ABSTRACT
A unique aspect of expression in higher eukaryotes is the choice in the species, variety, and the location and timing for expression. Firstly, there is a choice in the plant species to be used for expression. A choice no longer severely limited by transformation technology (see above). If the protein content of a crop (tissue) would correlate with the accumulation of the transgene protein, crops with higher protein contents would be more cost-effective for production. However, as far as we are aware, no proof is available that this is indeed the case. Moreover, the argument may be turned around-i.e., crops that are already accumulating high levels of endogenous protein may have little capacity left for the accumulation of heterologous protein. Secondly, there may even appear to be differences among varieties of one and the same species. It is well known that different germplasms differ in agronomic characteristics as yield, disease resistance, etc. It would therefore not be surprising if they would also differ in their ability to accumulate heterologous proteins. Thirdly, the enzyme can be expressed in various organs or tissues using appropriate promoters. Fourthly, expression in the plant, tissues, or organs can be limited to certain periods of development using promoters active only during these stages. Finally, the enzyme can be targeted to specific cellular compartments as the vacuoles, the protein bodies (in seed), the chloroplasts, the mitochondria, and the ER or to the apoplast using appropriate targeting signals described in the literature (47).
