Plants are increasingly becoming attractive bioreactors for production of commercially important proteins. Plastid transformation, which has been recently developed for this purpose, offers several advantages. (1) Each molecular unit of the chloroplast genome has a relatively small size, ca. 150 kbp, which facilitates homologous recombination and hence permits introduction of DNA modifications such as deletions or insertions at precise locations into the chloroplast chromosome. This is in contrast with the random insertion of DNA fragments into the nuclear genome obtained using the well-known method of gene transfer using Agrobacterium tumefaciens. (2) The overall organization of the chloroplast genome is of prokaryotic type, and therefore complex phenomenon such as position effects and epigenetic controls encountered in nuclear gene regulation do not exist. (3) The chloroplast chromosome has a large number – up to one hundred – of copies per mature chloroplast. For example, 10 000 chromosome copies per cell are present in mesophyll, which have approximately 100 chloroplasts/cell. Consequently, the amount of heterologous protein corresponding to a given transgene can reach a relative high level, e.g., in some cases up to 45 % of total soluble proteins.1 (4) Finally, most plant species of agronomic interest do not transmit the chloroplast genome by pollen, as their chloroplasts are maternally inherited. Maternal inheritance avoids gene dispersion and transfer to other, non-transformed, related plant species. Application of plastid transformation technology is limited by the difficulty of obtaining regulated, selective expression of the transgenes. Except for the blue light-inducible psbD promoter, there are no specific inducible endogenous promoters available; hence transgenes are constitutively expressed throughout plant development. As a consequence, plant growth is often heavily disturbed by th
