Processing and localization of bovine beta-casein expressed in transgenic soybean seeds under control of a soybean lectin expression cassette

Identity preserved plant molecular farming offers value-added opportunity for farmers

Bulk commodity row crop production in the United States is frequently subject to narrow profit margins, often complicated by weather, supply chains, trade, and other factors. Farmers seeking to increase profits and hedge against market volatility often seek to diversify their operations, including producing more lucrative or productive crop varieties. Recombinant plants producing animal or other non-native proteins (commonly referred to as plant molecular farming) present a value-added opportunity for row crop farmers. However, these crops must be produced under robust identity preserved systems to prevent comingling with bulk commodities to maintain the value for farmers, mitigate against market disruptions, and minimize any potential food, feed, or environmental risks.

Soybean seed protein storage vacuoles for expression of recombinant molecules

Soybean is one of the most important protein sources for human consumption and livestock feed. Soy production also allows the biosynthesis of edible oils, biodiesel, and biofertilizers. With the advent of modern agricultural biotechnology, soybean plants have also converted into bioreactors of therapeutic proteins and industrial enzymes. Soybean's characteristics, such as protein storage vacuoles (PSVs) and other unique organelles, allow the plant to be exploited as an accumulator of heterologous proteins under high stability and scalability conditions, and that maintains its basic functions. This review reports the main aspects of heterologous protein accumulation in soybean PSVs.

Analysis of expression characteristics of soybean leaf and root tissue-specific promoters in Arabidopsis and soybean

The characterization of tissue-specific promoters is critical for studying the functions of genes in a given tissue/organ. To study tissue-specific promoters in soybean, we screened tissue-specific expressed genes using published soybean RNA-Seq-based transcriptome data coupled with RT-PCR analysis. We cloned the promoters of three genes, GmADR1, GmBTP1, and GmGER1, and constructed their corresponding β-Glucuronidase (GUS) promoter-GUS reporter vectors. We generated transgenic Arabidopsis plants and examined the expression patterns of these promoters by GUS staining and RT-PCR analysis. We also transformed the promoter-GUS reporter vectors into soybean to obtain hairy roots, and examined promoter expression by GUS staining. We found a root-specific expression pattern of GmADR1 and GmBTP1 in both Arabidopsis and soybean, and the promoter of GmGER1 showed a leaf-specific pattern in transgenic Arabidopsis plants. To test the potential utility of these promoters in soybean improvement by transgenic means, we used the GmADR1 promoter to drive expression of a salt resistance gene in soybean, GmCaM4, by generating transgenic soybean plants. We found that the transgenic plants had significantly enhanced salt tolerance compared to non-transformed wild-type, suggesting that introducing endogenous promoters by transgenic means can drive the expression of functional genes in specific tissues and organs in soybean.

An embryo lethal transgenic line manifests global expression changes and elevated protein/oil ratios in heterozygous soybean plants

Understanding the molecular processes of seed development is important especially in agronomic crops that produce large amounts of nutrient reserves. Because soybean is a vital source of vegetable protein worldwide, producers are concerned about increasing the total amount of protein in the seed without substantially lowering the amount of oil, another economically important product. Here we describe a transgenic soybean line with increased protein and protein/oil ratio, containing an average of 42.2% protein vs. 38.5% in controls and with a protein/oil ratio of 2.02 vs. 1.76 in controls over several generations of greenhouse growth. Other phenotypic data show that the seeds are heavier, although there are overall lower yields per plant. We postulate these effects result from insertion site mutagenesis by the transgenic construct. As this line never achieves homozygosity and appears to be embryo lethal when homozygous, one functional copy of the gene is most likely essential for normal seed development. Global transcript analyses using RNA-Seq for 88,000 gene models over two stages of cotyledon development revealed that more genes are over-expressed in the transgenic line including ribosomal protein related genes and those in the membrane protein and transporters families. Localization of the insertion site should reveal the genes and developmental program that has been perturbed by the transgenic construct, resulting in this economically interesting increase in protein and the protein/oil ratio.

Seed specific expression and analysis of recombinant human adenosine deaminase (hADA) in three host plant species

The plant seed is a leading platform amongst plant-based storage systems for the production of recombinant proteins. In this study, we compared the activity of human adenosine deaminase (hADA) expressed in transgenic seeds of three different plant species: pea (Pisum sativum L.), Nicotiana benthamiana L. and tarwi (Lupinus mutabilis Sweet). All three species were transformed with the same expression vector containing the hADA gene driven by the seed-specific promoter LegA2 with an apoplast targeting pinII signal peptide. During the study, several independent transgenic lines were generated and screened from each plant species and only lines with a single copy of the gene of interest were used for hADA expression analysis. A stable transgenic canola line expressing the ADA protein, under the control of 35S constitutive promoter was used as both as a positive control and for comparative study with the seed specific promoter. Significant differences were detected in the expression of hADA. The highest activity of the hADA enzyme (Units/g seed) was reported in tarwi (4.26 U/g) followed by pea (3.23 U/g) and Nicotiana benthamiana (1.69 U/g). The expression of mouse ADA in canola was very low in both seed and leaf tissue compared to other host plants, confirming higher activity of seed specific promoter. Altogether, these results suggest that tarwi could be an excellent candidate for the production of valuable recombinant proteins.

Transgenic Soybean Production of Bioactive Human Epidermal Growth Factor (EGF)

Necrotizing enterocolitis (NEC) is a devastating condition of premature infants that results from the gut microbiome invading immature intestinal tissues. This results in a life-threatening disease that is frequently treated with the surgical removal of diseased and dead tissues. Epidermal growth factor (EGF), typically found in bodily fluids, such as amniotic fluid, salvia and mother's breast milk, is an intestinotrophic growth factor and may reduce the onset of NEC in premature infants. We have produced human EGF in soybean seeds to levels biologically relevant and demonstrated its comparable activity to commercially available EGF. Transgenic soybean seeds expressing a seed-specific codon optimized gene encoding of the human EGF protein with an added ER signal tag at the N' terminal were produced. Seven independent lines were grown to homozygous and found to accumulate a range of 6.7 +/- 3.1 to 129.0 +/- 36.7 μg EGF/g of dry soybean seed. Proteomic and immunoblot analysis indicates that the inserted EGF is the same as the human EGF protein. Phosphorylation and immunohistochemical assays on the EGF receptor in HeLa cells indicate the EGF protein produced in soybean seed is bioactive and comparable to commercially available human EGF. This work demonstrates the feasibility of using soybean seeds as a biofactory to produce therapeutic agents in a soymilk delivery platform.

Improved protein quality in transgenic soybean expressing a de novo synthetic protein, MB-16

To improve soybean [Glycine max (L.) Merrill] seed nutritional quality, a synthetic gene, MB-16 was introduced into the soybean genome to boost seed methionine content. MB-16, an 11 kDa de novo protein enriched in the essential amino acids (EAAs) methionine, threonine, lysine and leucine, was originally developed for expression in rumen bacteria. For efficient seed expression, constructs were designed using the soybean codon bias, with and without the KDEL ER retention sequence, and β-conglycinin or cruciferin seed specific protein storage promoters. Homozygous lines, with single locus integrations, were identified for several transgenic events. Transgene transmission and MB-16 protein expression were confirmed to the T5 and T7 generations, respectively. Quantitative RT-PCR analysis of developing seed showed that the transcript peaked in growing seed, 5-6 mm long, remained at this peak level to the full-sized green seed and then was significantly reduced in maturing yellow seed. Transformed events carrying constructs with the rumen bacteria codon preference showed the same transcription pattern as those with the soybean codon preference, but the transcript levels were lower at each developmental stage. MB-16 protein levels, as determined by immunoblots, were highest in full-sized green seed but the protein virtually disappeared in mature seed. However, amino acid analysis of mature seed, in the best transgenic line, showed a significant increase of 16.2 and 65.9 % in methionine and cysteine, respectively, as compared to the parent. This indicates that MB-16 elevated the sulfur amino acids, improved the EAA seed profile and confirms that a de novo synthetic gene can enhance the nutritional quality of soybean.

Differential expression and elution behavior of basic 7S globulin among cultivars under hot water treatment of soybean seeds

Basic 7S globulin (Bg7S), which accumulates in mature soybean (Glycine max) seeds, is an extracellular matrix protein. A large amount of Bg7S is synthesized de novo and is eluted from soybean seeds when immersed in 50-60°C water (hot water treatment, HWT). However, the Bg7S elution mechanism remains unclear. Under HWT, the seeds probably undergo heat stress and flooding stress. To obtain fundamental knowledge related to how Bg7S is eluted from hot-water-treated seeds, this study compared Bg7S elution among soybean cultivars having different flooding tolerance during pre-germination. The amounts of Bg7S eluted from seeds varied significantly among cultivars. Elution was suppressed by seed coats regarded as preventing the leakage of seed contents by rapid water imbibition. Furthermore, Bg7S expression levels differed among cultivars, although the difference did not result from any variation in Bg7S promoter sequences. However, the expression levels of Bg7S under HWT were not associated with the flooding tolerance level. Immunoelectron microscopy revealed that the Bg7S accumulated in the intercellular space of hot-water-treated seeds. Plasma membrane shrinkage was observed. The main proteins eluted from seeds under HWT were located in the extracellular space. This study clarified the mechanism of Bg7S elution from seeds under HWT.

Glyceollin is an important component of soybean plant defense against Phytophthora sojae and Macrophomina phaseolina

The response of soybean transgenic plants, with suppressed synthesis of isoflavones, and nontransgenic plants to two common soybean pathogens, Macrophomina phaseolina and Phytophthora sojae, was studied. Transgenic soybean plants of one line used in this study were previously generated via bombardment of embryogenic cultures with the phenylalanine ammonia lyase, chalcone synthase, and isoflavone synthase (IFS2) genes in sense orientation driven by the cotyledon-preferable lectin promoter (to turn genes on in cotyledons), while plants of another line were newly produced using the IFS2 gene in sense orientation driven by the Cassava vein mosaic virus constitutive promoter (to turn genes on in all plant parts). Nearly complete inhibition of isoflavone synthesis was found in the cotyledons of young seedlings of transgenic plants transformed with the IFS2 transgene driven by the cotyledon-preferable lectin promoter compared with the untransformed control during the 10-day observation period, with the precursors of isoflavone synthesis being accumulated in the cotyledons of transgenic plants. These results indicated that the lectin promoter could be active not only during seed development but also during seed germination. Downregulation of isoflavone synthesis only in the seed or in the whole soybean plant caused a strong inhibition of the pathogen-inducible glyceollin in cotyledons after inoculation with P. sojae, which resulted in increased susceptibility of the cotyledons of both transgenic lines to this pathogen compared with inoculated cotyledons of untransformed plants. When stems were inoculated with M. phaseolina, suppression of glyceollin synthesis was found only in stems of transgenic plants expressing the transgene driven by a constitutive promoter, which developed more severe infection. These results provide further evidence that rapid glyceollin accumulation during infection contributes to the innate soybean defense response.

Non-food/feed seeds as biofactories for the high-yield production of recombinant pharmaceuticals

We describe an attractive cloning system for the seed-specific expression of recombinant proteins using three non-food/feed crops. A vector designed for direct subcloning by Gateway® recombination was developed and tested in Arabidopsis, tobacco and petunia plants for the production of a chimeric form (GAD67/65) of the 65 kDa isoform of glutamic acid decarboxylase (GAD65). GAD65 is one of the major human autoantigens involved in type 1 diabetes (T1D). The murine anti-inflammatory cytokine interleukin-10 (IL-10) was expressed with the described system in Arabidopsis and tobacco, whereas proinsulin, another T1D major autoantigen, was expressed in Arabidopsis. The cost-effective production of these proteins in plants could allow the development of T1D prevention strategies based on the induction of immunological tolerance. The best yields were achieved in Arabidopsis seeds, where GAD67/65 reached 7.7% of total soluble protein (TSP), the highest levels ever reported for this protein in plants. IL-10 and proinsulin reached 0.70% and 0.007% of TSP, respectively, consistent with levels previously reported in other plants or tissues. This versatile cloning vector could be suitable for the high-throughput evaluation of expression levels and stability of many valuable and difficult to produce proteins.

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

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

Production of monoclonal antibodies with a controlled N-glycosylation pattern in seeds of Arabidopsis thaliana

Seed-specific expression is an appealing alternative technology for the production of recombinant proteins in transgenic plants. Whereas attractive yields of recombinant proteins have been achieved by this method, little attention has been paid to the intracellular deposition and the quality of such products. Here, we demonstrate a comparative study of two antiviral monoclonal antibodies (mAbs) (HA78 against Hepatitis A virus; 2G12 against HIV) expressed in seeds of Arabidopsis wild-type (wt) plants and glycosylation mutants lacking plant specific N-glycan residues. We demonstrate that 2G12 is produced with complex N-glycans at great uniformity in the wt as well as in the glycosylation mutant, carrying a single dominant glycosylation species, GnGnXF and GnGn, respectively. HA78 in contrast, contains additionally to complex N-glycans significant amounts of oligo-mannosidic structures, which are typical for endoplasmic reticulum (ER)-retained proteins. A detailed subcellular localization study demonstrated the deposition of both antibodies virtually exclusively in the extracellular space, illustrating their efficient secretion. In addition, although a KDEL-tagged version of 2G12 exhibited an ER-typical N-glycosylation pattern, it was surprisingly detected in protein storage vacuoles. The different antibody variants showed different levels of degradation with hardly any degradation products detectable for HA78 carrying GnGnXF glycans. Finally, we demonstrate functional integrity of the HA78 and 2G12 glycoforms using viral inhibition assays. Our data therefore demonstrate the usability of transgenic seeds for the generation of mAbs with a controlled N-glycosylation pattern, thus expanding the possibilities for the production of optimally glycosylated proteins with enhanced biological activities for the use as human therapeutics.

Advances in plant molecular farming

Plant molecular farming (PMF) is a new branch of plant biotechnology, where plants are engineered to produce recombinant pharmaceutical and industrial proteins in large quantities. As an emerging subdivision of the biopharmaceutical industry, PMF is still trying to gain comparable social acceptance as the already established production systems that produce these high valued proteins in microbial, yeast, or mammalian expression systems. This article reviews the various cost-effective technologies and strategies, which are being developed to improve yield and quality of the plant-derived pharmaceuticals, thereby making plant-based production system suitable alternatives to the existing systems. It also attempts to overview the different novel plant-derived pharmaceuticals and non-pharmaceutical protein products that are at various stages of clinical development or commercialization. It then discusses the biosafety and regulatory issues, which are crucial (if strictly adhered to) to eliminating potential health and environmental risks, which in turn is necessary to earning favorable public perception, thus ensuring the success of the industry.

Seed-based expression systems for plant molecular farming

The evolution of the seed system provides enormous adaptability to the gymnosperms and angiosperms, because of the properties of dormancy, nutrient storage and seedling vigour. Many of the unique properties of seeds can be exploited in molecular farming applications, particularly where it is desirable to produce large quantities of a recombinant protein. Seeds of transgenic plants have been widely used to generate a raw material for the extraction and isolation of proteins and polypeptides, which can be processed into valuable biopharmaceuticals. The factors that control high-level accumulation of recombinant proteins in seed are reviewed in the following paragraphs. These include promoters and enhancers, which regulate transcript abundance. However, it is shown that subcellular trafficking and targeting of the desired polypeptides or proteins play a crucial role in their accumulation at economically useful levels. Seeds have proven to be versatile hosts for recombinant proteins of all types, including peptides or short and long polypeptides as well as complex, noncontiguous proteins like antibodies and other immunoglobulins. The extraction and recovery of recombinant proteins from seeds is greatly assisted by their dormancy properties, because this allows for long-term stability of stored products including recombinant proteins and a decoupling of processing from the growth and harvest cycles. Furthermore, the low water content and relatively low bioload of seeds help greatly in designing cost-effective manufacturing processes for the desired active pharmaceutical ingredient. The development of cGMP processes based on seed-derived materials has only been attempted by a few groups to date, but we provide a review of the key issues and criteria based on interactions with Food and Drug Administration and European Medicines Agency. This article uses 'case studies' to highlight the utility of seeds as vehicles for pharmaceutical production including: insulin, human growth hormone, lysozyme and lactoferrin. These examples serve to illustrate the preclinical and, in one case, clinical information required to move these plant-derived molecules through the research phase and into the regulatory pathway en route to eventual approval.

Transgenic soybean seed as protein expression system: aqueous extraction of recombinant beta-glucuronidase

Soybean is one of the plant species with potential to be used as seed-based bioreactor. As part of the downstream processing (DSP) of this technology, extraction is a key step, since it defines the composition of the solution from which the recombinant product will be purified. In the present work, the characteristics of soybean seeds used as a bioreactor were evaluated from a process engineering standpoint through analysis of the influence of pH and ionic strength on the extraction of recombinant beta-glucuronidase (rGUS). Concentrations of recombinant protein and native soybean compounds were analyzed and compared with similar data from extraction studies using transgenic corn seeds as bioreactor. Efficient rGUS extraction was obtained at pH of around 5.5 with no addition of salt. Soybean seed extracts had lower levels of co-extracted native compounds, than corn seed extracts, and should be considered as a potential plant bioreactor in terms of DSP.

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.

Plants as bioreactors for the production of vaccine antigens

Plants have been identified as promising expression systems for commercial production of vaccine antigens. In phase I clinical trials several plant-derived vaccine antigens have been found to be safe and induce sufficiently high immune response. Thus, transgenic plants, including edible plant parts are suggested as excellent alternatives for the production of vaccines and economic scale-up through cultivation. Improved understanding of plant molecular biology and consequent refinement in the genetic engineering techniques have led to designing approaches for high level expression of vaccine antigens in plants. During the last decade, several efficient plant-based expression systems have been examined and more than 100 recombinant proteins including plant-derived vaccine antigens have been expressed in different plant tissues. Estimates suggest that it may become possible to obtain antigen sufficient for vaccinating millions of individuals from one acre crop by expressing the antigen in seeds of an edible legume, like peanut or soybean. In the near future, a plethora of protein products, developed through ‘naturalized bioreactors’ may reach market. Efforts for further improvements in these technologies need to be directed mainly towards validation and applicability of plant-based standardized mucosal and edible vaccines, regulatory pharmacology, formulations and the development of commercially viable GLP protocols. This article reviews the current status of developments in the area of use of plants for the development of vaccine antigens.

Establishment of a soybean (Glycine max Merr. L) transposon-based mutagenesis repository

Soybean is a major crop species providing valuable feedstock for food, feed and biofuel. In recent years, considerable progress has been made in developing genomic resources for soybean, including on-going efforts to sequence the genome. These efforts have identified a large number of soybean genes, most with unknown function. Therefore, a major research priority is determining the function of these genes, especially those involved in agronomic performance and seed traits. One means to study gene function is through mutagenesis and the study of the resulting phenotypes. Transposon-tagging has been used successfully in both model and crop plants to support studies of gene function. In this report, we describe efforts to generate a transposon-based mutant collection of soybean. The Ds transposon system was used to create activation-tagging, gene and enhancer trap elements. Currently, the repository houses approximately 900 soybean events, with flanking sequence data derived from 200 of these events. Analysis of the insertions revealed approximately 70% disrupted known genes, with the majority matching sequences derived from either Glycine max or Medicago truncatula sequences. Among the mutants generated, one resulted in male-sterility and was shown to disrupt the strictosidine synthase gene. This example clearly demonstrates that it is possible to disrupt soybean gene function by insertional mutagenesis and to derive useful mutants by this approach in spite of the tetraploid nature of the soybean genome.

Production and localization of recombinant pharmaceuticals in transgenic seeds

Among the many plant-based production systems that have been developed for pharmaceutical proteins, seeds have the useful advantage of accumulating proteins in a relatively small volume, and recombinant proteins are very stable in dry seeds allowing long-term storage and facilitating distribution before processing.To take full advantage of the natural ability of endosperm cells to store large amounts of protein in a protected subcellular environment, it is useful to target recombinant proteins to appropriate storage organelles. In this chapter, we describe the distinct types of protein storage organelles in the cereal endosperm and a protocol for the detection of recombinant proteins in these organelles by immunofluorescence and immunogold labelling.The use of food and feed crops for the production of pharmaceutical proteins such as edible vaccines implies the need for strict separation of the transgenic seeds from the food and feed chain. For improved traceability visual markers may be co-expressed with the gene of interest in engineered seeds. DsRed is one example for a fluorescent protein that can be detected with high sensitivity using low tech equipment. We therefore describe the generation of transgenic maize plants expressing DsRed in a constitutive manner, and we point out the advantages of using this marker during the process of transformation and selection of plant tissue and later during breeding of transgenic lines into elite germplasm.

Functional specialization of Medicago truncatula leaves and seeds does not affect the subcellular localization of a recombinant protein

A number of recent reports suggest that the functional specialization of plant cells in storage organs can influence subcellular protein sorting, so that the fate of a recombinant protein tends to differ between seeds and leaves. In order to test the general applicability of this hypothesis, we investigated the fate of a model recombinant glycoprotein in the leaves and seeds of a leguminous plant, Medicago truncatula. Detailed analysis of immature seeds by immunofluorescence and electron microscopy showed that recombinant phytase carrying a signal peptide for entry into the endoplasmic reticulum was efficiently secreted from storage cotyledon cells. A second version of the protein carrying a C-terminal KDEL tag for retention in the endoplasmic reticulum was predominantly retained in the ER of seed cotyledon cells, but some of the protein was secreted to the apoplast and some was deposited in storage vacuoles. Importantly, the fate of the recombinant protein in the leaves was nearly identical to that in the seeds from the same plant. This shows that in M. truncatula, the unanticipated partial vacuolar delivery and secretion is not a special feature of seed cotyledon tissue, but are conserved in different specialized tissues. Further investigation revealed that the unexpected fate of the tagged variant of phytase likely resulted from partial loss of the KDEL tag in both leaves and seeds. Our results indicate that the previously observed aberrant deposition of recombinant proteins into storage organelles of seed tissue is not a general reflection of functional specialization, but also depends on the species of plant under investigation. This discovery will have an impact on the production of recombinant pharmaceutical proteins in plants.

Authentic seed-specific activity of the Perilla oleosin 19 gene promoter in transgenic Arabidopsis

The Perilla (Perilla frutescens L. cv. Okdong) oleosin gene, PfOle19, produces a 19-kDa protein that is highly expressed only in seeds. The activity of the -2,015 bp 5'-upstream promoter region of this gene was investigated in transgenic Arabidopsis plants using the fusion reporter constructs of enhanced green fluorescent protein (EGFP) and beta-glucuronidase (GUS). The PfOle19 promoter directs Egfp expression in developing siliques, but not in leaves, stems or roots. In the transgenic Arabidopsis, EGFP fluorescence and histochemical GUS staining were restricted to early seedlings, indehiscent siliques and mature seeds. Progressive 5'-deletions up to the -963 bp position of the PfOle19 promoter increases the spatial control of the gene expression in seeds, but reduces its quantitative levels of expression. Moreover, the activity of the PfOle19 promoter in mature seeds is 4- and 5-fold greater than that of the cauliflower mosaic virus 35S promoter in terms of both EGFP intensity and fluorometric GUS activity, respectively.

The Quest to Understand the Basis and Mechanisms that Control Expression of Introduced Transgenes in Crop Plants

We discuss mechanisms and factors that influence levels and stability of expressed heterologous proteins in crop plants. We have seen substantial progress in this field over the past two decades in model experimental organisms such as Arabidopsis and tobacco. There is no question such studies have resulted in furthering our understanding of key processes in the plant cell and the elaboration of sophisticated models to explain underlying mechanisms that might influence the fate, levels and stability of expression of recombinant heterologous proteins in plants. However, very often, such information is not applicable outside these laboratory experimental models. In order to generate a knowledge basis that can be used to achieve high levels and stability of heterologous proteins in relevant crop plants it is imperative to perform such studies on the target crops. With this in mind, we discuss key elements of the process at the DNA, RNA and protein levels. We believe it is essential to discuss recombinant protein production in crops in a holistic manner in order to develop a comprehensive knowledge base that will in turn serve plant biotechnology applications well.

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.

Sowing the seeds of success: pharmaceutical proteins from plants

Among the many plant-based production systems that have been developed for pharmaceutical proteins, seeds have the useful advantage of accumulating proteins in a relatively small volume and in a stable environment in which they are protected from degradation. Several seed crops, including cereals, grain legumes and oilseeds, have been explored as production platforms, and the first commercial products -- all technical proteins and enzymes -- have already reached the market. Recent studies have explored the use of seeds for the production of pharmaceutical proteins, particularly replacement human proteins, recombinant antibodies and (oral) vaccines.

Storage protein accumulation in the absence of the vacuolar processing enzyme family of cysteine proteases

The role(s) of specific proteases in seed protein processing is only vaguely understood; indeed, the overall role of processing in stable protein deposition has been the subject of more speculation than direct investigation. Seed-type members of the vacuolar processing enzyme (VPE) family were hypothesized to perform a unique function in seed protein processing, but we demonstrated previously that Asn-specific protein processing in developing Arabidopsis seeds occurs independently of this VPE activity. Here, we describe the unexpected expression of vegetative-type VPEs in developing seeds and test the role(s) of all VPEs in seed storage protein accumulation by systematically stacking knockout mutant alleles of all four members (alphaVPE, betaVPE, gammaVPE, and deltaVPE) of the VPE gene family in Arabidopsis. The complete removal of VPE function in the alphavpe betavpe gammavpe deltavpe quadruple mutant resulted in a total shift of storage protein accumulation from wild-type processed polypeptides to a finite number of prominent alternatively processed polypeptides cleaved at sites other than the conserved Asn residues targeted by VPE. Although alternatively proteolyzed legumin-type globulin polypeptides largely accumulated as intrasubunit disulfide-linked polypeptides with apparent molecular masses similar to those of VPE-processed legumin polypeptides, they showed markedly altered solubility and protein assembly characteristics. Instead of forming 11S hexamers, alternatively processed legumin polypeptides were deposited primarily as 9S complexes. However, despite the impact on seed protein processing, plants devoid of all known functional VPE genes appeared unchanged with regard to protein content in mature seeds, relative mobilization rates of protein reserves during germination, and vegetative growth. These findings indicate that VPE-mediated Asn-specific proteolytic processing, and the physiochemical property changes attributed to this specific processing step, are not required for the successful deposition and mobilization of seed storage protein in the protein storage vacuoles of Arabidopsis seeds.

Increasing maize seed methionine by mRNA stability

The amino acid methionine is a common protein building block that is also important in other cellular processes. Plants, unlike animals, synthesize methionine de novo and are thus a dietary source of this nutrient. A new approach for using maize as a source of nutrient methionine is described. Maize seeds, a major component of animal feeds, have variable levels of protein-bound methionine. This variability is a result of post-transcriptional regulation of the Dzs10 gene, which encodes a seed-specific high-methionine storage protein. Here we eliminate methionine variability by identifying and replacing the cis-acting site for Dzs10 regulation using transgenic seeds. Interestingly, two different mechanisms affect mRNA accumulation, one dependent on and the other independent of the untranslated regions (UTRs) of Dzs10 RNA. Accumulation of chimeric Dzs10 mRNA was not reduced in hybrid crosses and was uncoupled from genomic imprinting by Dzr1, a regulator of Dzs10. Uniform high levels of Dzs10 protein were maintained over five backcross generations of the transgene. The increased level of methionine in these transgenic seeds allowed the formulation of a useful animal feed ration without the addition of synthetic methionine.