**4. Food for thought**

The first demonstration the 2A was active in plant cells used an artificial polyprotein which comprised two reporter proteins flanking 2A [38]. This co-expression system was soon adopted by plant virologists for use in both rod-shaped and icosahedral virus particles either as highlevel expression systems, or, to produce particles 'decorated' with fluorescent proteins, immunogens, single-chain antibodies etc. [137-147]. Here, plants are used simply as 'bioreac‐ tors' for production of recombinant proteins / virus particles – the plants are not transgenic. In the case of transgenic plants the first reports of the use of 2A to co-express multiple proteins were as a 'proof-of-principle' or research tools [38, 57], but within a few years plants were being genetically engineered to demonstrate how nutritional properties could be improved [105, 149]. Whilst the use of 2A rapidly expanded in the arenas of animal biotechnology and biomedicine (e.g. monoclonal antibody production, cancer gene therapies, production of pluripotent stem cells: reviewed in [25]), progress in transgenic plants was slower-due to a number of reasons, including the 'trickle-down' effects on plant biotechnology from the EU policies concerning genetically-modified plants. Over the past few years, however, the 2A coexpression system has been used in the development of methods to engineer plant genomes [149], the expression of high-value proteins, the improvement of plant tolerance to biotic and abiotic stresses, the improvement of nutritional properties through metabolome engineering [*vide supra*] and the expression of plant storage proteins with amino acid content more suited to human nutrition [150]. The drive to improve agricultural productivity through the devel‐ opment of 'dual-use' crops necessitates complex strategies of plant engineering and it seems clear that the use of 2A will continue to expand.
