Plant based recombinant protein expression systems have become serious competitors over traditional cell culture systems in the last two decades. They offer various advantages (Hellwig et al., 2004; Ma et al., 2005) including low production cost, low risk of contamination by mammalian pathogens and endotoxins, production of correctly folded and assembled multimeric proteins and the possibility for use as edible vaccines (Ma et al., 2003; Lico et al., 2008; Shih and Doran, 2009; Marsian and Lomonossoff, 2016).
Most plant secretary proteins undergo post translational modifications for proper functioning, localization and stability. Over 300 post translation modifications have been identified in plants (Stulemeijer and Joosten, 2008). Thus, plants have ability to perform post translational modifications including production of complicated glycoproteins through glycosylation, a modification important for the bioactivity and stability of those proteins. In contrast, bacterial or lower eukaryotic expression systems do not have this ability, cannot be used when expressing proteins with these specific post translational modification or the assembly of multimeric subunits for their biological activity (Webster and Thomas, 2012).
Most vaccines currently used (especially for viral diseases) come from lab cultured pathogens through attenuation or inactivation, making possible the risk of residual pathogenic activity. In this regard, recombinant proteins expressed in plant bio reactors are safer and more reliable, as they contain specific components of pathogens with the immunological properties of the original pathogen but not its pathogenic properties (Marsian and Lomonossoff, 2016; Giddings, 2001). Plant expression systems enormously simplify the delivery and administration of vaccines in several ways. Expressing subunit vaccine genes in edible tissues of plants facilitates the oral delivery of vaccines. Orally delivered plant made vaccines is protected by plant cells in the stomach through bio encapsulation. This simplifies the production and administration procedures of vaccines by eliminating the requirement of expensive purification steps as well as sterile injections. Vaccines expressed in plant bioreactors do not need cold storage and transportation nor sterile delivery facilities eliminating additional production costs (Daniell, 2006; Boehm, 2007; Daniell et al., 2009).