Temporal and spatial distribution of erythropoietin in transgenic tobacco plants
Plants have shown promise as bioreactors for the large-scale production of a wide variety of recombinant proteins. To increase the economic feasibility of this technology, numerous molecular approaches have been developed to enhance the production yield of these valuable proteins in plants. Alternatively, we chose to examine the temporal and spatial distribution of erythropoietin (EPO) accumulation during tobacco plant development, in order to establish the optimal harvesting time to further maximize heterologous protein recovery. EPO is used extensively worldwide for the treatment of anaemia and is currently the most commercially valuable biopharmaceutical on the market. Our results indicate that the concentration of recombinant EPO and endogenous total soluble protein (TSP) declined significantly for every leaf of the plant during maturation, although the rate of these declines was strongly dependent on the leaf's position on the plant. As a result, the amount of EPO produced in leaves relative to TSP content remained essentially unchanged over the course of the plant's life. Decreasing levels of recombinant protein in leaves was attributed to proteolytic degradation associated with tissue senescence since transgene silencing was not detected. We found that significantly higher concentrations of EPO within younger leaves more than compensated for their smaller size, when compared to their low-expressing, fully-grown counterparts. This suggests that fast-growing, young leaves should be periodically harvested from the plants as they continue to grow in order to maximize recombinant protein yield. These findings demonstrate that EPO accumulation is highly influenced by the plant's physiology and development.