This review article describes recent progress in research and development and practical applications of plant-made pharmaceutical (PMP) protein production in plant factories. We highlight key technologies for transgene expression in plants and the importance of environmental control in plant factories. As a new class of protein production platforms, plant-based systems have several advantages over conventional microbial and mammalian cell culture-based systems with regard to capital and

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... g costs, production scalability, and low risk of contamination by human pathogens during production. To date, five PMPs (two for humans and three for animals) have been approved by regulatory agencies worldwide. Among these, recent approval of a plant-derived COVID-19 vaccine (COVIFENZ ® ) in Canada marked a significant milestone in the PMP research field, providing a compelling case for the utility of the PMP technology to address emerging global public health problems. Currently, primary PMP production methods are built on either stable transformation or transient gene expression. The latter can be further classified into recombinant plant virus, agroinfiltration, and deconstructed virus (or agroinfection) systems based on the modes of transgene delivery and expression, and Nicotiana benthamiana is a de facto standard as host plant species. PMP manufacturing is operationally and regulatorily divided into upstream processing for plant biomass preparation and protein biosynthesis, and downstream processing for protein extraction, purification and fill/finish. Plant factories (including those with artificial lighting and greenhouses) are used for the upstream process, and there are several pilot-scale plant factories for commercial PMP production. Environmental control and cultural management before and after transgene introduction are essential engineering controls in transient gene expression systems in order to maximize the PMP protein productivity and quality. Finally, we offer our perspective and expectations for the future in this field.