Plastid transformation

Mahga, P., Plastid transformation in higher plants, Annu. Rev. Plant Biol. 55, 289,... 

Another requirement for oral delivery is the ability to express foreign proteins in plastids that are present in non-green tissues. One such example is the expression of an antibiotic resistance gene (aadA) in tomato chromoplasts [28]. More recently, stable and highly efficient plastid transformation has been achieved in the non-green... 

The plastid genome of modern higher plants remains well conserved, with little interspecific variation in genomic organization and coding capacity. Identical copies of the plastid genome are present in a diverse array of plastid transformation types proplastids (precursor plastids found in most plant cells and present predominantly in meristematic tissues). 

Another approach that has been explored for plastid transformation is microinjection. Microinjection into discrete plastids of intact plant cells entails the use of a syringe consisting of a submicron diameter glass capillary driven by the thermal expansion of galinstan, a liquid metal alloy... 

Heifetz, 2000). Although protoplasts that have been transformed in this way have been shown to exhibit transient gene expression, stable plastid transformation has not yet been successfully achieved (Knoblauch et al., 1999). 

There are many advantages to utilizing plastids for transformation some of them are listed in the following sections. 

Since thousands of copies of the plastid genome can be found per plant cell, extremely high levels of foreign protein are able to accumulate in plants that harbor transgenic chloroplasts. Expression levels greater than 40% of total soluble cellular protein, or 10-100 times higher than protein expression from nuclear transformants, have been detected from plastid transformants. 

A number of significant stumbling blocks have been identified that limit the practicality of plastid transformations the most noteworthy are mentioned here. 

Target Organelle for Foreign Protein Accumulation Transformation of plastids may be disadvantageous if the protein product is required to leave the plastid and go to another part of the plant cell. Unfortunately, little is known about the export of macromolecules from this organelle at present. 

Plastid transformation is highly dependent on the tissue culture process because it enables copies of the wild-type plastid genome to be selectively eliminated before plant regeneration (Maliga, 2003). However, many of the crop species regenerated in this way turn out to be sterile, a consequence of plant regeneration from tissue culture. As mentioned earlier, the transformation of Arabidopsis, tomato, potato, rice, and rape seed oil has been achieved at very low efficiencies, and the resulting transformants... 

Khan, M.S. et al. (1999). Fluorescent antibiotic resistance marker for tracking plastid transformation in higher plants. Nat. Biotechnol. 17 910-915. 

Maliga, P. (2003). Progress towards conunerciahzation of plastid transformation technology. Trends Biotechnol. 21(1) 20-28. 

Ruf, S., Hermann, M., Berger, I.J., Carrer, H., and Bock, R. (2001). Stable genetic transformation of tomato plastids and expression of a foreign protein in fruit. Nat. Biotechnol. 18 870-875. 

Svab, Z. and Maliga, R. (1993). High-frequency plastid transformation in tobacco by selection for a chimeric aadA gene. Proc. Natl. Acad. Sci. U.S.A. 90 913-917. 

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