Many recombinant proteins have been produced in those plants that are ideally suited for laboratory experiments, such as tobacco or Arabidopsis thaliana, but these are not suited to long-term or large-scale production. The global agricultural industry has focused on high-yield crops that have been optimised over generations to be cost-effective for the large-scale production of food, animal fodder and other products, such as fibres for clothing and pulp for paper-making. Therefore, research on molecular farming has moved towards evaluating these commercial species for the production of pharmaceuticals and other recombinant proteins.In their contribution, Stoger et al. first discuss the nature of the recombinant proteins expressed in plants before progressing to a detailed discussion of the issues that are related to choosing the most suitable species for protein expression. These include a meticulous review of the decisions that need to be taken during protein-expression projects and an indication of where the potential advantages lie with each crop species. Their contribution provides a solid framework and entry point for researchers new to molecular farming.
Industrial ‘Plants’ of the Future
In her chapter, Hood describes the exploitation of transgenic plants as a production system for proteins that are of industrial importance. These include recombinant enzymes, the use of which would benefit industrial production processes that are currently based on synthetic chemistry. The high specificity and activity of enzymes is what makes them attractive to industry, and the argument presented in the contribution is that plants, with their high biomass and large-scale production potential, are ideal for mass production of these proteins. The central thesis is that using plants will reduce the cost of these proteins and make them available for a wide number of applications in industries where they were previously unaffordable.
The use of plants to produce industrially relevant enzymes has been shown to be practical in a number of species. For example, phytase and αα-amylase have been produced in tobacco. However, the author argues that the expression of proteins in leaf tissue is not ideally suited to molecular farming. This is primarily because of the expense and the difficulties involved in extracting the proteins from leaves. She discusses, therefore, using other plant species, such as alfalfa or oilseed crops, to produce industrially important proteins. The author makes the case for maize (Zea) as a production system, her well-reasoned argument being based on a comparison of the production costs of various crops. Although alfalfa has the greatest potential for the production of recombinant protein per hectare, this is offset by the need to extract the protein from leaf material. Of the crops surveyed, soybean has the lowest cost for protein production, but the methods required for transformation are labour intensive and expensive. The cereal crops, such as rice and wheat, are shown to have positive advantages for expression but, for pragmatic reasons, maize was their selected production platform. First, maize is the largest crop produced in North America and its two major advantages are the low cost and ease of large-scale production. The rationale for the use of maize rests on the sophisticated existing production and harvesting infrastructure and on the advantages of using a seed-based production system.
One contentious and difficult issue covered by Hood is that of containment, which is of serious concern given the large amounts of maize produced within the United States for both human and animal consumption. The author presents a series of measures that can be used to control inadvertent mixing of corn destined for food or fodder with the transgenic variety. It can be argued that this is where industry has to be at its most vigilant, because the consequences of transgenic crops entering the food chain are potentially very large. Overall, however, with close control over its use, maize could become an acceptable and profitable species for use as an ‘industrial plant’ of the future.
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