The major difficulty in engineering plastid genome for production of transplastomic plants is in generating homoplasmic plants in which all the plastids are uniformly transformed, for that takes a long process of selection, thus hampering the production of genetically stable transplastomic plants (e.g. rice). This is due to the presence of about 10-100 plastids, each of which has up to 100 copies of the plastid genome, in one cell, that does not allow achieving homoplastomic state [73] . It was also stated that getting high level of protein expression, even though the gene copy number is high, is another problem. In 2005, however, Nguyen et al. [41] described the generation of homoplasmic plastid transformants of a commercial cultivar of potato (Solanum tuberosum L.) using two tobacco specific plastid transformation vectors, pZS197 (Prrn/aadA/psbA3 ) and pMSK18 (trc/gfp/Prrn/aadA/psbA3 ). Similarly, Liu et al. [74] were able to develop homoplasmic fertile plants of Brassica oleracea L. var. capitata L. (cabbage). Among other higher plants of which fertile homoplasmic plants with genetically modified plastid genomes have be been produced are Nicotiana tabacum (tobacco), Nicotiana plumbaginifolia (texmex tobacco), Solanum lycopersicum (tomato), Glycine max (soybean), Lesquerella fendleri (bladderpod), Gossypium hirsutum (cotton), Petunia hybrida (petunia), and Lactuca sativa (lettuce) [68] . The amino glycoside 3adenylyltransferase (aadA) gene, which confers dual resistance to spectinomycin-streptomycin antibiotics, is still the selectable marker that is routinely used efficiently for plastid transformation [58, 75, 76] . Since the antibiotic resistant genes used in transformation are not desirable in the final products, different strategies have been developed to eliminate the necessity of using such selectable markers [56, 77] .