2-DE proteome maps for the leaf apoplast of Nicotiana benthamiana

Iron Induces Resistance Against the Rice Blast Fungus Magnaporthe oryzae Through Potentiation of Immune Responses

Iron is an essential nutrient required for plant growth and development. The availability of iron might also influence disease resistance in plants. However, the molecular mechanisms involved in the plant response to iron availability and immunity have been investigated separately from each other. In this work, we found that exposure of rice plants to high iron enhances resistance to infection by the fungal pathogen Magnaporthe oryzae, the causal agent of blast disease. RNA-Seq analysis revealed that blast resistance in iron-treated rice plants was associated with superinduction of defense-related genes during pathogen infection, including Pathogenesis-Related genes. The expression level of genes involved in the biosynthesis of phytoalexins, both diterpene phytoalexins and the flavonoid phytoalexin sakuranetin, was also higher in iron-treated plants compared with control plants, which correlated well with increased levels of phytoalexins in these plants during M. oryzae infection. Upon pathogen infection, lipid peroxidation was also higher in iron-treated plants compared with non-treated plants. We also show that M. oryzae infection modulates the expression of genes that play a pivotal role in the maintenance of iron homeostasis. Histochemical analysis of M. oryzae-infected leaves revealed colocalization of iron and reactive oxygen species in cells located in the vicinity of fungal penetration sites (e.g. appressoria) in rice plants that have been exposed to iron. Together these findings support that ferroptosis plays a role in the response of iron-treated rice plants to infection by virulent M. oryzae. Understanding interconnected regulations between iron signaling and immune signaling in rice holds great potential for developing novel strategies to improve blast resistance in rice.

Prunus persica apoplastic proteome analysis reveals candidate proteins involved in the resistance and defense against Taphrina deformans

Taphrina deformans is the fungus responsible for the peach leaf curl disease. To gain insight into the molecular mechanisms involved in plant resistance and response to the fungus, apoplastic differentially abundant proteins (DAPs) in a resistant (DR) and/or in a susceptible genotype (FL) were identified after 12 and 96 h post inoculation (hpi) and compared to those at 0 hpi. The Prunus persica apoplastic proteome was assessed by LC-MS/MS analysis. Altogether 332 proteins were identified, and their molecular and biological functions were classified. In both genotypes, major changes occurred at 96 hpi when the fungus had achieved the filamentous form. However, at 96 hpi, DR exhibited a greater number of increased proteins than FL. DAPs were enriched in biotic stress response, with most of the proteins belonging to the pathogenesis related (PR)-type. PRs exhibited the greatest fold changes of induction in DR. While PRs acting on pathogen cell wall (PR2, PR3 and PR4) were increased in both susceptible and resistant genotypes, others were exclusively induced in DR, such as some isoforms of PR5, defensin and PR17. Proteins exclusively induced in DR upon T.deformans inoculation such as four berberine bridge enzymes, two snakins and a GDS-lipase were identified. Moreover, upon inoculation cuticle was thickened to a greater extent in DR than in FL. This work reveals the active role of the apoplast against T. deformans and not only contributes to the elucidation of responses involved in resistance to leaf curl disease but also improves the knowledge on peach defenses against pathogens.

Transient proteolysis reduction of Nicotiana benthamiana-produced CAP256 broadly neutralizing antibodies using CRISPR/Cas9

The hypersensitive response is elicited by Agrobacterium infiltration of Nicotiana benthamiana, including the induction and accumulation of pathogenesis-related proteins, such as proteases. This includes the induction of the expression of several cysteine proteases from the C1 (papain-like cysteine protease) and C13 (legumain-like cysteine protease) families. This study demonstrates the role of cysteine proteases: NbVPE-1a, NbVPE-1b, and NbCysP6 in the proteolytic degradation of Nicotiana benthamiana (glycosylation mutant ΔXTFT)-produced anti-human immunodeficiency virus broadly neutralizing antibody, CAP256-VRC26.25. Three putative cysteine protease cleavage sites were identified in the fragment crystallizable region. We further demonstrate the transient coexpression of CAP256-VRC26.25 with CRISPR/Cas9-mediated genome editing vectors targeting the NbVPE-1a, NbVPE-1b, and NbCysP6 genes which resulted in a decrease in CAP256-VRC26.25 degradation. No differences in structural features were observed between the human embryonic kidney 293 (HEK293)-produced and ΔXTFT broadly neutralizing antibodies produced with and without the coexpression of genome-editing vectors. Furthermore, despite the presence of proteolytically degraded fragments of plant-produced CAP256-VRC26.25 without the coexpression of genome editing vectors, no influence on the in vitro functional activity was detected. Collectively, we demonstrate an innovative in planta strategy for improving the quality of the CAP256 antibodies through the transient expression of the CRISPR/Cas9 vectors.

EffectorP 3.0: Prediction of Apoplastic and Cytoplasmic Effectors in Fungi and Oomycetes

Many fungi and oomycete species are devasting plant pathogens. These eukaryotic filamentous pathogens secrete effector proteins to facilitate plant infection. Fungi and oomycete pathogens have diverse infection strategies and their effectors generally do not share sequence homology. However, they occupy similar host environments, either the plant apoplast or plant cytoplasm, and, therefore, may share some unifying properties based on the requirements of these host compartments. Here, we exploit these biological signals and present the first classifier (EffectorP 3.0) that uses two machine-learning models: one trained on apoplastic effectors and one trained on cytoplasmic effectors. EffectorP 3.0 accurately predicts known apoplastic and cytoplasmic effectors in fungal and oomycete secretomes with low estimated false-positive rates of 3 and 8%, respectively. Cytoplasmic effectors have a higher proportion of positively charged amino acids, whereas apoplastic effectors are enriched for cysteine residues. The combination of fungal and oomycete effectors in training leads to a higher number of predicted cytoplasmic effectors in biotrophic fungi. EffectorP 3.0 expands predicted effector repertoires beyond small, cysteine-rich secreted proteins in fungi and RxLR-motif containing secreted proteins in oomycetes. We show that signal peptide prediction is essential for accurate effector prediction, because EffectorP 3.0 recognizes a cytoplasmic signal also in intracellular, nonsecreted proteins.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Engineering Approaches in Plant Molecular Farming for Global Health

Since the demonstration of the first plant-produced proteins of medical interest, there has been significant growth and interest in the field of plant molecular farming, with plants now being considered a viable production platform for vaccines. Despite this interest and development by a few biopharmaceutical companies, plant molecular farming is yet to be embraced by ‘big pharma’. The plant system offers a faster alternative, which is a potentially more cost-effective and scalable platform for the mass production of highly complex protein vaccines, owing to the high degree of similarity between the plant and mammalian secretory pathway. Here, we identify and address bottlenecks in the use of plants for vaccine manufacturing and discuss engineering approaches that demonstrate both the utility and versatility of the plant production system as a viable biomanufacturing platform for global health. Strategies for improving the yields and quality of plant-produced vaccines, as well as the incorporation of authentic posttranslational modifications that are essential to the functionality of these highly complex protein vaccines, will also be discussed. Case-by-case examples are considered for improving the production of functional protein-based vaccines. The combination of all these strategies provides a basis for the use of cutting-edge genome editing technology to create a general plant chassis with reduced host cell proteins, which is optimised for high-level protein production of vaccines with the correct posttranslational modifications.

BGAL1 depletion boosts the level of β-galactosylation of N- and O-glycans in N. benthamiana

Glyco-design of proteins is a powerful tool in fundamental studies of structure-function relationship and in obtaining profiles optimized for efficacy of therapeutic glycoproteins. Plants, particularly Nicotiana benthamiana, are attractive hosts to produce recombinant glycoproteins, and recent advances in glyco-engineering facilitate customized N-glycosylation of plant-derived glycoproteins. However, with exception of monoclonal antibodies, homogenous human-like β1,4-galactosylation is very hard to achieve in recombinant glycoproteins. Despite significant efforts to optimize the expression of β1,4-galactosyltransferase, many plant-derived glycoproteins still exhibit incomplete processed N-glycans with heterogeneous terminal galactosylation. The most obvious suspects to be involved in trimming terminal galactose residues are β-galactosidases (BGALs) from the glycosyl hydrolase family GH35. To elucidate the so far uncharacterized mechanisms leading to the trimming of terminal galactose residues from glycans of secreted proteins, we studied a N. benthamiana BGAL known to be active in the apoplast (NbBGAL1). Here, we determined the NbBGAL1 subcellular localization, substrate specificity and in planta biological activity. We show that NbBGAL1 can remove β1,4- and β1,3-galactose residues on both N- and O-glycans. Transient BGAL1 down-regulation by RNA interference (RNAi) and BGAL1 depletion by genome editing drastically reduce β-galactosidase activity in N. benthamiana and increase the amounts of fully galactosylated complex N-glycans on several plant-produced glycoproteins. Altogether, our data demonstrate that NbBGAL1 acts on galactosylated complex N-glycans of plant-produced glycoproteins.

Physiological and Proteomic Changes in the Apoplast Accompany Leaf Senescence in Arabidopsis

The apoplast, i.e. the cellular compartment external to the plasma membrane, undergoes important changes during senescence. Apoplastic fluid volume increases quite significantly in senescing leaves, thereby diluting its contents. Its pH elevates by about 0.8 units, similar to the apoplast alkalization in response to abiotic stresses. The levels of 159 proteins decrease, whereas 24 proteins increase in relative abundance in the apoplast of senescing leaves. Around half of the apoplastic proteins of non-senescent leaves contain a N-terminal signal peptide for secretion, while all the identified senescence-associated apoplastic proteins contain the signal peptide. Several of the apoplastic proteins that accumulate during senescence also accumulate in stress responses, suggesting that the apoplast may constitute a compartment where developmental and stress-related programs overlap. Other senescence-related apoplastic proteins are involved in cell wall modifications, proteolysis, carbohydrate, ROS and amino acid metabolism, signaling, lipid transport, etc. The most abundant senescence-associated apoplastic proteins, PR2 and PR5 (e.g. pathogenesis related proteins PR2 and PR5) are related to leaf aging rather than to the chloroplast degradation program, as their levels increase only in leaves undergoing developmental senescence, but not in dark-induced senescent leaves. Changes in the apoplastic space may be relevant for signaling and molecular trafficking underlying senescence.

pH Gradient Mitigation in the Leaf Cell Secretory Pathway Attenuates the Defense Response of Nicotiana benthamiana to Agroinfiltration

Partial neutralization of the Golgi lumen pH by the ectopic expression of influenza virus M2 proton channel is useful to stabilize acid-labile recombinant proteins in plant cells, but the impact of pH gradient mitigation on host cellular functions has not been investigated. Here, we assessed the unintended effects of M2 expression on the leaf proteome of Nicotiana benthamiana infiltrated with the bacterial gene vector Agrobacterium tumefaciens. An isobaric tags for relative and absolute quantification quantitative proteomics procedure was followed to compare the leaf proteomes of plants agroinfiltrated with either an "empty" vector or an M2-encoding vector. Leaves infiltrated with the empty vector had a low soluble protein content compared to noninfiltrated control leaves, associated with increased levels of stress-related proteins but decreased levels of photosynthesis-associated proteins. M2 expression partly compromised these effects of agroinfiltration to restore soluble protein content in the leaf tissue, associated with restored levels of photosynthesis-associated proteins and reduced levels of stress-related proteins in the apoplast. These data illustrate the cell-wide influence of the Golgi lumen pH homeostasis on the leaf proteome of N. benthamiana responding to microbial challenge. They also underline the relevance of assessing the eventual unintended effects of accessory proteins used to modulate specific cellular or metabolic functions in plant protein biofactories.

Expression of the Biologically Active Insulin Analog SCI-57 in Nicotiana Benthamiana

Diabetes mellitus is a growing problem worldwide; however, only 23% of low-income countries have access to insulin, and ironically it costs higher in such countries than high-income ones. Therefore, new strategies for insulin and insulin analogs production are urgently required to improve low-cost access to therapeutic products, so as to contain the diabetes epidemic. SCI-57 is an insulin analog with a greater affinity for the insulin receptor and lower thermal degradation than native insulin. It also shows native mitogenicity and insulin-like biological activity. In this work, SCI-57 was transiently expressed in the Nicotiana benthamiana (Nb) plant, and we also evaluated some of its relevant biological effects. An expression plasmid was engineered to translate an N-terminal ubiquitin and C-terminal endoplasmic reticulum-targeting signal KDEL, in order to increase protein expression and stability. Likewise, the effect of co-expression of influenza M2 ion channel (M2) on the expression of insulin analog SCI-57 (SCI-57/M2) was evaluated. Although using M2 increases yield, it tends to alter the SCI-57 amino acid sequence, possibly promoting the formation of oligomers. Purification of SCI-57 was achieved by FPLC cation exchange and ultrafiltration of N. benthamiana leaf extract (NLE). SCI-57 exerts its anti-diabetic properties by stimulating glucose uptake in adipocytes, without affecting the lipid accumulation process. Expression of the insulin analog in agroinfiltrated plants was confirmed by SDS-PAGE, RP-HPLC, and MS. Proteome changes related to the expression of heterologous proteins on N. benthamiana were not observed; up-regulated proteins were related to the agroinfiltration process. Our results demonstrate the potential for producing a biologically active insulin analog, SCI-57, by transient expression in Nb.

Transformation by growth onto agro-infiltrated tissues (TGAT), a simple and efficient alternative for transient transformation of the cucurbit powdery mildew pathogen Podosphaera xanthii

A major limitation of molecular studies in powdery mildew fungi (Erysiphales) is their genetic intractability. This is because they are obligate biotrophs. In these parasites, biotrophy is determined by the presence of haustoria, which are specialized structures of parasitism that play an essential role in the acquisition of nutrients and the deliverance of effectors. Podosphaera xanthii is the main causal agent of cucurbit powdery mildew and a major limitation for crop productivity. In a previous study using P. xanthii conidia, we showed, for the first time, the transformation of powdery mildew fungi by Agrobacterium tumefaciens. In this work, we hypothesized that the haustorium could also act as a natural route for the acquisition of DNA. To test our hypothesis, melon cotyledons were agro-infiltrated with A. tumefaciens that contained diverse transfer DNA (T-DNA) constructs harbouring different marker genes under the control of fungal promoters and, after elimination of the bacterium, the cotyledons were subsequently inoculated with P. xanthii conidia. Our results conclusively demonstrated the transfer of different T-DNAs from A. tumefaciens to P. xanthii, including two fungicide resistance markers (hph and tub2), a reporter gene (gfp) and a translational fusion (cfp-PxEC2). These results were further supported by the co-localization of translational fluorescent fusions of A. tumefaciens VirD2 and P. xanthii Rab5 proteins into small vesicles of haustorial and hyphal cells, suggesting endocytosis as the mechanism for T-DNA uptake, presumably by the haustorium. From our perspective, transformation by growth onto agro-infiltrated tissues (TGAT) is the easiest and most reliable method for the transient transformation of powdery mildew fungi.

On the Promising Role of Enzyme Activity Assay in Interpreting Comparative Proteomic Data in Plants

Comparative proteomics is widely used to detect protein changes, especially differential abundance proteins (DAPs) that are involved in plant responses to development, disease, or environment. Once DAPs are identified, it is essential to validate any change in their abundance, and their role in the biological process under study. In addition to common confirmation by quantitative RT-PCR, immunoblot, and multiple reaction monitoring analysis, it has been proposed that enzyme activity assay (EAA) can be complementary to the standard proteomics results, especially regarding the elucidation of protein (enzyme) function and the mechanism of enzyme-associated biochemical or metabolic pathways. The enzymes discussed here are the DAPs identified in comparative plant proteomics. Despite the small number of enzymes in a proteome, they often make up a substantial proportion of the DAPs identified in comparative studies. Currently, only a few studies have performed EAA to complement the interpretation of proteomic data, especially activity-based protein profiling. This viewpoint aims to arouse the attention of proteomic researchers on the promising role of EAA in plant proteomics and highlights the need for high-throughput assays of enzyme activities in comparative plant proteomics.

Genome-wide transcriptome analysis and identification of benzothiadiazole-induced genes and pathways potentially associated with defense response in banana

Background

Bananas (Musa spp.) are the most important fruit crops worldwide due to their high nutrition value. Fusarium wilt of banana, caused by fungal pathogen Fusarium oxysporum f. sp. cubense tropical race 4 (Foc 4), is considered as the most destructive disease in the world and results in extensive damage leading to productivity loss. The widespread use of plant resistance inducers (PRIs), such as benzothiadiazole (BTH), is a novel strategy to stimulate defense responses in banana plants to protect against pathogens infection. The recent focus on the crop defense against fungal infections has led to a renewed interest on understanding the molecular mechanisms of specific PRIs-mediated resistance. This transcriptome study aimed to identify genes that are associated with BTH-induced resistance. Patterns of gene expression in the leaves and roots of BTH-sprayed banana plants were studied using RNA-Seq.

Results

In this study, 18 RNA-Seq libraries from BTH-sprayed and untreated leaves and roots of the Cavendish plants, the most widely grown banana cultivar, were used for studying the transcriptional basis of BTH-related resistance. Comparative analyses have revealed that 6689 and 3624 differentially expressed genes were identified in leaves and roots, respectively, as compared to the control. Approximately 80% of these genes were differentially expressed in a tissue-specific manner. Further analysis showed that signaling perception and transduction, transcription factors, disease resistant proteins, plant hormones and cell wall organization-related genes were stimulated by BTH treatment, especially in roots. Interestingly, the ethylene and auxin biosynthesis and response genes were found to be up-regulated in leaves and roots, respectively, suggesting a choice among BTH-responsive phytohormone regulation.

Conclusions

Our data suggests a role for BTH in enhancing banana plant defense responses to Foc 4 infection, and demonstrates that BTH selectively affect biological processes associated with plant defenses. The genes identified in the study could be further studied and exploited to develop Foc 4-resistant banana varieties.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4830-7) contains supplementary material, which is available to authorized users.

The transcriptome, extracellular proteome and active secretome of agroinfiltrated Nicotiana benthamiana uncover a large, diverse protease repertoire

Infiltration of disarmed Agrobacterium tumefaciens into leaves of Nicotiana benthamiana (agroinfiltration) facilitates quick and safe production of antibodies, vaccines, enzymes and metabolites for industrial use (molecular farming). However, yield and purity of proteins produced by agroinfiltration are hampered by unintended proteolysis, restricting industrial viability of the agroinfiltration platform. Proteolysis may be linked to an immune response to agroinfiltration, but understanding of the response to agroinfiltration is limited. To identify the proteases, we studied the transcriptome, extracellular proteome and active secretome of agroinfiltrated leaves over a time course, with and without the P19 silencing inhibitor. Remarkably, the P19 expression had little effect on the leaf transcriptome and no effect on the extracellular proteome. 25% of the detected transcripts changed in abundance upon agroinfiltration, associated with a gradual up-regulation of immunity at the expense of photosynthesis. By contrast, 70% of the extracellular proteins increased in abundance, in many cases associated with increased efficiency of extracellular delivery. We detect a dynamic reprogramming of the proteolytic machinery upon agroinfiltration by detecting transcripts encoding for 975 different proteases and protease homologs. The extracellular proteome contains peptides derived from 196 proteases and protease homologs, and activity-based proteomics displayed 17 active extracellular Ser and Cys proteases in agroinfiltrated leaves. We discuss unique features of the N. benthamiana protease repertoire and highlight abundant extracellular proteases in agroinfiltrated leaves, being targets for reverse genetics. This data set increases our understanding of the plant response to agroinfiltration and indicates ways to improve a key expression platform for both plant science and molecular farming.

ApoplastP: prediction of effectors and plant proteins in the apoplast using machine learning

The plant apoplast is integral to intercellular signalling, transport and plant-pathogen interactions. Plant pathogens deliver effectors both into the apoplast and inside host cells, but no computational method currently exists to discriminate between these localizations. We present ApoplastP, the first method for predicting whether an effector or plant protein localizes to the apoplast. ApoplastP uncovers features of apoplastic localization common to both effectors and plant proteins, namely depletion in glutamic acid, acidic amino acids and charged amino acids and enrichment in small amino acids. ApoplastP predicts apoplastic localization in effectors with a sensitivity of 75% and a false positive rate of 5%, improving the accuracy of cysteine-rich classifiers by > 13%. ApoplastP does not depend on the presence of a signal peptide and correctly predicts the localization of unconventionally secreted proteins. The secretomes of fungal saprophytes as well as necrotrophic, hemibiotrophic and extracellular fungal pathogens are enriched for predicted apoplastic proteins. Rust pathogens have low proportions of predicted apoplastic proteins, but these are highly enriched for predicted effectors. ApoplastP pioneers apoplastic localization prediction using machine learning. It will facilitate functional studies and will be valuable for predicting if an effector localizes to the apoplast or if it enters plant cells.

Comparative Membrane-Associated Proteomics of Three Different Immune Reactions in Potato

Plants have evolved different types of immune reactions but large-scale proteomics about these processes are lacking, especially in the case of agriculturally important crop pathosystems. We have established a system for investigating PAMP-triggered immunity (PTI) and two different effector-triggered immunity (ETI; triggered by Avr2 or IpiO) responses in potato. The ETI responses are triggered by molecules from the agriculturally important Phytophthora infestans interaction. To perform large-scale membrane protein-based comparison of these responses, we established a method to extract proteins from subcellular compartments in leaves. In the membrane fractions that were subjected to quantitative proteomics analysis, we found that most proteins regulated during PTI were also regulated in the same way in ETI. Proteins related to photosynthesis had lower abundance, while proteins related to oxidative and biotic stress, as well as those related to general antimicrobial defense and cell wall degradation, were found to be higher in abundance. On the other hand, we identified a few proteins—for instance, an ABC transporter-like protein—that were only found in the PTI reaction. Furthermore, we also identified proteins that were regulated only in ETI interactions. These included proteins related to GTP binding and heterotrimeric G-protein signaling, as well as those related to phospholipase signaling.

The Botrytis cinerea elicitor protein BcIEB1 interacts with the tobacco PR5-family protein osmotin and protects the fungus against its antifungal activity

The broad-range phytopathogenic fungus Botrytis cinerea secretes hundreds of proteins during infection of its plant hosts. One of these proteins, BcIEB1, is abundantly secreted and is able to elicit plant defenses, probably as a pathogen-associated molecular pattern, although its native function in B. cinerea biology remains unknown. Pull-down experiments designed to isolate the molecular target of BcIEB1 in tobacco resulted in the identification of osmotin, a pathogenesis-related protein of family 5 that shows antifungal activity. The expression of osmotin in Escherichia coli allowed the verification of the BcIEB1-osmotin interaction with pure proteins by pull-down and far Western blot experiments, as well as the confirmation of the activity of osmotin against B. cinerea. Interestingly, B. cinerea Δbcieb1 mutants are more susceptible than the wild-type to osmotin, and the external addition of pure BcIEB1 protects the Δbcieb1 mutants, as well as Saccharomyces cerevisiae, from the antifungal action of osmotin, thus pointing at PR5 inhibition as the primary native function of BcIEB1. The question of whether osmotin is also involved in the activation of plant defenses by BcIEB1 is also addressed, and the data suggest that osmotin does not participate in the elicitation process.

Agroinfiltration contributes to VP1 recombinant protein degradation

There is a growing interest in applying tobacco agroinfiltration for recombinant protein production in a plant based system. However, in such a system, the action of proteases might compromise recombinant protein production. Protease sensitivity of model recombinant foot-and-mouth disease (FMD) virus P1-polyprotein (P1) and VP1 (viral capsid protein 1) as well as E. coli glutathione reductase (GOR) were investigated. Recombinant VP1 was more severely degraded when treated with the serine protease trypsin than when treated with the cysteine protease papain. Cathepsin L- and B-like as well as legumain proteolytic activities were elevated in agroinfiltrated tobacco tissues and recombinant VP1 was degraded when incubated with such a protease-containing tobacco extract. In silico analysis revealed potential protease cleavage sites within the P1, VP1 and GOR sequences. The interaction modeling of the single VP1 protein with the proteases papain and trypsin showed greater proximity to proteolytic active sites compared to modeling with the entire P1-polyprotein fusion complex. Several plant transcripts with differential expression were detected 24 hr post-agroinfiltration when the RNA-seq technology was applied to identify changed protease transcripts using the recently available tobacco draft genome. Three candidate genes were identified coding for proteases which included the Responsive-to-Desiccation-21 (RD21) gene and genes for coding vacuolar processing enzymes 1a (NbVPE1a) and 1b (NbVPE1b). The data demonstrates that the tested recombinant proteins are sensitive to protease action and agroinfiltration induces the expression of potential proteases that can compromise recombinant protein production.

Protein Dynamics in the Plant Extracellular Space

The extracellular space (ECS or apoplast) is the plant cell compartment external to the plasma membrane, which includes the cell walls, the intercellular space and the apoplastic fluid (APF). The present review is focused on APF proteomics papers and intends to draw information on the metabolic processes occurring in the ECS under abiotic and biotic stresses, as well as under non-challenged conditions. The large majority of the proteins detected are involved in "cell wall organization and biogenesis", "response to stimulus" and "protein metabolism". It becomes apparent that some proteins are always detected, irrespective of the experimental conditions, although with different relative contribution. This fact suggests that non-challenged plants have intrinsic constitutive metabolic processes of stress/defense in the ECS. In addition to the multiple functions ascribed to the ECS proteins, should be considered the interactions established between themselves and with the plasma membrane and its components. These interactions are crucial in connecting exterior and interior of the cell, and even simple protein actions in the ECS can have profound effects on plant performance. The proteins of the ECS are permanently contributing to the high dynamic nature of this plant compartment, which seems fundamental to plant development and adaptation to the environmental conditions.

Plant fluid proteomics: Delving into the xylem sap, phloem sap and apoplastic fluid proteomes

The phloem sap, xylem sap and apoplastic fluid play key roles in long and short distance transport of signals and nutrients, and act as a barrier against local and systemic pathogen infection. Among other components, these plant fluids contain proteins which are likely to be important players in their functionalities. However, detailed information about their proteomes is only starting to arise due to the difficulties inherent to the collection methods. This review compiles the proteomic information available to date in these three plant fluids, and compares the proteomes obtained in different plant species in order to shed light into conserved functions in each plant fluid. Inter-species comparisons indicate that all these fluids contain the protein machinery for self-maintenance and defense, including proteins related to cell wall metabolism, pathogen defense, proteolysis, and redox response. These analyses also revealed that proteins may play more relevant roles in signaling in the phloem sap and apoplastic fluid than in the xylem sap. A comparison of the proteomes of the three fluids indicates that although functional categories are somewhat similar, proteins involved are likely to be fluid-specific, except for a small group of proteins present in the three fluids, which may have a universal role, especially in cell wall maintenance and defense. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

Tackling Unwanted Proteolysis in Plant Production Hosts Used for Molecular Farming

Although the field of molecular farming has significantly matured over the last years, some obstacles still need to be resolved. A major limiting factor for a broader application of plant hosts for the production of valuable recombinant proteins is the low yield of intact recombinant proteins. These low yields are at least in part due to the action of endogenous plant proteases on the foreign recombinant proteins. This mini review will present the current knowledge of the proteolytic enzymes involved in the degradation of different target proteins and strategies that are applied to suppress undesirable proteolytic activities in order to safeguard recombinant proteins during the production process.

The Plant Ovule Secretome: A Different View toward Pollen-Pistil Interactions

During plant sexual reproduction, continuous exchange of signals between the pollen and the pistil (stigma, style, and ovary) plays important roles in pollen recognition and selection, establishing breeding barriers and, ultimately, leading to optimal seed set. After navigating through the stigma and the style, pollen tubes (PTs) reach their final destination, the ovule. This ultimate step is also regulated by numerous signals emanating from the embryo sac (ES) of the ovule. These signals encompass a wide variety of molecules, but species-specificity of the pollen-ovule interaction relies mainly on secreted proteins and their receptors. Isolation of candidate genes involved in pollen-pistil interactions has mainly relied on transcriptomic approaches, overlooking potential post-transcriptional regulation. To address this issue, ovule exudates were collected from the wild potato species Solanum chacoense using a tissue-free gravity-extraction method (tf-GEM). Combined RNA-seq and mass spectrometry-based proteomics led to the identification of 305 secreted proteins, of which 58% were ovule-specific. Comparative analyses using mature ovules (attracting PTs) and immature ovules (not attracting PTs) revealed that the last maturation step of ES development affected almost half of the ovule secretome. Of 128 upregulated proteins in anthesis stage, 106 were not regulated at the mRNA level, emphasizing the importance of post-transcriptional regulation in reproductive development.

Leaf proteome rebalancing in Nicotiana benthamiana for upstream enrichment of a transiently expressed recombinant protein

A key factor influencing the yield of biopharmaceuticals in plants is the ratio of recombinant to host proteins in crude extracts. Postextraction procedures have been devised to enrich recombinant proteins before purification. Here, we assessed the potential of methyl jasmonate (MeJA) as a generic trigger of recombinant protein enrichment in Nicotiana benthamiana leaves before harvesting. Previous studies have reported a significant rebalancing of the leaf proteome via the jasmonate signalling pathway, associated with ribulose 1,5-bisphosphate carboxylase oxygenase (RuBisCO) depletion and the up-regulation of stress-related proteins. As expected, leaf proteome alterations were observed 7 days post-MeJA treatment, associated with lowered RuBisCO pools and the induction of stress-inducible proteins such as protease inhibitors, thionins and chitinases. Leaf infiltration with the Agrobacterium tumefaciens bacterial vector 24 h post-MeJA treatment induced a strong accumulation of pathogenesis-related proteins after 6 days, along with a near-complete reversal of MeJA-mediated stress protein up-regulation. RuBisCO pools were partly restored upon infiltration, but most of the depletion effect observed in noninfiltrated plants was maintained over six more days, to give crude protein samples with 50% less RuBisCO than untreated tissue. These changes were associated with net levels reaching 425 μg/g leaf tissue for the blood-typing monoclonal antibody C5-1 expressed in MeJA-treated leaves, compared to less than 200 μg/g in untreated leaves. Our data confirm overall the ability of MeJA to trigger RuBisCO depletion and recombinant protein enrichment in N. benthamiana leaves, estimated here for C5-1 at more than 2-fold relative to host proteins.

Isolation and Analysis of Cell Wall Proteome in Elsholtzia splendens Roots Using ITRAQ with LC-ESI-MS/MS

Cell wall proteins (CWPs) are a prime site for signal perception and defense responses to environmental stresses. To gain further insights into CWPs and their molecular function, traditional techniques (e.g., two-dimensional gel electrophoresis) may be ineffective for special proteins. Elsholtzia splendens is a copper-tolerant plant species that grow on copper deposits. In this study, a fourplex isobaric tag was used for relative and absolute quantitation with liquid chromatography-tandem mass spectrometry approach to analyze the root CWPs of E. splendens. A total of 479 unique proteins were identified, including 121 novel proteins. Approximately 80.79 % of the proteins were extracted in the CaCl2 fraction, 16.08 % were detected in the NaCl fraction, and 3.13 % were identified in both fractions. The identified proteins have been involved in various processes, including cell wall remodeling, signal transduction, defense, and carbohydrate metabolism, thereby indicating a complex regulatory network in the apoplast of E. splendens roots. This study presents the first large-scale analysis of CWPs in metal-tolerant plants, which may be of paramount importance to understand the molecular functions and metabolic pathways in the root cell wall of copper-tolerant plants.

Protein profile of Beta vulgaris leaf apoplastic fluid and changes induced by Fe deficiency and Fe resupply

The fluid collected by direct leaf centrifugation has been used to study the proteome of the sugar beet apoplastic fluid as well as the changes induced by Fe deficiency and Fe resupply to Fe-deficient plants in the protein profile. Plants were grown in Fe-sufficient and Fe-deficient conditions, and Fe resupply was carried out with 45 μM Fe(III)-EDTA for 24 h. Protein extracts of leaf apoplastic fluid were analyzed by two-dimensional isoelectric focusing-SDS-PAGE electrophoresis. Gel image analysis revealed 203 consistent spots, and proteins in 81% of them (164) were identified by nLC-MS/MS using a custom made reference repository of beet protein sequences. When redundant UniProt entries were deleted, a non-redundant leaf apoplastic proteome consisting of 109 proteins was obtained. TargetP and SecretomeP algorithms predicted that 63% of them were secretory proteins. Functional classification of the non-redundant proteins indicated that stress and defense, protein metabolism, cell wall and C metabolism accounted for approximately 75% of the identified proteome. The effects of Fe-deficiency on the leaf apoplast proteome were limited, with only five spots (2.5%) changing in relative abundance, thus suggesting that protein homeostasis in the leaf apoplast fluid is well-maintained upon Fe shortage. The identification of three chitinase isoforms among proteins increasing in relative abundance with Fe-deficiency suggests that one of the few effects of Fe deficiency in the leaf apoplast proteome includes cell wall modifications. Iron resupply to Fe deficient plants changed the relative abundance of 16 spots when compared to either Fe-sufficient or Fe-deficient samples. Proteins identified in these spots can be broadly classified as those responding to Fe-resupply, which included defense and cell wall related proteins, and non-responsive, which are mainly protein metabolism related proteins and whose changes in relative abundance followed the same trend as with Fe-deficiency.

New insights into regulation of proteome and polysaccharide in cell wall of Elsholtzia splendens in response to copper stress

BACKGROUND AND AIMS

Copper (Cu) is an essential micronutrient for plants. However, excess amounts of Cu are toxic and result in a wide range of harmful effects on the physiological and biochemical processes of plants. Cell wall has a crucial role in plant defense response to toxic metals. To date, the process of cell wall response to Cu and the detoxification mechanism have not been well documented at the proteomic level.

METHODS

An recently developed 6-plex Tandem Mass Tag was used for relative and absolute quantitation methods to achieve a comprehensive understanding of Cu tolerance/detoxification molecular mechanisms in the cell wall. LC-MS/MS approach was performed to analyze the Cu-responsive cell wall proteins and polysaccharides.

KEY RESULTS

The majority of the 22 up-regulated proteins were involved in the antioxidant defense pathway, cell wall polysaccharide remodeling, and cell metabolism process. Changes in polysaccharide amount, composition, and distribution could offer more binding sites for Cu ions. The 33 down-regulated proteins were involved in the signal pathway, energy, and protein synthesis.

CONCLUSIONS

Based on the abundant changes in proteins and polysaccharides, and their putative functions, a possible protein interaction network can provide new insights into Cu stress response in root cell wall. Cu can facilitate further functional research on target proteins associated with metal response in the cell wall.

Plant Cell Wall Proteins: A Large Body of Data, but What about Runaways?

Plant cell wall proteomics has been a very dynamic field of research for about fifteen years. A full range of strategies has been proposed to increase the number of identified proteins and to characterize their post-translational modifications. The protocols are still improving to enlarge the coverage of cell wall proteomes. Comparisons between these proteomes have been done based on various working strategies or different physiological stages. In this review, two points are highlighted. The first point is related to data analysis with an overview of the cell wall proteomes already described. A large body of data is now available with the description of cell wall proteomes of seventeen plant species. CWP contents exhibit particularities in relation to the major differences in cell wall composition and structure between these plants and between plant organs. The second point is related to methodology and concerns the present limitations of the coverage of cell wall proteomes. Because of the variety of cell wall structures and of the diversity of protein/polysaccharide and protein/protein interactions in cell walls, some CWPs can be missing either because they are washed out during the purification of cell walls or because they are covalently linked to cell wall components.

Uncovering plant-pathogen crosstalk through apoplastic proteomic studies

Plant pathogens have evolved by developing different strategies to infect their host, which in turn have elaborated immune responses to counter the pathogen invasion. The apoplast, including the cell wall and extracellular space outside the plasma membrane, is one of the first compartments where pathogen-host interaction occurs. The plant cell wall is composed of a complex network of polysaccharides polymers and glycoproteins and serves as a natural physical barrier against pathogen invasion. The apoplastic fluid, circulating through the cell wall and intercellular spaces, provides a means for delivering molecules and facilitating intercellular communications. Some plant-pathogen interactions lead to plant cell wall degradation allowing pathogens to penetrate into the cells. In turn, the plant immune system recognizes microbial- or damage-associated molecular patterns (MAMPs or DAMPs) and initiates a set of basal immune responses, including the strengthening of the plant cell wall. The establishment of defense requires the regulation of a wide variety of proteins that are involved at different levels, from receptor perception of the pathogen via signaling mechanisms to the strengthening of the cell wall or degradation of the pathogen itself. A fine regulation of apoplastic proteins is therefore essential for rapid and effective pathogen perception and for maintaining cell wall integrity. This review aims to provide insight into analyses using proteomic approaches of the apoplast to highlight the modulation of the apoplastic protein patterns during pathogen infection and to unravel the key players involved in plant-pathogen interaction.

Proteolysis of recombinant proteins in bioengineered plant cells

Plants are increasingly used as alternative expression hosts for the production of recombinant proteins offering many advantages including higher biomass and the ability to perform post-translational modifications on complex proteins. Key challenges for optimized accumulation of recombinant proteins in a plant system still remain, including endogenous plant proteolytic activity, which may severely compromise recombinant protein stability. Several strategies have recently been applied to improve protein stability by limiting protease action such as recombinant protein production in various sub-cellular compartments or application of protease inhibitors to limit protease action. A short update on the current strategies applied is provided here, with particular focus on sub-cellular sites previously selected for recombinant protein production and the co-expression of protease inhibitors to limit protease activity.

Plant secretomics: identification, isolation, and biological significance under environmental stress

Plant secretomes are the proteins secreted by the plant cells and are involved in the maintenance of cell wall structure, relationship between host and pathogen, communication between different cells in the plant, etc. Amalgamation of methodologies like bioinformatics, biochemical, and proteomics are used to separate, classify, and outline secretomes by means of harmonizing in planta systems and in vitro suspension cultured cell system (SSCs). We summed up and explained the meaning of secretome, methods used for the identification and isolation of secreted proteins from extracellular space and methods for the assessment of purity of secretome proteins in this review. Two D PAGE method and HPLC based methods for the analysis together with different bioinformatics tools used for the prediction of secretome proteins are also discussed. Biological significance of secretome proteins under different environmental stresses, i.e., salt stress, drought stress, oxidative stress, etc., defense responses and plant interactions with environment are also explained in detail.

Protection of recombinant mammalian antibodies from development-dependent proteolysis in leaves of Nicotiana benthamiana

The expression of clinically useful proteins in plants has been bolstered by the development of high-yielding systems for transient protein expression using agroinfiltration. There is a need now to know more about how host plant development and metabolism influence the quantity and quality of recombinant proteins. Endogenous proteolysis is a key determinant of the stability and yield of recombinant proteins in plants. Here we characterised cysteine (C1A) and aspartate (A1) protease profiles in leaves of the widely used expression host Nicotiana benthamiana, in relation with the production of a murine IgG, C5-1, targeted to the cell secretory pathway. Agroinfiltration significantly altered the distribution of C1A and A1 proteases along the leaf age gradient, with a correlation between leaf age and the level of proteolysis in whole-cell and apoplast protein extracts. The co-expression of tomato cystatin SlCYS8, an inhibitor of C1A proteases, alongside C5-1 increased antibody yield by nearly 40% after the usual 6-days incubation period, up to ~3 mg per plant. No positive effect of SlCYS8 was observed in oldest leaves, in line with an increased level of C1A protease activity and a very low expression rate of the inhibitor. By contrast, C5-1 yield was greater by an additional 40% following 8- to 10-days incubations in younger leaves, where high SlCYS8 expression was maintained. These findings confirm that the co-expression of recombinant protease inhibitors is a promising strategy for increasing recombinant protein yields in plants, but that further opportunity exists to improve this approach by addressing the influence of leaf age and proteases of other classes.

Plant cell wall proteomics: the leadership of Arabidopsis thaliana

Plant cell wall proteins (CWPs) progressively emerged as crucial components of cell walls although present in minor amounts. Cell wall polysaccharides such as pectins, hemicelluloses, and cellulose represent more than 90% of primary cell wall mass, whereas hemicelluloses, cellulose, and lignins are the main components of lignified secondary walls. All these polymers provide mechanical properties to cell walls, participate in cell shape and prevent water loss in aerial organs. However, cell walls need to be modified and customized during plant development and in response to environmental cues, thus contributing to plant adaptation. CWPs play essential roles in all these physiological processes and particularly in the dynamics of cell walls, which requires organization and rearrangements of polysaccharides as well as cell-to-cell communication. In the last 10 years, plant cell wall proteomics has greatly contributed to a wider knowledge of CWPs. This update will deal with (i) a survey of plant cell wall proteomics studies with a focus on Arabidopsis thaliana; (ii) the main protein families identified and the still missing peptides; (iii) the persistent issue of the non-canonical CWPs; (iv) the present challenges to overcome technological bottlenecks; and (v) the perspectives beyond cell wall proteomics to understand CWP functions.

Plant secretome - from cellular process to biological activity

Recent studies suggest that plants secrete a large number of proteins and peptides into the extracellular space. Secreted proteins play a crucial role in stress response, communication and development of organisms. Here we review the current knowledge of the secretome of more than ten plant species, studied in natural conditions or during (a)biotic stress. This review not only deals with the classical secretory route via endoplasmic reticulum and Golgi followed by proteins containing a known N-terminal signal peptide, but also covers new findings about unconventional secretion of leaderless proteins. We describe alternative secretion pathways and the involved compartments like the recently discovered EXPO. The well characterized secreted peptides that function as ligands of receptor proteins exemplify the biological significance and activity of the secretome. This article is part of a Special Issue entitled: An Updated Secretome.

Large-scale proteomic analysis of the grapevine leaf apoplastic fluid reveals mainly stress-related proteins and cell wall modifying enzymes

BACKGROUND

The extracellular space or apoplast forms a path through the whole plant and acts as an interface with the environment. The apoplast is composed of plant cell wall and space within which apoplastic fluid provides a means of delivering molecules and facilitates intercellular communications. However, the apoplastic fluid extraction from in planta systems remains challenging and this is particularly true for grapevine (Vitis vinifera L.), a worldwide-cultivated fruit plant. Large-scale proteomic analysis reveals the protein content of the grapevine leaf apoplastic fluid and the free interactive proteome map considerably facilitates the study of the grapevine proteome.

RESULTS

To obtain a snapshot of the grapevine apoplastic fluid proteome, a vacuum-infiltration-centrifugation method was optimized to collect the apoplastic fluid from non-challenged grapevine leaves. Soluble apoplastic protein patterns were then compared to whole leaf soluble protein profiles by 2D-PAGE analyses. Subsequent MALDI-TOF/TOF mass spectrometry of tryptically digested protein spots was used to identify proteins. This large-scale proteomic analysis established a well-defined proteomic map of whole leaf and leaf apoplastic soluble proteins, with 223 and 177 analyzed spots, respectively. All data arising from proteomic, MS and MS/MS analyses were deposited in the public database world-2DPAGE. Prediction tools revealed a high proportion of (i) classical secreted proteins but also of non-classical secreted proteins namely Leaderless Secreted Proteins (LSPs) in the apoplastic protein content and (ii) proteins potentially involved in stress reactions and/or in cell wall metabolism.

CONCLUSIONS

This approach provides free online interactive reference maps annotating a large number of soluble proteins of the whole leaf and the apoplastic fluid of grapevine leaf. To our knowledge, this is the first detailed proteome study of grapevine apoplastic fluid providing a comprehensive overview of the most abundant proteins present in the apoplast of grapevine leaf that could be further characterized in order to elucidate their physiological function.

Wounding, insect chewing and phloem sap feeding differentially alter the leaf proteome of potato, Solanum tuberosum L

Background

Various factors shape the response of plants to herbivorous insects, including wounding patterns, specific chemical effectors and feeding habits of the attacking herbivore. Here we performed a comparative proteomic analysis of the plant's response to wounding and herbivory, using as a model potato plants (Solanum tuberosum L.) subjected to mechanical wounding, defoliation by the Colorado potato beetle Leptinotarsa decemlineata Say, or phloem sap feeding by the potato aphid Macrosiphum euphorbiae Thomas.

Results

Out of ~500 leaf proteins monitored by two-dimensional gel electrophoresis (2-DE), 31 were up- or downregulated by at least one stress treatment compared to healthy control plants. Of these proteins, 29 were regulated by beetle chewing, 8 by wounding and 8 by aphid feeding. Some proteins were up- or downregulated by two different treatments, while others showed diverging expression patterns in response to different treatments. A number of modulated proteins identified by mass spectrometry were typical defense proteins, including wound-inducible protease inhibitors and pathogenesis-related proteins. Proteins involved in photosynthesis were also modulated, notably by potato beetle feeding inducing a strong decrease of some photosystem I proteins. Quantitative RT PCR assays were performed with nucleotide primers for photosynthesis-related proteins to assess the impact of wounding and herbivory at the gene level. Whereas different, sometimes divergent, responses were observed at the proteome level in response to wounding and potato beetle feeding, downregulating effects were systematically observed for both treatments at the transcriptional level.

Conclusions

These observations illustrate the differential impacts of wounding and insect herbivory on defense- and photosynthesis-related components of the potato leaf proteome, likely associated with the perception of distinct physical and chemical cues in planta.

A protease activity-depleted environment for heterologous proteins migrating towards the leaf cell apoplast

Recombinant proteins face major constraints along the plant cell secretory pathway, including proteolytic processing compromising their structural integrity. Here, we demonstrate the potential of protease inhibitors as in situ stabilizing agents for recombinant proteins migrating towards the leaf apoplast. Genomic data for Arabidopsis, rice and Nicotiana spp. were assessed to determine the relative incidence of protease families in the cell secretory pathway. Transient expression assays with the model platform Nicotiana benthamiana were then performed to test the efficiency of protease inhibitors in stabilizing proteins targeted to the apoplast. Current genomic data suggest the occurrence of proteases from several families along the secretory pathway, including A1 and A22 Asp proteases; C1A and C13 Cys proteases; and S1, S8 and S10 Ser proteases. In vitro protease assays confirmed the presence of various proteases in N. benthamiana leaves, notably pointing to the deposition of A1- and S1-type activities preferentially in the apoplast. Accordingly, transient expression and secretion of the A1/S1 protease inhibitor, tomato cathepsin D inhibitor (SlCDI), negatively altered A1 and S1 protease activities in this cell compartment, while increasing the leaf apoplast protein content by ∼45% and improving the accumulation of a murine diagnostic antibody, C5-1, co-secreted in the apoplast. SlCYS9, an inhibitor of C1A and C13 Cys proteases, had no impact on the apoplast proteases and protein content, but stabilized C5-1 in planta, presumably upstream in the secretory pathway. These data confirm, overall, the potential of protease inhibitors for the in situ protection of recombinant proteins along the plant cell secretory pathway.

Comparative evaluation of extraction methods for apoplastic proteins from maize leaves

Proteins in the plant apoplast are essential for many physiological processes. We have analysed and compared six different infiltration solutions for proteins contained in the apoplast to recognize the most suitable method for leaves and to establish proteome maps for each extraction. The efficiency of protocols was evaluated by comparing the protein patterns resolved by 1-DE and 2-DE, and revealed distinct characteristics for each infiltration solution. Nano-LC-ESI-Q-TOF MS analysis of all fractions was applied to cover all proteins differentially extracted by infiltration solutions and led to the identification of 328 proteins in total in apoplast preparations. The predicted subcellular protein localisation distinguished the examined infiltration solutions in those with high or low amounts of intracellular protein contaminations, and with high or low quantities of secreted proteins. All tested infiltration solution extracted different subsets of proteins, and those implications on apoplast-specific studies are discussed.

Comparative proteomics analysis of the root apoplasts of rice seedlings in response to hydrogen peroxide

Background

Plant apoplast is the prime site for signal perception and defense response, and of great importance in responding to environmental stresses. Hydrogen peroxide (H₂O₂) plays a pivotal role in determining the responsiveness of cells to stress. However, how the apoplast proteome changes under oxidative condition is largely unknown. In this study, we initiated a comparative proteomic analysis to explore H₂O₂-responsive proteins in the apoplast of rice seedling roots.

Methodology/Principal Findings

14-day-old rice seedlings were treated with low concentrations (300 and 600 µM) of H₂O₂ for 6 h and the levels of relative electrolyte leakage, malondialdehyde and H₂O₂ were assayed in roots. The modified vacuum infiltration method was used to extract apoplast proteins of rice seedling roots, and then two-dimensional electrophoresis gel analysis revealed 58 differentially expressed protein spots under low H₂O₂ conditions. Of these, 54 were successfully identified by PMF or MS/MS as matches to 35 different proteins including known and novel H₂O₂-responsive proteins. Almost all of these identities (98%) were indeed apoplast proteins confirmed either by previous experiments or through publicly available prediction programs. These proteins identified are involved in a variety of processes, including redox homeostasis, cell wall modification, signal transduction, cell defense and carbohydrate metabolism, indicating a complex regulative network in the apoplast of seedling roots under H₂O₂ stress.

Conclusions/Significance

The present study is the first apoplast proteome investigation of plant seedlings in response to H₂O₂ and may be of paramount importance for the understanding of the plant network to environmental stresses. Based on the abundant changes in these proteins, together with their putative functions, we proposed a possible protein network that provides new insights into oxidative stress response in the rice root apoplast and clues for the further functional research of target proteins associated with H₂O₂ response.

Rapid transient production in plants by replicating and non-replicating vectors yields high quality functional anti-HIV antibody

BACKGROUND

The capacity of plants and plant cells to produce large amounts of recombinant protein has been well established. Due to advantages in terms of speed and yield, attention has recently turned towards the use of transient expression systems, including viral vectors, to produce proteins of pharmaceutical interest in plants. However, the effects of such high level expression from viral vectors and concomitant effects on host cells may affect the quality of the recombinant product.

METHODOLOGY/PRINCIPAL FINDINGS

To assess the quality of antibodies transiently expressed to high levels in plants, we have expressed and characterised the human anti-HIV monoclonal antibody, 2G12, using both replicating and non-replicating systems based on deleted versions of Cowpea mosaic virus (CPMV) RNA-2. The highest yield (approximately 100 mg/kg wet weight leaf tissue) of affinity purified 2G12 was obtained when the non-replicating CPMV-HT system was used and the antibody was retained in the endoplasmic reticulum (ER). Glycan analysis by mass-spectrometry showed that the glycosylation pattern was determined exclusively by whether the antibody was retained in the ER and did not depend on whether a replicating or non-replicating system was used. Characterisation of the binding and neutralisation properties of all the purified 2G12 variants from plants showed that these were generally similar to those of the Chinese hamster ovary (CHO) cell-produced 2G12.

CONCLUSIONS

Overall, the results demonstrate that replicating and non-replicating CPMV-based vectors are able to direct the production of a recombinant IgG similar in activity to the CHO-produced control. Thus, a complex recombinant protein was produced with no apparent effect on its biochemical properties using either high-level expression or viral replication. The speed with which a recombinant pharmaceutical with excellent biochemical characteristics can be produced transiently in plants makes CPMV-based expression vectors an attractive option for biopharmaceutical development and production.