SummaryWe describe here the development of a reproducible plastid transformation system for potato and regeneration of plants with uniformly transformed plastids. Two distinct tobacco-speci®c plastid vectors, pZS197 (Prrn/aadA/TpsbA) and pMON30125 (Prrn/GFP/ Trps16::PpsbA/aadA/TpsbA), designed for integration into the large single copy and inverted repeat regions of the plastid genome, respectively, were bombarded into leaf explants of potato line FL1607. A total of three transgenic lines were selected out of 46 plates bombarded with pZS197 and three transgenic lines out of 104 plates were obtained with pMON30125. Development of a high frequency leaf-based regeneration system, a stringent selection scheme and optimization of biolistic transformation protocol were critical for recovery of plastid transformants. Plastidexpressed green¯uorescent protein was used as a visual marker for identi®cation of plastid transformants at the early stage of selection and shoot regeneration. The establishment of a plastid transformation system in potato, which has several advantages over routinely used nuclear transformation, offers new possibilities for genetic improvement of this crop.
Efficient production of seedling-derived Type I callus was demonstrated for several corn genotypes including commercial inbred lines. Seeds were germinated on MS-based medium containing 10 mg l(-1) picloram and 3 mg l(-1) 6-benzylaminopurine, which induced the development of axillary buds in the area of coleoptilar node. Nodal sections of 7-10-day old seedlings were isolated, split longitudinally, and placed on callus induction medium supplemented with 2.2 mg l(-1) picloram and 0.5 mg l(-1) 2,4-dichlorophenoxyacetic acid. For lines L4 and L9 the frequency of embryogenic callus induction was 38-42% based on calli per split nodal section. Frequency of callus induction from split nodal sections of seeds germinated on media without growth regulators was 0-3%. Seedling-derived callus of five genotypes was used for Agrobacterium-mediated transformation. Two constructs containing the green fluorescence protein gene and genes for either neomycin phosphotransferase II or glyphosate selection were used in transformation experiments. Transformation frequency varied from 2 to 11% and about 60% of the T(0) plants had 1-2 copies of transgenes.
Petiole protoplasts from in vitro-grown carrot plants are a very good alternative to traditionally obtained protoplasts from suspension cultures. High plating and regeneration efficiencies were obtained in most of the breeding lines that were tested. The embedding of the protoplasts in alginate was crucial for initiating cell division and further development. Several streptomycin resistant and chlorophyll-deficient plant lines were selected for using the petiole protoplast system. Maternally inherited streptomycin resistance was demonstrated by sexual crosses. Protoplast fusion of several chlorophyll-deficient lines did not result in complementation, indicating the cytoplasmic nature of the mutations. Petiole protoplasts were used for direct transformation with plasmid DNA pNUNV containing NPTII as a selectable marker. High transformation frequencies (up to 1%) were obtained after PEG treatment of the protoplasts. Kanamycin resistance was shown to be inherited as a single dominant nuclear trait.
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