Transplastomic Plants and Chloroplast Engineering
In, we discussed methods for transfer of genes to the nuclear genome in higher plants. In we also described the achievements made in the production and commercialization of transgenic plants, with particular emphasis on improvement of agronomically important traits in crop plants. However, transgenic plants produced in these cases almost invariably involved insertion of foreign genes in the nuclear genome, and only in rare cases, foreign genes could be successfully inserted in the genomes of plastids. Perhaps no examples are known (till 2001), where mitochondrial genome could be modified using recombinant DNA technology.

Chloroplast engineering or manipulation really means changes in the chloroplast genome or plastid proteins. When changes are brought about only at the protein level, genes for these plastid proteins are located either in the nuclear genome or in the chloroplast genome itself. In the former case, the proteins are synthesized in the cytoplasm and targeted to the plastids, while in the latter, the proteins are synthesized within the chloroplasts themselves. It is also known that plastid genome is small in size and a vast majority of the plastid proteins are really coded by nuclear genes. In view of this, introduction of novel proteins or enzymes into plastids can be achieved by either of the two following routes: (i) insertion of a foreign gene directly into the chloroplast genome for expression by the plastid's own protein synthesis machinery or (ii) insertion of a chimeric foreign gene construct (containing the gene of interest fused to sequences coding for a plastid transit peptide plus appropriate transcriptional control signals) into the nuclear genome. In the latter case, the gene product after its synthesis in the cytoplasm, is targeted to the chloroplast. Successful efforts following both these routes have been made.