The specificity of vesicle trafficking: coat proteins and SNAREs

Molecular Evolution of SNAREs in Vitis vinifera and Expression Analysis under Phytohormones and Abiotic Stress

SNARE proteins (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) play a key role in mediating a variety of plant biological processes. Currently, the function of the SNARE gene family in phytohormonal and abiotic stress treatments in grapevine is currently unknown, making it worthwhile to characterize and analyze the function and expression of this family in grapevine. In the present study, 52 VvSNARE genes were identified and predominantly distributed on 18 chromosomes. Secondary structures showed that the VvSNARE genes family irregular random coils and α-helices. The promoter regions of the VvSNARE genes were enriched for light-, abiotic-stress-, and hormone-responsive elements. Intraspecific collinearity analysis identified 10 pairs collinear genes within the VvSNARE family and unveiled a greater number of collinear genes between grapevine and apple, as well as Arabidopsis thaliana, but less associations with Oryza sativa. Quantitative real-time PCR (qRT-PCR) analyses showed that the VvSNARE genes have response to treatments with ABA, NaCl, PEG, and 4 °C. Notably, VvSNARE2, VvSNARE14, VvSNARE15, and VvSNARE17 showed up-regulation in response to ABA treatment. VvSNARE2, VvSNARE15, VvSNARE18, VvSNARE19, VvSNARE20, VvSNARE24, VvSNARE25, and VvSNARE29 exhibited significant up-regulation when exposed to NaCl treatment. The PEG treatment led to significant down-regulation of VvSNARE1, VvSNARE8, VvSNARE23, VvSNARE25, VvSNARE26, VvSNARE31, and VvSNARE49 gene expression. The expression levels of VvSNARE37, VvSNARE44, and VvSNARE46 were significantly enhanced after exposure to 4 °C treatment. Furthermore, subcellular localization assays certified that VvSNARE37, VvSNARE44, and VvSNARE46 were specifically localized at the cell membrane. Overall, this study showed the critical role of the VvSNARE genes family in the abiotic stress response of grapevines, thereby providing novel candidate genes such as VvSNARE37, VvSNARE44, and VvSNARE46 for further exploration in grapevine stress tolerance research.

TaCRT3 Is a Positive Regulator of Resistance to Blumeria graminis f. sp. tritici in Wheat

Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is one of the most prevalent diseases of wheat worldwide and can lead to severe yield reductions. Identifying genes involved in powdery mildew resistance will be useful for disease resistance breeding and control. Calreticulin (CRT) is a member of multigene family widely found in higher plants and is associated with a variety of plant physiological functions and defense responses. However, the role of CRT in wheat resistance to powdery mildew remains unclear. TaCRT3 was identified from the proteomic sequence of an incompatible interaction between the wheat (Triticum aestivum) cultivar Xingmin 318 and the Bgt isolate E09. Following analysis of transient expression of the GFP-TaCRT3 fusion protein in Nicotiana benthamiana leaves, TaCRT3 was localized in the nucleus, cytoplasm, and cell membrane. Transcript expression levels of TaCRT3 were significantly upregulated in the wheat-Bgt incompatible interaction. More critically, knockdown of TaCRT3 using virus-induced gene silencing resulted in attenuated resistance to Bgt in wheat. Histological analysis showed a significant increase in Bgt development in TaCRT3-silenced plants, whereas the pathogen-related gene was significantly downregulated in TaCRT3-silenced leaves. In addition, overexpression of TaCRT3 in wheat enhanced the resistance to powdery mildew, the growth of Bgt was significantly inhibited, and the area of H2O2 near the infection site and the expression of defense-related genes of the salicylic acid pathway significantly increased. These findings imply that TaCRT3 may act as a disease resistance factor that positively regulates resistance to powdery mildew, during which SA signaling is probably activated.

Extracellular vesicles: an emerging player in retinal homeostasis

Extracellular vesicles (EVs) encompass secreted membrane vesicles of varied sizes, including exosomes (-30-200 nm) and microvesicles (MVs) that are ∼100-1,000 nm in size. EVs play an important role in autocrine, paracrine, and endocrine signaling and are implicated in myriad human disorders including prominent retinal degenerative diseases, like age related macular degeneration (AMD) and diabetic retinopathy (DR). Studies of EVs in vitro using transformed cell lines, primary cultures, and more recently, induced pluripotent stem cell derived retinal cell type(s) (e.g., retinal pigment epithelium) have provided insights into the composition and function of EVs in the retina. Furthermore, consistent with a causal role of EVs in retinal degenerative diseases, altering EV composition has promoted pro-retinopathy cellular and molecular events in both in vitro and in vivo models. In this review, we summarize the current understanding of the role of EVs in retinal (patho)physiology. Specifically, we will focus on disease-associated EV alterations in specific retinal diseases. Furthermore, we discuss the potential utility of EVs in diagnostic and therapeutic strategies for targeting retinal diseases.

Caleosin/peroxygenases: multifunctional proteins in plants

BACKGROUND

Caleosin/peroxygenases (CLO/PXGs) are a family of multifunctional proteins that are ubiquitous in land plants and are also found in some fungi and green algae. CLO/PXGs were initially described as a class of plant lipid-associated proteins with some similarities to the oleosins that stabilize lipid droplets (LDs) in storage tissues, such as seeds. However, we now know that CLO/PXGs have more complex structures, distributions and functions than oleosins. Structurally, CLO/PXGs share conserved domains that confer specific biochemical features, and they have diverse localizations and functions.

SCOPE

This review surveys the structural properties of CLO/PXGs and their biochemical roles. In addition to their highly conserved structures, CLO/PXGs have peroxygenase activities and are involved in several aspects of oxylipin metabolism in plants. The enzymatic activities and the spatiotemporal expression of CLO/PXGs are described and linked with their wider involvement in plant physiology. Plant CLO/PXGs have many roles in both biotic and abiotic stress responses in plants and in their responses to environmental toxins. Finally, some intriguing developments in the biotechnological uses of CLO/PXGs are addressed.

CONCLUSIONS

It is now two decades since CLO/PXGs were first recognized as a new class of lipid-associated proteins and only 15 years since their additional enzymatic functions as a new class of peroxygenases were discovered. There are many interesting research questions that remain to be addressed in future physiological studies of plant CLO/PXGs and in their recently discovered roles in the sequestration and, possibly, detoxification of a wide variety of lipidic xenobiotics that can challenge plant welfare.

A defined clathrin-mediated trafficking pathway regulates sFLT1/VEGFR1 secretion from endothelial cells

FLT1/VEGFR1 negatively regulates VEGF-A signaling and is required for proper vessel morphogenesis during vascular development and vessel homeostasis. Although a soluble isoform, sFLT1, is often mis-regulated in disease and aging, how sFLT1 is trafficked and secreted from endothelial cells is not well understood. Here we define requirements for constitutive sFLT1 trafficking and secretion in endothelial cells from the Golgi to the plasma membrane, and we show that sFLT1 secretion requires clathrin at or near the Golgi. Perturbations that affect sFLT1 trafficking blunted endothelial cell secretion and promoted intracellular mis-localization in cells and zebrafish embryos. siRNA-mediated depletion of specific trafficking components revealed requirements for RAB27A, VAMP3, and STX3 for post-Golgi vesicle trafficking and sFLT1 secretion, while STX6, ARF1, and AP1 were required at the Golgi. Depletion of STX6 altered vessel sprouting in a 3D angiogenesis model, indicating that endothelial cell sFLT1 secretion is important for proper vessel sprouting. Thus, specific trafficking components provide a secretory path from the Golgi to the plasma membrane for sFLT1 in endothelial cells that utilizes a specialized clathrin-dependent intermediate, suggesting novel therapeutic targets.

Developmental program-independent secretory granule degradation in larval salivary gland cells of Drosophila

Both constitutive and regulated secretion require cell organelles that are able to store and release the secretory cargo. During development, the larval salivary gland of Drosophila initially produces high amount of glue-containing small immature secretory granules, which then fuse with each other and reach their normal 3-3.5 μm in size. Following the burst of secretion, obsolete glue granules directly fuse with late endosomes or lysosomes by a process called crinophagy, which leads to fast degradation and recycling of the secretory cargo. However, hindering of endosome-to-TGN retrograde transport in these cells causes abnormally small glue granules which are not able to fuse with each other. Here, we show that loss of function of the SNARE genes Syntaxin 16 (Syx16) and Synaptobrevin (Syb), the small GTPase Rab6 and the GARP tethering complex members Vps53 and Scattered (Vps54) all involved in retrograde transport cause intense early degradation of immature glue granules via crinophagy independently of the developmental program. Moreover, silencing of these genes also provokes secretory failure and accelerated crinophagy during larval development. Our results provide a better understanding of the relations among secretion, secretory granule maturation and degradation and paves the way for further investigation of these connections in other metazoans.

Arabidopsis SYP121 acts as an ROP2 effector in the regulation of root hair tip growth

Tip growth is an extreme form of polarized cell expansion that occurs in all eukaryotic kingdoms to generate highly elongated tubular cells with specialized functions, including fungal hyphae, animal neurons, plant pollen tubes, and root hairs (RHs). RHs are tubular structures that protrude from the root epidermis to facilitate water and nutrient uptake, microbial interactions, and plant anchorage. RH tip growth requires polarized vesicle targeting and active exocytosis at apical growth sites. However, how apical exocytosis is spatially and temporally controlled during tip growth remains elusive. Here, we report that the Qa-Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) SYP121 acts as an effector of Rho of Plants 2 (ROP2), mediating the regulation of RH tip growth. We show that active ROP2 promotes SYP121 targeting to the apical plasma membrane. Moreover, ROP2 directly interacts with SYP121 and promotes the interaction between SYP121 and the R-SNARE VAMP722 to form a SNARE complex, probably by facilitating the release of the Sec1/Munc18 protein SEC11, which suppresses the function of SYP121. Thus, the ROP2-SYP121 pathway facilitates exocytic trafficking during RH tip growth. Our study uncovers a direct link between an ROP GTPase and vesicular trafficking and a new mechanism for the control of apical exocytosis, whereby ROP GTPase signaling spatially regulates SNARE complex assembly and the polar distribution of a Q-SNARE.

Inositol Signaling in the Basidiomycete Fungus Schizophyllum commune

Intracellular signaling is conserved in eukaryotes to allow for response to extracellular signals and to regulate development and cellular functions. In fungi, inositol phosphate signaling has been shown to be involved in growth, sexual reproduction, and metabolic adaptation. However, reports on mushroom-forming fungi are lacking so far. In Schizophyllum commune, an inositol monophosphatase has been found up-regulated during sexual development. The enzyme is crucial for inositol cycling, where it catalyzes the last step of inositol phosphate metabolism, restoring the inositol pool from the monophosphorylated inositol monophosphate. We overexpressed the gene in this model basidiomycete and verified its involvement in cell wall integrity and intracellular trafficking. Strong phenotypes in mushroom formation and cell metabolism were evidenced by proteome analyses. In addition, altered inositol signaling was shown to be involved in tolerance towards cesium and zinc, and increased metal tolerance towards cadmium, associated with induced expression of kinases and repression of phosphatases within the inositol cycle. The presence of the heavy metals Sr, Cs, Cd, and Zn lowered intracellular calcium levels. We could develop a model integrating inositol signaling in the known signal transduction pathways governed by Ras, G-protein coupled receptors, and cAMP, and elucidate their different roles in development.

A Review of Plant Vacuoles: Formation, Located Proteins, and Functions

Vacuoles, cellular membrane-bound organelles, are the largest compartments of cells, occupying up to 90% of the volume of plant cells. Vacuoles are formed by the biosynthetic and endocytotic pathways. In plants, the vacuole is crucial for growth and development and has a variety of functions, including storage and transport, intracellular environmental stability, and response to injury. Depending on the cell type and growth conditions, the size of vacuoles is highly dynamic. Different types of cell vacuoles store different substances, such as alkaloids, protein enzymes, inorganic salts, sugars, etc., and play important roles in multiple signaling pathways. Here, we summarize vacuole formation, types, vacuole-located proteins, and functions.

Glandular trichomes of the leaves in three Doronicum species (Senecioneae, Asteraceae): morphology, histochemistry, and ultrastructure

Two types of glandular tichomes (GTs) develop on the leaves in three Doronicum species. The purpose of the work was to establish common and distinctive morphological, anatomical, histochemical, and ultrustructural features of the trichomes. It turned out that differences between types of trichomes are more significant than interspecific ones. For each Doronicum species, differences between GTs of two types include the dimensions, intensity of coloration by histochemical dyes, as well as ultrastructural features of the cells. The GTs of the first type are higher than GTs of the second type. Two to three upper cell layers of the first trichomes develop histochemical staining, whereas in the second ones, only apical cells give a positive histochemical reaction. In all trichomes, polysaccharides, polyphenols, and terpenoids are detected. In the GTs of the first type, polysaccharides are synthesized in larger quantity; in the GTs of the second type, synthesis of the secondary metabolites predominates. Main ultrastructural features of the GTs of the first type include proliferation of RER and an activity of Golgi apparatus denoting the synthesis of enzymes and pectin; however, development of SER, diversiform leucoplasts with reticular sheaths, and chloroplasts with peripheral plastid reticulum also demonstrate the synthesis of lipid substances. The ultrastructural characteristics of the second type GTs indicate the primary synthesis of lipid components. Secretion is localized in a periplasmic space of the upper cell layers. The secretory products pass through the cell wall, accumulate in the subcuticular cavity, and rupture it.

The use of quantitative imaging to investigate regulators of membrane trafficking in Arabidopsis stomatal closure

Expansion of gene families facilitates robustness and evolvability of biological processes but impedes functional genetic dissection of signalling pathways. To address this, quantitative analysis of single cell responses can help characterize the redundancy within gene families. We developed high-throughput quantitative imaging of stomatal closure, a response of plant guard cells, and performed a reverse genetic screen in a group of Arabidopsis mutants to five stimuli. Focussing on the intersection between guard cell signalling and the endomembrane system, we identified eight clusters based on the mutant stomatal responses. Mutants generally affected in stomatal closure were mostly in genes encoding SNARE and SCAMP membrane regulators. By contrast, mutants in RAB5 GTPase genes played specific roles in stomatal closure to microbial but not drought stress. Together with timed quantitative imaging of endosomes revealing sequential patterns in FLS2 trafficking, our imaging pipeline can resolve non-redundant functions of the RAB5 GTPase gene family. Finally, we provide a valuable image-based tool to dissect guard cell responses and outline a genetic framework of stomatal closure.

Regulation and mechanisms of extracellular vesicle biogenesis and secretion

EV (extracellular vesicle) biology is a rapidly expanding field. These heterogeneous membrane vesicles, which are shed from virtually all cell types, collectively represent a new dimension of intercellular communication in normal physiology and disease. They have been shown to deliver infectious and pathogenic agents to non-infected cells whereas in cancers they are thought to condition the tumor microenvironment. Their presence in body fluids and inherent capacity for systemic delivery point to their clinical promise. All of the above only intensifies the need to better understand the classification, mode of biogenesis, and contents of the different subtypes of EVs. This article focusses on vesicle subtypes labeled as exosomes and MVs (microvesicles) and discusses the biogenesis and release of these vesicles from cells.

A cell surface display fluorescent biosensor for measuring MMP14 activity in real-time

Despite numerous recent advances in imaging technologies, one continuing challenge for cell biologists and microscopists is the visualization and measurement of endogenous proteins as they function within living cells. Achieving this goal will provide a tool that investigators can use to associate cellular outcomes with the behavior and activity of many well-studied target proteins. Here, we describe the development of a plasmid-based fluorescent biosensor engineered to measure the location and activity of matrix metalloprotease-14 (MMP14). The biosensor design uses fluorogen-activating protein technology coupled with a MMP14-selective protease sequence to generate a binary, “switch-on” fluorescence reporter capable of measuring MMP14 location, activity, and temporal dynamics. The MMP14-fluorogen activating protein biosensor approach is applicable to both short and long-term imaging modalities and contains an adaptable module that can be used to study many membrane-bound proteases. This MMP14 biosensor promises to serve as a tool for the advancement of a broad range of investigations targeting MMP14 activity during cell migration in health and disease.

Cell differentiation in nitrogen-fixing nodules hosting symbiosomes

The nitrogen-fixing nodule is a unique ecological niche for rhizobia, where microaerobic conditions support functioning of the main enzyme of nitrogen fixation, nitrogenase, which is highly sensitive to oxygen. To accommodate bacteria in a symbiotic nodule, the specialised infected cells increase in size owing to endoreduplication and are able to shelter thousands of bacteria. Bacteria are isolated from the cytoplasm of the plant cell by a membrane-bound organelle-like structure termed the symbiosome. It is enclosed by a symbiosome membrane, mainly of plant origin but with some inclusion of bacterial proteins. Within the symbiosome, bacterial cells differentiate into bacteroids a form that is specialised for nitrogen fixation. In this review, we briefly summarise recent advances in studies of differentiation both of symbiosomes and of the infected cells that accommodate them. We will consider the role of CCS52A, DNA topoisomerase VI, tubulin cytoskeleton rearrangements in differentiation of infected cells, the fate of the vacuole, and the distribution of symbiosomes in the infected cells. We will also consider differentiation of symbiosomes, paying attention to the role of NCR peptides, vesicular transport to symbiosomes, and mutant analysis of symbiosome development in model and crop legumes. Finally, we conclude that mechanisms involved in redistribution organelles, including the symbiosomes, clearly merit much more attention.

Interchromosomal Transfer of Immune Regulation During Infection of Barley with the Powdery Mildew Pathogen

Powdery mildew pathogens colonize over 9500 plant species, causing critical yield loss. The Ascomycete fungus, Blumeria graminis f. sp. hordei (Bgh), causes powdery mildew disease in barley (Hordeum vulgare L.). Successful infection begins with penetration of host epidermal cells, culminating in haustorial feeding structures, facilitating delivery of fungal effectors to the plant and exchange of nutrients from host to pathogen. We used expression Quantitative Trait Locus (eQTL) analysis to dissect the temporal control of immunity-associated gene expression in a doubled haploid barley population challenged with Bgh. Two highly significant regions possessing trans eQTL were identified near the telomeric ends of chromosomes (Chr) 2HL and 1HS. Within these regions reside diverse resistance loci derived from barley landrace H. laevigatum (MlLa) and H. vulgare cv. Algerian (Mla1), which associate with the altered expression of 961 and 3296 genes during fungal penetration of the host and haustorial development, respectively. Regulatory control of transcript levels for 299 of the 961 genes is reprioritized from MlLa on 2HL to Mla1 on 1HS as infection progresses, with 292 of the 299 alternating the allele responsible for higher expression, including Adaptin Protein-2 subunit μ AP2M and Vesicle Associated Membrane Protein VAMP72 subfamily members VAMP721/722. AP2M mediates effector-triggered immunity (ETI) via endocytosis of plasma membrane receptor components. VAMP721/722 and SNAP33 form a Soluble N-ethylmaleimide-sensitive factor Attachment Protein REceptor (SNARE) complex with SYP121 (PEN1), which is engaged in pathogen associated molecular pattern (PAMP)-triggered immunity via exocytosis. We postulate that genes regulated by alternate chromosomal positions are repurposed as part of a conserved immune complex to respond to different pathogen attack scenarios.

Firmly Planted, Always Moving

I was a budding pianist immersed in music in Leningrad, in the Soviet Union (now Saint Petersburg, Russia), when I started over, giving up sheet music for the study of ciliates. In a second starting-over story, I emigrated to the United States, where I switched to studying carbohydrate-binding plant lectin proteins, dissecting plant vesicular trafficking, and isolating novel glycosyltransferases responsible for making cell wall polysaccharides. I track my journey as a plant biologist from student to principal investigator to founding director of the Center for Plant Cell Biology and then director of the Institute for Integrative Genome Biology at the University of California, Riverside. I discuss implementing a new vision as the first and (so far) only female editor in chief of Plant Physiology, as well as how my laboratory helped develop chemical genomics tools to study the functions of essential plant proteins. Always wanting to give back what I received, I discuss my present efforts to develop female scientist leadership in Chinese universities and a constant theme throughout my life: a love of art and travel.

Transcriptome Analysis of Chilling-Imbibed Embryo Revealed Membrane Recovery Related Genes in Maize

The delayed seed germination and poor seedling growth caused by imbibitional chilling injury was common phenomenon in maize seedling establishment. In this study, RNA sequencing technology was used to comprehensively investigate the gene expressions in chilling-imbibed maize embryo and to reveal the underlying mechanism of chilling injury at molecular level. Imbibed seeds for 2 h at 5°C (LT2) were selected and transcriptomic comparative analysis was performed. Among 327 DEGs indentified between dry seed (CK0) and LT2, 15 specific genes with plasma membrane (PM) relevant functions belonging to lipid metabolism, stress, signaling and transport were characterized, and most of them showed down-regulation pattern under chilling stress. When transferred to 25°C for recovery (LT3), remarkable changes occurred in maize embryo. There were 873 DEGs including many PM related genes being identified between LT2 and LT3, some of which showing significant increase after 1 h recovery. Moreover, 15 genes encoding intracellular vesicular trafficking proteins were found to be exclusively differential expressed at recovery stage. It suggested that the intracellular vesicle trafficking might be essential for PM recovery through PM turnover. Furthermore, transcriptome analyses on imbibed embryos under normal condition (25°C) were also made as a contrast. A total of 651 DEGs were identified to mainly involved in protein metabolism, transcriptional regulation, signaling, and energy productions. Overall, the RNA-Seq results provided us a deep knowledge of imbibitional chilling injury on plasma membrane and a new view on PM repaired mechanism during early seed imbibition at transcriptional level. The DEGs identified in this work would be useful references in future seed germination research.

The plant secretory pathway seen through the lens of the cell wall

Secretion in plant cells is often studied by looking at well-characterised, evolutionarily conserved membrane proteins associated with particular endomembrane compartments. Studies using live cell microscopy and fluorescent proteins have illuminated the highly dynamic nature of trafficking, and electron microscopy studies have resolved the ultrastructure of many compartments. Biochemical and molecular analyses have further informed about the function of particular proteins and endomembrane compartments. In plants, there are over 40 cell types, each with highly specialised functions, and hence potential variations in cell biological processes and cell wall structure. As the primary function of secretion in plant cells is for the biosynthesis of cell wall polysaccharides and apoplastic transport complexes, it follows that utilising our knowledge of cell wall glycosyltransferases (GTs) and their polysaccharide products will inform us about secretion. Indeed, this knowledge has led to novel insights into the secretory pathway, including previously unseen post-TGN secretory compartments. Conversely, our knowledge of trafficking routes of secretion will inform us about polarised and localised deposition of cell walls and their constituent polysaccharides/glycoproteins. In this review, we look at what is known about cell wall biosynthesis and the secretory pathway and how the different approaches can be used in a complementary manner to study secretion and provide novel insights into these processes.

Visualization of vesicular transport from the endoplasmic reticulum to lysosome using an amidine derived two-photon probe

An amidine-based small molecule two-photon fluorescent probe for monitoring vesicle transport from the ER to lysosome in live cells was reported.

NtGNL1a ARF-GEF acts in endocytosis in tobacco cells

BACKGROUND

Processes of anterograde and retrograde membrane trafficking play an important role in cellular homeostasis and dynamic rearrangements of the plasma membrane (PM) in all eukaryotes. These processes depend on the activity of adenosine ribosylation factors (ARFs), a family of GTP-binding proteins and their guanine exchange factors (GEFs). However, knowledge on the function and specificity of individual ARF-GEFs for individual steps of membrane trafficking pathways is still limited in plants.

RESULTS

In this work, treatments with various trafficking inhibitors showed that the endocytosis of FM 4-64 is largely dynamin-dependent and relies on proteins containing endocytic tyrosine-based internalization motif and intact cytoskeleton. Interestingly, brefeldin A (BFA), reported previously as an inhibitor of anterograde membrane trafficking in plants, appeared to be the most potent inhibitor of endocytosis in tobacco. In concert with this finding, we demonstrate that the point mutation in the Sec7 domain of the GNOM-LIKE protein1a (NtGNL1a) confers intracellular trafficking pathway-specific BFA resistance. The internalization of FM 4-64 and trafficking of PIN-FORMED1 (PIN1) auxin efflux carrier in BY-2 tobacco cells were studied to reveal the function of the ARF-GEF NtGNL1a in these.

CONCLUSIONS

Altogether, our observations uncovered the role of NtGNL1a in endocytosis, including endocytosis of PM proteins (as PIN1 auxin efflux carrier). Moreover these data emphasize the need of careful evaluation of mode of action of non-native inhibitors in various species. In addition, they demonstrate the potential of tobacco BY-2 cells for selective mapping of ARF-GEF-regulated endomembrane trafficking pathways.

Functional homologies in vesicle tethering

The HOPS multisubunit tethering factor (MTC) is a macromolecular protein complex composed of six different subunits. It is one of the key components in the perception and subsequent fusion of multivesicular bodies and vacuoles. Electron microscopy studies indicate structural flexibility of the purified HOPS complex. Inducing higher rigidity into HOPS by biochemically modifying the complex declines the potential to mediate SNARE-driven membrane fusion. Thus, we propose that integral flexibility seems to be not only a feature, but of essential need for the function of HOPS. This review focuses on the general features of membrane tethering and fusion. For this purpose, we compare the structure and mode of action of different tethering factors to highlight their common central features and mechanisms.

Lipid trafficking in plant cells

Plant cells contain unique organelles such as chloroplasts with an extensive photosynthetic membrane. In addition, specialized epidermal cells produce an extracellular cuticle composed primarily of lipids, and storage cells accumulate large amounts of storage lipids. As lipid assembly is associated only with discrete membranes or organelles, there is a need for extensive lipid trafficking within plant cells, more so in specialized cells and sometimes also in response to changing environmental conditions such as phosphate deprivation. Because of the complexity of plant lipid metabolism and the inherent recalcitrance of membrane lipid transporters, the mechanisms of lipid transport within plant cells are not yet fully understood. Recently, several new proteins have been implicated in different aspects of plant lipid trafficking. While these proteins provide only first insights into limited aspects of lipid transport phenomena in plant cells, they represent exciting opportunities for further studies.

Arabidopsis SNAREs SYP61 and SYP121 coordinate the trafficking of plasma membrane aquaporin PIP2;7 to modulate the cell membrane water permeability

Plant plasma membrane intrinsic proteins (PIPs) are aquaporins that facilitate the passive movement of water and small neutral solutes through biological membranes. Here, we report that post-Golgi trafficking of PIP2;7 in Arabidopsis thaliana involves specific interactions with two syntaxin proteins, namely, the Qc-SNARE SYP61 and the Qa-SNARE SYP121, that the proper delivery of PIP2;7 to the plasma membrane depends on the activity of the two SNAREs, and that the SNAREs colocalize and physically interact. These findings are indicative of an important role for SYP61 and SYP121, possibly forming a SNARE complex. Our data support a model in which direct interactions between specific SNARE proteins and PIP aquaporins modulate their post-Golgi trafficking and thus contribute to the fine-tuning of the water permeability of the plasma membrane.

Study of seed hair growth in Populus tomentosa, an important character of female floral bud development

Background

Poplar seed hair is an environmental annoyance in northern China due to its abundance and widespread airborne distribution after maturation. The morphogenesis and molecular mechanisms of its development are not well understood, and little attention has been focused on the dynamics of its development. To better understand the mechanism of poplar seed hair development, paraffin sections were used to examine the initiation and elongation of poplar seed hairs. RNA-seq technology was also employed to provide a comprehensive overview of transcriptional changes that occur during seed hair development.

Results

The placenta at the base of ovary, was identified as the origin of seed hair development, which is in sharp contrast to cotton fibers that originate from epidermal cells of the seed coat. An enlarged cell nucleus in seed hair cells was also observed, which was supported by our gene ontology enrichment analysis. The significant enriched GO term of “endoreduplication” indicated that cycles of endoreduplication, bypassing normal mitosis, is the underlying mechanisms for the maintenance of the uni-cellular structure of seed hairs. By analyzing global changes in the transcriptome, many genes regulating cell cycle, cell elongation, cell well modification were identified. Additionally, in an analysis of differential expression, cellulose synthesis and cell wall biosynthesis-related biological processes were enriched, indicating that this component of fiber structure in poplar seed hairs is consistent with what is found in cotton fibers. Differentially expressed transcription factors exhibited a stage-specific up-regulation. A dramatic down-regulation was also revealed during the mid-to-late stage of poplar seed hair development, which may point to novel mechanisms regulating cell fate determination and cell elongation.

Conclusions

This study revealed the initiation site of poplar seed hairs and also provided a comprehensive overview of transcriptome dynamics during the process of seed hair development. The high level of resolution on dynamic changes in the transcriptome provided in this study may serve as a valuable resource for developing a more complete understanding of this important biological process.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-475) contains supplementary material, which is available to authorized users.

Proteomic dissection of the Arabidopsis Golgi and trans-Golgi network

The plant Golgi apparatus and trans-Golgi network are major endomembrane trafficking hubs within the plant cell and are involved in a diverse and vital series of functions to maintain plant growth and development. Recently, a series of disparate technical approaches have been used to isolate and characterize components of these complex organelles by mass spectrometry in the model plant Arabidopsis thaliana. Collectively, these studies have increased the number of Golgi and vesicular localized proteins identified by mass spectrometry to nearly 500 proteins. We have sought to provide a brief overview of these technical approaches and bring the datasets together to examine how they can reveal insights into the secretory pathway.

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).

Monoubiquitin-dependent endocytosis of the iron-regulated transporter 1 (IRT1) transporter controls iron uptake in plants

Plants take up iron from the soil using the iron-regulated transporter 1 (IRT1) high-affinity iron transporter at the root surface. Sophisticated regulatory mechanisms allow plants to tightly control the levels of IRT1, ensuring optimal absorption of essential but toxic iron. Here, we demonstrate that overexpression of Arabidopsis thaliana IRT1 leads to constitutive IRT1 protein accumulation, metal overload, and oxidative stress. IRT1 is unexpectedly found in trans-Golgi network/early endosomes of root hair cells, and its levels and localization are unaffected by iron nutrition. Using pharmacological approaches, we show that IRT1 cycles to the plasma membrane to perform iron and metal uptake at the cell surface and is sent to the vacuole for proper turnover. We also prove that IRT1 is monoubiquitinated on several cytosol-exposed residues in vivo and that mutation of two putative monoubiquitination target residues in IRT1 triggers stabilization at the plasma membrane and leads to extreme lethality. Together, these data suggest a model in which monoubiquitin-dependent internalization/sorting and turnover keep the plasma membrane pool of IRT1 low to ensure proper iron uptake and to prevent metal toxicity. More generally, our work demonstrates the existence of monoubiquitin-dependent trafficking to lytic vacuoles in plants and points to proteasome-independent turnover of plasma membrane proteins.

AtRabD2b and AtRabD2c have overlapping functions in pollen development and pollen tube growth

BACKGROUND

Rab GTPases are important regulators of endomembrane trafficking, regulating exocytosis, endocytosis and membrane recycling. Many Rab-like proteins exist in plants, but only a subset have been functionally characterized.

RESULTS

Here we report that AtRabD2b and AtRabD2c play important roles in pollen development, germination and tube elongation. AtrabD2b and AtrabD2c single mutants have no obvious morphological changes compared with wild-type plants across a variety of growth conditions. An AtrabD2b/2c double mutant is also indistinguishable from wild-type plants during vegetative growth; however its siliques are shorter than those in wild-type plants. Compared with wild-type plants, AtrabD2b/2c mutants produce deformed pollen with swollen and branched pollen tube tips. The shorter siliques in the AtrabD2b/2c double mutant were found to be primarily due to the pollen defects. AtRabD2b and AtRabD2c have different but overlapping expression patterns, and they are both highly expressed in pollen. Both AtRabD2b and AtRabD2c protein localize to Golgi bodies.

CONCLUSIONS

These findings support a partially redundant role for AtRabD2b and AtRabD2c in vesicle trafficking during pollen tube growth that cannot be fulfilled by the remaining AtRabD family members.

Plant secretome: unlocking secrets of the secreted proteins

Plant secretomics is a newly emerging area of the plant proteomics field. It basically describes the global study of secreted proteins into the extracellular space of plant cell or tissue at any given time and under certain conditions through various secretory mechanisms. A combination of biochemical, proteomics and bioinformatics approaches has been developed to isolate, identify and profile secreted proteins using complementary in vitro suspension-cultured cells and in planta systems. Developed inventories of secreted proteins under normal, biotic and abiotic conditions revealed several different types of novel secreted proteins, including the leaderless secretory proteins (LSPs). On average, LSPs can account for more than 50% of the total identified secretome, supporting, as in other eukaryotes, the existence of novel secretory mechanisms independent of the classical endoplasmic reticulum-Golgi secretory pathway, and suggesting that this non-classical mechanism of protein expression is, for as yet unknown reasons, more massively used than in other eukaryotic systems. Plants LSPs, which seem to be potentially involved in the defense/stress responses, might have dual (extracellular and/or intracellular) roles as most of them have established intracellular functions, yet presently unknown extracellular functions. Evidence is emerging on the role of glycosylation in the apical sorting and trafficking of secretory proteins. These initial secretome studies in plants have considerably advanced our understanding on secretion of different types of proteins and their underlying mechanisms, and opened a door for comparative analyses of plant secretomes with those of other organisms. In this first review on plant secretomics, we summarize and discuss the secretome definition, the applied approaches for unlocking secrets of the secreted proteins in the extracellular fluid, the possible functional significance and secretory mechanisms of LSPs, as well as glycosylation of secreted proteins and challenges involved ahead. Further improvements in existing and developing strategies and techniques will continue to drive forward plant secretomics research to building comprehensive and confident data sets of secreted proteins. This will lead to an increased understanding on how cells couple the concerted action of secreted protein networks to their internal and external environments.

A conceptual mathematical model of the dynamic self-organisation of distinct cellular organelles

Formation, degradation and renewal of cellular organelles is a dynamic process based on permanent budding, fusion and inter-organelle traffic of vesicles. These processes include many regulatory proteins such as SNAREs, Rabs and coats. Given this complex machinery, a controversially debated issue is the definition of a minimal set of generic mechanisms necessary to enable the self-organization of organelles differing in number, size and chemical composition. We present a conceptual mathematical model of dynamic organelle formation based on interacting vesicles which carry different types of fusogenic proteins (FP) playing the role of characteristic marker proteins. Our simulations (ODEs) show that a de novo formation of non-identical organelles, each accumulating a different type of FP, requires a certain degree of disproportionation of FPs during budding. More importantly however, the fusion kinetics must indispensably exhibit positive cooperativity among these FPs, particularly for the formation of larger organelles. We compared different types of cooperativity: sequential alignment of corresponding FPs on opposite vesicle/organelles during fusion and pre-formation of FP-aggregates (equivalent, e.g., to SNARE clusters) prior to fusion described by Hill kinetics. This showed that the average organelle size in the system is much more sensitive to the disproportionation strength of FPs during budding if the vesicular transport system gets along with a fusion mechanism based on sequential alignments of FPs. Therefore, pre-formation of FP aggregates within the membranes prior to fusion introduce robustness with respect to organelle size. Our findings provide a plausible explanation for the evolution of a relatively large number of molecules to confer specificity on the fusion machinery compared to the relatively small number involved in the budding process. Moreover, we could speculate that a specific cooperativity which may be described by Hill kinetics (aggregates or Rab/SNARE complex formation) is suitable if maturation/identity switching of organelles play a role (bistability).

Plasma membrane repair in plants

Resealing is the membrane-repair process that enables cells to survive disruption, preventing the loss of irreplaceable cell types and eliminating the cost of replacing injured cells. Given that failure in the resealing process in animal cells causes diverse types of muscular dystrophy, plasma membrane repair has been extensively studied in these systems. Animal proteins with Ca(2+)-binding domains such as synaptotagmins and dysferlin mediate Ca(2+)-dependent exocytosis to repair plasma membranes after mechanical damage. Until recently, no components or proof for membrane repair mechanisms have been discovered in plants. However, Arabidopsis SYT1 is now the first plant synaptotagmin demonstrated to participate in Ca(2+)-dependent repair of membranes. This suggests a conservation of membrane repair mechanisms between animal and plant cells.

Analysis of potato virus X replicase and TGBp3 subcellular locations

Potato virus X (PVX) infection leads to certain cytopathological modifications of the host endomembrane system. The subcellular location of the PVX replicase was previously unknown while the PVX TGBp3 protein was previously reported to reside in the ER. Using PVX infectious clones expressing the green fluorescent protein reporter, and antisera detecting the PVX replicase and host membrane markers, we examined the subcellular distribution of the PVX replicase in relation to the TGBp3. Confocal and electron microscopic observations revealed that the replicase localizes in membrane bound structures that derive from the ER. A subset of TGBp3 resides in the ER at the same location as the replicase. Sucrose gradient fractionation showed that the PVX replicase and TGBp3 proteins co-fractionate with ER marker proteins. This localization represents a region where both proteins may be synthesized and/or function. There is no evidence to indicate that either PVX protein moves into the Golgi apparatus. Cerulenin, a drug that inhibits de novo membrane synthesis, also inhibited PVX replication. These combined data indicate that PVX replication relies on ER-derived membrane recruitment and membrane proliferation.

The structure, function, and biosynthesis of plant cell wall pectic polysaccharides

Plant cell walls consist of carbohydrate, protein, and aromatic compounds and are essential to the proper growth and development of plants. The carbohydrate components make up approximately 90% of the primary wall, and are critical to wall function. There is a diversity of polysaccharides that make up the wall and that are classified as one of three types: cellulose, hemicellulose, or pectin. The pectins, which are most abundant in the plant primary cell walls and the middle lamellae, are a class of molecules defined by the presence of galacturonic acid. The pectic polysaccharides include the galacturonans (homogalacturonan, substituted galacturonans, and RG-II) and rhamnogalacturonan-I. Galacturonans have a backbone that consists of alpha-1,4-linked galacturonic acid. The identification of glycosyltransferases involved in pectin synthesis is essential to the study of cell wall function in plant growth and development and for maximizing the value and use of plant polysaccharides in industry and human health. A detailed synopsis of the existing literature on pectin structure, function, and biosynthesis is presented.

Chemical dissection of endosomal pathways

Membrane trafficking and associated signal transduction pathways are critical for plant development and responses to environment. These transduction pathways, including those for brassinosteroids and auxins, require endocytosis to endosomes and recycling back to the plasma membrane. A major challenge toward understanding these processes and their biological roles has been the highly dynamic nature of endomembrane trafficking. To effectively study endocytosis and recycling, which occur in a time frame of minutes, bioactive chemicals provide a powerful and exacting tool. Pharmacological inhibitors such as Brefeldin A (BFA) and the newly identified Endosidin 1 (ES1) have been used to define endosome compartments. ES1 is a clear example of the ability of chemicals to dissect even distinct subpopulations of endosomes involved in trafficking and signal transduction. The ability to characterize and dissect such highly dynamic pathways in a temporal and spatial manner is possible only using pharmacological reagents which can act rapidly and reversibly.

Powerful partners: Arabidopsis and chemical genomics

Chemical genomics (i.e. genomics scale chemical genetics) approaches capitalize on the ability of low molecular mass molecules to modify biological processes. Such molecules are used to modify the activity of a protein or a pathway in a manner that it is tunable and reversible. Bioactive chemicals resulting from forward or reverse chemical screens can be useful in understanding and dissecting complex biological processes due to the essentially limitless variation in structure and activities inherent in chemical space. A major advantage of this approach as a powerful addition to conventional plant genetics is the fact that chemical genomics can address loss-of-function lethality and redundancy. Furthermore, the ability of chemicals to be added at will and to act quickly can permit the study of processes that are highly dynamic such as endomembrane trafficking. An important aspect of utilizing small molecules effectively is to characterize bioactive chemicals in detail including an understanding of structure-activity relationships and the identification of active and inactive analogs. Bioactive chemicals can be useful as reagents to probe biological pathways directly. However, the identification of cognate targets and their pathways is also informative and can be achieved by screens for genetic resistance or hypersensitivity in Arabidopsis thaliana or other organisms from which the results can be translated to plants. In addition, there are approaches utilizing "tagged" chemical libraries that possess reactive moieties permitting the immobilization of active compounds. This opens the possibility for biochemical purification of putative cognate targets. We will review approaches to screen for bioactive chemicals that affect biological processes in Arabidopsis and provide several examples of the power and challenges inherent in this new approach in plant biology.

The secretory system of Arabidopsis

Over the past few years, a vast amount of research has illuminated the workings of the secretory system of eukaryotic cells. The bulk of this work has been focused on the yeast Saccharomyces cerevisiae, or on mammalian cells. At a superficial level, plants are typical eukaryotes with respect to the operation of the secretory system; however, important differences emerge in the function and appearance of endomembrane organelles. In particular, the plant secretory system has specialized in several ways to support the synthesis of many components of the complex cell wall, and specialized kinds of vacuole have taken on a protein storage role-a role that is intended to support the growing seedling, but has been co-opted to support human life in the seeds of many crop plants. In the past, most research on the plant secretory system has been guided by results in mammalian or fungal systems but recently plants have begun to stand on their own as models for understanding complex trafficking events within the eukaryotic endomembrane system.

A majority of cotton genes are expressed in single-celled fiber

Multicellular eukaryotes contain a diversity of cell types, presumably differing from one another in the suite of genes expressed during development. At present, little is known about the proportion of the genome transcribed in most cell types, nor the degree to which global patterns of expression change during cellular differentiation. To address these questions in a model plant system, we studied the unique and highly exaggerated single-celled, epidermal seed trichomes ("cotton") of cultivated cotton (Gossypium hirsutum). By taking advantage of advances in expression profiling and microarray technology, we evaluated the transcriptome of cotton fibers across a developmental time-course, from a few days post-anthesis through primary and secondary wall synthesis stages. Comparisons of gene expression in populations of developing cotton fiber cells to genetically complex reference samples derived from 6 different cotton organs demonstrated that a remarkably high proportion of the cotton genome is transcribed, with 75-94% of the total genome transcribed at each stage. Compared to the reference samples, more than half of all genes were up-regulated during at least one stage of fiber development. These genes were clustered into seven groups of expression profiles that provided new insight into biological processes governing fiber development. Genes implicated in vesicle coating and trafficking were found to be overexpressed throughout all stages of fiber development studied, indicating their important role in maintaining rapid growth of this unique plant cell.

Identification and mitotic partitioning strategies of vacuoles in the unicellular red alga Cyanidioschyzon merolae

Cyanidioschyzon merolae is considered as a suitable model system for studies of organelle differentiation, proliferation and partitioning. Here, we have identified and characterized vacuoles in this organism and examined the partitioning of vacuoles using fluorescence and electron microscopy. Vacuoles were stained with the fluorescent aminopeptidase substrate 7-amino-4-chloromethylcoumarin L: -arginine amide, acidotrophic dyes quinacrine and LysoTracker, and 4',6-diamidino-2-phenyl indole, which, at a high concentration, stains polyphosphate. Vacuoles have been shown to be approximately 500 nm in diameter with a mean of around five per interphase cell. The vacuolar H(+)-ATPase inhibitor concanamycin A blocked the accumulation of quinacrine in the vacuoles, suggesting the presence of the enzyme on these membranes. Electron microscopy revealed that the vacuoles were single membrane-bound organelles with an electron-dense substance, often containing a thick layer surrounding the membrane. Immunoelectron microscopy using an anti-vacuolar-H(+)-pyrophosphatase antibody revealed the presence of the enzyme on these membranes. In interphase cells, vacuoles were distributed in the cytoplasm, while in mitotic cells they were localized adjacent to the mitochondria. Filamentous structures were observed between vacuoles and mitochondria. Vacuoles were distributed almost evenly to daughter cells and redistributed in the cytoplasm after cytokinesis. The change in localization of vacuoles also happened in microtubule-disrupted cells. Since no actin protein or filaments have been detected in C. merolae, this result suggests an intrinsic mechanism for the movement of vacuoles that differs from commonly known mechanisms mediated by microtubules and actin filaments.

How rhizobial symbionts invade plants: the Sinorhizobium-Medicago model

Nitrogen-fixing rhizobial bacteria and leguminous plants have evolved complex signal exchange mechanisms that allow a specific bacterial species to induce its host plant to form invasion structures through which the bacteria can enter the plant root. Once the bacteria have been endocytosed within a host-membrane-bound compartment by root cells, the bacteria differentiate into a new form that can convert atmospheric nitrogen into ammonia. Bacterial differentiation and nitrogen fixation are dependent on the microaerobic environment and other support factors provided by the plant. In return, the plant receives nitrogen from the bacteria, which allows it to grow in the absence of an external nitrogen source. Here, we review recent discoveries about the mutual recognition process that allows the model rhizobial symbiont Sinorhizobium meliloti to invade and differentiate inside its host plant alfalfa (Medicago sativa) and the model host plant barrel medic (Medicago truncatula).

Cell death in seedlings of the interspecific hybrid of Nicotiana gossei and N. tabacum; possible role of knob-like bodies formed on tonoplast in vacuolar-collapse-mediated cell death

Vacuolar collapse plays a direct role in the cell death of the interspecific hybrid of Nicotiana gossei Domin xN. tabacum L. which exhibits hybrid lethality at the seedling stage. We have previously reported that cell death in these seedlings began at the base of hypocotyls and spread throughout the plant (Mino et al. 2002). A light microscopic analysis revealed that the process involved disruption of the intra-cellular membranes, plasmolysis, and retraction of the wall of the cell in hypocotyls. A transmission electron microscopic analysis showed that there were several abnormal structures, i.e. knob-like bodies on the tonoplast and small vesicles in the cytoplasm, and the disintegration of the tonoplast, in the cells of seedlings grown at 26 degrees C. However, no such cytological defects were observed in the seedlings grown at 37 degrees C, at which temperature the expression of lethality was suppressed. The activity levels of vacuolar processing enzyme (VPE), which might be involved in the vacuolar collapse of plant cells, temporarily increased in the seedlings grown at 26 degrees C before apparent cell death proceeded, but it remained unchanged in the seedlings grown at 37 degrees C. Applications of acetyl-L: -tyrosyl-L: -valyl-L: -alanyl-L: -aspart-1-aldehyde, an inhibitor for VPE, and cycloheximide to the seedlings suppressed VPE's activities, the formation of knob-like bodies on the tonoplast, and cell death. VPE might be involved in the structural anomalies on the tonoplast which lead to cell death triggered by vacuolar collapse in hybrid seedlings.

How pollen tubes grow

Sexual reproduction of flowering plants depends on delivery of the sperm to the egg, which occurs through a long, polarized projection of a pollen cell, called the pollen tube. The pollen tube grows exclusively at its tip, and this growth is distinguished by very fast rates and reaches extended lengths. Thus, one of the most fascinating aspects of pollen biology is the question of how enough cell wall material is produced to accommodate such rapid extension of pollen tube, and how the cell wall deposition and structure are regulated to allow for rapid changes in the direction of growth. This review discusses recent advances in our understanding of the mechanism of pollen tube growth, focusing on such basic cellular processes as control of cell shape and growth by a network of cell wall-modifying enzymes, molecular motor-mediated vesicular transport, and intracellular signaling by localized gradients of second messengers.

Medicago truncatula syntaxin SYP132 defines the symbiosome membrane and infection droplet membrane in root nodules

Symbiotic association of legume plants with rhizobia bacteria culminates in organogenesis of nitrogen-fixing root nodules. In indeterminate nodules, plant cells accommodate rhizobial infection by enclosing each bacterium in a membrane-bound, organelle-like compartment called the symbiosome. Numerous symbiosomes occupy each nodule cell; therefore an enormous amount of membrane material must be delivered to the symbiosome membrane for its development and maintenance. Protein delivery to the symbiosome is thought to rely on the plant secretory system; however, the targeting mechanisms are not well understood. In this study, we report the first in-depth analysis of a syntaxin localized on symbiosome membranes. Syntaxins help define a biochemical identity to each compartment in the plant secretory system and facilitate vesicle docking and fusion. Here, we present biochemical and cytological evidence that the SNARE MtSYP132, a Medicago truncatula homologue of Arabidopsis thaliana Syntaxin of Plants 132, localizes to the symbiosome membrane. Using a specific anti-MtSYP132 peptide antibody, we also show that MtSYP132 localizes to the plasma membrane surrounding infection threads and is most abundant on the infection droplet membrane. These results indicate that MtSYP132 may function in infection thread development or growth and the early stages of symbiosome formation.

cDNA microarrays as a tool for identification of biomineralization proteins in the coccolithophorid Emiliania huxleyi (Haptophyta)

Marine unicellular coccolithophore algae produce species-specific calcite scales otherwise known as coccoliths. While the coccoliths and their elaborate architecture have attracted the attention of investigators from various scientific disciplines, our knowledge of the underpinnings of the process of biomineralization in this alga is still in its infancy. The processes of calcification and coccolithogenesis are highly regulated and likely to be complex, requiring coordinated expression of many genes and pathways. In this study, we have employed cDNA microarrays to investigate changes in gene expression associated with biomineralization in the most abundant coccolithophorid, Emiliania huxleyi. Expression profiling of cultures grown under calcifying and noncalcifying conditions has been carried out using cDNA microarrays corresponding to approximately 2,300 expressed sequence tags. A total of 127 significantly up- or down-regulated transcripts were identified using a P value of 0.01 and a change of >2.0-fold. Real-time reverse transcriptase PCR was used to test the overall validity of the microarray data, as well as the relevance of many of the proteins predicted to be associated with biomineralization, including a novel gamma-class carbonic anhydrase (A. R. Soto, H. Zheng, D. Shoemaker, J. Rodriguez, B. A. Read, and T. M. Wahlund, Appl. Environ. Microbiol. 72:5500-5511, 2006). Differentially regulated genes include those related to cellular metabolism, ion channels, transport proteins, vesicular trafficking, and cell signaling. The putative function of the vast majority of candidate transcripts could not be defined. Nonetheless, the data described herein represent profiles of the transcription changes associated with biomineralization-related pathways in E. huxleyi and have identified novel and potentially useful targets for more detailed analysis.

Overexpression of the Arabidopsis syntaxin PEP12/SYP21 inhibits transport from the prevacuolar compartment to the lytic vacuole in vivo

Golgi-mediated transport to the lytic vacuole involves passage through the prevacuolar compartment (PVC), but little is known about how vacuolar proteins exit the PVC. We show that this last step is inhibited by overexpression of Arabidopsis thaliana syntaxin PEP12/SYP21, causing an accumulation of soluble and membrane cargo and the plant vacuolar sorting receptor BP80 in the PVC. Anterograde transport proceeds normally from the endoplasmic reticulum to the Golgi and the PVC, although export from the PVC appears to be compromised, affecting both anterograde membrane flow to the vacuole and the recycling route of BP80 to the Golgi. However, Golgi-mediated transport of soluble and membrane cargo toward the plasma membrane is not affected, but a soluble BP80 ligand is partially mis-sorted to the culture medium. We also observe clustering of individual PVC bodies that move together and possibly fuse with each other, forming enlarged compartments. We conclude that PEP12/SYP21 overexpression specifically inhibits export from the PVC without affecting the Golgi complex or compromising the secretory branch of the endomembrane system. The results provide a functional in vivo assay that confirms PEP12/SYP21 involvement in vacuolar sorting and indicates that excess of this syntaxin in the PVC can be detrimental for further transport from this organelle.

Rab11 GTPase-regulated membrane trafficking is crucial for tip-focused pollen tube growth in tobacco

Pollen tube growth is a polarized growth process whereby the tip-growing tubes elongate within the female reproductive tissues to deliver sperm cells to the ovules for fertilization. Efficient and regulated membrane trafficking activity incorporates membrane and deposits cell wall molecules at the tube apex and is believed to underlie rapid and focused growth at the pollen tube tip. Rab GTPases, key regulators of membrane trafficking, are candidates for important roles in regulating pollen tube growth. We show that a green fluorescent protein-tagged Nicotiana tabacum pollen-expressed Rab11b is localized predominantly to an inverted cone-shaped region in the pollen tube tip that is almost exclusively occupied by transport vesicles. Altering Rab11 activity by expressing either a constitutive active or a dominant negative variant of Rab11b in pollen resulted in reduced tube growth rate, meandering pollen tubes, and reduced male fertility. These mutant GTPases also inhibited targeting of exocytic and recycled vesicles to the pollen tube inverted cone region and compromised the delivery of secretory and cell wall proteins to the extracellular matrix. Properly regulated Rab11 GTPase activity is therefore essential for tip-focused membrane trafficking and growth at the pollen tube apex and is pivotal to reproductive success.

YKT6 is a Core Constituent of Membrane Fusion Machineries at the Arabidopsis trans-Golgi Network

SNARE complex formation is essential for membrane fusion in exocytotic and vacuolar trafficking pathways. Vesicle-associated (v-) SNARE associates with a target membrane (t-) SNARE to form a SNARE complex bridging two membranes, which may facilitate membrane fusion. The Arabidopsis genome encodes a large number of predicted SNARE proteins that might function primarily as fusogens for vesicle transport in endomembrane systems. The SNAREs SYP41, SYP61 and VTI12 reside in the trans-Golgi network and have been proposed to function together in vesicle fusion with this organelle. Here, we use a liposome fusion assay to demonstrate that VTI12 and either SYP41 or SYP61, but not both, are required for membrane fusion. This indicates that SYP41 and SYP61 are likely to function in independent vesicle fusion reactions in Arabidopsis. In addition, we have identified two new functionally interchangeable components, YKT61 and YKT62, that show sequence similarity to the multifunctional yeast SNARE YKT6. Both YKT61 and YKT62 interact with SYP41 and are essential for membrane fusion mediated by either SYP41 or SYP61. These results therefore define the core constituents required for membrane fusion at the Arabidopsis trans-Golgi network.