Recombinant proteins in plants

Background Over the last few decades, several studies have shown that plants can be a viable option for producing functional recombinant proteins with a wide range of structural characteristics [1]. In addition, the potential benefit for developing countries is a prominent feature that we have recently addressed [2]. Plant-produced recombinant proteins can already be considered a novel component in sustainable food production [3]. A major reason for this optimism relates to cost. Indeed, it is widely recognized that plants used as bioreactors to produce recombinant proteins would enable a significant reduction in overall manufacturing costs [2]. Although recombinant proteins can be functionally expressed in different plant systems, it is imperative to determine the platform that offers the most advantageous conditions for the expression and recovery of a particular protein [1]. In addition, because plant pathogenic organisms cannot cause human disease, this opens the possibility of exploiting plants and edible fruits as potential candidates for the production of orally administered antigens [1]. Basically, there are three strategies for recombinant protein production in plant-based systems: (1) use of cell-culture-based systems that are equivalent to mammalian, microbial and insect cell systems; (2) transient expression of foreign genes in plant tissues that are transformed by either agroinjection or by viral infection and (3) development of transgenic plants carrying stably integrated transgenes [4,5]. Here, I will focus on some of our recent results on transient expression and soybean seed as bioreactor-based systems. Transient expression systems are very useful for research and are now being routinely used for the rapid production of valuable proteins. These systems allow high throughput production and straightforward manipulation, permitting the rapid validation of expression constructs and the production of large amounts of recombinant protein within a few weeks. As a direct consequence, the protein yields from transient expression in plants are normally higher than yields observed in other recombinant plant systems. Transient technology is based on the insertion of transgenes into plant cells using plant viruses, commonly the tobacco mosaic virus (TMV) and the potato virus × (PVX) as well as transgenic Agrobacterium tumefaciens. Transgene insertion occurs without stable chromosomal integration, resulting in non-permanent and non-inheritable gene expression. Because the transfer rates of Agrobacterium T-DNA and viral-carried genes can reach a very high number of plant cells after infection. Tobacco leaves are the dominant choice for the development of commercial platforms using transient expression [1]. Seeds as bioreactors also provide a potential economical platform for the large-scale production and storage of recombinant proteins [1,5]. Soybean seed storage proteins are of great interest for the development of regulated tissue-specific genes products of commercial interest through recombinant DNA technology. The 7S globulins are comprised of b-conglycinin subunits. b-conglycinin regulatory sequences are seed tissue-specific, temporally regulated and expressed in both the embryonic axis and cotyledons of developing seeds.