Plastid transformation in flowering plants.
Genomics of Chloroplasts and Mitochondria. 35:393-414. Abstract
The plastid genome of higher plants is relatively small, 120–230-kb in size, and present in up to 10,000 copies per cell. Standard protocols for the introduction of transforming DNA employ biolistic DNA delivery or polyethylene glycol treatment. Genetically stable, transgenic plants are obtained by modification of the plastid genome by homologous recombination, followed by selection for the transformed genome copy by the expression of marker genes that protect the cells from selective agents. Commonly used selective agents are antibiotics, including spectinomycin, streptomycin, kanamycin and chloramphenicol. Selection for resistance to amino acid analogues has also been successful. The types of plastid genome manipulations include gene deletion, gene insertion, and gene replacement, facilitated by specially designed transformation vectors. Methods are also available for post-transformation removal of marker genes. The model species for plastid genetic manipulation is Nicotiana tabacum, in which most protocols have been tested. Plastid transformation is also available in several solanaceous crops (tomato, potato, eggplant) and ornamental species (petunia, Nicotianasylvestris). Significant progress has been made with Brasssicaceae including cabbage, oilseed rape and Arabidopsis. Recent additions to the crops in which plastid transformation is reproducibly obtained are lettuce, soybean and sugar beet. The monocots are a taxonomic group recalcitrant to plastid transformation; initial inroads have been made only in rice.