Plastid transformation systems

Transformation of protoplasts by PEG has been demonstrated to work well for chloroplasts as well as plant nuclei. Since chloroplasts tend to be appressed to the cell membranes in protoplasts, they can be penetrated easily by this method. However, this approach is technically more demanding than biolistic delivery and involves a considerably longer period of time for the selection process (Heifetz, 2000). 

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In conclusion, plastid-based expression systems provide for potentially higher expression levels than the majority of nuclear expression systems, and with a higher level of transgene containment, but the platform provides limited post-translational processing choices. Also, despite reports of successful plastid transformation of new species beyond Nicotiana, the choice of production host is stiU predominantly restricted to the Nicotiana family. However, this hmita-tion does not present a serious drawback for production of pharmaceutical proteins (except perhaps for edible vaccines ) since tobacco is a non-feed/non-food crop with a weU-estabhshed agriculture. 

Since the first successful transformation of tobacco chloroplasts [83], expression systems based on the transformation of plant plastids has attracted the attention of plant biotechnologists. The features that make this technology so attractive are its potential for high protein yield, along with inherent biosafety features such as limited plastid transfer via pollen (due to maternal inheritance of plastid-encoded genes) [84] and the relatively low probability of transgene movement from the chloroplast to the nucleus [85, 86]. 

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