The endoplasmic reticulum is the main membrane source for biogenesis of the lytic vacuole in Arabidopsis

TWS1, a Novel Small Protein, Regulates Various Aspects of Seed and Plant Development

Small proteins have long been overlooked due to their poor annotation and the experimental challenges they pose. However, in recent years, their role in various processes has started to emerge, opening new research avenues. Here, we present the isolation and characterization of two allelic mutants, twisted seed1-1 (tws1-1) and tws1-2, which exhibit an array of developmental and biochemical phenotypes in Arabidopsis (Arabidopsis thaliana) seeds. We have identified AT5G01075 as the subtending gene encoding a small protein of 81 amino acids localized in the endoplasmic reticulum. TWS1 is strongly expressed in seeds, where it regulates both embryo development and accumulation of storage compounds. TWS1 loss-of-function seeds exhibit increased starch, sucrose, and protein accumulation at the detriment of fatty acids. TWS1 is also expressed in vegetative and reproductive tissues, where it is responsible for proper epidermal cell morphology and overall plant growth. At the cellular level, TWS1 is responsible for cuticle deposition on epidermal cells and organization of the endomembrane system. Finally, we show that TWS1 is a single-copy gene in Arabidopsis, and it is specifically conserved among angiosperms.

Structural basis for the reversibility of proton pyrophosphatase

Proton Pyrophosphatase (H⁺-PPase) is an evolutionarily conserved enzyme regarded as a bona fide vacuolar marker. However, H⁺-PPase also localizes at the plasma membrane of the phloem, where, evidence suggests that it functions as a Pyrophosphate Synthase and participates in phloem loading and photosynthate partitioning. We believe that this pyrophosphate synthesising function of H⁺-PPase is fundamentally rooted to its molecular structure, and here we postulate, on the basis of published crystal structures of membrane-bound pyrophosphatases, a plausible mechanism of pyrophosphate synthesis.

COPII Paralogs in Plants: Functional Redundancy or Diversity?

In eukaryotes, the best-described mechanism of endoplasmic reticulum (ER) export is mediated by coat protein complex II (COPII) vesicles, which comprise five conserved cytosolic components [secretion-associated, Ras-related protein 1 (Sar1), Sec23-24, and Sec13-31]. In higher organisms, multiple paralogs of COPII components are created due to gene duplication. However, the functional diversity of plant COPII subunit isoforms remains largely elusive. Here we summarize and discuss the latest findings derived from studies of various arabidopsis COPII subunit isoforms and their functional diversity. We also put forward testable hypotheses on distinct populations of COPII vesicles performing unique functions in ER export in developmental and stress-related pathways in plants.

Receptor-mediated sorting of soluble vacuolar proteins: myths, facts, and a new model

To prevent their being released to the cell exterior, acid hydrolases are recognized by receptors at some point in the secretory pathway and diverted towards the lytic compartment of the cell (lysosome or vacuole). In animal cells, the receptor is called the mannosyl 6-phosphate receptor (MPR) and it binds hydrolase ligands in the trans-Golgi network (TGN). These ligands are then sequestered into clathrin-coated vesicles (CCVs) because of motifs in the cytosolic tail of the MPR which interact first with monomeric adaptors (Golgi-localized, Gamma-ear-containing, ARF-binding proteins, GGAs) and then with tetrameric (adaptin) adaptor complexes. The CCVs then fuse with an early endosome, whose more acidic lumen causes the ligands to dissociate. The MPRs are then recycled back to the TGN via retromer-coated carriers. Plants have vacuolar sorting receptors (VSRs) which were originally identified in CCVs isolated from pea (Pisum sativum L.) cotyledons. It was therefore assumed that VSRs would have an analogous function in plants to MPRs in animals. Although this dogma has enjoyed wide support over the last 20 years there are many inconsistencies. Recently, results have been published which are quite contrary to it. It now emerges that VSRs and their ligands can interact very early in the secretory pathway, and dissociate in the TGN, which, in contrast to its mammalian counterpart, has a pH of 5.5. Multivesicular endosomes in plants lack proton pump complexes and consequently have an almost neutral internal pH, which discounts them as organelles of pH-dependent receptor-ligand dissociation. These data force a critical re-evaluation of the role of CCVs at the TGN, especially considering that vacuolar cargo ligands have never been identified in them. We propose that one population of TGN-derived CCVs participate in retrograde transport of VSRs from the TGN. We also present a new model to explain how secretory and vacuolar cargo proteins are effectively separated after entering the late Golgi/TGN compartments.

Sorting Motifs Involved in the Trafficking and Localization of the PIN1 Auxin Efflux Carrier

In contrast with the wealth of recent reports about the function of μ-adaptins and clathrin adaptor protein (AP) complexes, there is very little information about the motifs that determine the sorting of membrane proteins within clathrin-coated vesicles in plants. Here, we investigated putative sorting signals in the large cytosolic loop of the Arabidopsis (Arabidopsis thaliana) PIN-FORMED1 (PIN1) auxin transporter, which are involved in binding μ-adaptins and thus in PIN1 trafficking and localization. We found that Phe-165 and Tyr-280, Tyr-328, and Tyr-394 are involved in the binding of different μ-adaptins in vitro. However, only Phe-165, which binds μA(μ2)- and μD(μ3)-adaptin, was found to be essential for PIN1 trafficking and localization in vivo. The PIN1:GFP-F165A mutant showed reduced endocytosis but also localized to intracellular structures containing several layers of membranes and endoplasmic reticulum (ER) markers, suggesting that they correspond to ER or ER-derived membranes. While PIN1:GFP localized normally in a μA (μ2)-adaptin mutant, it accumulated in big intracellular structures containing LysoTracker in a μD (μ3)-adaptin mutant, consistent with previous results obtained with mutants of other subunits of the AP-3 complex. Our data suggest that Phe-165, through the binding of μA (μ2)- and μD (μ3)-adaptin, is important for PIN1 endocytosis and for PIN1 trafficking along the secretory pathway, respectively.

Biogenesis of Plant Prevacuolar Multivesicular Bodies

Plant prevacuolar compartments (PVCs), or multivesicular bodies (MVBs), are single membrane-bound organelles that play important roles in mediating protein trafficking to vacuoles in the secretory pathway. PVC/MVB also serves as a late endosome in the endocytic pathway in plants. Since the plant PVC was identified as an MVB more than 10 years ago, great progress has been made toward the understanding of PVC/MVB function and biogenesis in plants. In this review, we first summarize previous research into the identification and characterization of plant PVCs/MVBs, and then highlight recent advances on the mechanisms underlying intraluminal vesicle formation and maturation of plant PVCs/MVBs. In addition, we discuss the possible crosstalk that appears to occur between PVCs/MVBs and autophagosomes during autophagy in plants. Finally, we list some open questions and present future perspectives in this field.

Maize ZmVPP5 is a truncated Vacuole H(+) -PPase that confers hypersensitivity to salt stress

In plants, Vacuole H(+) -PPases (VPPs) are important proton pumps and encoded by multiple genes. In addition to full-length VPPs, several truncated forms are expressed, but their biological functions are unknown. In this study, we functionally characterized maize vacuole H(+) -PPase 5 (ZmVPP5), a truncated VPP in the maize genome. Although ZmVPP5 shares high sequence similarity with ZmVPP1, ZmVPP5 lacks the complete structure of the conserved proton transport and the inorganic pyrophosphatase-related domain. Phylogenetic analysis suggests that ZmVPP5 might be derived from an incomplete gene duplication event. ZmVPP5 is expressed in multiple tissues, and ZmVPP5 was detected in the plasma membrane, vacuole membrane and nuclei of maize cells. The overexpression of ZmVPP5 in yeast cells caused a hypersensitivity to salt stress. Transgenic maize lines with overexpressed ZmVPP5 also exhibited the salt hypersensitivity phenotype. A yeast two-hybrid analysis identified the ZmBag6-like protein as a putative ZmVPP5-interacting protein. The results of bimolecular luminescence complementation (BiLC) assay suggest an interaction between ZmBag6-like protein and ZmVPP5 in vivo. Overall, this study suggests that ZmVPP5 might act as a VPP antagonist and participate in the cellular response to salt stress. Our study of ZmVPP5 has expanded the understanding of the origin and functions of truncated forms of plant VPPs.

Male functions and malfunctions: the impact of phosphoinositides on pollen development and pollen tube growth

Phosphoinositides in pollen. In angiosperms, sexual reproduction is a series of complex biological events that facilitate the distribution of male generative cells for double fertilization. Angiosperms have no motile gametes, and the distribution units of generative cells are pollen grains, passively mobile desiccated structures, capable of delivering genetic material to compatible flowers over long distances and in an adverse environment. The development of pollen (male gametogenesis) and the formation of a pollen tube after a pollen grain has reached a compatible flower (pollen tube growth) are important aspects of plant developmental biology. In recent years, a wealth of information has been gathered about the molecular control of cell polarity, membrane trafficking and cytoskeletal dynamics underlying these developmental processes. In particular, it has been found that regulatory membrane phospholipids, such as phosphoinositides (PIs), are critical regulatory players, controlling key steps of trafficking and polarization. Characteristic features of PIs are the inositol phosphate headgroups of the lipids, which protrude from the cytosolic surfaces of membranes, enabling specific binding and recruitment of numerous protein partners containing specific PI-binding domains. Such recruitment is globally an early event in polarization processes of eukaryotic cells and also of key importance to pollen development and tube growth. Additionally, PIs serve as precursors of other signaling factors with importance to male gametogenesis. This review highlights the recent advances about the roles of PIs in pollen development and pollen function.

The Arabidopsis tonoplast is almost devoid of glycoproteins with complex N-glycans, unlike the rat lysosomal membrane

The distribution of the N-glycoproteome in integral membrane proteins of the vacuolar membrane (tonoplast) or the plasma membrane of Arabidopsis thaliana and, for further comparison, of the Rattus norvegicus lysosomal and plasma membranes, was analyzed. In silico analysis showed that potential N-glycosylation sites are much less frequent in tonoplast proteins. Biochemical analysis of Arabidopsis subcellular fractions with the lectin concanavalin A, which recognizes mainly unmodified N-glycans, or with antiserum against Golgi-modified N-glycans confirmed the in silico results and showed that, unlike the plant plasma membrane, the tonoplast is almost or totally devoid of N-glycoproteins with Golgi-modified glycans. Lysosomes share with vacuoles the hydrolytic functions and the position along the secretory pathway; however, our results indicate that their membranes had a divergent evolution. We propose that protection against the luminal hydrolases that are abundant in inner hydrolytic compartments, which seems to have been achieved in many lysosomal membrane proteins by extensive N-glycosylation of the luminal domains, has instead been obtained in the vast majority of tonoplast proteins by limiting the length of such domains.

The biosensor toolbox for plant developmental biology

Plant development is highly interconnected with the metabolic state of tissues and cells. Current research efforts focus on the identification of the links and mechanisms that govern the interplay between metabolic and gene-regulatory networks. Genetically encoded sensors that allow detection of small molecules in vivo and at high spatio-temporal resolution promise to be the tools of choice for quantifying and visualizing the dynamics of metabolite flux in plants. We provide an overview about current approaches to measure signaling molecules, such as hormones, calcium and sugars, as well as for monitoring the metabolic state via energy equivalents and pH. Biosensors show great potential to address questions of plant development but there are also limitations where alternative approaches are needed.

Apoplasmic loading in the rice phloem supported by the presence of sucrose synthase and plasma membrane-localized proton pyrophosphatase

BACKGROUND AND AIMS

Although Oryza sativa (rice) is one of the most important cereal crops, the mechanism by which sucrose, the major photosynthate, is loaded into its phloem is still a matter of debate. Current opinion holds that the phloem loading pathway in rice could involve either a symplasmic or an apoplasmic route. It was hypothesized, on the basis of a complementary body of evidence from arabidopsis, which is an apoplasmic loader, that the membrane specificity of proton pyrophosphatases (H(+)-PPases; OVPs) in the sieve element-companion cell (SE-CC) complexes of rice source leaves would support the existence of either of the aforementioned phloem loading mechanisms. Additionally, it was contended that the presence of sucrose synthase in the SE-CC complexes would be consistent with an apoplasmic sucrose loading route in rice.

METHODS

Conventional chemical fixation methods were used for immunohistochemical localization of H(+)-PPases and sucrose synthase in rice and arabidopsis at the light microscopy level, while ultrastructural immunogold labelling of H(+)-PPases and sucrose synthase was performed on high-pressure frozen source leaves of rice.

KEY RESULTS

Using immunogold labelling, it was found that OVPs predominantly localize at the plasma membrane (PM) of the SE-CC complexes in rice source leaf minor veins, while in the root meristematic cells, OVPs preferentially localize at the vacuoles. The PM specificity of OPVs in the SE-CC complexes was deemed to support apoplasmic loading in the rice phloem. Further backing for this interpretation came from the sucrose synthase-specific immunogold labelling at the SE-CC complexes of rice source leaves.

CONCLUSION

These findings are consistent with the idea that, in the same way as in arabidopsis and a majority of grasses, sucrose is actively loaded into the SE-CC complexes of rice leaves using an apoplasmic step.

Actin-dependent vacuolar occupancy of the cell determines auxin-induced growth repression

The cytoskeleton is an early attribute of cellular life, and its main components are composed of conserved proteins. The actin cytoskeleton has a direct impact on the control of cell size in animal cells, but its mechanistic contribution to cellular growth in plants remains largely elusive. Here, we reveal a role of actin in regulating cell size in plants. The actin cytoskeleton shows proximity to vacuoles, and the phytohormone auxin not only controls the organization of actin filaments but also impacts vacuolar morphogenesis in an actin-dependent manner. Pharmacological and genetic interference with the actin-myosin system abolishes the effect of auxin on vacuoles and thus disrupts its negative influence on cellular growth. SEM-based 3D nanometer-resolution imaging of the vacuoles revealed that auxin controls the constriction and luminal size of the vacuole. We show that this actin-dependent mechanism controls the relative vacuolar occupancy of the cell, thus suggesting an unanticipated mechanism for cytosol homeostasis during cellular growth.

Novel Vein Patterns in Arabidopsis Induced by Small Molecules

The critical role of veins in transporting water, nutrients, and signals suggests that some key regulators of vein formation may be genetically redundant and, thus, undetectable by forward genetic screens. To identify such regulators, we screened more than 5000 structurally diverse small molecules for compounds that alter Arabidopsis (Arabidopsis thaliana) leaf vein patterns. Many compound-induced phenotypes were observed, including vein networks with an open reticulum; decreased or increased vein number and thickness; and misaligned, misshapen, or nonpolar vascular cells. Further characterization of several individual active compounds suggests that their targets include hormone cross talk, hormone-dependent transcription, and PIN-FORMED trafficking.

Job Sharing in the Endomembrane System: Vacuolar Acidification Requires the Combined Activity of V-ATPase and V-PPase

The presence of a large central vacuole is one of the hallmarks of a prototypical plant cell, and the multiple functions of this compartment require massive fluxes of molecules across its limiting membrane, the tonoplast. Transport is assumed to be energized by the membrane potential and the proton gradient established by the combined activity of two proton pumps, the vacuolar H(+)-pyrophosphatase (V-PPase) and the vacuolar H(+)-ATPase (V-ATPase). Exactly how labor is divided between these two enzymes has remained elusive. Here, we provide evidence using gain- and loss-of-function approaches that lack of the V-ATPase cannot be compensated for by increased V-PPase activity. Moreover, we show that increased V-ATPase activity during cold acclimation requires the presence of the V-PPase. Most importantly, we demonstrate that a mutant lacking both of these proton pumps is conditionally viable and retains significant vacuolar acidification, pointing to a so far undetected contribution of the trans-Golgi network/early endosome-localized V-ATPase to vacuolar pH.

Proteomic analysis of lipid body from the alkenone-producing marine haptophyte alga Tisochrysis lutea

Lipid body (LB) is recognized as the cellular carbon and energy storage organelle in many organisms. LBs have been observed in the marine haptophyte alga Tisochrysis lutea that produces special lipids such as long‐chain (C₃₇‐C₄₀) ketones (alkenones) with 2–4 trans‐type double bonds. In this study, we succeeded in developing a modified method to isolate LB from T. lutea. Purity of isolated LBs was confirmed by the absence of chlorophyll auto‐fluorescence and no contamination of the most abundant cellular protein ribulose‐1,5‐bisphosphate carboxylase/oxygenase. As alkenones predominated in the LB by GC‐MS analysis, the LB can be more appropriately named as “alkenone body (AB).” Extracted AB‐containing proteins were analyzed by the combination of 1DE (SDS‐PAGE) and MS/MS for confident protein identification and annotated using BLAST tools at National Center for Biotechnology Information. Totally 514 proteins were identified at the maximum. The homology search identified three major proteins, V‐ATPase, a hypothetical protein EMIHUDRAFT_465517 found in other alkenone‐producing haptophytes, and a lipid raft‐associated SPFH domain‐containing protein. Our data suggest that AB of T. lutera is surrounded by a lipid membrane originating from either the ER or the ER‐derived four layer‐envelopes chloroplast and function as the storage site of alkenones and alkenes.

Molecular Composition of Plant Vacuoles: Important but Less Understood Regulations and Roles of Tonoplast Lipids

The vacuole is an essential organelle for plant growth and development. It is the location for the storage of nutrients; such as sugars and proteins; and other metabolic products. Understanding the mechanisms of vacuolar trafficking and molecule transport across the vacuolar membrane is of great importance in understanding basic plant development and cell biology and for crop quality improvement. Proteins play important roles in vacuolar trafficking; such proteins include Rab GTPase signaling proteins; cargo recognition receptors; and SNAREs (Soluble NSF Attachment Protein Receptors) that are involved in membrane fusion. Some vacuole membrane proteins also serve as the transporters or channels for transport across the tonoplast. Less understood but critical are the roles of lipids in vacuolar trafficking. In this review, we will first summarize molecular composition of plant vacuoles and we will then discuss our latest understanding on the role of lipids in plant vacuolar trafficking and a surprising connection to ribosome function through the study of ribosomal mutants.

The Monoterpene Carvacrol Generates Endoplasmic Reticulum Stress in the Pathogenic Fungus Candida albicans

The monoterpene carvacrol, the major component of oregano and thyme oils, is known to exert potent antifungal activity against the pathogenic yeast Candida albicans. This monoterpene has been the subject of a considerable number of investigations that uncovered extensive pharmacological properties, including antifungal and antibacterial effects. However, its mechanism of action remains elusive. Here, we used integrative chemogenomic approaches, including genome-scale chemical-genetic and transcriptional profiling, to uncover the mechanism of action of carvacrol associated with its antifungal property. Our results clearly demonstrated that fungal cells require the unfolded protein response (UPR) signaling pathway to resist carvacrol. The mutants most sensitive to carvacrol in our genome-wide competitive fitness assay in the yeast Saccharomyces cerevisiae expressed mutations of the transcription factor Hac1 and the endonuclease Ire1, which is required for Hac1 activation by removing a nonconventional intron from the 3' region of HAC1 mRNA. Confocal fluorescence live-cell imaging revealed that carvacrol affects the morphology and the integrity of the endoplasmic reticulum (ER). Transcriptional profiling of pathogenic yeast C. albicans cells treated with carvacrol demonstrated a bona fide UPR transcriptional signature. Ire1 activity detected by the splicing of HAC1 mRNA in C. albicans was activated by carvacrol. Furthermore, carvacrol was found to potentiate antifungal activity of the echinocandin antifungal caspofungin and UPR inducers dithiothreitol and tunicamycin against C. albicans. This comprehensive chemogenomic investigation demonstrated that carvacrol exerts its antifungal activity by altering ER integrity, leading to ER stress and the activation of the UPR to restore protein-folding homeostasis.

Salt-induced remodeling of spatially restricted clathrin-independent endocytic pathways in Arabidopsis root

Endocytosis is a ubiquitous cellular process that is characterized well in animal cells in culture but poorly across intact, functioning tissue. Here, we analyze endocytosis throughout the Arabidopsis thaliana root using three classes of probes: a lipophilic dye, tagged transmembrane proteins, and a lipid-anchored protein. We observe a stratified distribution of endocytic processes. A clathrin-dependent endocytic pathway that internalizes transmembrane proteins functions in all cell layers, while a sterol-dependent, clathrin-independent pathway that takes up lipid and lipid-anchored proteins but not transmembrane proteins is restricted to the epidermal layer. Saline stress induces a third pathway that is clathrin-independent, nondiscriminatory in its choice of cargo, and operates across all layers of the root. Concomitantly, small acidic compartments in inner cell layers expand to form larger vacuole-like structures. Plants lacking function of the Rab-GEF (guanine nucleotide exchange factor) VPS9a (vacuolar protein sorting 9A) neither induce the third endocytic pathway nor expand the vacuolar system in response to salt stress. The plants are also hypersensitive to salt. Thus, saline stress reconfigures clathrin-independent endocytosis and remodels endomembrane systems, forming large vacuoles in the inner cell layers, both processes correlated by the requirement of VPS9a activity.

FYVE1 is essential for vacuole biogenesis and intracellular trafficking in Arabidopsis

The plant vacuole is a central organelle that is involved in various biological processes throughout the plant life cycle. Elucidating the mechanism of vacuole biogenesis and maintenance is thus the basis for our understanding of these processes. Proper formation of the vacuole has been shown to depend on the intracellular membrane trafficking pathway. Although several mutants with altered vacuole morphology have been characterized in the past, the molecular basis for plant vacuole biogenesis has yet to be fully elucidated. With the aim to identify key factors that are essential for vacuole biogenesis, we performed a forward genetics screen in Arabidopsis (Arabidopsis thaliana) and isolated mutants with altered vacuole morphology. The vacuolar fusion defective1 (vfd1) mutant shows seedling lethality and defects in central vacuole formation. VFD1 encodes a Fab1, YOTB, Vac1, and EEA1 (FYVE) domain-containing protein, FYVE1, that has been implicated in intracellular trafficking. FYVE1 localizes on late endosomes and interacts with Src homology-3 domain-containing proteins. Mutants of FYVE1 are defective in ubiquitin-mediated protein degradation, vacuolar transport, and autophagy. Altogether, our results show that FYVE1 is essential for plant growth and development and place FYVE1 as a key regulator of intracellular trafficking and vacuole biogenesis.

Auxin regulates SNARE-dependent vacuolar morphology restricting cell size

The control of cellular growth is central to multicellular patterning. In plants, the encapsulating cell wall literally binds neighbouring cells to each other and limits cellular sliding/migration. In contrast to its developmental importance, growth regulation is poorly understood in plants. Here, we reveal that the phytohormone auxin impacts on the shape of the biggest plant organelle, the vacuole. TIR1/AFBs-dependent auxin signalling posttranslationally controls the protein abundance of vacuolar SNARE components. Genetic and pharmacological interference with the auxin effect on vacuolar SNAREs interrelates with auxin-resistant vacuolar morphogenesis and cell size regulation. Vacuolar SNARE VTI11 is strictly required for auxin-reliant vacuolar morphogenesis and loss of function renders cells largely insensitive to auxin-dependent growth inhibition. Our data suggests that the adaptation of SNARE-dependent vacuolar morphogenesis allows auxin to limit cellular expansion, contributing to root organ growth rates.

DOI:http://dx.doi.org/10.7554/eLife.05868.001

In plants and animals, the way that cells grow is carefully controlled to enable tissues and organs to form and be maintained. This is especially important in plants because the cells are attached to each other by their cell walls. This means that, unlike some animal cells, plant cells are not able to move around as a plant's organs develop.

Plant cells contain a large storage compartment called the vacuole, which occupies 30–80% of a cell's volume. The volume of the vacuole increases as the cell increases in size, and some researchers have suggested that the vacuole might help to control cell growth. A plant hormone called auxin can alter the growth of plant cells. However, this hormone's effect depends on the position of the cell in the plant; for example, it inhibits the growth of root cells, but promotes the growth of cells in the shoots and leaves. Nevertheless, it is not clear precisely how auxin controls plant cell growth.

Here, Löfke et al. studied the effect of auxin on the appearance of vacuoles in a type of plant cell—called the root epidermal cell—on the surface of the roots of a plant called Arabidopsis thaliana. The experiments show that auxin alters the appearance of the vacuoles in these cells so they become smaller in size. At the same time, auxin also inhibits the growth of these cells.

Löfke et al. found that auxin increases the amount of certain proteins in the membrane that surrounds the vacuole. These proteins belong to the SNARE family and one SNARE protein in particular, called VTI11, is required for auxin to be able to both alter the appearance of the vacuoles and restrict the growth of root epidermal cells. Enzymes called PI4 kinases were also shown to be involved in the control of the SNARE proteins in response to the auxin hormone.

Löfke et al.'s findings suggest that auxin restricts the growth of root epidermal cells by controlling the amount of SNARE proteins in the vacuole membrane. The next challenge will be to understand precisely how the shape of the vacuole is controlled and how it contributes to cell growth.

DOI:http://dx.doi.org/10.7554/eLife.05868.002

The endoplasmic reticulum: a dynamic and well-connected organelle

The endoplasmic reticulum forms the first compartment in a series of organelles which comprise the secretory pathway. It takes the form of an extremely dynamic and pleomorphic membrane-bounded network of tubules and cisternae which have numerous different cellular functions. In this review, we discuss the nature of endoplasmic reticulum structure and dynamics, its relationship with closely associated organelles, and its possible function as a highway for the distribution and delivery of a diverse range of structures from metabolic complexes to viral particles.

Molecular dissection of Phaseolus vulgaris polygalacturonase-inhibiting protein 2 reveals the presence of hold/release domains affecting protein trafficking toward the cell wall

The plant endomembrane system is massively involved in the synthesis, transport and secretion of cell wall polysaccharides and proteins; however, the molecular mechanisms underlying trafficking toward the apoplast are largely unknown. Besides constitutive, the existence of a regulated secretory pathway has been proposed. A polygalacturonase inhibitor protein (PGIP2), known to move as soluble cargo and reach the cell wall through a mechanism distinguishable from default, was dissected in its main functional domains (A, B, C, D), and C sub-fragments (C1-10), to identify signals essential for its regulated targeting. The secretion patterns of the fluorescent chimeras obtained by fusing different PGIP2 domains to the green fluorescent protein (GFP) were analyzed. PGIP2 N-terminal and leucine-rich repeat domains (B and C, respectively) seem to operate as holding/releasing signals, respectively, during PGIP2 transit through the Golgi. The B domain slows down PGIP2 secretion by transiently interacting with Golgi membranes. Its depletion leads, in fact, to the secretion via default (Sp2-susceptible) of the ACD-GFP chimera faster than PGIP2. Depending on its length (at least the first 5 leucine-rich repeats are required), the C domain modulates B interaction with Golgi membranes allowing the release of chimeras and their extracellular secretion through a Sp2 independent pathway. The addition of the vacuolar sorting determinant Chi to PGIP2 diverts the path of the protein from cell wall to vacuole, suggesting that C domain is a releasing rather than a cell wall sorting signal.

mRNA-based protein targeting to the endoplasmic reticulum and chloroplasts in plant cells

The targeting of proteins to subcellular organelles is specified by the presence of signal/leader peptide sequences normally located on the N-terminus. In the past two decades, messenger RNA (mRNA) localization, a pathway driven by cis-acting localization elements within the RNA sequence, has emerged as an alternative mechanism for protein targeting to specific locations in the cytoplasm, on the endoplasmic reticulum or to mitochondria and chloroplasts. In this review, we will summarize studies on mRNA-based protein targeting to the endoplasmic reticulum and chloroplast within plant cells.

pH in the plant endomembrane system-an import and export business

pH homeostasis is an essential process in all plant cells and the maintenance of correct luminal pH in the compartments of the endomembrane system is important not only for secondary active transport but also for a variety of cellular functions including protein modification, sorting, and trafficking. Due to their electrogenicity primary H(+)-pumps cannot establish and control the often large proton-gradients single-handedly but require the co-action of other ion transporters that serve as either shunt conductances or proton-leaks. Here, I will thus focus on recent results that highlight the interplay of proton-pumps and proton-coupled transporters in controlling pH in the compartments of the plant endomembrane system.

Plant vacuolar trafficking driven by RAB and SNARE proteins

Membrane-bounded organelles are connected to each other by membrane trafficking, which is accomplished by membrane fusion between transport vesicles and target organelles mediated by RAB GTPases and SNARE proteins. Of those trafficking pathways networking plant organelles, the vacuolar trafficking pathway has recently been shown to be uniquely diversified from non-plant systems, most likely reflecting unique functions of plant vacuoles such as the storage of proteins and other organic compounds, generation of turgor pressure, and space-filling to enlarge plant bodies. Plant-unique trafficking machineries in addition to evolutionarily conserved molecular components are allocated to this trafficking pathway in distinctive ways. In this review, we summarize recent findings on SNARE proteins and RAB GTPases mediating vacuolar transport in plants, especially focusing on the functions and regulation of two distinct trans-SNARE complexes and RAB5 and RAB7 in multiple vacuolar trafficking pathways.

ER - the key to the highway

The endoplasmic reticulum (ER) is the key organelle at the start of the secretory pathway and the list of its functions is continually growing. The ER organization as a tubular/cisternal network at the cortex of plant cells has recently been shown to be governed by the membrane tubulation proteins of the reticulon family working alongside plant atlastin homologues, members of the RHD3 group of proteins. Such a network has intimate connections with other organelles such as peroxisomes via peroxules, chloroplasts, Golgi bodies and at the cell cortex to the plasma membrane with cytoskeleton at so called 'anchor/contact sites'. The ER network is by no means static displaying a range of different movements and acting as a subcellular highway supports the motility of organelles such as peroxisomes, mitochondria and Golgi bodies plus the transport of macromolecules such as viral movement proteins, nucleocapsid proteins and RNA. Here we highlight recent and exciting discoveries on the maintenance of the ER structure and its role on movement and biology of other organelles.

Update on transparent testa mutants from Arabidopsis thaliana: characterisation of new alleles from an isogenic collection

We present a comprehensive overview on flavonoid-related phenotypes of A. thaliana tt and tds mutants, provide tools for their characterisation, increase the number of available alleles and demonstrate that tds3 is allelic to tt12 and tds5 to aha10. Flavonoid biosynthesis is one of the best-studied secondary metabolite pathways in plants. In the model system Arabidopsis thaliana it leads to the synthesis of three phenolic compound classes: flavonol glycosides, anthocyanins and proanthocyanidins (PAs). PAs appear brown in their oxidised polymeric forms, and most A. thaliana mutants impaired in flavonoid accumulation were identified through screens for lack of this seed coat pigmentation. These mutants are referred to as transparent testa (tt) or tannin-deficient seed (tds). More than 20 mutants of these types have been published, probably representing most of the genes relevant for PA accumulation in A. thaliana. However, data about the genes involved in PA deposition or oxidation are still rather scarce. Also, for some of the known mutants it is unclear if they represent additional loci or if they are allelic to known genes. For the present study, we have performed a systematic phenotypic characterisation of almost all available tt and tds mutants and built a collection of mutants in the genetic background of the accession Columbia to minimise effects arising from ecotype variation. We have identified a novel tt6 allele from a forward genetic screen and demonstrated that tds3 is allelic to tt12 and tds5 to aha10.

GFS9/TT9 contributes to intracellular membrane trafficking and flavonoid accumulation in Arabidopsis thaliana

Flavonoids are the most important pigments for the coloration of flowers and seeds. In plant cells, flavonoids are synthesized by a multi-enzyme complex located on the cytosolic surface of the endoplasmic reticulum, and they accumulate in vacuoles. Two non-exclusive pathways have been proposed to mediate flavonoid transport to vacuoles: the membrane transporter-mediated pathway and the vesicle trafficking-mediated pathway. No molecules involved in the vesicle trafficking-mediated pathway have been identified, however. Here, we show that a membrane trafficking factor, GFS9, has a role in flavonoid accumulation in the vacuole. We screened a library of Arabidopsis thaliana mutants with defects in vesicle trafficking, and isolated the gfs9 mutant with abnormal pale tan-colored seeds caused by low flavonoid accumulation levels. gfs9 is allelic to the unidentified transparent testa mutant tt9. The responsible gene for these phenotypes encodes a previously uncharacterized protein containing a region that is conserved among eukaryotes. GFS9 is a peripheral membrane protein localized at the Golgi apparatus. GFS9 deficiency causes several membrane trafficking defects, including the mis-sorting of vacuolar proteins, vacuole fragmentation, the aggregation of enlarged vesicles, and the proliferation of autophagosome-like structures. These results suggest that GFS9 is required for vacuolar development through membrane fusion at vacuoles. Our findings introduce a concept that plants use GFS9-mediated membrane trafficking machinery for delivery of not only proteins but also phytochemicals, such as flavonoids, to vacuoles.

Lysosome-related organelles as mediators of metal homeostasis

Metal ion assimilation is essential for all forms of life. However, organisms must properly control the availability of these nutrients within the cell to avoid inactivating proteins by mismetallation. To safeguard against an imbalance between supply and demand in eukaryotes, intracellular compartments contain metal transporters that load and unload metals. Although the vacuoles of Saccharomyces cerevisiae and Arabidopsis thaliana are well established locales for the storage of copper, zinc, iron, and manganese, related compartments are emerging as important mediators of metal homeostasis. Here we describe these compartments and review their metal transporter complement.

Plant vacuole morphology and vacuolar trafficking

Plant vacuoles are essential organelles for plant growth and development, and have multiple functions. Vacuoles are highly dynamic and pleiomorphic, and their size varies depending on the cell type and growth conditions. Vacuoles compartmentalize different cellular components such as proteins, sugars, ions and other secondary metabolites and play critical roles in plants response to different biotic/abiotic signaling pathways. In this review, we will summarize the patterns of changes in vacuole morphology in certain cell types, our understanding of the mechanisms of plant vacuole biogenesis, and the role of SNAREs and Rab GTPases in vacuolar trafficking.

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.

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.

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.

Homotypic vacuole fusion requires VTI11 and is regulated by phosphoinositides

Most plant cells contain a large central vacuole that is essential to maintain cellular turgor. We report a new mutant allele of VTI11 that implicates the SNARE protein VTI11 in homotypic fusion of protein storage and lytic vacuoles. Fusion of the multiple vacuoles present in vti11 mutants could be induced by treatment with Wortmannin and LY294002, which are inhibitors of Phosphatidylinositol 3-Kinase (PI3K). We provide evidence that Phosphatidylinositol 3-Phosphate (PtdIns(3)P) regulates vacuole fusion in vti11 mutants, and that fusion of these vacuoles requires intact microtubules and actin filaments. Finally, we show that Wortmannin also induced the fusion of guard cell vacuoles in fava beans, where vacuoles are naturally fragmented after ABA-induced stomata closure. These results suggest a ubiquitous role of phosphoinositides in vacuole fusion, both during the development of the large central vacuole and during the dynamic vacuole remodeling that occurs as part of stomata movements.

Delivering of proteins to the plant vacuole--an update

Trafficking of soluble cargo to the vacuole is far from being a closed issue as it can occur by different routes and involve different intermediates. The textbook view of proteins being sorted at the post-Golgi level to the lytic vacuole via the pre-vacuole or to the protein storage vacuole mediated by dense vesicles is now challenged as novel routes are being disclosed and vacuoles with intermediate characteristics described. The identification of Vacuolar Sorting Determinants is a key signature to understand protein trafficking to the vacuole. Despite the long established vacuolar signals, some others have been described in the last few years, with different properties that can be specific for some cells or some types of vacuoles. There are also reports of proteins having two different vacuolar signals and their significance is questionable: a way to increase the efficiency of the sorting or different sorting depending on the protein roles in a specific context? Along came the idea of differential vacuolar sorting, suggesting a possible specialization of the trafficking pathways according to the type of cell and specific needs. In this review, we show the recent advances in the field and focus on different aspects of protein trafficking to the vacuoles.

Characterization of the putative amino acid transporter genes AtCAT2, 3 &4: the tonoplast localized AtCAT2 regulates soluble leaf amino acids

The plant vacuole constitutes a large transient storage compartment for nutrients, proteins and metabolites, and is a major cellular sink for toxic waste compounds. Amino acids can cross the vacuolar membrane via specific transport proteins, which are molecularly not well characterized. Two members of a small subfamily of the cationic amino acid transporters, AtCAT2 and AtCAT4, were primarily localized at the tonoplast when tagged with GFP. The closely related AtCAT3, by contrast, was detected in the endoplasmic reticulum membrane. The exchange of a di-acidic motif at the carboxy-tail affected their sub-cellular localization, with larger effects visible in transiently transformed protoplasts compared to stably expressing plant lines. The genes have broad, partially overlapping tissue expression, with CAT2 dominating in most tissues. Loss-of-function mutants of individual CATs showed no visible phenotype under various conditions, but the overall tissue concentration of amino acids was increased in soil-grown cat2 mutants. The data suggest that CAT2 is a critical target of leaf amino acid concentrations and manipulation of this tonoplast transporter can significantly alter total tissue amino acid concentrations.

Control of vacuolar dynamics and regulation of stomatal aperture by tonoplast potassium uptake

Stomatal movements rely on alterations in guard cell turgor. This requires massive K(+) bidirectional fluxes across the plasma and tonoplast membranes. Surprisingly, given their physiological importance, the transporters mediating the energetically uphill transport of K(+) into the vacuole remain to be identified. Here, we report that, in Arabidopsis guard cells, the tonoplast-localized K(+)/H(+) exchangers NHX1 and NHX2 are pivotal in the vacuolar accumulation of K(+) and that nhx1 nhx2 mutant lines are dysfunctional in stomatal regulation. Hypomorphic and complete-loss-of-function double mutants exhibited significantly impaired stomatal opening and closure responses. Disruption of K(+) accumulation in guard cells correlated with more acidic vacuoles and the disappearance of the highly dynamic remodelling of vacuolar structure associated with stomatal movements. Our results show that guard cell vacuolar accumulation of K(+) is a requirement for stomatal opening and a critical component in the overall K(+) homeostasis essential for stomatal closure, and suggest that vacuolar K(+) fluxes are also of decisive importance in the regulation of vacuolar dynamics and luminal pH that underlie stomatal movements.

Autophagy-related direct membrane import from ER/cytoplasm into the vacuole or apoplast: a hidden gateway also for secondary metabolites and phytohormones?

Transportation of low molecular weight cargoes into the plant vacuole represents an essential plant cell function. Several lines of evidence indicate that autophagy-related direct endoplasmic reticulum (ER) to vacuole (and also, apoplast) transport plays here a more general role than expected. This route is regulated by autophagy proteins, including recently discovered involvement of the exocyst subcomplex. Traffic from ER into the vacuole bypassing Golgi apparatus (GA) acts not only in stress-related cytoplasm recycling or detoxification, but also in developmentally-regulated biopolymer and secondary metabolite import into the vacuole (or apoplast), exemplified by storage proteins and anthocyanins. We propose that this pathway is relevant also for some phytohormones’ (e.g., auxin, abscisic acid (ABA) and salicylic acid (SA)) degradation. We hypothesize that SA is not only an autophagy inducer, but also a cargo for autophagy-related ER to vacuole membrane container delivery and catabolism. ER membrane localized enzymes will potentially enhance the area of biosynthetic reactive surfaces, and also, abundant ER localized membrane importers (e.g., ABC transporters) will internalize specific molecular species into the autophagosome biogenesis domain of ER. Such active ER domains may create tubular invaginations of tonoplast into the vacuoles as import intermediates. Packaging of cargos into the ER-derived autophagosome-like containers might be an important mechanism of vacuole and exosome biogenesis and cytoplasm protection against toxic metabolites. A new perspective on metabolic transformations intimately linked to membrane trafficking in plants is emerging.

Assessment and optimization of autophagy monitoring methods in Arabidopsis roots indicate direct fusion of autophagosomes with vacuoles

Autophagy is a degradation pathway that recycles cell materials upon encountering stress conditions or during specific developmental processes. To better understand the physiological roles of autophagy, proper monitoring methods are very important. In mammals and yeast, monitoring of autophagy is often performed with a green fluorescent protein (GFP)-ATG8 fusion protein or with acidotropic dyes such as monodansylcadaverine (MDC) and LysoTracker Red (LTR). To evaluate these monitoring methods, here we examined these systems by inducing autophagy in Arabidopsis thaliana roots as a model for monitoring autophagy in planta. Under carbon- and nitrogen-starved conditions, the number and size of vesicles labeled by GFP-ATG8 was increased for several hours and then gradually decreased to a level higher than that observed before the start of the experiment. We also observed the disappearance of GFP-ATG8-labeled vesicles after treatment with wortmannin, a phosphatidylinositol 3-kinase inhibitor known as an autophagy inhibitor, showing that the GFP-ATG8 transgenic line constitutes an excellent method for monitoring autophagy. These data were compared with plants stained with MDC and LTR. There was no appreciable MDC/LTR staining of small organelles in the root under the induction of autophagy. Some vesicles were eventually observed in the root tip only, but co-localization experiments, as well as experiments with autophagy-deficient atg mutants, provided the evidence that these structures were located in the vacuole and were not manifestly autophagosomes and/or autolysosomes. Extreme caution should therefore be used when monitoring autophagy with the aid of MDC/LTR. Additionally, our observations strongly suggest that autophagosomes fuse directly to vacuoles in Arabidopsis roots.

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.

Multiple vacuoles in impaired tonoplast trafficking3 mutants are independent organelles

Plant vacuoles are essential and dynamic organelles, and mechanisms of vacuole biogenesis and fusion are not well characterized. We recently demonstrated that Wortmannin, an inhibitor of Phosphatidylinositol 3-Kinase (PI3K), induces the fusion of plant vacuoles both in roots of itt3/vti11 mutant alleles and in guard cells of wild type Arabidopsis and Fava bean. Here we used Fluorescence Recovery After Photobleaching (FRAP) to demonstrate that the vacuoles in itt3/vti11 are independent organelles. Furthermore, we used fluorescent protein reporters that bind specifically to Phosphatidylinositol 3-Phosphate (PtdIns(3)P) or PtdIns(4)P to show that Wortmannin treatments that induce the fusion of vti11 vacuoles result in the loss of PtdIns(3)P from cellular membranes. These results provided supporting evidence for a critical role of PtdIns(3)P in vacuole fusion in roots and guard cells.

Involvement of autophagy in the direct ER to vacuole protein trafficking route in plants

Trafficking of proteins from the endoplasmic reticulum (ER) to the vacuole is a fundamental process in plants, being involved both in vacuole biogenesis as well as with plant growth and response to environmental stresses. Although the canonical transport of cellular components from the ER to the vacuole includes the Golgi apparatus as an intermediate compartment, there are multiple lines of evidence that support the existence of a direct ER-to-vacuole, Golgi-independent, trafficking route in plants that uses the autophagy machinery. Plant autophagy was initially described by electron microscopy, visualizing cellular structures that are morphologically reminiscent of autophagosomes. In some of these reports these structures were shown to transport vacuole residing proteins, particularly seed storage proteins, directly from the ER to the vacuole. More recently, following the discovery of the proteins of the core autophagy machinery, molecular tools were implemented in deciphering the involvement of autophagy in this special trafficking route. Here we review the relatively older and more recent scientific observations, supporting the involvement of autophagy in the special cellular trafficking pathways of plants.

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.

Trafficking of endoplasmic reticulum-retained recombinant proteins is unpredictable in Arabidopsis thaliana

A wide variety of recombinant proteins has been produced in the dicot model plant, Arabidopsis thaliana. Many of these proteins are targeted for secretion by means of an N-terminal endoplasmic reticulum (ER) signal peptide. In addition, they can also be designed for ER retention by adding a C-terminal H/KDEL-tag. Despite extensive knowledge of the protein trafficking pathways, the final protein destination, especially of such H/KDEL-tagged recombinant proteins, is unpredictable. In this respect, glycoproteins are ideal study objects. Microscopy experiments reveal their deposition pattern and characterization of their N-glycans aids in elucidating the trafficking. Here, we combine microscopy and N-glycosylation data generated in Arabidopsis leaves and seeds, and highlight the lack of a decent understanding of heterologous protein trafficking.

The dynamic behavior of storage organelles in developing cereal seeds and its impact on the production of recombinant proteins

Cereal endosperm is a highly differentiated tissue containing specialized organelles for the accumulation of storage proteins, which are ultimately deposited either within protein bodies derived from the endoplasmic reticulum, or in protein storage vacuoles (PSVs). During seed maturation endosperm cells undergo a rapid sequence of developmental changes, including extensive reorganization and rearrangement of the endomembrane system and protein transport via several developmentally regulated trafficking routes. Storage organelles have been characterized in great detail by the histochemical analysis of fixed immature tissue samples. More recently, in vivo imaging and the use of tonoplast markers and fluorescent organelle tracers have provided further insight into the dynamic morphology of PSVs in different cell layers of the developing endosperm. This is relevant for biotechnological applications in the area of molecular farming because seed storage organelles in different cereal crops offer alternative subcellular destinations for the deposition of recombinant proteins that can reduce proteolytic degradation, allow control over glycan structures and increase the efficacy of oral delivery. We discuss how the specialized architecture and developmental changes of the endomembrane system in endosperm cells may influence the subcellular fate and post-translational modification of recombinant glycoproteins in different cereal species.

Reprogramming cells to study vacuolar development

During vegetative and embryonic developmental transitions, plant cells are massively reorganized to support the activities that will take place during the subsequent developmental phase. Studying cellular and subcellular changes that occur during these short transitional periods can sometimes present challenges, especially when dealing with Arabidopsis thaliana embryo and seed tissues. As a complementary approach, cellular reprogramming can be used as a tool to study these cellular changes in another, more easily accessible, tissue type. To reprogram cells, genetic manipulation of particular regulatory factors that play critical roles in establishing or repressing the seed developmental program can be used to bring about a change of cell fate. During different developmental phases, vacuoles assume different functions and morphologies to respond to the changing needs of the cell. Lytic vacuoles (LVs) and protein storage vacuoles (PSVs) are the two main vacuole types found in flowering plants such as Arabidopsis. Although both are morphologically distinct and carry out unique functions, they also share some similar activities. As the co-existence of the two vacuole types is short-lived in plant cells, how they replace each other has been a long-standing curiosity. To study the LV to PSV transition, LEAFY COTYLEDON2, a key transcriptional regulator of seed development, was overexpressed in vegetative cells to activate the seed developmental program. At the cellular level, Arabidopsis leaf LVs were observed to convert to PSV-like organelles. This presents the opportunity for further research to elucidate the mechanism of LV to PSV transitions. Overall, this example demonstrates the potential usefulness of cellular reprogramming as a method to study cellular processes that occur during developmental transitions.