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Plants offer a novel system for the production of pharmaceutical proteins on a commercial scale that is more scalable, cost-efficient and safe than current expression paradigms. In this study, we report a simple and convenient, yet scalable approach to introduce target-gene containing Agrobacterium tumefaciens into plants for protein transient expression.
Mammalian cell culture is the major platform for commercial production of human vaccines and therapeutic proteins. However, it cannot meet the increasing worldwide demand for pharmaceuticals due to its limited scalability and high cost. Plants have shown to be one of the most promising alternative pharmaceutical production platforms that are robust, scalable, low-cost and safe. The recent development of virus-based vectors has allowed rapid and high-level transient expression of recombinant proteins in plants. To further optimize the utility of the transient expression system, we demonstrate a simple, efficient and scalable methodology to introduce target-gene containing Agrobacterium into plant tissue in this study. Our results indicate that agroinfiltration with both syringe and vacuum methods have resulted in the efficient introduction of Agrobacterium into leaves and robust production of two fluorescent proteins; GFP and DsRed. Furthermore, we demonstrate the unique advantages offered by both methods. Syringe infiltration is simple and does not need expensive equipment. It also allows the flexibility to either infiltrate the entire leave with one target gene, or to introduce genes of multiple targets on one leaf. Thus, it can be used for laboratory scale expression of recombinant proteins as well as for comparing different proteins or vectors for yield or expression kinetics. The simplicity of syringe infiltration also suggests its utility in high school and college education for the subject of biotechnology. In contrast, vacuum infiltration is more robust and can be scaled-up for commercial manufacture of pharmaceutical proteins. It also offers the advantage of being able to agroinfiltrate plant species that are not amenable for syringe infiltration such as lettuce and Arabidopsis. Overall, the combination of syringe and vacuum agroinfiltration provides researchers and educators a simple, efficient, and robust methodology for transient protein expression. It will greatly facilitate the development of pharmaceutical proteins and promote science education.
Since the 1970s, plants have been explored as alternatives to mammalian, insect, and bacterial cell cultures for the commercial production of recombinant proteins and protein therapeutics 1. Plant-based systems for the expression of biopharmaceuticals have shown promise in recent years as several novel treatments for diseases, like Gaucher's Disease 2, and avian H5N1 influenza 3, have shown success in clinical trials. The development of competent mechanisms for recombinant protein expression in plants in the decades since those initial experiments has created the potential for plant-based systems to alter the current paradigm of protein production for three primary reasons. Firstly, there is a notable decrease in cost as mammalian, insect, and bacterial bioreactors require considerable startup costs, expensive growth media, and complicated processes for downstream purification 4. The creation of stable transgenic plant lines also allows them to outpace the scalability of other expression systems as protein expressing plants could be grown and harvested on an agricultural scale 5. Secondly, plant-based expression systems significantly reduce the risk of transmitting a human or animal pathogen from the protein-expressing host to humans, demonstrating superiority in public safety 6. Lastly, plants utilize a eukaryotic endomembrane system that is similar to mammalian cells, allowing for proper post-translational modification of proteins including glycosylation and the assembly of multiple-subunit proteins 7. This ability puts plant-based systems ahead of those based on prokaryotic systems, such as bacteria, since a wider number of pharmaceutical recombinant proteins, including monoclonal antibodies (mAbs), have a more complicated structure and require extensive posttranslational modifications or assembly 8.
There are two major approaches to expressing recombinant proteins in plants. The first is the development of a stably transgenic line, where DNA coding for the target protein is cloned into an expression cassette and introduced to either the nuclear or chloroplast genomes. In doing so, the foreign DNA becomes heritable through succeeding generations and allows for tremendously improved scalability, far beyond that of other expression systems 1. Introduction of exogenous DNA to the nuclear genome is usually achieved by Agrobacterium tumefaciens infection of plant tissue or, less often, by microprojectile bombardment of the tissue 9. Plant hormones are then used to induce differentiation and growth of transgenic plant tissue such as roots and leaves. Transformation of the chloroplast genome cannot be achieved with A. tumefaciens, but relies entirely on gold or tungsten particles coated with DNA fired ballistically into plant cells. The second method of expressing recombinant protein in plants is through transient expression 10. In this scenario, virus-derived vectors harboring the gene of interest are delivered via A. tumefaciens to fully developed plants through a process called agroinfiltration. Instead of integrating into the plant genome, the delivered gene construct will then begin to direct the transient production of the desired protein, which can be harvested and isolated after a short incubation period. Transient gene expression offers the advantage of greater overall protein accumulation as well as an improved time of protein production, as plants will be ready to harvest approximately 1-2 weeks after agroinfiltration 11. This is significantly faster than the processes of generation, selection, and confirmation of stable transgenic plant lines, which can take several months to a year. This however, is also the limitation of the transient expression system, as it will not yield genetically stable plant lines that can be used to generate a seed bank for large scale commercial production. Despite this, approaches have been developed to improve large scale transient expression. Here we demonstrate one method of generation of transient protein-expressing Nicotiana benthamiana plants using deconstructed viral vectors delivered by A. tumefaciens.
Two major methods are being developed for the delivery of A. tumefaciens into plant tissue: bench scale infiltration via syringe and large scale infiltration via vacuum chamber. Both protocols are described here using N. benthamiana, which is closely related to the common tobacco plant, as the host plant for transient expression of two fluorescent proteins: the green fluorescent protein (GFP) from jellyfish Aequorea victoria and the red fluorescent protein from Discosoma coral (DsRed) 12,13. N. benthamiana is the most common host plant for recombinant protein because it is amenable to genetic transformation, can yield high amounts of biomass rapidly, and is a prolific seed producer for scale-up production 14. Another advantage of using N. benthamiana as hosts for protein expression is the availability of a variety of expression vectors 2,5. In this study, two deconstructed viral vectors, one based on a tobacco mosaic virus (TMV) RNA replicon system (MagnICON vectors) and the other derived from the bean yellow dwarf virus (BeYDV) DNA replicon system (geminiviral vectors) 4,11,15-18 , are used to carry the GFP and DsRed gene and deliver them into N. benthamiana cells via A. tumefaciens. Three DNA constructs will be used for GFP or DsRed expression with MagnICON vectors. They include the 5' module (pICH15879) containing the promoter and other genetic elements for driving the expression of the target gene, the 3' module containing the gene of interest (pICH-GFP or pICH-DsRed), and the integrase module (pICH14011) coding for an enzyme that integrates the 5' and 3' modules together upon expression 8,15. Three DNA constructs are also needed for expression with geminiviral vectors. In addition to vectors containing the replicon of the target gene ( pBYGFP or pBYDsRed), a vector coding for the replication protein (pREP110) is required for the amplification of the target replicon 11,14,16. Furthermore, the inclusion of a vector encoding the silencing suppressor p19 from tomato bushy stunt virus is desired for high level target gene expression 11,16.
There are generally three major steps for the introduction of genes of recombinant proteins into plant cells by agroinfiltration including plant growth, A. tumefaciens culture preparation, and infiltration. As every step is critical for the ultimate success of this procedure, therefore, a detailed description for each is provided for both syringe infiltration and vacuum infiltration below.
1. Plant Growth
2. A. tumefaciens Culture Preparation
2.1 Preparation for Syringe Infiltration
2.2 Preparation for Vacuum Infiltration
3. Infiltration
3.1 Syringe Infiltration
3.2 Vacuum Infiltration
4. Fluorescent Protein Detection and Photography
1. Expression of Fluorescent Proteins by Syringe Infiltration
To demonstrate the effectiveness of syringe infiltration of Agrobacterium into plant tissue, we tested the expression of two fluorescent proteins - GFP and DsRed - by two different deconstructed plant viral vectors - geminiviral and MagnICON - in N. benthamiana. For N. benthamiana leaves that were entirely infiltrated with Agrobacteria containing geminiviral vectors, GFP expression was observed over ...
The increasing demands for protein-based pharmaceuticals worldwide require new production platforms that are robust, scalable, low-cost and safe. Plants have shown to be one of the most promising alternative production systems for pharmaceutical protein production. In recent years, the development of deconstructed virus-based vectors has enabled transient expression of proteins in plants, which greatly enhances the speed and yield of plant expression systems 2,10. To further optimize the utility of the transie...
The authors have no competing financial interests.
We thank R. Sun and other students of Chen's Laboratory for their contribution to plant material generation. We also thank Dr. D. Green for his support of undergraduate research in the College of Technology and Innovation (CTI). This research was supported in part by NIH grants U01 AI075549 and 1R21AI101329 to Q. Chen, and a SSE grant from CTI of Arizona State University to Q. Chen. K. Leuzinger, M. Dent, J. Hurtado, and J. Stahnke are undergraduate students supported by the SSE grant.
Name | Company | Catalog Number | Comments |
Reagents | |||
GFP | Invitrogen | V353-20 | www.invitrogen.com See reference: Lico and Chen, et al 2008 |
DsRed | Clontech | 632152 | www.clontech.com See reference: Baird and Zacharias, et al 2000 |
MagnICON Vector | Icon Genetics | n/a | www.icongenetics.com See reference: giritch and Marillonnet, et al 2006 |
Geminiviral Vector | Author's Lab | n/a | See reference: Chen and He, et al 2011 |
N. benthamiana | Author's Lab | n/a | herbalistics.com.au |
Agrobacterium tumefaciens strain gv3101 | Author's Lab | n/a | See reference: Lai and Chen 2012 |
LB Agar Carbenicillin-100, plates | Sigma | L0418 | www.sigmaaldrich.com |
LB Agar Kanamycin-50, plates | Sigma | L0543 | www.sigmaaldrich.com |
Magnesium sulfate hepa hydrate | Sigma | M2773-500 g | www.sigmaaldrich.com |
Bacto-Tryptone | Fisher | 73049-73-7 | www.fishersci.com |
Bacto Yeast Extract | Becton, Dickinson & CO. | REF 212750 | www.bd.com |
Difco Nutrient Broth | Becton, Dickinson & CO. | REF 234000 | www.bd.com |
MES hydrate Buffer | Sigma | M8250-1kg | www.sigmaaldrich.com |
Carbenicillin | Sigma | C1613-1ML | www.sigmaaldrich.com |
Kanamycin | Sigma | 70560-51-9 | www.sigmaaldrich.com |
Sodium Hydroxide | Sigma | 221465 | www.sigmaaldrich.com |
Jack's Fertilizer | Hummert International | Jul-25 | www.hummert.com |
Equipment | |||
Vacuubrand MD4 Vacuum Pump | Fisher | 13-878-113 | www.fishersci.com |
Vacuum Air Regulator Valve | Fisher | NC9386590 | www.fishersci.com |
Desiccator 12 1/8" with O ring | Fisher | 08-594-15C | www.fishersci.com |
3 L Tub | Rubber-Maid | n/a | Rubbermaid Servn' Saver Bowl, 10-cup will work |
plate/shelf 230ML | Fisher | NC9489269 | www.fishersci.com |
Peat Pellet | Hummert International | 14-2370-1 | www.hummert.com |
Propagation Tray Dome | hydrofarm | 132052 | www.hydroponics.net |
Propagaiton Tray | hydrofarm | 138758 | www.hydroponics.net |
Virbo Hand Seeder | Gro-Mor INC | n/a | www.gro-morent.com |
Flora Cart 4 shelf | Hummert International | 65-6924-1 | www.hummert.com |
15 ml Round Bottom Culture Tubes | Sigma | CLS430172-500EA | http://www.sigmaaldrich.com |
Spectrophotometer | Bio-Rad | 170-2525 | www.bio-rad.com |
Spectrophotometer Cuvettes | Bio-Rad | 223-9950 | www.bio-rad.com |
Microcentrifuge Tubes | USA Scientific | 1415-2500 | www.usascientific.com |
Benchtop Centrifuge | Bio-Rad | 166-0602EDU | www.bio-rad.com |
Incubator/Shaker | Eppendorf | Excella E25 | www.eppendorf.com |
Ultraviolet Light Model#: UVGL-25 | UVP | 95-0021-12 | www.uvp.com |
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