The arcelin-5 gene of Phaseolus vulgaris directs high seed-specific expression in transgenic Phaseolus acutifolius and Arabidopsis plants

Production of Designer VHH-Based Antibodies in Plants

Simplified monoclonal antibodies can be produced by fusing a VHH or nanobody, derived from camelid heavy-chain-only antibodies to the Fc domain of either IgG (VHH-IgG), IgA (VHH-IgA), or IgY (VHH-IgY). These recombinant antibodies are encoded by a single gene and their production can be easily scaled up in plants. This chapter contains methods for Gateway cloning of VHH-Fc fusions into the binary T-DNA vectors pEAQ-HT-DEST1 and pPhasGW, electroporation of Agrobacterium with the resulting constructs, transient antibody expression in Nicotiana benthamiana leaves, and stable antibody expression in Arabidopsis thaliana seeds. The properties of chimeric VHH-based antibodies produced in plants enable novel passive immunization treatments, such as in-feed oral delivery or intravenous injection.

Overexpression of rice glutaredoxin genes LOC_Os02g40500 and LOC_Os01g27140 regulate plant responses to drought stress

Glutaredoxins (Grxs) are small (10-15 kDa) glutathione (GSH) - dependent redox proteins. The role of Grxs are well documented in tolerance to heavy metal stress in prokaryotic and mammalian systems and a few plant genera, but is poorly understood in plants against drought. In the present study, two rice glutaredoxin (Osgrx) genes (LOC_Os02g40500 and LOC_Os01g27140) responsible for tolerance against heavy metal stress have been studied for investigating their role against drought. Each glutaredoxin gene was over-expressed in Arabidopsis thaliana to reveal their role in drought stress. The relative expression of both Osgrx genes was higher in the transgenic lines. Transgenic lines of both Osgrxs showed longer roots, higher seed germination, and survival efficiency during drought stress. The physiological parameters (P_N, g_s, E, WUE, qP, NPQ and ETR), antioxidant enzymes (GRX, GR, GPX, GST, APX, POD, SOD, CAT, DHAR, and MDHAR), antioxidant molecules (ascorbate and GSH) and stress-responsive amino acids (cysteine and proline) levels were additionally increased in transgenic lines of both Osgrxs to provide drought tolerance. The outcomes from this study strongly determined that each Osgrx gene participated in the moderation of drought and might be utilized in biological engineering strategies to overcome drought conditions in different crops.

Over-expression of chickpea glutaredoxin (CaGrx) provides tolerance to heavy metals by reducing metal accumulation and improved physiological and antioxidant defence system

Glutaredoxins (Grxs) are small multifunctional redox proteins. Grxs have glutathione-dependent oxidoreductase activity in the presence of glutathione reductase and NADPH. The role of Grxs is well studied in heavy metal tolerance in prokaryotic and mammalian systems but not in plant genera. In the present study, a chickpea glutaredoxin (CaGrx) gene (LOC101493651) has been investigated against metal stress based on its primary screening in chickpea which revealed higher up-regulation of CaGrx gene under various heavy metals (AsIII-25 μM, AsV-250 μM, Cr(VI)-300 μM, and Cd-500 μM) stress. This CaGrx gene was overexpressed in Arabidopsis thaliana and investigated various biochemical and physiological performances under each metal stress. Transgenic plants showed significant up-regulation of the CaGrx gene during qRT-PCR analysis as well as longer roots, higher seed germination, and survival efficiency during each metal stress. The levels of stress markers, TBARS, H₂O_2, and electrolyte leakage were found to be less in transgenic lines as compared to WT revealed less toxicity in transgenics. The total accumulation of AsIII, AsV, and Cr(VI) were significantly reduced in all transgenic lines except Cd, which was slightly reduced. The physiological parameters such as net photosynthetic rate (P_N), stomatal conductance (g_s), transpiration (E), water use efficiency (WUE), photochemical quenching (qP), and electron transport rate (ETR), were maintained in transgenic lines during metal stress. Various antioxidant enzymes such as glutaredoxin (GRX), glutathione reductase (GR), glutathione peroxidase (GPX), glutathione-S-transferase (GST), ascorbate peroxidase (APX), superoxide dismutase (SOD), catalase (CAT), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), antioxidant molecules (ascorbate, GSH) and stress-responsive amino acids (proline and cysteine) levels were significantly increased in transgenic lines which provide metal tolerance. The outcome of this study strongly indicates that the CaGrx gene participates in the moderation of metal stress in Arabidopsis, which can be utilized in biotechnological interventions to overcome heavy metal stress conditions in different crops.

Seeds as Economical Production Platform for Recombinant Proteins

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The cost-effective production of high-quality and biologically active recombinant molecules especially proteins is extremely desirable. Seed-based recombinant protein production platforms are considered as superior choice owing to lack of human/animal pathogenic organisms, lack of cold chain requirements for transportation and long-term storage, easy scalability and development of edible biopharmaceuticals in plants with objective to be used in purified or partially processed form is desirable. This review article summarizes the exceptional features of seed-based biopharming and highlights the needs of exploiting it for commercial purposes. Plant seeds offer a perfect production platform for high-value molecules of industrial as well as therapeutic nature owing to lower water contents, high protein storage capacity, weak protease activity and long-term storage ability at ambient temperature. Exploiting extraordinarily high protein accumulation potential, vaccine antigens, antibodies and other therapeutic proteins can be stored without effecting their stability and functionality up to years in seeds. Moreover, ability of direct oral consumption and post-harvest stabilizing effect of seeds offer unique feature of oral delivery of pharmaceutical proteins and vaccine antigens for immunization and disease treatment through mucosal as well as oral route.

Cytokinin inhibits cotton fiber initiation by disrupting PIN3a-mediated asymmetric accumulation of auxin in the ovule epidermis

Auxin-dependent cell expansion is crucial for initiation of fiber cells in cotton (Gossypium hirsutum), which ultimately determines fiber yield and quality. However, the regulation of this process is far from being well understood. In this study, we demonstrate an antagonistic effect between cytokinin (CK) and auxin on cotton fiber initiation. In vitro and in planta experiments indicate that enhanced CK levels can reduce auxin accumulation in the ovule integument, which may account for the defects in the fiberless mutant xu142fl. In turn, supplementation with auxin can recover fiber growth of CK-treated ovules and mutant ovules. We further found that GhPIN3a is a key auxin transporter for fiber-cell initiation and is polarly localized to the plasma membranes of non-fiber cells, but not to those of fiber cells. This polar localization allows auxin to be transported within the ovule integument while specifically accumulating in fiber cells. We show that CKs antagonize the promotive effect of auxin on fiber cell initiation by undermining asymmetric accumulation of auxin in the ovule epidermis through down-regulation of GhPIN3a and disturbance of the polar localization of the protein.

A vacuolar sorting receptor-independent sorting mechanism for storage vacuoles in soybean seeds

The seed storage proteins of soybean (Glycine max) are composed mainly of glycinin (11S globulin) and β-conglycinin (7S globulin). The subunits of glycinin (A1aB1b, A1bB2, A2B1a, A3B4, and A5A4B3) are synthesized as a single polypeptide precursor. These precursors are assembled into trimers with a random combination of subunits in the endoplasmic reticulum, and are sorted to the protein storage vacuoles. Proteins destined for transport to protein storage vacuoles possess a vacuolar sorting determinant, and in this regard, the A1aB1b subunit contains a C-terminal peptide that is sufficient for its sorting to protein storage vacuoles. The A3B4 subunit, however, lacks a corresponding C-terminal sorting determinant. In this study, we found that, unlike the A1aB1b subunit, the A3B4 subunit does not bind to previously reported vacuolar sorting receptors. Despite this difference, we observed that the A3B4 subunit is sorted to protein storage vacuoles in a transgenic soybean line expressing the A3B4 subunit of glycinin. These results indicate that a protein storage vacuolar sorting mechanism that functions independently of the known vacuolar sorting receptors in seeds might be present in soybean seeds.

Auxin Regulates Cotton Fiber Initiation via GhPIN-Mediated Auxin Transport

Cotton fibers are seed trichomes that make cotton unique compared with other plants. At anthesis, IAA, a major auxin in plants, accumulates in the fiber cell to promote cell initiation. However, many important aspects of this process are not clear. Here, auxin distribution patterns indicated by auxin-dependent DR5::GUS (β-glucuronidase) expression in cotton ovules were studied during fiber cell differentiation and cell initiation [-2 to 2 DPA (days post-anthesis)]. The nucellus and fiber cell were two major sites where auxin accumulates. The accumulation in the nucellus started from -1 DPA, and that in fiber cells from 0 DPA. Immunolocalization analysis further suggests that the IAA accumulation in fiber initials began before flower opening. Furthermore, we demonstrate that accumulated IAA in fiber initials was mainly from efflux transport and not from in situ synthesis. Eleven auxin efflux carrier (GhPIN) genes were identified, and their expression during ovule and fiber development was investigated. Ovule-specific suppression of multiple GhPIN genes in transgenic cotton inhibited both fiber initiation and elongation. In 0 DPA ovules, GhPIN3a, unlike other GhPIN genes, showed additional localization of the transcript in the outer integument. Collectively, these results demonstrate the important role of GhPIN-mediated auxin transport in fiber-specific auxin accumulation for fiber initiation.

The pharmaceutics from the foreign empire: the molecular pharming of the prokaryotic staphylokinase in Arabidopsis thaliana plants

Here, we present the application of microbiology and biotechnology for the production of recombinant pharmaceutical proteins in plant cells. To the best of our knowledge and belief it is one of few examples of the expression of the prokaryotic staphylokinase (SAK) in the eukaryotic system. Despite the tremendous progress made in the plant biotechnology, most of the heterologous proteins still accumulate to low concentrations in plant tissues. Therefore, the composition of expression cassettes to assure economically feasible level of protein production in plants remains crucial. The aim of our research was obtaining a high concentration of the bacterial anticoagulant factor—staphylokinase, in Arabidopsis thaliana seeds. The coding sequence of staphylokinase was placed under control of the β-phaseolin promoter and cloned between the signal sequence of the seed storage protein 2S2 and the carboxy-terminal KDEL signal sequence. The engineered binary vector pATAG-sak was introduced into Arabidopsis thaliana plants via Agrobacterium tumefaciens-mediated transformation. Analysis of the subsequent generations of Arabidopsis seeds revealed both presence of the sak and nptII transgenes, and the SAK protein. Moreover, a plasminogen activator activity of staphylokinase was observed in the protein extracts from seeds, while such a reaction was not observed in the leaf extracts showing seed-specific activity of the β-phaseolin promoter.

Expression of bioactive anti-CD20 antibody fragments and induction of ER stress response in Arabidopsis seeds

Seed-based expression system is an attractive platform for the production of recombinant proteins in molecular farming. Despite the many advantages of molecular farming, little is known about the effect of the different subcellular accumulation of recombinant proteins on the endoplasmic reticulum (ER) quality control system in host plants. In this study, we analyzed the expression of anti-CD20 antibody fragments in seeds of Arabidopsis thaliana (ecotype Columbia) and corresponding glycosylation mutants, and evaluated the influence of three different signal sequences on the expression levels of scFv-Fc of C2B8. The highest protein accumulation level, with a maximum of 6.12 % total soluble proteins, was observed upon fusing proteins to the signal peptide of Arabidopsis seed storage albumin 2. The ER stress responses in developing seeds at 13 days post-anthesis were also compared across different transgenic lines under normal and heat shock conditions. Based on the gene expression profiles of ER stress transducers, our results suggest that accumulation of antibody fragments in the ER exerts more stress on ER homeostasis. In addition, quantitative PCR results also implicate enhanced activation of ER-associated degradation in transgenic lines. Last but not the least, we also demonstrate the anti-tumor potency of plant-derived proteins by showing the anti-tumor activity of purified scFv-Fc proteins against Daudi cells. Together, our data implies that better understanding of the interaction between exogenous protein production and the cellular quality control system of the host plant is necessary for the development of an optimal expression strategy that will be especially beneficial to commercial protein manufacturing.

Seed-to-seed-to-seed growth and development of Arabidopsis in microgravity

Arabidopsis thaliana was grown from seed to seed wholly in microgravity on the International Space Station. Arabidopsis plants were germinated, grown, and maintained inside a growth chamber prior to returning to Earth. Some of these seeds were used in a subsequent experiment to successfully produce a second (back-to-back) generation of microgravity-grown Arabidopsis. In general, plant growth and development in microgravity proceeded similarly to those of the ground controls, which were grown in an identical chamber. Morphologically, the most striking feature of space-grown Arabidopsis was that the secondary inflorescence branches and siliques formed nearly perpendicular angles to the inflorescence stems. The branches grew out perpendicularly to the main inflorescence stem, indicating that gravity was the key determinant of branch and silique angle and that light had either no role or a secondary role in Arabidopsis branch and silique orientation. Seed protein bodies were 55% smaller in space seed than in controls, but protein assays showed only a 9% reduction in seed protein content. Germination rates for space-produced seed were 92%, indicating that the seeds developed in microgravity were healthy and viable. Gravity is not necessary for seed-to-seed growth of plants, though it plays a direct role in plant form and may influence seed reserves.

Boosting in planta production of antigens derived from the porcine reproductive and respiratory syndrome virus (PRRSV) and subsequent evaluation of their immunogenicity

Porcine reproductive and respiratory syndrome (PRRS) is a disease of swine, caused by an arterivirus, the PRRS virus (PRRSV). This virus infects pigs worldwide and causes huge economic losses. Due to genetic drift, current vaccines are losing their power. Adaptable vaccines could provide a solution to this problem. This study aims at producing in planta a set of antigens derived from the PRRSV glycoproteins (GPs) to be included in a subunit vaccine. We selected the GP3, GP4 and GP5 and optimized these for production in an Arabidopsis seed platform by removing transmembrane domains (Tm) and/or adding stabilizing protein domains, such as the green fluorescent protein (GFP) and immunoglobulin (IgG) ‘Fragment crystallizable’ (Fc) chains. Accumulation of the GPs with and without Tm was low, reaching no more than 0.10% of total soluble protein (TSP) in homozygous seed. However, addition of stabilizing domains boosted accumulation up to a maximum of 2.74% of TSP when GFP was used, and albeit less effectively, also the Fc chains of the porcine IgG3 and murine IgG2a increased antigen accumulation, to 0.96% and 1.81% of TSP respectively, while the murine IgG3 Fc chain did not. Antigens with Tm were less susceptible to these manipulations to increase yield. All antigens were produced in the endoplasmic reticulum and accordingly, they carried high-mannose N-glycans. The immunogenicity of several of those antigens was assessed and we show that vaccination with purified antigens did elicit the production of antibodies with virus neutralizing activity in mice but not in pigs.

Strong seed-specific protein expression from the Vigna radiata storage protein 8SGα promoter in transgenic Arabidopsis seeds

Vigna radiata (mung bean) is an important crop plant and is a major protein source in developing countries. Mung bean 8S globulins constitute nearly 90% of total seed storage protein and consist of three subunits designated as 8SGα, 8SGα' and 8SGβ. The 5'-flanking sequences of 8SGα' has been reported to confer high expression in transgenic Arabidopsis seeds. In this study, a 472-bp 5'-flanking sequence of 8SGα was identified by genome walking. Computational analysis subsequently revealed the presence of numerous putative seed-specific cis-elements within. The 8SGα promoter was then fused to the gene encoding β-glucuronidase (GUS) to create a reporter construct for Arabidopsis thaliana transformation. The spatial and temporal expression of 8SGα∷GUS, as investigated using GUS histochemical assays, showed GUS expression exclusively in transgenic Arabidopsis seeds. Quantitative GUS assays revealed that the 8SGα promoter showed 2- to 4-fold higher activity than the Cauliflower Mosaic Virus (CaMV) 35S promoter. This study has identified a seed-specific promoter of high promoter strength, which is potentially useful for directing foreign protein expression in seed bioreactors.

Generation of VHH antibodies against the Arabidopsis thaliana seed storage proteins

Antibodies and antibody derived fragments are excellent tools for the detection and purification of proteins. However, only few antibodies targeting Arabidopsis seed proteins are currently available. Here, we evaluate the process to make antibody libraries against crude protein extracts and more particularly to generate a VHH phage library against native Arabidopsis thaliana seed proteins. After immunising a dromedary with a crude Arabidopsis seed extract, we cloned the single-domain antigen-binding fragments from their heavy-chain only antibodies in a phage display vector and selected nanobodies (VHHs) against native Arabidopsis seed proteins. For 16 VHHs, the corresponding antigens were identified by affinity purification and MS/MS analysis. They were shown to bind the major Arabidopsis seed storage proteins albumin and globulin (14 to albumin and 2 to globulin). All 16 VHHs were suitable primary reagents for the detection of the Arabidopsis seed storage proteins by ELISA. Furthermore, several of the anti-albumin VHHs were used successfully for storage protein localisation via electron microscopy. The easy cloning, selection and production, together with the demonstrated functionality and applicability, strongly suggest that the VHH antibody format will play a more prominent role in future protein research, in particular for the study of native proteins.

Fusion of an Fc chain to a VHH boosts the accumulation levels in Arabidopsis seeds

Nanobodies® (VHHs) provide powerful tools in therapeutic and biotechnological applications. Nevertheless, for some applications, bivalent antibodies perform much better, and for this, an Fc chain can be fused to the VHH domain, resulting in a bivalent homodimeric VHH-Fc complex. However, the production of bivalent antibodies in Escherichia coli is rather inefficient. Therefore, we compared the production of VHH7 and VHH7-Fc as antibodies of interest in Arabidopsis seeds for detecting prostate-specific antigen (PSA), a well-known biomarker for prostate cancer in the early stages of tumour development. The influence of the signal sequence (camel versus plant) and that of the Fc chain origin (human, mouse or pig) were evaluated. The accumulation levels of VHHs were very low, with a maximum of 0.13% VHH of total soluble protein (TSP) in homozygous T3 seeds, while VHH-Fc accumulation levels were at least 10- to 100-fold higher, with a maximum of 16.25% VHH-Fc of TSP. Both the camel and plant signal peptides were efficiently cleaved off and did not affect the accumulation levels. However, the Fc chain origin strongly affected the degree of proteolysis, but only had a slight influence on the accumulation level. Analysis of the mRNA levels suggested that the low amount of VHHs produced in Arabidopsis seeds was not due to a failure in transcription, but rather to translation inefficiency, protein instability and/or degradation. Most importantly, the plant-produced VHH7 and VHH7-Fc antibodies were functional in detecting PSA and could thus be used for diagnostic applications.

Production of active human glucocerebrosidase in seeds of Arabidopsis thaliana complex-glycan-deficient (cgl) plants

There is a clear need for efficient methods to produce protein therapeutics requiring mannose-termination for therapeutic efficacy. Here we report on a unique system for production of active human lysosomal acid β-glucosidase (glucocerebrosidase, GCase, EC 3.2.1.45) using seeds of the Arabidopsis thaliana complex-glycan-deficient (cgl) mutant, which are deficient in the activity of N-acetylglucosaminyl transferase I (EC 2.4.1.101). Gaucher disease is a prevalent lysosomal storage disease in which affected individuals inherit mutations in the gene (GBA1) encoding GCase. A gene cassette optimized for seed expression was used to generate the human enzyme in seeds of the cgl (C5) mutant, and the recombinant GCase was mainly accumulated in the apoplast. Importantly, the enzymatic properties including kinetic parameters, half-maximal inhibitory concentration of isofagomine and thermal stability of the cgl-derived GCase were comparable with those of imiglucerase, a commercially available recombinant human GCase used for enzyme replacement therapy in Gaucher patients. N-glycan structural analyses of recombinant cgl-GCase showed that the majority of the N-glycans (97%) were mannose terminated. Additional purification was required to remove ∼15% of the plant-derived recombinant GCase that possessed potentially immunogenic (xylose- and/or fucose-containing) N-glycans. Uptake of cgl-derived GCase by mouse macrophages was similar to that of imiglucerase. The cgl seed system requires no addition of foreign (non-native) amino acids to the mature recombinant GCase protein, and the dry transgenic seeds represent a stable repository of the therapeutic protein. Other strategies that may completely prevent plant-like complex N-glycans are discussed, including the use of a null cgl mutant.

Overproduction of recombinant proteins in plants

Recombinant protein production in microbial hosts and animal cell cultures has revolutionized the pharmaceutical and industrial enzyme industries. Plants as alternative hosts for the production of recombinant proteins are being actively pursued, taking advantage of their unique characteristics. The key to cost-efficient production in any system is the level of protein accumulation, which is inversely proportional to the cost. Levels of up to 5 g/kg biomass have been obtained in plants, making this production system competitive with microbial hosts. Increasing protein accumulation at the cellular level by varying host, germplasm, location of protein accumulation, and transformation procedure is reviewed. At the molecular level increased expression by improving transcription, translation and accumulation of the protein is critically evaluated. The greatest increases in protein accumulation will occur when various optimized parameters are more fully integrated with each other. Because of the complex nature of plants, this will take more time and effort to accomplish than has been the case for the simpler unicellular systems. However the potential for plants to become one of the major avenues for protein production appears very promising.

Production of camel-like antibodies in plants

Transgenic plants for the production of high-value recombinant complex and/or glycosylated proteins are a promising alternative for conventional systems, such as mammalian cells and bacteria. Many groups use plants as production platform for antibodies and antibody fragments. Here, we describe how bivalent camel-like antibodies can be produced in leaves and seeds. Camel-like antibodies are fusions of the antigen-binding domain of heavy chain camel antibodies (VHH) with an Fc fragment of choice. Transient expression in Nicotiana benthamiana leaves allows the production of VHH-Fc antibodies within a few days after the expression plasmid has been obtained. Generation of stable Arabidopsis thaliana transformants allows production of scalable amounts of VHH-Fc antibodies in seeds within a year. Further, we describe how the in planta-produced VHH-Fc antibodies can be quantified by Western blot analysis with Fc-specific antibodies.

Genetic analysis of morphological traits in a new, versatile, rapid-cycling Brassica rapa recombinant inbred line population

A recombinant inbred line (RIL) population was produced based on a wide cross between the rapid-cycling and self-compatible genotypes L58, a Caixin vegetable type, and R-o-18, a yellow sarson oil type. A linkage map based on 160 F7 lines was constructed using 100 Single nucleotide polymorphisms (SNPs), 130 AFLP®, 27 InDel, and 13 publicly available SSR markers. The map covers a total length of 1150 centiMorgan (cM) with an average resolution of 4.3 cM/marker. To demonstrate the versatility of this new population, 17 traits, related to plant architecture and seed characteristics, were subjected to quantitative trait loci (QTL) analysis. A total of 47 QTLs were detected, each explaining between 6 and 54% of the total phenotypic variance for the concerned trait. The genetic analysis shows that this population is a useful new tool for analyzing genetic variation for interesting traits in B. rapa, and for further exploitation of the recent availability of the B. rapa whole genome sequence for gene cloning and gene function analysis.

Plant seeds as bioreactors for recombinant protein production

Plants are attractive expression systems for the economic production of recombinant proteins. Among the different plant-based systems, plant seed is the leading platform and holds several advantages such as high protein yields and stable storage of target proteins. Significant advances in using seeds as bioreactors have occurred in the past decade, which include the first commercialized plant-derived recombinant protein. Here we review the current progress on seeds as bioreactors, with focus on the different food crops as production platforms and comprehensive strategies in optimizing recombinant protein production in seeds.

Production of antibody fragments in Arabidopsis seeds

Plants offer a number of attractive benefits over conventional mammalian or bacterial cell culture systems for the production of valuable pharmaceutical and industrial proteins. Currently, antibodies and their derived fragments represent the largest and most important group of biotechnological products in clinical trials. In particular, single-chain antibodies are an interesting class of biopharmaceuticals because they are able to overcome specific problems associated with full-length antibodies. Another valuable antibody format is the scFv-Fc: fusion of the Fc domain to a single-chain variable fragment restores antibody effector functions, allows purification, and mimics, despite being a 'single-gene' product, the bivalent properties of a full-length IgG. Although many different plant-based production platforms have been evaluated for antibody production, seeds are especially attractive because, as natural storage organs, they provide an optimal biochemical environment for the accumulation and long-term storage of large amounts of functional proteins. This chapter describes how to achieve high-level seed-specific expression of antibody fragments, how to select the best transgenic lines, and how to evaluate the accumulation level in the seed stocks from the selected lines.

Arcelins from an Indian wild pulse, Lablab purpureus, and insecticidal activity in storage pests

A partially purified protein fraction was isolated from seed flour of the Indian wild bean, Lablab purpureus, by ion exchange and size-exclusion chromatographies. Partially purified L. purpureus proteins had hemagglutination and glycoslyation properties similar to those of lectins or lectin-like proteins from other pulses. Data obtained from two-dimensional gel electrophoresis, MALDI-TOF, and MALDI-TOF/TOF and N-terminal protein sequencing of the isolated polypeptides from L. purpureus demonstrated that the extract contained proteins similar to isoforms of arcelins 3 and 4 and pathogenesis-related protein 1 (PvPR1) of Phaseolus vulgaris. L. purpureus proteins were resistant to degradation by the commercial enzymes trypsin and chymotrypsin and were moderately resistant to pepsin, but were readily hydrolyzed to smaller peptides by papain. Insect feeding bioassays of the extract with the storage pests Rhyzopertha dominica and Oryzaephilus surinamensis, internal and external feeders of grain, respectively, demonstrated that L. purpureus proteins at 2% in the diet resulted in retarded development. However, a 5% dose of the L. purpureus fraction resulted in complete mortality of all larvae in both species. This study has demonstrated that proteins in the partially purified L. purpureus extract have the potential to control storage pests in cereals transformed with L. purpureus defense-related genes, but the need for more studies regarding efficacy and safety is discussed.

Mutated TATA-box/TATA binding protein complementation system for regulated transgene expression in tobacco

A two-component expression system was developed to achieve tightly regulated expression of transgenes in plants. One component functioned as an expression module whereas the other functioned as a regulatory module. The expression module comprised a highly expressing TATA-dependent seed-specific promoter in which the TATA motif in the core promoter was mutated to TGTA. The regulatory module expressed a mutated general transcription factor TBPm(3) that recognized TGTA and initiated transcription. Vectors were designed using component one alone or in combination with component two, and were transformed into tobacco. The TGTA mutation in the TATA-box completely inactivated the promoter, making component one non-functional. This non-functional module became transcriptionally active in the presence of the component two that expressed TBPm(3). The reporter gene gusA was expressed from the TGTA-containing chimeric legumin promoter, in a tightly seed-specific manner, in transgenic tobacco plants in the presence of TBPm(3) that was expressed from a constitutive promoter. The results show that the TGTA and TBPm(3) combination can be used to achieve high-level tissue-specific expression of TATA-dependent promoters.

Ectopic expression of a conifer Abscisic Acid Insensitive3 transcription factor induces high-level synthesis of recombinant human alpha-L-iduronidase in transgenic tobacco leaves

We are examining various plant-based systems to produce enzymes for the treatment of human lysosomal storage disorders. Constitutive expression of the gene encoding the human lysosomal enzyme, alpha-L-iduronidase (IDUA; EC 3.2.1.76) in leaves of transgenic tobacco plants resulted in low-enzyme activity, and the protein appeared to be subject to proteolysis. Toward enhancing production of this recombinant enzyme in vegetative tissues, transgenic tobacco plants were generated to co-express a CaMV35S:Chamaecyparis nootkatensis Abscisic Acid Insensitive3 (CnABI3) gene construct, along with the human gene construct. The latter contained regulatory sequences of the Phaseolus vulgaris arcelin 5-I gene (5'-flanking, signal-peptide-encoding, and 3'-flanking regions). Ectopic synthesis of the CnABI3 protein led to the transactivation of the arcelin promoter and accordingly high activity (e.g., 25,000 pmol/min/mg total soluble protein) and levels of recombinant IDUA mRNA and protein were induced in leaves of transgenic tobacco, particularly in the presence of 150-200 microM S-(+)-ABA. Synthesis of human IDUA containing a carboxy-terminal ER retention (SEKDEL) sequence was also inducible by ABA in leaves co-transformed with the CnABI3 gene. As compared to the natural S-(+)-ABA, two persistent ABA analogues, (+)-8' acetylene ABA and (+)-8'methylene ABA, led to greater levels of beta-glucuronidase (GUS) reporter activities in leaves co-expressing the CnABI3 gene and a vicilin:GUS chimeric gene. In contrast, (+)-8' acetylene ABA and natural ABA appeared to be equally effective in stimulating the CnABI3-induced expression of an arcelin:GUS gene, and of the human IDUA gene, the latter also driven by arcelin-gene-regulatory sequences. Various stress-related treatments, particularly high concentrations of NaCl, had an even greater effect than ABA in promoting accumulation of human IDUA in co-transformed tobacco leaves. This strategy provides the means of enhancing the yields of recombinant proteins in transgenic plant vegetative tissues and potentially in cultured plant cells. The human recombinant protein can be readily induced in the presence of chemicals such as NaCl that can be added to cell cultures or even whole plants without a significant increase in production costs.

Aberrant localization and underglycosylation of highly accumulating single-chain Fv-Fc antibodies in transgenic Arabidopsis seeds

Production of high-value recombinant proteins in transgenic seeds is an attractive and economically feasible alternative to conventional systems based on mammalian cells and bacteria. In contrast to leaves, seeds allow high-level accumulation of recombinant proteins in a relatively small volume and a stable environment. We demonstrate that single-chain variable fragment (scFv)-Fc antibodies, with N-terminal signal sequence and C-terminal KDEL tag, can accumulate to very high levels as bivalent IgG-like antibodies in Arabidopsis thaliana seeds and illustrate that a plant-produced anti-hepatitis A virus scFv-Fc has similar antigen-binding and in vitro neutralizing activities as the corresponding full-length IgG. As expected, most scFv-Fc produced in seeds contained only oligomannose-type N-glycans, but, unexpectedly, 35-40% was never glycosylated. A portion of the scFv-Fc was found in endoplasmic reticulum (ER)-derived compartments delimited by ribosome-associated membranes. Additionally, consistent with the glycosylation data, large amounts of the recombinant protein were deposited in the periplasmic space, implying a direct transport from the ER to the periplasmic space between the plasma membrane and the cell wall. Aberrant localization of the ER chaperones calreticulin and binding protein (BiP) and the endogenous seed storage protein cruciferin in the periplasmic space suggests that overproduction of recombinant scFv-Fc disturbs normal ER retention and protein-sorting mechanisms in the secretory pathway.

Plants as factories for production of biopharmaceutical and bioindustrial proteins: lessons from cell biologyThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

Transgenic plants, seeds, and cultured plant cells are potentially one of the most economical systems for large-scale production of recombinant proteins for industrial and pharmaceutical uses. Biochemical, technical, and economic concerns with current production systems have generated enormous interest in developing plants as alternative production systems. However, various challenges must be met before plant systems can fully emerge as suitable, viable alternatives to current animal-based systems for large-scale production of biopharmaceuticals and other products. Aside from regulatory issues and developing efficient methods for downstream processing of recombinant proteins, there are at least two areas of challenge: (1) Can we engineer plant cells to accumulate recombinant proteins to sufficient levels? (2) Can we engineer plant cells to post-translationally modify recombinant proteins so that they are structurally and functionally similar to the native proteins? Attempts to improve the accumulation of a recombinant protein in plant cells require an appreciation of the processes of gene transcription, mRNA stability, processing, and export, and translation initiation and efficiency. Likewise, many post-translational factors must be considered, including protein stability, protein function and activity, and protein targeting. Moreover, we need to understand how the various processes leading from the gene to the functional protein are interdependent and functionally linked. Manipulation of the post-translational processing machinery of plant cells, especially that for N-linked glycosylation and glycan processing, is a challenging and exciting area. The functions of N-glycan heterogeneity and microheterogeneity, especially with respect to protein function, stability, and transport, are poorly understood and this represents an important area of cell biology.

Synthesis of enzymatically active human alpha-L-iduronidase in Arabidopsis cgl (complex glycan-deficient) seeds

As an initial step to develop plants as systems to produce enzymes for the treatment of lysosomal storage disorders, Arabidopsis thaliana wild-type (Col-0) plants were transformed with a construct to express human alpha-l-iduronidase (IDUA; EC 3.2.1.76) in seeds using the promoter and other regulatory sequences of the Phaseolus vulgaris arcelin 5-I gene. IDUA protein was easily detected on Western blots of extracts from the T(2) seeds, and extracts contained IDUA activity as high as 2.9 nmol 4-methylumbelliferone (4 MU)/min/mg total soluble protein (TSP), corresponding to approximately 0.06 microg IDUA/mg TSP. The purified protein reacted with an antibody specific for xylose-containing plant complex glycans, indicating its transit through the Golgi complex. In an attempt to avoid maturation of the N-linked glycans of IDUA, the same IDUA transgene was introduced into the Arabidopsis cgl background, which is deficient in the activity of N-acetylglucosaminyl transferase I (EC 2.4.1.101), the first enzyme in the pathway of complex glycan biosynthesis. IDUA activity and protein levels were significantly higher in transgenic cgl vs. wild-type seeds (e.g. maximum levels were 820 nmol 4 MU/min/mg TSP, or 18 microg IDUA/mg TSP). Affinity-purified IDUA derived from cgl mutant seeds showed a markedly reduced reaction with the antibody specific for plant complex glycans, despite transit of the protein to the apoplast. Furthermore, gel mobility changes indicated that a greater proportion of its N-linked glycans were susceptible to digestion by Streptomyces endoglycosidase H, as compared to IDUA derived from seeds of wild-type Arabidopsis plants. The combined results indicate that IDUA produced in cgl mutant seeds contains glycans primarily in the high-mannose form. This work clearly supports the viability of using plants for the production of human therapeutics with high-mannose glycans.

A reproducible genetic transformation system for cultivated Phaseolus acutifolius (tepary bean) and its use to assess the role of arcelins in resistance to the Mexican bean weevil

A reproducible Agrobacterium tumefaciens-mediated genetic transformation method that delivers fertile and morphologically normal transgenic plants was developed for cultivated tepary bean (Phaseolus acutifolius L. Gray). Factors contributing to higher transformation efficiencies include (1) a low initial concentration of bacteria coupled with a longer cocultivation period with callus, (2) an initial selection of callus on a medium containing low levels of the selectable agent, (3) omission of the selectable agent from the medium during callus differentiation to shoots and (4) the efficient conversion of transgenic shoots into fertile plants. All plants regenerated with this procedure (T0) were stably transformed, and the introduced foreign genes were inherited in a Mendelian fashion in most of the 33 independent transformants. Integration, stable transmission and high expression levels of the transgenes were observed in the T1 and/or T3 progenies of the transgenic lines. The binary transformation vectors contained the beta-glucuronidase reporter gene, the neomycin phosphotransferase II selectable marker gene and either an arcelin 1 or an arcelin 5 gene. Arcelins are seed proteins that are very abundant in some wild P. vulgaris L. genotypes showing resistance to the storage insect Zabrotes subfasciatus (Boheman) (Coleoptera, Bruchidae). Transgenic beans from two different cultivated P. acutifolius genotypes with high arcelin levels were infested with Z. subfasciatus, but they were only marginally less susceptible to infestation than the non-transgenic P. acutifolius. Hence, the arcelin genes tested here are not major determinants of resistance against Z. subfasciatus.

Boosting heterologous protein production in transgenic dicotyledonous seeds using Phaseolus vulgaris regulatory sequences

Over the past decade, several high value proteins have been produced in different transgenic plant tissues such as leaves, tubers, and seeds. Despite recent advances, many heterologous proteins accumulate to low concentrations, and the optimization of expression cassettes to make in planta production and purification economically feasible remains critical. Here, the regulatory sequences of the seed storage protein gene arcelin 5-I (arc5-I) of common bean (Phaseolus vulgaris) were evaluated for producing heterologous proteins in dicotyledonous seeds. The murine single chain variable fragment (scFv) G4 (ref. 4) was chosen as model protein because of the current industrial interest in producing antibodies and derived fragments in crops. In transgenic Arabidopsis thaliana seed stocks, the scFv under control of the 35S promoter of the cauliflower mosaic virus (CaMV) accumulated to approximately 1% of total soluble protein (TSP). However, a set of seed storage promoter constructs boosted the scFv accumulation to exceptionally high concentrations, reaching no less than 36.5% of TSP in homozygous seeds. Even at these high concentrations, the scFv proteins had antigen-binding activity and affinity similar to those produced in Escherichia coli. The feasibility of heterologous protein production under control of arc5-I regulatory sequences was also demonstrated in Phaseolus acutifolius, a promising crop for large scale production.

Cotton alpha-globulin promoter: isolation and functional characterization in transgenic cotton, Arabidopsis, and tobacco

Globulins are the most abundant seed storage proteins in cotton and, therefore, their regulatory sequences could potentially provide a good source of seed-specific promoters. We isolated the putative promoter region of cotton alpha-globulin B gene by gene walking using the primers designed from a cotton staged embryo cDNA clone. PCR amplified fragment of 1108 bp upstream sequences was fused to gusA gene in the binary vector pBI101.3 to create the test construct. This was used to study the expression pattern of the putative promoter region in transgenic cotton, Arabidopsis, and tobacco. Histochemical GUS analysis revealed that the promoter began to express during the torpedo stage of seed development in tobacco and Arabidopsis, and during cotyledon expansion stage in cotton. The activity quickly increased until embryo maturation in all three species. Fluorometric GUS analysis showed that the promoter expression started at 12 and 15 dpa in tobacco and cotton, respectively, and increased through seed maturation. The strength of the promoter expression, as reflected by average GUS activity in the seeds from primary transgenic plants, was vastly different amongst the three species tested. In Arabidopsis, the activity was 16.7% and in tobacco it was less than 1% of the levels detected in cotton seeds. In germinating seedlings of tobacco and Arabidopsis, GUS activity diminished until it was completely absent 10 days post imbibition. In addition, absence of detectable level of GUS expression in stem, leaf, root, pollen, and floral bud of transgenic cotton confirmed that the promoter is highly seed-specific. Analysis of GUS activity at individual seed level in cotton showed a gene dose effect reflecting their homozygous or hemizygous status. Our results show that this promoter is highly tissue-specific and it can be used to control transgene expression in dicot seeds.

Highly efficient targeting and accumulation of a F(ab) fragment within the secretory pathway and apoplast of Arabidopsis thaliana

To further improve antibody production in plants, constructs were designed to minimize transgene silencing and to retain a F(ab) fragment within the secretory pathway of transgenic Arabidopsis thaliana plants. The levels of antibody accumulation suggest that placing the sequences that encode Fd and light chain under the control of nonidentical 3' regions reduces susceptibility to post-transcriptional gene silencing compared with when the individual polypeptide-encoding sequences are placed under the control of identical 3' regions. High levels of accumulation (up to 6% of total soluble protein) were found for both secreted and intracellularly targeted antibody fragments. Immunofluorescence microscopic analysis showed that F(ab) fragments devoid of any additional C-terminal sequence were efficiently secreted, whereas retention of F(ab) fragments within the endomembrane system of the secretory pathway was achieved by C-terminal fusion of the DIKDEL sequence to the antibody light chain. Furthermore, analysis by immunoprecipitation and ELISA showed that intracellular retention of antibody fragments did not affect antigen-binding activity, and more than 80% of the isolated antibody fragments were found to bind antigen. Taken together, our results provide improvements to the technology of recombinant antibody production in transgenic plants.

Analysis of bruchid resistance in the wild common bean accession G02771: no evidence for insecticidal activity of arcelin 5

Arcelins are abundant seed storage proteins thought to be implicated in the resistance of wild Phaseolus vulgaris (L.) genotypes against Zabrotes subfasciatus (Boheman), an important storage insect pest of common bean. Here, the insecticidal activity of the arcelin-5 variant that is present in the highly resistant P. vulgaris accession G02771 was investigated. No correlation could be established between the presence of arcelin 5 and the insecticidal effects observed in G02771 seeds. Insect feeding assays with artificial seeds into which purified arcelin-5 protein was incorporated and with transgenic P. acutifolius (A. Gray) seeds in which the arcelin-5 genes were expressed, showed that the presence of arcelin-5 proteins, even at elevated levels, was not sufficient to achieve adequate resistance against Z. subfasciatus. The same might apply to other arcelin variants. Nevertheless, as resistance is clearly closely linked to the presence of the arcelin-1 or arcelin-5 locus, arcelins remain useful markers in breeding programmes aimed at introgressing high levels of resistance to Z. subfasciatus in P. vulgaris cultivars.

Transgene silencing in monocots

Plant gene silencing was originally thought to be a quirk of transformation procedures, but is now recognized to be a facet of vitally important gene regulatory systems, present in all organisms. Monocot plants, especially the grasses, play a foremost role in the agricultural economy of all nations, and their biotechnological manipulation offers great potential for both developed and developing countries. Here, we review reported instances of transgene silencing in monocots and relate the processes of transcriptional and post-transcriptional gene silencing (TGS, PTGS) in perspective to the rapidly burgeoning knowledge of these phenomena in many organisms. Recent findings include the involvement of an RNA-dependent RNA polymerase and a nuclease in PTGS systems and the close relationship between methylation and chromatin structure in TGS events.

Plants as bioreactors for protein production: avoiding the problem of transgene silencing

Plants are particularly attractive as large-scale production systems for proteins intended for therapeutical or industrial applications: they can be grown easily and inexpensively in large quantities that can be harvested and processed with the available agronomic infrastructures. The effective use of plants as bioreactors depends on the possibility of obtaining high protein accumulation levels that are stable during the life cycle of the transgenic plant and in subsequent generations. Silencing of the introduced transgenes has frequently been observed in plants, constituting a major commercial risk and hampering the general economic exploitation of plants as protein factories. Until now, the most efficient strategy to avoid transgene silencing involves careful design of the transgene construct and thorough analysis of transformants at the molecular level. Here, we focus on different aspects of the generation of transgenic plants intended for protein production and on their influence on the stability of heterologous gene expression.

Plants as bioreactors for protein production: avoiding the problem of transgene silencing

Plants are particularly attractive as large-scale production systems for proteins intended for therapeutical or industrial applications: they can be grown easily and inexpensively in large quantities that can be harvested and processed with the available agronomic infrastructures. The effective use of plants as bioreactors depends on the possibility of obtaining high protein accumulation levels that are stable during the life cycle of the transgenic plant and in subsequent generations. Silencing of the introduced transgenes has frequently been observed in plants, constituting a major commercial risk and hampering the general economic exploitation of plants as protein factories. Until now, the most efficient strategy to avoid transgene silencing involves careful design of the transgene construct and thorough analysis of transformants at the molecular level. Here, we focus on different aspects of the generation of transgenic plants intended for protein production and on their influence on the stability of heterologous gene expression.

Transgene expression systems in plant, a natural bioreactor

Plants are important resources that have been providing us food from the earliest times. The rapid advances that have taken place in plant genetic engineering have made it possible to modify plants to increase food production and contribute to environmental purification. Transgenic plants are gaining increasing attention from the industry as a natural bioreactor for the production of industrial and chemical products. Useful expression systems based on promoters to optimize transgene expression in plant cells, hold the key to maximizing the potential of this concept of molecular-farming or industrial plants. This review, which is devoted to the use of plants for heterologous protein production, is divided into three parts. First, we introduce the nature of plant promoters and strategies for the isolation of novel promoters. In the second part, various promoters showing high-level constitutive, organ-specific, or inducible expression, are summarized as useful tools for realizing the efficient transcription of transgenes. Finally, problems in the expression of foreign gene in plant cells and future prospects in plant biotechnology are discussed.