cell instantaneously has been used to avoid damages caused by convention chemical fixation. Despite the advantages of HPF, cells could still be damaged during dissection prior to HPF. Therefore, it is critical to isolate cells/tissues of interest quickly and carefully. The samples frozen by HPF are often processed by freeze substitution (FS), and FS should be carried out under appropriate conditions. Here we describe dissection, HPF, and FS methods that we have utilized to prepare plant samples for electron tomography/immuno-electron microscopy.

A multi-colour/multi-affinity marker set to visualize phosphoinositide dynamics in Arabidopsis

Phosphatidylinositolphosphates (PIPs) are phospholipids that contain a phosphorylated inositol head group. PIPs represent a minor fraction of total phospholipids, but are involved in many regulatory processes, such as cell signalling and intracellular trafficking. Membrane compartments are enriched or depleted in specific PIPs, providing a unique composition for these compartments and contributing to their identity. The precise subcellular localization and dynamics of most PIP species is not fully understood in plants. Here, we designed genetically encoded biosensors with distinct relative affinities and expressed them stably in Arabidopsis thaliana. Analysis of this multi-affinity 'PIPline' marker set revealed previously unrecognized localization of various PIPs in root epidermis. Notably, we found that PI(4,5)P2 is able to localize PIP2 -interacting protein domains to the plasma membrane in non-stressed root epidermal cells. Our analysis further revealed that there is a gradient of PI4P, with the highest concentration at the plasma membrane, intermediate concentration in post-Golgi/endosomal compartments, and the lowest concentration in the Golgi. Finally, we also found a similar gradient of PI3P from high in late endosomes to low in the tonoplast. Our library extends the range of available PIP biosensors, and will allow rapid progress in our understanding of PIP dynamics in plants.

The Arabidopsis pi4kIIIβ1β2 double mutant is salicylic acid-overaccumulating: a new example of salicylic acid influence on plant stature

Growth is the best visible sign of plant comfort. If plants are under stress, a difference in growth with control conditions can indicate that something is going wrong (or better). Phytohormones such as auxin, cytokinins, brassinosteroids or giberellins, are important growth regulators and their role in plant growth was extensively studied. On the other hand the role of salicylic acid (SA), a phytohormone commonly connected with plant defense responses, in plant growth is under-rated. However, studies with SA-overaccumulating mutants directly showed an influence of SA on plant growth. Recently we characterized an Arabidopsis SA-overaccumulating mutant impaired in phosphatidylinositol-4-kinases (pi4kIIIβ1β2). This mutant is dwarf. The crossing with mutants impaired in SA signaling revealed that pi4kIIIβ1β2 stunted rosette is due to high SA, while the short root length is not. This brings into evidence that upper and lower parts of the plants, even though they may share common phenotypes, are differently regulated.

The cell biology of cellulose synthesis

Plant stature and development are governed by cell proliferation and directed cell growth. These parameters are determined largely by cell wall characteristics. Cellulose microfibrils, composed of hydrogen-bonded β-1,4 glucans, are key components for anisotropic growth in plants. Cellulose is synthesized by plasma membrane-localized cellulose synthase complexes. In higher plants, these complexes are assembled into hexameric rosettes in intracellular compartments and secreted to the plasma membrane. Here, the complexes typically track along cortical microtubules, which may guide cellulose synthesis, until the complexes are inactivated and/or internalized. Determining the regulatory aspects that control the behavior of cellulose synthase complexes is vital to understanding directed cell and plant growth and to tailoring cell wall content for industrial products, including paper, textiles, and fuel. In this review, we summarize and discuss cellulose synthesis and regulatory aspects of the cellulose synthase complex, focusing on Arabidopsis thaliana.

Immunogold labeling and electron tomography of plant endosomes

High-resolution imaging of endosomal compartments and associated organelles can be achieved using state-of-the-art electron microscopy techniques, such as the combination of cryofixation/freeze-substitution for sample processing and electron tomography for three-dimensional (3D) analysis. This chapter deals with the main steps associated with these imaging techniques: selection of samples suitable for studying plant endosomes, sample preparation by high-pressure freezing/freeze-substitution, and electron tomography of plastic sections. In addition, immunogold approaches for identification of subcellular localization of endosomal and cargo proteins are also discussed.

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

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

Reconstructing plant cells in 3D by serial section electron tomography

In micrographs acquired with a transmission electron microscope, 3-dimensional (3D) objects are superimposed onto a 2D screen. This reduction in dimension necessarily leads to a degradation of image resolution. To overcome this problem, 3D microscopy techniques, such as tomography and single particle analysis, have been developed. Tomography has been used to visualize cells in 3D, and single particle analysis has been used to investigate macromolecules and viral particles. In this chapter we will describe how we have collected tilting series micrographs from plant cells and how we have reconstructed the cellular volumes using dual axis electron tomography.

3-D analysis of dictyosomes and multivesicular bodies in the green alga Micrasterias denticulata by FIB/SEM tomography

In the present study we employ FIB/SEM tomography for analyzing 3-D architecture of dictyosomes and formation of multivesicular bodies (MVB) in high pressure frozen and cryo-substituted interphase cells of the green algal model system Micrasterias denticulata. The ability of FIB/SEM of milling very thin ‘slices’ (5–10 nm), viewing the block face and of capturing cytoplasmic volumes of several hundred μm³ provides new insight into the close spatial connection of the ER–Golgi machinery in an algal cell particularly in z-direction, complementary to informations obtained by TEM serial sectioning or electron tomography.

Our FIB/SEM series and 3-D reconstructions show that interphase dictyosomes of Micrasterias are not only closely associated to an ER system at their cis-side which is common in various plant cells, but are surrounded by a huge “trans-ER” sheath leading to an almost complete enwrapping of dictyosomes by the ER. This is particularly interesting as the presence of a trans-dictyosomal ER system is well known from mammalian secretory cells but not from cells of higher plants to which the alga Micrasterias is closely related. In contrast to findings in plant storage tissue indicating that MVBs originate from the trans-Golgi network or its derivatives our investigations show that MVBs in Micrasterias are in direct spatial contact with both, trans-Golgi cisternae and the trans-ER sheath which provides evidence that both endomembrane compartments are involved in their formation.

Myosin XIK of Arabidopsis thaliana accumulates at the root hair tip and is required for fast root hair growth

Myosin motor proteins are thought to carry out important functions in the establishment and maintenance of cell polarity by moving cellular components such as organelles, vesicles, or protein complexes along the actin cytoskeleton. In Arabidopsis thaliana, disruption of the myosin XIK gene leads to reduced elongation of the highly polar root hairs, suggesting that the encoded motor protein is involved in this cell growth. Detailed live-cell observations in this study revealed that xik root hairs elongated more slowly and stopped growth sooner than those in wild type. Overall cellular organization including the actin cytoskeleton appeared normal, but actin filament dynamics were reduced in the mutant. Accumulation of RabA4b-containing vesicles, on the other hand, was not significantly different from wild type. A functional YFP-XIK fusion protein that could complement the mutant phenotype accumulated at the tip of growing root hairs in an actin-dependent manner. The distribution of YFP-XIK at the tip, however, did not match that of the ER or several tip-enriched markers including CFP-RabA4b. We conclude that the myosin XIK is required for normal actin dynamics and plays a role in the subapical region of growing root hairs to facilitate optimal growth.

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

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

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

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

Identification of myosin XI receptors in Arabidopsis defines a distinct class of transport vesicles

To characterize the mechanism through which myosin XI-K attaches to its principal endomembrane cargo, a yeast two-hybrid library of Arabidopsis thaliana cDNAs was screened using the myosin cargo binding domain as bait. This screen identified two previously uncharacterized transmembrane proteins (hereinafter myosin binding proteins or MyoB1/2) that share a myosin binding, conserved domain of unknown function 593 (DUF593). Additional screens revealed that MyoB1/2 also bind myosin XI-1, whereas myosin XI-I interacts with the distantly related MyoB7. The in vivo interactions of MyoB1/2 with myosin XI-K were confirmed by immunoprecipitation and colocalization analyses. In epidermal cells, the yellow fluorescent protein-tagged MyoB1/2 localize to vesicles that traffic in a myosin XI-dependent manner. Similar to myosin XI-K, MyoB1/2 accumulate in the tip-growing domain of elongating root hairs. Gene knockout analysis demonstrated that functional cooperation between myosin XI-K and MyoB proteins is required for proper plant development. Unexpectedly, the MyoB1-containing vesicles did not correspond to brefeldin A-sensitive Golgi and post-Golgi or prevacuolar compartments and did not colocalize with known exocytic or endosomal compartments. Phylogenomic analysis suggests that DUF593 emerged in primitive land plants and founded a multigene family that is conserved in all flowering plants. Collectively, these findings indicate that MyoB are membrane-anchored myosin receptors that define a distinct, plant-specific transport vesicle compartment.

A three-stage model of Golgi structure and function

The Golgi apparatus contains multiple classes of cisternae that differ in structure, composition, and function, but there is no consensus about the number and definition of these classes. A useful way to classify Golgi cisternae is according to the trafficking pathways by which the cisternae import and export components. By this criterion, we propose that Golgi cisternae can be divided into three classes that correspond to functional stages of maturation. First, cisternae at the cisternal assembly stage receive COPII vesicles from the ER and recycle components to the ER in COPI vesicles. At this stage, new cisternae are generated. Second, cisternae at the carbohydrate synthesis stage exchange material with one another via COPI vesicles. At this stage, most of the glycosylation and polysaccharide synthesis reactions occur. Third, cisternae at the carrier formation stage produce clathrin-coated vesicles and exchange material with endosomes. At this stage, biosynthetic cargo proteins are packaged into various transport carriers, and the cisternae ultimately disassemble. Discrete transitions occur as a cisterna matures from one stage to the next. Within each stage, the structure and composition of a cisterna can evolve, but the trafficking pathways remain unchanged. This model offers a unified framework for understanding the properties of the Golgi in diverse organisms.

Plant TGNs: dynamics and physiological functions

In all eukaryotic cells, a membrane trafficking system connects the post-Golgi organelles, including the trans-Golgi network (TGN), endosomes, and vacuoles. This complex network plays critical roles in several higher-order functions in multicellular organisms. The TGN, one of the important organelles for protein transport in the post-Golgi network, functions as a sorting station, where cargo proteins are directed to the appropriate post-Golgi compartments. The TGN has been considered to be a compartment belonging to the Golgi apparatus, located on the trans side of the Golgi apparatus. However, in plant cells, recent studies have suggested that the TGN is an independent, dynamic organelle that possesses features different than those of TGNs in animal and yeast cells. In this review, we summarize recent progress regarding the dynamics and physiological functions of the plant TGN.

Trans-Golgi network localized ECHIDNA/Ypt interacting protein complex is required for the secretion of cell wall polysaccharides in Arabidopsis

The secretion of cell wall polysaccharides through the trans-Golgi network (TGN) is required for plant cell elongation. However, the components mediating the post-Golgi secretion of pectin and hemicellulose, the two major cell wall polysaccharides, are largely unknown. We identified evolutionarily conserved YPT/RAB GTPase Interacting Protein 4a (YIP4a) and YIP4b (formerly YIP2), which form a TGN-localized complex with ECHIDNA (ECH) in Arabidopsis thaliana. The localization of YIP4 and ECH proteins at the TGN is interdependent and influences the localization of VHA-a1 and SYP61, which are key components of the TGN. YIP4a and YIP4b act redundantly, and the yip4a yip4b double mutants have a cell elongation defect. Genetic, biochemical, and cell biological analyses demonstrate that the ECH/YIP4 complex plays a key role in TGN-mediated secretion of pectin and hemicellulose to the cell wall in dark-grown hypocotyls and in secretory cells of the seed coat. In keeping with these observations, Fourier transform infrared microspectroscopy analysis revealed that the ech and yip4a yip4b mutants exhibit changes in their cell wall composition. Overall, our results reveal a TGN subdomain defined by ECH/YIP4 that is required for the secretion of pectin and hemicellulose and distinguishes the role of the TGN in secretion from its roles in endocytic and vacuolar trafficking.

Insights into plant plasma membrane aquaporin trafficking

Plasma membrane intrinsic proteins (PIPs) are plant aquaporins that facilitate the diffusion of water and small uncharged solutes through the cell membrane. Deciphering the network of interacting proteins that modulate PIP trafficking to and activity in the plasma membrane is essential to improve our knowledge about PIP regulation and function. This review highlights the most recent advances related to PIP subcellular routing and dynamic redistribution, identifies some key molecular interacting proteins, and indicates exciting directions for future research in this field. A better understanding of the mechanisms by which plants optimize water movement might help in identifying new molecular players of agronomical relevance involved in the control of cellular water uptake and drought tolerance.

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

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

Defective chloroplast development inhibits maintenance of normal levels of abscisic acid in a mutant of the Arabidopsis RH3 DEAD-box protein during early post-germination growth

The plastid has its own translation system, and its ribosomes are assembled through a complex process in which rRNA precursors are processed and ribosomal proteins are inserted into the rRNA backbone. DEAD-box proteins have been shown to play roles in multiple steps in ribosome biogenesis. To investigate the cellular and physiological roles of an Arabidopsis DEAD-box protein, RH3, we examined its expression and localization and the phenotypes of rh3-4, a T-DNA insertion mutant allele of RH3. The promoter activity of RH3 is strongest in the greening tissues of 3-day and 1-week-old seedlings but reduced afterwards. Cotyledons were pale and seedling growth was retarded in the mutant. The most obvious abnormality in the mutant chloroplasts was their lack of normal ribosomes. Electron tomography analysis indicated that ribosome density in the 3-day-old mutant chloroplasts is only 20% that of wild-type chloroplasts, and the ribosomes in the mutant are smaller. These chloroplast defects in rh3-4 were alleviated in 2-week-old cotyledons and true leaves. Interestingly, rh3-4 seedlings have lower amounts of abscisic acid prior to recovery of their chloroplasts, and were more sensitive to abiotic stresses. Transcriptomic analysis indicated that nuclear genes for chloroplast proteins are down-regulated, and proteins mediating chloroplast-localized steps of abscisic acid biosynthesis are expressed to a lower extent in 1-week-old rh3-4 seedlings. Taken together, these results suggest that conversion of eoplasts into chloroplasts in young seedlings is critical for the seedlings to start carbon fixation as well as for maintenance of abscisic acid levels for responding to environmental challenges.

Cis-Golgi cisternal assembly and biosynthetic activation occur sequentially in plants and algae

The cisternal progression/maturation model of Golgi trafficking predicts that cis-Golgi cisternae are formed de novo on the cis-side of the Golgi. Here we describe structural and functional intermediates of the cis cisterna assembly process in high-pressure frozen algae (Scherffelia dubia, Chlamydomonas reinhardtii) and plants (Arabidopsis thaliana, Dionaea muscipula; Venus flytrap) as determined by electron microscopy, electron tomography and immuno-electron microscopy techniques. Our findings are as follows: (i) The cis-most (C1) Golgi cisternae are generated de novo from cisterna initiators produced by the fusion of 3-5 COPII vesicles in contact with a C2 cis cisterna. (ii) COPII vesicles fuel the growth of the initiators, which then merge into a coherent C1 cisterna. (iii) When a C1 cisterna nucleates its first cisterna initiator it becomes a C2 cisterna. (iv) C2-Cn cis cisternae grow through COPII vesicle fusion. (v) ER-resident proteins are recycled from cis cisternae to the ER via COPIa-type vesicles. (vi) In S. dubia the C2 cisternae are capable of mediating the self-assembly of scale protein complexes. (vii) In plants, ∼90% of native α-mannosidase I localizes to medial Golgi cisternae. (viii) Biochemical activation of cis cisternae appears to coincide with their conversion to medial cisternae via recycling of medial cisterna enzymes. We propose how the different cis cisterna assembly intermediates of plants and algae may actually be related to those present in the ERGIC and in the pre-cis Golgi cisterna layer in mammalian cells.

Growth mechanisms in tip-growing plant cells

Tip growth is employed throughout the plant kingdom. Our understanding of tip growth has benefited from modern tools in molecular genetics, which have enabled the functional characterization of proteins mediating tip growth. Here we first discuss the evolutionary role of tip growth in land plants and then describe the prominent model tip-growth systems, elaborating on some advantages and disadvantages of each. Next we review the organization of tip-growing cells, the role of the cytoskeleton, and recent developments concerning the physiological basis of tip growth. Finally, we review advances in the understanding of the extracellular signals that are known to guide tip-growing cells.

Eat in or take away? How phosphatidylinositol 4-kinases feed the phospholipase C pathway with substrate

Phosphatidylinositol 4-kinases (PI4Ks) catalyze the first step in the synthesis of phosphoinositide pools hydrolysed by phosphoinositide-dependent phospholipase C (PI-PLC) and thus constitute a potential key regulation point of this pathway. Twelve putative PI4K isoforms, divided as type-II (AtPI4KIIγ1- 8) and type-III PI4Ks (AtPI4KIIIα1- 2 and AtPI4KIIIβ1- 2), have been identified in Arabidopsis genome. By a combination of pharmalogical and genetic approaches we recently evidenced that AtPI4KIIIβ1 and AtPI4KIIIβ2 contribute to supply PI-PLC with substrate and that AtPI4KIIIα1 is probably also involved in this process. Given the current knowledge on PI-PLC and type-III PI4Ks localization in plant cells it raises the question whether type-III PI4Ks produce phosphatidylinositol 4-phosphate at the site of its consumption by the PI-PLC pathway. We therefore discuss the spatial organization of substrate supply to PI-PLC in plant cells with reference to recent data evidenced in mammalian cells.

Distinct and overlapping roles for AP-1 and GGAs revealed by the "knocksideways" system

Although adaptor protein complex 1 (AP-1) and Golgi-localized, γ ear-containing, ADP-ribosylation factor-binding proteins (GGAs) are both adaptors for clathrin-mediated intracellular trafficking, the pathways they mediate and their relationship to each other remain open questions. To tease apart the functions of AP-1 and GGAs, we rapidly inactivated each adaptor using the "knocksideways" system and then compared the protein composition of clathrin-coated vesicle (CCV) fractions from control and knocksideways cells. The AP-1 knocksideways resulted in a dramatic and unexpected loss of GGA2 from CCVs. Over 30 other peripheral membrane proteins and over 30 transmembrane proteins were also depleted, including several mutated in genetic disorders, indicating that AP-1 acts as a linchpin for intracellular CCV formation. In contrast, the GGA2 knocksideways affected only lysosomal hydrolases and their receptors. We propose that there are at least two populations of intracellular CCVs: one containing both GGAs and AP-1 for anterograde trafficking and another containing AP-1 for retrograde trafficking. Our study shows that knocksideways and proteomics are a powerful combination for investigating protein function, which can potentially be used on many different types of proteins.

Trying to make sense of retromer

Retromer is a cytosolic protein complex which binds to post-Golgi organelles involved in the trafficking of proteins to the lytic compartment of the cell. In non-plant organisms, retromer mediates the recycling of acid hydrolase receptors from early endosomal (EE) compartments. In plants, retromer components are required for the targeting of vacuolar storage proteins, and for the recycling of endocytosed PIN proteins. However, there are contradictory reports as to the localization of the sorting nexins and the core subunit of retromer. There is also uncertainty as to the identity of the organelles from which vacuolar sorting receptors (VSRs) and endocytosed plasma membrane (PM) proteins are recycled. In this review we try to resolve some of these conflicting observations.

Wortmannin treatment induces changes in Arabidopsis root proteome and post-Golgi compartments

Wortmannin is a widely used pharmaceutical compound which is employed to define vesicular trafficking routes of particular proteins or cellular compounds. It targets phosphatidylinositol 3-kinase and phosphatidylinositol 4-kinases in a dose-dependent manner leading to the inhibition of protein vacuolar sorting and endocytosis. Combined proteomics and cell biological approaches have been used in this study to explore the effects of wortmannin on Arabidopsis root cells, especially on proteome and endomembrane trafficking. On the subcellular level, wortmannin caused clustering, fusion, and swelling of trans-Golgi network (TGN) vesicles and multivesicular bodies (MVBs) leading to the formation of wortmannin-induced multivesicular compartments. Appearance of wortmannin-induced compartments was associated with depletion of TGN as revealed by electron microscopy. On the proteome level, wortmannin induced massive changes in protein abundance profiles. Wortmannin-sensitive proteins belonged to various functional classes. An inhibition of vacuolar trafficking by wortmannin was related to the downregulation of proteins targeted to the vacuole, as showed for vacuolar proteases. A small GTPase, RabA1d, which regulates vesicular trafficking at TGN, was identified as a new protein negatively affected by wortmannin. In addition, Sec14 was upregulated and PLD1 alpha was downregulated by wortmannin.

AGD1, a class 1 ARF-GAP, acts in common signaling pathways with phosphoinositide metabolism and the actin cytoskeleton in controlling Arabidopsis root hair polarity

The Arabidopsis thaliana AGD1 gene encodes a class 1 adenosine diphosphate ribosylation factor-gtpase-activating protein (ARF-GAP). Previously, we found that agd1 mutants have root hairs that exhibit wavy growth and have two tips that originate from a single initiation point. To gain new insights into how AGD1 modulates root hair polarity we analyzed double mutants of agd1 and other loci involved in root hair development, and evaluated dynamics of various components of root hair tip growth in agd1 by live cell microscopy. Because AGD1 contains a phosphoinositide (PI) binding pleckstrin homology (PH) domain, we focused on genetic interactions between agd1 and root hair mutants altered in PI metabolism. Rhd4, which is knocked-out in a gene encoding a phosphatidylinositol-4-phosphate (PI-4P) phosphatase, was epistatic to agd1. In contrast, mutations to PIP5K3 and COW1, which encode a type B phosphatidylinositol-4-phosphate 5-kinase 3 and a phosphatidylinositol transfer protein, respectively, enhanced the root hair defects of agd1. Enhanced root hair defects were also observed in double mutants to AGD1 and ACT2, a root hair-expressed vegetative actin isoform. Consistent with our double-mutant studies, targeting of tip growth components involved in PI signaling (PI-4P), secretion (RABA4b) and actin regulation (ROP2), were altered in agd1 root hairs. Furthermore, tip cytosolic calcium ([Ca²⁺](cyt) ) oscillations were disrupted in root hairs of agd1. Taken together, our results indicate that AGD1 links PI signaling to cytoskeletal-, [Ca²⁺](cyt-) , ROP2-, and RABA4b-mediated root hair development.

Arabidopsis type-III phosphatidylinositol 4-kinases β1 and β2 are upstream of the phospholipase C pathway triggered by cold exposure

Phosphatidylinositol-4-phosphate (PtdIns4P) is the most abundant phosphoinositide in plants and the precursor of phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P(2)]. This lipid is the substrate of phosphoinositide-dependent phospholipase C (PI-PLC) that produces diacylglycerol (DAG) which can be phosphorylated to phosphatidic acid (PtdOH). In plants, it has been suggested that PtdIns4P may also be a direct substrate of PI-PLC. Whether PtdIns4P is the precursor of PtdIns(4,5)P(2) or a substrate of PI-PLC, its production by phosphatidylinositol-4-kinases (PI4Ks) is the first step in generating the phosphoinositides hydrolyzed by PI-PLC. PI4Ks can be divided into type-II and type-III. In plants, the identity of the PI4K upstream of PI-PLC is unknown. In Arabidopsis, cold triggers PI-PLC activation, resulting in PtdOH production which is paralleled by decreases in PtdIns4P and PtdIns(4,5)P(2). In suspension cells, both the PtdIns4P decrease and the PtdOH increase in response to cold were impaired by 30 μM wortmannin, a type-III PI4K inhibitor. Type-III PI4Ks include AtPI4KIIIα1, β1 and β2 isoforms. In this work we show that PtdOH resulting from the PI-PLC pathway is significantly lowered in a pi4kIIIβ1β2 double mutant exposed to cold stress. Such a decrease was not detected in single pi4kIIIβ1 and pi4kIIIβ2 mutants, indicating that AtPI4KIIIβ1 and AtPI4KIIIβ2 can both act upstream of the PI-PLC. Although several short-term to long-term responses to cold were unchanged in pi4kIIIβ1β2, cold induction of several genes was impaired in the double mutant and its germination was hypersensitive to chilling. We also provide evidence that de novo synthesis of PtdIns4P by PI4Ks occurs in parallel to PI-PLC activation.

Isolation and proteomic analysis of the SYP61 compartment reveal its role in exocytic trafficking in Arabidopsis

The endomembrane system is a complex and dynamic intracellular trafficking network. It is very challenging to track individual vesicles and their cargos in real time; however, affinity purification allows vesicles to be isolated in their natural state so that their constituent proteins can be identified. Pioneering this approach in plants, we isolated the SYP61 trans-Golgi network compartment and carried out a comprehensive proteomic analysis of its contents with only minimal interference from other organelles. The proteome of SYP61 revealed the association of proteins of unknown function that have previously not been ascribed to this compartment. We identified a complete SYP61 SNARE complex, including regulatory proteins and validated the proteome data by showing that several of these proteins associated with SYP61 in planta. We further identified the SYP121-complex and cellulose synthases, suggesting that SYP61 plays a role in the exocytic trafficking and the transport of cell wall components to the plasma membrane. The presence of proteins of unknown function in the SYP61 proteome including ECHIDNA offers the opportunity to identify novel trafficking components and cargos. The affinity purification of plant vesicles in their natural state provides a basis for further analysis and dissection of complex endomembrane networks. The approach is widely applicable and can afford the study of several vesicle populations in plants, which can be compared with the SYP61 vesicle proteome.

Myosin XIK is a major player in cytoplasm dynamics and is regulated by two amino acids in its tail

It has recently been found that among the 17 Arabidopsis myosins, six (XIC, XIE, XIK, XI-I, MYA1, and MYA2) have a major role in the motility of Golgi bodies and mitochondria in Nicotiana benthamiana and Nicotiana tabacum. Here, the same dominant negative tail fragments were also found to arrest the movement of Gogi bodies when transiently expressed in Arabidopsis plants. However, when a Golgi marker was transiently expressed in plants knocked out in these myosins, its movement was dramatically inhibited only in the xik mutant. In addition, a tail fragment of myosin XIK could inhibit the movement of several post-Golgi organelles, such as the trans-Golgi network, pre-vacuolar compartment, and endosomes, as well as total cytoplasmic streaming, suggesting that myosin XIK is a major player in cytoplasm kinetics. However, no co-localization of myosin tails with the arrested organelles was observed. Several deletion truncations of the myosin XIK tail were generated to corroborate function with localization. All deletion mutants possessing an inhibitory effect on organelle movement exhibited a diffuse cytoplasmic distribution. Point mutations in the tail of myosin XIK revealed that Arg1368 and Arg1443 are essential for its activity. These residues correspond to Lys1706 and Lys1779 from mouse myosin Va, which mediate the inhibitory head-tail interaction in this myosin. Therefore, such an interaction might underlie the dominant negative effect of truncated plant myosin tails and explain the mislocalization with target organelles.

Phosphoinositide signaling

"All things flow and change…even in the stillest matter there is unseen flux and movement." Attributed to Heraclitus (530-470 BC), from The Story of Philosophy by Will Durant. Heraclitus, a Greek philosopher, was thinking on a much larger scale than molecular signaling; however, his visionary comments are an important reminder for those studying signaling today. Even in unstimulated cells, signaling pathways are in constant metabolic flux and provide basal signals that travel throughout the organism. In addition, negatively charged phospholipids, such as the polyphosphorylated inositol phospholipids, provide a circuit board of on/off switches for attracting or repelling proteins that define the membranes of the cell. This template of charged phospholipids is sensitive to discrete changes and metabolic fluxes-e.g., in pH and cations-which contribute to the oscillating signals in the cell. The inherent complexities of a constantly fluctuating system make understanding how plants integrate and process signals challenging. In this review we discuss one aspect of lipid signaling: the inositol family of negatively charged phospholipids and their functions as molecular sensors and regulators of metabolic flux in plants.

Structure and function of endosomes in plant cells

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

Dynamic behavior of clathrin in Arabidopsis thaliana unveiled by live imaging

Clathrin-coated vesicles (CCV) are necessary for selective transport events, including receptor-mediated endocytosis on the plasma membrane and cargo molecule sorting in the trans-Golgi network (TGN). Components involved in CCV formation include clathrin heavy and light chains and several adaptor proteins that are conserved among plants. Clathrin-dependent endocytosis has been shown to play an integral part in plant endocytosis. However, little information is known about clathrin dynamics in living plant cells. In this study, we have visualized clathrin in Arabidopsis thaliana by tagging clathrin light chain with green fluorescent protein (CLC-GFP). Quantitative evaluations of colocalization demonstrate that the majority of CLC-GFP is localized to the TGN, and a minor population is associated with multivesicular endosomes and the Golgi trans-cisternae. Live imaging further demonstrated the presence of highly dynamic clathrin-positive tubules and vesicles, which appeared to mediate interactions between the TGNs. CLC-GFP is also targeted to cell plates and the plasma membrane. Although CLC-GFP colocalizes with a dynamin isoform at the plasma membrane, these proteins exhibit distinct distributions at newly forming cell plates. This finding indicates independent functions of CLC (clathrin light chains) and dynamin during the formation of cell plates. We have also found that brefeldin A and wortmannin treatment causes distinctly different alterations in the dynamics and distribution of clathrin-coated domains at the plasma membrane. This could account for the different effects of these drugs on plant endocytosis.

Polarized cell growth in Arabidopsis requires endosomal recycling mediated by GBF1-related ARF exchange factors

Polarized tip growth is a fundamental cellular process in many eukaryotic organisms, mediating growth of neuronal axons and dendrites or fungal hyphae. In plants, pollen and root hairs are cellular model systems for analysing tip growth. Cell growth depends on membrane traffic. The regulation of this membrane traffic is largely unknown for tip-growing cells, in contrast to cells exhibiting intercalary growth. Here we show that in Arabidopsis, GBF1-related exchange factors for the ARF GTPases (ARF GEFs) GNOM and GNL2 play essential roles in polar tip growth of root hairs and pollen, respectively. When expressed from the same promoter, GNL2 (in contrast to the early-secretory ARF GEF GNL1) is able to replace GNOM in polar recycling of the auxin efflux regulator PIN1 from endosomes to the basal plasma membrane in non-tip growing cells. Thus, polar recycling facilitates polar tip growth, and GNL2 seems to have evolved to meet the specific requirement of fast-growing pollen in higher plants.

Plant endosomal trafficking pathways

Endosomes regulate both the recycling and degradation of plasma membrane (PM) proteins, thereby modulating many cellular responses triggered at the cell surface. Endosomes also play a role in the biosynthetic pathway by taking proteins to the vacuole and recycling vacuolar cargo receptors. In plants, the trans-Golgi network (TGN) acts as an early/recycling endosome whereas prevacuolar compartments/multivesicular bodies (MVBs) take PM proteins to the vacuole for degradation. Recent studies have demonstrated that some of the molecular complexes that mediate endosomal trafficking, such as the retromer, the ADP-ribosylation factor (ARF) machinery, and the Endosomal Sorting Complexes Required for Transport (ESCRTs) have both conserved and specialized functions in plants. Whereas there is disagreement on the subcellular localization of the plant retromer, its function in recycling vacuolar sorting receptors (VSRs) and modulating the trafficking of PM proteins has been well established. Studies on Arabidopsis ESCRT components highlight the essential role of this complex in cytokinesis, plant development, and vacuolar organization. In addition, post-translational modifications of plant PM proteins, such as phosphorylation and ubiquitination, have been demonstrated to act as sorting signals for endosomal trafficking.

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

The trans-Golgi network (TGN) in plant cells is an independent organelle, displaying rapid association and dissociation with Golgi bodies. In plant cells, the TGN is the site where secretory and endocytic membrane trafficking meet. Cell wall components, signaling molecules and auxin transporters have been found to undergo intracellular trafficking around the TGN. However, how different trafficking pathways are regulated and how different cargoes are sorted in the TGN is poorly defined in plant cells. Using a combined approach of genetic and in vivo imaging, we recently demonstrated that Arabidopsis TRAPPII acts in the TGN and is required for polar targeting of PIN2, but not PIN1, auxin efflux carrier in root tip cells. Here, we report that, TRAPPII in Arabidopsis is required for polar distribution of AUX1, an auxin influx carrier in protophloem cells and epidermal cells of Arabidopsis root tips. In yeast cells, TRAPPII serves as a guanine-nucleotide exchange factor (GEF) for Ypt1 and Ypt31/32 in late Golgi trafficking, while in mammalian cells, TRAPPII acts as a GEF for Rab1 (homolog of yeast Ypt1) in early Golgi trafficking. We show here that TRAPPII in Arabidopsis is functionally linked to Rab-A proteins, homologs of yeast Ypt31/32, but not Rab-D proteins, homologs of yeast Ypt1 and animal Rab1 proteins.

Signals and mechanisms affecting vesicular trafficking during root growth

Vesicular trafficking is mediated by distinct exocytic and endocytic routes in eukaryotic cells. These pathways involve RAB family proteins, ADP-ribosylation factor, RHO proteins of the Ras superfamily, and SNAREs (soluble N-ethylmaleimide-sensitive factor adaptors). Studies have shown that vesicular trafficking plays a crucial role in protein localization and movement, signal transduction, and multiple developmental processes. Here we summarize the role of vesicular trafficking in root and root hair growth and in auxin-mediated root development, focusing on the regulation of the polarized subcellular distribution of the PIN proteins (auxin efflux carriers).

A specific role for Arabidopsis TRAPPII in post-Golgi trafficking that is crucial for cytokinesis and cell polarity

Cytokinesis and cell polarity are supported by membrane trafficking from the trans-Golgi network (TGN), but the molecular mechanisms that promote membrane trafficking from the TGN are poorly defined in plant cells. Here we show that TRAPPII in Arabidopsis regulates the post-Golgi trafficking that is crucial for assembly of the cell plate and cell polarity. Disruptions of AtTRS120 or AtTRS130, two genes encoding two key subunits of TRAPPII, result in defective cytokinesis and cell polarity in embryogenesis and seedling development. In attrs120 and attrs130, the organization and trafficking in the endoplasmic reticulum (ER)-Golgi interface are normal. However, post-Golgi trafficking to the cell plate and to the cell wall, but not to the vacuole, is impaired. Furthermore, TRAPPII is required for the selective transport of PIN2, but not PIN1, to the plasma membrane. We revealed that AtTRS130 is co-localized with RAB-A1c. Expression of constitutively active RAB-A1c partially rescues attrs130. RAB-A1c, which resides at the TGN, is delocalized to the cytosol in attrs130. We propose that TRAPPII in Arabidopsis acts upstream of Rab-A GTPases in post-Golgi membrane trafficking in plant cells.

Green light for polyphosphoinositide signals in plants

Plant genomes lack homologues of the inositol 1,4,5-trisphosphate receptor and protein kinase C, which are important components of the canonical phospholipase C signalling system in animals. Instead, plants seem to utilize alternative downstream signalling molecules, that is, InsP(6) and phosphatidic acid. Inositol lipids may also function as second messengers themselves. By reversible phosphorylation of the inositol headgroup, five biologically active plant polyphosphoinositides can be detected. Protein targets interact with specific polyphosphoinositide isomers via selective lipid-binding domains, thereby altering their intracellular localization and/or enzymatic activity. Such lipid-binding domains have also been used to create GFP based-lipid biosensors to visualize PPIs dynamics in vivo. Here, we highlight some recent advances and ideas on PPIs' role in plant signalling.