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Over the last three decades, the expression of recombinant proteins in plants and plant cells has been promoted as an alternative cost-effective production platform. However, the market is still dominated by prokaryotic and mammalian expression systems, the former offering high production capacity at a low cost, and the latter favored for the production of complex biopharmaceutical products. Although plant systems are now gaining widespread acceptance as a platform for the larger-scale production of recombinant proteins, there is still resistance to commercial uptake. This partly reflects the relatively low yields achieved in plants, as well as inconsistent product quality and difficulties with larger-scale downstream processing. Furthermore, there are only a few cases in which plants have demonstrated economic advantages compared to established and approved commercial processes, so industry is reluctant to switch to plant-based production. Nevertheless, some plant-derived proteins for research or cosmetic/pharmaceutical applications have reached the market, showing that plants can excel as a competitive production platform in some niche areas. Here, we discuss the strengths of plant expression systems for specific applications, but mainly address the bottlenecks that must be overcome before plants can compete with conventional systems, enabling the future commercial utilization of plants for the production of valuable proteins.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel coronavirus responsible for an ongoing human pandemic . There is a massive international effort underway to develop diagnostic reagents, vaccines, and antiviral drugs in a bid to slow down the spread of the disease and save lives. One part of that international effort involves the research community working with plants, bringing researchers from all over the world together with commercial enterprises to achieve the rapid supply of protein antigens and antibodies for diagnostic kits, and scalable production systems for the emergency manufacturing of vaccines and antiviral drugs. Here, we look at some of the ways in which plants can and are being used in the fight against COVID-19. COVID-19: How Can Plant Biotechnology Help?An outbreak of potentially lethal coronavirus (SARS-CoV-2; see Glossary) in Wuhan, China, in December 2019, has created a pandemic (COVID-19) that has provoked governments across the world to introduce emergency containment and control measures. The aim of these measures is to delay the spread of infection, thus reducing the acute pressure on hospital beds, frontline medical staff, and resources. Slowing down the rate of infection and thereby reducing the total number of acute cases at any one time can help to prevent the collapse of national healthcare systems. These tactics also give researchers more time to develop effective testing assays to identify carriers, treatments that reduce the severity of symptoms and resolve infections more quickly, and vaccine candidates to protect the unexposed segment of the population. Researchers working on the applications of plants can have a key role during this critical time by using their knowledge and infrastructure as a means to develop and produce new diagnostics and therapeutics. Indeed, plants may offer the only platform that can be used to manufacture such reagents at scale in a timeframe of weeks, compared with months or even years for cellbased systems. Here, we look at three areas where plants could make major contributions: diagnostic reagents to identify infected and recovered individuals, vaccines to prevent infection, and antivirals to treat symptoms.

To evaluate the expression of different forms of a tumor-specific antibody in plants, we adapted a recently described Agrobacterium-mediated transient expression system. A recombinant single-chain Fv antibody (scFvT84.66) and a full-size mouse͞human chimeric antibody (cT84.66) derived from the parental murine mAb T84.66 specific for the human carcinoembryonic antigen were engineered into a plant expression vector. Chimeric T84.66 heavy and light chain genes were constructed by exchanging the mouse light and heavy chain constant domain sequences with their human counterparts and cloned into two independent plant expression vectors. In vivo assembly of full-size cT84.66 was achieved by simultaneous expression of the light and heavy chains after vacuum infiltration of tobacco leaves with two populations of recombinant Agrobacterium. Upscaling the transient system permitted purification of functional recombinant antibodies from tobacco leaf extracts within a week. His6-tagged scFvT84.66 was purified by immobilized metal affinity chromatography and cT84.66 by protein A affinity chromatography. Sufficient amounts of recombinant antibodies were recovered for detailed characterization by SDS͞PAGE, Western blotting, and ELISA.Monoclonal antibodies are essential tools in biology, biochemistry, and medicine. Their high affinity and specificity make them invaluable for diagnostic and therapeutic applications. However, the therapeutic use of murine mAbs is limited because they elicit a human anti-mouse antibody response and large amounts of antibody are required for therapy. These limitations may be overcome by engineering humanized antibodies and by producing these proteins in plants.The human anti-mouse antibody response can be reduced by using recombinant antibody (rAb) technology to replace the murine light and heavy chain constant domains with the corresponding human domains and the remaining murine variable domains to maintain the antigen specificity and affinity of the original mAb. A second approach is to use single-chain Fv antibody fragments (scFvs) where the constant domains have been removed and the variable domains are joined by a flexible linker (1). Compared with the full-size antibodies, scFvs display better tumor penetration and faster serum clearance but exhibit no effector functions. A critical step in testing the potential therapeutic use of these molecules is the development of a reproducible and efficient method for large-scale antibody production.Plants are potentially the most economical system for largescale production of rAbs (2, 3). rAbs are efficiently folded and assembled within the endoplasmic reticulum (ER) of plant cells (4-6) and retain the antigen binding properties of the antibodies produced by plasma or hybridoma cells (2,5,(7)(8)(9). Since the first report of antibody expression in transgenic plants (7), different engineered antibodies have been produced successfully, including full-size antibodies (8-11), Fab fragments (12), scFvs (13-21), and single-domain antibodies (22).Regene...