Metabolic Engineering of Plant Cells in a Space Environment

Metabolic engineering aims to optimize yields of comlnercially valuable products by controlling enzymatic, transport, and cell regulatory functions by indirect or direct genetic intervention. Many natural products from plants on earth are now being metabolically engineered (eg Grotewold et al., 1998; Knauf, 1998; Gnlys et al., 1998). While large-scale culture is technically feasible (Verpoorte el al., 1998), plant cell bioreactors have historically given ecollotnically low and unreliable yields (Alfermann and Petersen, 1995; Fowler, 1988). Shikonin and berberine (Fujita, 1988) raised hopes for products that were eventually not commercially sustainable. The recovery of products froln vacuoles requires harvesting and disruption of cells. Processes are sought where products are released directly into the culture medium. Slow growth and low product levels are major constraints. Space environments offer new opportunities for the metabolic engineering of plant cells. Bioreactor process controls will differ significantly from production facilities on earth because of constraints in gravitational fields, the increased use of small modular designs, and physical Iinlitations to downstream processing capabilities. For the International Space Station (ISS), spacecraft construction and maintenance of crew support systems will initially limit the time for basic and applied research. This review evaluates factors for the development and testing of lTIodels for drug producing plant cells, given the constraints of space environlnents. Mission priorities are given