The use of plants as expression hosts for the large-scale production of recombinant proteins is a recent innovation that has potential advantages of economy, scalability and safety over traditional expression systems.
Many pharmaceutical proteins have been expressed in plants as part of 'proof of principle' studies, including human and animal proteins, recombinant subunit vaccines and recombinant antibodies. Only a few of these proteins have reached advanced stages of development and even fewer have begun clinical trials.
Most pharmaceutical proteins have been produced in transgenic tobacco plants, because tobacco has a long history as a model organism and robust expression constructs are available. However, there is increasing interest in the use of other species, particularly cereals, legumes, fruit and vegetables.
Diverse plant-based expression systems, such as transient expression, plant cell-suspension cultures, recombinant plant viruses and the chloroplast transgenic system are being investigated.
Pharmaceutical proteins that are expressed in dry cereal and legume seeds are highly stable and can be stored for long periods at room temperature with no loss of activity.
The expression of antibodies and vaccines in edible fruit and vegetables might allow oral administration in partially processed plant tissues.
There are some differences between the glycan structures of recombinant glycoproteins that are produced in animals and plants, but so far there is no evidence that such differences cause adverse reactions in human patients.
The acceptability of plant-derived pharmaceutical proteins depends on the production of such proteins under cGMP conditions, in line with other expression systems.
Imagine a world in which any protein, either naturally occurring or designed by man, could be produced safely, inexpensively and in almost unlimited quantities using only simple nutrients, water and sunlight. This could one day become reality as we learn to harness the power of plants for the production of recombinant proteins on an agricultural scale. Molecular farming in plants has already proven to be a successful way of producing a range of technical proteins. The first plant-derived recombinant pharmaceutical proteins are now approaching commercial approval, and many more are expected to follow.
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The authors are grateful to R. Twyman for critical assessment and help with manuscript preparation.
- SINGLE-CHAIN FV FRAGMENTS
(scFvs). Monoclonal antibody derivatives that comprise a single polypeptide in which the variable regions of the heavy and light immunoglobulin chains are joined together by a flexible linker. scFvs are advantageous because only one transgene is required, and the molecules themselves are small and lack the effector functions of normal antibodies; however, a disadvantage is that they are univalent, whereas serum antibodies are divalent.
- MOLECULAR FARMING
The large-scale production of recombinant proteins in living cells or organisms; frequently applied to the use of crop plants or domestic animals as expression hosts because of the allusion to agriculture.
In the context of this article, a gene or protein that is not derived from the species in which it is expressed.
A transgenic plant in which the transgene is found in the plastid genome rather than the nuclear genome.
A recombinant antibody that comprises the heavy- and light-chain variable regions joined by a flexible peptide linker. The linker is long enough to allow separation of the domains so that two of the polypeptides can assemble into a dimer, making the antibody divalent.
A recombinant antibody in which the heavy- and light-chain variable regions are part of the same polypeptide chain, which also includes the heavy-chain hinge region and one heavy-chain constant domain.
- AGROINFILTRATED LEAVES
Usually leaves of tobacco (although many other species can be used) that are transiently transformed with Agrobacterium tumefaciens, which results in the transient expression of recombinant proteins. This is a useful strategy for testing expression constructs and obtaining small amounts of protein for analysis before going to the expense of transgenics.
Producing toxins in the gut that specifically affect the intestinal mucosa.
- DICOLYLEDONOUS PLANTS
(Dicots). Broad-leaf flowering plants the seeds of which contain two cotyledons (embryonic seed leaves that either remain in the seed when the plant germinates or emerge and become green). Examples include potato, tomato, tobacco and all peas and beans.
- MONOCOTYLEDONOUS PLANTS
Narrow-leaf plants the seeds of which contain one cotyledon. Examples include cereals, grasses, orchids and lilies.
A single antigenic determinant on a protein that is recognized by an antibody. A single protein can have many epitopes.
- LEYDIG CELLS
Interstitial cells in the testis that are responsible for the production of male sex hormones, such as testosterone, and are important in male sexual differentiation.
- SITE-DIRECTED MUTAGENESIS
An in vitro mutagenesis procedure that is often carried out using the polymerase chain reaction in which specific mutations are introduced into a DNA molecule.
- SIGNAL PEPTIDE
A short sequence of mainly hydrophobic amino acids at the N-terminus of secreted proteins. This peptide is captured by a signal-recognition particle as it emerges from the ribosome, which allows the ribosome to be transported to the endoplasmic reticulum.
- MOLECULAR CHAPERONES
Proteins the function of which is to ensure correct folding of other proteins during or after synthesis, or the refolding of denatured proteins.
The extracellular space. In plants, this is a large and continuous network of cavities under the cell wall. Proteins that are secreted from the cell often remain trapped here.
- POSITION EFFECTS
When transgenes integrate into genomic DNA, the expression level is often influenced by the surrounding chromatin. Local regulatory elements, such as enhancers, also influence transgene expression. Position effects lead to wide variations in transgene expression levels, even in plants that are transformed with identical constructs.
- AFFINITY TAGS
Short peptide sequences added to recombinant proteins, which bind strongly to particular affinity matrices and can be used to purify recombinant proteins.
- AGROBACTERIUM-MEDIATED TRANSFORMATION
Transformation that is achieved using the natural gene-transfer mechanism of Agrobacterium tumefaciens.
- WHISKER TRANSFORMATION
Transformation that is achieved by mixing walled plant cells with silicon carbide fibres that penetrate the cell wall and membrane, which generate pores through which DNA can be taken up into the cell.
Transformation that is achieved by exposing cells or protoplasts to a brief pulse of electricity, which results in the formation of transient membrane pores through which DNA can be taken up into the cell.
- PROTOPLAST TRANSFORMATION
Any technique for introducing DNA into unwalled plant cells (protoplasts), such as calcium phosphate transfection, PEG transfection or electroporation.
- T-DNA BORDER SEQUENCES
Imperfect 25 bp direct repeat sequences that flank the piece of DNA that is transferred to the plant genome by Agrobacterium tumefaciens. These sequences are recognized by the bacterial VIRD1 and VIRD2 proteins, which form an endonuclease complex. Cleavage of the border sequences initiates T-DNA transfer.
- HYBRIDOMA CELLS
A hybrid cell line that is created by fusing a mortal antibody-producing B-lymphocyte with an immortalized myeloma line. The hybridoma line is immortal and produces a continuous supply of a particular monoclonal antibody.
A family of flowering plants (order Solanales) that comprise ∼100 genera and ∼2,500 species, many of which are economically important as food or medicinal crops. Examples include tobacco, potato and tomato.
- FRIABLE CALLUS TISSUE
Callus tissue is undifferentiated plant tissue, which grows when seeds or explants are cultured on media that contains an appropriate balance of plant hormones. Friable callus tissue is easily broken into fragments.
- BATCH, FED-BATCH, PERFUSION AND CONTINUOUS FERMENTATION
Batch fermentation is a closed system in which all of the substrate is added at the beginning, whereas in the fed-batch process the substrate is added in increments as fermentation proceeds. Continuous fermentation is an open system in which substrate is added continuously at a steady rate. Perfusion fermentation is a continuous process that allows cells to be grown at high density, and so results in increased biomass and product yields.
The soil zone that surrounds plant roots, which is rich in microorganisms and in which interactions occur between plants and microbes.
- LEAF GUTTATION FLUID
Fluid that seeps from the apoplast onto the leaf surface. In plants with large leaves, such as tobacco, large amounts of guttation fluid can be produced each day.
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Ma, J., Drake, P. & Christou, P. The production of recombinant pharmaceutical proteins in plants. Nat Rev Genet 4, 794–805 (2003). https://doi.org/10.1038/nrg1177
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