The changing fate of a secretory glycoprotein in developing maize endosperm

Imaging the ER and Endomembrane System in Cereal Endosperm

The cereal endosperm is a complex structure comprising distinct cell types, characterized by specialized organelles for the accumulation of storage proteins. Protein trafficking in these cells is complicated by the presence of several different storage organelles including protein bodies (PBs) derived from the endoplasmic reticulum (ER) and dynamic protein storage vacuoles (PSVs). In addition, trafficking may follow a number of different routes depending on developmental stage, showing that the endomembrane system is capable of massive reorganization. Thus, developmental sequences involve progressive changes of the endomembrane system of endosperm tissue and are characterized by a high structural plasticity and endosomal activity.Given the technical dexterity required to access endosperm tissue and study subcellular structures and SSP trafficking in cereal seeds, static images are the state of the art providing a bulk of information concerning the cellular composition of seed tissue. In view of the highly dynamic endomembrane system in cereal endosperm cells, it is reasonable to expect that live cell imaging will help to characterize the spatial and temporal changes of the endomembrane system. The high resolution achieved with electron microscopy perfectly complements the live cell imaging.We therefore established an imaging platform for TEM as well as for live cell imaging. Here, we describe the preparation of different cereal seed tissues for live cell imaging concomitant with immunolocalization studies and ultrastructure.

Generation of multi-layered protein bodies in N. benthamiana for the encapsulation of vaccine antigens

The ability of plants to assemble particulate structures such as virus-like particles and protein storage organelles allows the direct bioencapsulation of recombinant proteins during the manufacturing process, which holds promise for the development of new drug delivery vehicles. Storage organelles found in plants such as protein bodies (PBs) have been successfully used as tools for accumulation and encapsulation of recombinant proteins. The fusion of sequences derived from 27-kDa-γ-zein, a major storage protein of maize, with a protein of interest leads to the incorporation of the chimeric protein into the stable and protected environment inside newly induced PBs. While this procedure has proven successful for several, but not all recombinant proteins, the aim of this study was to refine the technology by using a combination of PB-forming proteins, thereby generating multi-layered protein assemblies in N. benthamiana. We used fluorescent proteins to demonstrate that up to three proteinaceous components can be incorporated into different layers. In addition to 27-kDa-γ-zein, which is essential for PB initiation, 16-kDa-γ-zein was identified as a key element to promote the incorporation of a third zein-component into the core of the PBs. We show that a vaccine antigen could be incorporated into the matrix of multi-layered PBs, and the protein microparticles were characterized by confocal and electron microscopy as well as flow cytometry. In future, this approach will enable the generation of designer PBs that serve as drug carriers and integrate multiple components that can be functionalized in different ways.

Multiscale imaging reveals the presence of autophagic vacuoles in developing maize endosperm

Cereal endosperm is solely devoted to the storage of proteins and starch that will be used by the embryo upon germination. The high degree of specialization of this tissue is reflected in its endomembrane system, in which ER derived protein bodies and protein storage vacuoles (PSVs) are of particular interest. In maize seeds, the main storage proteins are zeins, that form transport incompetent aggregates within the ER lumen and finally build protein bodies that bud from the ER. In contrast to the zeins, the maize globulins are not very abundant and the vacuolar storage compartment of maize endosperm is not fully described. Whereas in other cereals, including wheat and barley, the PSV serves as the main protein storage compartment, only small, globulin-containing PSVs have been identified in maize so far. We present here a multi-scale set of data, ranging from live-cell imaging to more sophisticated 3D electron microscopy techniques (SBF-SEM), that has allowed us to investigate in detail the vacuoles in maize endosperm cells, including a novel, autophagic vacuole that is present in early developmental stages.

Maize Endosperm Development: Tissues, Cells, Molecular Regulation and Grain Quality Improvement

Maize endosperm plays important roles in human diet, animal feed and industrial applications. Knowing the mechanisms that regulate maize endosperm development could facilitate the improvement of grain quality. This review provides a detailed account of maize endosperm development at the cellular and histological levels. It features the stages of early development as well as developmental patterns of the various individual tissues and cell types. It then covers molecular genetics, gene expression networks, and current understanding of key regulators as they affect the development of each tissue. The article then briefly considers key changes that have occurred in endosperm development during maize domestication. Finally, it considers prospects for how knowledge of the regulation of endosperm development could be utilized to enhance maize grain quality to improve agronomic performance, nutrition and economic value.

Progressive Aggregation of 16 kDa Gamma-Zein during Seed Maturation in Transgenic Arabidopsis thaliana

Prolamins constitute a unique class of seed storage proteins, present only in grasses. In the lumen of the endoplasmic reticulum (ER), prolamins form large, insoluble heteropolymers termed protein bodies (PB). In transgenic Arabidopsis (Arabidopsis thaliana) leaves, the major maize (Zea mays) prolamin, 27 kDa γ-zein (27γz), assembles into insoluble disulfide-linked polymers, as in maize endosperm, forming homotypic PB. The 16 kDa γ-zein (16γz), evolved from 27γz, instead forms disulfide-bonded dispersed electron-dense threads that enlarge the ER lumen without assembling into PB. We have investigated whether the peculiar features of 16γz are also maintained during transgenic seed development. We show that 16γz progressively changes its electron microscopy appearance during transgenic Arabidopsis embryo maturation, from dispersed threads to PB-like, compact structures. In mature seeds, 16γz and 27γz PBs appear very similar. However, when mature embryos are treated with a reducing agent, 27γz is fully solubilized, as expected, whereas 16γz remains largely insoluble also in reducing conditions and drives insolubilization of the ER chaperone BiP. These results indicate that 16γz expressed in the absence of the other zein partners forms aggregates in a storage tissue, strongly supporting the view that 16γz behaves as the unassembled subunit of a large heteropolymer, the PB, and could have evolved successfully only following the emergence of the much more structurally self-sufficient 27γz.

Sweet Modifications Modulate Plant Development

Plant development represents a continuous process in which the plant undergoes morphological, (epi)genetic and metabolic changes. Starting from pollination, seed maturation and germination, the plant continues to grow and develops specialized organs to survive, thrive and generate offspring. The development of plants and the interplay with its environment are highly linked to glycosylation of proteins and lipids as well as metabolism and signaling of sugars. Although the involvement of these protein modifications and sugars is well-studied, there is still a long road ahead to profoundly comprehend their nature, significance, importance for plant development and the interplay with stress responses. This review, approached from the plants' perspective, aims to focus on some key findings highlighting the importance of glycosylation and sugar signaling for plant development.

Seed-produced anti-globulin VHH-Fc antibodies retrieve globulin precursors in the insoluble fraction and modulate the Arabidopsis thaliana seed subcellular morphology

Nanobody-heavy chain (VHH-Fc) antibody formats have the potential to immunomodulate even highly accumulating proteins and provide a valuable tool to experimentally modulate the subcellular distribution of seed storage proteins. Recombinant antibodies often obtain high accumulation levels in plants, and thus, besides being the actual end-product, antibodies targeting endogenous host proteins can be used to interfere with the localization and functioning of their corresponding antigens. Here, we compared the effect of a seed-expressed nanobody-heavy chain (VHH-Fc) antibody against the highly abundant Arabidopsis thaliana globulin seed storage protein cruciferin with that of a VHH-Fc antibody without endogenous target. Both antibodies reached high accumulation levels of around 10% of total soluble protein, but strikingly, another significant part was present in the insoluble protein fraction and was recovered only after extraction under denaturing conditions. In seeds containing the anti-cruciferin antibodies but not the antibody without endogenous target, the amount of soluble, processed globulin subunits was severely reduced and a major part of the cruciferin molecules was found as precursor in the insoluble fraction. Moreover, in these seeds, aberrant vacuolar phenotypes were observed that were different from the effects caused by the depletion of globulins in knock-out seeds. Remarkably, the seeds with strongly reduced globulin amounts are fully viable and germinate with frequencies similar to wild type, illustrating how flexible seeds can retrieve amino acids from the stored proteins to start germination.

3D Electron Microscopy Gives a Clue: Maize Zein Bodies Bud From Central Areas of ER Sheets

Zeins are the main storage proteins in maize seed endosperm, and the onset of zein synthesis in young seeds challenges the endomembrane system and results in the formation of storage organelles. Even though zeins lack a conventional endoplasmic reticulum (ER) retention signal, they accumulate within the ER and assemble in conspicuous ER-derived protein bodies (PBs) stabilized by disulfide bridge formation and hydrophobic interaction between zein chains. Zein body formation during seed development has been extensively studied, as well as the mechanisms that lead to the initiation of PBs. However, the exact course of the PB formation process and the spatial relationship with the ER remain unclear. The development of serial block face scanning electron microscopy (SBF-SEM) techniques that allow three-dimensional imaging combined with the high resolution of electron microscopy provides new perspectives on the study of the plant endomembrane system. Here, we demonstrate that (i) the ER of maize seeds is mainly formed by massive sheets and (ii) PBs are not budding from tubules or the edge of sheets, but protrude from the entire surface of the ER sheet.

Towards coeliac-safe bread

Gluten-free foods cannot substitute for products made from wheat flour. When wheat products are digested, the remaining peptides can trigger an autoimmune disease in 1% of the North American and European population, called coeliac disease. Because wheat proteins are encoded by a large gene family, it has been impossible to use conventional breeding to select wheat varieties that are coeliac-safe. However, one can test the properties of protein variants by expressing single genes in coeliac-safe cereals like maize. One source of protein that can be considered as coeliac-safe and has bread-making properties is teff (Eragrostis tef), a grain consumed in Ethiopia. Here, we show that teff α-globulin3 (Etglo3) forms storage vacuoles in maize that are morphologically similar to those of wheat. Using transmission electron microscopy, immunogold labelling shows that Etglo3 is almost exclusively deposited in the storage vacuole as electron-dense aggregates. Of maize seed storage proteins, 27-kDa γ-zein is co-deposited with Etglo3. Etglo3 polymerizes via intermolecular disulphide bonds in maize, similar to wheat HMW glutenins under non-reducing conditions. Crossing maize Etglo3 transgenic lines with α-, β- and γ-zein RNA interference (RNAi) lines reveals that Etglo3 accumulation is only dramatically reduced in γ-zein RNAi background. This suggests that Etglo3 and 27-kDa γ-zein together cause storage vacuole formation and behave similar to the interactions of glutenins and gliadins in wheat. Therefore, expression of teff α-globulins in maize presents a major step in the development of a coeliac-safe grain with bread-making properties.

Protein sorting into protein bodies during barley endosperm development is putatively regulated by cytoskeleton members, MVBs and the HvSNF7s

Cereal endosperm is a short-lived tissue adapted for nutrient storage, containing specialized organelles, such as protein bodies (PBs) and protein storage vacuoles (PSVs), for the accumulation of storage proteins. During development, protein trafficking and storage require an extensive reorganization of the endomembrane system. Consequently, endomembrane-modifying proteins will influence the final grain quality and yield. However, little is known about the molecular mechanism underlying endomembrane system remodeling during barley grain development. By using label-free quantitative proteomics profiling, we quantified 1,822 proteins across developing barley grains. Based on proteome annotation and a homology search, 94 proteins associated with the endomembrane system were identified that exhibited significant changes in abundance during grain development. Clustering analysis allowed characterization of three different development phases; notably, integration of proteomics data with in situ subcellular microscopic analyses showed a high abundance of cytoskeleton proteins associated with acidified PBs at the early development stages. Moreover, endosomal sorting complex required for transport (ESCRT)-related proteins and their transcripts are most abundant at early and mid-development. Specifically, multivesicular bodies (MVBs), and the ESCRT-III HvSNF7 proteins are associated with PBs during barley endosperm development. Together our data identified promising targets to be genetically engineered to modulate seed storage protein accumulation that have a growing role in health and nutritional issues.

ESCRTing in cereals: still a long way to go

The multivesicular body (MVB) sorting pathway provides a mechanism for the delivery of cargo destined for degradation to the vacuole or lysosome. The endosomal sorting complex required for transport (ESCRT) is essential for the MVB sorting pathway by driving the cargo sorting to its destination. Many efforts in plant research have identified the ESCRT machinery and functionally characterised the first plant ESCRT proteins. However, most studies have been performed in the model plant Arabidopsis thaliana that is genetically and physiologically different to crops. Cereal crops are important for animal feed and human nutrition and have further been utilized as promising candidates for recombinant protein production. In this review, I summarize the role of plant ESCRT components in cereals that are involved in efficient adaptation to environmental stress and grain development. A special focus is on barley (Hordeum vulgare L.) ESCRT proteins, where recent studies show their quantitative mapping during grain development, e.g. associating HvSNF7.1 with protein trafficking to protein bodies (PBs) in starchy endosperm. Thus, it is indispensable to identify the molecular key-players within the endomembrane system including ESCRT proteins to optimize and possibly enhance tolerance to environmental stress, grain yield and recombinant protein production in cereal grains.

Maize 16-kD γ-zein forms very unusual disulfide-bonded polymers in the endoplasmic reticulum: implications for prolamin evolution

In the lumen of the endoplasmic reticulum (ER), prolamin storage proteins of cereal seeds form very large, ordered heteropolymers termed protein bodies (PBs), which are insoluble unless treated with alcohol or reducing agents. In maize PBs, 16-kD γ-zein locates at the interface between a core of alcohol-soluble α-zeins and the outermost layer mainly composed of the reduced-soluble 27-kD γ-zein. 16-kD γ-zein originates from 27-kD γ-zein upon whole-genome duplication and is mainly characterized by deletions in the N-terminal domain that eliminate most Pro-rich repeats and part of the Cys residues involved in inter-chain bonds. 27-kD γ-zein also forms insoluble PBs when expressed in transgenic vegetative tissues. We show that in Arabidopsis leaves, 16-kD γ-zein assembles into disulfide-linked polymers that fail to efficiently become insoluble. Instead of forming PBs, these polymers accumulate as very unusual threads that markedly enlarge the ER lumen, resembling amyloid-like fibers. Domain-swapping between the two γ-zeins indicates that the N-terminal region of 16-kD γ-zein has a dominant effect in preventing full insolubilization. Therefore, a newly evolved prolamin has lost the ability to form homotypic PBs, and has acquired a new function in the assembly of natural, heteropolymeric PBs.

Imaging the ER and Endomembrane System in Cereal Endosperm

The cereal endosperm is a complex structure comprising distinct cell types, characterized by specialized organelles for the accumulation of storage proteins. Protein trafficking in these cells is complicated by the presence of several different storage organelles including protein bodies (PBs) derived from the endoplasmic reticulum (ER) and dynamic protein storage vacuoles (PSVs). In addition, trafficking may follow a number of different routes depending on developmental stage, showing that the endomembrane system is capable of massive reorganization. Thus, developmental sequences involve progressive changes of the endomembrane system of endosperm tissue and are characterized by a high structural plasticity and endosomal activity.Given the technical dexterity required to access endosperm tissue and study subcellular structures and (seed storage protein) SSP trafficking in cereal seeds, static images are the state of the art providing a bulk of information concerning the cellular composition of seed tissue. In view of the highly dynamic endomembrane system in cereal endosperm cells, it is reasonable to expect that live cell imaging will help to characterize the spatial and temporal changes of the system. The high resolution achieved with electron microscopy perfectly complements the live cell imaging.We therefore established an imaging platform for TEM as well as for live cell imaging. Here, we describe the preparation of different cereal seed tissues for live cell imaging concomitant with immunolocalization studies and ultrastructure.

Glyco-Engineering of Plant-Based Expression Systems

Most secreted proteins in eukaryotes are glycosylated, and after a number of common biosynthesis steps the glycan structures mature in a species-dependent manner. Therefore, human therapeutic proteins produced in plants often carry plant-like rather than human-like glycans, which can affect protein stability, biological function, and immunogenicity. The glyco-engineering of plant-based expression systems began as a strategy to eliminate plant-like glycans and produce human proteins with authentic or at least compatible glycan structures. The precise replication of human glycans is challenging, owing to the absence of a pathway in plants for the synthesis of sialylated proteins and the necessary precursors, but this can now be achieved by the coordinated expression of multiple human enzymes. Although the research community has focused on the removal of plant glycans and their replacement with human counterparts, the presence of plant glycans on proteins can also provide benefits, such as boosting the immunogenicity of some vaccines, facilitating the interaction between therapeutic proteins and their receptors, and increasing the efficacy of antibody effector functions. Graphical Abstract Typical structures of native mammalian and plant glycans with symbols indicating sugar residues identified by their short form and single-letter codes. Both glycans contain fucose, albeit with different linkages.

Transgenic Production of an Anti HIV Antibody in the Barley Endosperm

Barley is an attractive vehicle for producing recombinant protein, since it is a readily transformable diploid crop species in which doubled haploids can be routinely generated. High amounts of protein are naturally accumulated in the grain, but optimal endosperm-specific promoters have yet to be perfected. Here, the oat GLOBULIN1 promoter was combined with the legumin B4 (LeB4) signal peptide and the endoplasmic reticulum (ER) retention signal (SE)KDEL. Transgenic barley grain accumulated up to 1.2 g/kg dry weight of recombinant protein (GFP), deposited in small roundish compartments assumed to be ER-derived protein bodies. The molecular farming potential of the system was tested by generating doubled haploid transgenic lines engineered to synthesize the anti-HIV-1 monoclonal antibody 2G12 with up to 160 μg recombinant protein per g grain. The recombinant protein was deposited at the periphery of protein bodies in the form of a mixture of various N-glycans (notably those lacking terminal N-acetylglucosamine residues), consistent with their vacuolar localization. Inspection of protein-A purified antibodies using surface plasmon resonance spectroscopy showed that their equilibrium and kinetic rate constants were comparable to those associated with recombinant 2G12 synthesized in Chinese hamster ovary cells.

Review/N-glycans: The making of a varied toolbox

Asparagine (N)-linked protein glycosylation is one of the most crucial, prevalent, and complex co- and post-translational protein modifications. It plays a pivotal role in protein folding, quality control, and endoplasmic reticulum (ER)-associated degradation (ERAD) as well as in protein sorting, protein function, and in signal transduction. Furthermore, glycosylation modulates many important biological processes including growth, development, morphogenesis, and stress signaling processes. As a consequence, aberrant or altered N-glycosylation is often associated with reduced fitness, diseases, and disorders. The initial steps of N-glycan synthesis at the cytosolic side of the ER membrane and in the lumen of the ER are highly conserved. In contrast, the final N-glycan processing in the Golgi apparatus is organism-specific giving rise to a wide variety of carbohydrate structures. Despite our vast knowledge on N-glycans in yeast and mammals, the modus operandi of N-glycan signaling in plants is still largely unknown. This review will elaborate on the N-glycosylation biosynthesis pathway in plants but will also critically assess how N-glycans are involved in different signaling cascades, either active during normal development or upon abiotic and biotic stresses.

The Induction of Recombinant Protein Bodies in Different Subcellular Compartments Reveals a Cryptic Plastid-Targeting Signal in the 27-kDa γ-Zein Sequence

Naturally occurring storage proteins such as zeins are used as fusion partners for recombinant proteins because they induce the formation of ectopic storage organelles known as protein bodies (PBs) where the proteins are stabilized by intermolecular interactions and the formation of disulfide bonds. Endogenous PBs are derived from the endoplasmic reticulum (ER). Here, we have used different targeting sequences to determine whether ectopic PBs composed of the N-terminal portion of mature 27 kDa γ-zein added to a fluorescent protein could be induced to form elsewhere in the cell. The addition of a transit peptide for targeting to plastids causes PB formation in the stroma, whereas in the absence of any added targeting sequence PBs were typically associated with the plastid envelope, revealing the presence of a cryptic plastid-targeting signal within the γ-zein cysteine-rich domain. The subcellular localization of the PBs influences their morphology and the solubility of the stored recombinant fusion protein. Our results indicate that the biogenesis and budding of PBs does not require ER-specific factors and therefore, confirm that γ-zein is a versatile fusion partner for recombinant proteins offering unique opportunities for the accumulation and bioencapsulation of recombinant proteins in different subcellular compartments.

Protein accumulation in aleurone cells, sub-aleurone cells and the center starch endosperm of cereals

There are mainly three endosperm storage tissues in the cereal endosperm: aleurone cells, sub-aleurone cells and the center starch endosperm. The protein accumulation is very different in the three endosperm storage tissues. The aleurone cells accumulate protein in aleurone granules. The sub-aleurone cells and the center starch endosperm accumulate protein in endoplasmic reticulum-derived protein bodies and vacuolar protein bodies. Proteins are deposited in different patterns within different endosperm storage tissues probably because of the special storage properties of these tissues. There are several special genes and other molecular factors to mediate the protein accumulation in these tissues. Different proteins have distinct functions in the protein body formation and the protein interactions determine protein body assembly. There are both cooperation and competition relationships between protein, starch and lipid in the cereal endosperm. This paper reviews the latest investigations on protein accumulation in aleurone cells, sub-aleurone cells and the center starch endosperm. Useful information will be supplied for future investigations on the cereal endosperm development.

Fusion, rupture, and degeneration: the fate of in vivo-labelled PSVs in developing barley endosperm

Cereal endosperm is a highly differentiated tissue containing specialized organelles for the accumulation of storage proteins. The endosperm of barley contains hordeins, which are ultimately deposited within protein storage vacuoles (PSVs). These organelles have been characterized predominantly by the histochemical analysis of fixed immature tissue samples. However, little is known about the fate of PSVs during barley endosperm development, and in vivo imaging has not been attempted in order to gain further insight. In this report, young seeds were followed through development to characterize the dynamic morphology of PSVs from aleurone, subaleurone, and central starchy endosperm cells. TIP3-GFP was used as a PSV membrane marker and several fluorescent tracers were used to identify membranes and monitor endomembrane organelles in real time. Whereas the spherical appearance of strongly labelled TIP3-GFP PSVs in the aleurone remained constant, those in the subaleurone and central starchy endosperm underwent substantial morphological changes. Fusion and rupture events were observed in the subaleurone, and internal membranes derived from both the tonoplast and endoplasmic reticulum were identified within these PSVs. TIP3-GFP-labelled PSVs in the starchy endosperm cells underwent a dramatic reduction in size, so that finally the protein bodies were tightly enclosed. Potential desiccation-related membrane-altering processes that may be causally linked to these dynamic endomembrane events in the barley endosperm are discussed.

A comparative glycoproteome study of developing endosperm in the hexose-deficient miniature1 (mn1) seed mutant and its wild type Mn1 in maize

In maize developing seeds, transfer cells are prominently located at the basal endosperm transfer layer (BETL). As the first filial cell layer, BETL is a gateway to sugars, nutrients and water from mother plant; and anchor of numerous functions such as sucrose turnover, auxin and cytokinin biosynthesis/accumulation, energy metabolism, defense response, and signaling between maternal and filial generations. Previous studies showed that basal developing endosperms of miniature1 (mn1) mutant seeds lacking the Mn1-encoded cell wall invertase II, are also deficient for hexose. Given the role of glucose as one of the key sugars in protein glycosylation and proper protein folding; we performed a comparative large scale glycoproteome profiling of total proteins of these two genotypes (mn1 mutant vs. Mn1 wild type) using 2D gel electrophoresis and glycosylation/total protein staining, followed by image analysis. Protein identification was done by LC-MS/MS. A total of 413 spots were detected; from which, 113 spots matched between the two genotypes. Of these, 45 showed >20% decrease/increase in glycosylation level and were selected for protein identification. A large number of identified proteins showed decreased glycosylation levels in mn1 developing endosperms as compared to the Mn1. Functional classification of proteins, showed mainly of post-translational modification, protein turnover, chaperone activities, carbohydrate and amino acid biosynthesis/transport, and cell wall biosynthesis. These proteins and activities were related to endoplasmic reticulum (ER) stress and unfolded protein response (UPR) as a result of the low glycolsylation levels of the mutant proteins. Overall, these results provide for the first time a global glycoproteome profile of maize BETL-enriched basal endosperm to better understand their role in seed development in maize.

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.

Two glycosylated vacuolar GFPs are new markers for ER-to-vacuole sorting

Vacuolar Sorting Determinants (VSDs) have been extensively studied in plants but the mechanisms for the accumulation of storage proteins in somatic tissues are not yet fully understood. In this work we used two mutated versions of well-documented vacuolar fluorescent reporters, a GFP fusion in frame with the C-terminal VSD of tobacco chitinase (GFPChi) and an N-terminal fusion in frame with the sequence-specific VSD of the barley cysteine protease aleurain (AleuGFP). The GFP sequence was mutated to present an N-glycosylation site at the amino-acid position 133. The reporters were transiently expressed in Nicotiana tabacum protoplasts and agroinfiltrated in Nicotiana benthamiana leaves and their distribution was identical to that of the non-glycosylated versions. With the glycosylated GFPs we could highlight a differential ENDO-H sensitivity and therefore differential glycan modifications. This finding suggests two different and independent routes to the vacuole for the two reporters. BFA also had a differential effect on the two markers and further, inhibition of COPII trafficking by a specific dominant-negative mutant (NtSar1h74l) confirmed that GFPChi transport from the ER to the vacuole is not fully dependent on the Golgi apparatus.

Efficient recovery of recombinant proteins from cereal endosperm is affected by interaction with endogenous storage proteins

Cereal seeds are versatile platforms for the production of recombinant proteins because they provide a stable environment for protein accumulation. Endogenous seed storage proteins, however, include several prolamin-type polypeptides that aggregate and crosslink via intermolecular disulfide bridges, which could potentially interact with multimeric recombinant proteins such as antibodies, which assemble in the same manner. We investigated this possibility by sequentially extracting a human antibody expressed in maize endosperm, followed by precipitation in vitro with zein. We provide evidence that a significant proportion of the antibody pool interacts with zein and therefore cannot be extracted using non-reducing buffers. Immunolocalization experiments demonstrated that antibodies targeted for secretion were instead retained within zein bodies because of such covalent interactions. Our findings suggest that the production of soluble recombinant antibodies in maize could be enhanced by eliminating or minimizing interactions with endogenous storage proteins.

Plant species and organ influence the structure and subcellular localization of recombinant glycoproteins

Many plant-based systems have been developed as bioreactors to produce recombinant proteins. The choice of system for large-scale production depends on its intrinsic expression efficiency and its propensity for scale-up, post-harvest storage and downstream processing. Factors that must be considered include the anticipated production scale, the value and intended use of the product, the geographical production area, the proximity of processing facilities, intellectual property, safety and economics. It is also necessary to consider whether different species and organs affect the subcellular trafficking, structure and qualitative properties of recombinant proteins. In this article we discuss the subcellular localization and N-glycosylation of two commercially-relevant recombinant glycoproteins (Aspergillus niger phytase and anti-HIV antibody 2G12) produced in different plant species and organs. We augment existing data with novel results based on the expression of the same recombinant proteins in Arabidopsis and tobacco seeds, focusing on similarities and subtle differences in N-glycosylation that often reflect the subcellular trafficking route and final destination, as well as differences generated by unique enzyme activities in different species and tissues. We discuss the potential consequences of such modifications on the stability and activity of the recombinant glycoproteins.

Endosperm development: dynamic processes and cellular innovations underlying sibling altruism

The endosperm is a product of fertilization that evolved to support and nourish its genetic twin sibling embryo. Cereal endosperm accumulates starch and protein stores, which later support the germinating seedling. These nutritional stores prompted the domestication of cereals and are the focus of ongoing efforts for crop improvement and biotechnological innovations. Endosperm development entails several novel modifications to basic cellular and developmental processes. Cereals display nuclear endosperm development, which begins with a period of free nuclear division to generate a coenocyte. Cytoskeletal arrays distribute nuclei around the periphery of the cytoplasm and direct the subsequent deposition of cell wall material during cellularization. Positional cues and signaling systems function dynamically in the specification of the four major cell types: transfer cells, embryo-surrounding cells, starchy endosperm (SE), and aleurone. Genome balance, epigenetic gene regulation, and parent-of-origin effects are essential for directing these processes. Transfer cells transport solutes, including sugars and amino acids, from the maternal plant tissues into the developing grain where they are partitioned between embryo and SE cells. Cells of the embryo-surrounding region appear to coordinate development of the embryo and endosperm. As the seed matures, SE cells assimilate starch and protein stores, undergo DNA endoreduplication, and finally undergo programmed cell death. In contrast, aleurone cells follow a maturation program similar to the embryo, allowing them to survive desiccation. At germination, the aleurone cells secrete amylases and proteases that hydrolyze the storage products of the SE to nourish the germinating seedling.

The formation, function and fate of protein storage compartments in seeds

Seed storage proteins (SSPs) have been studied for more than 250 years because of their nutritional value and their impact on the use of grain in food processing. More recently, the use of seeds for the production of recombinant proteins has rekindled interest in the behavior of SSPs and the question how they are able to accumulate as stable storage reserves. Seed cells produce vast amounts of SSPs with different subcellular destinations creating an enormous logistic challenge for the endomembrane system. Seed cells contain several different storage organelles including the complex and dynamic protein storage vacuoles (PSVs) and other protein bodies (PBs) derived from the endoplasmic reticulum (ER). Storage proteins destined for the PSV may pass through or bypass the Golgi, using different vesicles that follow different routes through the cell. In addition, trafficking may depend on the plant species, tissue and developmental stage, showing that the endomembrane system is capable of massive reorganization. Some SSPs contain sorting signals or interact with membranes or with other proteins en route in order to reach their destination. The ability of SSPs to form aggregates is particularly important in the formation or ER-derived PBs, a mechanism that occurs naturally in response to overloading with proteins that cannot be transported and that can be used to induce artificial storage bodies in vegetative tissues. In this review, we summarize recent findings that provide insight into the formation, function, and fate of storage organelles and describe tools that can be used to study them.

Expression of antibody fragments with a controlled N-glycosylation pattern and induction of endoplasmic reticulum-derived vesicles in seeds of Arabidopsis

Intracellular trafficking and subcellular deposition are critical factors influencing the accumulation and posttranslational modifications of proteins. In seeds, these processes are not yet fully understood. In this study, we set out to investigate the intracellular transport, final destination, N-glycosylation status, and stability of the fusion of recombinant single-chain variable fragments to the crystallizing fragment of an antibody (scFv-Fc) of two antiviral monoclonal antibodies (2G12 and HA78). The scFv-Fcs were expressed in Arabidopsis (Arabidopsis thaliana) seeds and leaves both as secretory molecules and tagged with an endoplasmic reticulum (ER) retention signal. We demonstrate differential proteolytic degradation of scFv-Fcs in leaves versus seeds, with higher degradation in the latter organ. In seeds, we show that secretory versions of HA78 scFv-Fcs are targeted to the extracellular space but are deposited in newly formed ER-derived vesicles upon KDEL tagging. These results are in accordance with the obtained N-glycosylation profiles: complex-type and ER-typical oligomannosidic N-glycans, respectively. HA78 scFv-Fcs, expressed in seeds of an Arabidopsis glycosylation mutant lacking plant-specific N-glycans, exhibit custom-made human-type N-glycosylation. In contrast, 2G12 scFv-Fcs carry exclusively ER-typical oligomannosidic N-glycans and were deposited in newly formed ER-derived vesicles irrespective of the targeting signals. HA78 scFv-Fcs exhibited efficient virus neutralization activity, while 2G12 scFv-Fcs were inactive. We demonstrate the efficient generation of scFv-Fcs with a controlled N-glycosylation pattern. However, our results also reveal aberrant subcellular deposition and, as a consequence, unexpected N-glycosylation profiles. Our attempts to elucidate intracellular protein transport in seeds contributes to a better understanding of this basic cell biological mechanism and is a step toward the versatile use of Arabidopsis seeds as an alternative expression platform for pharmaceutically relevant proteins.

Delivery of prolamins to the protein storage vacuole in maize aleurone cells

Zeins, the prolamin storage proteins found in maize (Zea mays), accumulate in accretions called protein bodies inside the endoplasmic reticulum (ER) of starchy endosperm cells. We found that genes encoding zeins, α-globulin, and legumin-1 are transcribed not only in the starchy endosperm but also in aleurone cells. Unlike the starchy endosperm, aleurone cells accumulate these storage proteins inside protein storage vacuoles (PSVs) instead of the ER. Aleurone PSVs contain zein-rich protein inclusions, a matrix, and a large system of intravacuolar membranes. After being assembled in the ER, zeins are delivered to the aleurone PSVs in atypical prevacuolar compartments that seem to arise at least partially by autophagy and consist of multilayered membranes and engulfed cytoplasmic material. The zein-containing prevacuolar compartments are neither surrounded by a double membrane nor decorated by AUTOPHAGY RELATED8 protein, suggesting that they are not typical autophagosomes. The PSV matrix contains glycoproteins that are trafficked through a Golgi-multivesicular body (MVB) pathway. MVBs likely fuse with the multilayered, autophagic compartments before merging with the PSV. The presence of similar PSVs also containing prolamins and large systems of intravacuolar membranes in wheat (Triticum aestivum) and barley (Hordeum vulgare) starchy endosperm suggests that this trafficking mechanism may be common among cereals.