Genetic Transformation of Triticeae Cereals for Molecular Farming

Abstract

The in planta production of recombinant proteins is a newly emerging area. The use of transgenic crops enjoys several comparative advantages over established heterologous protein production systems based on bacteria, yeasts, mammalian or plant cells, particularly in terms of cost and practicality. Thanks to the development of effective transformation protocols, the generation of recombinant vaccines, antibodies and enzymes in the grains of the Triticeae cereals has become a feasible proposition in recent years. A further advantage of in planta synthesized recombinant proteins over bacterial and yeast-derived ones relates to post-translational modifications, in particular glycosylation. Since the majority of pharmaceutically active proteins are glycoproteins, their synthesis in bacteria and yeast is not possible. Therefore most of these proteins are currently synthesized in mammalian cell cultures. Since such cultures need complex (and therefore expensive) media, they also bear the risks of contamination by human pathogens. At present, about a dozen plant-derived pharmaceuticals are in the clinical phase of testing. Beside that a secretory IgA targeting tooth decay (CaroRxTM-from Planet Biotechnology Inc, Ma et al., 1998, 2005) and a human intrinsic factor targeted as a dietary supplement to alleviate vitamin B-12 deficiency (Cobento Biotech AS) are already approved for human use (Faye & Gomord, 2010). A number of field trials are currently underway to investigate and validate additional products (Dunwell, 2009; APHIS, 2011). The Triticeae family includes the major temperate crop species barley and wheat, which have been intensively bred over many decades to become well adapted to a wide range of growing environments. Although the major end-use of the temperate cereal grain is for food and feed, a significant focus of certain improvement programmes is aimed at the bioenergy market. Barley is seen as a more suitable host than wheat for transgenic applications because it is more easily transformed. An important advantage of barley and wheat in the context of biosafety is that they are largely self-pollinating, and so have been accorded G.R.A.S. (generally recognized as safe) status by the European regulatory agency EFSA. The infrastructure associated with cereal grain production, harvest and post-harvest storage is well established, and production volume is readily scalable by simply adjusting acreage. A number of transgene expression systems are available, some designed to restrict expression to the grain, but others allowing ubiquitous expression (for review, see Hensel et al., 2011). The purification of heterologous products can be a costly process, although in some situations this step is not needed; a good example is provided by the feeding to poultry of