Gibberellic acid-induced aleurone layers responding to heat shock or tunicamycin provide insight into the N-glycoproteome, protein secretion, and endoplasmic reticulum stress

N-Linked Glycoproteome Analysis of Diosorea alata Tuber Shows Atypical Glycosylation and Indicates Central Role of Glycosylated Proteins in Tuber Maturation

Glycosylation is an important post translational modification in plants. First analysis of N-linked glycosylated proteins of Dioscorea alata using Concanavalin A lectin affinity chromatography enrichment coupled with label free quantification is presented. In total, 114 enriched glycoproteins were detected. Signal P and sub-cellular localization showed 42.2% of proteins to be secretory. These included peroxidases, endochitinases, calreticulin, calnexin, thaumatins and lipid transfer proteins. Gene Ontology and MapMan analysis predicted the enriched glycoproteins to be involved in processes essential for tuber maturation namely: signal transduction, lignification, protein trafficking, endoplasmic reticulum quality control and cell wall remodeling. This was supported by biochemical validation of the essential glycoproteins. Interestingly, out of the two dioscorin isoforms, Dio B was the only N-glycosylated form. In silico analysis showed O-glycosylation sites in the other form, Dio A suggesting its similarity with sporamin, the storage protein of sweet potato. Absence of signal peptide in Dio B and the presence of non-canonical motif hints towards its atypical glycosylation. The analysis revealed that N-glycosylation of Dio B isoform maintains the activities associated with Dioscorin at maturity and provides an overview of protein N-glycosylation, enriching the glycoproteome database of plants especially tubers.

Upregulation of protein N-glycosylation plays crucial roles in the response of Camellia sinensis leaves to fluoride

The tea plant (Camellia sinensis) is one of the three major beverage crops in the world with its leaves consumption as tea. However, it can hyperaccumulate fluoride with about 98% fluoride deposition in the leaves. Our previously studies found that cell wall proteins (CWPs) might play a central role in fluoride accumulation/detoxification in C. sinensis. CWP is known to be glycosylated, however the response of CWP N-glycosylation to fluoride remains unknown in C. sinensis. In this study, a comparative N-glycoproteomic analysis was performed through HILIC enrichment coupled with UPLC-MS/MS based on TMT-labeling approach in C. sinensis leaves. Totally, 237 N-glycoproteins containing 326 unique N-glycosites were identified. 73.4%, 18.6%, 6.3% and 1.7% of these proteins possess 1, 2, 3, and ≥4 modification site, respectively. 93.2% of these proteins were predicted to be localized in the secretory pathway and 78.9% of them were targeted to the cell wall and the plasma membrane. 133 differentially accumulated N-glycosites (DNGSs) on 100 N-glycoproteins (DNGPs) were detected and 85.0% of them exhibited upregulated expression after fluoride treatment. 78.0% DNGPs were extracellular DNGPs, which belonged to CWPs, and 53.0% of them were grouped into protein acting on cell wall polysaccharides, proteases and oxido-reductases, whereas the majority of the remaining DNGPs were mainly related to N-glycoprotein biosynthesis, trafficking and quality control. Our study shed new light on the N-glycoproteome study, and revealed that increased N-glycosylation abundance of CWPs might contribute to fluoride accumulation/detoxification in C. sinensis leave.

CRISPR/Cas9 and Transgene Verification of Gene Involvement in Unfolded Protein Response and Recombinant Protein Production in Barley Grain

The use of plants as heterologous hosts to produce recombinant proteins has some intriguing advantages. There is, however, the potential of overloading the endoplasmic reticulum (ER) capacity when producing recombinant proteins in the seeds. This leads to an ER-stress condition and accumulating of unfolded proteins. The unfolded protein response (UPR) is activated to alleviate the ER-stress. With the aim to increase the yield of human epidermal growth factor (EGF) and mouse leukemia inhibitory factor (mLIF) in barley, we selected genes reported to have increased expression during ER-induced stress. The selected genes were calreticulin (CRT), protein disulfide isomerase (PDI), isopentenyl diphosphate isomerase (IPI), glutathione-s-transferase (GST), HSP70, HSP26, and HSP16.9. These were knocked out using CRISPR/Cas9 or overexpressed by conventional transgenesis. The generated homozygous barley lines were crossed with barley plants expressing EGF or mLIF and the offspring plants analyzed for EGF and mLIF protein accumulation in the mature grain. All manipulated genes had an impact on the expression of UPR genes when plantlets were subjected to tunicamycin (TN). The PDI knockout plant showed decreased protein body formation, with protein evenly distributed in the cells of the endosperm. The two genes, GST and IPI, were found to have a positive effect on recombinant protein production. mLIF expression was increased in a F2 homozygous GST knockout mutant background as compared to a F2 GST wild-type offspring. The overexpression of IPI in a F1 cross showed a significant increase in EGF expression. We demonstrate that manipulation of UPR related genes can have a positive effect on recombinant protein accumulation.

Exogenous phytohormones in the regulation of growth and development of cereals under abiotic stresses

Abiotic stresses, among which extreme temperatures, salinity, drought, UV radiation, heavy metal pollution, etc., adversely affect the growth and yield of cereals, the most important group of monocotyledonous plants that have met the nutritional and other needs of mankind for thousands of years. To cope with stress, plants deploy certain adaptive strategies that combine morphological, physiological, and biochemical responses, and on which growth and productivity depend. An important place in the formation of such strategies is occupied by phytohormones - signaling biomolecules of a different chemical structure and physicochemical properties, which act in nanomolar concentrations and regulate most physiological and metabolic processes of plants. In this review, the latest literature data concerning the growth and development regulation by exogenous phytohormones in cereals under abiotic stresses have been analyzed and summarized. The effects of priming and foliar treatment with abscisic acid, gibberellins, auxins, cytokinins, brassinosteroids, jasmonic and salicylic acids on the cultivated cereals tolerance to different abiotic stressors are discussed. Peculiarities of bilateral and multilateral hormonal signaling in the formation of responses of cultivated cereals to abiotic stressors after application of exogenous phytohormones are considered. The issue of exogenous phytohormones effects on molecular mechanisms controlling the synthesis of endogenous hormones, their signaling and activity are singled out. It is emphasized that phytohormonal engineering opens new opportunities to increase yields and is seen as an important promising approach to overcoming the cereal losses caused by adverse external factors.

Investigation of cell wall proteins of C. sinensis leaves by combining cell wall proteomics and N-glycoproteomics

BACKGROUND

C. sinensis is an important economic crop with fluoride over-accumulation in its leaves, which poses a serious threat to human health due to its leaf consumption as tea. Recently, our study has indicated that cell wall proteins (CWPs) probably play a vital role in fluoride accumulation/detoxification in C. sinensis. However, there has been a lack in CWP identification and characterization up to now. This study is aimed to characterize cell wall proteome of C. sinensis leaves and to develop more CWPs related to stress response. A strategy of combined cell wall proteomics and N-glycoproteomics was employed to investigate CWPs. CWPs were extracted by sequential salt buffers, while N-glycoproteins were enriched by hydrophilic interaction chromatography method using C. sinensis leaves as a material. Afterwards all the proteins were subjected to UPLC-MS/MS analysis.

RESULTS

A total of 501 CWPs and 195 CWPs were identified respectively by cell wall proteomics and N-glycoproteomics profiling with 118 CWPs in common. Notably, N-glycoproteomics is a feasible method for CWP identification, and it can enhance CWP coverage. Among identified CWPs, proteins acting on cell wall polysaccharides constitute the largest functional class, most of which might be involved in cell wall structure remodeling. The second largest functional class mainly encompass various proteases related to CWP turnover and maturation. Oxidoreductases represent the third largest functional class, most of which (especially Class III peroxidases) participate in defense response. As expected, identified CWPs are mainly related to plant cell wall formation and defense response.

CONCLUSION

This was the first large-scale investigation of CWPs in C. sinensis through cell wall proteomics and N-glycoproteomics. Our results not only provide a database for further research on CWPs, but also an insight into cell wall formation and defense response in C. sinensis.

N-linked glycoproteome analysis reveals central glycosylated proteins involved in wheat early seedling growth

Glycosylation is an important protein post-translational modification in eukaryotic organisms. It is involved in many important life processes, such as cell recognition, differentiation, development, signal transduction and immune response. This study carried out the first N-linked glycosylation proteome analysis of wheat seedling leaves using HILIC glycosylation enrichment, chemical deglycosylation, HPLC separation and tandem mass spectrometric identification. In total, we detected 308 glycosylated peptides and 316 glycosylated sites corresponding to 248 unique glycoproteins. The identified glycoproteins were mainly concentrated in plasma membranes (25.6%), cell wall (16.8%) and extracellular area (16%). In terms of molecular function, 65% glycoproteins belonged to various enzymes with catalytic activity such as kinase, carboxypeptidase, peroxidase and phosphatase, and, particularly, 25% of glycoproteins were related to binding functions. These glycoproteins are involved in cell wall reconstruction, biomacromolecular metabolism, signal transduction, endoplasmic reticulum quality control and stress response. Analysis indicated that 57.66% of glycoproteins were highly conserved in other plant species while 42.34% of glycoproteins went unidentified among the conserved glycosylated homologous proteins in other plant species; these may be the new N-linked glycosylated proteins first identified in wheat. The glycosylation sites generally occurred on the random coil, which could play roles in maintaining the structural stability of proteins. PNGase F digestion and glycosylation site mutations further verified the glycosylation modification and glycosylation sites of LRR receptor-like serine/threonine-protein kinase (LRR-RLK) and Beta-D-glucan exohydrolase (β-D-GEH). Our results indicated that N-linked glycosylated proteins could play important roles in the early seedling growth of wheat.

Comparative and evolutionary analyses of the divergence of plant oligosaccharyltransferase STT3 isoforms

STT3 is a catalytic subunit of hetero-oligomeric oligosaccharyltransferase (OST), which is important for asparagine-linked glycosylation. In mammals and plants, OSTs with different STT3 isoforms exhibit distinct levels of enzymatic efficiency or different responses to stressors. Although two different STT3 isoforms have been identified in both plants and animals, it remains unclear whether these isoforms result from gene duplication in an ancestral eukaryote. Furthermore, the molecular mechanisms underlying the functional divergences between the two STT3 isoforms in plant have not been well elucidated. Here, we conducted phylogenetic analysis of the major evolutionary node species and suggested that gene duplications of STT3 may have occurred independently in animals and plants. Across land plants, the exon-intron structure differed between the two STT3 isoforms, but was highly conserved for each isoform. Most angiosperm STT3a genes had 23 exons with intron phase 0, while STT3b genes had 6 exons with intron phase 2. Characteristic motifs (motif 18 and 19) of STT3s were mapped to different structure domains in the plant STT3 proteins. These two motifs overlap with regions of high nonsynonymous-to-synonymous substitution rates, suggesting the regions may be related to functional difference between STT3a and STT3b. In addition, promoter elements and gene expression profiles were different between the two isoforms, indicating expression pattern divergence of the two genes. Collectively, the identified differences may result in the functional divergence of plant STT3s.

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.

Isolation, Purity Assessment, and Proteomic Analysis of Endoplasmic Reticulum

Subcellular proteomics include, in its experimental workflow, steps aimed at purifying organelles. The purity of the subcellular fraction should be assessed before mass spectrometry analysis, in order to confidently conclude the presence of associated specific proteoforms, deepening the knowledge of its biological function. In this chapter, a protocol for isolating endoplasmic reticulum (ER) and purity assessment is reported, and it precedes the proteomic analysis through a gel-free/label-free proteomic approach. Dysfunction of quality-control mechanisms of protein metabolism in ER leads to ER stress. Additionally, ER, which is a calcium-storage organelle, is responsible for signaling and homeostatic function, and calcium homeostasis is required for plant tolerance. With such predominant cell functions, effective protocols to fractionate highly purified ER are needed. Here, isolation methods and purity assessments of ER are described. In addition, a gel-free/label-free proteomic approach of ER is presented.

Coumarin impairs redox homeostasis in wheat aleurone layers

Many plant families produce coumarin (COU) and its derivatives as secondary metabolites via the phenylpropanoid biosynthetic pathway. This ubiquitous group of phytochemicals was shown to have diverse physiological effects on cellular, tissue, and organ levels. So far, research dealing with the hormonal like behavior of COU and its interaction with the activity and/or transport of phytohormones is very limited. In the current study, the impact of COU on redox homeostasis in aleurone layers of wheat grains was investigated. Aleurone layers were incubated in either 1000 μM COU or 5 μM gibberellic acid (GA₃) alone or in combination with 5 μM abscisic acid (ABA). Results revealed that both COU and GA₃ treatments induced the production of α-amylase but inhibited the activities of superoxide dismutase, catalase and ascorbate peroxidase. The downregulation of antioxidant enzymes that is provoked by COU and GA₃ was accompanied by significant accumulation of both H₂O₂ and malondialdehyde. In contrast with the effect of ABA, both COU and GA₃ treatments resulted in a significant reduction in cell viability as revealed by trypan blue staining. These results suggest that COU could disrupt the redox balance in aleurone layers through downregulation of the enzymatic antioxidant system. Therefore, the current study provides evidence for the gibberellin like activity of COU.

Dynamic N-glycoproteome analysis of maize seedling leaves during de-etiolation using Concanavalin A lectin affinity chromatography and a nano-LC-MS/MS-based iTRAQ approach

The identification of N -glycosylated proteins with information about changes in the level of N -glycosylation during de-etiolation provides a database that will aid further research on plant N -glycosylation and de-etiolation. N-glycosylation is one of the most prominent and abundant protein post-translational modifications in all eukaryotes and in plants it plays important roles in development, stress tolerance and immune responses. Because light-induced de-etiolation is one of the most dramatic developmental processes known in plants, seedlings undergoing de-etiolation are an excellent model for investigating dynamic proteomic profiles. Here, we present a comprehensive, quantitative N-glycoproteomic profile of maize seedlings undergoing 12 h of de-etiolation obtained using Concanavalin A (Con A) lectin affinity chromatography enrichment coupled with a nano-LC-MS/MS-based iTRAQ approach. In total, 1084 unique N-glycopeptides carrying 909 N-glycosylation sites and corresponding to 609 proteins were identified and quantified, including 186 N-glycosylation sites from 162 proteins that were significantly regulated over the course of the 12 h de-etiolation period. Based on hierarchical clustering analysis, the significantly regulated N-glycopeptides were divided into seven clusters that showed different N-glycosylation patterns during de-etiolation. We found no obvious difference in the enriched MapMan bincode categories for each cluster, and these clustered significantly regulated N-glycoproteins (SRNPs) are enriched in miscellaneous, protein, cell wall and signaling, indicating that although the N-glycosylation regulation patterns of these SRNPs might differ, they are involved in similar biological processes. Overall, this study represents the first large-scale quantitative N-glycoproteome of the model C4 plant, maize, which is one of the most important cereal and biofuel crops. Our results greatly expand the maize N-glycoproteomic database and also shed light on the potential roles of N-glycosylation modification during the greening of maize leaves.

Maintaining the factory: the roles of the unfolded protein response in cellular homeostasis in plants

Much like a factory, the endoplasmic reticulum (ER) assembles simple cellular building blocks into complex molecular machines known as proteins. In order to protect the delicate protein folding process and ensure the proper cellular delivery of protein products under environmental stresses, eukaryotes have evolved a set of signaling mechanisms known as the unfolded protein response (UPR) to increase the folding capacity of the ER. This process is particularly important in plants, because their sessile nature commands adaptation for survival rather than escape from stress. As such, plants make special use of the UPR, and evidence indicates that the master regulators and downstream effectors of the UPR have distinct roles in mediating cellular processes that affect organism growth and development as well as stress responses. In this review we outline recent developments in this field that support a strong relevance of the UPR to many areas of plant life.

Mapping the N-linked glycosites of rice (Oryza sativa L.) germinating embryos

Germination is a key event in the angiosperm life cycle. N-glycosylation of proteins is one of the most common post-translational modifications, and has been recognized to be an important regulator of the proteome of the germinating embryo. Here, we report the first N-linked glycosites mapping of rice embryos during germination by using a hydrophilic interaction chromatography (HILIC) glycopeptides enrichment strategy associated with high accuracy mass spectrometry identification. A total of 242 glycosites from 191 unique proteins was discovered. Inspection of the motifs and sequence structures involved suggested that all the glycosites were concentrated within [NxS/T] motif, while 82.3% of them were in a coil structure. N-glycosylation preferentially occurred on proteins with glycoside hydrolase activities, which were significantly enriched in the starch and sucrose metabolism pathway, suggesting that N-glycosylation is involved in embryo germination by regulating carbohydrate metabolism. Notably, protein-protein interaction analysis revealed a network with several Brassinosteroids signaling proteins, including XIAO and other BR-responsive proteins, implying that glycosylation-mediated Brassinosteroids signaling may be a key mechanism regulating rice embryo germination. In summary, this study expanded our knowledge of protein glycosylation in rice, and provided novel insight into the PTM regulation in rice seed germination.

Monitoring intra- and extracellular redox capacity of intact barley aleurone layers responding to phytohormones

Redox regulation is important for numerous processes in plant cells including abiotic stress, pathogen defence, tissue development, seed germination and programmed cell death. However, there are few methods allowing redox homeostasis to be addressed in whole plant cells, providing insight into the intact in vivo environment. An electrochemical redox assay that applies the menadione-ferricyanide double mediator is used to assess changes in the intracellular and extracellular redox environment in living aleurone layers of barley (Hordeum vulgare cv. Himalaya) grains, which respond to the phytohormones gibberellic acid and abscisic acid. Gibberellic acid is shown to elicit a mobilisation of electrons as detected by an increase in the reducing capacity of the aleurone layers. By taking advantage of the membrane-permeable menadione/menadiol redox pair to probe the membrane-impermeable ferricyanide/ferrocyanide redox pair, the mobilisation of electrons was dissected into an intracellular and an extracellular, plasma membrane-associated component. The intracellular and extracellular increases in reducing capacity were both suppressed when the aleurone layers were incubated with abscisic acid. By probing redox levels in intact plant tissue, the method provides a complementary approach to assays of reactive oxygen species and redox-related enzyme activities in tissue extracts.

Plant subcellular proteomics: Application for exploring optimal cell function in soybean

Plants have evolved complicated responses to developmental changes and stressful environmental conditions. Subcellular proteomics has the potential to elucidate localized cellular responses and investigate communications among subcellular compartments during plant development and in response to biotic and abiotic stresses. Soybean, which is a valuable legume crop rich in protein and vegetable oil, can grow in several climatic zones; however, the growth and yield of soybean are markedly decreased under stresses. To date, numerous proteomic studies have been performed in soybean to examine the specific protein profiles of cell wall, plasma membrane, nucleus, mitochondrion, chloroplast, and endoplasmic reticulum. In this review, methods for the purification and purity assessment of subcellular organelles from soybean are summarized. In addition, the findings from subcellular proteomic analyses of soybean during development and under stresses, particularly flooding stress, are presented and the proteins regulated among subcellular compartments are discussed. Continued advances in subcellular proteomics are expected to greatly contribute to the understanding of the responses and interactions that occur within and among subcellular compartments during development and under stressful environmental conditions.

BIOLOGICAL SIGNIFICANCE

Subcellular proteomics has the potential to investigate the cellular events and interactions among subcellular compartments in response to development and stresses in plants. Soybean could grow in several climatic zones; however, the growth and yield of soybean are markedly decreased under stresses. Numerous proteomics of cell wall, plasma membrane, nucleus, mitochondrion, chloroplast, and endoplasmic reticulum was carried out to investigate the respecting proteins and their functions in soybean during development or under stresses. In this review, methods of subcellular-organelle enrichment and purity assessment are summarized. In addition, previous findings of subcellular proteomics are presented, and functional proteins regulated among different subcellular are discussed. Subcellular proteomics contributes greatly to uncovering responses and interactions among subcellular compartments during development and under stressful environmental conditions in soybean.

The Maltase Involved in Starch Metabolism in Barley Endosperm Is Encoded by a Single Gene

During germination and early seedling growth of barley (Hordeum vulgare), maltase is responsible for the conversion of maltose produced by starch degradation in the endosperm to glucose for seedling growth. Despite the potential relevance of this enzyme for malting and the production of alcoholic beverages, neither the nature nor the role of maltase is fully understood. Although only one gene encoding maltase has been identified with certainty, there is evidence for the existence of other genes and for multiple forms of the enzyme. It has been proposed that maltase may be involved directly in starch granule degradation as well as in maltose hydrolysis. The aim of our work was to discover the nature of maltase in barley endosperm. We used ion exchange chromatography to fractionate maltase activity from endosperm of young seedlings, and we partially purified activity for protein identification. We compared maltase activity in wild-type barley and transgenic lines with reduced expression of the previously-characterised maltase gene Agl97, and we used genomic and transcriptomic information to search for further maltase genes. We show that all of the maltase activity in the barley endosperm can be accounted for by a single gene, Agl97. Multiple forms of the enzyme most likely arise from proteolysis and other post-translational modifications.

Expression and characterization of protein disulfide isomerase family proteins in bread wheat

BACKGROUND

The major wheat seed proteins are storage proteins that are synthesized in the rough endoplasmic reticulum (ER) of starchy endosperm cells. Many of these proteins have intra- and intermolecular disulfide bonds. In eukaryotes, the formation of most intramolecular disulfide bonds in the ER is thought to be catalyzed by protein disulfide isomerase (PDI) family proteins. The cDNAs that encode eight groups of bread wheat (Triticum aestivum L.) PDI family proteins have been cloned, and their expression levels in developing wheat grains have been determined. The purpose of the present study was to characterize the enzymatic properties of the wheat PDI family proteins and clarify their expression patterns in wheat caryopses.

RESULTS

PDI family cDNAs, which are categorized into group I (TaPDIL1Aα, TaPDIL1Aβ, TaPDIL1Aγ, TaPDIL1Aδ, and TaPDIL1B), group II (TaPDIL2), group III (TaPDIL3A), group IV (TaPDIL4D), and group V (TaPDIL5A), were cloned. The expression levels of recombinant TaPDIL1Aα, TaPDIL1B, TaPDIL2, TaPDIL3A, TaPDIL4D, and TaPDIL5A in Escherichia coli were established from the cloned cDNAs. All recombinant proteins were expressed in soluble forms and purified. Aside from TaPDIL3A, the recombinant proteins exhibited oxidative refolding activity on reduced and denatured ribonuclease A. Five groups of PDI family proteins were distributed throughout wheat caryopses, and expression levels of these proteins were higher during grain filling than in the late stage of maturing. Localization of these proteins in the ER was confirmed by fluorescent immunostaining of the immature caryopses. In mature grains, the five groups of PDI family proteins remained in the aleurone cells and the protein matrix of the starchy endosperm.

CONCLUSIONS

High expression of PDI family proteins during grain filling in the starchy endosperm suggest that these proteins play an important role in forming intramolecular disulfide bonds in seed storage proteins. In addition, these PDI family proteins that remain in the aleurone layers of mature grains likely assist in folding newly synthesized hydrolytic enzymes during germination.

N-linked glycoproteome profiling of seedling leaf in Brachypodium distachyon L

Brachypodium distachyon L., a model plant for cereal crops, has become important as an alternative and potential biofuel grass. In plants, N-glycosylation is one of the most common and important protein modifications, playing important roles in signal recognition, increase in protein activity, stability of protein structure, and formation of tissues and organs. In this study, we performed the first glycoproteome analysis in the seedling leaves of B. distachyon. Using lectin affinity chromatography enrichment and mass-spectrometry-based analysis, we identified 47 glycosylation sites representing 46 N-linked glycoproteins. Motif-X analysis showed that two conserved motifs, N-X-T/S (X is any amino acid, except Pro), were significantly enriched. Further functional analysis suggested that some of these identified glycoproteins are involved in signal transduction, protein trafficking, and quality control and the modification and remodeling of cell-wall components such as receptor-like kinases, protein disulfide isomerase, and polygalacturonase. Moreover, transmembrane helices and signal peptide prediction showed that most of these glycoproteins could participate in typical protein secretory pathways in eukaryotes. The results provide a general overview of protein N-glycosylation modifications during the early growth of seedling leaves in B. distachyon and supplement the glycoproteome databases of plants.

Advances in plant proteomics toward improvement of crop productivity and stress resistancex

Abiotic and biotic stresses constrain plant growth and development negatively impacting crop production. Plants have developed stress-specific adaptations as well as simultaneous responses to a combination of various abiotic stresses with pathogen infection. The efficiency of stress-induced adaptive responses is dependent on activation of molecular signaling pathways and intracellular networks by modulating expression, or abundance, and/or post-translational modification (PTM) of proteins primarily associated with defense mechanisms. In this review, we summarize and evaluate the contribution of proteomic studies to our understanding of stress response mechanisms in different plant organs and tissues. Advanced quantitative proteomic techniques have improved the coverage of total proteomes and sub-proteomes from small amounts of starting material, and characterized PTMs as well as protein-protein interactions at the cellular level, providing detailed information on organ- and tissue-specific regulatory mechanisms responding to a variety of individual stresses or stress combinations during plant life cycle. In particular, we address the tissue-specific signaling networks localized to various organelles that participate in stress-related physiological plasticity and adaptive mechanisms, such as photosynthetic efficiency, symbiotic nitrogen fixation, plant growth, tolerance and common responses to environmental stresses. We also provide an update on the progress of proteomics with major crop species and discuss the current challenges and limitations inherent to proteomics techniques and data interpretation for non-model organisms. Future directions in proteomics research toward crop improvement are further discussed.

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.