Storage globulins pass through the Golgi apparatus and multivesicular bodies in the absence of dense vesicle formation during early stages of cotyledon development in mung bean

The LCG1-OsBP5/OsEBP89-Wx module regulates the grain chalkiness and taste quality in rice

It is well known that the overall quality of japonica/geng rice is superior to that of indica/xian rice varieties. However, the molecular mechanisms underlying the quality disparities between these two subspecies of rice are still largely unknown. In this study, we have pinpointed a gene homologous to SLR1, termed LCG1, exhibiting significant expression during early caryopsis development and playing a specific role in regulating rice chalkiness and taste by affecting the accumulation of grain storage components, starch granule structure and chain length distribution of amylopectin. LCG1 physically interacts with OsBP5 and indirectly influences the expression of the amylose synthesis gene Waxy (Wx) by hindering the transcriptional activity of the OsBP5/OsEBP89 complex. Notably, sequence variations in the promoter region of LCG1 result in enhanced transcription in japonica rice accessions. This leads to elevated LCG1 expression in CSSL-LCG1Nip, thereby enhancing rice quality. Our research elucidates the molecular mechanism underlying the impact of the LCG1-OsBP5/OsEBP89-Wx regulatory pathway on rice chalkiness and taste quality, offering new genetic resources for improving the indica rice quality.

Plant ESCRT protein ALIX coordinates with retromer complex in regulating receptor-mediated sorting of soluble vacuolar proteins

The endosomal sorting complex required for transport (ESCRT) machinery in multicellular organisms plays canonical functions in multivesicular body (MVB) biogenesis and membrane protein sorting. Nonetheless, its critical role in the sorting of soluble vacuolar proteins and its interplay with endosomal recycling machinery have yet to be reported. In this study, we demonstrate that Arabidopsis ESCRT-associated ALIXinteracts with the retromer core subunitsto regulate their recruitment onto endosome membrane for recycling of vacuolar sorting receptors (VSRs) for efficient sorting of soluble vacuolar proteins. This work provides molecular insights into the unique properties of ALIX in regulating vacuolar transport of soluble proteins, thus shedding new light on the crosstalk and coordination between the vacuolar trafficking and endosomal recycling pathways in plants.

Vacuolar proteins play essential roles in plant physiology and development, but the factors and the machinery regulating their vesicle trafficking through the endomembrane compartments remain largely unknown. We and others have recently identified an evolutionarily conserved plant endosomal sorting complex required for transport (ESCRT)-associated protein apoptosis-linked gene-2 interacting protein X (ALIX), which plays canonical functions in the biogenesis of the multivesicular body/prevacuolar compartment (MVB/PVC) and in the sorting of ubiquitinated membrane proteins. In this study, we elucidate the roles and underlying mechanism of ALIX in regulating vacuolar transport of soluble proteins, beyond its conventional ESCRT function in eukaryotic cells. We show that ALIX colocalizes and physically interacts with the retromer core subunits Vps26 and Vps29 in planta. Moreover, double-mutant analysis reveals the genetic interaction of ALIX with Vps26 and Vps29 for regulating trafficking of soluble vacuolar proteins. Interestingly, depletion of ALIX perturbs membrane recruitment of Vps26 and Vps29 and alters the endosomal localization of vacuolar sorting receptors (VSRs). Taken together, ALIX functions as a unique retromer core subcomplex regulator by orchestrating receptor-mediated vacuolar sorting of soluble proteins.

Subunit E isoform 1 of vacuolar H+-ATPase OsVHA enables post-Golgi trafficking of rice seed storage proteins

Dense vesicles (DVs) are Golgi-derived plant-specific carriers that mediate post-Golgi transport of seed storage proteins in angiosperms. How this process is regulated remains elusive. Here, we report a rice (Oryza sativa) mutant, named glutelin precursor accumulation8 (gpa8) that abnormally accumulates 57-kDa proglutelins in the mature endosperm. Cytological analyses of the gpa8 mutant revealed that proglutelin-containing DVs were mistargeted to the apoplast forming electron-dense aggregates and paramural bodies in developing endosperm cells. Differing from previously reported gpa mutants with post-Golgi trafficking defects, the gpa8 mutant showed bent Golgi bodies, defective trans-Golgi network (TGN), and enlarged DVs, suggesting a specific role of GPA8 in DV biogenesis. We demonstrated that GPA8 encodes a subunit E isoform 1 of vacuolar H+-ATPase (OsVHA-E1) that mainly localizes to TGN and the tonoplast. Further analysis revealed that the luminal pH of the TGN and vacuole is dramatically increased in the gpa8 mutant. Moreover, the colocalization of GPA1 and GPA3 with TGN marker protein in gpa8 protoplasts was obviously decreased. Our data indicated that OsVHA-E1 is involved in endomembrane luminal pH homeostasis, as well as maintenance of Golgi morphology and TGN required for DV biogenesis and subsequent protein trafficking in rice endosperm cells.

Antioxidant Potential of Mung Bean (Vigna radiata) Albumin Peptides Produced by Enzymatic Hydrolysis Analyzed by Biochemical and In Silico Methods

The objective of this study was to investigate the biochemical antioxidant potential of peptides derived from enzymatically hydrolyzed mung bean (Vigna radiata) albumins using an 2,2'-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging assay, a ferrous ion chelating assay and an oxygen radical absorbance capacity (ORAC) assay. Peeled raw mung bean was ground into flour and mixed with buffer (pH 8.3, 1:20 w/v ratio) before being stirred, then filtered using 3 kDa and 30 kDa molecular weight cut-off (MWCO) centrifugal filters to obtain albumin fraction. The albumin fraction then underwent enzymatic hydrolysis using either gastrointestinal enzymes (pepsin and pancreatin) or thermolysin. Peptides in the hydrolysates were sequenced. The peptides showed low ABTS radical-scavenging activity (90-100 μg ascorbic acid equivalent/mL) but high ferrous ion chelating activity (1400-1500 μg EDTA equivalent/mL) and ORAC values (>120 μM Trolox equivalent). The ferrous ion chelating activity was enzyme- and hydrolysis time-dependent. For thermolysin hydrolysis, there was a drastic increase in ferrous ion chelating activity from t = 0 (886.9 μg EDTA equivalent/mL) to t = 5 min (1559.1 μg EDTA equivalent/mL) before plateauing. For pepsin-pancreatin hydrolysis, there was a drastic decrease from t = 0 (878.3 μg EDTA equivalent/mL) to t = 15 (138.0 μg EDTA equivalent/mL) after pepsin was added, but this increased from t = 0 (131.1 μg EDTA equivalent/mL) to t = 15 (1439.2 μg EDTA equivalent/mL) after pancreatin was added. There was no significant change in ABTS radical scavenging activity or ORAC values throughout different hydrolysis times for either the thermolysin or pepsin-pancreatin hydrolysis. Overall, mung bean hydrolysates produced peptides with high potential antioxidant capacity, being particularly effective ferrous ion chelators. Other antioxidant assays that use cellular lines should be performed to measure antioxidant capacity before animal and human studies.

Mung bean proteins and peptides: nutritional, functional and bioactive properties

To date, no extensive literature review exists regarding potential uses of mung bean proteins and peptides. As mung bean has long been widely used as a food source, early studies evaluated mung bean nutritional value against the Food and Agriculture Organization of the United Nations (FAO)/the World Health Organization (WHO) amino acids dietary recommendations. The comparison demonstrated mung bean to be a good protein source, except for deficiencies in sulphur-containing amino acids, methionine and cysteine. Methionine and cysteine residues have been introduced into the 8S globulin through protein engineering technology. Subsequently, purified mung bean proteins and peptides have facilitated the study of their structural and functional properties. Two main types of extraction methods have been reported for isolation of proteins and peptides from mung bean flours, permitting sequencing of major proteins present in mung bean, including albumins and globulins (notably 8S globulin). However, the sequence for albumin deposited in the UniProt database differs from other sequences reported in the literature. Meanwhile, a limited number of reports have revealed other useful bioactivities for proteins and hydrolysed peptides, including angiotensin-converting enzyme inhibitory activity, anti-fungal activity and trypsin inhibitory activity. Consequently, several mung bean hydrolysed peptides have served as effective food additives to prevent proteolysis during storage. Ultimately, further research will reveal other nutritional, functional and bioactive properties of mung bean for uses in diverse applications.

Trafficking routes to the plant vacuole: connecting alternative and classical pathways

Due to the numerous roles plant vacuoles play in cell homeostasis, detoxification, and protein storage, the trafficking pathways to this organelle have been extensively studied. Recent evidence, however, suggests that our vision of transport to the vacuole is not as simple as previously imagined. Alternative routes have been identified and are being characterized. Intricate interconnections between routes seem to occur in various cases, complicating the interpretation of data. In this review, we aim to summarize the published evidence and link the emerging data with previous findings. We discuss the current state of information on alternative and classical trafficking routes to the plant vacuole.

Multiple internal sorting determinants can contribute to the trafficking of cruciferin to protein storage vacuoles

Trafficking of seed storage proteins to protein storage vacuoles is mediated by carboxy terminal and internal sorting determinants (ISDs). Protein modelling was used to identify candidate ISDs residing near surface-exposed regions in Arabidopsis thaliana cruciferin A (AtCruA). These were verified by AtCruA fusion to yellow fluorescent protein (YFP) and expression in developing embryos of A. thaliana. As the presence of endogenous cruciferin was found to mask the effects of weaker ISDs, experiments were conducted in a line that was devoid of cruciferin. In total, nine ISDs were discovered and a core determinant defined using a series of alanine scanning and deletion mutant variants. Coupling of functional data from AtCruA ISD-YFP fusions with statistical analysis of the physiochemical properties of analogous regions from several 11/12S globulins revealed that cruciferin ISDs likely adhere to the following rules: (1) ISDs are adjacent to or within hydrophilic, surface-exposed regions that serve to present them on the protein's surface; (2) ISDs generally have a hydrophobic character; (3) ISDs tend to have Leu or Ile residues at their core; (4) ISDs are approximately eight amino acids long with the physiochemical consensus [hydrophobic][preferably charged][small or hydrophobic, but not tiny][IL][polar, preferably charged][small, but not charged][hydrophobic, not charged, preferably not polar][hydrophobic, not tiny, preferably not polar]. Microscopic evidence is also presented for the presence of an interconnected protein storage vacuolar network in embryo cells, rather than discreet, individual vacuoles.

The Arabidopsis Endosomal Sorting Complex Required for Transport III Regulates Internal Vesicle Formation of the Prevacuolar Compartment and Is Required for Plant Development

We have established an efficient transient expression system with several vacuolar reporters to study the roles of endosomal sorting complex required for transport (ESCRT)-III subunits in regulating the formation of intraluminal vesicles of prevacuolar compartments (PVCs)/multivesicular bodies (MVBs) in plant cells. By measuring the distributions of reporters on/within the membrane of PVC/MVB or tonoplast, we have identified dominant negative mutants of ESCRT-III subunits that affect membrane protein degradation from both secretory and endocytic pathways. In addition, induced expression of these mutants resulted in reduction in luminal vesicles of PVC/MVB, along with increased detection of membrane-attaching vesicles inside the PVC/MVB. Transgenic Arabidopsis (Arabidopsis thaliana) plants with induced expression of ESCRT-III dominant negative mutants also displayed severe cotyledon developmental defects with reduced cell size, loss of the central vacuole, and abnormal chloroplast development in mesophyll cells, pointing out an essential role of the ESCRT-III complex in postembryonic development in plants. Finally, membrane dissociation of ESCRT-III components is important for their biological functions and is regulated by direct interaction among Vacuolar Protein Sorting-Associated Protein20-1 (VPS20.1), Sucrose Nonfermenting7-1, VPS2.1, and the adenosine triphosphatase VPS4/SUPPRESSOR OF K⁺ TRANSPORT GROWTH DEFECT1.

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.

GLUTELIN PRECURSOR ACCUMULATION3 encodes a regulator of post-Golgi vesicular traffic essential for vacuolar protein sorting in rice endosperm

In seed plants, a major pathway for sorting of storage proteins to the protein storage vacuole (PSV) depends on the Golgi-derived dense vesicles (DVs). However, the molecular mechanisms regulating the directional trafficking of DVs to PSVs remain largely elusive. Here, we report the functional characterization of the rice (Oryza sativa) glutelin precursor accumulation3 (gpa3) mutant, which exhibits a floury endosperm phenotype and accumulates excess proglutelins in dry seeds. Cytological and immunocytochemistry studies revealed that in the gpa3 mutant, numerous proglutelin-containing DVs are misrouted to the plasma membrane and, via membrane fusion, release their contents into the apoplast to form a new structure named the paramural body. Positional cloning of GPA3 revealed that it encodes a plant-specific kelch-repeat protein that is localized to the trans-Golgi networks, DVs, and PSVs in the developing endosperm. In vitro and in vivo experiments verified that GPA3 directly interacts with the rice Rab5a-guanine exchange factor VPS9a and forms a regulatory complex with Rab5a via VPS9a. Furthermore, our genetic data support the notion that GPA3 acts synergistically with Rab5a and VPS9a to regulate DV-mediated post-Golgi traffic in rice. Our findings provide insights into the molecular mechanisms regulating the plant-specific PSV pathway and expand our knowledge of vesicular trafficking in eukaryotes.

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.

Apical F-actin-regulated exocytic targeting of NtPPME1 is essential for construction and rigidity of the pollen tube cell wall

In tip-confined growing pollen tubes, delivery of newly synthesized cell wall materials to the rapidly expanding apical surface requires spatial organization and temporal regulation of the apical F-actin filament and exocytosis. In this study, we demonstrate that apical F-actin is essential for the rigidity and construction of the pollen tube cell wall by regulating exocytosis of Nicotiana tabacum pectin methylesterase (NtPPME1). Wortmannin disrupts the spatial organization of apical F-actin in the pollen tube tip and inhibits polar targeting of NtPPME1, which subsequently alters the rigidity and pectic composition of the pollen tube cell wall, finally causing growth arrest of the pollen tube. In addition to mechanistically linking cell wall construction and apical F-actin, wortmannin can be used as a useful tool for studying endomembrane trafficking and cytoskeletal organization in pollen tubes.

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

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

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

Successful transport to the vacuole of heterologously expressed mung bean 8S globulin occurs in seed but not in vegetative tissues

This study investigated the subcellular location of mung bean (Vigna radiata) 8S globulin in transient expression systems as well as in tobacco (Nicotiana tabacum) BY-2 cells and different tissues from a transgenic Arabidopsis (Arabidopsis thaliana) line stably expressing this storage globulin. When transiently expressed in protoplasts from both BY-2 cells and Arabidopsis suspension cultured cells, the 8S globulin located to structures that were neither Golgi nor pre-vacuolar compartments (PVCs). Immunogold electron microscopy of the transgenics reveals the 8S globulin-positive structures to be small, spherical, ribosome-covered endoplasmic reticulum (ER)-derived bodies. In BY-2 cells and all vegetative cells, the 8S globulin was present as a pro-form. However, in Arabidopsis embryos, with the onset of endogenous storage protein synthesis, the 8S globulin exited the ER and passed through the PVC to the protein storage vacuole where it was processed to its smaller mature form. These results clearly demonstrated that, when taken out of context and expressed in vegetative cells, the mung bean 8S storage globulin cannot exit the ER, and indicate that natural targeting of storage proteins to the vacuole should be better studied in the maturing seed.

Unconventional protein secretion

It is generally believed that protein secretion or exocytosis is achieved via a conventional ER (endoplasmic reticulum)-Golgi-TGN (trans-Golgi network)-PM (plasma membrane) pathway in the plant endomembrane system. However, such signal peptide (SP)-dependent protein secretion cannot explain the increasing number of SP-lacking proteins which are found outside of the PM in plant cells. The process by which such leaderless secretory proteins (LSPs) gain access to the cell exterior is termed unconventional protein secretion (UPS) and has been well-studied in animal and yeast cells, but largely ignored by the plant community. Here, we review the evidence for UPS in plants especially in regard to the recently discovered EXPO (exocyst-positive-organelle).

The Golgi-localized Arabidopsis endomembrane protein12 contains both endoplasmic reticulum export and Golgi retention signals at its C terminus

Endomembrane proteins (EMPs), belonging to the evolutionarily conserved transmembrane nine superfamily in yeast and mammalian cells, are characterized by the presence of a large lumenal N terminus, nine transmembrane domains, and a short cytoplasmic tail. The Arabidopsis thaliana genome contains 12 EMP members (EMP1 to EMP12), but little is known about their protein subcellular localization and function. Here, we studied the subcellular localization and targeting mechanism of EMP12 in Arabidopsis and demonstrated that (1) both endogenous EMP12 (detected by EMP12 antibodies) and green fluorescent protein (GFP)-EMP12 fusion localized to the Golgi apparatus in transgenic Arabidopsis plants; (2) GFP fusion at the C terminus of EMP12 caused mislocalization of EMP12-GFP to reach post-Golgi compartments and vacuoles for degradation in Arabidopsis cells; (3) the EMP12 cytoplasmic tail contained dual sorting signals (i.e., an endoplasmic reticulum export motif and a Golgi retention signal that interacted with COPII and COPI subunits, respectively); and (4) the Golgi retention motif of EMP12 retained several post-Golgi membrane proteins within the Golgi apparatus in gain-of-function analysis. These sorting signals are highly conserved in all plant EMP isoforms and, thus, likely represent a general mechanism for EMP targeting in plant cells.

Mechanisms and concepts paving the way towards a complete transport cycle of plant vacuolar sorting receptors

Delivery of proteins to the lytic vacuole in plants is a complex cascade of selective interactions that specifically excludes residents of the endoplasmic reticulum and secreted proteins. Vacuolar transport must be highly efficient to avoid mistargeting of hydrolytic enzymes to locations where they could be harmful. While plant vacuolar sorting signals have been well described for two decades, it is only during the last 5 years that a critical mass of data was gathered that begins to reveal how vacuolar sorting receptors (VSRs) may complete a full transport cycle. Yet, the field is far from reaching a consensus regarding the organelles that could be involved in vacuolar sorting, their potential biogenesis, and the ultimate recycling of membranes and protein machinery that maintain this pathway. This review will highlight the important landmarks in our understanding of VSR function and compare recent transport models that have been proposed so that an emerging picture of plant vacuolar sorting mechanisms can be drawn.

Multivesicular bodies in developing tobacco seed and mung bean are functionally equivalent

Protein storage vacuoles (PSVs) are the primarily storage organelles in cotyledon cells for protein preservation in seeds. Storage proteins are transported from the endoplasmic reticulum (ER) to the Golgi apparatus for subsequent delivery to PSVs via presumably Golgi-derived dense vesicles (DVs). However, recent studies demonstrated that storage proteins in early stage of developing cotyledon of mung beans reached the multivesicular bodies (MVBs) prior to the detection of DVs, indicating the possible involvement of MVBs in mediating transport of storage proteins during the early stage of seed development. Here, we further show that the MVBs in developing tobacco seeds are functionally and biochemically equivalent to those in developing mung beans. Thus, MVBs in developing tobacco seeds are structurally distinct from DVs, contain both vacuolar sorting receptors (VSRs) and storage proteins, and they are insensitive to treatments of wortmannin and brefeldin A (BFA).